Chapter 11 Organometallic Compounds
Old Faithful (Yellowstone National Park) David Richardson
“If a conclusion is not poetically balanced, it cannot be scientifically true.” Isacc Asimov
“Miracles are not contrary to nature but only contrary to what we
know about nature. ” Augustine of Hippo
2
Organometallic compounds contain an alkyl or aryl component in combination with a metal. These compounds are named as derivatives of the metal. The metal is the parent and the alkyl or aryl groups are substituent prefixes. For example, the name for compound I is cyclohexyl magnesium bromide.
Compound I
Compound II is phenyl magnesium iodide:
Compound II
When the compound does not contain a halogen, simply name the compound using the alkyl or aryl group followed by the name of the metal with no space between the alkyl or aryl group and the metal. For example, compound III would be called dicyclopentylmagnesium.
Compound III
3
Compound IV would be p-methoxyphenyllithium.
Compound IV
Compound V would be 1-butyn-1-yl sodium
Compound V
Another name for 1-butyn-1-yl sodium is sodium 1-butyn-1-yl. The metal is less electronegative than carbon in organometallic compounds; therefore, the carbon has a partial negative charge, and the metal has a partial positive charge.
This single factor provides organometallic compounds with their reactive abilities.
4
Preparation of organometallic compounds Reacting primary, secondary or tertiary alkyl halides with lithium or sodium in an anhydrous solvent serve as a source for synthesizing lithium and sodium organometallic compounds. . As you know, water will react with the metal to form hydrogen gas and the metal hydroxide. For example, sodium reacts violently with water to form sodium hydroxide and hydrogen gas, Na + 2 H2O (l) → 2 NaOH (aq) + H2 (g). Therefore, solvents used for preparing organometallic compounds cannot have protons attached to electronegative atoms; otherwise, reactions producing hydrogen gas similar to the one above will occur, e.g., 2 RCH2OH + 2 Li → 2 RCH2O- Li + H2. These solvents are aprotic solvents, i.e., they cannot have protons attached to electronegative atoms; therefore, they cannot be alcohols, water, primary or secondary amines, or mercaptans (RSH). Trace amounts of water, alcohols, ammonia, primary or secondary amines, hydrogen sulfide or mercaptans would destroy the organometallic reagent since small quantities of alcohols (primary, secondary or tertiary) will react with lithium or sodium to produce lithium or sodium alkoxides. The resulting organolithium or organosodium compound converts to a hydrocarbon. Also, the resulting lithium or sodium alkoxides would coat the surface of remaining lithium or sodium metal to prevent it from reacting with any remaining alkyl halide molecules. If water, ammonia, primary or secondary amines, hydrogen sulfide, or mercaptans are present in the reaction vessel, they would react with organolithium or organosodium compounds to produce hydrocarbons.
Therefore, aprotic solvents, i.e., solvents without hydrogen atoms attached to an electronegative atom, such as hydrocarbon solvents and ethers, are effective solvents for producing organolithium or organosodium compounds 5
from lithium or sodium and alkyl halides. Alkyl iodides react with lithium and sodium faster than alkyl bromides, and alkyl bromides react faster than alkyl chlorides, and alkyl fluorides do not react with lithium or sodium metal. Even though vinyl halides, H2C=CHX, and aryl halides, C6H5X, will not undergo nucleophilic substitution reactions and elimination reactions, they will react with lithium and sodium in the following manner:
The preferred solvent for reactions involving alkyl halides, vinyl halides and aryl halides is diethyl ether, CH3CH2OCH2CH3 or tetrahydrofuran. Chlorobenzene reacts with sodium metal to produce phenylsodium and sodium chloride. Tetrahydrofuran is the solvent for the system.
In tetrahydrofuran
6
2-iodo-1-butene, a vinyl halide, reacts with lithium metal to produce butenyllithium and lithium iodide. Diethyl ether is the solvent for the system.
In diethyl ether CH3CH2OCH2CH3
In diethyl ether CH3CH2OCH2CH3
1-bromobutane reacts with sodium metal to produce butylsodium and sodium bromide. Diethyl ether is the solvent for the system.
In diethyl ether CH3CH2OCH2CH3 The mechanism (the series of elementary steps that rationalize the formation of the compound) for the formation of organolithium and organosodium 7
compounds relates to the facile ability of sodium to lose an electron. The first step would involve the transfer of an electron to the alkyl group to form a radical anion. (1) The first step in the mechanism is the transfer of an electron from the metal to the alkyl halide to form the unstable radical anion intermediate.
The structure of the radical anion would have an electron in an antibonding molecular orbital of the carbon atom. (2) The resulting radical anion quickly dissociates (in a second step) to an alkyl radical and a halogen anion.
(3) The final step is a reaction between the alkyl radical and another sodium atom to form an alkyl anion molecule and sodium cation.
Adding equations 1-3 gives: RBr + 2 Na → R:- Na+ + Na+ Br-
Hydroboration-Oxidation Probably the most revolutionary and most important organometallic 8
compounds are the alkylboranes. As indicated in Chapter 3, Dr. Herbert C. Brown (05/22/1912-12/19/2004) pioneered organoborane exploration at Purdue University. He received the 1979 Nobel Prize in chemistry for his revolutionary work on the syntheses of organoborane from alkenes. Organoboranes are precursors to the syntheses of some important drugs, e.g., antidepressants and cholesterol-lowering drugs. In review, diborane adds to carbon-carbon double bonds. Treatment of the resulting organoboranes with hydrogen peroxide form alcohols with an apparent anti-Markovnikov’s arrangement. These reactions are hydroboration-oxidation reactions. The following is a general equation that represents the addition of diborane to alkenes to form alkylboranes.
Once the alkylborane has been formed, it can be oxidized with hydrogen peroxide in basic media to produce an alcohol where the OH group resides on the lesser alkylated carbon atom of the precursor alkene. The following is a general representation for this chemical reaction:
9
Following is an illustration of H. C. Brown’s hydroboration-oxidation reaction for the synthesis of 2-methylcyclohexanol from 1-methylcyclohexene.
1-methylcyclohexene
10
2-methylcyclohexanol
The product, 2-methylcyclohexanol, has the OH group on the carbon atom with the fewer number of carbon atoms. The following mechanism represents a pathway that explains hydroborationoxidation reactions. The product, 2-methylcyclohexanol, has the OH group on the carbon atom with the fewer number of carbon atoms.
(1)
1. HOOH + -OH → HOO- + HOH (2) 11
(3)
(4)
(5) HOOH + -OH → HOO- + HOH 12
(6)
(7)
(8)
(9) HOOH + -OH → HOO- + HOH 13
(10)
(11)
14
(12)
The sum of elementary steps (1) -(12) gives the reactants and products with their stoichiometric quantities. Hydroxide, -OH, is the catalyst for the reaction; therefore, it does not appear as a reactant or a product.
Grignard Reagents Grignard reagents are very important compounds used as precursors for the syntheses of multiple compounds. Professor Victor Grignard, the recipient of the 1912 Nobel Prize in Chemistry, reported that alkyl halides react with magnesium to produce organomagnesium halides, and these halides are precursors for synthesizing primary, secondary and tertiary alcohols.
“R� can be a primary alkyl group, a secondary alkyl group, a tertiary alkyl 15
group, cycloalkyl group, alkenyl groups, or aryl groups. The following is an example of a reaction where the aprotic solvent used is tetrahydrofuran (THF).
The reactivity of organohalide compounds with Mg is: RI > RBr > RCl > RF; and alkyl halides are more reactive than aryl and vinyl halides. The solvent of preference for vinyl and aryl halides is THF, because THF has a higher boiling point (339K) than ether (boiling point 308K).
The mechanism for the formation of the Grignard reagent is analogous to the mechanism for formation of organolithium and organosodium compounds with the formation of a radical anion intermediate. (1) Formation of the unstable radical anion intermediate.
(2) The rapid disintegration of the radical anion produces the alkyl radical. 16
(3) The alkyl radical can react with magnesium radical cation produced in step (1) to produce the Grignard reagent.
In a like manner that was described earlier, water; alcohols (primary, secondary or tertiary); ammonia; primary or secondary amines; hydrogen sulfide or mercaptans (R-SH) convert Grignard reagents to hydrocarbons.
Grignard reagents can react with the acidic protons of terminal alkynes to form alkyne magnesium halides.
17
Grignard reagents are precursors to the syntheses of primary alcohols, secondary alcohols, and tertiary alcohols. Grignard reagents and formaldehyde are precursors to the syntheses of primary alcohols. Aldehydes and Grignard reagents are precursors to the syntheses of secondary alcohols. Ketones and Grignard reagents are precursors to the syntheses of tertiary alcohols. Grignard reagents react with formaldehyde to form intermediate alkoxymagnesium halides, and then the alkoxymagnesium halides are hydrolyzed by mineral acids in water to produce primary alcohols. The following examples illustrate this reaction:
18
Secondary alcohols react with Grignard reagents and aldehydes in diethyl ether, and the resulting alkoxymagnesium halides are hydrolyzed by mineral acids in water to produce the desired secondary alcohols.
19
The following examples are illustrations of this process:
20
Grignard reagents react with ketones in diethyl ether, and the resulting alkoxymagnesium halides are hydrolyzed by mineral acids in water to produce the desired tertiary alcohols. The following examples are illustrations of this process.
21
Organolithium compounds are more reactive toward formaldehyde, aldehydes and ketones than Grignard reagents. The products, i.e., primary, secondary and tertiary alcohols, are analogous to the products produced by the Grignard reagents with formaldehyde, aldehydes and ketones.
22
The following sequence of chemical reactions where an acetylide reacts with a carbonyl group to produce acetylenic alcohols (compounds containing a triple bond and an “OH� group on an adjacent carbon atom).
Acetylenic Grignard reagents are prepared by reacting terminal acetylenes with alkyl Grignard reagents.
23
Also, acetylenic Grignard reagents react with formaldehyde, aldehydes and ketones to form primary, secondary and tertiary alkynols (compounds containing a triple bond an “OH” group on an adjacent carbon atom). For example, let’s look at one possible pathway for the synthesis of 4cylcohexyl-2-butyn-1-ol. The synthesis can be accomplished by treating 3cyclohexyl-1-propylmagnesium bromide with formaldehyde in dry ether, followed by the acid hydrolysis.
24
A similar synthesis could be used for the preparation of 6-cyclohexyl-4hexyn-3-ol; however, the alkynyl Grignard reagent would be treated with propionaldehyde (propanal) instead of formaldehyde.
25
Analogously, this process could be used to synthesize 6-cyclohexyl-3methyl-4-hexyn-3-ol where the alkynyl Grignard reagent reacts with 2butanone, a ketone.
26
Let’s design a synthesis for 2-phenyl-2-butanol. The process is easier if worked backwards. Working backwards is referred to as retrosynthesis.
There are three (3) pathways that could lead to the synthesis of 2-phenyl-2butanol.
(1) 27
From bromobenzene
In this synthesis schema, the product generated has a chiral center; therefore, the pathway leads to stereoisomers where x + y = 1.00 mole. 28
(2) From methyl bromide
The product generated has a chiral center; therefore, the pathway leads to 29
stereoisomers where x + y = 1.00 mole. (3) From ethyl bromide
The product generated has a chiral center; therefore, the pathway leads to stereoisomers where x + y = 1.00 mole.
30
Tertiary Alcohols can also be synthesized from esters. If two moles of the Grignard reagent react with a designated ester, after acid hydrolysis, the resulting compound is a tertiary alcohol.
The synthesis of 3-cyclohexyl-3-pentanol illustrates a synthesis process where two moles of a Grignard reagent lead to the formation of a tertiary alcohol.
31
Following is the mechanism that explains this reaction. (1)
32
(2)
(3)
Acid Hydrolysis:
33
The hydrolysis portion of the reaction is acid catalyzed.
Syntheses of Alkanes from Grignard reagents Alkanes result from the hydrolysis of Grignard reagents. The following are illustrations of the hydrolysis of Grignard reagents resulting in the formation of alkanes. Methane forms from the hydrolysis of methyl magnesium bromide.
2-Methylpropane forms from the hydrolysis of t-butyl magnesium bromide.
Propane forms from the hydrolysis of n-propyl magnesium iodide.
34
1-Deuteropropane forms from treating n-propyl magnesium iodide with deuterium oxide, D2O. Deuterium is an isotope of hydrogen. A hydrogen atom has one proton in its nucleus, and deuterium has one proton and one neutron in its nucleus.
As mentioned in Chapter 2, the reaction hydrolysis of methyl magnesium halide can be used to quantitatively determine the amount of water in some inert compounds. This is due to the stoichiometry of the reaction. There is a clear relationship between the number of moles of methane and the number of moles of water present in the sample, because the moles of methane formed from the hydrolysis is equivalent to the moles of water present in the sample.
The Corey-House Reaction Revisited As mentioned in Chapter 2, the Corey-House reaction involves the coupling of an alkyl halide with an organometallic compound, a lithium dialkylcuprate compound (the Gilman reagent) to produce an alkane. The reaction occurs between a primary or secondary alkyl halide (the reaction works best for a primary alkyl halide) and a lithium dialkylcuprate reagent, The following equation represents the overall reaction:.
R X + R '2 C u L i ÂŽ R R ' + R C u + L i X 35
The reaction is used to prepare unsymmetrical alkanes. The preparation of is from the corresponding alkyl halide. R’X + 2 Li → R’Li + LiX
2 R 'Li + CuX ® R '2CuLi + LiX This is not a simple coupling reaction, and the mechanism is not well understood. The alkyl group in the lithium dialkylcuprate reagent can be primary, secondary, or tertiary; however, as indicated previously, the reagent reacts best with a primary alkyl halide. The lithium diaklycuprate reagent will work fairly well on an unhindered secondary alkyl halide. For example, the synthesis of 2-methylpentane is an example of the CoreyHouse reaction.
C H
H
3
C H
C C H
+
C H
C
2 Li
I
3
C H
2
+
C C H
3
Li
+
L iI
Li
3
C H
H
3
H
3
3
C H
C uI C H
L iC u
3 2
36
+
L iI
Lithium dialkylcuprates react best with primary alkyl iodides.
Even though the mechanism is not clear, organocuprates work best with methyl and primary alkyl halides where the order of reactivity with halogens is I >Br > Cl >F. Alkyl p-toluenesulfonates are more reactive than alkyl halides. The reaction works best with primary alkyl halides and Gilman reagents, which are primary dialkycuprates.
Even though vinyl halides and aryl halides are not reactive toward SN2 mechanisms, they react in Corey-House reactions.
37
Formation of Cyclopropane from Organozinc Reagents, the Simmons-Smith Reaction Revisited Zinc is more electronegative than lithium or magnesium (Chart 1.1).
Metal Zn Mg Li
Electronegativit y 1.7 1.3 1.0
Therefore, the C-Zn bond would be less polar than C-Li or C-Mg bond. Organozinc reagents are less reactive toward aldehydes and ketones. A special organozinc reagent, iodomethylzinc iodide (ICH2ZnI), plays a role in organic synthesis. Iodomethylzinc iodide is prepared by reacting iodomethane and zinc(cooper) in diethyl ether as a solvent.
38
Iodomethylzinc iodide reacts with alkenes to form cyclopropane derivatives.
The above reaction is referred to as the Simmons-Smith reaction, and the reaction is stereospecific, i.e., cis alkenes will give rise to cis substituted cyclopropane, and trans alkenes will give rise to trans substituted cyclopropane:
39
Following is the mechanism (series of elementary steps) for the SimmonsSmith reaction.
(1)
C H
H
3
H
C C H 3C
(2)
40
H H
3
I
C
C
+
C H
H
I
C H Zn
C
I H 3C
2
Zn H
I
C H
H
3
C H C H 3C
C H
H
I
C
3
C
2
C H
Zn H
2
+
Z n I2
C I
H 3C
H
The reaction, in reality, is essentially a one step process that proceeds through the formation of a non-isolatable activated complex at the transition stage of the reaction. The reaction is represented as a two-step mechanism, because of the formation of the activated complex. The reaction follows a second order kinetic process. Additional information about the Simmons-Smith reaction can be found at the following website: http://www.organicchemistry.org/namedreactions/simmons-smith-reaction.shtm The Wurtz Reaction The Wurtz reaction is one of the oldest coupling reactions in organic chemistry. The Wurtz product is a dimer formed from two equivalent alkyl halides. 2 R-X + 2 Na → R-R + 2 NaX For example, n-octane can be synthesized from 1-bromobutane via the Wurtz Reaction.
41
By products of the Wurtz Reaction could be alkanes and alkenes.
The mechanism of the Wurtz Reaction could be as simple as a transfer of an electron to an antibonding orbital on a carbon atom to form a radical.
(1) The initial step takes place twice H
H
C C H 3C H 2C H
Br
2
H
42
+
.. . :Br: .. C
N a . C H 3C H 2C H
2
H
+
N a
+
H
H
C C H 3C H 2C H
+
Br
2
.. . :Br: .. C
N a . C H 3C H 2C H
+
N a
+
H
2
H
(2) This step also takes place twice
H .
C C H 3C H 2C H
H
2
.. :Br: .. H
C C H 3C H 2C H
C H 3C H 2C H
H .
C 2
H
2
H
.. :Br: ..
.
.. :Br: .. H
C C H 3C H 2C H
.. :Br: ..
. H
2
(3) H C C H 3C H 2C H
43
2
H . + H
C C H 3C H 2C H
2
C H 3C H 2C H
.
C H 2C H 2C H
2
C H
C H
H H
H
3
The Wurtz reaction can be used to synthesize strained ring compounds like bicyclo[1.1.0]butane.
Carbenes and Carbenoids Iodomethylzinc iodide is a carbenoid, i.e., it acts like a carbene. Carbenes have the formula R2C:. If the R groups are hydrogen, H2C:, the species is referred to as methylene. As expected, carbenes are very reactive chemical species. Carbenes will add to double bonds to produce cyclopropanes. Reactions one and two are examples of carbene insertion reactions.
Theoretically, the cyclopropane product, bicyclo[4.1.0]heptanes, in equation 44
2 can assume both a cis and trans form, but the reaction product is always cis. Therefore the reaction is stereospecific.
For obvious reasons, the cis isomer dominates. Carbenes are also of the type X2C:, where X represents Cl, I or Br. Carbenes are highly reactive; therefore, they are generated in situ in the presence of the alkene. The following reactions are examples of a couple of common carbene generators. 1. RHCI2 + Zn-Cu → [IRCHZnI] → ZnI + :CHR 2. K+ -OC(CH3)3 + CHX3 → X2C: + KX + HOC(CH3)3 In reaction 1, the diiodoalkane reacts with ZnCu alloy to give the unstable IRCHZnI which decomposes to the carbene. Reaction 2 uses the powerful base potassium tert-butoxide to generate the carbene from trihalomethanes. (CH3)3CO- K+
+ H:CCl3 → K+ - :CCl3
K+ - :CCl3 → KCl + :CCl2
45
Cyclopropanes can also be prepared via 1,3-dihalopropane.
Also, diazomethanes and ketenes are precursors for the generation of carbenes:
46
H C
C
O
UV light
:CH2
+ CO
H ketene The carbenes generated can insert into alkenes to form cyclopropanes.
The carbenes generated from diazomethanes and ketenes can have either a singlet state or a triplet state. The singlet state carbene has an unshared pair of electrons, i.e., electrons with opposite spins:
The C-H bond length is 1.12Çş The pairing of the electrons can be
Therefore, the term singlet is applied to this carbene. The triplet state carbene has unshared electrons which are not paired, i.e., 47
electrons with or without opposite spins.
Therefore, the possible spin arrangements for the two electrons could be:
1:2:1 hence the term triplet is applied to this carbene system. In the triplet state, the C-H bond length is 1.12 Çş. The singlet state carbenes add to alkenes stereospecifically to form cyclopropanes.
48
Carbenes in the triplet state add to alkenes in a nonstereospecific manner to form cyclopropane.
49
Diazomethane generates the triplet carbene, and ketene generates a triplet carbene. Transition-Metal Organometallic Compounds Diiron nonacarbonyl is a precursor to the synthesis of cubane. The synthesis of cubane was discussed in Chapter 8. This chapter presents a review of the synthesis of cubane, and an explanation of the structure of diiron nonacarbonyl, a neutral molecules with the formula Fe2(CO)9. Diiron nonacarbonyl is an orange solid prepared by the photolysis of an acetic acid solution of iron pentacarbonyl, Fe(CO)5. The two iron atoms in diiron nonacarbonyl have zero oxidation states, and the carbonyl molecules attached are ligands. As in many transition-metal organometallic compounds, the bonds between Fe and CO are coordinate 50
covalent bonds. However, in diiron nonacarbonyl, each iron atom has four coordinate covalent bonds and two single covalent bonds. The ylide, carbon monoxide,
, is a Lewis base. The Fe, a transition metal, is a Lewis acid. Four carbon monoxide molecules form coordinate covalent bonds with each iron atom. The two iron atoms share a carbon monoxide molecule via two single covalent bonds. Finally, the iron atoms form a single covalent with each other. The lone pair of electrons on the carbon of the carbonyl group (a carbon monoxide molecule) forms a coordinate covalent bond with an empty 4sp3d2 hybridized atomic orbital of Fe. The electron configuration for Fe is 1s2 2s2 3s2 3p6 4s2 3d6. Iron has eight valence electron (six electrons in the 3d atomic orbitals and 2 electrons in the 4s atomic orbital). There are two iron atoms in diiron nonacarbonyl. Each iron has four carbon monoxide molecules that form coordinate covalent bonds with it and one carbon atom with a single covalent bond. The two iron atoms connect by a single covalent bond; therefore, the total number of electrons provided by the carbon atoms and the other Fe atom is 10. The number of valence electrons on each iron atom is eight. The eight valence electrons and the 10 electrons provided by the carbonyl carbon atoms and the other Fe atom equal 18. Consequently, the transition-metal complex obeys the rule of 18. The rule of 18 for transition-metal complexes is that the number of electrons provided by the ligands and the valence electrons of the transition metal equals 18. First, let’s explain the hybridization in each Fe atom. The iron electrons in the 4s atomic orbital and 4p and 4d low lying atomic orbitals of iron can be arrange in the following manner: 51
One of the electrons in the 4s atomic orbital can be promoted to the 4p orbital:
The 4s atomic orbital, the three 4p atomic orbitals, and two of the 4d atomic orbitals hybridize to form six degenerate 4sp3d2 atomic orbitals:
Now, the 4 sp3d2 atomic orbital of iron linearly combines with the 4 sp3d2 atomic orbital of the other iron atom to form a bonding molecular orbital 52
where two electrons of opposite spins reside in the Fe-Fe single covalent bond. The 4 sp3d2 atomic orbital of iron linearly combines with the 2sp2 atomic orbital of a carbon atom to form bonding molecular orbital where two electrons of opposite spins reside in the Fe-C single covalent bond. Four carbon monoxide ylides with two paired electrons in 2sp molecular orbitals form coordinate covalent bonds with the four empty 4 sp3d2 atomic orbitals of Fe where two electrons of opposite spins reside in single coordinate covalent Fe-CO bonding molecular orbitals. These data suggest that the structure of diiron nonacarbonyl is:
53
or Fe2(CO)9 may be written as
Diiron nonacarbonyl reacts with 3,4-dichlorocyclobutene to produce cyclobutadiene tricarbonyliron (cyclobutadieneiron tricarbonyl). This compound obeys the 18-rule where four electrons are contributed by the cyclobutadiene, six from the carbonyl and eight valence iron electrons.
The cyclobutadienetricarbonyiron is a precursor for the synthesis of cubane via the following reaction schema (a re-visit of the discussed in Chapter 8):
54
(2)
(3)
(4)
55
(5)
(6)
(7)
56
(8)
Metallocenes Metallocenes are organometallic compounds with the aromatic carbanion, cyclopentadenide, sandwiching a metal cation. Cyclopentadenide can form a metallocene called ferrocene. The cyclopentadenide functions as a ligand in ferrocene where two cyclopentadenide entities sandwich a ferrous ion.
Ferrocene has the following structure.
57
Many metallocenes are known (http://en.wikipedia.org/wiki/Metallocene), and some have industrial use. The Ziegler Natta Catalyst The Ziegler-Natta catalyst named after Karl Ziegler and Giulio Natta is used in the preparation of isotactic, i.e., the “R” groups are in the cis position in the polymer.
R
H n
C H
C H
Z ie g le r - N a t t e r
R
R
R
R
C a t a ly s t n
Several decades ago, Karl Ziegler first published his studies on organoaluminum compounds as catalyst for the polymerization of ethylene. Natta discovered that the Ziegler’s catalyst produced polymers in which the side chains have the same direction, i.e., isotactic polymers. An effective catalyst for the isotactic polymerization of α alkenes is the Ziegler-Natta catalyst. The Ziegler-Natta catalyst is involved with three platforms, one platform uses 58
metallocenes and zirconium; however, a variety of metal catalysts (e.g., Ti, Zr, and Hf) are used in this process. Karl Ziegler and Giulio Natta received the Nobel Prize in 1963 for their pioneering work. The details of the mechanism are not well understood, because the reaction takes place on the surface of insoluble materials, and it is difficult to conduct kinetic experiments in the solid phase. Therefore, the series of elementary steps to rationalize the isotactic product are based on experimental evidence and scientific intuition. A popular Ziegler-Natta metal catalytic used for the isotactic polymerization of alpha substituted alkenes is Titanium with aluminum as a co-catalyst. The mechanism of the reaction could proceed via the following series of elementary steps. Step 1 involves the formation of a titanium-aluminum complex by reacting titanium tetrachloride with trimethylaluminum. Trimethyl aluminum is a dimer made by treating aluminum with methyl chloride in the presence of sodium.
Step 2 involves the reaction of the titanium-aluminum complex with the desired alkene.
59
Step 3 involves the rearrangement of the titanium-aluminum-alkene complex where a methyl group is moved to the alkene, and the methylene of the alkene replaces the methyl that is complexed with aluminum and titanium.
Step 4 involves multiple attachments of the alkene in such a manner that the alkyl groups are isotactic.
60
Step 5, the final step, releases the polymer and generates the titaniumaluminum complex formed in step 3.
61
Problems
1. Using benzene, methanol, and any other necessary inorganic materials suggest syntheses for the following compounds. (a)
(b)
(c)
(d)
62
2. Suggest syntheses for the following (a) 3-methyl-1-pentyn-3-ol from 2-butanone and any other necessary organic or inorganic material (b)
and any other necessary organic and inorganic compounds (c)
and any other necessary organic and inorganic compounds
63
3. Suggest products for the following reactions.
(a)
(b)
(c)
(d)
(e)
64
4. Suggest the major product for the reaction of t-butylmagnesium bromide with compound A followed by acid hydrolysis . What is the IUPAC name for this compound?
Compound A 5. Predict the major product expected from reacting the following compound with excess phenylmagnesium iodide followed by acid hydrolysis.
6. Compound I, C3H5Br, reacts with magnesium in dry
ether to form
C3H5MgBr. Treating C3H5MgBr with formaldehyde followed by acidic hydrolysis produces C4H8O, compound II. When three moles of C4H8O are treated with phosphorous tribromide, three moles of C4H7Br, compound III, are formed. Treating C4H7Br with magnesium in dry ether, followed by treatment with p-ethoxybenzaldehyde, resulted in the production of C13H17OMgBr. Acid hydrolysis of C13H17OMgBr produces compound IV. Suggest structures for compounds I, II, III, and IV.
65
C13H17OH,
7. Ferrocene is readily oxidized to ferrocenium ion by hydroxyl radicals or other kinds of free radicals. Consequently, ferrocene may serve as a therapeutic agent for treating cancer. The reaction for ferrocene as a chemical to treat cancer may be described by the following equation.
Suggest a synthesis for ferrocene from cyclopentadiene. 8. Suggest the product with the appropriate stereochemistry produced when (2S)-2-bromobutane reacts with lithium n-propylcuprate.
9. Suggest the major product expected for the following reaction.
Give a rational for your answer.
66
10. Suggest a synthesis for compound B from compound A.
compound B
compound A 11. Suggest syntheses that would lead to the major products for the following compounds from any necessary organic or inorganic reagents. (a)
1-deuteriobutane from butane
(b)
2-deuteriobutane from butane
(c)
2-deuterio-2-methylpropane from isobutane
(d)
1-deuterio--2-methylpropane from isobutane
12. 67
The name of the following compound
is pentacyclo[4.2.0.02.5 03,8.04,7]octane. Suggest an IUPAC name for the following compound:
13. Suggest structures for the product or products obtained in the following sequence of reactions.
68
14. Consider the following reaction schemas: (a)
+
A
M gB r
+
B
(b)
A
+
O
H 3O
+
C 8H
1 4O
(c)
C 8H
14
O + 2 N a + 2 N H
3
ÂŽ C 8H 14 + N a O H + N aN H 2 + N H 3
Suggest structures for A, B, C8H14O, the product of (b) and C8H14, the product of (c).
Solutions to Problems 69
Chapter 11
1. Using benzene, methanol, and any other necessary inorganic materials, suggest syntheses for the following compounds. (a)
(b)
70
(c)
71
72
(d)
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2. Suggest syntheses for the following (a) 3-methyl-1-pentyn-3-ol from 2-butanone and any other necessary organic or inorganic material and any other necessary organic and inorganic compounds.
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(b)
and any other necessary organic and inorganic compounds
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(c)
and any other necessary organic and inorganic compounds
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+ H2O
3. Suggest products for the following reactions. (a)
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(b)
(c)
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(d)
(e)
4.
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Suggest the major product for the reaction of t-butylmagnesium bromide with compound A followed by acid hydrolysis. What is the IUPAC name for this compound?
The major product is trans-(e,e)-1,4-di-t-buty-1-cyclohexanol
5. Predict the major product expected from reacting the following compound with excess phenylmagnesium iodide followed by acid hydrolysis.
6. 80
Compound I, C3H5Br, reacts with magnesium in dry
ether to form
C3H5MgBr. Treating C3H5MgBr with formaldehyde followed by acidic hydrolysis produces C4H8O, compound II. When three moles of C4H8O are treated with phosphorous tribromide, three moles of C4H7Br, compound III, are formed.
Treating C4H7Br with magnesium in dry ether, followed by
treatment with p-ethoxybenzaldehyde, resulted in the production of C13H17OMgBr. Acid hydrolysis of C13H17OMgBr produces compound IV. Suggest structures for compounds I, II, III, and IV.
compound I
compound II
compound III
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C13H17OH,
compound IV
7. Ferrocene is readily oxidized to ferrocenium ion by hydroxyl radicals or other kinds of free radicals. Consequently, ferrocene may serve as a therapeutic agent for treating cancer. The reaction for ferrocene as a chemical to treat cancer may be described by the following equation.
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Suggest a synthesis for ferrocene from cyclopentadiene.
8. Suggest the product with the appropriate stereochemistry produced when (2S)-2-bromobutane reacts with lithium n-propylcuprate. The Cory-House reaction works best when the lithium dialkylcuprate reagent reacts with a primary alkyl halide. The reaction works reasonably well for unhindered secondary alkyl halides. Therefore, keeping in mind that the mechanism of the Cory-House reaction has not yet been clearly elucidated, an assumption can be made that the reaction probably takes place by 83
backside attack of the lithium dialkylcuprate reagent. If this assumption is correct, then the product would be (3R)-3-methylhexane.
9. Suggest the major product expected for the following reaction.
compound I (cyclohexyl tosylate) Give a rational for your answer. The major product would be the elimination product:
The reaction would proceed through an elimination bimolecular mechanism, because the tosyl group is an excellent leaving group, and the reaction involves a strong base.
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or
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10. Suggest a synthesis for compound B from compound A.
compound B
compound A
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11. Suggest syntheses that would lead to the major products for the following compounds from any necessary organic or inorganic reagents. 1-deuteriobutane from butane
87
2-deuteriobutane from butane
2-deuterio-2-methylpropane from isobutane
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1-deuterio--2-methylpropane from isobutane
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12. The name of the following compound
is pentacyclo[4.2.0.02.5 03,8.04,7]octane. Suggest an IUPAC name for the following compound:
pentacyclo[6.2.0.02.7 03,10.06,9]decane
13. Suggest structures for the product or products obtained in the following sequence of reactions.
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14. Consider the following reaction schemas: Suggest structures for A, B, C8H14O, the product of (b); C8H14, the product of (c). (a)
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(b)
(c)
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C8H14
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94