Problem 11. Benzoporphyrin

The name "porphyrin" derives from the Greek word porphyra, meaning purple. Porphyrins are a group of macrocycle organic compounds, composed of four modified pyrrole subunits. They have a total of 26 π-electrons, 18 of which form a planar porphyrin ring structure. They are often described as aromatic. Metal complexes derived from porphyrins occur naturally. One of the best-known families of porphyrin complexes is heme, the pigment in red blood cells. A benzoporphyrin is a porphyrin with a benzene ring fused to pyrrole unit(s).

11.1. Benzoporphyrins can be prepared starting from a masked pyrrole derivative E. The synthesis of E starts with a reaction of cis-1,2-dichloroethene and thiophenol to give A. Oxidation of A yields B having phenylsulfonyl units. The cis-product B is then converted to its trans isomer C when treated with a catalytic amount of Br2 under UV light. The Diels–Alder reaction between C and 1,3-cyclohexadiene under thermal conditions gives the product D, which is converted to a pyrrole carboxylic acid ester when reacted with ethyl isocyanoacetate. Ester then is treated with TFA to give the pyrrole derivative E.

Draw the structures of compounds A–E including stereochemistry when necessary.

11.2. Porphyrins can easily be prepared via a cyclization reaction of pyrrole derivatives with aldehydes. Draw the structure of aldehyde F and determine the oxidation state of zinc in compound H.

11.3. When H is heated under vacuum, it can give a more conjugated product through a retroDiels–Alder reaction.

To complete the structure of I, draw the structures of the dashed circle part of I (all the circles are identical) and J.

Ammonia is a major metabolic compound and the importance of its sensitive detection has been emphasized recently because of its correlation with specific diseases. In normal physiological conditions, ammonia can be expelled from slightly alkaline blood and emanated through the skin or exhaled with the breath. Dysfunction in the kidney or liver that converts ammonia to urea can result in an increase in the ammonia concentration in breath or urine. Consequently, the detection of the ammonia present in breath or urine can be used for the early diagnostics of liver or stomach diseases. The development of sensor devices for measuring ammonia with a sensitivity of 50 ppb–2 ppm and with a fast response time is highly desired.

For that purpose, I was used to prepare a fiber-optic ammonia gas sensor. Exposure of this sensor to ammonia changes the transmittance of the fiber-optic. By using an appropriate spectrometer, ammonia gas in different concentrations was passed through the sensor and the change in transmittance was measured. The results of these measurements are listed in the Table below.

[NH3] (ppm) Sensor response (%)

0.500 1.00 2.00 4.00 7.00 9.00 11.0 20.0 25.0 30.0 –0.2540 –0.7590 –1.354 –1.838 –2.255 –2.500 –2.600 –2.947 –3.152 –3.256

11.4. Using the linear region of sensor response data prepare a calibration curve and find the calibration equation as �� =��+����.

11.5. This sensor is then used for the detection of ammonia in human breath. When a kidney patient’s breath was fed into the sensor, a –3.812% change in the response is observed. Calculate the ammonia concentration in the patient’s breath.

Solution:

11.1.

11.2.

F

Oxidation state of Zinc in G = 2+

11.3.

11.4.

log([NH3]) Sensor Response (%) –0.301 0.00 0.301 0.602 0.845 0.954 1.041 1.301 1.398 1.477 –0.2540 –0.759 –1.354 –1.838 –2.255 –2.500 –2.600 –2.947 –3.152 –3.256

�� = −0.8058– 1.6814��

12.5. −3.812=−0.8058−1.6814×log[����3]

log[����3]=1.788

[����3]=61.4������