Question

The proton nmr and the infrared spectra of DCC, DMAP, DCU, the product and the nmrs of the starting materials and products are shown. These are labeled Compounds A, B and C. a.Which nmr is the product?

b. Which is the nmr of the starting material?

c. What is the mixture of compounds shown as compound C?

tert-Butyl Ethyl Fumarate DCC, DMAP, CHCI OH Chemical Formula: C,H,O Chemical Formula: CH 0 Molecular Weight: 144.13 Molecular Weight: 74.12 Chemical Formula: CH 604 Molecular Weight: 200.23 Procedure: A 500-ml, one-necked flask equipped with a calcium chloride drying tube is charged with 28.83 8 (0.20 mol) of monoethyl fumarate, 200 mL of dry dichloromethane, 44.47 g (0.60 mol) offerte butyl alcohol, and 2.00 g (0.016 mol) of 4-dimethylaminopyridine. The solution is stirred and cooled in an ice bath to 0°C while 45.59 g (0.22 mol) of dicyclohexylcarbodiimide is added over a 5-min period. After a further 5 min at 0°C the ice bath is removed and the dark brown reaction mixture is stirred for 3 hr at room temperature. The dicyclohexylurea that has precipitated is removed by filtration through a fritted Büchner funnel, and the filtrate is washed with two 50-ml portions of 0.5 N hydrochloric acid and two 50-ml portions of saturated sodium bicarbonate solution. During this procedure some additional dicyclohexylurea is precipitated, which is removed by filtration of both layers to facilitate their separation. The organic solution is dried over anhydrous sodium sulfate and concentrated with a rotary evaporator. The concentrate is distilled under reduced pressure, affording, after a small forerun, 30.5-32.5 g (76-81%) of tert- butyl ethyl fumarate, bp 105-107°C (12 mm). Background: The procedure is an example of a Steglich Esterification. It is a modification for ester synthesis of the procedure usually used for amide synthesis (especially in peptide synthesis). The amide synthesis is covered in the McMurry textbook (p. 692, reaction mechanism on p. 693). In the reaction dicyclohexylcarbodiimide is converted to dicyclohexylurea. 4-Dimethylamino- pyridine is used as a catalyst and is constantly regenerated as the product forms. Catalysis reactions such as these are often described in a catalytic cycle mechanism scheme. The scheme for the esterification reaction is shown below. In this scheme, the carboxylic acid reacts with dicyclohexylcarbodiimide to give the reaction product of the addition of carboxylic acid across one of the C-N bonds. The first step is presumed to be deprotonation to form the carboxylate ion which then attacks the central carbon (partially positive, since it is between two electronegative nitrogen atoms). The resulting intermediate reacts rapidly to give amides in the case of amine nucleophiles but is unacceptably slow in the case of reaction to the ester with alcohol nucleophiles. The addition of 4-dimethyl- aminopyridine (DMAP) facilitates the reaction with alcohols to give esters by forming the acyl-

Answer 1

proton NMR spectrum of compound A belong to starting material carboxylic acid

CH=CH COOH CH2 CH3 PPY Compound A

spectrum of compound B belong to product

(CH3)3C CH=CH 2 CH2 Compound

c.

compound c is mixture of condensation product and dicyclohexyl urea. in the spectrum product signal is marked with red and dicyclohexyl urea signal is marked with black

Compound -H - - -- product dicyclohexyl urea