SYNTHESIS, NMR, AND THE ANION INDUCED RETRO DIELS-ALDER REACTION OF 7-SUBSTITUTED NORBORNENES (ENOLATES, DIANIONS, ALKALI CATIONS, CHEMICAL SHIFT)
Abstract
The anionic retro Diels-Alder reaction of bicyclo 2.2.1 hept-2- ene-7-carboxylic acid dianions was investigated. The dilithio dianions were prepared by deprotonation of the carboxylic acids with lithium diisopropylamide (LDA) (2 eq). Mixed potassium-lithium dianions were prepared by deprotonation of the potassium salts of the acids with LDA. The reaction was determined to proceed with retention of stereochemistry consistent with a concerted process. Chapter 1 presents a general overview of anionically accelerated reactions. It focuses on the mechanistic explanations of rate acceleration. Chapter 2 presents the synthesis of various bicyclo 2.2.1 hept-2-ene-7-carboxylic acids needed for this study. Over fifty 7-substituted norbornenes were prepared and the ('13)C and ('1)H-NMR spectra analyzed. The preparation of 5-n-hexylbicyclo 2.2.1 hept-2-ene-7-carboxylic acids is described in detail. This synthesis presented major problems: the introduction of the n-hexyl group via direct reaction of the bicyclo 2.2.1 hept-5-ene enolate with n-hexyl iodide was unsatisfactory because of rapid competing aldol condensation. Dimeric and trimeric aldol condensation products were isolated and identified. This problem was overcome via alkylation of the corre- sponding dimethylhydrazone anion which proceeded in high yield. Chapter 3 describes the retro Diels-Alder cleavage of bicyclo- 2.2.1 hept-2-ene-7-carboxylic acid dianions. The endo-5,6-dimethyl- bicyclo 2.2.1 hept-2-ene-7-carboxylic acid dianion was shown to initially fragment primarily to cis-2-butene which is consistent with a concerted mechanism. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) The 5-n-hexylbicyclo 2.2.1 hept-2-ene-7-carboxylic acid dianion fragmented to 1-octene which isomerized under the reaction conditions. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) The rate of octene formation was found to depend on the couterion. The dilithio dianion reacted slowly (t(, 1/2) = 20 hours at 40(DEGREES)). The rate of the mixed potassium lithium salt was proportional to the K('+)/Li('+) ratio. Chapter 4 describes the preparation of carboxylic acid dianions by reaction of ketene (bis(trimethylsilyl) acetals with n-BuLi (2 equivalents) in DME. The intermediate trimethylsilyl ester enolate was observed by ('1)H-NMR at -50(DEGREES). The rate of desilylation of the ester enolate was only about five times slower than the desilylation of the ketene bis(trimethylsilyl) acetal (k/k(,2) (DBLTURN) 5/1). (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI)
Degree
Ph.D.
Subject Area
Organic chemistry
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