Mechanism of acid chloride decarbonylation with chlorotris(triphenylphosphine)rhodium(I). Stereochemistry and direction of elimination

The decarbonylation of erythro- and threo-2,3-diphenylbutanoyl chloride with chlorotris(triphenylphosphine)rhodium(I) gave exclusively trans- and cis- methylstilbene, respectively. These results are best explained either by an acyl- - alkyl rearrangement with retention and a cis BETA -hydride elimination, or a concented cis elimination. The rates of the decarbonylation of dichioro(3- perdeuteriophenyl-3,3-dideuteriopropionyl)bis(triphenylphosphine) rhodium(III) along with the undeuterated complex k/sub H/ = 3.34 x 10/sup -5/ sec/sup -1/, k/ sub D/ - 4.75 x 10/sup -6/ sec/sup -1/) showed a primary deuteriu m isotope effect of 7.04. Dec arbonylation of 2-methylpentanoyl chloride, 2,2- dimethylpentanoyl chloride;, 2-ethyl-;3-methylbutanoyl chloride, and 2,3- dimethylbutanoyl chloride with trans-chlorocarbonylbis(tri- phenlphosphine)rhodium(I) showed that this olefin-forming reaction preferred the Saytzeff elimination. (auth) (VIII), bis-p-nitrophenyl sulfite (IX), and phenyl p-nitrophenyl sulfite (X), in carboxylic acid buffers were examined as simple models for the corresponding pepsin-catalyzed processes. The observation of a substantial Bronsted BETA value, 0.85, for the monocarboxylate ion-catalyzed hydrolysis of VIII, as well as the finding that the kinetic solvent isotope effect (k/sub OAc//sup H/sub 2/O//(k/sub CAc)/s up D/sub 2/O = 1.32 are consistent with the hypothesis that these reactions of VIII occur by nucleophilic catalysis. Similarly, the acetate-catalyzed solvolysis of IX and X is suggested to occur through a nucleophilic pathway.more » Support for the proposal that the nucleophilic attack of the carboxylate ions on the sulfites leads to the formation of mixed anhydride intermediates (eq 7) has been obtalned by the detection of the acetohydroxamic acid produced when VIII, IX, and X are solvolyzed in acetate buffer in the presence of hydroxylamine. In the case of VIII measurement of the rate of acetohydroxamic acid production has indicated that under the conditions employed the rate-controlling step is mixed anhydride formation. In contrast to the solvolyses in carboxylic acid buffers, the direct kinetic detection of intermediates in the reactions of the sulfite esters VIII and X with hydroxylamine buffers has been accomplished. The rate constants for intermediate formation and decomposition were obtained by a computer-assisted kinetic analysis in the case of VIII and by observation of the reaction course at different wavelengths in the case of X. Through a combination of kinetic and synthetic experiments, the solvolysis of X in N,0-dimethylhydroxyl-amine buffer has been shown to proceed through the initial formation of phenyl methoxymethylamidosulfite (XI). The demonstration that reactive intermediates can be detected in model solvolytic reactions of sulfite esters provides encouragement for the search for conditions under which intermediates formed by nucleophilic attack by the active site enzymatic carboxylate groups of pepsin can be detected directly. (auth)« less