Steric Course of the Rhodium-Catalyzed Decarbonylation of Chiral 4-M ethyl-[1-³H,2-²H₁]pentanal

Abstract

Dedicated to Professor Helmut Simon on the occasion o f his 60th birthdayStereochemistry, Decarbonylation, Rhodium Catalyst, Chiral Methyl Group, 4-Methylpentanal (R)-and (5)-4-methyl-[l-3H,2-2H!]pentanal were prepared from l -and D-leucine via leucic acid and (S)-and (ft)-4-methyl-[2-2H,]pentanoic acid. Decarbonylation of these samples with tris-(triphenylphosphine)rhodium chloride followed by Kuhn-Roth oxidation of the resulting 2-methylbutane gave chiral acetic acid of 35% e.e. S and 31% e.e. R configuration, respectively. The decarbonylation reaction thus proceeds with net retention of configuration, possibly accom panied by some racemization.The stereochemistry of the decarbonylation of aldehydes catalyzed by tris-(triphenylphosphine)rhodium chloride has been examined in a few cases [1]. The reaction was found to proceed with high stereoselectivity, except when the a-carbon car ried an electron-donating group like -OCH3, and net retention of configuration. However, all the cases studied involved hindered aldehydes in which the formyl group was attached to a quaternary car bon. In the analogous decarbonylation of acid chlorides [2, 3] net retention of configuration was also observed, but these reactions were accompanied by a high degree of racemization. However, the cases studied here also involved less crowded systems in which the carbonyl group was attached to a primary or secondary carbon. These reactions represent the reversal of the so-called carbonyl insertions into transition metal-carbon bonds [4], which, mechanis tically, are actually migrations of a metal-bound alkyl group to an adjacent bound carbon monoxide. These, in the cases studied, also proceed with reten tion of configuration of the alkyl group [5, cf. 4, 6, 7].In the course of studies on stereochemical aspects of the biosynthesis of ergot alkaloids [8] we were faced with the problem of determining the configura tion of 4-methylpentanoic acid stereospecifically monodeuterated at C-2 on ^g samples obtained in an impure state from complex reaction mixtures. It oc curred to us that the limitation of quantity could be overcome by converting the acid to the 1-tritiated alcohol by reduction with tritiated LiBH4, followed by oxidation to the aldehyde and decarbonylation to give 2-methylbutane carrying a chiral methyl group at C-4 (Scheme I). In this sequence the tritiated, deuterated alcohol could be diluted with ample car rier material, allowing the subsequent chemical steps to be carried out on a more manageable scale.To explore the feasibility of this degradation se quence it was necessary to determine whether the decarbonylation of an uncrowded aldehyde like 4-methylpentanal also proceeds stereospecifically with retention of configuration. The results are reported here because of the current interest in an important biochemical carbonyl insertion reaction, the synthe sis of acetic acid in anaerobic organisms [9], and its stereochemistry [10]. ResultsTo examine the steric course of the transition metal-catalyzed decarbonylation of 4-methylpenta...