An MNDO SCF-MO study of the mechanism of the benzilic acid and related rearrangements

Abstract

Mechanisms for the benzilic acid and related rearrangements have been studied using the MNDO SCF-MO method. The barriers to concerted closed shell [1,2] migration of a substituent R in the initially formed intermediate (2) were found to display a much smaller range of values than is commonly found in rearrangements involving cations or radicals. Both the formation of (2) and its subsequent rearrangement are predicted to be enhanced if either R or the non-migrating group X bears electron-withdrawing substituents. The calculated hydrogen isotope effect and the Bell tunnelling correction for the rearrangement of glyoxal are both significantly larger than for the analogous hydride transfer in the Can n izzaro reaction of forma Idehyde. Alternative open s he1 I path ways involving intramolecular singleelectron-transfers (SET) to the carbonyl group are relatively high in energy if the non-migrating substituent X is not capable of stabilising an adjacent radical centre (i.e. X = H), but are much more favourable for e.g. the case of X = phenyl. When R bears an electron-withdrawing substituent, electron-transfer to this group is favoured. We suggest on this basis that an SET mechanism may explain the migration of hydrogen rather phenyl in the reaction of phenylglyoxal. Electron-transfer is also favoured for the rearrangement of benzoin to benzilic acid itself, and is predicted to result in the formation of low-energy cyclic intermediates. Intervention of a counter-ion was modelled with Li( H,O)l co-ordinating to two oxygen atoms during migration. The migratory barriers were actually increased as a result, and it is suggested that the role of the counter-ion in non-polar solvents is to alter the pre-equilibrium in favour of the intermediate (2).

Rearrangement of 3-Deoxy-d-erythro-hexos-2-ulose in Aqueous Solution: NMR Evidence of Intramolecular 1,2-Hydrogen Transfer

“…Acyclic forms were proposed as the reactive forms of 1 in the above 2KH mechanism (Scheme 5), modeled from previous theoretical studies of related benzylic acid rearrangements of glyoxal and similar substrates. 16 These latter compounds are unable to cyclize, unlike 1. Thus, consideration should be given to the possibility of cyclic forms of 1 serving as reactive substrates.…”

Semi-empirical AM1 and PM3 molecular orbital calculations on the mechanism of the hydrolysis of unsaturated lactones: substituted (E )-5,5′-diphenylbifuranylidenediones and 3,7-diphenylpyrano[4,3-c]pyran-1,5-diones

“…In the latter ones a stabilising intramolecular hydrogen bond between one H-atom of the approaching water molecule and the keto carbonyl function is present, leading to a lower activation energy (3-4 kcal mol Ϫ1 ) for TS (11-12) as compared to TS (8)(9). As for the opening of the first lactone ring AM1 calculations also lead to a significantly lower activation energy for opening of the second lactone ring [TS (9-10) and TS (12)(13)] than for addition of the nucleophile, especially for the enol tautomer. The respective transition states [see structure (b) in Fig.…”

“…The energetics for this reaction sequence are collected in Table 2. With respect to the structures of the transition states for addition of H 2 O to the pyranopyrandiones [TS (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)] as well as their frequencies [ν(AM1) ca. 1980 i, ν(PM3) ca.…”

“…For the bifuranylidenediones this effect only further strengthens the assumption of TS (8-9) being rate determining. Concerning the ratedetermining step [TS (6)(7)(8)(9)(10)(11)(12)(13)(14)(15) or TS (16)(17)] of the alkaline hydrolysis of the pyranopyrandiones the results are less clearcut. With both transition states a good description of substituent effects on reaction rates can be obtained.…”

Mechanistic and Synthetic Aspects of the Benzilic Acid and Ester Rearrangements

“…Rajyagura and Rzepa [10], carried out a MNDO SCF-MO study and they claimed that a SET mechanism was a distinct possibility for the benzilic acid and related rearrangements (Scheme 4, shows the most probable pathways, path a (classical) and paths b and c (SET processes). To surmount this difficulty Rajyagura and Rzepa [10] proposed an intramolecular SET mechanism (Scheme 4, pathway b and c) in which hydrogen migration was preferred over phenyl migration due to the ability of the phenyl group to stabilise the adjacent radical centre in intermediates (II) and (III) (Scheme 4, R = Ph). In the case of phenylglyoxal (5) (Scheme 4, R = H, R 1 = Ph) it was calculated on the basis of the classical mechanism that the Ph group should migrate preferentially, which is contrary to the experimental evidence [5].…”