Isotope effects carbonyl groups

There are at least two mechanisms available for aziridine cis-trans isomerism. The first is base-catalyzed and proceeds via an intermediate carbanion (235). The second mechanism can be either thermally or photochemically initiated and proceeds by way of an intermediate azomethine ylide. The absence of a catalytic effect and interception of the 1,3-dipole intermediate provide support for this route. A variety of aziridinyl ketones have been found to undergo equilibration when subjected to base-catalyzed conditions (65JA1050). In most of these cases the cis isomer is more stable than the trans. Base-catalyzed isotope exchange has also been observed in at least one molecule which lacks a stabilizing carbonyl group (72TL3591). 

An inverse isotope effect will occur if coordination at the reaction center increases in the transition state. The bending vibration will become more restricted. Entry 4 in Scheme 4.2 exenqilifies such a case involving conversion of a tricoordinate carbonyl group to a letravalent cyanohydrin. In this case the secondary isotope effect is 0.73. 

The oxidation of a-hydroxy acids by benzyltrimethylammonium tribromide (BTMAB) to the corresponding carbonyl compounds shows a substantial solvent isotope effect, A (H20)/A (D20) = 3.57, but no KIE for a-deuteromandelic acid.133 The oxidation of glucose by hypobromous acid is first order in glucose and the acid.134 [l,l-2H2]Ethanol shows a substantial kinetic isotope effect when oxidized by hexamethylenetetramine-bromine (HABR) in acetic acid to aldehyde.135 Kinetics of the oxidation of aliphatic aldehydes by hexamethylenetetramine-bromine have been studied by the same group.136 Dioxoane dibromide oxidizes y-tocopherol to 5-bromomethyl-y-tocopherylquinone, which spontaneously cyclizes to 5-formyl-y-tocopherol.137... 

The o-substituted benzoic acid methyl ester of formula (CH3)2NC6H4C02CH3 has been found to lose both the ester methyl and the amine methyl in metastable ion decompositions. It has been proposed that the bond cleavage to eliminate the ester methyl is accompanied by a hydrogen transfer from one of the other methyl groups to the carbonyl oxygen in a 7-membered cyclic transition state [83]. The mechanistic proposal rests upon isotope effects observed on substituting deuterium in methyl groups. 

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