Reactions of PhSCH₂Li and NCCH₂Li with Benzaldehyde and Benzophenone:  When Does the Mechanism Change from ET to Polar?

Abstract

The carbonyl-carbon kinetic isotope effect (KIE) and the substituent effect were measured for the reaction of phenylthiomethyllithium (PhSCH(2)Li, 1) with benzaldehyde and benzophenone, and cyanomethyllithium (NCCH(2)Li, 2) with benzaldehyde, and the results were compared with those for other lithium reagents such as MeLi, PhLi, CH(2)=CHCH(2)Li, and CH(2)=C(OLi)C(CH(3))(3). It was previously shown that the reactions of MeLi, PhLi, and CH(2)=CHCH(2)Li proceed via a rate-determining electron transfer (ET) process whereas the reaction of lithium pinacolone enolate goes through the polar (PL) mechanism. The reaction of 1 with benzaldehyde gave no carbonyl-carbon KIE ((12)k/(13)k = 0.999 +/- 0.004), similar to that measured previously for the MeLi reaction with benzophenone ((12)k/(14)k = 1.000). The effect of substituents of the aromatic ring of benzaldehyde and benzophenone on the reactivity gave very small Hammett rho values of 0.17 +/- 0.03 and 0.26 +/- 0.05, respectively. These small rho values are again similar to that observed for the reaction of MeLi. Likewise the reactions of 2 with benzaldehydes gave small KIE and the rho value ((12)k/(13)k = 0.996 +/- 0.004, rho = 0.14 +/- 0.02). Dehalogenation and enone-isomerization probe experiments for 2 showed no evidence for the presence of radical-ion pair of sufficient lifetime during the course of the reaction. It is concluded that the reactions of 1 and 2 with the aromatic carbonyl compounds proceed via the electron transfer-radical coupling mechanism with rate-determining ET as in the reactions of MeLi, PhLi, and CH(2)=CHCH(2)Li.

Carbon isotope evidence for the substrates and mechanisms of prebiotic synthesis in the early solar system

“…Metal oxide catalysts are present in Murchison and other CM2 meteorites (Bunch and Olsen, 1975;Hanowski and Brearley, 2000), supporting the plausibility of this scenario. Accounting for previously measured KIEs associated with addition reactions to aldehydes (a 0 to -19 ‰ KIE for carbonyl carbons; (Yamataka et al, 1997;Yamataka et al, 2001)) and the likely upper limit of a ~-30 ‰ KIE for the oxidation of a carbonyl carbon, the δ 13 C values of the C 1 -C 5 monocarboxylic acids can be calculated as a mixture of a 13 C-enriched carbonyl carbon and 13 C-depleted methyl carbons. The final predicted monocarboxylic acid molecular-average δ 13 C values vary little between the 0 ‰ and -30 ‰ isotope effects on the carbonyl carbon, so we will consider the -30 ‰ predictions that closely agree with previous measurements for the C 3 -C 5 species from Yuen et al (Yuen et al, 1984) and with the trends presented in more recent studies by (Huang et al, 2005) and (Aponte, Woodward, et al, 2019) ( Figure 4a).…”

Reactions of Allylmagnesium Halides with Carbonyl Compounds: Reactivity, Structure, and Mechanism

“…47 Although the single-electron transfer mechanism may account for these observations, other reactions of carbon nucleophiles that proceed by single-electron pathways are sensitive to electronic effects. 60,113,114 Experiments with cyclopropanecarboxaldehydes provide evidence that additions of allylmagnesium halides to alkyl aldehydes do not proceed via a single-electron-transfer mechanism (Scheme 12). 50 Additions by the single-electron transfer pathway should form ring-opened products because, if a ketyl intermediate were formed, the rate of ringopening (k ≈ 10 11 •s -1 ) should be similar to the rate of recombination of radical pairs.…”