78338-82-6Relevant academic research and scientific papers
Enantioselective α-Chlorination Reactions of in Situ Generated C1 Ammonium Enolates under Base-Free Conditions
Stockhammer, Lotte,Weinzierl, David,B?gl, Thomas,Waser, Mario
supporting information, p. 6143 - 6147 (2021/08/18)
The asymmetric α-chlorination of activated aryl acetic acid esters can be carried out with high levels of enantioselectivities utilizing commercially available isothiourea catalysts under base-free conditions. The reaction, which proceeds via the in situ formation of chiral C1 ammonium enolates, is best carried out under cryogenic conditions combined with a direct trapping of the activated α-chlorinated ester derivative to prevent epimerization, thus allowing for enantioselectivities of up to e.r. 99:1.
A kinetic evaluation of carbon-hydrogen, carbon-carbon, and carbon-silicon bond activation in benzylic radical cations
Freccero, Mauro,Pratt, Albert,Albini, Angelo,Long, Conor
, p. 284 - 297 (2007/10/03)
A detailed study of the competition between C-C, C-H, and C-Si bond fragmentation in a series of 4-methoxy-α-substituted toluene radical cations (1.+), involving both product studies and kinetic analysis, is presented. C-C bond fragmentation occurs with several radical cations in acetonitrile. The rate constants for such processes, determined by laser flash photolysis, varied from 2.8 x 104 (1c.+) to 1.53 x 106 (1f.+) s-1. The activation parameters for C-C bond fragmentation are characterized by low activation enthalpies on the order of 30 kJ mol-1 and negative activation entropies in the range -34 to -55 J mol-1 K-1. Deprotonation of the radical cations is always a second-order process induced by nucleophiles [cerium(IV) ammonium nitrate (CAN) or nitrate anion], with second-order rate constants from 7.7 x 107 (1h.+) to 8.8 x 108 (1i.+) M-1 s-1 in neat acetonitrile (CAN assisted) and from 0.4 x 108 (1j.+) to 7.1 x 108 (1i.+) M-1 s-1 in the presence of nitrate anion. The rate constant for nitrate-induced decarboxylation was higher, 13.6 x 108 M-1 s-1 (1d.+). In a few cases C-C (1e.+, 1f.+) and C-Si (1g.+) fragmentations occurred, also as second-order processes induced by nitrate, with rate constants from 4.4 x 108 (1f.+) to 8.2 x 108 (1g.+) M-1 s-1. ΔH and ΔS had opposing influences on C-H and C-C fragmentation, and in the case of 1e.+ a temperature-dependent product distribution was obtained. The activation parameters for the observed C-H, C-C, and C-Si fragmentations have been compared, and suggest a rationale for the mechanisms and selectively of such processes in radical cations.
