Alicyclic reaction mechanisms. 7. The predominance and quantification of steric effects in the solvolysis of secondary aliphatic esters

Scales of Nucleophilicity and Electrophilicity: A System for Ordering Polar Organic and Organometallic Reactions

“…In most investigations of hydride abstractions from CH bonds, the triphenylcarbenium ion (= trityl cation) has been employed as the hydride acceptor.15 31 Because of their importance in biochemistry, hydride transfers from NADH analogues have been extensively investigated, in particular, Nben~yl-l,4-dihydronicotinamide. [~~~ Bunton et al showed that the reactions of cations with this hydride donor obey the N , relationship of R i t~h i e .…”

“…In this section, discussion is limited to the abstraction of Hfrom hydrides of Main Group 4 elements, which have been studied in detail. In most investigations of hydride abstractions from CH bonds, the triphenylcarbenium ion (= trityl cation) has been employed as the hydride acceptor.15 31 Because of their importance in biochemistry, hydride transfers from NADH analogues have been extensively investigated, in particular, N - Bunton et al showed that the reactions of cations with this hydride donor obey the N , relationship of R i t~h i e . '~~] As these data are primarily used for the characterization of the electrophilicity of carbenium ions.…”

The controversy about nonclassical ions: Finished too early?

“…The center of the debate was the solvolysis rate of 2‐exo norbornyl derivatives which was considered to be exceptionally fast, and the much smaller reactivity of the endo‐isomer . The steroids 1a , b and 2a , b show that such rate differences are by no means restricted to norbornyl esters . The rate difference, measured in hexafluoroisopropanol (HFIP), between the 17 α /17‐ ß ‐tosylates exceeds those of exo/endo‐2‐norbornyl compounds even by an order of magnitude, with an up to 30.000‐fold rate enhancement (Scheme and Table ).…”

“…In 97% 1,1,1,3,3,3‐hexafluoroisopropanol at 25°C; constants, for 1 , 2 , 4 , 5 , 6 , 8 , and 9 from Schneider and Becker and for 3 (2‐norbornyl‐OTs) and 10 (cyclohexyl‐OTs) from Bentley et al; there for 4 (cyclopentyl‐OTs), a value of 415 × 10 −6 [s] −1 was reported.…”

“…Steric contributions to reactivity differences were pioneered by H. C. Brown, who stressed that stereoselectivities similar to those in the norbornyl case are quite frequent in other systems . Steric effects can be evaluated quantitatively with the aid of force field calculations, simulating, eg, the strain energy difference ΔSI sp3/sp2 between sp 3 and sp 2 states in alkane reactions with, eg, methyl or hydroxy groups, and oxo groups as model substituents, respectively. Figure shows a linear correlation (coefficient R 2 = 0.96) between ΔSI sp3/sp2 and activation free energy Δ G # for the great majority of alkyltosylate solvolysis in HFIP .…”