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Structure–reactivity relationship for alcohol oxidations via hydride transfer to a carbocationic oxidizing agent
Author(s) -
Lu Yun,
Bradshaw Joshua,
Zhao Yu,
Kuester William,
Kabotso Daniel
Publication year - 2011
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.1842
Subject(s) - chemistry , steric effects , alcohol , kinetic isotope effect , carbocation , hydride , reactivity (psychology) , alkyl , oxidizing agent , medicinal chemistry , photochemistry , reaction rate constant , organic chemistry , kinetics , hydrogen , deuterium , medicine , physics , alternative medicine , quantum mechanics , pathology
Second‐order rate constants were determined for the oxidation of 27 alcohols (R 1 R 2 CHOH) by a carbocationic oxidizing agent, 9‐phenylxanthylium ion, in acetontrile at 60 °C. Alcohols include open‐chain alkyl, cycloalkyl, and unsaturated alcohols. Kinetic isotope effects for the reaction of 1‐phenylethanol were determined at three H/D positions of the alcohol (KIE α‐D = 3.9, KIE β‐D3 = 1.03, KIE OD = 1.10). These KIE results are consistent with those we previously reported for the 2‐propanol reaction, suggesting that these reactions follow a hydride‐proton sequential transfer mechanism that involves a rate‐limiting formation of the α‐hydroxy carbocation intermediate. Structure–reactivity relationship for alcohol oxidations was deeply discussed on the basis of the observed structural effects on the formation of the carbocationic transition state (C δ+ OH). Efficiencies of alcohol oxidations are largely dependent upon the alcohol structures. Steric hindrance effect and ring strain relief effect win over the electronic effect in determining the rates of the oxidations of open‐chain alkyl and cycloalkyl alcohols. Unhindered secondary alkyl alcohols would be selectively oxidized in the presence of primary and hindered secondary alkyl alcohols. Strained C 7 C 11 cycloalkyl alcohols react faster than cyclohexyl alcohol, whereas the strained C 5 and C 12 alcohols react slower. Aromatic alcohols would be efficiently and selectively oxidized in the presence of aliphatic alcohols of comparable steric requirements. This structure–reactivity relationship for alcohol oxidations via hydride‐transfer mechanism is hoped to provide a useful guidance for the selective oxidation of certain alcohol functional groups in organic synthesis. Copyright © 2011 John Wiley & Sons, Ltd.