CARBENES AND THE O-H BOND - HYDROXYALKYL-SUBSTITUTED ARYLCARBENES

[2-(Hydroxymethyl)phenyl]carbene (4), [2-(hydroxymethyl)phenyl]phenylcarbene (19), and [2-(2-hydroxyethyl)phenyl]carbene (30) have been generated by photolysis of tosylhydrazone or diazo precursors in protic solvents. These carbenes give cyclic ethers (7, 18, 33) competitively with insertion into O-H bonds of the solvent. For comparison, the analogous benzyl cations (9, 17, 31) have been generated by solvolysis or dediazoniation. The cations are more sensitive to structural variation than their carbenic counterparts: 9 does not undergo intramolecular nucleophilic substitution, in contrast to 17 and 31. These observations are explicable in terms of high barriers for rotation about aryl-cation bonds, as compared with low barriers for rotation about aryl-carbene bonds. Two major effects of the solvent (ROH) and of the base (RONa) on product formation may be distinguished: (i) protonation of the carbene (or of its precursors) in the more acidic media leads to predominantly cationic processes; (ii) deprotonation of the OH group under strongly basic conditions enhances the nucleophilicity of the oxygen, and also facilitates insertion into the α-C-H bonds of 30. © 1990, American Chemical Society. All rights reserved.