Carbenes and the O-H Bond: Hydroxyalkyl-Substituted Arylcarbenes

Article abstract of DOI:10.1021/jo00295a018

<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.