Spectroscopic Evidence for Carbene Protonation

Quantum yields for the formation of 141 from 138 in TFE-MeCN were estimated by transient absorption actinometry (Table l).62 The data refer to solvated carbocations (141) since ion pairs (140) are too short-lived for detection on the ns time scale. The modest to poor yields of 141 could be due to predominant ion-pair recombination (140 - 142), or to parallel protonation (139 — 140) and insertion (139 — 142). Picosecond LFP studies on photoheterolyses of A CH-X in MeCN revealed that the ratio of collapse to escape (k /ki) for [Ar2CH+ X-] is slightly affected by p-substituents (H, Me, OMe) and by X (Cl, Br).66 In contrast, 4 M1 was found to increase by a factor of 17 as p-H (138d) was replaced with p-OMe (138a).62 Hence the ion-pair hypothesis seems difficult to reconcile with the effect of p-substituents on unless the strong nucleophile RO in 140 behaves differently from the weakly nucleophilic halide ions. 

Some direct information on ion pairing comes from picosecond LFP of 138a in ROH-MeCN mixtures.67 In the presence of EtOH, the transient absorption of 139a, A-max 400 nm, was apparent at 50 ps. At 150 ps, the 400 nm 

Aryl(cyclopmpyl)carbenes. The potential of the cyclopropyl group for conjugation 70 suggests that the nucleophilicity of carbenes should not be strongly 

Aryl(carbonyl)carbenes. The mechanism of proton transfer to the carbenes 168 was found to respond sensibly to the ApA a of the reactants. LFP of 167a generated 168a (A. = 355 nm) whose absorption was quenched rapidly by 

Acid catalysis was observed in the formation of 171 from 168, and the enol cation 170 was proposed as an (invisible) intermediate.830 

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