Carbenes production

Thermolysis of 58a in butanol affords, together with 17% of 60a (R = C4H9) which evidences the intermediacy of the thiophosphene 59 a, a variety of partly atypical products which seriously impede the desired rearrangement38. Photolysis of 58b in methanol is also found to give only 18 % 1,2-P/C shift to form the heterocumulene 59b, from which the thiophosphinic rater 60b (R = CH3) results 39). As already mentioned in connection with the photolysis of diazo compounds of type 36 (see Sect. 2.2), Wolff rearrangement (9%) and O/H insertion (6%) once again compete with thiophosphinic ester formation. Moreover, solvolysis of the P(S)/C(N2) bond 391 prevents a greater contribution of carbene products to the overall yield. 

In order to safely identify k0 with intramolecular carbenic reactions (e.g., k and the formation of alkene 4 in Scheme 1), product analysis should demonstrate that the yield of intramolecular products exceeds 90%, while dimer, azine, and solvent-derived (intermolecular) carbene products should be absent or minimal. If these conditions are not met, mechanistic interpretation is often ambiguous, a result that is well illustrated by the saga of benzylchlorocarbene (see below, Section IV.C). Less desirably, k0 can be corrected for competitive intermolecular carbenic reactions. Bimolecular reactions like dimerization and azine formation can be minimized by working at low carbene precursor concentrations, and careful experimental practice should include quantitative product studies at several precursor concentrations to highlight potential product contamination by intermolecular processes. 

The yields of carbene (and pyridinium ylide) are thus controlled by competition between simple nitrogen loss to the carbene or 1,2-H (or 1,2-C) shift linked to nitrogen loss in the excited state of the diazoalkane precursor 56 When R=i-C3H7, the a-CH bond is weak, there is much RIES upon diazoalkane photolysis, and carbene production is inefficient. When R=/-C4H9 (no a-CH bond) or R=cyclo-CjU5 (strong a-CH bond), diazoalkane photolysis gives little RIES and much carbene.56... 

In the same vein is the observation that the lifetime of dipropylcarbene (59) in CH2C12 or cyclohexane is 0.3 ns,84 which, after statistical correction is 48 times less than the lifetime ( 21 ns) of Me2C in pentane.22 This reflects promotion by the propyl bystander groups of 59 of the 1,2-H shift to Z- and E-3-heptene.84 (Dipropylcarbene can be photolytically generated from either an oxadiazoline (diazoalkane)84 or diazirine85 precursor, but RIES lowers the efficiency of carbene production in either case.) Recently reported LFP lifetimes for Et2C and MeCEt in cyclohexane or benzene are 0.6-3 ns (cyclohexane) or 1-5 ns (benzene),14 in accord with the lifetimes of S822 and S9.84 The rate constants for carbene disappearance in cyclohexane ( 3 x 108 to 2 x 109 s 1) are presumably limited by 1,2-H shifts.14... 

In principle, the heat of formation of carbenes can be determined from PAC heats of reaction and either the carbene precursor or the carbene products heats of formation (Scheme 1). 

Scheme 7.36). Some carbene product in the ortho isomer (the excess benzo-cyclobutene) actually comes not from the carbene, but from the carbene precursor (see Section 2.4 for much more on this general subject). 

A related mechanism is proposed for di(arylsulfonyl)-diazomethane initiation (9b). Excited dyes were shown to sensitize the decomposition of the diazo derivative to afford carbene products. The carbene product had been previously known as a reductant for photoreducible dyes (21). 

Earlier results also indicated that irradiation of this class of compounds gave CO loss. Thus Fischer and Fischer (168) noted in 1974 that irradiation of Cr- and W-alkylidene complexes in the presence of a series of tertiary phosphines led to CO loss and formation of a mixture of cis- and franj-M(CO)4(PR3)(carbene) products [Eq. (134)]. Low-temperature... 

Diazirines (isomeric with diazoaUcanes) give carbenes/ but aryhnethyl radicals have also been generated from diazirines. In a different study, thermolysis of diaryloxydiazirines gave the anticipated carbene products, but photolysis gave both carbenes and aryloxy radicals by a-scission. ... 

The dimethylamino-2//-azirine 39 undergoes exclusive C—C bond breaking by flash vacuum pyrolysis (350 C, 0.1 torr).50 In this case, the intermediate iminocarbene (or nitrile ylide) 40 is stabilized by the amino group. Only the carbene product 42 and not the nitrene product 41 was isolated (Eq. 13). 

Under more vigorous conditions, the Bamford-Stevens reaction yields diazoalkanes only as unobservable, metastable intermediates. The carbene formed is also metastable. This procedure is used, however, for the synthesis of compounds that are carbene products. 

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