Carbenes, generation substituent effects

Generation of 78 by thermolysis or photolysis of a diazoalkane or diazirine precursor, however, affords the singlet carbene, whose 1,2-H shift to ethene is opposed by a barrier of only 0.678 to 1.298 kcal/mol. Consequently, even in cryogenic matrices, singlet 78 rearranges more rapidly than it intersystem crosses to the triplet, which has therefore not been detected by UV or ESR in either an Ar matrix at 8 K or a Xe matrix at 15 K." The lifetime of singlet 78 at ambient temperature has been estimated at <0.5 ns.89,98b (Note the enormous spectator substituent effect of Cl the lifetime of MeCCl is 740 ns,60 at least 1500 times longer than that of MeCH.)... 

The treatment of the 2-methyl- 1-propenylbismuthonium salt with KOf-Bu in the presence of excess styrene gives alkylidenecyclopropanes in good yield (Scheme 7). The para substituent effect on the relative reactivity is very small, indicating that a free alkylidene carbene is generated as the active species. In this case, the alkylidene carbene is trapped by the styrene in solution. 

A range of (phosphino)(aryl)carbenes (20 R = i-Pr, c-hexyl V, W, X, Y, Z = various combinations of H, F, Me, CF3, NMe2) have been generated by photolysis of their diazo precursors. The substituent effects on carbene stability were investigated both experimentally and theoretically. The presence of an acceptor and also a spectator aryl substituent resulted in stable or at least persistent carbenes, which could be characterized by standard spectroscopic techniques, whereas a donor aryl ring produced a very fleeting carbene. 

The effect of substituents on the temperature dependence of a-carbonyl-carbene reactivity has been examined using carbenes generated by low-temperature photolysis of methyl diazophenylacetate. A correction to the literature on the photoreaction of isopropylidene diazomalonate (98) with 1,3,3-trimethyl-cyclohexane (99) has been reported. The photoproduct, originally thought to be a cyclopropane derivative, has now been shown to be the cyclobutanone (100), the formation of which presumably involves a photo-Wolff rearrangement as illustrated in Scheme 11. Substituent effects observed in the product distribution of diazo-amide photochemistry have been ascribed to conformational factors the jS-lactam, oxindole, and Wolff rearrangement products appear to arise directly from the excited singlet state of the sym-Z form of the diazo-amide itself. 

Computational studies have compared substituent effects on the stability of ketenes, allenes, diazomethanes, diazirines, and cyclopropenes. Ketenes belong to the first generation of reactive intermediates along with carbocations, carbanions, radicals, and carbenes, and are intensively studied members of the cumulene family, with many useful synthetic applications. Ketenes were first recognized in 1905, when diphenylketene, a stable and isolable example, was obtained from the dehalogenation of the a-bromodiphenylacetyl bromide (Scheme 7.37). The most characteristic reaction of ketene is cycloaddition, as in the formation of p-laclams. 

Irradiation of 7 in methanol gives a quantitative yield of a-methoxyphosphine oxide 18a il,14) however, when the same carbene (16a) is generated by thermolysis of 7 in ethanol then rearrangement to 17 a also takes place 15). In view of the behavior of 16a, para substituents in the phenyl groups of the phosphoryl group influence the rearrangement only if they exert a promoting effect. Thus it is understandable that... 

Interaction of an electrophilic carbene or carbenoid with R—S—R compounds often results in the formation of sulfonium ylides. If the carbene substituents are suited to effectively stabilize a negative charge, these ylides are likely to be isolable otherwiese, their intermediary occurence may become evident from products of further transformation. Ando 152 b) has given an informative review on sulfonium ylide chemistry, including their formation by photochemical or copper-catalyzed decomposition of diazocarbonyl compounds. More recent examples, including the generation and reactions of ylides obtained by metal-catalyzed decomposition of diazo compounds in the presence of thiophenes (Sect. 4.2), allyl sulfides and allyl dithioketals (Sect. 2.3.4) have already been presented. 

Although alkylidenecarbenes (R)2C=C and carbenoids 22-24 have an ip-hybridized carbene center similar to that of vinylidenecarbenes, the reactivity will be subject to the steric influence of substituents R3 and R4 because its location is closer to the carbene center than vinylidenecarbenes (Scheme 11). The steric effect was exerted in the reactions of 2-methylpropenylidene 22 generated from 2-methyl-1-chloropropene and butyllithium (BuLi) (Scheme ll).22 23 The results are summarized in Table 5. A more detailed discussion on the stereoselectivity of this reaction will be revisited in Section HI. A. 

The other method to further stabilize kinetically protected triplet DPCs is to use the electronic (thermodynamic) effect of/7ara-substituents. It is well-documented in radical chemistry that the abilities of various substituents to delocalize the spin on a carbon radical play an important role. [36, 37] Thus we generated bis(2,6-dimethylphenyl)carbene ( 31a ) having a series of substituents at para positions and studied the effect of /7ara-substituents on the stability of the carbene (Table 2, Scheme 9, Figure 6 ). [38-40]... 

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