Carbene precursors rearrangements

Diazirine, fluoromethoxy-nitrogen extrusion, 7, 224 Diazirine, methylvinyl-rearrangement, 7, 221 Diazirines addition reactions to Grignard compounds, 7, 2 0 as carbene precursors, 7, 236 IR spectra, 7, 203 microwave spectrum, 7, 199 molecular spectra, 7, 202-204 nitrogen extrusion, 7, 223 NMR, 7, 202 photoconversion to diazoalkanes, 7, 234 photoisomerization, 7, 221 photolysis, 7, 225-227 quantum chemical investigations, 7, 197 reactions... 

Carbenes and substituted carbenes add to double bonds to give cyclopropane derivatives ([1 -f 2]-cycloaddition). Many derivatives of carbene (e.g., PhCH, ROCH) ° and Me2C=C, and C(CN)2, have been added to double bonds, but the reaction is most often performed with CH2 itself, with halo and dihalocarbenes, " and with carbalkoxycarbenes (generated from diazoacetic esters). Alkylcarbenes (HCR) have been added to alkenes, but more often these rearrange to give alkenes (p. 252). The carbene can be generated in any of the ways normally used (p. 249). However, most reactions in which a cyclopropane is formed by treatment of an alkene with a carbene precursor do not actually involve free carbene... 

Knowledge of the intramolecular product distribution may allow for the partitioning of k between competitive intramolecular reactions, but one must be certain that noncarbenic routes to the products do not compete with the carbenic pathways. In particular, we must be concerned with the possible intervention of RIES (cf. Section m.C), especially when diazirines or diazoalkanes are the carbene precursors. Again, corrections for RIES can be made to quantitate the carbenic routes see, for example, the discussion of the cyclobutylhalocarbene rearrangements (Section m.C.1). 

Additional evidence for a second intermediate in supposed carbene reactions comes from numerous studies.17-29 In the earliest experimental approach, the carbene precursor, frequently a diazirine, was photolyzed in the presence of increasing quantities of an alkene, which trapped the carbene with the formation of a cyclopropane (5 in Scheme 1). If carbene 2 were the sole product-forming intermediate, as depicted in Scheme 1, then the ratio of its alkene addition product (5) to its 1,2-H shift rearrangement product (4) would vary linearly with alkene concentration Eq. 9. 

Clearly, rearrangements do occur in the excited states of diazirine and diazo carbene precursors. Kinetic studies of carbenic rearrangements need to consider the possible intervention of RIES when absolute rate constants are partitioned between competitive rearrangement pathways on the basis of product distributions.28... 

We have seen that 1,2-H migrations in singlet carbenes may be affected by (e.g.) the participation of carbene precursor excited states, QMT, stabilization of the hydride shift transition state by polar solvents, and temperature. Here, we consider our third principal theme, the effect of substituents on the kinetics of carbenic rearrangements. We first examine the influence of bystander and spectator substituents (as defined in Eq. 22) on 1,2-H rearrangements of alkyl, alkylchloro, and alkylacetoxycarbenes. 

Other possible carbene precursors (vicinal dihalides, peresters, ketenes, carbene adducts with stable hydrocarbons, etc.) cannot generally be used for in situ generation of carbenes, because the fragments are likely to recombine. However, they can be used, for example, in experiments involving pyrolysis or other forms of external carbene generation where the fragments get a chance to separate in the gas phase and become trapped in distant matrix sites. AU conceivable halo- and dihalocar-benes were made and smdied in this way (see, e.g., the 1993 review by Sander et al." ) However, such methods can only be applied to carbenes which resist thermal rearrangement to more stable products. 

Recent study on sulfonium ylide [2,3]-sigmatropic rearrangement has been focused on the development of new catalytic systems, including new catalysts and alternative carbene precursor other than commonly used a-diazocarbonyl compounds. Besides the most commonly used Cu(i) and Rh(ii) catalysts, Fe com-... 

The ester group in II is suggestive—although it is not a proof—of the intermediacy of a ketene, and ketene production in diazocarbonyl chemistry usually implies a Wolff rearrangement. The construction of a three-carbon chain on the other side of the ketone is a confirmation of this prediction. In turn, the Wolff rearrangement requires an a-keto carbene precursor that is the fate of diazo compounds exposed to ultraviolet light (wavelength lower than 3200 A). All this is translated into the mechanism depicted in Scheme 43.1. 

Free carbene 429 generated by photolysis or thermolysis of the diazo compound cyclizes into the sulfonium ylide, which undergoes a thia-Stevens rearrangement to 430 in 19% yield. The same result was obtained using the corresponding p-tosylhydrazone sodium salt as a carbene precursor (83JA6096). Rh(II)-catalyzed decomposition of sulfur-containing diazo... 

The migratory aptitude of R in (11) varies widely with its structure (see Section 3.9.2.1), the shift of an alkoxy group being among the slowest. The formation of alkoxyketenes in the photolysis of alkyl diazoacetates is a fairly recent discovery. The major competing reactions of the carbene precursor are insertions into the C—H and O—H bonds of alcohols employed as solvents and ketene traps. The extent of Wolff rearrangement varies with structure ethyl diazoacetate (20-25%), phenyl diazoacetate (45-60%), and A -methyldiazoacetamide (30%). These reactions are of limited synthetic interest at present. 

With the intent to selectively derivatize CyD oligosaccharides,80 Abelt enlisted nitrogenous carbene precursors as labile guests.81 The involvement of supramolecular carbenes could be inferred based on intramolecular products stemming from 1,2-H shifts. Of course, 3//-diazirines that possess a-C-H bonds, like 3-methyl-3-phenyl-3//-diazirine (6) (Scheme 2), are susceptible to rearrangements in the excited state (RIES) that mimic the results of carbene 1,2-H shifts,82 e.g., 6 —>10 (Scheme 2). 

However, a study of phenyl carbene generated directly and generated by a carbene-to-carbene rearrangement shows significant differences in product composition, so it is likely that at least some of the products reported as products of carbene reactions are formed directly from the carbene precursor, in a reaction which does not involve a carbene. 

Bis(dialkylamino)cyclopropanes, 14 in Scheme 6, are potential carbene precursors in what amounts to the reverse of the usual reaction of carbenes with aUcenes. Vilsmaier, Kristen and Tetzlaff reported [41 ] that the flash vacuum pyrolysis of several 7,7-bis(dialkylamino)bicyclo[4.1.0]heptanes gave cyclohexene and amidines resulting from rearrangement of diaminocarbenes, Scheme 8. It... 

Intriguingly, while vinyldiazoacetates are utilized as carbene precursors in the allylic C—H bond insertion reaction catalyzed by Rh complexes, the combined C—H bond functionalization/Cope rearrangement occurs readily. This has emerged as a reliable methodology for the construction of 1,5-diene compounds bearing two vicinal stereogenic centers and will be discussed in detail in Section 1.1.2.6. 

Both silene isomers 278 and 279 are ideal precursors for the generation of silylene 284, since their interconversion to 284 is spontaneous (in the case of 278) or can be easily induced by irradiation (in the case of 279). There are numerous well-established methods to prepare transient silylenes 279. Three important examples are shown in equation 69, namely the photolytic genCTation from a trisilane 280, thermolytic or photolytic decomposition of cyclic silanes 281 " " and degradation of diazidosilanes 282 56 -pjjg photolysis of the diazido silane 282 is an especially clean reaction which has been used in several spectroscopic studies. The photolysis of or-diazo compounds 283 is the only frequently used reaction path to sUenes 284 via a carbene-silene rearrangement. ... 

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