Rearrangement, allylic carbenes

As is the case for the [2,3] Wittig rearrangement, the stereochemical consequences of the [2,3] ylide rearrangement are sensitive to perturbation by external steric and stereoelectronic factors, presenting a useful opportunity for both substrate- and reagent-based asymmetric induction. Rearrangements of carbene-derived ylides of allylic sulfide 1 provide a simple example of substrate-directed diastereosclcction, in which diastereoface selectivity results from attack on the exocyclic olefin via the less-hindered equatorial approach vector112. 

Nucleophiles like lithium enolates and organocuprates can be added to the terminus of the allyl ligand of cationic cyclic allyl(carbene)iron complexes to give 4-substituted tricarbonyl[l,(3 )-diene]iron complexes. Subsequent to the nucleophilic addition, a ferra-Claisen rearrangement is supposed to be involved in the reaction mechanism. The free dienes can be released by treatment with ceric ammonium nitrate or alkaline hydrogen peroxide (Scheme 4-65). ... 

Ketenes react with tertiary allylic amines in the presence of Lewis acids to give zwitterionic intermediates which undergo [3,3]-sigmatropic rearrangement [119]. Photolysis of chromium carbene complexes in the presence of tertiary amines results in similar chemistry [120]. Cyclic (Table 21) and strained allylic amines (Eq. 34) work best, while acylic amines are less reactive (Eq. 35). 

Their precursors must be the tricarbonyl o-allenyls with the uncoordinated C=C bonds. Neither an allylic rearrangement nor cis-trans isomerization has been observed in the reaction of CpMo(CO)3(cw-CH2CH=CHMe) with PPhj, the product being CpMo(CO)2(PPh3)(cw-COCH2CH=CHMe) (81). The interesting reaction leading to the formation of cationic carbene compounds was mentioned earlier [Eq. (17) and Section V] (78). 

A mechanistic picture which reconciles the experimental results is given in Scheme 24. It is assumed that both the heteroatom and the double bond of the allyl halide compete for an electrophilic metal carbene. Heteroatom attack yields a metalated ylide 129, which may go on to ylide 131 by demetalation and/or to allylmetal complex 130. Symmetry-allowed [2,3] rearrangement of 131 accounts for product 132, and metal elimination from 130 gives rise to products 132 and 133, corresponding to [2,3] and [1,2] rearrangement, respectively, as well as haloacetate (if R3 = CHc ). 

Osborn and Green s elegant results are instructive, but their relevance to metathesis must be qualified. Until actual catalytic activity with the respective complexes is demonstrated, it remains uncertain whether this chemistry indeed relates to olefin metathesis. With this qualification in mind, their work in concert is pioneering as it provides the initial experimental backing for a basic reaction wherein an olefin and a metal exclusively may produce the initiating carbene-metal complex by a simple sequence of 7r-complexation followed by a hydride shift, thus forming a 77-allyl-metal hydride entity which then rearranges into a metallocyclobutane via a nucleophilic attack of the hydride on the central atom of the 7r-allyl species ... 

Ring expansion of cycloproparenes to cycloheptatrienes or tropones has been discussed in the context of electrophilic addition to cycloproparenes. When 1,1 -di-chloro-2,5-diphenylbenzocyclopropene (22) is thermolyzed in refluxing benzene, the dimer 373 is formed as a mixture of /Z-isomers. It is believed to arise via dimerization of the carbene 372, which, in turn results from an allylic rearrangement of22to371. ... 

Uemura and co-workers developed an interesting and unique catalytic system, in which the Rh(ii) intermediate is generated by Rh2(OAc)4-catalyzed reaction of conjugated ene-yne-carbonyl compounds (Equation (16)). The Rh(ii)-carbene is trapped by allyl sulfide to give [2,3]-sigmatropic rearrangement product 121 in good yields." ... 

This dominance of sulfur in the reactions with electrophiles is well brought out in the addition of carbenes to the-two monocycles. Tire allylic sulfide (5,6-dihydro-2jF/- thiopyran) only affords the products of reaction at sulfur, while the vinylic sulfide (3,4-dihydro-2f/-thiopyran), in which the alkene is a little more nucleophilic due to the small interaction with the heteroatom, shows dichotomous behaviour. Dichlorocarbene affords the cyclopropane product (78) in 70% yield, but the stabilized ylide (76) is produced from bismethoxycar-bonylmethylide and (75). In fact it is possible that the initial reaction with dichlorocarbene is reaction at sulfur and subsequent rearrangement of this less stabilized ylide. Schemes 6 and 7 illustrate the results and proposed mechanisms (77JOC3365,64JOC2211). 

The rearrangement exhibits some stereochemical preference for c/s-vinyl carbene complex (with respect to the metal) compared to the //ww-isomer. Thus, 2-methyl-2-m-vinyl cyclopropyl (methoxy) carbene chromium pentacarbonyl rearranges to 5-methyl-5-vinyl-2-methoxycyclopentenone approximately 4 times faster (THF, 52 °Q than the trans-isomer, which in turn rearranges faster than phenyl derivatives. This suggests that vinyl complexes undergo initial Cope-type rearrangement to form metallacycloheptadienes, which then rearrange to jt-allyl complexes. Subsequent CO insertion and reductive elimination leads to the vinylcyclopentenones (equation 89)150. 

Iodonium ylides (136), generated in situ with bisacetoxyiodobenzene, are converted to allyl- or benzyl-substituted oxonium or sulfonium ylides (137) via rhodium- or copper-catalysed carbene transfer.115 Such ylides undergo [1,2]- or [2,3]-rearrangement to the corresponding 2-substituted heterocycles (138). An example of the rhodium-catalysed reaction is reported in Scheme 36. 

As a supporting evidence, it is well-known that the electron-rich 0 6-arene)Ru complex of terminal alkyne 428 rearranges easily by the treatment with NaPR, of the )/ -vinylidenc complex 429, which is a strongly electrophilic carbene complex. Attack of ROH on the carbene carbon generates the the alkoxycarbene complex 431 via 430 [166]. Formation of ketone 427 by attack of the allylic alcohol is understanable by this mechanism. Formation of Ru-vinylidene complex 429 from the terminal alkyne has been proposed as the intermediate 432 of the reaction of terminal alkyne, amine and CO2 to form the vinyl carbamate 433 [167,168]. 

The chemistry of allyloxy(methoxy)carbene (55) depends on the temperature at which it is generated.55 At 110°C, the carbene homolytically dissociated yielding mainly allylic ester. At 50 °C, the carbene dimerized and underwent a Claisen rearrangement to (56). 

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