Allenes from carbenes

The discovery of carbene and carbenoid additions to olefins was the major breakthrough that initiated the tapping of this structural resource for synthetic purposes. Even so, designed applications of cyclopropane chemistry in total syntheses remain limited. Most revolve around electrophilic type reactions such as acid induced ring opening or solvolysis of cyclopropyl carbinyl alcohol derivatives. One notable application apart from these electrophilic reactions is the excellent synthesis of allenes from dibromocyclopropanes 2). 

Reactions of complexes of 1,2-cycloheptadienes have received only cursory attention. 1,2-cycloheptadiene is readily displaced from bisftriphenyl-phosphine)platinum(O)118 [Eq. (54)], but no reagent has been found that will displace the allene from iron.119 Reaction of the iron complex with alcohol in the presence of base (e.g., 312 — 322) is typical of Fp+ complexes of acyclic allenes.131132 The thermal chemistry of 312 is unusual in its decomposition to 324 (Scheme 41). This is probably attributable to the presence of the triflate counterion, since the corresponding fluoroborate salt is stable when warmed to 40°C for 16 h.119 A mechanism to 324 via carbene complex 321 appears likely. 

It is worth mentioning that the synthesis of allenes from propargyl halides in Grignard reactions is favored by transition metal catalysis more than by the participation of allenic carbene intermediates in as much as the level of metal impurities in magnesium turnings is not reduced to a minimum, or metal salts such as ferric chloride and other metal chelates such as nickel acetoacetonate are added to the reaction mixture. ... 

This sulfoxide-magnesium exchange reaction could be successfully applied to a new synthesis of allenes (Scheme 3.12) [6]. The procedure is a novel method for synthesis of allenes from three components, ketones, chloromethyl p-tolyl sulfoxide, and sulfones, in relatively short steps. A key step is an attack of the lithium a-sulfonyl carbanion on the electron-deficient carbene carbon. /1-Elimination of the sulfoxyl group then occurs to give the allene. 

Semi-empirical INDO calculations have been carried out on cycloheptatrienyli-dene, cycloheptatetraene, and related systems. The most stable structure for cyclo-heptatetraene was found to be non-planar with a C2 axis of symmetry, and should be more stable than the carbene form. Even though most of the chemistry of cyclo-heptatrienylidene is consistent with carbene behaviour, the allene and carbene forms may be in equilibrium, and the position of equilibrium solvent-dependent. In fact decomposition of the sodium salt of the toluene-p-sulphonyl-hydrazone (163) gives dimeric products formed from allene (164), and addition of cycloheptatrienylidene to tetracyclones gives products derived from the adduct (165). ... 

Vinyl Fischer carbenes can be used as three-carbon components in Ni(0)-mediated and Rh(l)-catalyzed [3 + 2 + 21-reactions with alkynes (Schemes 48 and 49)142 and with allenes (Schemes 50 and 51).143 All three of the proposed mechanisms for the [3 + 2 + 2]-cycloadditions involve an initial carbene transfer from chromium to nickel or rhodium (Schemes 49, 52, and 53). As is seen from the products of the two [3 + 2 + 2]-reactions with 1,1-dimethylallene, although the nickel and rhodium carbenes 147G and 147K appear similar, the initial insertion of the allene occurs with opposite regioselectivity. 

This chapter will cover only reactions in which the isomerization to the allene starts from a stable molecule and not from a reactive intermediate generated in situ by reactions which are not isomerizations, such as the Doering-Moore-Skattebol reaction or free carbenes. Metallotropic rearrangements also will not be covered many of these reactions can be found in Chapter 9. Furthermore, the allene should be the final product of the reaction and not only a transient species leading to other products (see, for example, Chapters 6 and 20). 

Both acridone and dibenzo[6,/]azepine produce unexpected products (Scheme 7.39) when reacted with dimethylvinylidene carbene (7.1.18.A). Acridone reacts initially at the nitrogen atom to produce the 10-(3,3-dimethylallenyl) derivative (13%) and a pyrroloacridone (10%) which, if the structure is correct, could be derived from the allene by sigmatropic shifts [16]. The dibenzoazepine reacts as expected to produce a cyclopropyl derivative but, under the reaction conditions, the adduct rearranges spontaneously to yield a 1,6-methanodibenzo[b,/]cyclo-prop [J]azepine, the structure of which was confirmed by X-ray crystallography [17]. 

The kinetics and mechanism of pyrrole pyrolysis were investigated by ab initio quantum-chemical calculations. It was revealed that pyrrole undergoes tautomerization to form 2H- and 37/-pyrroles prior to any thermal decomposition. It has been shown that the major product, HCN, arises from a hydrogen migration in pyrrole to form a cyclic carbene with the NH bond intact. Ring scission of the carbene leads to an allenic imine of HCN and propyne which is the lowest energy pathway. The 277-pyrrole... 

Over the years it became apparent, especially from matrix-isolation studies, that the seven-membered ring species was better formulated as cycloheptatetraene (72), a cyclic allene, rather than as the carbene (71). The question of the intermediacy of 73 remains unresolved Theory supports Baron s early suggestion, but the molecule itself has never been detected in a simple system. 

Preparation of allenic acetals from unsaturated carbenes [137]. 

This method has successfully been applied for the addition of chloro(l-chloro-2,2-dimethylvinyl)carbene — formed from 1,1,2-trichlo o-3,3-dimethylcyclopro-pane by dehydrochlorination and subsequent ring-opening — onto ketene methyl silyl acetals, the products of which are transformed to interesting allenic esters when treated with tetrabutylammonium fluoride [149]. 

Carbene 132 is implicated in the photolysis of 1 since the observed289 photodimerization to 9,10-dihydrophenanthrene and -anthracene is best explained by head-to-head and head-to-tail coupling of this species. Moreover, the fact that allene 134 is isolated289,290 as the major product from irradiation of diesters 31 (equation 35) is fully consistent with a photo-Wolff rearrangement of the carbene. The minor product here involves cyclization... 

When the carbene or carbenoid resulting from a dihalocyclopropane is unable to rearrange to the al-lene due to steric or other factors, insertion or addition reactions characteristic of carbenes take place. Thus dibromonorcarane on reaction with methyllithium gives a bicyclobutane derivative by insertion of the carbene into a 0-C—H bond (equation 57).178 Allene formation is sterically unfavorable in this case. Similarly, dibromotetramethylcyclopropane gives l,2,2-trimethylbicyclo[1.1.0]butane instead of tetra-methylallene (equation 58).179 181 An example involving a tricyclic dibromocyclopropane is given in equation (59).182... 

Cyclopropylidene reaction Neither the barrier nor the equilibrium constant for the cyclopro-pylidene/allene reaction have been measured. The only direct experimental information of these species come from the failure to observe cyclopropylidene at 77 K (Chapman OL (1974) Pure and Applied Chemistry 40 511). This and other experiments (references in Bettinger HF, Schleyer PvR, Schreiner PR, Schaefer HF (1997) J Org Chem 62 9267 and in Bettinger HF, Schreiner PR, Schleyer PvR, Schaefer HF (1996) J Phys Chem 100 16147) show that the carbene is much higher in energy than allene and rearranges very rapidly to the... 

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