Allenes carbenes

This allenic carbene is now added to the alcohol 366A. What will be the product ... 

An interesting variation appears when furan reacts with the allenic carbene generated by the action of potassium ter-butoxide upon l-bromo-3,3-dimethylallene. Though the yield is only 9%, one product is reasonably assigned a structure (Scheme 52) that could hardly be approached by way of a cyclopropane intermediate. The authors comment that in an allenic carbene in the singlet state two electrons will be accommodated in the sp... 

A third possibility is the fragmentation of the ring. Thus, in the case of 2,2-dimethyl-1,1-dihalocyclopropanes ring-opening occurs to give enynes and, in some cases, to produce allenic carbenes 64,76-78,69... 

In a synthetically useful variation of this reaction, allenic carbenes may also be generated from a range of 3,3-disubstituted dibromocyclopropanes by reaction with either aqueous or powdered sodium hydroxide and a phase transfer catalyst, and trapped in moderate to good yield by alkenes in a two-phase system, e.g.214 ... 

The advantage of this method is that the product vinylidenecyclopropanes carrying an olefinic hydrogen do not isomerize under the reaction conditions to alkynes. The final example represents the first solution trapping of the parent allenic carbene, an important component of interstellar space. 

While it is intellectually pleasing to find self-consistencies in this mechanism, such as the fact that XII, XIII, and XV would be the Grignard species formed anew that compete against II for proton abstraction in the first step (see experimental evidence, (2)), and that the allenic carbene approach is also attractive, there are still other alternative interpretations to be considered. First, the Sn2 pathway that accounts for II is supported by experimental evidence (3). The authors of the original paper contend, however, that this route is disfavored in the light of their extended studies about the transition metal salt catalyzed Grignard reactions of propargyl halides, wherefrom allenes invariably result. 

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. ... 

Pasto [29] undertook a detailed study of the reaction of terminal propargyl chlorides 40 with Grignard reagents, and found an allene carbene-zwitterion intermediate 41 that could best rationalize the formation of the two alkynes 42 and 44 and allene 43 observed [Eq. (14)], the 8 2 mechanism discarded, or at least being the minor process involved in such a reaction. 

Allenic carbenes react with methylpyrroles and indoles to give 3-alkenyl pyridines 185 [87JCS(P1)1347] and quinolines 185 [R -l-R = (CH=CH)2] [76JCS(P1)2103] in low yield. 

The reaction of a 1,2-dihalocyclopropene (halogen = Br, Cl) with methyllithium also results in the formation an allenic carbene, in this case by an overall dehalogenation such reactions are discussed in Section 2.B.2.1.4.2. 

The reaction of a number of 1,1,2-trihalocyelopropanes with 2 molar equivalents of methyl-lithium in the presence of an alkene gave the trapping produets 2 of an allenic carbene. Since the eyelopropanes react with 1 molar equivalent of methyllithium to produee a halo-cyclopropene, it may be construed that this represents an intermediate in the reaction. In the... 

Both l-bromo-3-methyl-l,2-butadiene (182) and 3-bromo-3-methyl-l-butyne (183) give in aqueous ethanol the same product mixture, containing mainly propargyl alcohol and ether, but no allenic derivative (Shiner and Humphrey, 1967). In the presence of both PhS and OH ions, an identical product mixture (32% allenic and 52% propargylic thioether) is again formed from both isomers. This and the rate acceleration by base were ascribed to the formation of a common allene-carbene intermediate which was written as (184a 184b) (Scheme 8). Since the... 

The easily accessible allene carbene can be trapped with prenol [139] to give the allene 14 which has to be reduced partially to chrysanthemol 75 [24], (Reaction scheme 8)... 

Cyclopropanations of alkenes with alkyl diazoacetates are catalysed by rhodium(ii) carboxylates, and Sasaki et al have shown that crown ethers have several advantages over quaternary ammoniurq ions for the catalytic synthesis of allene-cyclopropanes from addition of allene-carbenes to olehnic substrates. 

The generation of carbenes by PTC has been discussed earlier, and, not surprisingly, crown ethers catalyse the synthesis of dihalo- and other carbenes to give cyclopropanation in the presence of alkenes. > Reaction of chloroform with aqueous base and hydrazine, catalysed by 18-crown-6, provides a new synthesis of diazomethane (Equation 15), presumably via dichlorocarbene. Allene carbene (40) can also be generated by PTC and used in situ (Scheme 9), and crown ethers are found to be more efficient catalysts than quaternary ammonium ions. 

Terpenoid alkenylidenecyclopropanes of the type (10), which are easily synthesized by addition of the allene carbene (9) to various alkenes, have been found to be extremely useful intermediates in the synthesis of a range of functionalized head-to-tail and irregular monoterpenes (Scheme 2). ... 

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