Allenes synthesis from halides

In general, allenes are prepared by the same types of reactions which lead to alkenes elimination of hydrogen halides, halogens and water from adjacent carbon atoms. However, some methods are characteristic of allene synthesis, e.g. the dehalogenation of gem-dihalocyclopropanes, rearrangement of alkynes and the 1,4-addition to vinylalkynes. 

The base-induced dehydrohalogenation of vinyl halides and allyl halides often gives low yields of allenes because of the competing reaction to alkynes alkynes can either be formed by direct elimination from vinyl halides or by isomerization of the allene first formed to the isomeric alkyne. Since it has been established that anti elimination of hydrogen halide from vinyl halides to yield alkynes is much faster than syn elimination, the proper choice of the starting material is often important for a successful allene synthesis. When ( )-4-bromo-4-octene was treated with NaOMe, the sole product was 3,4-octadiene, whereas the conesponding Z-educt yielded 4-octyne (Scheme 66). ... 

Marshall, J. A. Synthesis and Reactions of Allylic, Allenic, Vinylic, and Arylmetal Reagents from Halides and Esters via Transient Organopalladium Intermediates. Chem. Rev. 2000, 100, 3163-3185. 

Marshall JA. Synthesis and reactions of allylic, allenic, vinylic, and arylmetal reagents from halides and esters via transient organopalladium intermediates. Chem. Rev. 2000 100 3163-3186. 

Scheme 9.1 Synthesis of aiienyimetai compounds from propargylic halides, alkynes, and allenes.

Allenyllithium reagents are commonly prepared through lithiation of propargylic halides or by deprotonation of alkynes or certain allenes (Eq. 9.1). Lithiated allenes often serve as precursors to stable allenylmetal compounds such as stannanes or silanes. They can also be employed for the in situ synthesis of allenylzinc, -titanium and -boronate compounds, which can be further transformed to substitution products not accessible from their allenyllithio precursors. 

Although the iron-catalyzed synthesis of allenes from propargylic halides was reported by Pasto and coworkers in 1978 [67], little progress was achieved in this field until recently [68]. In 2003, Fiirstner and coworkers discovered propargylic epoxides as valuable substrates for the reaction with Grignard reagents in presence of catalytic amounts of Fe(acac)3 to generate 2,3-allenol derivatives (Scheme 5.24) [69]. 

Palladium-catalyzed heteroannulation is illustrated by synthesis of substituted l//-benzo[rf]azonine 227, which was prepared from allene and tosylamide-containing aryl halide (Equation 26). The reaction was suggested to proceed by addition of an arylpalladium compound to the allene to generate a 7i-allylpalladium intermediate, which subsequently undergoes nucleophilic displacement of palladium at the less-hindered end of the 7i-allyl system <1998JOC6859>. 

Carbonyl compounds also react with allenes and, in fact, the first example of a gold-catalyzed addition of a nucleophile to allenes was the formation of lurans from allenones. The reaction was applied by Gevorgyan to the synthesis of substituted furans and in the case of bromide-substituted allenes halide migration was observed. Similar additions of... 

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. ... 

A facile synthesis of 2-aryl-1,3-dienes has been realized from aryl halides and allenes. Ionic liquids prove to be a superior media for the Heck reaction with aryl... 

The synthesis of allenes of the type RpCH C CHa is an easier task, now made even easier by a new method based on the reaction of perfluoroalkyl coppers with propargyl halides or alcohols. The alkyl copper is generated in situ from copper bronze and a perfluoroalkyl iodide in DMSO reaction with propargyl bromide is extremely rapid and a minor explosion occurred during work-up. 

Recently, the reaction of polymer-bound aryl halides with 2,4-disubstituted allene-carboxyUc acids leading to polymer-bound butenolides was reported (Scheme 7). After removal from the polymer, the substituted butenolides can serve as important building blocks for natural product synthesis. ... 

Prins cyclizations, which proceed by intramolecular addition of alkenes to oxocarbenium ions, provide a simple, efficient method for the stereoselective synthesis of carbocycles and cyclic ethers [77]. Halosilanes and (la) have been used for Prins cyclizations not only as Lewis acids but also as heteroatom nucleophiles. For instance, in the presence of MesSil or MesSiBr, and lutidine, mixed acetals (26) are efficiently cyclized to 4-halotetrahydropyrans (27) with high diastereoselectivity [78]. The halide is introduced into the axial site of the C(4) position. The proposed mechanism for the MesSiBr-promoted reaction involves the initial formation of a-bromoethers (28) from (26). Solvolysis of (28) provides the intimate ion pair (29). Cyclization to the chair transition structure (30) and proximal addition of the bromide produces the observed axial adduct (27). The role of lutidine is to suppress a less selective HBr-promoted cyclization (Scheme 9.23). Acetals bearing an alkyne or allene moiety also undergo the halosilane-promoted cyclization to form haloalkenes [79, 80]. 

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