Allenyl halide

Jacobs and Fenton (202) were the first to suggest the possible intermediacy of ion 242 in the hydrolysis of trisubstituted allenyl halides to give the corresponding propargyl alcohols as products. A detailed and elegant study of the solvolysis of a series of triarylchloroallenes, 243, has recently been carried out by Schiavelli and co-workers (203). Excellent first-order kinetics... 

Because of this large rate factor in favor of the solvolysis of an allenyl system, it was of interest to examine the solvolytic behavior of the unsubstituted parent allenyl halide. Stang and co-workers (205) have observed good pseudo-first-order rates for the solvolysis of allenyl bromide, 244, R) = = Rj = H,... 

Allenyl cations have been generated by solvolysis of allenic derivatives, by photolysis of allenyl halides and by reaction of metal salts with allenyl and propargyl halides. This review will delineate these reactions. The related butatrienyl cations are not many and they will be only briefly described. 

A copper reagent can also react directly with 1,2-allenyl halides to provide alkynes and/or allenes[68, 69]. 

In the early 1980s, one of the first preparations of substituted allenes was reported, which employed a palladium-catalyzed cross-coupling reaction of allenyl halides [9]. In this study, allenyl bromides 13 and various Grignard reagents 14 were coupled in the presence of catalytic amounts of a Pd(0) species, generated in situ by reduction of a Pd(II) salt. Trisubstituted allenes 15 were obtained with high regioselectivity (allene 15 alkyne 16 = 90 10 to 99 1) (Scheme 14.5). 

The cross-coupling reactions of allenes with components containing sp-carbon atoms are useful synthetic transformations since they provide yne-allenes and enyne-allenes, respectively. Due to the synthetic potential of these classes of carbon-rich unsaturated compounds, the scope and limitations were systematically investigated [1, 16-18]. The first synthetic application was reported in 1981, describing the preparation of alkynyl-substituted allenes by coupling of alkynylzinc chlorides with allenyl halides (Scheme 14.8) [11]. 

A complementary approach for cross-couplings with allenes was applied by using metallated allene species instead of allenyl halides, which have already been discussed in Sect. 14.2.1. Since allenyllithium compounds are readily available by deprotonation of allenes with n-butyllithium, successful cross-coupling reactions between lithiated allenes such as 54 or 57 and aryl or vinylic halides allowed convenient routes to aryl- and vinyl-substituted allenes, e.g. 55, 58 and 60 (Scheme 14.15) [30],... 

Cross-Coupling Reactions of Allenes Producing Compounds with an Intact 1,2-Diene Moiety 849 Cross-Coupling Reactions of Allenyl Halides 849 Cross-Coupling Reactions of Allenylmetal Compounds 856 Cross-Coupling Reactions of a-Substituted Allenes 860 Cross-Coupling Reactions of Allenes at the Central Position 862 Synthesis of Alkynes 870 Miscellaneous Reactions 872 Conclusion 873... 

The rearrangement occurs more readily when activating groups (aryl, carboxyl, etc.) are attached to the triple bond. Jacobs [38] reports that a reaction involving adsorption of an acetylenic compound on an active basic surface has led to the practical synthesis of arylallenes, allenyl ethers, allenyl halides, and other substituted allenes. 

In a similar process, nickel compounds also catalyze the carbonylation of allenyl halides, under phase-transfer conditions (PTC), to give allenyl acids in poor to reasonably good yields (equation 94)368. 

D. Cross-coupling Reactions with Allenyl Halides. 1283... 

The coupling reaction of a-keto esters with allyl, propargyl, and allenyl halides using indium metal in aqueous solvents affords a-hydroxy-y,<5-unsaturated esters (Equation (28)).197,198 1,3-Dicarbonyl compounds undergo efficient carbonyl allylation reactions in an aqueous medium through a Barbier-type reaction (Equation (29)). The reaction is general and a variety of 1,3-dicarbonyls has been alkylated using allyl bromide or allyl chloride in conjunction with indium.199... 

C. Coupling of Terminal Alkynes with Propargyl, Vinyl and Allenyl Halides... 

The catalytic pair Ni(CN)2/Bu4N Br readily catalyzes the biphasic carbonyla-tion of a-haloalkynes [144, 145] and allenyl halides [145]. The first reaction results in a mixture of allenic monoacids and unsaturated diacids, whereas allenyl halides transform to allenic acids with high regioselectivity. The carbonylation of gem-dibromocyclopropanes under PTC conditions is catalyzed by Ni" and Co" salts. The reaction gives the corresponding cyclopropanecarboxylic acids in fair yields [146]. 

The stereochemistry of the direct substitution reaction has been the subject of some debate. Most recently, it has been reported that reactions of alkylheterocuprates proceed with high syn selectivity, while inversion of allenyl configuration, or anti selectivity, is observ in reactions of phenylcopper reagents. The degree of selectivity is variable and may be a reflection of product isomerization under the reaction conditions. Predominant anti stereoselectivity (anti syn ratios range from 91 9 to >99 1) is observed also in Sn2 reactions of allenyl halides (see Scheme 3), a finding that is consistent with the known preference for anti substitution of allylic substrates (see Section I.5.2.4.5). This method for al-kyne preparation has found application in the leukotriene area, and also for the synthesis of alkoxy al-kynes. ... 

Carboxylation. Under phase-transfer conditions, propargyl and allenyl halides in the presence of CO and Ni(CN)2 are converted into allenic acids. Further reaction gives rise to 2-alkylidenesuccinic acids. 

K " propargyl and allenyl halides in allenic acids. Further reaction... 

Amides, carbamates, and ureas displayed excellent reactivity in the formation of allenamides. These substrates cross-coupled with allenyl halides in the presence of a catalytic amount of CuTC and a diamine ligand (eq 30). ... 

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