Allenes from propargyl alcohols

Palladium(0)-catalysed selective allene formation by hydrogenolysis is a convenient synthetic route to allenes from propargylic alcohols. The method offers a new and efficient preparative method for allenes from easily available propargylic alcohols after converting them to formates or carbonates. The best catalyst for smooth hydrogenolysis is prepared by mixing Pd(OAc)2 or Pd(acac)2 with -Bu3P in a ratio... 

Transpositional deoxygenation. o-Nitrobenzenesulfonylhydrazine supplies a hydride source to effect the transpositional deoxygenation of allylic alcohols via the V-sulfonyl-V-allylhydrazines, which are formed by an S 2 reaction. This reaction has been used to synthesize allenes from propargylic alcohols in one step. ... 

Molander and Sommers [115] reported a chromium(III)-catalyzed synthesis of allenes from propargyl alcohol derivatives and triaUcylaluminum reagents. When substituted and enantiomerically enriched propargyl alcohols 403 were treated with these reagents in the presence of the chromium complex 404, allenes 405 were obtained in good yields and with high levels of chirality transfer (Scheme 10.138). 

Reductive Transposition of Allylic Alcohols. Similar to the synthesis of allenes from propargylic alcohols, the Mitsunobu displacement of allylic alcohols with IPNBSH followed by hydrolysis, diazene formation, and sigmatropic loss of dinitrogen provides reductively transposed alkenes. This methodology has proven effective for the reductive transposition of a variety of allylic alcohols (eq 2). The overall transformation provides the desired olefin with high selectivity in the formation of the irans-alkene (eq 3). ... 

Synthesis.—Vinylidene adamantane (113) has been prepared and its reactions investigated.The synthesis of allenes from propargyl alcohols and their derivatives continues to provide a number of papers dealing with this extremely useful transformation. Ferric chloride catalyses the highly selective formation of allenes... 

Myers crafted a process involving a 1,5-H shift for the stereospecific synthesis of optically active allenes from propargylic alcohols (Scheme 16.31) [119, 120], Mitsonobu displacement of the alcohol in 229 by arylsulfonyl-hydrazine 230 afforded 231. At ambient temperature, elimination of sulfinic acid occured, and the resulting diazine 232 underwent extrusion of dinitrogen and 1,5-H-shift to afford allene 233 [119]. The sequence has been demonstrated to be enantiospecific, affording optically active allenes such as... 

TABLE 14. Allenic sulphoxides from propargylic alcohols and sulphenyl chlorides, RSC1... 

Enyne metathesis starting either from acetylenic boronates and homoallylic alcohols [104a,c] or from propargyl alcohols and allylboronates [104b] has recently been described. The resulting boronated dienes can be converted to allenes or cycloaddition products. The cross metathesis of vinylcyclopropyl-boronates directed toward the total synthesis of natural products has very recently been investigated by Pietruszka et al. [104d]. 

Acid-catalyzed hydrolysis of S-allenylsulfinylamines 425, easily accessible from propargyl alcohols (cf. Scheme 7.8), provides the alkynes 427 (Scheme 7.56) [108, 109]. This transformation is postulated to proceed via the intermediate allenic sulfinic acid 426. However, in some cases with R1 = R2 = alkyl, more complicated products are formed instead of simple alkynes 427 [372]. 

Similarly, the allenic derivatives 120 and 121 have been obtained from propargylic alcohols as shown in Scheme 7.34. °... 

Reactions with —OH Groups and Epoxides.—The formation of A -l,2-oxaphos-pholen derivatives from propargylic alcohols and phosphorus trichloride has been studied in detail. Intermediate phosphites (24) and allenic phosphonates (25) are described, and the A -l,2-oxaphosphoIen is produced in the final stage, as shown. Improved conditions have been outlined for the preparation of allylic bromides (26) from allylic alcohols and phosphorus tribromide. Related reactions of primary alcohols with the complex of phosphorus trichloride and DMF lead to the chloride (27) 22 addition of zinc bromide to the reaction results in the formation of alkyl bromides, but an attempt to extend this exchange to the preparation of cyanides was not successful. ... 

Allenyltins are conveniently prepared from the terminal alkynes which have a suitable leaving group. An example is shown in equation 19. A two-step one-pot procedure for the synthesis of allene by hydrostannation of alkynes has recently been reported starting from propargylic alcohols a hydrostannation and subsequent deoxystannylation generates the allenes 6 as shown in equation 20. A chiral version of this procedure has also been described in the paper. 

Myers AG, Zheng B (1996) New and Stereospecific Synthesis of Allenes in a Single Step from Propargylic Alcohols. J Am Chem Soc 118 4492... 

A mild and direct process for C-C bond formation from propargylic alcohols and olefins has been developed in the presence of a silver catalyst (Ji et al. 2009) (Scheme 11.1). In this reaction, trace amounts of water were necessary and allene alcohols and 1,3-dienes were obtained selectively. 

Marshall has employed an analogous sequence in the development of a general approach to optically active allenes, as shown for the stereospecific formation of allene 197 from propargylic alcohol 195 (Scheme 16.23)... 

Air-stable palladium(O) catalyst, [(Cy3P)2Pd(H)(H20)]BF4, catalyses carbonylation of propargylic alcohols to generate dienoic acids and esters (equation 167)286. Since propar-gyl alcohols are obtained from carbonyl compounds by acetyhde addition reactions, this sequence constitutes a three-carbon homologation. a-Allenic alcohols are converted to tt-vinylacrylic acids under similar conditions (equation 168)287. 

Various nonracemic allenylstannanes have been prepared from nonracemic propargylic mesylates and (Bu3Sn)2CuLi. The stereochemistry of the displacement was shown to be anti by correlation with an allenic stannane prepared through Claisen orthoester rearrangement of a propargylic alcohol of known configuration (Scheme 33)80. 

Allenic alcohols,3 In the presence of Sml2 and Pd[P(C6H3),]4, sec- and tert-propargylic acetates add to ketones to give allenic alcohols as the only or major product. A mixture of allenic and homopropargylic alcohols is formed from reactions of primary propargylic acetates. 

In this context, albeit not real isomerizations, the [2,3]-Wittig rearrangements induced by a tin-lithium exchange must also be mentioned. Starting from enantio-merically pure propargylic alcohols, high ee values for the axial chiral allenes could be observed as shown for 153 (Scheme 1.69) [505, 506],... 

---