Dienes from alkyne-alcohols

The addition of H—X, where X = F, Cl, Br and I, to alkenes, dienes and alkynes has been extensively studied from both mechanistic1-3 and synthetic standpoints. While it is one of the earliest methods employed for the synthesis of organic halides, it can often lead to mixtures of products, and the direct conversion of the corresponding alcohols is more commonly used nowadays. The early literature abounds with examples of this reaction in which either mixtures of products are formed, or the products are not well characterized. Few major advances have been reported in more recent times which overcome the synthetic disadvantages of this direct process. It can nevertheless be the method of choice for the synthesis of certain substrates. 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

Carbonyl compound alkylations. Allylic alcohols and homoallylic silyl ethers are prepared from alkynes and dienes, respectively. 

The regioselectivity of the allylation depends on the presence of an adjacent free hydroxyl group the predominant formation of linear 1,4-dienes (awti-Markovnikov products) is achieved from propargylic alcohols whereas simple terminal alkynes wifh a protected hydroxyl group give fhe corresponding branched 1,4-dienes (Markovnikov products) (Tab. 8.9) [57c]. 

Azamacrolide 32e was synthesized from epoxy alcohol 91 (Scheme 6.18). Base-mediated fragmentation of epoxy chloride 91a and alkylation followed by acetylene-zipper reaction gave terminal alkyne 93. Alkylation of 93 and hydrogenation furnished diene 94. Conversion of alcohol into aminoethanol derivative followed by oxidation afforded the acid 94, which on macrolactonization gave azamacrolide (—)-32e (Scheme 6.18). 

The thermal isomerization of higher terminal alkynes also delivered some allene, from 1-hexyne and 1-heptyne, for example, some 1,2-diene was formed [30]. With an ,/l-unsaturated unit in the alkyne 9, a photochemical isomerization to 10 was successful but delivered only a low yield and 11 as a significant side-product [31]. These reactions tolerate different functional groups alcohols, ethers or, as in 12, tertiary amines and nitriles have been used (Scheme 1.5) [32, 33],... 

Isomerizations. A convenient method for the conversion of alkynes to conjugated dienes is by treatment with Ph P. The synthetic application is shown in the preparation of 2,4-alkadienols from 2-alkynoic acids involving esterification with pentafluorophenol, isomerization with PhjP (PhMe, 50°), and reduction with DIBAL-H. Even propargyl bromide can be isomerized to give 1-bromopropadiene, albeit in 29% yield. Conjugated alkadienoic esters and amides are obtained in one step from the pentafluorophenyl alkynoates on reaction with alcohols and amines, respectively, after treatment with catalytic amount of PhjP. 

PdCE-catalyzed addition reaction of allyl chloride to alkynes is explained by chloropalladation of a triple bond, followed by insertion of the double bond of allyl chloride to generate 43. No jr-allyl complex is formed from allyl chloride and PdCl2. The final step is elimination of /3-Cl to afford 1-chloro-1,4-diene 44 with regeneration of Pd(ll) [37]. As another example, the Pd(0)-catalyzed Heck reaction of vinyl acetate affords stilbene in this reaction, the primary product is /3-phenylvinyl acetate (45), which reacts again with iodobenzene, and the last step is elimination of /S-OAc to give stilbene. At the same time, Pd(II) is generated, which is reduced to Pd(0) in situ [38]. However, elimination of /3-heteroatoms is not always faster than that of j8-H. For example, the Heck reaction of allyl alcohol with iodobenzene proceeds by preferential elimination of /3-H from the insertion product 46 to afford aldehyde 47, and no elimination of /3-OH from the same carbon occurs to give the alkene 48 [39,40]. 

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