Allene derivatives 3-hydride elimination

The reactions of type II proceed by transmetallation of the complex 5. The transmetallation of 5 with hard carbon nucleophiles M R (M = main group metals) such as Grignard reagents and metal hydrides MH generates 8. Subsequent reductive elimination gives rise to an allene derivative as the final product. Coupling reactions of terminal alkynes in the presence of Cul belong to Type II. 

Cyclopropyl-substituted allenes open the door to yet another reaction mode. When treated with aryl iodides in the presence of a typical Heck-catalyst system and a dienophile, cyclohexene derivatives 77 were obtained (Scheme 11) [53,54]. Thus, the initially formed arylpalladium iodide car-bopalladates 72 to form a a-allylpalladium intermediate 73. It swiftly undergoes the cyclopropylcarbinyl to homoallyl rearrangement yielding the ho-moallylpalladium species 74 which finally suffers /1-hydride elimination. The thus formed 2-aryl-1,3,5-hexatrienes 75 are prone to undergo polymerization, but can be efficiently trapped by an appropriate dienophile at the least steri-... 

For this reaction, a mechanism involving the addition-elimination of the ruthenium hydride is proposed. Allene derivatives are probably formed in the initial stage and the subsequent addition of the ruthenium hydride to the allene followed by the elimination of the ruthenium hydride forms a 1,3-diene derivative, which is stabilized due to conjugation with the siloxy group (Scheme 12.4). 

The second type of reactions proceed by transmetallation of the complexes 1. MR (M = main group metals) and metal hydrides MH undergo the transmetallation with 1 to generate 6. Subsequent reductive elimination gives the allene derivative 7. Also reactions of 1 with 1-alkynes in the presence of Cul to afford allenylalkynes belong to this type. 

As outlined in Sect. B and C, catalytic intermolecular carbopalladations of allenes followed by either /3-hydride elimination or intermolecular nucleophilic trapping provide 1,3-dienes or allyl derivatives bearing the nucleophile moiety, respectively, while an intermolecular carbopalladation followed by intramolecular trapping sequential reaction provides cyclic skeletons (Scheme 27). In Type I, the nucleophilic moiety is connected with the C—X bond, and in lype II it is attached to the allene moiety. 

Condensation between the allenic aldehydes 25 and the allenylboranes 24, derived from the allenylsilanes 23, also exhibited high diastereoselectivity (Scheme 20.7) [33-35], However, unlike 17, a reversal of diastereoselectivity in favor of the RR/SS pair of the a-silyl alcohols 26 occurred. Consequently, treatment of 26 with potassium hydride to promote the syn elimination furnished the enyne-allenes 27 having predominantly the E configuration (fc Z>% 4) for the central carbon-carbon double... 

Reactions of propynyl alcohols and their derivatives with metal hydrides, such as lithium aluminum hydride, constitute an important regio- and stereoselective approach to chiral allenes of high enantiomeric purity63-69. Formally, a hydride is introduced by net 1,3-substitution, however, when leaving groups such as amines, sulfonates and tetrahydropyranyloxy are involved, it has been established that the reaction proceeds by successive trans-1,2-addition and preferred anti-1,2-elimination reactions. The conformational mobility of the intermediate results in both syn- and ami- 1,2-elimination, which leads to competition between overall syn- and anti-1,3-substitution and hence lower optical yields and/or a reversal of the stereochemistry. 

Reductive deoxygenation-rearrangement of 2-yne-l,4-diols to 1,3-dienes is a useful synthetic procedure since a large variety of ynediols are available in a few steps by sequential reaction at both ends of acetylene with aldehydes. Acetylenic 1,4-diols can be deoxygenated reductively by lithium aluminium hydride to form conjugated dienes of high stereoisomeric purity (equation 9). A modification to this procedure is the use of acetylenic 1,4-diol mono-THP derivative. Allenic tertiary alcohols which are intermediates in the reaction can be separated and subjected to reductive elimination rearrangement... 

According to DFT calculations, the silver(I)-catalysed conversion of the prolinol-derived propargylic amine (270) into (-)-allene (273), proceeds via an initial silver(I) coordination to the C=C bond in an anft-manner with respect to nitrogen (271). The latter complex then undergoes a rate-limiting hydride transfer from the prolinol moiety (with AG =44.8 kJ mol ) to generate the vinyl-silver intermediate (272), which then affords the final allene (273) via an anri-periplanar elimination. 

The tetrahydropyranyl derivative of propaigyl alcohol, tetrahydro-2-(2-propynyloxy)-2/f-pyran, can be converted to methyl 4-hydroxy-2-butynoate in four steps. Diethyl [(2-tetrahydropyranyloxy)methyl]phosphonate is a convenient Wadsworth-Emmons reagent. Tetrahydropyranyl esters of a-bromo acids can be used in the Reformatsky reaction for the preparation of (3-hydroxy acids. Elimination of a tetrahydropy-ranyloxy moiety from butyne-l,4-diols with lithium hydride constitutes an efficient method for the synthesis of allenic alcohols. ... 

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