Hydroalumination side reactions

Alkynes are more reactive in hydroalumination than alkenes. Hence, unlike internal alkenes, internal acetylenes readily undergo hydroalumination. Side reactions, however, may occur. Proton-aluminum exchange in terminal alkynes, for instance, leads to substituted products, and geminal dialuminum derivatives are formed as a result of double hydroalumination. In addition, nonsymmetric alkynes usually give mixtures of regioisomers. Appropriate reaction conditions, however, allow selective hydroalumination. Thus, the addition of isoBu2AlH to alkynes is a... 

Alkynes are much more reactive toward hydroalumination than alkenes. Hence, they readily react with both dialkylaluminum hydrides and LiAlH4 under mild conditions in the absence of a catalyst [1]. However, it is not always possible to avoid side reactions and subsequent transformation of the vinylalanes formed in this transformation [81, 82]. In addition, ds-trans-isomerization of the metallated C=C bond can take place, thereby reducing the stereoselectivity of the overall reaction [83]. 

Despite this gamut of competing reactions, reaction conditions can usually be found for preparing either the syn or the anti adduct of many terminal alkynes with high regio- and stereo-selectivity (Table 2). Failing this the 1-alkynylsilanes can be employed as masked forms of 1-alkynes. In hydroalumination reactions, such silanes have little or no tendency to undergo the side reactions (equations 19-24) to... 

The second side reaction with alkenes is the configurational change at the jp -hybridized carbon-aluminum bond in the hydroalumination adduct (equation 31). ... 

More recent studies describe treatment of alcohols 189 with a CuI/LAH mixture, ostensibly providing a CuH reagent to effect a net Sn2 delivery, thus, placing hydrogen in a stereodefined allylic position (Eqn. 1-13). Secondary alcohols invariably gave products of -configuration. CuI/LAD mixtures led to >95 /o incorporation of deuterium. Hydroalumination of the strained olefin is not a competing side reaction in this chemistry. 

Unlike alkenes, disubstituted acetylenes are hydroaluminated under mild conditions by means of fBu2AlH and IBusAl without a catalyst However, often it is impossible to avoid undesired side reactions and the subsequent transformations of vinyl alanes. Hydrolysis of the vinylalanes gives rise to 1,2-disubstituted olefins of Z- or -configuration depending on the namre of the hydroaluminating reagent used [58], the structure of the initial acetylene [59], and the solvent [59]. 

The hydroalumination of terminal alkenes and dienes using BujAl as aluminum source can also be catalyzed by late transition metal complexes such as (PPh3)2PdCl2 [38]. The reactions are carried out at room temperature, the use of dichloromethane as a solvent is crucial because it probably regenerates the catalytically active Pd(II) species by reoxidizing Pd(0) complexes formed in side reactions. Internal C=C bonds are not hydroaluminated under these conditions. Functional groups such as Cl, Br, and PhS in a position remote to the terminal C=C bond are tolerated, however, certain substrates such as 4-bromo-l-butene, 11-iodo-l-undecene, allyl phenyl ether, allyl benzyl ether, and (3 )-l,3-decadiene did not provide the desired hydroa-... 

Hydroalumination of 1-alkynes generally proceeds without the aid of transition metal catalysts. However, the reaction sometimes suffers from undesirable side-processes, including the formation of alk-l-ynylalanes or protonolysis of the intermediate alk-l-enylalanes. In particular, these side-products increase significantly in the reaction of... 

The use of DIBAL-H in the presence of the zirconium catalyst 141 leads to a hydroalumination reaction and provides access to functionalized aluminum reagents that have been employed in subsequent carboalumination reactions (Scheme 13.20) [82]. Negishi employed this feature to introduce the two stereogenic centers in an elegant synthesis of the vitamin E side chain 151. 

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