Alkynes methylalumination

Most probably, Zr-catalyzed C—C bond formation by cyclic carbozirconation was first observed in 1978, when the Zr-catalyzed reaction of alkynes with Et3Al was reported [13,56]. An acyclic carbozirconation mechanism similar to that of methylalumination was initially... 

In 1978, Negishi et al. reported highly regio- and stereoselective methylalumination of alkynes with Me3Al using a zirconocene catalyst [59]. The involvement of cationic zirconocene species in the activation of carbon—carbon triple bonds was suggested in a reaction mechanism featuring electrophilic activation by aluminum (Scheme 8.30). 

Attempts to use the isobutyl group in the carbometalation of alkynes only give rise to hy-drometalated products, but ethyl and n-propyl groups can be successfully transferred from the corresponding dialkylaluminum chlorides. The regioselectivity in these reactions is lower than that for the methyl transfer. Indeed, the reaction mechanism may be different from that of methylalumination [62]. 

Alkynes are carbometallated with several organometallic compounds under mild conditions [3] but carbocupration has probably the greatest synthetic possibilities [4], associated with the zirconium-catalyzed methylalumination [5] reaction both proceed with very high stereoselectivity and afford the syn addition product as described in Scheme 7-2. 

Carboalumination of alkynes is possible in the presence of the zirconium catalyst Cp2ZrCl2. The method is particularly attractive for methylalumination using Me3 A1 and can tolerate a wide variety of functional groups. Terminal alkynes react with very good regioselectivity ( 95 5) and excellent stereoselectivity (>98%) in favour of the alkenyl aluminium species, such as 58 (2.69). Mixtures of regioisomers often result when using internal alkynes with two different substituents. 

Scheme 3.7 Zr-catalyzed methylalumination of alkynes (ZMA) with MejAl and subsequent conversions to demetallated trisubstituted alkenes [45, 47].

Catalytic methylalumination reactions provide an efficient entry for the carbometallation of alkynes [116]. In a recent paper. Ready et al. [117] reported a sequential process for the methylalumination-oxygenation of terminal alkynes as a logical extension of their previous work described earlier [97]. When simple... 

Organotitanium and -zirconium chemistry already has an established place in organic synthesis and many reactions are covered elsewhere in the Practical Approach series. Examples include reductive coupling of carbonyl compounds with low valent titanium to form 1,2-diols or alkenes methylenation of ester carbonyl groups with titanocene methylidene (Cp2Ti=CH2) zirconium-catalysed methylalumination of alkynes and hydrozirconation of alkynes and alkenes with the Schwartz reagent, Cp2ZrHCl. ... 

There is no doubt that one of the most useful functionalizations of alkynes is Negishi methylalumination of terminal alkynes [21]. This reaction is catalyzed by the zirconocene 1 and the addition of trimethylaluminium proceeds regioselectively through a stereoselective s/n-addition to give the cis-methylalkenylalanes 33, which can be used in further reactions with electrophiles or cross-coupling reactions (Scheme 13). This method will not be... 

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