Vinylaluminum intermediate

Conjugate reductions. This combination (usually 1 3 ratio) effects conjugate reduction of a,p-acetylenic ketones or esters to a, 3-enones or unsaturated esters at -50° with moderate (E)-selectivity. The HMPT is believed to function as a ligand to aluminum and thus to promote hydroalumination to give a vinylaluminum intermediate, which can be trapped by an allylic bromide (equation I).1 The re-... 

Reaction with Organometallic Derivatives. Similarly, the lesser reactivity of NIS compared with NBS allows it to be used for the s)uithesis of vinyl iodides by iodination of vinylaluminum intermediates in the presence of reactive alkenic groups. NIS has also been found to be an excellent reagent for the oxidative cyclization of dipeptide dianions to 8-lactams (eq 1). ... 

The preparation of vinyl iodide 39 first required the transformation of (E,E)-farnesyl acetone (28), performed according to Negishi, to the terminal alkyne 27 with 75% yield [35]. The latter then gave (E)-vinyl iodide 39 in dia-stereomerically pure form and 74% yield by Zr-catalyzed carboalumination with trimethylaluminum and trapping of the intermediate vinylaluminum species with iodine [36]. The alkyl iodide rac-29 necessary to ensure the coupling with 39 was obtained by selective monofunctionalization of 3-methyl-pentane-1,5-diol (26) in a few steps [37]. 

Insertion stereochemistries for the alkyne insertion process were found to be variable. Sometimes cis addition of the Ni-CHs bond occurred (e.g., for PhCsCCHs), sometimes trans addition occurred (e.g., for PhC=CPh), and sometimes a mixture of isomers was obtained. Allowing the complexes to stand led to a thermodynamically controlled mixture of cis and trans insertion products. Equilibration of cis and trans vinylaluminum compounds is known. Initial cis insertion of the alkyne yields a coor-dinately unsaturated vinylnickel intermediate that accounts for the different stereochemistry of products formed under kinetic control. Isomerization of the double bond occurs for this intermediate in competition with product formation. Thus the stereochemistry of the kinetic product does not necessarily give the stereochemistry of a preceding insertion step. 

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