Carboalumination of alkyne

At this point in time carboalumination of alkynes has been more widely applied in synthesis. The most frequently used catalyst is (Cp)2ZrCl2. It is believed that a bimetallic species is formed.227... 

Small amounts of water accelerate carboalumination of alkynes.228 This acceleration may be the result of formation of aluminoxanes. 

Petasis reported an efficient addition of vinyl boronic acid to iminium salts.92 While no reaction was observed when acetonitrile was used as solvent, the reaction went smoothly in water to give allyl amines (Eq. 11.54). The reaction of the boron reagent with iminium ions generated from glyoxylic acid and amines affords novel a-amino acids (Eq. 11.55). Carboalumination of alkynes in the presence of catalytic Cp2ZrCl2 and H2O affords vinylalane intermediates, which serve as nucleophiles in the subsequent addition to enantiomerically enriched... 

Scheme 4 Acyclic and cyclic mechanisms for the Zr-catalyzed carboalumination of alkynes.

The synthetic utility of the Zr-catalyzed carboalumination of alkynes may be indicated by over 100 natural products including highly complex ones that have been synthesized by using this reaction74 178 (Table 2). 

Scheme 1.24. Synthesis of alkenylalanes by Zr-catalyzed carboalumination of alkynes followed by Pd- or Ni-catalyzed cross-coupling.

Many dozen complex natural products have been synthesized in a highly selective manner by the use of Zr-catalyzed carboalumination of alkynes. Although most of them are not mentioned here due to space limitation, those that have been reported by the author s group over the past few years alone include freelingyne [160], coenzyme Q10 [159], and (3- and y-carotenes [163], which were synthesized with unprecedented efficiency and selectivity. 

The stereospecific construction of the trisubstituted double bond of the side chain at C-1 of carbazomadurins A (253) and B (254) was achieved using Negishi s zirconium-catalyzed carboalumination of alkynes 758 and 763, respectively. Reaction of 5-methyl-l-hexyne (758) with trimethylalane in the presence of zirconocene dichloride, followed by the addition of iodine, afforded the vinyl iodide 759 with the desired E-configuration of the double bond. Halogen-metal exchange with ferf-butyllithium, and reaction of the intermediate vinyllithium compound with tributyltin chloride, provided the vinylstannane 751a (603) (Scheme 5.79). 

CARBAMIC ACID, (2-METHYL-2-BUTENYL)-, METHYL ESTER, 65, 159 Carbamic add, methyl ester, 65,159 Carboalumination of alkynes. 66, 64 Carbocupration of alkynes, 66, 65... 

Alkenylmercuric chlorides. The reaction of alkenylalanes, available by Zr-catalyzed carboalumination of alkynes, with mercuric chloride provides a convenient synthesis of (E)-alkenylmercuric chlorides. 

Carboalumination of alkynes and alkenes proceeds slowly without a catalyst. The reaction is accelerated by Cp2ZrCl2 [177], Carboalumination of alkenes is catalysed by... 

Using ( )-methylalkenylalanes 140, obtained by highly stereo- and regioselective Zr-catalyzed carboalumination of alkynes, the stereochemically defined enynes 141 can also by synthesized by Pd-catalyzed cross-coupling reactions with 1-haloalkynes [Eq. (47)] [68]. 

Negishi, E.-i., Kondakov, D. Y., Choueiry, D., Kasai, K., Takahashi, T. Multiple Mechanistic Pathways for Zirconium-Catalyzed Carboalumination of Alkynes. Requirements for Cyclic Carbometalation Processes Involving C-H Activation. J. Am. Chem. Soc. 1996,118, 9577-9588. 

Carboabimination The carboalumination of alkynes and enynes generates aluminacyclopentenes which can be processed into alkenylcyclopropanes and 2-cyclopentenones. 

---