Hydroalumination-Protonolysis

The submitters report that Z-4-(trimethylsilyl)-3-buten-l-ol of >98% isomeric purity can be obtained in ca. 60% overall yield by a more lengthy sequence involving hydroalumination-protonolysis of the tetrahydropyranyl (THP) ether of 4-(trimethylsilyl)-3-butyn-l-ol followed by cleavage5 of the THP ether with pyrldinium p-toluenesulfonate in methanol. This sequence is less convenient for the tetrahydropyridine synthesis described In the next procedure, since the Isomeric purity of the vlnylsilane is not Important for the cycllzatlon reaction.6... 

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... 

Hydroalumination followed by protonolysis is less convenient for stereoselective cis hydrogenation of the double bond. Simple, vicinally disubstituted double bonds sluggishly undergo hydroalumination and the carbon-aluminum bond undergoes inversion in hydrocarbon solvents at moderate temperatures. Strained olefins are more reactive and can react stereoselec-tively under carefully controlled conditions126. 

As mostly discussed earlier. Type III alkenyl derivatives, that is, ( )-R CH= CHM(or X), are widely and satisfactorily generated by (i) alkyne hydrometallation (M = B, Zr or, in some cases, Al, etc.) (Table 3.2, Scheme 3.6), (ii) polar halogenation reactions ofalkynes (Eqs. (1), (2), and (7), Scheme 3.15), and additionally, (iii) anti bromoboration of ethyne [53] followed by Negishi coupling (Eq. (1), Scheme 3.12). On the other hand. Type IV alkenyl derivatives may be prepared by (i) Normant alkylcupration of ethyne [67, 68] (Eqs. (5) and (6), Scheme 3.11), (ii) Zr-catalyzed alkylalumination of ethyne, (iii) syn hydroboration of 1-halo-l-alkynes followed by hydride-induced inversion of configuration [59] (Scheme 3.9), (iv) hydroboration of 1-alkynes followed by brominolysis (but not iodinolysis) with inversion [95], and (v) syn hydrozirconation or syn hydroalumination of 1-boryl- or 1-silyl-l-alkynes followed by protonolysis of the C-Al or C-Zr bond [96-98]. 

Olefins are hydroaluminated by tri-isobutylalane at or below room temperature under the catalytic influence of Cl2ZrCp2 protonolysis then furnishes the corresponding alkanes in almost quantitative yield e.g. Scheme 7). The... 

The addition of trialkylalanes to alkenes is also catalysed by dicyclopentadienyl-zirconium dichloride to afford hydroalumination products that undergo protonolysis to yield alkanes. Thus, reaction of oct-l-ene with Bu gAl with a catalytic quantity of cpaZrCl2 at 0 °C gives n-octane in 97 % yield. Reduction of alkenes containing hydroxyl, phenylthiolato, and bromo functions can also be reduced in the same way. ... 

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