Vinyl alanes, addition

Concurrently, the butyltellurenyl bromide was obtained by the addition of a solution of bromine (0.16 g, 2.0 mmol) in benzene ( 10 mL) to the solution of dibutylditelluride (0.738 g, 2.0 mmol) in hexane cooled at 0°C under a nitrogen atmosphere. The mixture was stirred at 0°C for 10 min, then LiCl (0.196 g, 4.2 mmol) was added, the dark solution turned clear red and stirring was continued until LiCl was dissolved (10 min). The resulting solution was transferred to the flask containing the vinyl alane. The reaction was stirred for 2 h at room temperature and then a mixture of ice and water ( 60 mL) was added. The solids were filtered and the products extracted with hexane (3X70 mL) and ethyl acetate... 

Construction of the acyclic segment 395 was initiated by conversion of (-)-citronellal (392) into bromo aldehyde 393. Formation of the unsaturated aldehyde via the intermediate enamine was followed by reduction to the allylic alcohol 394. Elimination of the vinyl bromide in 394 to the acetylene was followed by the addition of trimethylaluminum to give the terminal vinyl alane. The intermediate alane was then quenched with iodine to furnish the corresponding iodo-vinyl alcohol, which was protected as the THP ether to afford 395. 

Reactions of terminal alkynes with DIBAL are very similar - syn addition leads to the Zs-vinyl alane and hence, by exchange with iodine or bromine, to A-vinyl halides.18... 

Vinyl aluminium compounds also carry out conjugate additions and illustrate well the difference in reactivity between vinyl alanes and the related ate complexes. If the enone can adopt the s-cis conformation, as in 175, a cyclic mechanism is possible in which aluminium both acts as a Lewis acid and delivers the vinyl nucleophile. Notice that the Zs-vinyl alane leads to the -y,8-unsaturated ketone 178 as this is again an SE2 reaction at the vinyl carbon.32... 

If the enone cannot adopt the s-cis conformation, it is necessary to convert the vinyl alane into an ate complex 179 before conjugate addition. Conjugate addition now occurs to s-trans enones, such as cyclic enones, to give the Zs-yS-unsaturated ketone 180 after aqueous work-up. Notice that this time the most stable anion is transferred - the choice is between Me, t-Bu, or vinyl. Aluminium is not a transition metal and simply releases the most stable anion as there must be some negative charge on the group being transferred. 

We have already seen examples of cuprates being used in stereochemically controlled conjugate additions to cyclopentenones and similar results can be achieved with ate complexes of vinyl alanes prepared by hydroalumination of alkynes to give e.g. 179 R = hexyl and hence the anti-A-product43 182. 

Another way to generated vinyl alanes is the transition metal catalyzed addition of trialkylaluminum reagents to alkynes. In Hoye s synthesis of elenic acid, 10 1-dodecyne was treated with Me3Al in the present of Cp2ZrCl2 and then MeLi. The resulting ate complex 730 was then reacted with triflate 731 to give a 96% yield of the alkene, 732. 

The reaction of tosylhydrazones of a -unsaturated enones with sodium boro-hydride in alcoholic solution leads not to reduction, but instead to elimination according to Scheme 35, to provide a synthesis of allyl ethers in good yields. Potassium carbonate or sodium alkoxides can replace borohydride as the base in this sequence. trans-A y ethyl ethers (74) may be synthesized stereoselectively by the addition of aluminium hydrides to 1-alkynes and subsequent reaction of the vinyl alane formed (Scheme 36). This route complements the same authors ... 

The new phosphine amine ligand (486) has been developed for the Cu-catalysed Michael addition of vinyl alanes R2A1C(R )C=CR R to A-substituted-2,3-dehydro-4-piperidones. The reaction gave the expected products with <97% ee ... 

Similar considerations apply to reactions between tri-l-alkyl alanes and other 1-alkenes. Vinyl and allyl triorganosilanes may also be dimerized in this way. Dimerization of the allyl derivative proceeds normally, i.e., the aluminum atom adds on to the terminal carbon atom of the allyl group, but a trialkyl or triaryl silyl group directly adjacent to the C=C double bond appears to cause reversal of the direction of addition of the A1—C bond (119) ... 

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