Hydride, diisobutylaluminum reaction with alkynes

Digitoxigenin, structure of, 1097 Digitoxin, structure of, 989 Dihalide, alkynes from, 261 Dihedral angle, 94 Diiodomelhane. Simmons-Smith reaction with, 228-229 Diisobutylaluminum hydride, reaction with esters, 812 structure of, 699 Diisopropylamine, pK.d of, 923 1,3-Diketone, pfCa of, 852 Dimethyl disulfide, bond angles in, 20 structure of, 20 Dimethyl ether, electrostatic... 

Once again, ( )-carvone was chosen as starting material (Scheme 48) (727). Aldol reaction with formaldehyde led to 6-hydroxymethylcarvone. The primary alcohol was protected as silylether 415 to permit addition of (cyanomethyl)lithium. The 5 1 mixture of epimeric tertiary alcohols was separated and the main product, 416, the alcohol generated by axial attack, was protected stereoselectively as bromoether 417 utilizing pyridinium hydrobromide perbromide. To introduce the necessary alkyne, the nitrile was reduced to the aldehyde with diisobutylaluminum hydride and subsequent hydrolysis. Addition of the alkyne led to a 2 1 mixture of... 

Cydopropanes. 1-Alkynes (I) can be converted into alkylcyclopropanes (4) by the following sequence. Reaction with diisobutylaluminum hydride gives a 1-alkcnylalane... 

Hydroalumination. The treatment of alkynes with diisobutylaluminum hydride in hydrocarbon solvents results in a aT-addition of the Al-H bond to the triple bond to produce stereodefined alkenylalanes. The hydroalumination of alkynes is more limited in scope than the corresponding hydroboration reaction of alkynes (see Chapter 5) with regard to accommodation of functional groups and regioselectivity. Whereas hydroalumination of 1-alkynes is highly regioselective, placing the aluminum at the terminal position of the triple bond, unsymmetrically substituted alkynes produce mixtures of isomeric alkenylalanes. 

In another interesting variant of this process, disilacyclopentenes (e.g., (57)) were prepared by the reaction of diisobutylaluminum hydride with alkynes such as (56) (Scheme 8) <70ZAAC(377)48>. In this case, cu-addition of A1—H to the alkyne triple bond results in an increased reactivity of the —CH2Br group to the extent that spontaneous ring closure takes place in solvents of weak Lewis basicity (ether or dioxane) to afford the corresponding disilacyclopentene after hydrolytic loss of aluminum. 

Reduction of mono- or disubstituted alkynes with diisobutylaluminum hydride proceeds, like the corresponding hydroboration reaction, via stereoselective cw-addition of the Al-H bond to the triple bond. Hydrolysis of the resultant tram-alkenylalanes produces 1-alkenes and cA-alkenes, respectively. 

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