TRANS-VINYLBORANES FROM 9-BORABICYCLO[3.3.1]NONANE THROUGH DEHYDROBORYLATION

The hydroboration of 1-alkynes (1) was reinvestigated by B-11 NMR unde r optimized conditions (THF, 18 h, 0-degrees-C) and found to provide t rans-vinyl-9-BBN adducts (2) together with variable amounts of 1, 1-di borylalkanes (3) depending both upon the excess of 1 employed and the nature of alkyne substitution. By contrast, the formation of 3 with 2 equiv of 9-BBN-H is quantitative. A new completely stereoselective rou te to 2 from 3 was discovered with its reaction with ArCHO in an elect rocyclic process (rho = 0.42). While analogous to the Midland reductio n, the term dehydroborylation is introduced to emphasize the olefinati on aspect of the reaction. Compound 3a (R = Me) is smoothly dehydrobor ylated at 25-degrees-C with PhCHO following second-order kinetics. Com petitive rate studies reveal its reaction to be slower than that of Al pineborane (7) (k7/k3a = 4.5) but faster than that of B-siamyl-9-BBN ( 6) (k6/k3a = 0.34). The value of the dehydroborylation approach to 2 a nd the advantages of using 9-BBN derivatives in vinylborane reactions are demonstrated with numerous examples. Thus, 1,8-nonadiyne is conver ted, through a bis(vinylborane) (11), to pure trans,trans-1,9-dideuter io-1,8-nonadiene (12). This transformation has not been previously pos sible for 9-BBN-H because of competitive dihydroboration. The dihydrob oration of 1-(triethylsilyl)-1-propyne, after thermal isomerization an d deuterolysis, affords trans-(3-deuterioallyl)silane (16), a most rem arkable overall conversion. The insertion of aromatic aldehydes into 2 was further demonstrated to provide a convenient entry to trans-allyl ic alcohols. The selective oxidation of 2 with TMANO produces trans-al kenyl-9-oxa-10-borabicyclo[3.3.2]decanes, 18, which resist further rea ction with ArCHO, oxidation in the atmosphere, and protonolysis. A 1 - -> 3 --> 2 --> 20 sequence was employed without the isolation of 2 in a one-pot Suzuki coupling with ArBr to provide trans-stilbenes (20, Ar = p-C6H4X, X = OMe (80%), NMe2 (60%)).