Boranes, alkenyl

Similai selectivity is observed in the synthesis of allylsilanes where X = CF3SO3 and Y = Si(CH3) 3 (304). Alkenyl- and alkynylborates containing a leaving group in the y-position rearrange to aUyhc and aUenic boranes, respectively (305). 

Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

Hydrolysis of 1-boronylallylstannanes, available by stannylation of alkenyl and allyl boranes. gives (Z)-allylstannanes stereoselectively34. 

Treatment of the optically active gem-borazirconocene alkanes with deuterium oxide followed by alkaline oxidation affords the corresponding optically active 1-deuterio primary alcohols. The enantiomeric excess of the resulting primary alcohols represents the diaster-eoselectivity of the asymmetric hydrozirconation (Scheme 7.13). Based on the cost and availability of optically active ligands, three types were explored monoterpenes, 1,2-diols, and 1,2-amino alcohols. Hydrozirconation of optically pure 1-alkenyl boranes 39 provided optically active 1,1-bimetallics 40. 

A number of procedures for stereoselective syntheses of alkenes involving alkenyl-boranes have been developed. For each of the procedures given below, give the structures of the intermediates and describe the mechanism in sufficient detail to account for the observed stereoselectivity. 

B-Bromo and B-iodo-9-borabicyclo[3.3.1]nonane add similarly in a cis fashion to terminal triple bonds 471 They do not react, however, with alkenes and internal acetylenic bonds. In contrast to the results mentioned above, phenyl-substituted chloroboranes (PhBCl2, Ph2BCl) do not participate in haloboration. Instead, the C—B bond adds across the multiple bond to form phenylalkyl-(alkenyl) boranes.466,468... 

More recently, a novel metal-substituted methylenecyclopropene (triafulvene) derivative was obtained when bis(propyne)zirconocene was treated with one equivalent of tris(pentafluorophenyl)borane, followed by excess of benzonitrile (equation 367)430. The first step involves alkynyl ligand coupling to give the isolable Cp2Zr(//-2,4-hexadiyne)B(C6F5)3 betaine. This undergoes a formal intramolecular nitrile insertion into the Zr—C(sp2) c-bond of the adjacent alkenyl zirconocene unit, leading to the zirconium-boron triafulvene-betaine. X-ray analysis of the triafulvene confirmed the planar... 

Another method for the preparation of acetylenic carboxylates involves the reaction of 1-alkenylboranes with CO and alcohols (equation 135). The alkenyl boranes are readily prepared from alkynes by addition of boranes. PdCl2 was again the catalyst, with NaOAc as base and benzoquinone as oxidant.541... 

---