[129493-14-7] · C10H17B · 9-Vinyl-9-borabicyclo[3.3.1]nonane · (MW 148.08)
(highly reactive and selective Diels-Alder dienophile1)
Physical Data: bp 28-30 °C/0.25 mmHg.1
Preparative Method: vinylboranes are most conveniently prepared from the reaction of vinylstannanes with bromo- and chloroboranes (eq 1).1 These preparations may be carried out in situ2 for Diels-Alder reactions.
Handling, Storage, and Precautions: volatile vinylboranes are extremely pyrophoric; many problems may be avoided using the in situ procedure.2 Vinyl-9-BBN dimerizes3 extensively over the course of days at rt, and slowly in a freezer, but some other vinyldialkylboranes are much more stable.3
Vinylboranes are highly reactive, regioselective, and endo-stereoselective dienophiles, and exhibit other useful and unusual properties in their Diels-Alder reactions. Vinyl-9-BBN is 200 times more reactive than Methyl Acrylate with 1,3-Butadiene (eq 2),1 and reacts with a wide variety of dienes at 25-55 °C. Vinyl-9-BBN is
omniphilic because it displays high and surprisingly invariant reactivity with electron-rich, electron-poor, and unactivated dienes.4
The regioselectivity of vinyl-9-BBN reactions is very high (eq 3).3 However, this selectivity appears mainly due to steric effects; less sterically demanding vinylboranes are much less selective.3 The large role of steric effects often results in unusual regioselectivities, such as the exclusive formation of the
meta product from piperylene and the formation of a single regioisomer from vinylcyclohexene (eq 4).1
Electronic effects can also give rise to unusual regioselectivities with vinylboranes. Although vinyldihaloboranes are more reactive than vinyldialkylboranes,5 their selectivity is low. However, 2-alkenyl- and cis-1-alkenyldibromoboranes are highly regioselective dienophiles and afford the unusual meta product (eq 5).6 A similar meta regioselectivity is observed with alkynic boranes.7 In both cases the unusual selectivity is attributed to a [4 + 3] transition state based on ab initio calculations.8 Since the corresponding alkenyl-BBN derivatives afford the para products, either regioisomer may be obtained by judicious choice of borane.
As shown in eq 4, the stereoselectivity with vinylboranes is generally >90% endo. Examples of lower selectivity have been improved by choice of vinyldialkylborane.3 An exception to this selectivity has recently been observed in the reaction of 2-propenyl-9-BBN with cyclopentadiene, which affords the exo product with high selectivity (eq 6).6
2-Trimethylsilylvinyl-9-BBN is also highly reactive.9 Acid-catalyzed elimination of the silyl alcohol products readily affords 1,4-cyclohexadienes (eq 7), making 2-trimethylsilylvinyl-9-BBN a highly reactive Acetylene equivalent for Diels-Alder reactions.
Daniel A. Singleton
Texas A & M University, College Station, TX, USA