Enantioselective diboration of olefins is a valuable strategy for transforming unsaturated hydrocarbons into useful chiral building blocks. Typically, the process is carried out via transition-metal based activation of diboron reagent. Prof. James Morken has developed metal-free, affordable, enantioselective, 1,2-diboration of alkenes cocatalyzed by carbohydrate-derived catalysts and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, 139009). This process employs pseudoenantiomeric glycols, 6-tert-butyldimethylsilyl-1,2-dihydroglucal (THS-DHG, 901235) and dihydrorhamnal (DHR, 901237), derived from D-glucal and L-rhamnal, respectively as chiral catalysts to enhance the reactivity of the diboron nucleus and to control the facial selectivity of the diboration reaction. Among various organoboronic esters studied for this reaction, 2,2′-bi-1,3,2-dioxaborinane (B2(pro)2, 901464) was found to be versatile in promoting reaction efficiency while maintaining high enantioselectivity. Reaction byproducts arising from B2(pro)2, namely, 1,3-propanediol and boric acid are relatively innocuous and easily removable by an aqueous wash. Carbohydrate-derived catalysts, TBS-DHG and DHR, offer not only greener means but also promote highly enantioselective 1,2-diboration of both terminal and internal alkenes at a modest temperature and a shorter reaction time.
THS-DHG catalyst 901235
DHR catalyst 901237
B2(pro)2 901464
Scheme 1:General scheme for the enantioselective diboration of alkenes with B2(pro)2 and carbohydrate-derived catalysts, TBS-DHG and DHR.
Scheme 2:Enantioselective diboration of terminal alkenes by carbohydrate-derived catalysts
Scheme 3:Enantioselective diboration of internal alkenes by a carbohydrate-derived catalyst, TBS-DHG
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