Ligand-Controlled, Norbornene-Mediated, Regio- and Diastereoselective Rhodium-Catalyzed Intramolecular Alkene Hydrosilylation Reactions

Abstract

Ligand-controlled, norbornene-mediated, regio- and diastereoselective rhodium-catalyzed intramolecular alkene hydrosilylation of homoallyl silyl ethers (1) exploiting either BINAP or 1,6-bis(diphenylphosphino)hexane (dpph) has been developed. This method permits selective access to either trans-oxasilacyclopentanes (trans-2) or oxasilacyclohexanes (3) at will. A substoichiometric amount of norbornene markedly increased both yield and selectivity. A norbornene-mediated hydride shuttle process is discussed.

Direct Double Arylation of Bridged Alkenes and Efficient Bis‐C–H Alkylation of Benzamides

“…These norbornenes were then subjected to the directed alkylation reaction we studied earlier. The brief kinetic studies have shown that different from the norbornenes we were using during our previous studies, norbornenes 9 a ‐ 9 c and 9 e have speeded the reactions up ,. More specifically, norbornene 9 b prior to the other norbornenes examined showed the best reactivity and bis‐alkylation product was formed in 91% NMR yield together with 9% of mono‐substituted benzamide 2 a ( Scheme ).…”

Rhodium‐Catalyzed Alkene Hydrosilylation via a Hydride Shuttle Process by Diene Ligands: Dramatic Enhancement of Regio‐ and Diastereoselectivity

“…The utility of this reaction is enhanced by a reasonably wide substrate scope, comprised of both electronically and sterically diverse aliphatic and aromatic substrates (Table 2). Both regio‐ and diastereoselectivity among all substrates were greatly enhanced vis‐à‐vis the nbe‐mediated reactions 7. We were able to lower the catalyst loading to 0.5 mol‐% without severely impacting the diastereoselectivity in trans ‐ 2a (24:1 rr , 18:1 dr ); the reaction simply took longer to complete (72 h).…”

“…As shown in Table 1, we proceeded to examine a series of cyclic “diene” donors (10 mol‐%) (entries 1–9). The hydrosilylation of 1a 11 employing [Rh( rac ‐binap)Cl] 2 and 3‐bromocyclohexene ( L2 , entry 2) resulted in the formation of trans ‐ 2a with both excellent regio‐ and diastereoselectivity 7. Chloro‐, iodo‐, acetate, and carbamate congeners (entries 1 and 3–5) provided significantly lower selectivities 12.…”

“…While norbornene‐mediated hydride shuttle processes have been described for Pd‐catalyzed reactions, they are not well‐explored in the context of Rh catalysis 7,8. To better understand the catalytic process, the hydrosilylation of L2 (20 mol‐%) and silyl ether 9 , which does not bear an alkene moiety, was investigated (Scheme ).…”

“…Readily modifiable metal‐ligand architectures often render unprecedented reactivity and selectivity to molecular catalysis by controlling electronic and steric properties of the catalyst through engaging favorable coordination 3. Our laboratory recently disclosed ligand‐controlled, norbornene‐mediated, regio‐ and diastereoselective rhodium‐catalyzed intramolecular alkene hydrosilylation reactions4–6 (Scheme , a) 7. The strained cycloalkene norbornene (nbe) acts as a functional ligand in the Rh I complex 4 , in conjunction with a choice of a supporting phosphine ligand (BINAP or dpph), to impact regio‐ and diastereoselectivity via a “hydride shuttle” process 8.…”