Copper-Catalyzed Enantioselective and Exo-Selective Addition of Silicon Nucleophiles to 7-Oxa- and 7-Azabenzonorbornadiene Derivatives

Abstract

An enantioselective formal hydrosilylation of 7-oxa- and 7-azabenzonorbornadiene derivatives is reported. The exo-selective addition of the silicon moiety across these strained alkenes is achieved under copper catalysis, employing Si–B reagents as silicon pronucleophiles in the presence of an alkoxide base. No ring opening is observed. While successful for those substrates, the same procedure cannot be applied to benzonorbornadiene, norbornadiene, or norbornene.

Activation of the Si–B interelement bond related to catalysis

“…Very recently, Cui and Oestreich successfully extended this asymmetric catalysis to the formal hydrosilylation of 7-oxa-and 7-azabenzonorbornadiene derivatives with a change of the ligand from Segphos derivative L40 to R-QuinoxP* (L41) (Scheme 112). 133 exo-Selective addition of the silicon moiety across these strained alkenes was achieved, and no ring opening was observed in these reactions. The reaction was general for various 7-oxa-and 7-azabenzonorbornadiene derivatives and Si-B compounds (342a-e -344a-e; 345a, b -346a, b, Scheme 112).…”

Recent Advances in Transition‐Metal‐Catalyzed Ring‐Retentive Addition Reactions of Oxa(aza)benzonorbornadienes

“…Regrettably, the substrate range of this hydrosilylation reaction was limited, the oxabenzonorbornadienes with substituent at the bridgeheads or azabenzonorbornadienes with electron-rich substituent at the benzene ring were not suitable for this protocol; in addition, benzonorbornadiene, norbornadiene, norbornene, or SiÀ B reagents with bulky groups at the silicon atom were also not compatible. [32] 2.9. Hydrophosphination Leung and co-workers realized the asymmetric addition of diarylphosphines to oxa (aza) benzonorbornadienes in the presence of a chiral phosphapalladacycle catalyst and a mild acid, the aimed asymmetric hydrophosphination products were obtained in excellent yields and good enantioselectivities (Scheme 12, eq 12.1).…”

“…Using Cu(MeCN) 4 PF 6 as the copper precatalyst and ( R,R)‐ QuinoxP as a chiral bidentate phosphine ligand, employing Si−B reagents 11 ‐ 1 as silicon pronucleophiles in the presence of an alkoxide base, the exo ‐selective addition products 11 ‐ 2 were furnished in good to excellent yields with high enantioselectivities. Regrettably, the substrate range of this hydrosilylation reaction was limited, the oxabenzonorbornadienes with substituent at the bridgeheads or azabenzonorbornadienes with electron‐rich substituent at the benzene ring were not suitable for this protocol; in addition, benzonorbornadiene, norbornadiene, norbornene, or Si−B reagents with bulky groups at the silicon atom were also not compatible [32] …”

Beyond Carbon: Enantioselective and Enantiospecific Reactions with Catalytically Generated Boryl- and Silylcopper Intermediates

“…For terminal alkenes, the sterically favored anti -Markovnikov-type products are predominantly formed, passing through the branched alkyl­copper intermediates V / VI with a stereo­genic carbon atom for R 1 ≠ R 2 (Scheme c). − The subsequent protonation occurs with retention of the configuration. However, the clever design of bulky chiral ligands enabled hydroboration reactions with Markovnikov regio­selectivity through VII / VIII , furnishing the corresponding α-chiral boronates and silanes. , The addition of Cu–B/Si species across internal alkenes remains challenging and is restricted to strained cycloalkenes − as well as acyclic alkenes − bearing a substituent that can stabilize the formed alkyl­copper intermediate (Scheme d). However, quantum-chemical calculations and experimental investigations have suggested that the migratory insertion of an internal double bond into Cu–B/Si bonds likely proceeds with a syn stereo­chemistry, resulting in the formation of the alkyl­copper species IX/X . − This also rationalizes the stereo­chemical outcome of borylative amination and arylation reactions later presented in section . ,, …”

“…However, the clever design of bulky chiral ligands enabled hydroboration reactions with Markovnikov regio­selectivity through VII / VIII , furnishing the corresponding α-chiral boronates and silanes. , The addition of Cu–B/Si species across internal alkenes remains challenging and is restricted to strained cycloalkenes − as well as acyclic alkenes − bearing a substituent that can stabilize the formed alkyl­copper intermediate (Scheme d). However, quantum-chemical calculations and experimental investigations have suggested that the migratory insertion of an internal double bond into Cu–B/Si bonds likely proceeds with a syn stereo­chemistry, resulting in the formation of the alkyl­copper species IX/X . − This also rationalizes the stereo­chemical outcome of borylative amination and arylation reactions later presented in section . ,, …”