Intramolecular Friedel–Crafts alkylation with a silylium-ion-activated cyclopropyl group: formation of tricyclic ring systems from benzyl-substituted vinylcyclopropanes and hydrosilanes

Abstract

A trityl-cation-initiated annulation of benzyl-substituted vinylcyclopropanes (VCPs) with hydrosilanes is reported. Two Si-C(sp3) bonds and one C(sp2)-C(sp3) bond are formed in this process where an intramolecular 6-endo-tet Friedel-Crafts alkylation of...

Recent Advances on Synthetic Methodology Merging C–H Functionalization and C–C Cleavage

“…Oestreich et al developed an interesting route to silicon-containing polycyclic structures 309 through the coupling of benzyl-substituted VCPs 307 and hydrosilanes 308 ( Scheme 40 ) [ 117 ]. The process employed the trityl cation derivative Ph 3 C + [B(C 6 F 5 ) 4 ] − as an initiator to generate a silylium-ion from the hydrosylanes 308 .…”

B(C6F5)3-Catalyzed Diastereoselective Formal (4 + 1)-Cycloaddition of Vinylcyclopropanes and Et2SiH2

“…At higher concentration, more catalyst loading or prolonged reaction time, the hydrosilylation product is further converted into the corresponding trans-configured cyclopropane-stabilized silylium ion with [HB­(C 6 F 5 ) 3 ] − as the counteranion. Ring opening by intermolecular attack of the borohydride (rather than the intramolecular processes outlined before , ) could explain the diastereoselective formation of the trans -3,4-disubstituted silolane. We present here a B­(C 6 F 5 ) 3 -catalyzed formal (4 + 1) cycloaddition of VCPs and Et 2 SiH 2 .…”

“…Our laboratory recently reported trityl-cation-initiated, silylium-ion-promoted cycloadditions of VCPs and dihydrosilanes. , For aryl-substituted VCPs, silicon-containing six-membered rings were obtained as a result of a formal (5 + 1)-cycloaddition; a cis-configured cyclopropane-stabilized silylium ion is the key intermediate in this skeletal reorganization (R = Ph; Scheme , top) . In turn, a benzyl instead of an aryl group steered the reaction toward an intramolecular Friedel–Crafts alkylation involving the intermediacy of a trans-configured cyclopropane-stabilized silylium ion (R = Bn; Scheme , top) . For both, an initially formed β-silicon-stabilized cyclopropylcarbinyl cation is not captured by external hydride, that is, excess dihydrosilane, but instead undergoes a 1,3-hydride shift from silicon to carbon to generate either of the aforementioned cyclopropane-stabilized silylium ions that can exist as interconverting cis- and trans-diastereomers. , However, the situation changes completely with B­(C 6 F 5 ) 3 /hydrosilane combinations .…”

“…In turn, a benzyl instead of an aryl group steered the reaction toward an intramolecular Friedel–Crafts alkylation involving the intermediacy of a trans-configured cyclopropane-stabilized silylium ion (R = Bn; Scheme , top) . For both, an initially formed β-silicon-stabilized cyclopropylcarbinyl cation is not captured by external hydride, that is, excess dihydrosilane, but instead undergoes a 1,3-hydride shift from silicon to carbon to generate either of the aforementioned cyclopropane-stabilized silylium ions that can exist as interconverting cis- and trans-diastereomers. , However, the situation changes completely with B­(C 6 F 5 ) 3 /hydrosilane combinations . The in situ-formed borohydride [HB­(C 6 F 5 ) 3 ] − , which is a better hydride donor than hydrosilanes, traps the β-silicon-stabilized cyclopropylcarbinyl cation to yield the conventional hydrosilylation product (Scheme , bottom left). , A new (4 + 1)-cycloaddition has now been discovered under reaction conditions similar to those of that B­(C 6 F 5 ) 3 -catalyzed hydrosilylation (Scheme , bottom right).…”

“…Conversely, VCPs 1c ( n = 2) and 1d ( n = 1) with less methylene groups in their carbon chains reacted in significantly lower yields of 40% for 2c and 30% for 2d . Byproducts were a previously observed tricylic compound in the case of 1c (13% yield; see Tables S1 and S2) and the ring-opened hydrosilylation product in the case of 1d (9% yield; see Table S3). The phenyl-substituted VCP 1e ( n = 0) gave the conventional hydrosilylation product 3e in 73% yield (not shown). , …”