Mild partial deoxygenation of esters catalyzed by an oxazolinylborate-coordinated rhodium silylene

Abstract

An electrophilic, coordinatively unsaturated rhodium complex supported by borate-linked oxazoline, oxazoline-coordinated silylene, and N-heterocyclic carbene donors [{κ(3)-N,Si,C-PhB(Ox(Me2))(Ox(Me2)SiHPh)Im(Mes)}Rh(H)CO][HB(C6F5)3] (, Ox(Me2) = 4,4-dimethyl-2-oxazoline; Im(Mes) = 1-mesitylimidazole) is synthesized from the neutral rhodium silyl {PhB(Ox(Me2))2Im(Mes)}RhH(SiH2Ph)CO () and B(C6F5)3. The unusual oxazoline-coordinated silylene structure in is proposed to form by rearrangement of an unobserved isomeric cationic rhodium silylene species [{PhB(Ox(Me2))2Im(Mes)}RhH(SiHPh)CO][HB(C6F5)3] generated by H abstraction. Complex catalyzes reductions of organic carbonyl compounds with silanes to give hydrosilylation products or deoxygenation products. The pathway to these reactions is primarily influenced by the degree of substitution of the organosilane. Reactions with primary silanes give deoxygenation of esters to ethers, amides to amines, and ketones and aldehydes to hydrocarbons, whereas tertiary silanes react to give 1,2-hydrosilylation of the carbonyl functionality. In contrast, the strong Lewis acid B(C6F5)3 catalyzes the complete deoxygenation of carbonyl compounds to hydrocarbons with PhSiH3 as the reducing agent.

An H‐Substituted Rhodium Silylene

“…29 Si)i ndicated as ingle 29 Si environment as a doublet of doublet of doublets( ddd) at d = 54.6 ( Figure 1b), a marked downfield shift comparedt ob oth 2-SiH 2 Ph (d = À8.3) and the previously mentioned Rh III silylene described by Sadow (vide supra) (d = 6.6). [11] The 29 Si nucleusi sc oupled to two chemically-unique 31 Pn uclei ( 2 J SiÀP = 10 Hz and 3 J SiÀP = 2.6 Hz), consistentw ith ap hosphinimine-stabilizedR h I silylene.…”

“…With silyl hydride complexes 2-SiH 2 Ph and 2-SiHPh 2 in hand, attempts to generate Rh silylenes via H À abstraction using B(C 6 F 5 ) 3 ,i nl ine with work by Tilley and Sadow,w ere conducted. [9][10][11] Specifically,astirring bromobenzene-d 5 solution of 2-SiH 2 Ph was combined with 1equivalent of B(C 6 F 5 ) 3 ,f orming the anticipated hydridoborate anion [HB(C 6 F 5 ) 3 ] À ,a sc onfirmed by 11 Ba nd 19 FNMR spectroscopy.H owever,t he expected metal-containing cation arising from a-H migration from Si to Rh was not observed ( 1 Ha nd 31 PNMR spectra indicated formation of multiple products). Exhaustive efforts to isolate aR hs ilylene species from this mixture provedu nsuccessful.…”

Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

“…R An NHC-bis-oxazolinylborate rhodium silylene complex has recently been reported in which silicon is stabilized by intramolecular coordination of an oxazoline group to the silylene ligand. [105] This stabilized rhodium silylene complex was active as a catalyst for deoxygenation of a variety of carbonyl substrates, but it is unclear whether or not the base-stabilized silylene ligand participates in hydrosilations in the same way as an unstabilized silylene complex might. Nevertheless, it is notable that a terminal silylene complex of rhodium was isolated with similar supporting ligands to those used by Gade for catalytic hydrosilation studies.…”

Boron‐Centered Scorpionate‐Type NHC‐Based Ligands and Their Metal Complexes

“…Alternatively, the rhodium dicarbonyl 138 or iridium isocyanide 144 reacted with PhSiH 3 in benzene solution at 60 °C in the dark to form the silyl compound [PhB(ox c Me₂ ) 2 (Im Mes )]RhH(SiH 2 Ph)CO ( 145 ) or at room temperature to form [PhB(ox c Me₂ ) 2 (Im Mes )]IrH(SiH 2 Ph)(CN t Bu) ( 146 ) . The reaction between [PhB(ox c Me₂ ) 2 (Im Mes )]RhH(SiH 2 Ph)CO ( 145 ) and B(C 6 F 5 ) 3 produced compound 147 (Figure ) . Complex 147 catalyzes the deoxygenation of esters to ethers, amides to amines, and ketones to alkanes in the presence of primary organosilanes, which act as oxygen acceptors and reducing agents .…”