Borane catalysed ring opening and closing cascades of furans leading to silicon functionalized synthetic intermediates

The conversion of renewable biomass resources to synthetically valuable chemicals is highly desirable, but remains a formidable challenge in regards to the substrate scope and reaction conditions. Here we present the development of tris(pentafluorophenyl)borane–catalysed conversion of furans via ring-opening and closing cascade processes to afford silicon-functionalized synthetic chemicals under transition metal-free conditions. The furan ring-opening with hydrosilanes is highly efficient (TON up to 2,000) and atom-economical without forming any byproduct to give rise to α-silyloxy-(Z)-alkenyl silanes. Additional equivalents of silane smoothly induce a subsequent B(C6F5)3-catalysed cyclization of initially formed olefinic silane compounds to produce anti-(2-alkyl)cyclopropyl silanes, another versatile synthon being potentially applicable in the synthesis of natural products and pharmacophores.


Proposed Mechanistic Pathway for the Silylative Reduction of Furan
In regard to the silylative ring-opening of 2-methylfuran, a catalytic cycle was proposed on the basis of the above deuterium experimental data. Ring-opening of furan is assumed to proceed mainly via two steps: facile formation of a

Supplementary Discussion 2 Proposed Mechanistic Pathway for the Silylative Reduction of 2,3-Dimethylfuran
In relevance to the mechanistic pathway of 2-methylfuran, a catalytic cycle of ring-opening process of 2,3-dimethylfuran is proposed on the basis of the above present deuterium experiment. Ring-opening of 2,3-dimethylfuran is assumed to be mainly dependent on two factors: a facile formation of (silyl)oxonium intermediates and subsequent stereoselective attack of borohydride nucleophile.
Upon the in situ generation of a borane-silane adduct I, 2,3-dimethylfuran attacks the silylium species to afford an oxonium species II that immediately reacts with borohydride leading to a partially reduced furan intermediate III bearing a sp 3 C-Si bond next to an oxygen atom. The second hydrosilylation of III is proposed to occur formin g an O-silyl oxonium species IV which is immediately reacted with borohydride at the silicon attached carbon center leading to V to minimize the steric repulsion between substituents and borohydride nucleophile. Upon hydrolysis, intermediate V is smoothly converted to another silylated enol intermediate VI that is immediately tautomerized to γ-silylketone VII. In fact, the deuterium labeling experiment is fully consistent with this mechanistic reasoning.

Supplementary Discussion 3 Proposed Mechanistic Pathway for the Silylative Reduction of 2-Methylbenzofuran
In relevance to the mechanistic pathway of 2-methylfuran, a catalytic cycle of ring-opening process of 2methylbenzofuran is proposed on the basis of the above present deuterium experiment.

Initial-Rate Kinetic Measurements (Fig. 3E): General Considerations for Kinetic Experiments:
Reactions used for the kinetic analysis were set up in an Ar-filled J-Young NMR tube. Kinetic analysis of the NMR scale reaction was carried out by collecting multiple (7 ~ 20) data points early in the reaction before the substrate concentrations were depleted. Reaction progress was monitored by 1 H NMR (400 MHz, CD 2 Cl 2 ) analysis at 54 sec intervals over 1 h at the specified temperature (25 O C). The kinetic data obtained from intensity increase in the terminal methyl group of cyclopropane over time (up to 20% conversion) on the basis of the internal standard of mesitylene provided the initial rate. Data were fit by least-squares analysis (R 2 > 0.983).

Representative Procedure for Initial-Rate Kinetics
Four different reactions were carried out to determine the initial rate kinetics. B(C 6 F 5 ) 3  seconds, and was quickly put into the NMR probe that was pre-adjusted to the target temperature, 25 o C (298 K). Then, the ring closing of Z-2a was monitored by 1 H NMR at 54 seconds intervals for 1 h to determine an initial rate (ν i ) for the appearance of anti-3a (up to ~20% conversion) at 25 o C.