Electron‐Transfer and Hydride‐Transfer Pathways in the Stoltz–Grubbs Reducing System (KOtBu/Et₃SiH)

Abstract

Recent studies by Stoltz, Grubbse tal. have shown that triethylsilane and potassium tert-butoxide react to form ahighly attractive and versatile system that shows (reversible) silylation of arenes and heteroarenes as well as reductive cleavage of C À Ob onds in aryl ethers and C À Sb onds in aryl thioethers.Their extensive mechanistic studies indicate acomplex network of reactions with an umber of possible intermediates and mechanisms,b ut their reactions likely feature silyl radicals undergoing addition reactions and S H 2reactions. This paper focuses on the same system, but through computational and experimental studies,r eports complementary facets of its chemistry based on a) single-electron transfer (SET), and b) hydride delivery reactions to arenes.Recently,Stoltz, Grubbs et al. [1] have discovered as imple and elegant system comprising Et 3 SiH (2)a nd KOtBu which achieves an umber of remarkable reactions:1 )converting arenes and heteroarenes,a nd their alkylated counterparts, into silyl-substituted products,o ften with excellent regiocontrol [1a-c] (e.g. 1!3;S cheme 1);2 )achieving reductive C À S bond cleavage in aryl thioethers (e.g. 4!5)i nareaction which has potential importance in removing sulfur traces from hydrocarbon fuels; [1d] 3) triggering reductive CÀObond cleavage in aryl ethers (e.g. 6!7)inareaction with potential applications to controlled lignin degradation. [1a,d] Anumber of intermediates likely arise from reaction of these two reagents, and spectroscopic evidence has resulted in informed proposals being made for their structures.T hese reactions have proved puzzling, but arecent coordinated study by synthetic, mechanistic, and computational chemists has allowed significant advances to be made.[1e,f] Thec onclusions are:1 )the combination of Et 3 SiH and KOtBu leads to triethylsilyl radicals which have am ajor role to play in the reductive cleavage of the C À Oand C À Sbonds, [1d] 2) triethylsilyl radicals are also likely to be involved in the silylation reactions, although nonradical routes to the silylation have also been considered in depth and may also play acentral role.[1e,f] The mechanistic details are not fully in place,for example,onhow formation of the silyl radicals occurs,b ut rational working hypotheses have been advanced. [1e] We had wondered if single-electron transfer mechanisms were playing as ignificant role in some of these reactions, notably for the cleavage of CÀOa nd CÀSb onds.A ne arly suggestion [1a] mentioned pentavalent silicates (e.g. 13 b;s ee Scheme 2) as reagents that were likely involved in the CÀO cleavage,b ut the more recent computational studies on the substrates 4 and 6 instead support an alternative mechanism.[1d] In this regard, Scheme 1s hows ipso addition to the carbon atom of the CÀOb ond by triethylsilyl radicals, followed by CÀOb ond cleavage in conversion of 6 into 7.Our recent interest in reductive chemistry carried out by reactions involving KOtBu attracted us to this area.[2] Studies mentioned above [1e] suggest that the r...

An isolable, crystalline complex of square-planar silicon(IV)

“…61,62 Indeed, tetracoordinated silicon radical anions remain elusive as they are presumed to be powerful electron donors, even stronger than alkali metals. 63 A chemical reduction of 2 was attempted with decamethylcobaltocene (À1.9 V versus Fc/Fc + ) as a reducing agent to support the electroanalytical results. Performed in C 6 D 6 , a rapid reaction to a mixture of unidentified products indicated a redox process to happen.…”