Decatungstate‐Photocatalyzed Si−H/C−H Activation in Silyl Hydrides: Hydrosilylation of Electron‐Poor Alkenes

Abstract

Tetrabutylammonium decatungstate has been used for the photocatalytic activation of the Si−H bond in trisubstituted silanes and applied for the hydrosilylation of electron‐poor alkenes. The mechanism that occurs depends on the silyl hydride used, as supported by laser flash photolysis and EPR trapping experiments. Homolytic Si−H cleavage through a hydrogen atom transfer from the silane to the excited catalyst operates with dimethylphenylsilane and methyldiphenylsilane, for which the hydrosilylation yields were satisfactory. If we used tertiary silanes that have more labile Si−H bonds, such as triphenylsilane or tris(trimethylsilyl)silane, the reaction worked to some extent even under uncatalyzed conditions because of a radical chain reaction. Competition between C−H and Si−H cleavage was observed, however, if we used trialkylsilanes (e.g., triethylsilane). In favorable cases, the process was also efficient under flow conditions or if promoted by sunlight.

Visible‐Light‐Mediated Metal‐Free Hydrosilylation of Alkenes through Selective Hydrogen Atom Transfer for Si−H Activation

“…[9] Fagnoni, Ravelli, and co-workers recently disclosed an elegant hydrosilylation of electron-poor alkenes through the use of tetrabutylammonium decatungstate as ad irect HAT catalyst under UV light (310 nm) irradiation. [10] Notably, mixtures of the SiÀHa nd CÀHa ctivation products were obtained with trialkylsilanes (Scheme 1B), likely owing to the similar BDEs of the SiÀHa nd CÀHb onds in these compounds. [11] As silicon is more electropositive than carbon (electronegativity 1.90 vs.2 .55 on the Pauling scale), Si À H bonds are generally more hydridic than C À Hb onds.W e speculated that by incorporating ah ighly electrophilic HAT catalyst, such as the quinuclidinium radical cation used by the MacMillan group, [12] the SiÀHb ond should be selectively activated in the presence of multiple CÀHbonds with similar bond strengths,t herefore enabling selective hydrosilylation (Scheme 1C).…”

“…After ac areful investigation of various photocatalysts and solvents (see the Supporting Information, Table S1), the combination of acatalytic amount of the organophotoredox catalyst 1,2,3,5tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN, 1) [13] and quinuclidin-3-yl acetate (2)i na cetonitrile (MeCN) was found to provide the best result, and the hydrosilylation product 4 was isolated in 81 %y ield. Remarkably,u nlike in the study by Fagnoni and Ravelli, [10] only the hydrosilylation product was obtained, and C À Hactivation side products were not detected, highlighting the effectiveness of the electrophilic quinuclidinium radical cation HATc atalyst for selective SiÀHactivation. Replacing the photocatalyst with Ir[dF-(CF 3 )ppy] 2 (dtbpy)PF 6 ,which has asimilar redox potential as 4CzIPN,delivered acomparable result.…”

“…[14] Employing Et 3 SiH and Me 2 PhSiH as two representative silane reagents,adiverse range of electron-deficient alkenes were examined. As depicted in Scheme 2a,acrylates with different steric and electronic properties were suitable substrates (4)(5)(6)(7)(8)(9)(10)(11)(12), and functional groups such as silane (10), ether (11), and epoxide (12)m oieties were tolerated. Dimethyl maleate and al actone were also viable substrates,r eadily affording the corresponding hydrosilylation products in 84 %a nd 37 % yield, respectively (13 and 14).…”

Visible‐Light‐Mediated Metal‐Free Hydrosilylation of Alkenes through Selective Hydrogen Atom Transfer for Si−H Activation

“…[8] In the context of HAT, polarity plays av ital role to enable selective hydrogen atom abstraction beyond the control of the relative bond dissociation energies (BDEs). [10] Notably, mixtures of the SiÀHa nd CÀHa ctivation products were obtained with trialkylsilanes (Scheme 1B), likely owing to the similar BDEs of the SiÀHa nd CÀHb onds in these compounds. [10] Notably, mixtures of the SiÀHa nd CÀHa ctivation products were obtained with trialkylsilanes (Scheme 1B), likely owing to the similar BDEs of the SiÀHa nd CÀHb onds in these compounds.…”

Selective C(sp3)−H Aerobic Oxidation Enabled by Decatungstate Photocatalysis in Flow

“…[12, 13b] Among the different reported photocatalysts,d ecatungstate (DT) has proved to be aversatile and inexpensive HAT catalyst which can readily perform hydrogen abstraction on C(sp 3 ) À Hfragments upon activation by irradiation with nearultraviolet light. [14] This strategy allowed for the construction of numerous CÀC, [15] CÀSi, [16] CÀN, [17] and CÀF [18] bonds. [15a,19] Intrigued by these seminal reports,w ew ondered if DT catalysis could be used to effect selective C(sp 3 )ÀHa erobic oxidation.…”