Phosphines as Silylium Ion Carriers for Controlled C–O Deoxygenation: Catalyst Speciation and Turnover Mechanisms

Abstract

We report studies delineating the speciation, kinetics, and deoxygenation catalysis of phosphine-modified mixtures of B(C 6 F 5 ) 3 (BCF) and R 3 SiH. Combinations of BCF, a tertiary silane, and PAr 3 generate the [H−B(C 6 F 5 ) 3 − ][R 3 Si−PAr 3 + ] ion pair with conversions that depend on the silane and the phosphine. Smaller silanes enhance the ionization of the Si−H, as judged by heteronuclear NMR spectroscopy. Kinetic studies indicate that from BCF•PPh 2 (p-tol), formation of the borohydride/silyl phosphonium ion pair is α [Et 3 SiH] 1 [PPh 2 (p-tol)] 0 . DFT calculations confirmed the intermediacy of the weakly coordinated BCF•••H−SiEt 3 adduct en route to the silyl phosphonium. For the catalytic deoxygenation of anisole with Et 3 SiH, phosphine additives slow the reaction relative to phosphine-free conditions. In situ monitoring confirmed the presence of [H− B(C 6 F 5 ) 3 − ][Et 3 Si−PAr 3 + ] at early times, but this slowly converts to [H−B(C 6 F 5 ) 3 − ][H 3 C−PAr 3 + ], which is catalytically inactive. These data are reconciled by invoking a competitive demethylation of a key PhOMe(SiEt 3 ) + oxonium ion intermediate by H− B(C 6 F 5 ) 3 − (productive) or phosphine (nonproductive).

Reversible Silylium Transfer between P‐H and Si‐H Donors

“…Silane adducts comparable to 3 have been reported for a fluorinated boraindene, [34b] a cationic iridium fragment, [36] and a series of silylium ions [37] . Phosphine adducts of silylium ions containing direct P‐Si donor‐acceptor interactions are known, [7g, 38] but we have found no previous examples of silylium ions stabilized by η 1 ‐P‐H interactions, although a similar structure has been proposed as an intermediate in oxidative insertion of the phosphenium‐like cation [Cp*PH] + into a Et 3 Si−H bond [7g] . DFT‐calculated structures of 3 a , b show approximately linear P‐H‐Si linkages in these complexes, with P−H and Si−H bond lengthening consistent with delocalized bridge bonding, at least in these gas‐phase models (see Supporting Information).…”

“…Reversible exchange of silylium ions between Lewis bases is well known, [38, 41] but very few examples involve η 1 ‐E−H bond adducts like 3 and 4 . Silane adduct 4 itself undergoes self‐exchange with free silane in solution [37a] .…”

“…Reversible exchange of silylium ions between Lewis bases is well known, [38,41] but very few examples involve h 1 -E À H bond adducts like 3 and 4.S ilane adduct 4 itself undergoes self-exchange with free silane in solution. [37a] Theresting state for an iridium system active for the catalytic reduction of ethers by silanes involves aw ell-characterized equilibrium that transfers R 3 Si + between an Ir-H donor and the substrate ether;t he equilibrium lies toward the silyloxonium ether adduct.…”

Enhanced Activation of Coordinated Dinitrogen with p‐Block Lewis Acids

“…It was postulated that the size of the [(dppe) 2 M(N 2 )] fragment might facilitate dissociation of the borane, which would allow subsequent BÀHa nd SiÀHb ond splitting and functionalization of the N 2 ligand with boryl ands ilyl groups in 14-17.S uch reactivity again compares well with that of usual nitrogen/boron (N/B) FLPs (Scheme 7, top). [28][29][30] No borylation or silylation of N 2 occursi nt he absence of B(C 6 F 5 ) 3 .A kin to 9 and 10,t he NÀN bonds in the functionalized complexes are rather long (1.23-1.27 for the Wc omplexes),w ith n NN below 1670 cm À1 .T hese products can be seen as adducts of d 6 dinitrogen complexes with the elusive [Et 3 Si] + [HB(C 6 F 5 ) 3 ] À silylium cation, [31] or the un-known [(C 6 F 5 ) 2 B] + [HB(C 6 F 5 ) 3 ] À borinium ion, [32] but we proposed to describet hese ionic species 14-17 as boryl-or silyl-diazenido complexes with metal centers in the + 2o xidation state. [33] In the silyldiazenido complexes 16 and 17,t he NMR resonances of the 29 Si nucleus are shiftedt oh igh field, strongly suggesting that the positive chargei sn ot carried by the Si atom.…”

Silylated cyclopentadienes as competent silicon Lewis acid catalysts

“…stability, solubility, and chirality) beyond what these simple species allow. To this end, notable advances have been made in the development of effective chiral anions, 4 extremely stable anions such as carboranes 5 and perfluoroborates, 6 and complex counterions generated via anion binding. 7 Despite these important advances, there remains an important need for new, highly stable, readily accessible, and broadly diversifiable anion frameworks.…”