Article pubs.acs.org/Organometallics
Phosphines as Silylium Ion Carriers for Controlled C−O Deoxygenation: Catalyst Speciation and Turnover Mechanisms Anton Gudz, Philippa R. Payne, and Michel R. Gagné* Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599, United States S Supporting Information *
ABSTRACT: We report studies delineating the speciation, kinetics, and deoxygenation catalysis of phosphine-modified mixtures of B(C6F5)3 (BCF) and R3SiH. Combinations of BCF, a tertiary silane, and PAr3 generate the [H−B(C6F5)3−][R3Si−PAr3+] 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·PPh2(p-tol), formation of the borohydride/silyl phosphonium ion pair is α [Et3SiH]1[PPh2(p-tol)]0. DFT calculations confirmed the intermediacy of the weakly coordinated BCF···H−SiEt3 adduct en route to the silyl phosphonium. For the catalytic deoxygenation of anisole with Et3SiH, phosphine additives slow the reaction relative to phosphine-free conditions. In situ monitoring confirmed the presence of [H− B(C6F5)3−][Et3Si−PAr3+] at early times, but this slowly converts to [H−B(C6F5)3−][H3C−PAr3+], which is catalytically inactive. These data are reconciled by invoking a competitive demethylation of a key PhOMe(SiEt3)+ oxonium ion intermediate by H− B(C6F5)3− (productive) or phosphine (nonproductive).
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INTRODUCTION In recent years, the chemistry of frustrated Lewis pairs (FLPs) has emerged as a productive concept for the catalytic heterolytic activation of reactants such as H21 or R3SiH.2 Metal-free FLP catalysts have been utilized in a wide variety of applications,3 including catalytic hydrogenation of imines4 and carbonyls,5 and the hydrosilylation of carbonyls,6,7 ethers,8 imines,9−11 pyridines,12 and enones.13 Most recently, BCF and its derivatives have shown superb chemoselectivities in the deoxygenation of complex biomass-derived sugars,14−16 ethercontaining alkyl tosylates,17 and natural products.18 The commercially available perfluoroaryl borane catalyst B(C6F5)3 (BCF) is the most common Lewis acid component of the reported FLPs, though non-BCF-catalyzed FLP reactions are also emerging,19−21 The Lewis basic partner is typically more variable and includes nitrogen and phosphine-derived compounds.22,23 As noted above, BCF-type catalysts are active for the hydrosilylative reduction of a variety of C−O bonds.24,2 This reactivity is enabled by the muted oxophilicity and heightened hydride affinity of fluoroaryl boranes, which enables efficient transformations in functional group rich environments. The key intermediate in the Piers mechanism for BCF activation of a silane is the weak BCF···H−SiR3 adduct (A, Figure 1a), which is susceptible to nucleophilic attack at Si by a Lewis base (e.g., ether or carbonyl) to generate a reactive ion pair composed of an H−B(C6F5)3− nucleophile and a R3Si−LB+ electrophile. © XXXX American Chemical Society
Their recombination leads to C−O reduction. This mechanistic scheme has been supported by the isolation of the BCF−silane adduct with an exceptionally electrophilic version of BCF,25 by stereochemical studies demonstrating that attack of the Lewis base is invertive at Si26 and by computational studies of ketone hydrosilylation.27 Highlighting its unique reactivity, BCF and its variants can also mediate chemoselective reductions in complex natural product environments.18 In these studies, it was noted that selectivity and functional group tolerance could be improved with phosphine additives. It was previously noted by Klankermayer,4 and then Du,28,29 that phosphine additives enhance turnover rates and selectivities for the enantioselective hydrosilylations of imines. The mechanistic role of added phosphine in the mediation of reactivity and selectivity is not well understood and presents an opportunity for catalyst development and optimization. Extensive mechanistic investigations on the hydrosilylation of ethers have led to the mechanism in Figure 1a, where the BCF preferentially activates the Si−H bond of the silane via A.8 This activated borane−silane species transfers R3Si+ to the substrate to generate the undetectable ion pair R2O−SiR3+ and H− B(C6F5)3−. The C−O bond of the silyl oxonium is then cleaved by H−B(C6F5)3− to regenerate the BCF catalyst. Most of the Received: September 8, 2017
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DOI: 10.1021/acs.organomet.7b00689 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Figure 1. BCF-catalyzed reduction of ethers. (a) Piers mechanism: BCF activates the Si−H bond of the trialkylsilane via A, followed by nucleophilic attack of ether to generate a silyl oxonium species, which is reduced by the borohydride. (b) Proposed phosphine-modified cycle. Phosphine intercepts A to form the stable H−B(C6F5)3−/R3Si−PR3+ ion pair, which acts as the silylium source.
catalyst thus speciates as free BCF, and the ionic compounds are all reactive intermediates. In the phosphine-modified version of this reaction, Klankermayer and co-workers have shown that the combination of a trialkylsilane, a moderately basic triaryl Mes3P phosphine, and BCF will stoichiometrically ionize the silane into H−B(C6F5)3− and R3Si−PMes3+. Although the mechanistic role of these additives has not been determined, it can be surmised that if the BCF/PR3 pair is (at least partially) frustrated then an equilibrium accessible η1silane adduct of B(C6F5)3 can react with free phosphine to yield the thermodynamically favorable ion pair.30−32 Transfer of silylium from the phosphine to the imine substrate would generate a similarly activated substrate poised to undergo hydride attack. A mechanism for the reduction of ethers consistent with these observations is shown in Figure 1b. Consistent with the Klankermayer work on imine reduction, in situ monitoring of the BCF/PAr3-catalyzed reduction of anisole to PhOTES (eq 1) showed at early times that the
Figure 2. Potential speciation outcomes of BCF, silane, and phosphine reaction mixtures.
The combination of BCF and 1.1 equiv of PPh3 in CH2Cl2 rapidly precipitates the Lewis adduct as a white solid. The addition of a 50-fold excess of Et3SiH dissolves the solid in
