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Tetrahydroxydiboron-Mediated Palladium

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Tetrahydroxydiboron-Mediated Palladium Tetrahydroxydiboron-Mediated Palladium-Catalyzed Transfer Hydrogenation and Deuteriation of Alkenes and Alkynes Us- ing Water as the Stoichiometric H or D Atom Donor Steven P. Cummings, Thanh-Ngoc Le, Gilberto E. Fernandez, Lorenzo G. Quiambao, and Benjamin J. Stokes* School of Natural Sciences, University of California, Merced, 5200 N. Lake Road, Merced, CA 95343, USA Supporting Information Placeholder ABSTRACT: There are few examples of catalytic transfer hydro- Scheme 1. The Use of Water as the H Atom Donor in genations of simple alkenes and alkynes that use water as a stoichi- Catalytic Transfer Hydrogenations of Alkenes ometric H or D atom donor. We have found that diboron reagents A. Homolytic transfer hydrogenation from water mediated by Ti(III) (ref. 2). efficiently mediate the transfer of H or D atoms from water directly Cp2TiCl2 (2.5 equiv) onto unsaturated C–C bonds using a palladium catalyst. This reac- Mn (8.0 equiv) tion is conducted on a broad variety of alkenes and alkynes at am- [Pd] (10 mol %) bient temperature, and boric acid is the sole byproduct. Mechanis- THF, rt, 24 h B. This work: Transfer hydrogenation from water using B (OH) . tic experiments suggest that this reaction is made possible by a rate- 2 4 B2(OH)4 determining H atom transfer event that generates a Pd–hydride [Pd] catalyst R R intermediate. Importantly, complete deuterium incorporation from H2O + R R A rt B stoichiometric D2O has also been achieved. C. Proposed mechanism of H atom transfer from water using B2(OH)4. [Pd] H–OH H–OH B(OH)3 X2B–BX2 X2B–[Pd]–BX2 X2B–[Pd]–BX2 X2B–[Pd]–H (X = OH) C D E The catalytic hydrogenation of alkenes or alkynes is most often H [Pd] H B(OH) H OH [Pd] BX [Pd] 3 H–OH 2 executed in batches by direct application of hydrogen gas. Alterna- R R [Pd] BX2 R R R R tively, transfer hydrogenation (TH) appeals to many bench chem- R R R R ists because it does not require the use of a flammable gas. A wide H H H B H G H F A variety of H or D atom donors are available for use in both homo- bonds (Scheme 1B). Mechanistically, we hypothesized that, follow- and heterogeneous catalytic TH reactions,1 but there are few exam- ing oxidative addition to give C,7 water could coordinate to one ples of TH reactions that use water stoichiometrically to hydrogen- boron atom to give adduct D (Scheme 1C). Then, H atom transfer ate an unsaturated carbon-carbon bond2 despite the safety and cost could liberate boric acid to afford Pd-hydride E.8 Migratory inser- benefits associated with the use of H2O and D2O. While stoichio- tion of an unsaturated substrate, such as A, could afford Pd-alkyl metric TH of unactivated alkenes and alkynes using H2O could be intermediate F, which could coordinate another equivalent of wa- broadly useful for reductions, the transfer deuteriation (TD) of ter (G) and generate Pd-hydride H following another H atom alkenes and alkynes using D2O is of special interest because deuter- transfer event. Reductive elimination from H could generate the ium-labeled small molecules are useful for the study of synthetic second C–H bond, and afford B. Below, we describe the successful mechanistic pathways,3 for pharmacokinetic studies towards drug development of this concept into an efficient hydrogenation and discovery,4 and for preparing LC-MS internal standards.5 deuteriation reaction that avoids the necessity of H2 or D2 gas and The only method we are aware of that uses a near-stoichiometric reduces a wide variety of alkenes and alkynes to alkanes quantita- amount of water as the putative H atom donor relies on a single tively at ambient temperature.9 electron transfer from titanocene chloride to activate the O–H We discovered that untreated palladium on carbon (Pd/C) bond for metal hydride generation (Scheme 1A).2 This reaction could quantitatively transfer hydrogen atoms from water to 1,1- suffers from inconsistent yields and incomplete deuterium incorpo- diphenylethylene 1a at ambient temperature in dichloromethane ration from D2O. We envisioned developing a complementary in the presence of tetrahydroxydiboron and just a slight excess (2.1 approach to metal hydride generation from water by harnessing the equivalents total, with one equivalent required per B atom) of wa- versatility of B2X4 reagents, which can serve as both a transition ter (Table 1, entry 1). This result suggests that H2 gas is not being metal oxidant and as a Lewis acid to engage water. generated by B2(OH)4 hydrolysis in situ, as H2 would be likely to Tetrahydroxydiboron, which is infrequently used for non- escape from solution and lead to low yield if this were the case. 6 borylative purposes, was envisioned as the ideal diboron reagent Alternatively, B2(OH)4 hydrolysis would lead to the intermediate to transfer H atoms from water across unsaturated carbon-carbon H–B(OH)2, which could potentially hydroborate the alkene to give Table 1. Reaction Optimization Table 2. Scope of The Hydrogenation of Alkenes additive (1.1 equiv) Ph catalyst (5 mol %) Ph B2(OH)4 (1.1 equiv) 1 3 1 3 + H2O R R Pd/C (5 mol %) R R Ph solvent (0.3 M) Ph + H2O CH Cl (0.3 M) rt, 18 h R2 R4 (5 equiv) 2 2 R2 R4 1a 2a rt, 5 h a 1 2 H2O Yield (%) Entry Catalyst Additive Solvent (equiv) of 2a terminal non-styrenyl alkenes EtO2C 1 Pd/C B2(OH)4 DCM 2.1 >95 O n-Hex HO 3 BnO 3 2 Pd/C B2(OH)4 DCM 0 0 Et EtO2C 3 Pd/C B2(OH)4 MeOH 0 65 1b, >95%a 1c, >95%a 1d, 97% 1e, 91% 1f, >95%a 4 Pd/C B2(OH)4 THF 0 50 cyclic alkenes C(OH)Me2 b Boc 5 Pd/C B2(OH)4 DCM 5 0 O N 6 Pd(PPh ) B (OH) DCM 5 0 3 4 2 4 Cl c 7 Pd/C B2(OH)4 DCM 5 10 Me 1g, >95%a 1h, >95%a 1i, >95%a 1j, 97%b 1k, tracea 1l, 69%a 8 Pd(OAc)2 B2(OH)4 DCM 5 >95 (64% conv.) (75:25 trans:cis) 9 Pd/C B cat DCM 5 >95 2 2 acyclic polysubstituted alkenes 10 Pd/C B2pin2 DCM 5 <5 CO2Et OH OBn OTIPS These reactions employed untreated catalysts unless otherwise noted, Ph Ph Ph Ph and were conducted on 2.0 mmol scale. In all cases, the substrate con- 1m, 90% 1n, 21%a 1o, 90%a 1p, 95% a 1 (58% conv.) (93% conv.) version matched the yield. Determined by H NMR analysis of reac- Ph t-Bu Ph tion mixtures upon filtration using 1,3,5-trimethoxybenzene as an in- Ph Ph ternal standard. b Reduced Pd/C was used in this experiment. c This Ph Ph Ph b b reaction was conducted in the presence of 145 equivalents of Hg per 1q, 97% 1r, 95% 1s, 95% 1a, 92% CO H Pd. Ph Ph Ph Me Me 2 alkyl boronic acids, but alkyl boronic acids do not proteode- Ph NHAc Ph Me Ph Ph Me Me 10 boronate under our reaction conditions. No background hydro- 1t, 99% 1u, 88%a,c 1v, >95%a 1w, 88%a,d genation occurred in dichloromethane in the absence of water (en- (88% conv.) (95% conv.) try 2), whereas hydrogenation did occur in anhydrous MeOH (en- styrenes p- try 3) and anhydrous THF (entry 4) in the absence of added water. C6H5 p-OMe(C6H4) p-Cl(C6H4) CF3(C6H4) p-NO2(C6H4) a Turning our attention to the catalyst itself, we found that a pre- 1x, 96% 1y, 96% 1z, 96% 1aa, 93% 1ab, 38% (55% conv.) reduced form of Pd/C did not afford a detectable amount of prod- Br uct (Table 1, entry 5), suggesting that the reaction may be cata- p-N3(C6H4) p-Br(C6H4) m-Br(C6H4) o- Br(C6H4) 11 C6H5 lyzed by metal nanoparticles. The use of Pd(PPh3)4 as catalyst 1ac, 6%a 1ad, 32%a 1ae, 51%a 1af, 25%a 1ag, 0%a also resulted in no substrate conversion, further implicating catalyt- (77% conv.) (82% conv.) (66% conv.) (84% conv.) (66% conv.) ically active Pd nanoparticles, which would be poisoned by phos- phines (entry 6).7 As a final piece of evidence in support of the Isolated yields are reported, and starting material was consumed in full, nanoparticulate nature of the catalyst, a mercury drop experiment unless otherwise noted. a Determined by 1H NMR analysis of the crude was performed, and resulted in a significant decrease in the conver- reaction mixtures using 1,3,5-trimethoxybenzene as an internal stand- 12 b sion of 1a (entry 7). In addition to untreated Pd/C, Pd(OAc)2 ard. 60 hour reaction of 5.0 mmol of the indicated substrate using 0.5 c d was also observed to catalyze the reaction (entry 8). Pt/C and mol % Pd/C. Reaction time was 18 h. H2O was used as the solvent. Rh/C are also effective catalysts.13 As our final piece of optimiza- (+)-terpineol 1l afforded reasonable product yield with modest tion, we probed the influence of the electronic nature of the dibo- diastereoselectivity favoring the trans diastereomer. ron reagent, and found that the arene-stabilized Most of the acyclic polysubstituted alkenes that we evaluated bis(catecholato)diboron variant functioned as effeciently as tetra- (1m–1w) were cleanly converted to product, with the exception of hydroxydiboron (Table 1, entry 9).
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