- Mechanistic Insights into the ReIO2(PPh3)2-Promoted Reductive Coupling of Alcohols
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A mechanistic investigation of the reductive coupling of benzylic and allylic alcohols by triphenylphosphine catalyzed by ReIO2(PPh3)2 (1) is disclosed utilizing (1) stoichiometric reaction studies of 1 with alcohols, with PPh3 and with OPPh3; (2) rate law determination of the reaction of benzhydrol with PPh3 catalyzed by 1; (3) substrate structure-dependent reactivity/selectivity studies; and (4) DFT computational analysis of various potential reaction pathways in the benzyl alcohol/PPh3 reaction. In situ NMR monitoring of reactions of 1 with PPh3 and various alcohols demonstrate (a) facile, reversible PPh3 dissociation from 1; (b) association of various alcohols to form Re-alcohol/alkoxide complexes, (Ph3P)IReO2(ROH) and (Ph3P)IReO(OH)(OR); and (c) thermal conversion of these alcohol(ate)-rhenium complexes to Ph2CH-CHPh2 and OPPh3 at >50 °C. Under pseudo-first-order conditions, the initial rate kinetics of reductive coupling of Ph2CHOH/PPh3 catalyzed by 1 shows (a) a reaction rate that is first-order each in ROH, catalyst and first-order (or higher) in PPh3 and (b) the reaction is inhibited by OPPh3. Alcohol structure effects show (a) relative reactivity of sec-, tert-benzylic = allylic > prim-benzylic/allylic sec-, prim-alkyl and (b) low regioselectivity of the dimers from unsymmetrical allylic alcohols. A DFT computational study of the reaction of benzyl alcohol/PPh3 with 1 reveals a preferred pathway involving: (a) formation of rhenium-alcohol and alkoxide intermediates, (Ph3P)IReO2(ROH) and (Ph3P)IReO(OH)(OR); (b) reduction of the latter by PPh3 to form (OPPh3)(Ph3P)IRe(OH)(OBn) (E); (c) association of a second BnOH with E to give (Ph3P)IRe(OBn)2 (K); (d) facile dissociation of a benzyl radical from K by C-O homolysis; and (e) a second rhenium-O-Bn homolysis from (PPh3)IRe(H2O)(OBn) (O), giving bibenzyl via benzyl radical recombination and regenerating (PPh3)ReIO2.
- Boucher-Jacobs, Camille,Liu, Peng,Nicholas, Kenneth M.
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- Oxo-Rhenium-Catalyzed Radical Addition of Benzylic Alcohols to Olefins
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Although carbon radicals generated from a variety of alcohol derivatives have proven valuable in coupling and addition reactions, the direct use of alcohols as synthetically useful radical sources is less known. In this report, benzylic alcohols are shown to be effective radical precursors for addition reactions to alkenes when treated with triphenylphosphine or piperidine with the catalyst ReIO2(PPh3)2 (I).
- Bandari, Chandrasekhar,Nicholas, Kenneth M.
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p. 3320 - 3327
(2020/03/23)
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- Water-Soluble Rhenium Phosphine Complexes Incorporating the Ph2C(X) Motif (X = O-, NH-): Structural and Cytotoxicity Studies
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Reaction of [ReOCl3(PPh3)2] or [ReO2I(PPh3)2] with 2,2′-diphenylglycine (dpgH2) in refluxing ethanol afforded the air-stable complex [ReO(dpgH)(dpg)(PPh3)] (1). Treatment of [ReO(OEt)I2(PPh3)2] with 1,2,3-triaza-7-phosphaadamantane (PTA) afforded the complex [ReO(OEt)I2(PTA)2] (2). Reaction of [ReOI2(PTA)3] with dpgH2 led to the isolation of the complex [Re(NCPh2)I2(PTA)3]·0.5EtOH (3·0.5EtOH). A similar reaction but using [ReOX2(PTA)3] (X = Cl, Br) resulted in the analogous halide complexes [Re(NCPh2)Cl2(PTA)3]·2EtOH (4·2EtOH) and [Re(NCPh2)(PTA)3Br2]·1.6EtOH (5·1.6EtOH). Using benzilic acid (2,2′-diphenylglycolic acid, benzH) with 2 afforded the complex [ReO(benz)2(PTA)][PTAH]·EtOH (6·EtOH). The potential for the formation of complexes using radioisotopes with relatively short half-lives suitable for nuclear medicine applications by developing conditions for [Re(NCPh2)(dpg)I(PTA)3] (7)[ReO4]- in a 4 h time scale was investigated. A procedure for the technetium analog of complex [Re(NCPh2)I2(PTA)3] (3) from 99mTc[TcO4]- was then investigated. The molecular structures of 1-7 are reported; complexes 3-7 have been studied using in vitro cell assays (HeLa, HCT116, HT-29, and HEK 293) and were found to have IC50 values in the range of 29-1858 μM.
- Alshamrani, Abdullah F.,Archibald, Stephen J.,Burke, Benjamin P.,Higham, Lee J.,Prior, Timothy J.,Redshaw, Carl,Roberts, David P.,Stasiuk, Graeme
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p. 2367 - 2378
(2020/03/05)
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- Analysis of an unprecedented mechanism for the catalytic hydrosilylation of carbonyl compounds
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This work details an in-depth evaluation of an unprecedented mechanism for the hydrosilylation of carbonyl compounds catalyzed by (PPh3) 2Re(O)2I. The proposed mechanism involves addition of a silane Si-H bond across one of the rhenium-oxo bonds to form siloxyrhenium hydride intermediate 2 that reacts with a carbonyl substrate to generate siloxyrhenium alkoxide 4, which, in turn, affords the silyl ether product. Compelling evidence for the operation of this pathway includes the following: (a) isolation and structural characterization by X-ray diffraction of siloxyrhenium hydride intermediate 2, (b) demonstration of the catalytic competence of intermediate 2 in the hydrosilylation reaction, (c) 1H and 31P{1H} NMR and ESI-MS evidence for single-turnover conversion of 2 into 1, (d) observation of intermediate 2 in the working catalyst system, and (e) kinetic analysis of the catalytic hydrosilylation of carbonyl compounds by 1.
- Nolin, Kristine A.,Krumper, Jennifer R.,Pluth, Michael D.,Bergman, Robert G.,Toste, F. Dean
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p. 14684 - 14696
(2008/09/20)
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