A novel reduction reaction for the conversion of aldehydes, ketones and primary, secondary and tertiary alcohols into their corresponding alkanes

A novel one-pot reaction has been developed for the reduction of aldehydes, ketones and primary, secondary and tertiary alcohols into their corresponding alkyl function. This is also the first reported method which can efficiently reduce primary, secondary, or tertiary alcohols, without affecting carbon-carbon double bonds, into their corresponding alkyl function in high yields. The reduction utilises either diethylsilane or n-butylsilane as the reducing agent in the presence of the Lewis acid catalyst tris(pentafluorophenyl)borane.     

A direct reduction of aliphatic aldehyde, acyl chloride, ester, and carboxylic functions into a methyl group

The aliphatic carboxylic group was efficiently reduced to the methyl group by HSiEt(3) in the presence of catalytic amounts of B(C(6)F(5))(3). To the best of our knowledge, this is the first example of a direct exhaustive reduction of aliphatic carboxylic function. Aliphatic aldehydes, acyl chlorides, anhydrides, and esters also underwent complete reduction under similar reaction conditions. Aromatic carboxylic acids, as well as other carbonyl functional equivalents, underwent smooth partial reduction to the corresponding TES-protected benzylic alcohols. It was shown that, unlike the reduction of aliphatic substrates, the exhaustive reduction of aromatic substrates was not straightforward: a concurrent Friedel-Crafts-like alkylation process competed with the reduction yielding trace to notable amounts of dimeric products, thus decreasing the overall selectivity of the reduction process.     

Direct chlorination of alcohols with chlorodimethylsilane catalyzed by a gallium trichloride/tartrate system under neutral conditions

The reaction of secondary alcohols 1 with chlorodimethylsilane (HSiMe(2)Cl) proceeded in the presence of a catalytic amount of GaCl(3)/diethyl tartrate to give the corresponding organic chlorides 3. In the catalytic cycle, the reaction of diethyl tartrate 4a with HSiMe(2)Cl 2 gives the chlorosilyl ether 5 with generation of H(2). Alcohol-exchange between the formed chlorosilyl ether 5 and the substrate alcohol 1 affords alkoxychlorosilane 6, which reacts with catalytic GaCl(3) to give the chlorinated product 3. The moderate Lewis acidity of GaCl(3) facilitates chlorination. Strong Lewis acids did not give product due to excessive affinity for the oxy-functionalities. Although tertiary alcohols were chlorinated by this system even in the absence of diethyl tartrate, certain alcohols that are less likely to give carbocationic species were effectively chlorinated using the GaCl(3)/diethyl tartrate system.     

Rapid and highly efficient trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS) catalyzed by reusable zirconyl triflate, [ZrO(OTf)2]

In this paper, rapid and efficient trimethylsilylation of alcohols and phenols with hexamethyldisilazane in the presence of catalytic amounts of ZrO(OTf)₂ is reported. Primary, secondary and tertiary alcohols as well as phenols were efficiently converted to their corresponding TMS ethers in short reaction times at room temperature. It is noteworthy that this method can be used for chemoselective silylation of primary alcohols in the presence of secondary and tertiary alcohols and phenols.     

Rapid,highly efficient and chemoselective trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS) catalyzed by reusable electron‐deficient tin(IV)porphyrin

In this paper, rapid and highly efficient trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS) in the presence of catalytic amounts of high‐valent [Sn^IV(TPP)(OTf)₂] is reported. This catalytic system catalyzes trimethylsilylation of primary, secondary and tertiary alcohols as well as phenols, and the corresponding TMS‐ethers were obtained in high yields and short reaction times at room temperature. It is noteworthy that this method can be used for chemoselective silylation of primary alcohols in the presence of secondary and tertiary alcohols and phenols. The catalyst was reused several times without loss of its catalytic activity. Copyright © 2009 John Wiley & Sons, Ltd.     

Direct reduction of alcohols: highly chemoselective reducing system for secondary or tertiary alcohols using chlorodiphenylsilane with a catalytic amount of indium trichloride.

The direct reduction of alcohols using chlorodiphenylsilane as a hydride source in the presence of a catalytic amount of indium trichloride is described. Benzylic alcohols, secondary alcohols, and tertiary alcohols were effectively reduced to give the corresponding alkanes in high yields. A compound bearing both primary and secondary hydroxyl groups was reduced only at the secondary site to afford the primary alcohol after workup with Bu(4)NF. This system showed high chemoselectivity only for the hydroxyl group while not reducing other functional groups that are readily reduced by standard reducing systems. Thus alcohols bearing ester, chloro, bromo, or nitro groups, which are sensitive to LiAlH(4) or Zn/H(+), were selectively reduced only at the hydroxyl sites by the chlorodiphenylsilane/InCl(3) system. NMR studies revealed the reaction course. The hydrodiphenylsilyl ether is initially formed and then, with InCl(3) acting as a Lewis acid, forms an oxonium complex, which accelerates the desiloxylation with donation of the hydrogen to the carbon.     

Selective tetrahydropyranylation of alcohols and phenols using titanium(IV) salophen trifluoromethanesulfonate as an efficient catalyst

Titanium(IV) salophen trifluoromethanesulfonate, [Ti^IV(salophen)(OSO₂CF₃)₂], as a catalyst enables selective tetrahydropyranylation of alcohols and phenols with 3,4‐dihydro‐2H‐pyran. Using this catalytic system, primary, secondary and tertiary alcohols, as well as phenols, were converted to their corresponding tetrahydropyranyl ethers in high yields and short reaction times at room temperature. Investigation of the chemoselectivity of this method showed discrimination between the activity of primary alcohols in the presence of secondary and tertiary alcohols and phenols. This heterogenized catalyst could be reused several times without loss of its catalytic activity. Copyright © 2016 John Wiley & Sons, Ltd.     

Polystyrene‐Gallium Trichloride Complex: A Mild,Highly Efficient,and Recyclable Polymeric Lewis Acid Catalyst for Chemoselective Silylation of Alcohols and Phenols with Hexamethyldisilazane

Polystyrene‐supported gallium trichloride (PS/GaCl₃) as a highly active and reusable heterogeneous Lewis acid effectively activates hexamethyldisilazane (HMDS) for the efficient silylation of alcohols and phenols at room temperature. In this heterogeneous catalytic system, primary, secondary, and tertiary alcohols as well as phenols were converted to their corresponding trimethylsilyl ethers with short reaction times and high yields under mild reaction conditions. The heterogenized catalyst is of high reusability and stability in the silylation reactions and was recovered several times with negligible loss in its activity or a negligible catalyst leaching, and also there is no need for regeneration. It is noteworthy that this method can be used for chemoselective silylation of different alcohols and phenols with high yields.     

Aerobic oxidation of primary alcohols catalyzed by copper complexes of 1,10-phenanthroline-derived ligands

Five copper complexes [(L(1))(2)Cu(H(2)O)](ClO(4))(2) (1), [(L(1))Cu(H(2)O)(3)](ClO(4))(2) (1a), [(L(3))(2)Cu(H(2)O)](ClO(4))(2) (2), [(L(5))(2)Cu(H(2)O)](ClO(4))(2) (3) and [(L(6))(2)Cu](ClO(4)) (4) (where L(1) = 1,10-phenanthroline, L(3) = 1,10-phenanthroline-5,6-dione, L(5) = 1,10-phenanthrolinefuroxan and L(6) = 2,9-dimethyl-1,10-phenanthrolinefuroxan), and in situ prepared copper complexes of 2,9-dimethyl-1,10-phenanthroline (L(2)) or 2,9-dimethyl-1,10-phenanthrolinedione (L(4)) were used for aerial oxidation of primary alcohols to the corresponding aldehydes under ambient conditions. The copper catalysts have been found to catalyze a series of primary alcohols including one secondary alcohol with moderate turnover numbers and selectivity towards primary alcohols. Copper(ii) complexes 1 (or 1a) and 2 were found to be the better catalysts among all other systems explored in this study. A copper(ii)-superoxo species is implicated to initiate the oxidation reaction. Structural and electronic factors of 1,10-phenanthroline-based ligands affecting the catalytic results for aerial oxidation of alcohols are discussed.     

Direct Trifluoromethylation of Alcohols Using a Hypervalent Iodosulfoximine Reagent

The direct trifluoromethylation of a variety of aliphatic alcohols using a hypervalent iodosulfoximine reagent afforded the corresponding ethers in moderate to good yields (14–72 %). Primary, secondary, and even tertiary alcohols, including examples derived from natural products, underwent this transformation in the presence of catalytic amounts of zinc bis(triflimide). Typical reaction conditions involved a neat mixture of 6.0 equivalents of the alcohol with 1.0 equivalent of the reagent, with the majority of reactions complete within 2 h with 2.5 mol % of the Lewis acid catalyst. Furthermore, experimental evidence was provided that the C−O bond-forming process occurred via the coordination of the alcohol to the iodine atom and subsequent reductive elimination.     

12-Tungstophosphoric acid supported on inorganic oxides as heterogeneous and reusable catalysts for the selective preparation of alkoxymethyl ethers and their deprotections under different reaction conditions

A wide variety of primary and secondary alcohols were efficiently converted to their corresponding methoxymethyl (MOM) and ethoxymethyl (EOM) ethers in the presence of catalytic amounts of supported H₃PW₁₂O₄₀ on silica gel and zirconia at room temperature and under microwave irradiation at solvent-free conditions, whereas, phenols and tertiary alcohols remained intact under the same reaction conditions. Deprotection of these ethers to their parent alcohols was also achieved using these heterogeneous catalysts in ethanol, as a green solvent, under reflux conditions and microwave irradiation. Selective deprotection of primary and secondary MOM- and EOM-ethers in the presence of phenolic and tertiary ones, methyl and benzyl ethers, esters and trimethylsilyl ethers was achieved by these reagent systems. The present methodology offers several advantages such as short reaction times, high yields, simple procedure, heterogeneous reaction conditions, selectivity, non-toxicity and reusability of the catalysts.     

Facile coupling of propargylic, allylic and benzylic alcohols with allylsilane and alkynylsilane, and their deoxygenation with Et(3)SiH, catalyzed by Bi(OTf)(3) in [BMIM][BF(4)] ionic liquid (IL), with recycling and reuse of the IL

Allyltrimethylsilane (allyl-TMS) reacts with propargylic alcohols in the presence of 10% Bi(OTf)(3) in [BMIM][BF(4)] solvent to furnish the corresponding 1,5-enynes in respectable isolated yields (87-93%) at room temperature. The utility of Bi(OTf)(3) as a superior catalyst was demonstrated in a survey study on coupling of allyl-TMS with employing several metallic triflates (Bi, Ln, Al, Yb) as well as, B(C(6)F(5))(3), Zn(NTf(2))(2) and Bi(NO(3))(3)·5H(2)O. Coupling of cyclopropyl substituted propargylic alcohol with allyl-TMS gave the skeletally intact 1,5-enyne and a ring opened derivative as a mixture. Coupling of propargylic/allylic alcohol with allyl-TMS resulted in allylation at both benzylic (2 isomers) and propargylic positions, as major and minor products respectively. The scope of this methodology for allylation of a series of allylic and benzylic alcohols was explored. Chemoselective reduction of a host of propargylic, propagylic/allylic, bis-allylic, allylic, and benzylic alcohols with Et(3)SiH was achieved in high yields with short reaction times. The same approach was successfully applied to couple representative propargylic and allylic alcohols with 1-phenyl-2-trimethylsilylacetylene. The recovery and reuse of the ionic liquid (IL) was gauged in a case study with minimal decrease in isolated yields after six cycles.     

Electrophilic chemistry of propargylic alcohols in imidazolium ionic liquids: propargylation of arenes and synthesis of propargylic ethers catalyzed by metallic triflates [Bi(OTf)3, Sc(OTf)3, Yb(OTf)3], TfOH, or B(C6F5)3

Metallic triflates M(OTf)(3) (M = Bi, Sc, Yb), immobilized in imidazolium ionic liquids [BMIM][BF(4)], [BMIM][PF(6)] and [BMIM][OTf] are efficient systems for one-pot reactions of propargylic alcohols 1,3-diphenyl-2-propyn-1-ol Ia, 1-methyl-3-phenyl-2-propyn-1-ol Ib, and 2-pentyn-1-ol Ic, with a wide range of arenes bearing activating substituents, under mild conditions. The [BMIM][PF(6)]/B(C(6)F(5))(3) and [BMIM][PF(6)]/TfOH systems were superior in propargylation with Ib and Ic, while reaction of 3-phenyl-2-propyn-1-ol Id with activated aromatics resulted in the formation of diaryl-propanones instead. Propargylation of anisole with Ib under M(OTf)(3) catalysis is highly para selective, but with TfOH or B(C(6)F(5))(3) as catalyst the ortho isomer was also formed. Steric influence of the propargylic moiety on substrate selectivity is reflected in the lack of ortho propargylation for phenol and ethylbenzene by using propargylic alcohol Ia, and notable formation of the ortho isomer employing alcohol Ib. In the later case para selectivity could be increased by running the reaction at r. t. for 10 h. The Bi(OTf)(3)-catalyzed reaction of 1,3-dimethoxybenzene with Ia led to minor formation of dipropargylated derivative, along with the monopropargyl product. Propargylation of the less reactive arenes (mesitylene, ethylbenzene, toluene), using Sc(OTf)(3) as catalyst, led increasingly to the formation of dipropargylic ethers and propargyl ketones, with no ring propargylation product with toluene. Concomitant formation of dipropargylic ether was also observed in Yb(OTf)(3)-catalyzed propargylation of β-naphthol, whereas propargylation of 2-nitro and 4-nitro-aniline led to N-propargylation. The recycling/reuse of the IL was demonstrated in representative cases with no appreciable decrease in the conversions over 3 cycles. It was also shown that recycled IL could be used to propargylate a different aromatic compound. The efficacy of IL/M(OTf)(3) and IL/TfOH systems for cross-breeding two propargylic alcohols or a propargylic alcohol with a non-propargylic alcohol and/or self-coupling, to form a wide variety of functionalized ethers is also demonstrated.     

Synthesis and reactivity of the phosphinoboranes R2PB(C6F5)2

The phosphinoboranes [R(2)PB(C(6)F(5))(2)](2) (R = Et 1, Ph 2) and R(2)PB(C(6)F(5))(2) (R = tBu 3, Cy 4, Mes 5) were synthesized from the reaction of (C(6)F(5))(2)BCl and the corresponding lithium phosphide. The relationships between B-P distance, P pyramidality, and the extent of BP multiple bonding were further explored computationally. Natural Bond Order (NBO) analyses of 3 and 4 showed that the π-bonding highest occupied molecular orbitals (HOMOs) were highly polarized. In addition the Lewis acid-base adducts, R(2)(H)P·B(H)(C(6)F(5))(2) (R = Et 6; Ph 7; tBu 8; Cy 9; Mes 10) were prepared via the reaction of the phosphines R(2)PH with the borane HB(C(6)F(5))(2). Compounds 1 and 2 showed no signs of reaction with H(2); however, reaction of compounds 3 and 4 with H(2) was observed to give 8 and 9. In a related set of reactions compounds 3 and 4 were reacted with H(3)NBH(3) or Me(2)(H)NBH(3) also led to the generation of 8 and 9, respectively. The reaction profile of the reaction of (CF(3))(2)BPR(2) with H(2) was examined computationally and shown to be exothermic. Efforts to effect the reverse reaction, that is, dehydrogenation of adducts 6-10 were unsuccessful. Compound 4 was also shown to react with 4-tert-butylpyridine to give Cy(2)PB(C(6)F(5))(2)(4-tBuC(5)H(4)N) 11 while reactions of 3 and 4 with the Lewis acid BCl(3) gave the dimers (R(2)PBCl(2))(2) (R = tBu 12, Cy 13) and the byproduct ClB(C(6)F(5))(2).     

Dihydrogen activation with (t)Bu3P/B(C6F5)3: a chemically competent indirect mechanism via in situ-generated p-(t)Bu2P-C6F4-B(C6F5)2

A chemically competent indirect pathway for the activation of dihydrogen by the nonmetal Lewis acid/Lewis base pair (t)Bu(3)P/B(C(6)F(5))(3) is described. The reaction between (t)Bu(3)P and B(C(6)F(5))(3) produces [(t)Bu(3)PH](+)[FB(C(6)F(5))(3)](-) and the known phosphinoborane p-(t)Bu(2)P-C(6)F(4)-B(C(6)F(5))(2) (1-(t)Bu) with elimination of isobutylene. At 1:1 stoichiometry, 1-(t)Bu is produced rapidly in detectable quantities and can act as a catalyst for the formation of [(t)Bu(3)PH](+)[HB(C(6)F(5))(3)](-) from (t)Bu(3)P and B(C(6)F(5))(3) in the presence of H(2). The extent to which this indirect path competes with the direct path is explored.     

A bimetallic catalyst and dual role catalyst: synthesis of N-(alkoxycarbonyl)indoles from 2-(alkynyl)phenylisocyanates

3-allyl-N-(alkoxycarbonyl)indoles are synthesized via the reaction of 2-(alkynyl)phenylisocyanates and allyl carbonates in the presence of Pd(PPh(3))(4) (1 mol %) and CuCl (4 mol %) bimetallic catalyst. It is most probable that Pd(0) acts as a catalyst for the formation of a pi-allylpalladium alkoxide intermediate and Cu(I) behaves as a Lewis acid to activate the isocyanate, and the cyclization step proceeds with a cooperative catalytic activity of Pd and Cu. On the other hand, N-(alkoxycarbonyl)indoles are produced via the reaction of 2-(alkynyl)phenylisocyanates and alcohols under a catalytic amount of Na(2)PdCl(4) (5 mol %) or PtCl(2) (5 mol %). Pd(II) or Pt(II) catalyst exhibits dual roles; it acts as a Lewis acid to accelerate the addition of alcohols to isocyanates and as a typical transition-metal catalyst to activate the alkyne for the subsequent cyclization.     

Allylation of alcohols and carboxylic acids with allyl acetate catalyzed by [Ir(cod)2](+)BF4- complex

A facile method for the synthesis of allyl alkyl ethers from alcohols with allyl acetate was developed by the use of [Ir(cod)(2)](+)BF(4)(-) complex. For instance, the reaction of allyl acetate with n-octyl alcohol in the presence of a catalytic amount of [Ir(cod)(2)](+)BF(4)(-) complex afforded allyl octyl ether in quantitative yield. Allyl carboxylates were also prepared by the exchange reaction between carboxylic acids and allyl acetate in good yields. The [Ir(cod)(2)](+)BF(4)(-) complex catalyzed the reaction of alkyl and aromatic amines with allyl acetate to lead to the corresponding allylamines in fair to good yields.     

Efficient O-trimethylsilylation of alcohols and phenols with trimethylsilyl azide catalyzed by tetrabutylammonium bromide under neat conditions

A very efficient procedure for the trimethylsilylation of a wide variety of alcohols, including primary, allylic, benzylic, secondary, hindered secondary, tertiary, and phenols is reported. The reactions were carried out under neat conditions with trimethylsilyl azide (TMSN(3)) and, when necessary, in the presence of a catalytic amount (20 mol %) of tetrabutylammonium bromide (TBABr) at 30 or 70 degrees C. Under catalytic conditions, the yields of the corresponding trimethylsilyl ethers were greater than 91%. This procedure also allows the selective protection of primary and secondary alcohols in the presence of tertiary ones.     

Studies on Cu(I)-catalyzed synthesis of simple 3-substituted 1,2-allenes and optically active 2-substituted secondary 2,3-allenols

The sequential treatment of terminal alkynes or propargylic alcohols with n-BuLi and MOMCl afforded the corresponding propargylic methyl ethers, which would react with primary alkyl Grignard reagents under the catalysis of CuBr to afford 3-substituted 1,2-allenes or 2-substituted secondary 2,3-allenols, respectively. The reaction may be applied to the synthesis of optically active 2-substituted secondary 2,3-allenols with up to >99% ee without any protection to the free hydroxyl group in the starting 4-hydroxy-2-alkynyl methyl ethers.     

Coordination of Lewis acid to eta(2)-enonepalladium(0) leading to continuous structure variation from eta(2)-olefin type to eta(3)-allyl type.

The reaction of alpha,beta-unsaturated carbonyl compounds, a palladium(0) complex, and Lewis acids led to the formation of a new class of complexes showing a wide variety of structures with eta(2)-type and eta(3)-type coordination of the carbonyl compounds. The reaction of Pd(PhCH=CHCOCH(3))(PPh(3))(2) with BF(3).OEt(2) or B(C(6)F(5))(3) quantitatively gave palladium complexes 1a,b having BX(3)-coordinated eta(2)-enonepalladium structure, as revealed by X-ray structure analysis of the B(C(6)F(5))(3) adduct 1b. On the other hand, the reaction of Pd(PhCH=CHCHO)(PPh(3))(2) with BF(3).OEt(2) or B(C(6)F(5))(3) gave distorted zwitterionic eta(3)-allylpalladium complexes 3a,b, where the Pd-carbonyl carbon distance in 3a (2.413(4) A) is much shorter than that (2.96(1) A) in 1b. The values of the P-P coupling constant and (13)C chemical shift for carbonyl carbon are useful criteria for predicting how the eta(3)-coordination mode contributes to the structure of the enone-palladium-Lewis acid system. Molecular orbital calculations on the series of model complexes suggest that orbital overlap in the highest occupied molecular orbital between the palladium and carbonyl carbon is enlarged by coordination of the Lewis acid to the carbonyl group. Palladium-catalyzed conjugate addition of R-M (R-M = AlMe(3), AlEt(3), ZnEt(2)) and its plausible reaction path are also reported.