Reductive homocoupling

Homocoupling, reduction or aromatization of the vinyl triflate are often competing side-reactions that need to be suppressed. Recently, Larhed and coworkers addressed these issues using microwave heating and efficient fiuorous techniques to enable rapid synthesis... 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

An approach to the preparation of asymmetrically 1,2-disubstituted 1,2-diamines has been reported the zinc-copper-promoted reductive coupling of two different N-(4-substituted)phenyl aromatic imines, one bearing a 4-methoxy and the other a 4-chloro substituent, in the presence of either boron trifluoride or methyltrichlorosilane, gave a mixture of the three possible 1,2-diamines, where the mixed one predominated [31 ]. Low degrees of asymmetric induction were observed using 1-phenylethylamine, phenylglycinol and its 0-methyl ether, and several a-amino acid esters as the chiral auxiharies meanwhile the homocoupling process was not avoided (M.Shimizu, personal communication). 

The homocoupling of aryl halides and triflates can be made catalytic in nickel by using zinc as a reductant for in situ regeneration of the active Ni(0) species. 

The reaction is considered to proceed via a silyl anion mechanism, although the possibility of a radical-based mechanism has also been discussed.115,125 In order to clarify the mechanism, coupling experiments on a 1 1 mixture of chlorotrimethylsilane, 27 (reduction potential <—3.0 V),126 and chlorotriphenylsilane, 28 (reduction potential vs. standard calomel electrode (SCE) < —3.0 V),120 were performed, in which the mixed coupling product 1,1,1-trimethyl-2,2,2-triphenyldisilane, 29, and the homocoupling product hexaphenyldisilane, 30, only, were found,125 as indicated in Scheme 15. 

In the presence of Pd(PPh3)4, reductive homocoupling of 3-silylpropargyl carbonate 50 proceeded to give a mixture of allenenyne 51 and diyne 52 [65], The highest allene selectivity (51 52 = 95 5) was achieved for the reaction of 50 with R = SiiPr3 and R = Et (Scheme 3.29). 

Regioselective syntheses of 1,3,5-unsymmetrically substituted benzenes (309) are catalyzed by Pd(dba)2/PPh3 mixed alkyne/diyne reactants give mixtures containing homocoupled and mixed products (24 21 from HC CPh + HC=CC= CC Hn). The probable mechanism involves oxidative addition to the Pd(0) center, insertion of the second diyne into the Pd—H bond, reductive coupling and subsequent jr-complexation of this product to Pd(0), followed by Diels-Alder cycloaddition of the third diyne and elimination of product. 

Treatment of an iV-(iV, A -dialkylaminoalkyl)benzotriazole, derived from an aldehyde and a secondary amine, with Sml2 affords a vicinal diamine via generation of the corresponding imine, one-electron reduction of the imine, and homocoupling (Equation (62)). ... 

Low-valent palladium complex, for example, Pd(0), is readily prepared in situ by reaction of PdCl2(PPh3)2/Zn. This is shown to induce the similar homocoupling polycondensation of metaphenylene 183 at 95 °C in DMF. Fluorinated alkyl esters 180 polymerize smoothly in supercritical CO2 1,4-dichlorophenylene 184 having a keto group also reductively polymerized similarly. 

The slow addition of the arylzinc compound to a solution containing both an aryl hahde, Ar X, and the palladium complex furnished the biaryl in good to excellent yields. The very reactive Pd(0) complex was likely formed in situ by reduction of the starting palladium(II) complex by ArZnX. The reactions were very rapid (ca 1 or 2 h) compared to most usual Pd-catalyzed reactions involving ArZnX (ca 24 h). The reduction of Pd(II) could account for the formation of a small amount of Ar-Ar (2-5%) in the last non-electrochemical step while no homocoupling of Ar X occurred. 

Diaryl ketones.2 In the presence of this complex (1 equivalent) S-(2-pyridyl) aryl thioates (1 equivalent), prepared from aryl carboxylic acids, undergo reductive homocoupling to diaryl ketones (equation I). 

Reduction of diaryl thioketones with ytterbium metal affords a mixture of the corresponding thiols and diarylmethanes together with products arising from homocoupling reactions (equation 119)433. 

Scheme 8.10. Possible mechanisms of the Pd-catalyzed reduction and homocoupling of organic halides. X= halide, triflate Y = R2(T, NHR, O. ...

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