Coupling, pinacol reactions

The use of reducing metals nowadays is mainly restricted to acyloin and pinacol coupling reactions (see p. 53f.) and Birch reductions of arenes (A.A. Akhrcm, 1972 see p. 103f.) and activated C—C multiple bonds (see p. 103f.). 

Scheme 5 Catalytic pinacol coupling reaction using [(t-BuO)3pe]K 26 [7]...

Mn. Manganese is also effective for mediating aqueous carbonyl ally-lations and pinacol-coupling reactions. Manganese offers a higher reactivity and complete chemoselectivity toward allylation of aromatic aldehydes.178... 

Catalytic Reductive Coupling of Carbonyl Compounds -The Pinacol Coupling Reaction and Beyond... 

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

It is important to select stoichiometric co-reductants or co-oxidants for the reversible cycle of a catalyst. A metallic co-reductant is ultimately converted to the corresponding metal salt in a higher oxidation state, which may work as a Lewis acid. Taking these interactions into account, the requisite catalytic system can be attained through multi-component interactions. Stereoselectivity should also be controlled, from synthetic points of view. The stereoselective and/or stereospecific transformations depend on the intermediary structure. The potential interaction and structural control permit efficient and selective methods in synthetic radical reactions. This chapter describes the construction of the catalytic system for one-electron reduction reactions represented by the pinacol coupling reaction. 

Hirao T (2007) Catalytic Reductive Coupling of Carbonyl Compounds - The Pinacol Coupling Reaction and Beyond. 279 53-75 Hirayama N, see Sakai K (2007) 269 233-271 Hirst AR, Smith DK (2005) Dendritic Gelators. 256 237-273 Holzwarth AR, see Balaban TS (2005) 258 1-38... 

Titanium-mediated pinacol coupling reactions have been reviewed until 2000.80 81 Since then, various intermole-cular pinacol couplings have been reported with aldehydes, - ketones, a-ketoesters, and imines, as well as asymmetric versions thereof.101-104 Scheme 29 shows one example of an asymmetric pinacol coupling of aromatic aldehydes, promoted and catalyzed by the new chiral titanium complex (A)-75, that has been developed by Riant and co-workers.101 Yields for pinacol products 76 are generally high. Under catalytic conditions, ee is moderate (up to 63%), while stoichiometric conditions allow to obtain up to 91% ee. 

Titanium(iv) iodide" " or a combination of a titanium(iv) salt and an iodide source promotes pinacol coupling reactions of aromatic aldehydes. The combination of the reagents is considered to generate titanium(m) species along with U. 

In several pinacol coupling reactions, the titanium(ll) species is assumed to be generated by treatment of TiCU with hexamethyldisilane. " " However, the original procedure " was corrected in 1998 to show that the titanium species generated by this method was TiCls. ... 

Gansauer, and a titanocene complex, TiCl3(THF)3, is also applicable as disclosed by Nelson and co-workers Catalytic pinacol coupling reactions occur with TiCU and zinc (or aluminum) in the presence of an acylating reagent such as acetic anhydride or acetyl chloride. ... 

Pinacol coupling reactions of aromatic aldehydes with commercially available TiCU /> a mixed solvent of THE and dichloromethane show high 47-selectivities." Also, high 47-selectivities are observed with TiCU-BuUTe in DME and TiCU-Bu"Li in Et20 at -50 °C. (see Table 1). 

Table 1 Pinacol coupling reactions of benzaldehyde promoted by titanium reagents... 

For pinacol coupling reactions with a catalytic amount of Cp2TiCl2 and reductants such as zinc and manganese, high, V7-selectivities are observed frequently. The mechanism for the stereoselectivities has been explained by a... 

Asymmetric pinacol coupling reactions with a stoichiometric amount of titanium salts have been observed using a combination of TiGL and chiral amines such as A,A,A,A-tetramethyl-l,2-cyclohexanediamine and 2-methoxy-methylpyrrolidine. " " In these reports, the titanium salt is considered to be TiGl2- However, the titanium salt prepared by reduction of TiGU with McsSiSiMcs has proved to be TiGL. The enantioselectivities of the reactions using the chiral amines are modest (Table 2). 

Addition of LiBr or LiCl to a solution of Sml2 in THF causes a color change from blue to purple. Oxidation potential of Sml2 in THF changes from —1.33 V to —1.98 0.01 V upon addition of I2 or LiBr (more than 1 equiv.), or to —2.11 0.01 V by addition of 12 or more equiv. of LiCl. In the presence of 4-12 equiv. of the bromide or chloride salt, the pinacol coupling reaction of cyclohexanone is accelerated. These salts should be dried before use otherwise, simple reduction to cyclohexanol occurs. The co-existing lithium cation can also act as a Lewis acid to activate the carbonyl group by coordination. ... 

Some additives accelerate the pinacol coupling reactions. Addition of Me3SiCl to Sml2 also accelerates the pinacol coupling reactions of aliphatic ketones and aldehydes. Pinacol coupling reactions are also promoted with samarium metal and a Lewis acid such as Et2AlCl or MesSiCl. Coordination of such a Lewis acid to a carbonyl oxygen facilitates the one-electron reduction by samarium. 

Diastereomeric ratios of pinacol dimers obtained by coupling of benzaldehyde and acetophenone with samarium reagents are summarized in Tables 3 and 4, respectively. In general, the stereochemical control of pinacol coupling reactions with a stoichiometric or catalytic samarium(ll) reagent is poor, giving nearly a 1 1 dljmeso mixture. 

The pinacol coupling reactions of keto amides with samarium(ll) can be extended to an asymmetric version by substrate control. Treatment of asymmetric keto amide 13 with samarium(ll) iodide, HMPA, and /-butyl alcohol in THF gives one of the r//-diastereomers in 79% yield exclusively (Equation (37)). ... 

Although yields and diastereoselectivities are moderate, a cross-pinacol coupling reaction between a 1,2-diketone and an aldehyde is accomplished with Sml2 in the presence of HMPA. ... 

Low-valent niobium and tantalum are also effective for homocoupling of aldehydes leading to 1,2-diols. " Not only commercially available NbClglDME) but also a combination of NbCls and zinc can be used for the pinacol coupling. Reactions in a mixed solvent of 1,4-dioxane and toluene (1 4) give better diastereoselectivity than those in DME or THE (Equation (52)). ... 

Robertson, G. M. Pinacol Coupling Reactions. In Comprehensive Organic Synthesis-, Trost, B. M., Fleming, I., Eds. Pergamon Oxford, 1991 ... 

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