Reductive coupling, of aldehydes

The reductive coupling of aldehydes or ketones with 01, -unsaturated carboxylic esters by > 2 mol samarium(II) iodide (J.A. Soderquist, 1991) provides a convenient route to y-lactones (K. Otsubo, 1986). Intramolecular coupling of this type may produce trans-2-hy-droxycycloalkaneacetic esters with high stereoselectivity, if the educt is an ( )-isomer (E.J. Enholm, 1989 A, B). 

Other syntheses of oxiranes which are occasionally useful and involve a species (64) are the dehydration of 1,2-diols (Scheme 69) (78TL5153, 7808(58)12) and the reductive coupling of aldehydes (Scheme 70) (B-73MI50500). 

A number of chiral monodentate phosphines have been examined in asymmetric nickel-catalyzed reductive couplings of aldehydes and alkynes. The best results to date have been obtained with (+)-NMDPP (16) [33]. Aromatic internal alkynes and branched aldehydes participate with excellent enantiose-lectivity (Scheme 15), although yields and enantioselectivities were somewhat lower with other combinations of aldehydes and alkynes. In a complemen-... 

A broad array of mechanistic pathways may be considered in the different variants of nickel-catalyzed reductive couplings of aldehydes and alkynes, and a generalized overview of possible mechanisms has been previously described [10]. Whereas a comprehensive mechanistic study has not been presented, a number of key observations have been illustrated that provide insight into how the nickel-catalyzed reductive couplings of aldehydes and alkynes proceed. It should be stressed at the outset that the different reaction variants may proceed by different mechanisms. 

The nickel-catalyzed reductive coupling of aldehydes and alkynes is only one member of a growing class of processes that involve the three-component addition of two rr-components and a reducing agent (Scheme 24). An... 

A variety of alternate methods for the reductive coupling of aldehydes and alkynes have been developed. A number of important hydrometallative strategies have been developed, although most of these methods require the stoichiometric formation of a vinyl metal species or metallacycle. A very attractive hydrogenative coupling method has recently been developed, and its scope is largely complementary to the nickel-catalyzed methods. A very brief overview of these methods is provided below. 

Riant et al. applied the catalytic system [CuF(Ph3P)3]2MeOH (0.1-1 mol%) and PhSiH3 (140 mol %) in the combination with Ph-TANIAPHOS as ligand to the syn-selective reductive coupling of aldehydes and methyl... 

Montgomery J, Sormunen GJ (2007) Nickel-Catalyzed Reductive Couplings of Aldehydes and Alkynes. 279. 1-23... 

Iridium-catalyzed transfer hydrogenation of aldehyde 73 in the presence of 1,1-dimethylallene promotes tert-prenylation [64] to form the secondary neopentyl alcohol 74. In this process, isopropanol serves as the hydrogen donor, and the isolated iridium complex prepared from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and (S)-SEGPHOS is used as catalyst. Complete levels of catalyst-directed diastereoselectivity are observed. Exposure of neopentyl alcohol 74 to acetic anhydride followed by ozonolysis provides p-acetoxy aldehyde 75. Reductive coupling of aldehyde 75 with allyl acetate under transfer hydrogenation conditions results in the formation of homoallylic alcohol 76. As the stereochemistry of this addition is irrelevant, an achiral iridium complex derived from [Ir(cod)Cl]2, allyl acetate, m-nitrobenzoic acid, and BIPHEP was employed as catalyst (Scheme 5.9). 

Reductive coupling of aldehydes 36 in the presence of triethylsilyl hydride leading to ethers 37 is found to be mediated by EGA Ethers 37 are obtained in a CH Clj— —LiClO4—BU4NCIO4—(Pt) system in 80-90% yield which is comparable to those of the triphenylmethyl perchlorate catalyzed reaction... 

Procter reported a solid-phase variant of Fukuzawa s asymmetric y-butyr-olactone synthesis (see Chapter 5, Section 5.2) that involves the intermolecular reductive coupling of aldehydes and ketones with a,p-unsaturated esters, immobilised using an ephedrine chiral resin 12.44 For example, treatment of acrylate 13 and crotonate 14 with cyclohexanecarboxaldehyde, employing Sml2 in THF, with tert-butanol as a proton source, gave lactones 15 and 16, respectively, in moderate yield and good to high enantiomeric excess (Scheme 7.11).44 The ephedrine resin 12 can be conveniently recovered and recycled.45... 

Isomerization of alkenes is catalyzed by bis(j7 -allyl)(jj -cyclopentadienyl)vanadium(IV). Cyclohexene nndergoes catalytic stereoselective aerobic oxidation with CpV(CO)4. Reductive coupling (see Reductive Coupling) of aldehydes and aldimines is catalyzed by a Cp-substituted vanadium compound in the presence of a chlorosilane and zinc or aluminum. Vanadocene or bis( ] -cyclopentadienyl)dimethylvanadium(IV) catalyzes the synthesis of 1,2-diphenyldisilane and 1,2,3-triphenyltrisilane from phenylsilane. ... 

These reagents (particularly reagent I) effect direct reductive coupling of aldehydes and ketones to olefins. For example, benzaldehyde can be converted into stilbene in 70-76% yield by treatment with excess reagent I. 

Pinacol formation. The reductive coupling of aldehydes by the metal reagent in the presence of MejSiCl is syn-selective. Aliphatic aldehydes give 2,4,5-trisubstituted dioxolanes. ... 

---