Reduction Reductive coupling

Huang et al. recently developed a one-pot protocol for the sequential reduction/reductive-coupling of cyclic and acyclic Al-acylcarbamates with activated alkenes. Initial studies focussed on the coupling of simple amides with electron-deficient alkenes to give a-substituted amines in... 

When pyridine is treated with zinc dust and acetic anhydride, a type of reductive coupling occurs and the product is diacetyltetrahydrodipyridyl (I) this undergoes a curious change on heating yielding pyridine and a new diacetyl compound, 1 4 diacetyl 1 4-dihydropyridine (II). The latter is reduced by zinc and acetic acid to 4-ethylpyridine (III). 

Reductive coupling of carbonyl compounds to yield olefins is achieved with titanium (0), which is freshly prepared by reduction of titanium(III) salts with LiAIH4 or with potassium. The removal of two carbonyl oxygen atoms is driven by T1O2 formation- Yields are often excellent even with sensitive or highly hindered olefins. (J.E. McMurry, 1974, 1976A,B). 

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). 

Organotelluriumfll and IV) compounds undergo transmetallation with Pd(II)[414], The carbonylation of the alkenylphenyltellurium(II) 459 gives the a,/3-Unsaturated ester 460 and benzoate, 460 being the main product[415], Reductive coupling of diaryl, dialkyl, and aryl alkyltellurides 461 to give 462 proceeds by treatment with Pd(OAc)2[416,417],... 

The reductive coupling of the 7r-allylpalladium enolates 679 gives the allylated ketones. This reaction is also possible thermally, as the Carroll reaction, which... 

Category Ih cyclizations effect closure of the C2 C3 bond. Scheme 3.1 depicts retrosynthetic transformations corresponding to syntheses in category lb. Included are three variations of the intramolecular aldol condensation and reductive coupling of o,/V-diacylanilines,... 

Divalent molybdenum compounds occur in mononuclear, dinuclear, and hexanuclear forms. Selected examples are shown in Figure 6. The mononuclear compounds are mostiy in the realm of organometaUic chemistry (30—32). Seven-coordinate complexes are common and include MoX2(CO)2(PR3)2, where X = Cl, Br, and I, and R = alkyl MoCl2(P(CH3)3)4, heptakis(isonitrile) complexes of the form Mo(CNR) 2 (Fig. 6d), and their chloro-substituted derivatives, eg, Mo(CNR)3CR. The latter undergo reductive coupling to form C—C bonds in the molybdenum coordination sphere (33). 

Compounds with Tin—Tin Bonds. The most important class of catenated tin compounds is the hexaorganoditins. The ditin compounds are usually prepared by reductive coupling of a triorganotin haUde with sodium in Hquid ammonia ... 

Among the appHcations of lower valent titanium, the McMurry reaction, which involves the reductive coupling of carbonyl compounds to produce alkenes, is the most weU known. An excellent review of lower valent titanium reactions is available (195). Titanium(II)-based technology is less well known. A titanium(II)-based complex has been used to mediate a stetio- and regio-specific reduction of isolated conjugated triple bonds to the corresponding polyenes (196). 

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). 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

The McMurry reagent reductively couples trifluoroacetophenones to the eorresponding stilbenes [67] (equation 55), and cross-coupling of ketones is also reported [68] (equation 56). 

Reductive Coupling of 4-Methoxy-a,ct,OrTnfIuoroacetophenone cis-and trans-l,1,1,4,4,4-Hexafluoro-2,3-bis(4-methoxyphenyl)-2-butene [67 ... 

Reductive coupling reaction of fluonnated vinyl iodides or bromides has been used as a route to fluorinated dienes [246, 247, 248, 249, 250. Generally, the vinyl iodide is heated with copper metal in DMSO or DMF no 1 ntermediate perfluorovmy I-copper reagent is detected. Typical examples are shown m equations 163-165 [246, 247, 249. The X-ray crystal structure of perfluorotetracyclobutacyclooctatetraene, prepared via coupling of tetrafluoro-l,2-diiodocyclobutene with copper, is planar... 

Reaction with alkali metals in liquid NH3 leads to reductive coupling to give colourless crystals of... 

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