Cross-McMurry coupling

Fig. 14.50. Crossed McMurry reactions ketone/ester coupling (top) and ketone/amide coupling (bottom).

Titanium A reductive cross-coupling of acrylonitriles CH2=CHCN with cyclo-hexenone and other enones, which connects the jS-carbons of both substrates, can be catalysed by Ti(III), generated from Cp2TiCl2 by reduction with zinc (in reminiscence of McMurry coupling). A plausible mechanism, involving a redox umpolung of the enone, has been proposed. " ... 

Annulenoid TTF of 53, 54 were reported by Markl and coworkers [56]. Two terminal DT rings fix the central annulene rings, whose electronic structures can be tuned by redox reactions of its DT rings via the cross-conjugated system (Scheme 8.7). Cyclization reactions by titanium-catalyzed McMurry coupling of the aldehyde precursors leads to a mixture of several stereoisomers. 

If both carbonyl substrates have sufficiently different redox potentials, a reasonable yield of crossed product is obtained without resorting to using one substrate in excess. This result was unexpected given the relative ease that a bisaromatic ketone should undergo ketyl formation followed by self pinacol coupling and was one of the first results that indicated more than one mechanism may be operative in McMurry couplings. In this case, the diaryl ketone may be reduced selectively to a dianion, which then undergoes nucleophilic addition to the saturated ketone. 

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

It is difficult to obtain cross-coupling products of two different carbonyl compounds by an intermolecular version of the McMurry reaction. Examples that use excess amounts of one carbonyl component are few. "" When one carbonyl component is replaced by a 1,1-dihalo compound or dithioacetal and the alternative is reduced with a low-valent metal such as low-valent titanium or chromium(ii), cross-coupling products, that is, Wittig-type olefins, are produced in high yields. Because the alternative approach is described elsewhere, we concentrate on only its important features here. 

McMurry enol triflate/cuprate cross coupling... 

The McMurry-Scott cross coupling reaction22 between enol triflates and organocuprates generally proceeds with retention of the configuration in the parent enol triflate component. In these reactions it is widely assumed that a planar alkenylcopper(III) intermediate is formed by oxidative... 

A two-step synthesis of cicerfuran was achieved using McMurry cross coupling of 4-benzyloxy-2-hydroxy-benzoaldehyde with 6-methoxybenzo 1 3]-dioxole-5-carbaldehyde, followed by an oxidative cyclization <07JOC10283>. 

The main advantage of this reaction over the McMurry reaction (see section on McMurry alkenation ) is the notion that the reaction can take place with two different ketones. In this regard the diazo-thioketone coupling is a cross-coupling rather than a homocoupling. 

The mixed coupling of aliphatic compounds is also well established. Through the use of an excess (usually 10-fold) of one of the reactants cross-coupling of aliphatic carbonyls is highly effective. This protocol was first developed by (Torey and then applied to reductive pinacolic couplings by McMurry. Li et al. applied this method to the synthesis of the alkaloid isoharringtonine (83 equation... 

Scheme 2 Synthesis of the cyclopentadienyirhenium tricarbonyl derivatives of OH-tamoxifen 2a-e by McMurry cross-coupling.

Some recent highlights include adaptation of Katritzky s benzo-triazole syntheses to ether preparation , generation of acyl radicals by carbonylation , and oxidative ring closures with Fe(C104)3 (as an alternative to the harsher Mn(III) route) . Bis(phosphonium) anhydrides are finding diverse applications, and a number of new vanadium reagents have materialized, e.g. a V(II) complex dimer for cross-coupling of aldehydes as an alternative to the McMurry route . ... 

Arylbenzo[b]furans (20) are obtained by iodine-mediated cyclization of (o-hydroxy)stilbenes (17) prepared by selective McMurry cross-coupling of salicylic aldehyde and aryl aldehydes [47] ... 

McMurry cross-coupling reactions have also been found to be useful for the synthesis of the unsymmetrically substituted bisporphyrin 68 and the porphyrin-chlorin heterodimer 69, which were isolated from mixtures of the corresponding homodimers [55] (Figure 6.14). 

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