Sodium amalgam reductive dimerization

The first reported synthesis (9,76) (78%) of Cp2V2(CO)5 (7) involved protonation of the dianion [CpV(CO)3]2- made by sodium amalgam reduction of CpV(CO)4. No vanadium hydride is isolated in this reaction. A slightly better yield (89%) results from the photolysis of CpV(CO)4 in THF using a falling-film photoreactor (77). The product 7 is produced via dimerization of an intermediate solvent complex CpV(CO)3(THF) followed by a further carbonyl loss (35). The corresponding Cp complex 7 is also available by an exactly analogous photolysis of Cp V(CO)4 or by protonation of [Cp V(CO)3]2- (JO). 

The synthetically important dimeric anions [ Co( i.-CO)(i7-C5H4R) 2] [47] (R = H, SiMea, or SiMePh2) are prepared by sodium amalgam reduction of the appropriate monomer [Co(CO)2(tj-C5H4R)] (207, 208). The ESR spectra of the monoanions showed equal hyperfine coupling to... 

The first sodium amalgam-induced dimerization of acrylonitrile to adiponitrile [29] was performed in aqueous solution, and the yield was discouragingly low (5%). Mainly, a base-induced addition of water resulting in p, -bis(cyanoethyl) ether occurred together with a reduction to propionitrile (PN). 

It is a volatile orange-red crystalline solid (m.p. 30°C), stable to over 100°C. On reduction with tertiary phosphines or sodium amalgam, Os(NBu )3 is formed, which is dimeric (ButN)2Os(/x-NBut)2Os(NBu )2. This can be oxidized to the osmium(VII) dication with concomitant shortening in the Os-Os distance from 3.1 to 2.68 A. 

Dihydro-1-methyIquinoline can be prepared by brief treatment of 1-methyl-quinolinium salts with sodium amalgam in water under alkaline conditions [87]. Reduction of quinolimum salts in acid solution gives dimeric compounds with structures analogous to 22, but with N-alkyl substituents [83]. 

Weber and coworkers reported the synthesis of novel substituted borazoles. The reaction of boron trichloride with 1,2-diimines 367 provide a mixture of products. The formation of dimeric borazole 369 was explained based on a nucleophilic attack of the chloroborane 370 on the ate complex 368 followed by dehydrohalogenation. Alternatively, a controlled addition of BCR to the diimine 367 results in a clean formation of the imine-borane complex 373 that upon reduction with sodium amalgam and calcium hydride provided the chloroborazole 375 (Scheme 59) <2006EJI5048>. 

A compound similar to (XIII) has also been prepared from the pyrocatechol compound Ti(02C6H4)Cl2 by reduction with sodium amalgam in THF, and has been formulated to be similar to (XIII) with the two phenoxide groups at each titanium being replaced by the pyrocatecholate dianion. The ESR signal from both these complexes suggests the presence of one unpaired electron per dimer (50). 

The reduction of l-methyl-2-carbomethoxypyridinium iodide (2, I ) with sodium amalgam leads to the pyridinyl radical dimer (2-2) (see sect. 4.1). The dimer is in equilibrium with the monomer (2 ) (K jssoc ca- 1.3-2.0 x 10 M by titration or EPR measurements) Dissociation of the dimer yields 2 , isolated via low temperature distillation (ca. 40-50 °C) with condensation at 77 K (Eq. 5). Dimerization occurs on warming the condensate above ca. 120 K. The spectroscopic properties of the radical 2" are determined with a new thin film spectroKopy apparatus, a device which dramatically simplifies experiments on stable radicals (S rt. 3.1.1). 

Before the introduction of metal-ammonia solutions for the reduction of a,p-unsaturated carbonyl compounds,sodium, sodium amalgam, or zinc in protic media were most commonly employed for this purpose. Some early examples of their use include the conversion of carvone to dihydrocarvone with zinc in acid or alkaline medium, and of cholest-4-en-3-one to cholestanone with sodium in alcohol. These earlier methods are complicated by a variety of side reactions, such as over-reduction, dimerization, skeletal rearrangements, acid- or base-catalyzed isomerizations and aldol condensations, most of which can be significantly minimized by metal-ammonia reduction. 

Methyl 2-naphthoate is reduced [86] in the presence of boric acid in methanolic solution at a mercury cathode to form 2-naphthalenemethanol in 19% yield, a mixture of diastereomeric pinacols from further reduction of the intermediate aldehyde in 19% yield, and a mixture of methyl 1,2- and l,4-dihydro-2-naphthoates in 42% yield. Methyl 1-naphthoate under the same conditions [87] gives methyl-1,4-dihydro-1-naphthoate (43%), a 4,4 -dimer of the 1,4-dihydro derivative (20%), di-l-naphthylglyoxal (5%), and 1-naphthalenemethanol (5%). Because mercury cathodes are used with sodium salt electrolytes, it is possible that electrogenerated sodium amalgam is the reducing agent. 

The generation of the precursors for cyclopentadienyl-silanol-functionalized iron complexes involves the formation of the corresponding iron anion in a first step [6]. 2a is obtained by reductive cleavage of the methoxysilyl-cyclopentadienyl-substituted iron dimer 1 with sodium amalgam in THF (Scheme 1). This reaction is restricted to alkoxysilyl-cyclopentadienyl-fiinctionalized iron anions because of the limited access to the corresponding Si-H-functionalized iron dimers. 

---