Dimerization reactions oxidative addition

These 17-electron systems are excellent nucleophiles and undergo a variety of displacement, dimerization and oxidative addition reactions [Co(PMe3)4] is especially reactive (Scheme 40)435... 

A number of low-valent, diphosphine bridged dimers undergo oxidative addition of conventional substrates, such as the halogens according to reaction 13. 

As far as the reactions with benzyl chlorides are concerned (74), the oxidative addition of benzyl chloride and substituted benzyl chlorides to palladium atoms yields rj -benzylpalladium chloride dimers. The parent compound, bis(l,2,3-7 -benzyl)di-/i,-chloro-palladium(II), quantitatively adds four molecules of PEts by first forcing the rj -benzyl-iy -benzyl transformation, with subsequent breakage of the Pd-Cl bridges to form trans-bistPEtsKbenzyDchloroPddI). The spectral characteristics of the parent molecule are indicative of the allylic type of bonding. Similar i7 -benzyl compounds were formed from 4-methylbenzyl chloride, 2-chloro-l,l,l-trifluoro-2-phenylethane, and 3,4-dimethylbenzyl chloride. 

Substantially more work has been done on reactions of square-planar nickel, palladium, and platinum alkyl and aryl complexes with isocyanides. A communication by Otsuka et al. (108) described the initial work in this area. These workers carried out oxidative addition reactions with Ni(CNBu )4 and with [Pd(CNBu )2] (. In a reaction of the latter compound with methyl iodide the complex, Pd(CNBu )2(CH3)I, stable as a solid but unstable in solution, was obtained. This complex when dissolved in toluene proceeds through an intermediate believed to be dimeric, which then reacts with an additional ligand L (CNBu or PPh3) to give PdL(CNBu )- C(CH3)=NBu I [Eq. (7)]. 

The use of well-defined complexes has been widespread in this reaction, despite intriguing studies by Beller and others that have shown that in situ catalytic systems often give better yields in comparison to isolated carbene-Pd(O) complexes [147-149]. Since the mechanism consists of an oxidative addition on a Pd(0)-monocarbene species, efforts in catalyst synthesis have been directed towards Pd(ll)-monocarbene complexes with other labile groups that can be easily released leading to the formation of Pd(0). This is the case for dimers of the type [Pd( j,-C1)C1(NHC)]2, a family of pre-catalysts effective under aerobic conditions [150], the [Pd(acac)Cl(NHC)] complexes [151] and related palladacycles [152-154],... 

Treatment of a-tocopherol (1) with elemental bromine provided quantitative yields of 5a-bromo-a-tocopherol (46). The reaction was assumed to proceed according to a radical mechanism, but later a nonradical oxidation-addition mechanism was proven (Fig. 6.33). Bromine oxidized a-tocopherol (1) to the intermediate ortho-qainone methide (3), which in turn added the HBr produced in the oxidation step.60 If the HBr was removed by flushing with nitrogen, the spiro dimer (9) became the main product, and if it was purged by HC1 gas, mainly 5a-chloro-a-tocopherol was produced. 

The electrode reaction of an organic substance that does not occur through electrocatalysis begins with the acceptance of a single electron (for reduction) or the loss of an electron (for oxidation). However, the substance need not react in the form predominating in solution, but, for example, in a protonated form. The radical formed can further accept or lose another electron or can react with the solvent, with the base electrolyte (this term is used here rather than the term indifferent electrolyte) or with another molecule of the electroactive substance or a radical product. These processes include substitution, addition, elimination, or dimerization reactions. In the reactions of the intermediates in an anodic process, the reaction partner is usually nucleophilic in nature, while the intermediate in a cathodic process reacts with an electrophilic partner. 

The Ir11 dimer [Ir(oep)]2 (oep = octaethylporphyrin) has been prepared in low yield by photolysis of (oep)IrCH3 in C6D6.473 This preparation has been improved by Chan et al.474, as shown in Reaction Scheme 24, where TEMPO = 2,2,6,6-tetramethyl-l-piperidinyloxy, free radical. The dimer undergoes several organometallic reactions, including oxidative addition of alkyl C 11 bonds and alkene insertions.475... 

While the aforementioned reaction works well for aminopyridines and alkoxypyridines, it is not operative for most electron-deficient pyridines as well as 2- and 4-bromopyridines. One of the possible reasons for its failure with 2-halopyridines is the formation of an unreactive dimer complex from the oxidative addition intermediate [130]. 

Rawal s group developed an intramolecular aryl Heck cyclization method to synthesize benzofurans, indoles, and benzopyrans [83], The rate of cyclization was significantly accelerated in the presence of bases, presumably because the phenolate anion formed under the reaction conditions was much more reactive as a soft nucleophile than phenol. In the presence of a catalytic amount of Herrmann s dimeric palladacyclic catalyst (101) [84], and 3 equivalents of CS2CO3 in DMA, vinyl iodide 100 was transformed into ortho and para benzofuran 102 and 103. In the mechanism proposed by Rawal, oxidative addition of phenolate 104 to Pd(0) is followed by nucleophilic attack of the ambident phenolate anion on o-palladium intermediate 105 to afford aryl-vinyl palladium species 106 after rearomatization of the presumed cyclohexadienone intermediate. Reductive elimination of palladium followed by isomerization of the exocyclic double bond furnishes 102. 

The reaction of a metal-dimer with H2 can also be regarded as an oxidative addition reaction. For instance, a dimer of a d7 metal complex reacts with dihydrogen to give two d6 species. In this process dihydrogen also gives formally two hydride anions. A well-known example in the present context is the conversion of dicobaltoctacarbonyl into hydridocobalttetracarbonyl ... 

Complex 77 has also been reported to catalyze the oxidative dimerization of alcohols to esters when the reactions are performed in the presence of base [76]. The presence of base presumably encourages the reversible attack of the alcohol onto the initially formed aldehyde to give a hemiacetal, which is further oxidized to give the ester product. Alcohols 87 and 15 were converted into esters 88 and 89 with good isolated yields (Scheme 20). Alternative iridium catalysts have been used for related oxidative dimerization reactions, and the addition of base is not always a requirement for the reaction to favor ester formation over aldehyde formation [77, 78]. 

The original work by Monsanto identified [Rh(CO)2l2] as the major Rh spedes under their process conditions and reaction of Mel with this complex as rate controlling for the process [3]. However, the proposed primary product of oxidative addition of Mel to [Rh(CO)2l2r, [RhMe(CO)2l3] , was not observed in early work. Forster [10] studied the reaction of Mel with [Rh(CO)2l2] by IR and found that it gave the acyl complex [Rh(C(0)Me)(C0)l3] , which was also isolated and characterized in the solid state as a dimer. The reaction could be followed quantitatively but the observed rate constant would be antidpated to be a composite of the rate constants for the formation of the intermediate [RhMe(CO)2l3r and its further reaction to [Rh(C(0)Me)(C0)l3] (Eq. (15)) and (Eq. (16)). 

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