Phosphine-substituted dimers

Spectral characterization of the phosphine-substituted rhenium dimers included electronic spectra and infrared data (124). Detailed assignments of electronic absorption bands were not proposed by the authors, but characteristic structure-spectra features were identified. For the Re2+ unit the lowest energy absorption was in the range of 695- 760 nm, similar to results in previous Ref+... 

Practical examples include nitration of aromatics, olefin hydroformylation with cobalt hydrocarbonyls and phosphine-substituted hydrocarbonyls as catalysts, and ethyne dimerization. 

Thus, a careful analysis of the rate constants for back electron transfer, hgand substitution, and dimerization leads to the conclusion that ligand exchange in the 17-electron radical (/cgub in Eq. 44) lowers the rate of back electron transfer from the acceptor radical (A ) (/c et in Eq. 43) to such an extent that dimerizations (and other possible follow-up reactions [118]) now become competitive and effect permanent photochemical transformations. The decrease of the back electron transfer rates is due to the attenuated reduction potentials of the phosphine-substituted radicals [176]. 

Catalysts which have been found to promote dimerization of phenyl isocyanate include pyridine (11), methylpyridine (12), triethylamine (13), X-methyl- (or ethyl-)morpholine, triethylphosphine, and other alkyl or alkyl-arylphosphines (14, 15). Alkylphosphines bring about a very violent polymerization since they act as active catalysts and the polymerization is quite exothermic. Triphenylphosphine is inactive. Alkyl-arylphosphines are not as active as alkylphosphines and permit better control of the reaction. Another convenient method (14, 16) for control of phosphine-catalyzed dimerization involves the addition of an alkylating agent such as benzyl chloride in an amount stoichioraetrically equivalent to the substituted phosphine present. Complete deactivation of the catalyst results. By this means the reaction may be mitigated or even quenched and then activated by the addition of more catalyst. 

An active area of metal carbonyl chemistry involves replacement of CO with group VB donor ligands, especially phosphites and phosphines. The direct replacement of CO by the ligand is the simplest means of preparation of the substituted dimers ... 

The catalysts found active in the norbornadiene dimerization are various nickel tt complexes, among them Ni(CO)4, Ni(CH2=CHCN)2, Ni(0) complexes of 1,1-dicyanoethylenes, and the corresponding phosphine substitution or addition compounds (3, 32, 33, 47). Other active catalysts are Co2(CO)8, Fe2(CO)9, Co2(CO)g 2P(CgH5)3, Co(CO)3NO, and Fe(CO)2(NO)2 (3, 4, 32, 33, 44, 48). Efficient catalysts were also obtained by the in situ reduction of Ni, Co, or Fe chelates with organo-aluminum compounds (46, 49). Finally, dimerization of norbornadiene could also be effected with Rh on carbon (45). The nickel catalysts... 

The dimeric tertiary-phosphine substituted compounds undergo a Co—Co bond cleavage and Na/Hg reduction in tetrahydrofuran ... 

Later work would prove this observation correct. Ligand substitution by PPh3 was originally proposed to yield a phosphine-substituted metal-metal-bonded dimer. 

The reaction of metal carbonyl dimers with silicon hydrides also probably involves an initial oxidative addition step. Chiral silyl-cobalt and silyl-manganese carbonyl complexes have been obtained through the reaction of optically active organosilicon hydrides with metal carbonyls65 68 (equation 15 and 16). Phosphine-substituted cobalt complexes were similarly obtained by reaction of a chiral hydrosilane with Co2(CO)6L2 [L = PPh3, P(OPh)3, P(c-C6Hn)3]69. 

Organometallic compounds with a 17-electron configuration are often labile toward associative ligand exchange. Radical chain mechanisms are well established for phosphine substitution on metal carbonyl hydrides (Scheme 23), the 17-electron chain carrier being in most cases non hydridic. This mechanism, however, was also shown to operate for OsH2(CO)4 via the 17-electron hydride complex OsH(CO)4 [137]. Thus, phosphine addition to the radical prevails over the dimerization, which indeed occurs in the absence of phosphine [33] (section 6.5.7), and over other possible decomposition pathways. The second step of the chain propagation process in Scheme 23, for this osmium system, is another example of atom transfer to a hydride radical (section 6.5.6). 

Jenner " has reported phosphine-catalyzed dimerization of methyl acrylate and acrylonitrile under MBH conditions at ambient pressure (Scheme 2.96). He also noticed that the p-substituted derivatives generally require high pressures. However, cyclohex-2-en-l-one underwent dimerization smoothly under these conditions with 100% yield (Scheme 2.97). 

The major species in the catalytic system with added phosphine is different from that in the system lacking added phosphine. Infrared spectroscopic analyses under steady-state conditions for hydroformylation catalyzed by the phosphine-modified complexes provide no evidence for the accumulation of an acylcobalt complex only phosphine-substituted cobalt carbonyl dimers and hydride complexes are observed.- " ... 

Phosphine substitution on the phosphonium carbene was found to affect the initiation rate. Phosphine bulk helps stabilize the carbene complexes with respect to decomposition and kinetic deactivation by dimerization pathways [41]. All the complexes were synthesized through the trichloride intermediate 29 to prevent the decomposition of the complexes bearing the less bulky phosphine groups. The active, 14-electron complexes were then generated via the addition of B(C5F5)3 to abstract the chloride Hgand (Scheme 9.3). In solution, precatalysts bearing bulky phosphines were all monomeric, while the mixed phosphine cases tended to reversibly dimerize in solution. An illustrative dimerization is shown for catalyst 30, which possesses intermediate steric bulk at the phosphonium moiety. 

An extensive study on the triruthenium chemistry of 2-amino-6-methylpyridine (Hampy) has been performed by Gabeza, Riera, and co-workers. The unsubstituted complex (/U-H)Ru3(GO)9( 3"Hampy) undergoes phosphine substitution, protonation, and methoxidation reactions.The complex Ru3(GO)8(M3"Hampy)(PhG=GHPh) 93 catalyzes the hydrogenation of diphenylacetylene and, in the presence of hydrogen, also forms a dimer 94 linked through... 

Formic acid behaves differently. The expected octadienyl formate is not formed. The reaction of butadiene carried out in formic acid and triethylamine affords 1,7-octadiene (41) as the major product and 1,6-octadiene as a minor product[41-43], Formic acid is a hydride source. It is known that the Pd hydride formed from palladium formate attacks the substituted side of tt-allylpalladium to form the terminal alkene[44] (see Section 2.8). The reductive dimerization of isoprene in formic acid in the presence of Et3N using tri(i)-tolyl)phosphine at room temperature afforded a mixture of dimers in 87% yield, which contained 71% of the head-to-tail dimers 42a and 42b. The mixture was treated with concentrated HCl to give an easily separable chloro derivative 43. By this means, a- and d-citronellol (44 and 45) were pre-pared[45]. 

When a bidentate phosphine is used as a ligand for the reaction of J-keto esters or /i-diketones, no dimerization takes place. Only a 2-butenyl group is introduced to give 68[49,62], Substituted dienes such as isoprene, 1,3-cyclohexa-diene, and ocimene react with carbon nucleophiles to give a mixture of possible regio- and stereoisomers of 1 1 adducts when dppp is used as a ligand[63,64]. 

A mixture of (C H ) , TiCl, and AlCl is useful for polymerizing C —olefins (85). The dimerization of propylene is accompHshed by using catalysts such as Ni(PR2)4 (86). Alkylphosphines such as / fZ-butylphosphine [2501-94-2] have been proposed as a substitute for high purity phosphine in the production of the semiconductor gallium phosphide (87). 

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