Cobalt complexes phosphine oxides

Several reports have appeared on the effect of additives on the Pauson-Khand reaction employing an alkyne-Co2(CO)6 complex. For example, addition of phosphine oxide improves the yields of cyclopentenones 119], while addition of dimethyl sulfoxide accelerates the reaction considerably [20]. Furthermore, it has been reported that the Pauson-Khand reaction proceeds even at room temperature when a tertiary amine M-oxide, such as trimethylamine M-oxide or N-methylmorpholine M-oxide, is added to the alkyne-Co2(CO)6 complex in the presence of alkenes [21]. These results suggest that in the Pauson-Khand reaction generation of coordinatively unsaturated cobalt species by the attack of oxides on the carbonyl ligand of the alkyne-Co2(CO)6 complex [22] is the key step. With this knowledge in mind, we examined further the effect of various other additives on the reaction to obtain information on the mechanism of this rearrangement. 

Table 49 Structures Some Phosphine Oxide Complexes of Cobalt(II)...

The chromium carbonyl linkers 1.40 (98) and 1.41 (99) were prepared from commercial triphenylphospine resin and respectively from pre-formed p-arene chromium carbenes and Fischer chromium amino carbenes. Their SP elaboration is followed by cleavage with pyridine at reflux for 2 h (1.40) and with iodine in DCM for 1 h at rt (1.41) both linkers produce the desired compounds in good yields. A similar cobalt carbonyl linker 1.42 (100) was prepared as a mixmre of mono- (1.42a) and bis- (1.42b) phosphine complex, either from pre-formed alkyne complexes on triphenylphosphine resin or by direct alkyne loading on the bisphosphine cobalt complex traceless cleavage was obtained after SP transformations by aerial oxidation (DCM, O2, hp, 72 h, rt) and modified alkynes were released with good yields and... 

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. 

The two-electron reductions of [Fe(CO)2(NO)(>/3-C3H4R)] (R = Cl, Br, or Me) (248) and [Co(CO)2L(>/3-allyl)] (L = phosphine or phosphite) (249) result in metal-allyl bond cleavage. For the cobalt complexes there is a linear correlation between Elj2 and the half neutralization potential of L. The rhodium complex [Rh P(OMe)3 2(fj3-allyl)] reacts with AgPF6 to give [Rh P(OMe)3 2(f 4-C6Hi0)] in which the two C3 fragments have been oxidatively coupled (250). 

Complexes of cobalt have also been used as catalysts for the oxidation of tertiary phosphines with molecular oxygen. In methanol solvent the compound Co2(CN)4(PMe2Ph)502 reacts with PMe2Ph, converting it into the oxide.Since the oxygen compound is readily formed again from the product complex Co(CN)2(PMe2Ph)3, a catalytic cycle can be obtained (28, 29) for the phosphine oxidation. 

The mechanisms of the oxidation of phosphines and arsines by chromium(VI) have been examined both in solution and on a diatomite support. Kinetic parameters are presented for both supported and solution reactions. A ruthenium complex of 1,4,8,1 l-tetramethyl-l,4,8,ll-tetraazacyclotetradecane has been utilized to oxidize triphenylphosphine in acetonitrile. Although a limited temperature range was utilized, a AH value of 8.7 0.8 kcal mor and a A5 value of -20 2 cal K mor were calculated. The secondary phosphine oxides, HP(0)R (R = n-butyl, isobutyl, cyclohexyl) and 9H-9-phosphabicyclononane-9-oxide, react with cobaltocene to yield dihydrogen and cobalt(I) compounds. With the less bulky phosphorus ligands at elevated temperatures trinuclear cobalt(III, II) complexes may be obtained. Arsenious acid may be utilized to catalyze the oxygen atom... 

Certain cobalt complexes which form binuclear dioxygen adducts (6) are also known to oxidize phosphine ligands 34, 35). However,... 

Cobalt complexes have been used to catalytically oxidize triphenylphosphine [126-128]. Schmidt and Yoke [126] have found that the sole product of the slow but quantitative oxidation of [CoCl2(PEt3)2] in several organic solvents is the phosphine oxide complex, equation (79). 

Although this system appears to possess all of the attributes necessary for the catalytic oxidation of phosphine to phosphine oxide, the catalytic activity of these cobalt complexes was not reported. 

The transition metal-catalyzed [2+2+2] cycloaddition has been applied to the atroposelective biaryl synthesis. The first example is the chiral cobalt(I) complex-catalyzed [2+2+2] cycloaddition of aryl-diynes with nitriles to produce axially chiral arylpyridines [19a]. Subsequently, the enantioselective synthesis of axially chiral biaryl phosphine oxides has also been achieved (Scheme 21.16) [19b]. 

In their earlier studies, Halpern and coworkers [171] investigated the oxidation mechanism of phosphines with different metal complexes. Thus, in the cobalt(ll)-mediated reaction, in a first step an oxygen-phosphine exchange was assumed, which led to an 02-bridged binuclear Co complex (Scheme 2.51). In a second step, external phosphine was converted into the corresponding phosphine oxide. 

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