Hydroxymethyl complexes

Pd, or Ni (Scheme 5-3). First, P-H oxidative addition of PH3 or hydroxymethyl-substituted derivatives gives a phosphido hydride complex. P-C bond formation was then suggested to occur in two possible pathways. In one, formaldehyde insertion into the M-H bond gives a hydroxymethyl complex, which undergoes P-C reductive elimination to give the product. Alternatively, nucleophilic attack of the phosphido group on formaldehyde gives a zwitterionic species, followed by proton transfer to form the O-H bond [7]. 

This was the first example in which models for presumed Fischer-Tropsch intermediates have been isolated and their sequential reduction demonstrated. Neither methane nor methanol was observed from further reduction of the methyl and the hydroxymethyl complexes. The use of THF/H20 as solvent was crucial in this sytem in THF alone CpRe(C0)(N0)CH3 was the only species observed, probably because the initial formyl complex was further reduced by BH3.— When multihydridic reagents are reacted with metal carbonyl complexes, formyl species are usually not observed. The rapid hydrolysis of BH3 by aqueous THF allowed NaBH to act as a... 

Sodium cyanoborohydfide reaction with 1 entails the intermediacy of a thermally unstable n1-hydroxymethyl complex CpFe(CO)2(CH2OH) (4). When the reduction sequence was carried out in ethanol at room temperature, only trace amounts of... 

The observed instability of CpFe(CO)2CH2OH (4) augments a growing body of evidence concerning thermal instability of a-hydroxyalkyl complexes (38,39,40,41,42). A similar hydroxymethyl complex CpRe(C0)N0(CH20H), the only fully characterized a-hy-droxymethyl complex to date (38 39), likewise converts to its methoxymethyl complex in methanol. 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

The hydroxymethyl complex LCoCH2OH yielded CH20 and LCo1 by heterolytic cleavage of the Co-C bond, k 0.1 s 1 in the pH range 1 < pH < 7. Subsequent rapid oxidation of LCo1 with N20 present in these solutions produced the stable LCo111. 

Hydroxymeihylene would also be responsible for methanol formation (62) by a further reduction to an hydroxymethyl complex and hydtogenolysts to CHjOH. Methane would be the result of further dehydration of a hydroxymethyl hydride complex to a methylene followed by hydrogenation to give a methyl group and hydrogenolysis to give methane (63) ... 

Similar results were obtained using the hydroxymethyl complex 102 (92). Irradiation of this complex in THF-dg or cyclohexane-dn solution gave the hydride complex 103 in essentially quantitative yield [Eq. (92)]. 

Similar methoxycarbonyl complexes were prepared by MeOH addition to Ir(CO)(PMe3)4. Reductive elimination was not observed for these complexes. Reaction of the formyl complex with HBF4 produced the hydroxymethyl complex ... 

Based on these experiments, mechanistic steps were suggested, which explain, independent of the metal used (M = Co, Rh), the formation of the typical reaction products such as formaldehyde, methyl formate, methanol, and the desired ethylene glycol (Scheme 6.115) [11]. A key role in this mechanism is played by formaldehyde, which is produced by the hydrogenolysis of a metal formyl intermediate. Either it reacts afterwards to methyl formate via a methoxy complex or, alternatively, a transient hydroxymethyl complex is formed which becomes the starting point for next transformations. In the absence of CO, preferentially methanol is released. Only by coupling with a further CO equivalent the C2-unit is constructed. Upon hydrolysis, ethylene glycol is released. 

The formyl complex reacts with dihydrogen to give 4.51, a complex where formaldehyde acts as a ligand. Two different insertion reactions of formaldehyde into the metal hydrogen bond leading to the formation of 4.52 or 4.53 are possible. The hydroxymethyl complex 4.53 can undergo further CO insertion to give 4.54. 

Sketch (a) ibuprofen, (b) j3-lactam, (c) j3-lactone, (d) cyclopentenone, and (e) a CO-inserted hydroxymethyl complex. Explain their relevance to carbonylation reactions using homogeneous catalysts. What is the (f) supercritical state of CO2 and (g) the adsorbent in the Cosorb process ... 

This mechanistic scheme was supported by the observation that the formyl complex reacts with BHa-THF to yield the methyl complex. When a tetrahydrofuran/water mixture is used as the solvent, the reaction can be stopped at each stage, including the step of hydroxymethyl complex formation. Scheme 10.13 ... 

Hydroxymethyl complexes are often the most difficult members of the series to prepare and isolate. In some cases, such complexes can be s)mthesized through reaction of carbonyl complexes with NaEBHaCN) in alcoholic solutions. For example, the reduction of [M(CO)3(ri5-Cp)]+ (M = Ru, Fe) with a four-fold excess of Na[BH3CN] in methanol at 25 C produced the neutral hydrox)nnethyl complexes, [M(C0)2(CH20H)(Ti5-Cp)], in yields of 55% (Ru) and 45% (Fe).74 it has been shown S that [Fe(CO)2(CH20H)(ii5-Cp)]+ reacts further with Na[BH3CN] in methanol or ethanol to produce the alkoxymethyl derivative. Scheme 10.14 ... 

Nelson has similarly shown76 that the neutral hydroxymethyl complex, [Ru(C0)2(CH20H)(Ti5-Cp )], is formed by reduction of [Ru(CO)3( n5-Cp )][BF4] with a three-fold excess of Na[BH3CN] in methanol. The reaction of the starting material with an equimolar quantity of NafBHsCN] in methanol yields the alkoxymethyl complex, [Ru(C0)2(CH20Me)(Ti5-Cp )], as the major... 

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