CpMo

Fig. 5. Representative structures for compounds of molybdenum(III) (a) hexacholoromolybdenum(III) ion, MoClg (b) bexabis(dimethylamiHo)dimo1ybdeniim (TTT), Mo (N(CH ) ) (c) the Mo S thiocubane core stmcture (d) dichlorocyclopentadienyl triaIkylphosphinedichloromolybdenum(III), CpMo(PR2)Cl2, where Cp = cyclopentadienyl and R = alkyl.

Fig. 7. Representive stmctures for compounds of molybdenum(0) (a) Mo(CO)g (b) tris(acetonitrile)tris(carbonyl)molybdenum(0) (c) bis(l,2-diphenylphosphinoethane) bis (dinitrogen) molybdenum(0), [R2PCH2CH2PR2]2Mo(N2)2, where R = CgH, also known as Mo(dppe)2(N2)2, where dppe = 1,2 — diphenylphosphinoethane (d) cyclopentadienyl tricarbonyl molybdenum(0) anion, CpMo(CO)3, where Cp = cyclopentadienyl (e)...

Table 9.3 Desymmetrization of prochiral cycloketones to enantiocomplementaiy lactones by CHMO- (CHMO cmeto ind CHMOermi) and CPMO-type (CHMOerewZ and CPMOcoma) enzymes (representative examples).

All chiral products as well as enantiomerically enriched substrate ketones from such transformations are valuable building blocks in asymmetric synthesis [182,183]. While CHMO-type enzymes in general display such a behavior, CPMO-type biocatalysts give... 

Belonging to group (i) are alkylmetal carbonyls and cyclopentadienylmetal alkyl carbonyls of formula RMn(CO)5, CpFe(CO)2R, and CpMo(CO)3R. Solvent dependence of the reaction of MeMn(CO)5 with CjHi,NH2 is illustrated also in Table I. The rate varies markedly with the dielectric constant and with the nucleophilic power of the solvent. For example, on going from dimethylformamide to mesitylene, the rate of insertion is reduced by 10. Similarly, the sequence MeCN > MejCO > THF > CHCI3 > CjHj was reported for the reaction of MeMn(CO)5 with P(0CH2)3CR (R = Me and Et) in various solvents (97). Analogous trends were observed for the insertion reactions of CpFe(CO)2R and CpMo(CO)3R (48, 80, 98). 

Evidence for the reactive intermediate in Eq. (20) is strictly of a kinetic nature. Since attempts at its detection by proton NMR spectroscopy starting with RMn(CO)j or CpMo(CO)jR were not successful (80, 81, 97), such a species must be present in low concentrations. 

Few quantitative data are available on the relative nucleophilicities of L toward various alkyl carbonyls. The rates of the reaction of CpMo(CO)3Me with L in toluene (Table II) decrease as a function of the latter reactant P( -Bu)3 > P( -OBu)j > PPhj > P(OPh)j, but the spread is relatively small (<8). The above order is that customarily observed for 8 2 reactions of low-valent transition metal complexes (J, 214). Interestingly, neither CpMo(CO)3Me nor CpFe(CO)2Me reacts with 1 or N, S, and As donor ligands 28, 79). This is in direct contrast to the insertion reactions of MeMn(CO)5 which manifest much less selectivity toward various L (see Section VI,B,C,D for details). 

Rather limited information is available on how the nature of R affects the rate of CO insertion, all other factors being constant. A generalization that ethylmetal complexes react faster than the corresponding methyl carbonyls derives from investigations on four systems RIr(CO)2(AsPh3)Cl2 (92), RMn(CO)5 (51), CpFe(CO)2R (98), and CpMo(CO)jR (80). When R = Et the reactions with CO or P and As donor ligands proceed at least 6 times faster than when R = Me. 

No comparative kinetic study has been made on the same alkyl carbonyl system for two members of a given transition metal triad. Qualitative data show that the middle member is more reactive than the heaviest one e.g., CpMo(CO)jR > CpW(CO)jR (Section VI,B), Rh(III) > Ir(III) (Section VI,E), and Pd(II) > Pt(II) (Section VI,F). However, the extreme unreactivity of CpW(CO)jR and a considerable difference in lability between most alkyls of Rh(III) and Ir(III), as well as those of Pd(II) and Pt(II), have prevented detailed investigations. Surprisingly, no kinetic studies have been conducted on insertion reactions of RRe(CO)5, which would seem readily amenable to such investigations. 

Within group (i), square-planar EtPt(CO)(AsPh3)Cl inserts more rapidly than six-coordinate EtIr(CO)2(AsPh3)Cl2. In THE at 40°C, the relative k s are 9 and 1. Comparison of group (ii) alkyl carbonyls reveals the order MeMn(CO)5 > CpMo(CO)3Me > CpFe(CO)2Me. The ratios of the k s are 23 I and 100 1, respectively, in THF at 25° and 50.7°C. The higher reactivity of manganese than of molybdenum is a consequence of the relative entropies, whereas the lowest reactivity of iron is caused by its Jff (Table III). 

Replacement of CO in MeCOMn(CO)5 with PPh3 seems to have little effect on the rate of the decarbonylation. As shown in Table IV, MeCO-Mn(CO)4PPh3 (an isomeric mixture) reacts only slightly faster than MeCOMn(CO)5 after provision is made for the difference in temperature 169). However, a recent kinetic study on the decarbonylation of CpMo-(CO)2L(COMe) (L = a tertiary phosphine) has shown that both inductive and steric properties of L are important 19a). Sterically demanding and weakly a-bonding phosphines increase the reaction rate. 

In some instances the decarbonylation can be effected, under mild conditions, by using a stoichiometric amount of a CO-abstracting metal complex. This method has proved to be fruitful for several CpFe(CO)2COR compounds (7, 2, 2a) and for CpMo(CO)2(PPh3)COPh 189) in conjunction with Rh(PPh3)jCl. 

A stereochemical point of considerable interest concerns the position of entry of L with respect to the newly formed acyl moiety. This has been investigated for RMn(CO)5 and CpMo(CO)3R. 

Reactions of CpMo(CO)3R with L also yield products in which COR and L occupy closest possible positions. Thus, CpMo(CO)3CH2CH2CH2Br and PPh3 [Eq. (18)] afford the cis product (X) which then isomerizes to the... 

The alkyls investigated have been principally of the type CpMo(CO)jR, their dicarbonyl substitution products, and 7r-indenyl analogs. 

High-pressure carbonylation of CpMo(CO)jR yields the very unstable acyls. The acetyl was detected by infrared spectroscopy but not isolated (70), whereas the propionyl decomposes readily to [CpMo(CO)3]2 (172). 

Iodide 48), As(OCH2)3CMe, S(t-Bu)2 (79), and pyridines 80) do not react with CpMo(CO)3Me in THE or MeCN. However, interaction between Na[CpMo(CO)3] and 2-(chloromethyl)pyridine has afforded (XIX) 144),... 

Their precursors must be the tricarbonyl o-allenyls with the uncoordinated C=C bonds. Neither an allylic rearrangement nor cis-trans isomerization has been observed in the reaction of CpMo(CO)3(cw-CH2CH=CHMe) with PPhj, the product being CpMo(CO)2(PPh3)(cw-COCH2CH=CHMe) (81). The interesting reaction leading to the formation of cationic carbene compounds was mentioned earlier [Eq. (17) and Section V] (78). 

Kinetic studies have been made of the reaction of CpMo(CO)j R (R = Me, Et, CH2Ph, and CH2CH=CH2) (48, 80, 81) and 7r-X2C9H5Mo(CO)jMe (X = H or OMe) (108) with a variety of P donor ligands L. Solvents employed ranged from nonpolar hexane to polar THF and MeCN. Generally, the mechanism is very sensitive to the coordinating ability of the solvent and the nucleophilicity of L. 

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