Molybdenum complexes substitution reactions

When methylene chloride was used as a solvent, it was found that 28 are obtained in minor amounts, while the dominating product is the -coordinated chloro-carbyne species [(> -Tp )Mo(CO)2(=CCl)], whose yield increases abruptly with substitution in the pyrazol-l-yl fragments (3-methyl-, 3,4,5-trimethyl-, and 3,5-dimethyl-4-chloro derivatives) [90AX(C)59,95JCS(D) 1709]. The tungsten analog can be prepared similarly. The chlorocarbyne molybdenum complex follows also from the reaction of the parent anion with triphenylsulfonium cation but conducted in dichloromethane. The bromo- and iodocarbyne derivatives are made similarly. 

Reaction of the 5-aminochromene 1044 with the complex salt 577 provided via an electrophilic aromatic substitution regio- and diastereoselectively the molybdenum complex 1050. The oxidative cyclization of complex 1050 with concomitant aromatization and demetalation using activated manganese dioxide led directly to girinimbine (115) in 50% yield. Oxidation of girinimbine (115) with DDQ in methanol afforded murrayacine (124) in 64% yield (660) (Scheme 5.161). 

An interesting allylic substitution reaction of ( )-cinnamyl methyl carbonate 143 has been examined by Pfaltz s group. The use of a molybdenum complex of ligand 144 resulted in 145 in 88% yield with an ee of 99% [for the (R) isomer] (Fig. 9.45). 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

CpCl2[P(OMe)3]WCOCH2(c-Pr) in 90% yield (equation 100)158. Molybdenum complexes are more vulnerable to ring-opening reactions with HC1, particularly the acyl-substituted complexes (see below). 

Ligand exchange reactions with nitrido molybdenum complex 56 has been documented (Eq. (19)). Treatment of 56 with 2 equiv. of HX (X = Cl or Br) leads to the formation of 57 which, following treatment with triethylamine, afforded 58, wherein a halide has been substituted for the azide ligand found in 56 [27]. The introduction of a nitride onto a Mov complex has been carried out by treatment of Mom or Mo ... 

The molybdenum complex [(CF3)2MeCO]2Mo(NAr)(=CHCMe2Ph) has been observed to be a more efficient catalyst for cyclization of vinyl silyl ether dienes than the ruthenium complex Cl2(PCy3)2Ru(=CHPh), probably because this type of alkene is sterically more demanding (than allyl derivatives) and therefore requires a catalyst less sensitive to steric bulkiness near the reaction center. However, some examples of the RCM of substituted vinylsilanes catalyzed by ruthenium complexes have been reported [127, 131] (Eq. 74). For more examples see Ref. [127]. 

Oxidation of molybdenum hexacarbonyl with acetic acid produces the carboxylate-bridged quadruple-bonded dimers, as shown inequation (35). This complex can undergo a number of substitution reactions that leave the metal metal bond intact. ... 

These complexes are excellent models for theoretical studies. The octacyano complexes of molybdenum and tungsten are stable and inert toward substitution reactions and therefore very suitable for theoretical studies of redox reactions and application of the Marcus theory. The photoreactivity of these systems is also proving to be important. The 0X0- and nitridocyano complexes of Mo(IV), W(IV), Tc(V), Re(V), and Os(VI) are very good candidates for kinetic studies of substitution reactions with both mono- and bidentate ligands and are of interest especially in view of the large variations in the observed reactivity. 

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