Molybdenum substitution reactions

Stable enolates such as diethyl malonate anions react with allyl sulfones (or acetates) in the presence of nickel complexes to give a mixture of the a- and /-product83. The regioselectivity is generally poor in the nickel-catalyzed reaction, but the molybdenum-catalyzed reaction is selective for alkylation at the more substituted allylic site, thereby creating a quaternary carbon center84. 

The work cited in sections 2.4 and 2.5 is representative of the SN1 substitution reactions of metal carbonyls. However, a much more extensive and detailed account has recently been published covering similar reactions of vanadium, chromium, molybdenum, tungsten, rhenium, iron and nickel carbonyls in addition to those of manganese and cobalt2 9a. 

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. 

The substitution reactions of molybdenum hexacar-bonyl, Mo(CO)6, with a variety of other molecules L are believed to occur by the following mechanism ... 

The substitution reactions on action of phosphorus trihalids or acylhalids on molybdenum (II) alkoxides have also been investigated in detail ... 

Metals other than palladium and molybdenum can be used for allylic substitution reactions. For example, nickel in the presence of the oxazolinylferrocenylphosphine 9 provides good asymmetric induction for the reaction of a Grignard reagent with allylic electrophilic systems such as acetates.151... 

The various allylic substitution reactions are illustrated in a synthesis of Tipranavir (11), an HIV protease inhibitor. A palladium-catalyzed opening of a vinyl epoxide set the quaternary stereogenic center (Scheme 22.23). A molybdenum-catalyzed allylic nuclophilic displacement was used to access the benzylioc stereogenic center (Scheme 22.24).153... 

In addition, allenes can act as the olefinic part of the reaction [32], Al-lenynes like 12 may react with both double bonds. Brummond established the substitution patterns for the reaction with either the external or the internal bond of the allenic fragment, that give products with different ring sizes (13— 14) [33]. This group has applied these studies to the synthesis of hydroxy-methylfulvalene (17), a potent anticancer agent related with illudines, a natural sesquiterpene family. The key step was the synthesis of 16 from 15 with a PKR mediated by molybdenum carbonyl (Scheme 6) [34,35]. In addition they have developed an asymmetric version of the reaction. They have transferred efficiently chirality from a non-racemic allene to an a-alkylidene and an a-silylidene cyclopentenone in a molybdenum mediated reaction [36-38]. 

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. 

Pastore, H.O., Ozin, G.A., Poe, and A.J. 1993. Intrazeolite metal carbonyl kinetics Substitution reactions of molybdenum carbonyl (Moj2(CO)6) in sodium zeolite Y. Journal of the American Chemical Society 115, 1215-1230. 

Since the molydenum compounds described above demonstrated mechanistic variation in photochemically initiated substitution reactions, the molybdenum dimetallacycle [(CO)4Mo(p-PMD)2Mo(CO)4] was selected for a study of the nature of the substitution mechanism associated with MC and MLCT excitation.285 PMD is pentamethylenediazirine. The substituting ligands were bpy, phen and l,2-bis(dip-henylphosphino)ethane (dppe) for the reaction in toluene solution. For reaction with... 

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