Molybdenum dioxo

Sharpless et al. and Muccigrosso et al. were probably the first to perform the direct allylic amination of nonfunctionalized alkenes in the presence of a metal complex [60]. The complex used for the allylic amination was the molibdooscaziridine 109 prepared by reaction of the molybdenum dioxo complex 107 and phenyl hydroxy-lamine 108 (Eq. (25)). 

Sharpless et al. found that heating a solution of 109 with 2-methyl-2-hexene 110 afforded an allylic amination reaction leading to the formation of 111 in 57 % yield together with high yields of the parent molybdenum dioxo complex 107 (Eq. (26)). 

A variety of different metal complexes have been screened as catalysts for allylic amination using phenyl hydroxylamine 108 as the nitrogen fragment donor, and it was found that iron-complexes have better redox capacity compared to molybdenum [64]. With the iron compounds, higher yields and a lower amount of hydroxylamine-derived byproducts are obtained. These byproducts constitute one of the problems in this type of allylic amination reactions in general, as their formation is difficult to suppress. The allylic amination reaction of a-methyl styrene 112 with 108 can, e.g., be catalyzed by the molybdenum dioxo complex 107, iron phthalocyanine 114, or by the combination of the iron chlorides 115 [64,65]. It appears from the results in... 

Fig. 1. Bond lengths, in pm, in complexes containing uranium and molybdenum dioxo groups...

The probable steps of the catalytic reaction have been studied individually. It has been shown that the molybdenum dioxo complex, X, reacts with triphenylphosphine in an inert atmosphere to give triphenylphosphine oxide and the red molybdenum rV complex, [MoO(S2CN(n-Pr)2)2], equation (89), which can be separated by chromatography on AI2O3 in an oxygen free atmosphere. 

Interesting results of NMR study of Schiff base molybdenum(VI) ds-dioxo complexes, derivatives of a or p-D-glucosamines [36] have been published by Zhao et al.87... 

Scheme 9 Heterogenisation of dioxo and oxodiperoxo molybdenum complexes on MCM functionalised with M-ligands or N,AT-ligands...

In contrast to the situation a decade ago, many incomplete cubane-type clusters with Mo304 S cores have been prepared and the structures have been determined by X-ray structure analyses. The results obtained are summarized in Tables I—III. The formal oxidation state of molybdenum in the compounds cited here is in all cases IV. Unlike Mo(VI) and Mo(V) compounds, mononuclear oxo or dioxo compounds of the Mo(IV) state are relatively rare and all the incomplete cubane-type compounds cited here have no terminal oxo ligand. Three Mo atoms form an equilateral triangle, and three single bonds exist between each Mo. Except for the compounds 1, 8, and 31 (Table III), and excluding Mo—Mo bonds, each molybdenum is octahedrally coordinated. 

Nitrate reductase from Chlorella, an assimilatory enzyme, is a homotetramer of molecular weight 360 000 and contains one each of Mo, heme and FAD per subunit. The nitrate reductase from E. coli is a dissimilatory enzyme. EXAFS data are available on the molybdenum sites in both enzymes (Table 24).1050 The environment of the molybdenum in the assimilatory enzyme is similar to that found for sulfite oxidase, with at least two sulfur ligands near the molybdenum and a shuttle between monoxo and dioxo forms with redox change in the enzyme. This allows a similar mechanism to be put forward for the assimilatory nitrate reductase,1051 shown in equation (57), where an oxo group is transferred from nitrate to MoIV with production of nitrite and MoVI. 

MolvO(bdt)22 can be converted to the dioxo molybdenum(VI) complex in good yield by the following reaction ... 

Of the complexes studied, those with ancillary ligands that contain sulfur atoms have received the most attention. Among the most studied are the bis(dial-kyldithiocarbamate) molybdenum oxido complexes [196,197], Unfortunately, although these complexes react with certain enzyme substrates, their common problem as models for Moco active sites involves the propensity of the Movl dioxo and MoIV monoxo complexes to form a comproportionated Mov p-oxido dimer (Eq. 13 dtc = diethyldithiocarbamate). 

The crystal structure of dioxobis-(l,3-diphenylpropanedionato)molybdenum(vi) has been determined by X-ray crystallography. The two cis-dioxo-groups involve Mo—O bond lengths of 169.6 pm, and the length of the Mo—O bonds in the chelate rings average 216.7 pm trans to the oxo-atoms, and 199.3 pm cis to these atoms.534... 

This can be achieved by using the cis molybdenum(VI)dioxo ion instead of titanium(IV). In this ion two comers of the octahedron at the molybdenum are already blocked by oxygen atoms and only two catechol units can be bound to the metal. Thus, reaction of ligand 3-H4 with MoC>2(acac)2 in the presence of potassium carbonate leads to a mononuclear macrocydic complex [3Mo0212 in which a loop-type conformation is stabilized at the peptide (Scheme 1.3.3) [23]. (Similar... 

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