Molybdenum complexes dioxo

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

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... 

Molybdenum(VI) dioxo complexes have also been used for the catalytic conversion of 1,2-diphenylhydrazine to azobenzene (795), and the oxidation of 2-hydroxy-2-phenylacetophenone (benzoin) to benzil (1025). In the latter, however, the reaction is rapidly deactivated as the catalyst [Mo02(S2CNEt2)2] is converted into the inactive molybdenum(V) dimer, [MoO(p-0)(S2CNEt2)]2-Attempts to oxidize aldehydes using these catalysts have generally been unsuccessful (795). 

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... 

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). 

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... 

The possible role of oxygen atom transfer in molybdenum enzyme catalysis was recognized in the early 1970s (190-194). In the ensuing years, a wealth of chemistry has established molybdenum as the premier exponent of such reactions (7, 195). Importantly, related dioxo-Mo(VI) and oxo-Mo(IV) complexes are interconverted by oxygen atom transfer reactions (Eq. (13)). These reactions are effected by reductants (X) such as tertiary alkyl and aryl compounds of the group 15 elements (especially phosphines) and oxidants (XO) such as S- and N-oxides. In many cases, however, the Mo(VI) and Mo(IV) compounds participate in a comproportionation reaction yielding dinuclear Mo(V) complexes (Eq. (15)). 

---