Molybdenum complexes, carbene chemistry

It is over eight years since the last comprehensive review appeared on metal isocyanide chemistry. In the interim, reviews have appeared on specific aspects of isocyanide chemistry. Lippard reviewed seven and eight coordination in molybdenum isocyanide compounds (7), Yamamoto reviewed metal(O)- isocyanide complexes (2), and in related reviews on car-bene complexes by Cotton and Lukehart (5), Lappert et al. (4), and Casey (5), mention was made of carbenes synthesized from metal isocyanides. 

In organic chemistry one surely thinks at once of the construction of cyclopropane derivatives from olefins and carbenes. Indeed, it has been shown that this also is possible with our complexes and with C=C double bonds that are electron-poor and arc either polarized or easily polarizable (77-81). As an example of this, I would like to cite the reaction of penta-carbonyl[methoxy (phenyl) carbene]chromium (0), -molybdenum (0), or -tungsten(0) with ethyl vinyl ether (79). One obtains the corresponding cyclopropane derivatives in this case, however, only when one removes... 

His proposal involved a metal carbene and a metallocyclobutane intermediate and was the first proposed mechanism consistent with all experimental observations to date. Later, Grubbs and coworkers performed spectroscopic studies on reaction intermediates and confirmed the presence of the proposed metal carbene. These results, along with the isolation of various metal alkyli-dene complexes from reaction mixtures eventually led to the development of well-defined metal carbene-containing catalysts of tungsten and molybdenum [23-25] (Fig. 2). After decades of research on olefin metathesis polymerization, polymer chemists started to use these well-defined catalysts to create novel polymer structures, while the application of metathesis in small molecule chemistry was just beginning. These advances in the understanding of metathesis continued, but low catalyst stability greatly hindered extensive use of the reaction. 

Besides the conventional methods, the metallo-carbene route to access cyclic compounds has become a versatile tool in sugar chemistry. Synthesis of stavudine 112, an antiviral nucleoside, from an allyl alcohol [101] is realized by a Mo(CO)5-mediated cyclization reaction (O Scheme 26). Molybdenum hexacarbonyl smoothly reacts with the triple bond of 113 to generate the intermediate Mo-carbene, which undergoes a clean cyclorearrangement to yield the furanoid glycal 114. Alkynol isomerization is effected by group-6 transition metal carbonyl complexes [102]. 

Fischer-type carbenes are known as potential carbene transfer reagents to electron-rich and electron-deficient alkenes. Little is known about the chemistry of carbene complexes with silicon substituents at the carbene C-atom, whereas complexes with germanium, tin, or lead have not yet been prepared. The tungsten-carbene complexes 6 react with an excess of ethyl vinyl ether to give l,2-diethoxy-l-(trialkylsilyl)cyclopropanes 7." Only the f-isomers were formed and similar results can be achieved by using the corresponding molybdenum or chromium complexes. On the other hand, no reaction takes place with 2,3-dihydrofuran or ethyl ( )-but-2-enoate. ... 

We must however keep in mind that some of the above reactions may not be simple reactions at the silicon atom, since transition metal complexes show multicenter reactivity (metal atom, ligands) as exemplified in the chemistry of triphenylgermyl-carbene complexes of cobalt carbonyl (253). Thus, displacements of a silyl ligand may result from a multistep process and a thorough examination of these reactions has to be made. An example can be drawn from molybdenum-germanium chemistry (247). As shown in Scheme 59, germanium is displaced from complex 167 by HO with retention of configuration. Actually,... 

The initial observation of a metal carbene that reacted with an alkene to give a metallacyclobutane complex was reported by Osborn and coworkers for the reaction shown in equation (10). This reaction was observed by NMR spectroscopy at low temperature (—70°C). When this reaction mixture was allowed to warm to higher temperature, polynorbornene was produced in high yield. Shortly after this discovery, the titanocene complex (4) was shown to be an efficient catalyst for the synthesis of monodisperse polynorbornenes. These discoveries, along with the synthesis of a new family of tungsten (5a), molybdenum (5b), and rhenium (6) catalysts,shown in Figure 1, have opened a new era of ROMP chemistry in which the polymer synthesis is guided by the selection of a catalyst... 

The generally accepted mechanism for olefin cross-metathesis is outlined for the case of propene in Mechanism 14.4. The catalyst belongs to a class of organometallics known as a metallocarbene, carbene complex, or alkylidene complex. Its structure is characterized by a carbon-metal double bond. In olefin metathesis the metal is typically ruthenium (Ru), tungsten (W), or molybdenum (Mo). Transition-metal carbene complexes were first prepared by Ernst O. Fischer (Munich) who shared the 1973 Nobel Prize in Chemistry with Geoffrey Wilkinson. 

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