Metallacyclobutane synthesis

R. J. Puddephatt No. Nobody has prepared such complexes and the synthesis is not trivial. Substitution of halide ligands in octahedral platinum(IV) derivatives is typically very slow, and a better route (suggested by J. K. Kochi) might involve oxidation of platinum(II) metallacyclobutanes with peroxides. It would certainly be worthwhile to attempt this synthesis in view of the promise of enhanced reactivity. 

Another convenient method for the synthesis of metallacyclobutanes is the reaction of di-Grignard reagents with Me2MCl2 (M = Si, Ge, Sn) (Scheme 69) <1983JA3336, 1984JOM(277)319, 1987JOM(321)291>. 

Consistent with metallacyclobutene synthesis, metallacyclobutane complexes can also be prepared by y-hydogen elimination (Equation 73), despite the greater entropic disadvantage. 

Among more recent innovations, the alkylative metallacyclobutane and metallacyclobutene synthesis, involving central carbon addition and addition/elimination reactivity patterns, holds considerable promise for near-term synthetic developments (Section 2.12.12). This is equally true in both the nucleophilic and free radical versions of this process, for both catalytic and stoichiometric transformations and multistep reaction cascades. 

Olefin metathesis is a useful tool for the formation of unsaturated C-C bonds in organic synthesis, and the reaction has been generally accepted to proceed through a series of metallacyclobutanes and carbene complexe intermediates [40-43]. For this type of reaction, the most widely used catalysts include an alkoxyl imido molybdenum complex (Schrock catalyst) [44] and a benzylidene ruthenium complex (Grubbs catalyst) [43]. The former is air- and moisture-sensitive and has some other drawbacks such as intolerance to many functional groups and impurities the latter has increased tolerance to water and many reactions have been used in aqueous solution without any loss of catalytic efficiency. 

The following procedure illustrates the use of di-Grignard reagents for the synthesis of metallacycles. In spite of the difficulty in preparing the di-Grignard reagent (see Section 3.1, p. 29), this is one of the most important routes to metallacyclobutanes [69]. 

More recent developments in the mechanistic aspects of the alkene metathesis reaction include the observation that the alkene coordinates to the metal carbene complex prior to the formation of the metallacyclobutane complex. Thns a 2 - - 2 addition reaction of the alkene to the carbene is very unlikely, and a vacant coordination site appears to be necessary for catalytic activity. It has also been shown that the metal carbene complex can exist in different rotameric forms (equation 11) and that the two rotamers can have different reactivities toward alkenes. " The latter observation may explain why similar ROMP catalysts can produce polymers that have very different stereochemistries. Finally, the synthesis of a well-defined Ru carbene complex (equation 12) that is a good initiator for ROMP reactions suggests that carbenes are probably the active species in catalysts derived from the later transition elements. ... 

A very good method for the synthesis of metallacycles with two metal-carbon a bonds is the reaction of dihalogenometal complexes with a,w-di-lithio- or di-Grignard-alkanes (Scheme 7). According to this procedure, metallacyclobutanes, -pentanes, -hexanes, and -heptanes of tita-... 

Another example of this type starts from exo,< xo-tetracyclo[3.3.1.0. 0 ]nonane, which reacted with Zeise s dimer, [(C2H4)PtCl2]2, to form an insoluble platinum complex. This product is solubilized with pyridine and was shown to be a metallacyclobutane complex 4. Synthesis of cxo-tricyclo[3.3.1.0 ]non-6-ene was achieved in 45-55% yield if 4 was treated with dimethyl sulfoxide. ... 

Besides rearrangements, ligand exchange, formation of alkanes, alkenes and other products, release of cyclopropanes is one of the most important reactions of metallacyclobutanes. Of course, this latter reaction is only useful in cyclopropane synthesis if the product is not identical with the starting material used to form the metallacyclobutane. Nevertheless, the discovery of a complex formed from hexachloroplatinic acid and cyclopropane and later structural elucidations have initiated intensive investigations on the conversion of cyclopropanes to metallacyclobutanes and release of cyclopropanes from the latter. These results have been thoroughly discussed in several reviews. " Therefore in this section only some general aspects of cyclopropane formation from metallacyclobutanes and selected synthetically useful methods are discussed. 

Metallacyclobutanes formed from cyclopropanes upon reductive elimination usually regenerate the starting material. Thus, reactions of this type are not useful in synthesis of new cyclopropanes. In some selected cases, however, strained hydrocarbons with cyclopropane subunits yield metallacyclobutanes which upon decomplexation give cyclopropanes not identical with the starting material. These conversions, however, are hard to generalize and often lead to unselective product formation. 

Similarly, a number of metallacyclobutanes (78-85) of transition metals were obtained from 60b (Scheme 11.26). In this context, the synthesis of 31 and 32 from the formal 2-titana-1,3-di-Grignard reagent 24 (Scheme 11.11) should also be mentioned. 

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... 

Evidence indicates that the stereodetermining step in the catalytic cycle is metallacyclobutane formation and that the stereochemistry of the metallacycle is transferred to the product (Fig. 4.28). Consequently, trans metallacyclobutanes rearrange to yield trans olefins, and cis metallacyclobutanes yield cis olefins. The synthesis of five-coordinate adducts of the general formula (NAr)(OR)(L)M= CH(BuO (e.g., L = PMes), which are not metathesis active, demonstrated that binding of a donor ligand (in a substrate, for example) was detrimental to catalytic activity, and also confirmed the reactivity patterns of different alkylidene conformations. 

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