Complex metallacyclobutane

Fig. 30. Mechanism for C-C activation of propene. Decay of the allyl hydride complex may proceed via migration of the metal-bound H atom to the /3-carbon atom in the allyl moiety (i.e. reverse /3-H migration), leading to formation of the same metallacyclobutane complex implicated in the Y + cyclopropane reaction. The dynamically most favorable decay pathway is to YCH2 + C2H4.

R.R. Schrock, M.I.T. Have you or anyone else prepared platinum(IV) metallacyclobutane complexes with alkoxide ligands in place of chlorides One might expect the alkoxide complexes to behave considerably differently than the chloro complexes, perhaps like early transition metal complexes. 

A complex such as Ta(CHCMe3)(PMe3)2Cl3 reacts readily with ethylene, propylene, or styrene to give all of the possible products (up to four) which can be formed by rearrangement of Intermediate metallacyclobutane complexes (two for substituted olefins) by a p-hydride elimination process (e.g., equation 2) ( ). We saw... 

On the basis of these studies and some calculations by Rappe and Goddard (19) it would seem incontrovertible that the oxo ligand prevents reduction of the metal and perhaps also enhances the rate of reforming an alkylidene complex from a metallacyclobutane complex. The next question was whether other strong n-donor ligands such as alkoxides could take over the oxo s function (lib). 

Feldman, Jerald and Schrock, Richard R., Recent Advances in the Chemistry of "d° Alkylidene and Metallacyclobutane Complexes. 39 1... 

The expected intermediate for the metathesis reaction of a metal alkylidene complex and an alkene is a metallacyclobutane complex. Grubbs studied titanium complexes and he found that biscyclopentadienyl-titanium complexes are active as metathesis catalysts, the stable resting state of the catalyst is a titanacyclobutane, rather than a titanium alkylidene complex [15], A variety of metathesis reactions are catalysed by the complex shown in Figure 16.8, although the activity is moderate. Kinetic and labelling studies were used to demonstrate that this reaction proceeds through the carbene intermediate. 

Over the past 15 years the understanding of the mechanism of these reactions has been greatly enhanced through the preparation of metal carbene complexes, particularly of Mo, W and Ru, that are both electronically unsaturated (<18e) and coordinatively unsaturated (usually <6 ligands), and which can act directly as initiators of olefin metathesis reactions. The intermediate metallacyclobutane complexes can also occasionally be observed. Furthermore, certain metallacyclobutane complexes can be used as initiators. 

When catalysts of the type 10/GaBr3 are used to initiate the ROMP of norbornene derivatives at low temperature (—50 °C) the intermediate transoid metallacyclobutane complexes (the precursors to the formation of trans double bonds) may be observed. The corresponding cisoid complexes are not stable enough to be detected. No metallacyclobutane complexes are observed in the absence of GaBr3100 112-114. 

Intermediate molybdacyclobutane complexes have also been detected in the reactions of 7 with 21-24115. Only in the case of 21 is the ultimate product a long-chain polymer, but in all cases one may observe, at 0-60 °C, a clean first-order rearrangement of the initial metallacyclobutane complex to the first metal carbene adduct, consisting of an equilibrium mixture of syn and anti rotamers in the ratio 9 1 (see below). Except in the case of 21, the metal carbene complexes do not survive for very long. For 21, however, ROMP is propagated, and distinct H NMR signals are seen for the longer-chain metal carbene complexes in both syn and anti forms. 

It has generally been assumed that in olefin metathesis reactions the olefin first coordinates to the metal carbene complex, en route to the formation of the intermediate metallacyclobutane complex, and that after cleavage of this intermediate the newly formed double bond is temporarily coordinated to the metal centre. A number of stable metal-carbene-olefin complexes are known see elsewhere116,117 for earlier references. They are mostly stabilized by chelation of the olefin and/or by heteroatom substituents on the carbene, although some have been prepared which enjoy neither of these modes of stabilization118,119. 

Table 8 Reductive formation of cyclopropanes from metallacyclobutane complexes...

Equilibrium cycloreversion of metallacyclobutane complexes to alkylidene intermediates is similar to that of metallacyclobutene complexes (Section 2.12.6.1.3). Metallacyclobutane complexes thus provide convenient progenitors of reactive alkylidene intermediates. The a-methylenetitanacyclobutane complex 79 decomposes into the vinylidene intermediate 103 (Scheme 17), which manifests nucleophilic character at the ct-carbon, consistent with... 

Metallacyclobutane complexes can undergo net reductive rearrangement to form alkene complexes or undergo decomposition with the liberation of free alkene. The rhodacyclobutane complex 112, for example, rearranges thermally to give propene complex 113 (Equation 33) <19980M4484>. 

Platinacyclobutane complex 118 undergoes equilibrium heterolytic scission of the exocyclic carbon-carbon bond to form a cationic allyl complex and the organic enolate ion (Equation 35) <1993OM3019>. Similar dissociative ionization was previously reported for rearrangements of iridium and rhodium metallacyclobutane complexes formed by nucleophilic alkylation < 1990JA6420>. This carbon-carbon bond activation is generally associated with reversible central carbon alkylation of Jt-allyl complexes (Section 2.12.9.3.3), but the homolytic equivalent has recently been... 

The cycloaddition of alkenes with metal alkylidene complexes remains the most common entry into the metallacyclobutane structural class. Consistent with metallacyclobutane intermediacy in the olefin metathesis reaction, the [2+2] cycloaddition is generally reversible a propensity for cycloreversion (Section 2.12.6.2.4), however, can significantly limit the utility of metallacyclobutane complexes as intermediates in other synthetic transformations. 

During the course of metathesis-related investigations, a range of metallacyclobutane complexes of other metals have recently been isolated and characterized, at least transiently. The observed complexes are understandably poor catalysts, but the structural and mechanistic insight obtained from such systems has assisted in the development of improved catalysts. 

Other active methylene compounds also react with both palladium(ll) <2002POL2653> and gold(m) <1997JOM243> to produce metallacyclobutane complexes (Scheme 34). The gold complex is not sufficiently stable for isolation. In these reactions, the silver oxide functions both as a base and a reagent for halide abstraction. In the gold series, 1,1,3,3-tetracyanopropane is also a competent pro-nucleophile <1999JOM219>. 

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

Cyclopropane oxidative addition to low-valent transition metals has been intensively investigated in the decades since the first metallacyclobutane complex was prepared by this methodology <1996CHEC-II(lb)887>. Comprehensive reviews on this topic are available <1980CGR149, 1994CRV2241>. 

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