Metallacyclobutane

The possibility of being involved in olefin metathesis is one of the most important properties of Fischer carbene complexes. [2+2] Cycloaddition between the electron-rich alkene 11 and the carbene complex 12 leads to the intermediate metallacyclobutane 13, which undergoes [2+2] cycloreversion to give a new carbene complex 15 and a new alkene 14 [19]. The (methoxy)phenylcar-benetungsten complex is less reactive in this mode than the corresponding chromium and molybdenum analogs (Scheme 3). 

The ADMET cycle involves the formation of two metallacyclobutane intermediates [D, F], whereas the ROMP mechanism contains only one. 

A second metallacyclobutane [F] is formed via die reassociation of terminal olefin from discrete oligomers (or monomer) with the active methylidene, produced in [E] (see above). 

Upon collapse of this metallacyclobutane [F], ethylene is driven off and an intermediate alkylidene is formed which bears the growing polymer chain [C]. 

Attempts to exploit the reaction of the dianion with alkyl halides to produce a c/.v-dialkyl complex by using 1,2- or 1,3-dihaloalkanes did not indeed give this result. The reaction of Ru(Por) " with 1,2-dibromoethane was sucessful, but the resulting metallacyclopropane product is better formulated as a /r-complex of ethene, and will be discussed below in the section on alkenc and alkyne complexes. The corresponding reaction of the diiinion with 1,3-dichloropropane gave no evidence for a metallacyclobutane. but instead free cyclopropane was detected by GC analysis and the porphyrin product was Ru(TTP)(THF)2. ... 

The stoichiometry of this conversion is in accordance with a carbene starting structure. An alternating alkyUdene/metallacyclobutane mechanism [102, 131-133], which has precedent in the ethylene polymerization catalyzed by a Ta(III) neopentilydene complex [134], has been proposed where the chro-miiun alkyUdenes undergo [2-1-2] cycloaddition to give chromacyclobutane intermediates (mechanism III in Scheme 7). 

They correspond to the cross-metathesis of propylene with the neopentyli-dene fragment (Scheme 18), and their relative ratio corresponds to a photograph of the active site as they are formed. Depending on how propylene will approach the carbene, it will generate different metallacyclobutanes, whose stabilities can direct the relative amounts of cross-metathesis (and selfmetathesis) products. This model is based on the following the favoured cross-metathesis product arises from the reaction pathway, in which [1,2]-interactions are avoided and [1,3]-interactions are minimized (here shown with both substituents in equatorial positions) [83]. 

It also explains the /Z selectivity of products at low conversions (kinetic ratio. Scheme 19). In the case of propene, a terminal olefin, E 2-butene is usually favoured (E/Z - 2.5 Scheme 19), while Z 3-heptene is transformed into 3-hexene and 4-octene with EjZ ratios of 0.75 and 0.6, respectively, which shows that in this case Z-olefins are favoured (Scheme 20). At full conversion, the thermodynamic equilibriums are reached to give the -olefins as the major isomers in both cases. For terminal olefins, the E olefin is the kinetic product because the favoured pathway involved intermediates in which the [ 1,2]-interactions are minimized, that is when both substituents (methyls) are least interacting. In the metathesis of Z-olefins, the metallacyclobutanes are trisubstituted, and Z-olefins are the kinetic products because they invoke reaction intermediates in which [1,2] and especially [1,3] interactions are minimized. 

In the case of olefin metathesis, the selectivity in initiation products can be understood in terms of minimization of the steric interactions in the metal-lacyclobutane intermediates (vide supra), which are governed by the relative position of the substituents the metallacyclobutane with substituents in pos-... 

The product selectivities in propane metathesis can also be explained by using the same model in which [1,3]- and [1,2]-interactions determine the ratio of products. For instance, the butane/pentane ratios are 6.2 and 4.8 for [(= SiO)Ta(= CHfBu)(CH2tBu)2] and [(= SiO)2Ta - H], respectively (Table 5). A similar trend is observed for the isobutane/isopentane ratio, which are 4.1 and 3.0, respectively. The higher selectivity in butanes (the transfer of one carbon via metallacyclobutanes involving [l,3]-interactions) than that of pentanes (the transfer of two carbons via metallacyclobutanes involving [1,2]-interactions) is consistent with this model (Scheme 28). 

Further improvements in activity of the imidazol-2-ylidene Ru complexes might be attained by the incorporation of a better a-donor substituents with larger steric requirements. The ligands that most efficiently promote catalytic activity are those that stabilize the high-oxidation state (14 e") of the ruthenium metallacyclobutane intermediate [7]. Both ligand-to-metal a-donation and bulkiness of the NHC force the active orientation of the carbene moiety and thus contribute to the rapid transformation into metallacyclobutane species [7b]. Both can be realized by incorporation of alkyl groups in 3,4-position of imidazol-2-ylidene moiety, lyie Me. Me... 

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.

Confirmation that the polymerizations proceed via metallacyclic intermediates was obtained by studying the ROMP of functionalized 7-oxanorbornadienes. These polymerize slower than their norbornene analogs, allowing NMR identification of the metallacyclobutane resonances and subsequent monitoring of ring opening to the first insertion product. In addition, the X-ray crystallographic structure of complex (212) has been reported.533... 

The metal-carbenoid intermediates, especially ones derived from a-diazocarbonyl compounds, are electrophilic, and electron-rich olefins in general react more easily with the carbenoid intermediates than electron-deficient olefins. For the interaction of metal carbenoid and olefin, three different mechanisms have been proposed, based on the stereochemistry of the reactions and the reactivity of the substrates (Figure 12) 21 (i) a nonconcerted, two-step process via a metallacyclobutane 226,264... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

Metallacyclobutanes have been proposed as intermediates in a number of catalytic reactions, and model studies with isolated transition metallacyclobutanes have played a large part in demonstrating the plausibility of the proposed mechanisms. Since the mechanisms of heterogeneously catalysed reactions are especially difficult to determine by direct study, model studies are particularly valuable. This article describes results which may be relevant to the mechanisms of isomerization of alkanes over metallic platinum by the bond shift process and of the oligomerization or polymerization of alkenes. 

At this point it should be noted that this mechanism is unexpected. Simple platinum alkyls decompose by 3-elimination whenever possible and there are no well-established examples of a-elimination [10]. All previous studies have indicated that metallacyclobutanes decompose by 3-elimination, even for tantalum and titanium derivatives for which a-elimination is a frequent mechanism for decomposition of the simple alkyls [11, 12]. There is even a labelling study which appears to prove the 3-elimination mechanism for decomposition of platinacyclobutanes (equation 3) [13]. 

Since 8-elimination reactions are often rapid and reversible, it is surprising that no examples of metallacyclobutane to metallacyclopentane ring expansion reactions according to equation (6) have been found. 

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. 

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. 

E.O. Fischer s discovery of (CO)sW[C(Ph)(OMe)D in 1964 marks the beginning of the development of the chemistry of metal-carbon double bonds (1). At about this same time the olefin metathesis reaction was discovered (2), but It was not until about five years later that Chauvln proposed (3) that the catalyst contained an alkylidene ligand and that the mechanism consisted of the random reversible formation of all possible metallacyclobutane rings. Yet low oxidation state Fischer-type carbene complexes were found not to be catalysts for the metathesis of simple olefins. It is now... 

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). 

These results at least demonstrate that ethylene can be polymerized by an alkylidene hydride catalyst, probably by forming a metallacyclobutane hydride intermediate. The extent to which this is relevant to the more classical Ziegler-Natta polymerization systems (27) is unknown. Recent results in lutetium chemistry (28), where alkylidene hydride complexes are thought to be unlikely, provide strong evidence for the classical mechanism. 

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