Metallocyclobutane intermediate

Second, several investigations have implied that the formation of cyclopropanes is related to olefin metathesis (5, 12, 13). Extrication of the metal from the metallocyclobutane intermediate has been suggested (5) as a route to the three-membered ring ... 

Soluble metathesis catalysts yield trans products in reactions with // / v-2-pentene, but generally are not very stereospecific with c/.v-2-pen-tene. In the latter case, the initially formed butenes and hexenes are typically about 60 and 50% cis, respectively. Basset noted (19) that widely diverse catalyst systems behaved similarily, and so it was suggested that the ligand composition about the transition metal was not a significant factor in the steric course of these reactions. Subsequently, various schemes to portray the stereochemistry have been proposed which deal only with interactions involving alkyl substituents on the reacting olefin or on the presumed metallocyclobutane intermediate. 

Initiation involves coordination of the double bond of monomer with the transition metal (imino and OR ligands not shown), cleavage of the 7t-bond with formation of a 4-membered metallocyclobutane intermediate, followed by rearrangement to form a metal-carbene propagating center ... 

Figure 20.11 Mechanistic scheme for n-heptane isomerization according to a bond-shift mechanism with a metallocyclobutane intermediate.

Two resonance-contributing structures (3a and 3b), in the formalism of ylide structures, can be used to describe metal carbene intermediates. The highly electrophilic character of those derived from Cu and Rh catalysts suggests that the contribution from the metal-stabilized carbocation 3b is important in the overall evaluation of the reactivities and selectivities of these metal carbene intermediates. Emphasis on the metal carbene structure 3a has led to the subsequently discounted proposal that cyclopropane formation from reactions with alkenes occurs through the intervention of a metallocyclobutane intermediate [18]. The metal-stabilized carbocation structure 3b is consistent with the cyclopropanation mechanism in which LnM dissociates from the carbene as bond-formation occurs between the carbene and the reacting alkene (Eq. 5.4) [7,15]. 

The extensive data accumulated by Nakamura and Otsuka, although interpreted by them as being due to the intervention of metal carbene and metallocyclobutane intermediates, can also be rationalized by an alternative mechanism in which coordination of the chiral Co(II) catalyst with the alkene activates the alkene for electrophilic addition to the diazo compound (Scheme 5.4). Subsequent ring closure can be envisioned to occur via a diazonium ion intermediate, without involving at any stage a metal carbene intermediate. 

The catalytic cycle proposed for the rhodium-porphyrin-based catalyst is shown in Fig. 7.18. In the presence of alkene the rhodium-porphyrin precatalyst is converted to 7.69. Formations of 7.70 and 7.71 are inferred on the basis of NMR and other spectroscopic data. Reaction of alkene with 7.71 gives the cyclopropanated product and regenerates 7.69. As in metathesis reactions, the last step probably involves a metallocyclobutane intermediate that collapses to give the cyclopropane ring and free rhodium-porphyrin complex. This is assumed to be the case for all metal-catalyzed diazo compound-based cyclo-propanation reactions. 

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. 

Catalysts comprising a metal-carbon double bond (metallocarbenes, or metallocenes) are efficient. With these initiators, the polymerization mechanism appears to involve coordination of the C=C double bond in the cycio- or dicycloalkene at a vacant d orbital on the metal. The metallocyclobutane intermediate which is formed decomposes to produce a new metal carbene and a new C=C bond. Propagation consists of repealed insertions of cycloalkenes at the metal carbene. 

RCM may proceed by the formation of a series of metallocyclobutane intermediates that break apart to form new alkenes and new metal carbenes, which propagate the reaction. The cycloalkene product accumulates as the reaction proceeds since the reverse reaction, ring-opening metathesis, is kinetically disfavored. 

Schrock and co-workers note that the chain mechanism is almost certainly correct, but major questions remain unanswered. They are conducting studies with alkyhdene complexes of niobium, tantalum, and tungsten, directed towards understanding in detail how and why metathesis catalysts work. From preliminary results they predict that the olefin co-ordinates to the metal before a metallocyclobutane complex can be formed, that rearrangement of metallocyclobutane is slow relative to the rate of metathesis, and that important chain-termination steps are rearrangement of metallocyclobutane intermediates and bimolecular decomposition of methylene complexes. For these systems, co-catalysts such as the alkyl-aluminium chlorides are not necessary the initial alkyl group on the metal... 

Finally, the partially selective Mechanism C in hydrogenolysis of cyclopentanes has a counterpart in dehydrocyclization of methylpentanes and n-hexane. The intervention of this mechanism, involving metallocyclobutane intermediates, is strongly supported by studies of aromatization (see Section V). 

Schulz and Achtsnit (77) state that this transfer process cannot be a secondary cracking, as this would lead to a different distribution pattern. Moreover, the absence of cracking under these conditions had been established separately (20). They, therefore, conclude that a metathesis reaction must take place. This is a very interesting suggestion, as the mechanism of olefin metathesis is well established. There seems to be general agreement in the literature on metathesis (65, 69) that the mechanism involves metal carbenes and a metallocyclobutane intermediate, for instance,... 

Alkene metathesis is a transition-metal-catalyzed reaction in which alkene bonds are cleaved and redistributed to form new alkenes [1-3]. The reaction proceeds through the formal [2 + 2] cycloaddition of an alkene and a metal alkylidene to yield a metallocyclobutane intermediate (Scheme 1). The productive retrocydoad-dition of this intermediate generates a new metal alkylidene and a new alkene product. These processes are generally reversible, and the reaction is under thermodynamic control. 

C2 comer of electronegatively substituted cyclopropanes. The coordination complexes thus formed, 36 and 37, then will rearrange to the metallocyclobutane intermediates of type 35. Energy levels of HOMOs would be associated with the ease with which the hydrogenolysis takes place. Further detailed experimental and theoretical scmtinies are required on this problem. 

Fig. 27.5 Initial steps in the mechanism of alkene metathesis involving first and second generation Grubbs catalysts. Two possibilities for the formation of the metallocyclobutane intermediates are shown.

In addition, some monometallic mechanisms based on a different mode of monomer insertion were also proposed. An example is a reaction mechanism that was proposed by Ivin and co-woikers. This mechanism is based on an insertion mechanism involving an a-hydrogen reversible shift, carbene, and a metallocyclobutane intermediate ... 

The generally accepted catalytic cycle for this change partners dance was proposed by Yves Chauvin and is believed to involve metallocyclobutane intermediates that result from reaction of metal alkylidenes (also called metal carbenes) with alkenes. The catalysts themselves are metal alkylidenes, in fact. Chauvin s catalytic cycle for olefin metathesis is summarized here. 

The Grubbs catalyst and the starting alkene undergo a [2 -1- 2] cycloaddition reaction (Section 8.19). This reaction forms two different metallocyclobutane intermediates because the metal can bond to either sp carbon of the alkene. 

Each of the metallocyclobutane intermediates undergoes a ring-opening reaction. Two metal-containing intermediates are formed (I and II). 

A major advance in polymerization via metathesis occurred when Schwab et al7 and Schrock unveiled well-defined transition metal catalysts. Molybdenum- and rathenium-based catalysts are highly active in olefin metathesis reactions. The general mechanism that underlies their reactivity is outlined in Figure 2. The catalyst reacts with the alkene via a (2 + 2] cycloaddition reaction to afford a metallocyclobutane intermediate. Cycloreversion with concomitant ring opening relieves strain and unleashes a new metal carbene that reacts with subsequent monomers to yield the polymer chain. 

This reaction was first used for the synthesis of higher olefins. The mechanism of metathesis proposed by Chauvin (Fig. 2.14) includes reaction of the metal methylene (metal aUcyH-dene) with the olefin, forming a metallocyclobutane intermediate. This intermediate then cleaves, yielding ethylene and a new metal alkyhdene. The ethylene formed contains one methylene from the catalyst and one from the starting olefin. The new metal alkyhdene contains the metal with its Hgands (indicated by the brackets around the metal) and an... 

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