Alkene metathesis intermediate alkylidene

The alkene metathesis reaction see Alkene Metathesis) exchanges alkylidene groups between different alkenes, and is catalyzed by a variety of high oxidation state, early transition metal species (equation 40). The reaction is of interest because it is the strongest bond in the alkene, the C=C bond, that is broken during the reaction. It is also commercially important in the Shell higher olefins process and in the polymerization of cycloalkenes. It is relevant to this article because carbenes are the key intermediates, and the best-known catalyst, (1), is a carbene complex. 

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. 

Alkene metathesis catalysis involves intermediates in which a transition metal is multiply bonded to carbon. These species are often referred to as nucleophilic caibenes when the carbon atom is negatively polarized. A more functional description is to name these compounds as alkylidene complexes, since they react to transfer an alkylidene moiety from a transition metid to a substrate carbon atom. Previous sections of this chapter have focused on a common example of this chemistry the process of metathesis that involves transition metal mediated interaction of carbon-carbon multiple bonds. 

Schrock carbene complexes undergo reaction with multiple bonds via four-center metallacyclic intermediates (51). Chapter 11 will consider what occurs when alkylidenes react with alkenes, a reaction known as alkene metathesis. Below are examples of Schrock carbenes reacting with polar multiple bonds such as C-N and C=0. 

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. 

Metal alkylidene complexes see Schrock type Carbene Complexes have been proposed as intermediates in many catalytic reactions, including alkene metathesis see Organic Synthesis Using Metal-mediated Metathesis Reaction, ... 

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

Computational studies have shown that alkane metathesis cannot occur via the o-bond metathesis between the C-C a-bonds and the M-C a-bonds originally proposed [101]. Experimental evidence has also suggested that the reaction mechanism must involve alkene metathesis as the key step and alkylidene hydrido metal complexes as associated intermediates [92, 93, 102]. To date, only few computational studies on alkane metathesis have been reported [103-106]. 

Alkyne polymerization can be initiated by metal-alkylidenes or metal-alkylidyne complexes. The mechanism involves metallacyclobutene or metallacyclobutadiene key intermediates, in the same way as alkene metathesis and ROM involve metal-lacyclobutane intermediates Katz mechanism, bottom of this page, an extension of the hauvin mechanism for alkyne metathesis. For instance, Schrock s catalyst W - u O - u 3, shown in section. as catalyst of disynunetrical alkyne metathesis, also initiates the polymerization of acetylene and terminal alkynes rather than metathesizing them. Indeed, the molecules of terminal alkynes successively insert into the W- bond of the metallacyclobutadiene intermediate... 

Alkyl complexes are intermediates in a number of homogeneous catalytic processes, such as carbonylation, alkene polymerization, and hydrogenation. Aryl complexes play a pivotal role in C-C bond-forming reactions such as Heck and Suzuki couplings (see Section 7.4). Alkylidene or carbene intermediates are involved in the versatile alkene metathesis reactions (see Section 7.3). 

From the above, a parallel appears to exist between the metathesis of alkenes and alkynes. Both reactions result in a redistribution of, respectively, alkylidene and alkylidyne groups. Moreover, the results obtained so far suggest that in both cases the reaction might proceed via a metallocyclic intermediate. 

Crowe proposed that benzylidene 6 would be stabilised, relative to alkylidene 8, by conjugation of the a-aryl substituent with the electron-rich metal-carbon bond. Formation of metallacyclobutane 10, rather than 9, should then be favoured by the smaller size and greater nucleophilicity of an incoming alkyl-substituted alkene. Electron-deficient alkyl-substituents would stabilise the competing alkylidene 8, leading to increased production of the self-metathesis product. The high trans selectivity observed was attributed to the greater stability of a fra s- ,p-disubstituted metallacyclobutane intermediate. 

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. 

There are no mechanistic details known from intermediates of copper, like we have seen in the studies on metathesis, where both metal alkylidene complexes and metallacyclobutanes that are active catalysts have been isolated and characterised. The copper catalyst must fulfil two roles, first it must decompose the diazo compound in the carbene and dinitrogen and secondly it must transfer the carbene fragment to an alkene. Copper carbene species, if involved, must be rather unstable, but yet in view of the enantioselective effect of the ligands on copper, clearly the carbene fragment must be coordinated to copper. It is generally believed that the copper carbene complex is rather a copper carbenoid complex, as the highly reactive species has reactivities very similar to free carbenes. It has not the character of a metal-alkylidene complex that we have encountered on the left-hand-side of the periodic table in metathesis (Chapter 16). Carbene-copper species have been observed in situ (in a neutral copper species containing an iminophosphanamide as the anion), but they are still very rare [9],... 

Olefin metathesis (olefin disproportionation) is the reaction of two alkenes in which the redistribution of the olelinic bonds takes place with the aid of transition metal catalysts (Scheme 7.7). The reaction proceeds with an intermediate formation of a metallacyclobutene. This may either break down to provide two new olefins, or open up to generate a metal alkylidene species which -by multiple alkene insertion- may lead to formation of alkylidenes with a polymeric moiety [21]. Ring-opening metathesis polymerization (ROMP) is the reaction of cyclic olefins in which backbone-unsaturated polymers are obtained. The driving force of this process is obviously in the relief of the ring strain of the monomers. 

Although the quasi-cyclobutane mechanism accounts fully for the products of the metathesis of alkenes at equilibrium, see above, some workers were uncomfortable with the unconventional nature of the bonding in the proposed intermediate. An alternative mechanism which brings about the same net alkylidene exchange (lower pathway in Scheme 12.14)... 

Olefin metathesis provides the principal synthetic context for metallacyclobutane reactivity this catalytic reaction proceeds by the transient, and reversible, formation of a metallacyclobutane intermediate from a metal alkylidene and an alkene. The olefin metathesis reaction has been exhaustively reviewed and is not directly discussed here... 

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