Carbene Complexes from Olefin Metathesis Reactions

Metal-Carbene Complexes from Olefin Metathesis Reactions. 

In scrutinizing the various proposed reaction sequences in Eq. (26), one may classify the behavior of carbene complexes toward olefins according to four intimately related considerations (a) relative reactivities of various types of olefins (b) the polar nature of the metal-carbene bond (c) the option of prior coordination of olefin to the transition metal, or direct interaction with the carbene carbon and (d) steric factors, including effects arising from ligands on the transition metal as well as substituents on the olefinic and carbene carbons. Information related to these various influences is by no means exhaustive at this point. Consequently, some apparent contradictions exist which seem to cast doubt on the relevance of various model compound studies to conventional catalysis of the metathesis reaction, a process which unfortunately involves species which elude direct structural determination. 

Studies on these carbene complexes, especially those of the Schrock type, have attracted special interest in connection with the mechanism of catalytic olefin metathesis reactions. The formation of metallacyclobutane intermediate from the oxidative cycloaddition reaction between carbene complex and olefin was found to be an important key step in the catalytic cycle (eq. (5)). 

In the 1970 s an intense interest in the olefin metathesis reaction [35] served as a driving force for research concerning metal-carbon double bonds. In its simplest form the olefin metathesis reaction consists of a redistribution of alkylidene components of olefins (Eq. 6). It was known to be promoted by tungsten, molybdenum, and rhenium, although at the time the catalytic reactions were black boxes, with nothing known about the mechanism or the detailed nature of the catalyst. Among the proposals was one that consisted of a reaction between an alkylidene complex (or carbene complex at that time) and an olefin to give a metallacyclobutane intermediate, from which an olefin could be lost and a new alkylidene complex formed, [36] as shown in... 

Tebbe found that titanocene complexes promoted olefin metathesis in addition to carbonyl olefination. Despite the fact that these complexes have low activity, they proved to be excellent model systems. For example, the Tebbe complex exchanges methylene units with a labeled terminal methylene at a slow rate that can be easily monitored (Eq. 4.6) [54]. This exchange is the essential transformation of olefin metathesis. When reactions with olefins are performed in the presence of a Lewis base, the intermediate titanium metallacycle can be isolated and even structurally characterized (Eq. 4.7) [61] These derivatives were not only the first metathesis-active metallacyclobutane complexes ever isolated, but they were also the first metallacyclobutanes isolated from the cycloaddition of a metal-carbene complex with an olefin. These metallacycles participate in all the reactions expected of olefin metathesis catalysts, especially exchange with olefins... 

Carbene Complexes.— Protonation of the chelated iron-carbene complex (59) in the presence of ethylene yields (60), the first example of a metal-carbene-olefin complex of the type postulated as intermediates in olefin metathesis reactions. Hydride abstraction from (61) affords (62). The new carbene complexes (63), (64), and (65) have been prepared by treatment of [Fe(jj-C5H5)(SnPh3)-(CO)(CS)l, [Fe( -C6H6) C(S)OPh (CO)3], and [Fe( -CS3) P(OMe)3 (CO)3] with ethylenediamine, methylfluorosulphonate (followed by methanol and PF e ions),... 

The intermediacy of carbene complexes in the olefin metathesis reaction has led to the identification of several interesting species, either synthesized as models or isolated from catalytic systems (See also bibliography). Noteworthy are compounds (5) and (9), both of which have been structurally characterized. 

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

The potential synthetic utility of titanium-based olefin metathesis and related reactions is evident from the extensive documentation outlined above. Titanium carbene complexes react with organic molecules possessing a carbon—carbon or carbon—oxygen double bond to produce, as metathesis products, a variety of acyclic and cyclic unsaturated compounds. Furthermore, the four-membered titanacydes formed by the reactions of the carbene complexes with alkynes or nitriles serve as useful reagents for the preparation of functionalized compounds. Since various types of titanium carbene complexes and their equivalents are now readily available, these reactions constitute convenient tools available to synthetic chemists. 

It has been shown that ruthenium carbene complex lb developed for olefin metathesis can catalyze RCM of enynes. Using this catalyst lb, five- to nine-membered ring compounds 3 are synthesized from enyne 2 (Scheme j) i Sa-iSc The reaction procedure for RCM of an enyne is very simple. A benzene solution of enyne 2b is stirred in the presence of 1 mol% of ruthenium carbene complex lb at room temperature (RT) under argon gas to give cyclic compound 3b having a diene moiety. 

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