Olefin metathesis cyclopropane formation

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 ... 

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 high selectivity of olefin metathesis with (2a) was key in the formation of diverse nonracemic vinyl cyclopropanes from chiral homoaUyhc alcohols and allyl chlorodimethylsilane in three steps, or allyl trimethylsilane in two steps, giving diastereomerically pnre cis or trans... 

Rhodium-based catalysis suffers from the high cost of the metal and quite often from a lack of stereoselectivity. This justifies the search for alternative catalysts. In this context, ruthenium-based catalysts look rather attractive nowadays, although still poorly documented. Recently, diruthenium(II,II) tetracarboxylates [42], polymeric and dimeric diruthenium(I,I) dicarboxylates [43], ruthenacarbor-ane clusters [44], and hydride and silyl ruthenium complexes [45 a] and Ru porphyrins [45 b] have been introduced as efficient cyclopropanation catalysts, superior to the Ru(II,III) complex Ru2(OAc)4Cl investigated earlier [7]. In terms of efficiency, electrophilicity, regio- and (partly) stereoselectivity, the most efficient ruthenium-based catalysts compare rather well with the rhodium(II) carboxylates. The ruthenium systems tested so far seem to display a slightly lower level of activity but are somewhat more discriminating in competitive reactions, which apparently could be due to the formation of less electrophilic carbenoid species. This point is probably related to the observation that some ruthenium complexes competitively catalyze both olefin cyclopropanation and olefin metathesis [46], which is at variance with what is observed with the rhodium catalysts. 

Cyclopropanes and oxiranes can play an important role in initiation of olefin metathesis reactions via the formation of four-membered metallacycles and their subsequent cleavage sequences (13a, 13b). Conversely, the reverse of such reactions may sometimes be responsible for terminating a metathesis chain reaction. 

The cyclopropanation of electron-rich alkenes requires modified reaction conditions. Warming a solution of a metal carbene and an enol ether at atmospheric pressure results in the formation of an alkene which apparently is formed in an olefin metathesis reaction. However, good yields of cyclopropanation products are obtained under CO pressure. Under these conditions the nucleophilic alkene is supposed to add to the carbene carbon to generate zwitterion 30 in which... 

The reaction of a metal-carbene complex with an olefin may lead to either cyclopropane or metathesis products, depending on the metal center and its ancillary ligands. In the case of olefin metathesis, the reaction may occur in variations that have enormous numbers of synthetic applications. Although these products are diverse in structure, they are all related by the same basic metal-carbene-mediated mechanism of formation. Because we provide only an overview of applications in this section and do not specify the particular catalyst used in each case, we direct the reader to the extensive reviews that are available for more information [32]. 

At this early stage of comprehension of the interrelation between metathesis and cyclopropanation, many questions remain. Why is the formation of cyclopropanes such a rare occurrence with typical metathesis catalysts, yet favored with some zero-valent carbene complexes What is the role of prior complexation of the olefin with the metal in determining the reaction course for metal-carbene species How are typical metathesis carbenes polarized, and how does this polarization influence selectivity of metathesis reactions (e.g., regenerative metathesis of a-... 

Both the intramolecular and the intermolecular secondary metathesis reactions affect the polymerisation kinetics by decreasing the rate of polymerisation, because a fraction of the active sites that should be available as propagation species are involved in these non-productive metathesis reactions. The kinetics of polymerisation in the presence of metal alkyl-activated and related catalysts shows in some cases a tendency towards retardation, again due to gradual catalyst deactivation [123]. Moreover, several other specific reactions can influence the polymerisation. Among them, the addition of carbene species to an olefinic double bond, resulting in the formation of cyclopropane derivatives [108], and metallacycle decomposition via reductive elimination of cyclopropane [109] deserve attention. 

The factors that direct a metallacyclobutane intermediate along one of these pathways are only partially understood. For example, in the case shown in Fig. 4.10, both metathesis and cyclopropanation are mediated by the same tungsten-carbene complex when the reaction is performed in a non-coordinating solvent [31]. These conditions are consistent with reaction via a metallacyclobutane because they allow the olefin to coordinate to the metal center, a prerequisite for [2 -I- 2] cycloaddition. In contrast, only cyclopropanation occurs when the reaction is performed in a coordinating solvent, presumably because formation of a metal-solvent adduct prevents olefin coordination and leaves only a direct carbene transfer mechanism available. 

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