Metallacyclobutenes

It is useful to consider the possible formulations of alkyne and allyl bonding to metals in terms of Green s MLX formalism.64 Coordination of an alkyne in a simple dative two-electron fashion is denoted ML, whereas the limit of metallacyclobutene formation is denoted MX2. For the allyl ligand, three imaginable coordinations are possible simple q1 coordination is denoted MX, butq3 coordination can encompass both MLX (one a bond plus a dative alkene coordination) and MX3 (three M—C a bonds). 

Initial [2 + 2] cycloaddition of the alkyne to the saturated chromium carbene complex followed by [2 + 2] cycloreversion to yield a l-chroma-l,3,5-hexatriene may be an energetically more favorable reaction pathway (Figure 2.25). However, no energy minima could be located on the path from the starting carbene complex to the chromahexatriene. Hence, the metallacyclobutene shown in Figure 2.25 does not represent an intermediate but only a transition state. 

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. 

Direct metallacyclobutene formation by reaction of free tetrafluorocyclopropene and trigonal platinum complexes299 or tetragonal iridium complexes (equation 230)300 is also reported in the literature. These reactions may proceed via a labile -cyclopropene complex intermediate. However, the stereoselectivity observed in the product complexes of... 

The intermediacy of a metallacyclobutene is proposed upon reaction of the diphenylcy-clopropenone dimer spirolactone with CpCo(CO)2, ultimately yielding a >j4-vinylketene complex (equation 23 l)295a. Unlike the analogous iron complex (Section IV.B.2.a), no vinyl carbene complex was observed, and hence formation of the metallacyclobutene seems to be more likely. 

Reaction of the carbene complex 148 with alkyne affords vinylcarbene 150 via metallacyclobutene 149. In the intramolecular reaction of enyne 152, catalysed by carbene complex 151, the triple bond is converted to vinylcarbene 153 which then reacts with the double bond to give the conjugated diene 154. Generation of 154 is expected by the formation and cleavage of cyclobutene 155 as a hypothetical intermediate. Based on this reaction, Ru-catalysed intramolecular metathesis of enyne 156 gave the N-containing cyclic diene 157, from which (—)-stemoamide (158) was synthesiszed. The reaction can be understood by assuming the formation of the hypothetical cyclobutene 159 from 156 [52],... 

The carbene complex 253 reacts with alkyne to give vinylcarbene complex 255 via the metallacyclobutene 254. The triple bond in allylpropargylamine 256 reacts at first to form vinylcarbene 257, and cyclopropanation of the double bond gives 258 [82],... 

By controlling the stoichiometry of the reaction, selective conversion of metallacyclobutenes to the corresponding 7i-allyl complexes can be accomplished, illustrated for both rhenacyclobutene 25 (Equation 8) <1998JA722> and platinacyclobutene 26 (Equation 9) <19980M2953>. 

The presence of a transition metal is not necessarily required for hydrocarbon insertion. Alkyne incorporation has been reported for boracyclobutenes, as well as metallacyclobutene complexes of the transition elements. Boracyclobutene 51, a reactive intermediate prepared in situ (Section 2.12.9.2.1), inserts an additional equivalent of trimethylsilylacetylene into the B-C(sp2) bond to yield boracyclohexadiene 52 (Scheme 7). This isomerizes to the interesting bridged compound 53, an analogue of a nonclassical carbocation <1994AGE2306>. The related boracyclobutene 7 inserts the alkyne to yield a persistent boracyclohexadiene 54, but this product clearly arises from insertion into the boracyclobutene carbon-carbon bond rather than a boron-carbon bond <1994AGE1487>. 

Gycloreversion to the vinylalkylidene is rarely observed directly, despite being a commonly proposed step in carbene or alkylidene/alkyne reaction cascades. Bridged titanacyclobutene complex 57, however, clearly resists the formation of a strained internal alkylidene from alkyne extrusion, equilibrating instead with vinylalkylidene complex 58, stabilized by the addition of trimethylphosphine (Equation 20) <2003ICA27>. This system provides a very rare simultaneous observation of interconverting metallacyclobutene and vinylalkylidene isomers. 

Equilibrium cycloreversion of metallacyclobutane complexes to alkylidene intermediates is similar to that of metallacyclobutene complexes (Section 2.12.6.1.3). Metallacyclobutane complexes thus provide convenient progenitors of reactive alkylidene intermediates. The a-methylenetitanacyclobutane complex 79 decomposes into the vinylidene intermediate 103 (Scheme 17), which manifests nucleophilic character at the ct-carbon, consistent with... 

Among unique transformations on the metallacyclobutene framework, allene complexes of cobalt can be prepared by fluoride-induced desilylation of cobaltacyclobutene complex 39 three isomeric complexes bearing the same disubsituted allene are obtained (Equation 41) <1998JA1100>. 

Although the [2+2] cycloaddition continues to dominate the methodology for the synthesis of boracyclobutene and metallacyclobutene complexes, conceptually new and potentially general alternatives have recently been introduced. In particular, the central carbon alkylation of electrophilic propargyl and allenyl complexes has significantly enriched the palette of available metallacyclobutenes, raising considerable promise for the development of new reactions of relevance to organic synthesis. 

Metallacyclobutene complexes of both early and late transition metals can, in some cases, be prepared by intramolecular 7-hydrogen elimination, although the intimate mechanism of the reaction varies across the transition series. For low-valent late metals, the reaction is generally assumed to proceed via the oxidative addition of an accessible 7-C-H bond (Scheme 28, path A), but for early metals and, presumably, any metal in a relatively high oxidation state, a concerted cr-bond metathesis is considered most probable (path B). In this process, the 7-C-H bond interacts directly with an M-X fragment (typically a second hydrocarbyl residue) to produce the metallacycle with the extrusion of H-X (i.e., a hydrocarbon). Either sp3- or spz-hybridized C-H bonds can participate in the 7-hydrogen elimination. 

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