Metallacyclobutenes intermediate

In addition to the stoichiometric preparative and reactivity manifolds discussed throughout this chapter, metallacyclobutane and metallacyclobutene intermediates play an essential role in a range of synthetically valuable, or potentially valuable, synthetic transformations, both catalytic and stoichiometric. The tremendous range of... 

The reaction of alkyl-substituted tungsten-carbene complexes of the type (88b) have been reported by Macomber to react with alkynes to give dienes of the type (319). One mechanism that has been proposed to account for this product is a 3-hydride elimination from the metallacyclobutene intermediate (320) and subsequent reductive elimination in the metal hydride species (321). An additional example of this type of reaction has been reported by Rudler, also for an alkyl tungsten carbene complex. Chromium complexes have not been observed to give diene products of this type the reaction of the analogous chromium complex (88a) with diphenylacetylene gives a cyclobutenone as the only reported product (see Scheme 31). Acyclic products are observed for both tungsten and chromium complexes in their reactions with ynamines. These reactions produce amino-stablized carbene complexes that are the result of the formal insertion of the ynamine into the metal-carbene bond. ... 

Metal carbenes are also initiators for the metathetical polymerization of alkynes by, e. g., catalysts such as M0CI5 and WClg. Reaction of the metal carbene with an alkyne gives a metallacyclobutene intermediate, followed by opening of this intermediate metallacycle into a new metal carbene that in its turn can interact with another alkyne molecule, and so on eq. (13). Metathesis of acetylenes proceeds through reactions between an alkylidyne complex and an alkyne via a metallacyclobutadiene intermediate (eq. (14)). 

Aratani has rationalized the stereoselectivity of his catalysts by a model based on a metallacyclobutene intermediate (cf. Scheme 4, reaction b) [18]. For semi-corrin and related ligands, formation of a metallacyclobutane seems less attractive because of steric hindrance, and a different pathway, analogous to reaction a in Scheme 4, has been proposed [36,40,53b]. Fig. 2 shows the postulated ster-... 

Finally, Katz and subsequently Rooney (386-388) showed that (CO)sW [C(OMe)Ph] catalyzes the polymerization of alkynes via carbene-alkyne and purported metallacyclobutene intermediates (Scheme 34). Later computational work by Hofmann (388b) called into question the formation of the latter species, suggesting that the reaction proceeds directly via the vinyl-carbene. Formation of the alkyne-carbene intermediate by low-temperature photolysis of (CO)5W[C—(OMe)Ph] in the presence of alkynes leads to polymerization upon warming. The use of a pre-formed carbene complex is not required since active polymerization catalysts can be formed by photolysis of W(CO)5 in the presence of terminal alkynes in hydrocarbon solutions. A key step in catalyst generation is rearrangement of a coordinated alkyne to a vinylidene ligand (389). 

Chromium oxide catalysts, supported on AI2O3 and activated by LiAlH4, have been claimed to cause ROMP of cycloalkenes but the yields are low (Eleuterio 1957). Cr(CO)3(mesitylene) initiates the polymerization of alkynes, possibly via a metallacyclobutene intermediate (Farona 1974 Woon 1974) see Ch. 10. 

The formation of a high percentage of five-mem-bered rings by tail-to-tail cyclopolymerizations via a-substituted metallacyclobutene intermediates, in the classical systems as well as the well-defined systems, is further evidence that the disubstituted alkylidene intermediate is, in fact, surprisingly reac-... 

Cr(CO)s =C(OMeX HCH2CH2 and alkynes results in a [4+2+1-2] cycloaddition process leading to cyclopentenones. The proposed mechanism is complex and involves a metallacyclobutene intermediate.286 The reaction of 1,6- or 1,7-enynes with Mo(CO)5 =C(OMe)Bu" leads to vinylcyclopropanes in high yield. The yields of the corresponding chromium complexes are... 

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... 

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 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>. 

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... 

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

A cycloaddition process between the Rh Ca bond of the allenylidene derivative 38 and the C=C bond of the terminal alkyne has been evoked in the formation of the zwitterionic 7t-allyl-allenyl complexes 81 (Scheme 28), the initially formed metallacyclobutenes 80 evolving into 81 by formation of carbene intermediate [RhCl(P/-Pr3)2(=CHCR=C=C=CPh2)] (R = Ph, p-MeC6H4, SiMca) and subsequent migration of one of the phosphine ligands from the metal to the carbene carbon atom [205]. 

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