Metallacyclobutadiene complexes

In reaction 71 the products are (i) EtC=CCMe3, resulting from the metathesis reaction, and (ii) a metallacyclobutadiene complex, produced by addition of a second molecule of... 

The crystal structure of a Fischer-type metallacyclobutadiene complex of rhenium 6 has also been reported <1993JA9986, 1990JOMC1>. The bond lengths between Re and the -carbons (2.18 and 2.13 A) are very similar, but the two C-C bond lengths are considerably different (C(l)-C(2) 1.36 A, C(2)-C(3) 1.45 A). In addition, the C(3)-0(4) bond distance (1.30 A) is much longer than C(5)-0(6) (1.19 A) but shorter than C(5)-0(7) (1.34 A). Therefore, the structure of rhenacyclobutadiene 6 is expected to be best represented by significant contributions from the resonance structures 6a and 6b and a smaller contribution from structure 6c. 

The reversible [2+2] cycloaddition of metal alkylidyne or Fischer-type metal carbyne complexes remains the only general methodology for the synthesis of metallacyclobutadiene complexes. Recent literature revolves principally around the heavier group 6 metals and the investigation of intermediates in catalytic alkyne metathesis (Scheme 25 Equation 45) <1996CHEC-II(lb)887> (W <2005OM4684>, Mo <2003JOM56>). 

The reaction of the rhenium alkylidyne complex 277 with diisopropyl-acetylene and with diethylacetylene [Eq. (196)] demonstrates the sensitivity of metathesis reactions toward steric factors (57). With diisopropylace-tylene an alkylidyne complex is obtained whereas the reaction with diethylacetylene gives a metallacyclobutadiene. In the metathesis reactions the alkyne with the bulkiest groups cleaves most easily from intermediate metallacyclobutadiene complexes. The rhenacyclobutadienes with the smallest substituents thus become sinks and slow down the effective rate of metathesis. The alkylidyne alkylidene rhenium complex 278 is an active olefin metathesis catalyst (52). Reaction with hexene transforms the neo-pentylidene group into a propylidene group as shown in Eq. (197). 

Cyclopropenyl systems can be formed from metallacyclobutadiene complexes. Here no decomplexation of unsaturated cyclopropanes is observed. The tungstacyclobutadiene complex 12 is converted into a cyclopropenyl complex 13 upon addition of a nitrogen nucleophile. ... 

Alkynes can also undergo metathesis reactions catalyzed by transition-metal carbyne complexes. The intermediates in these reactions are believed to be metallacyclobutadiene species, formed from the addition of an alkyne across a metal-carbon triple bond of the carbyne (Figure 14.35). The structures of a variety of metallacyclobutadiene complexes have been determined, and some have been shown to catalyze alkyne metathesis. 

Alkylidyne complexes also undergo [2+2] reactions. The tris(aryloxy)tungsten neo-pentylidyne complex in Scheme 13.30 imdergoes a [2+2] reaction with 3-hexyne to generate a symmetric metallacyclobutadiene complex. This product is a resonance hybrid of the two metallacyclobutadienes shown in the equation. 

The metallo-square hauvin s mechanism is still operating, as demonstrated by the isolation of metallacyclobutadiene complexes formed by cycloaddition of alkynes to alkylidyne complexes. The metallacyclobutadiene complexes themselves can also serve as alkyne metathesis catalysts confirming their intermediacy in the catalytic reactions starting from the alkylidyne complexes. Interestingly, they do not react readily with alkenes, rendering alkyne metathesis selective in the presence of olefmic bonds. ... 

An orbital interaction diagram for the tungsten-metallacyclobutadiene complex 18.36 at a square geometry. The orbitals that contribute to the a framework are indicated with dotted lines. 

The currently known carbometallation chemistry of the group 6 metals is dominated by the reactions of metal-carbene and metal-carbyne complexes with alkenes and alkynes leading to the formation of four-membered metallacycles, shown in Scheme 1. Many different fates of such species have been reported, and the readers are referred to reviews discussing these reactions.253 An especially noteworthy reaction of this class is the Dotz reaction,254 which is stoichiometric in Cr in essentially all cases. Beyond the formation of the four-membered metallacycles via carbometallation, metathesis and other processes that may not involve carbometallation appear to dominate. It is, however, of interest to note that metallacyclobutadienes containing group 6 metals can undergo the second carbometallation with alkynes to produce metallabenzenes, as shown in Scheme 53.255 As the observed conversion of metallacyclobutadienes to metallabenzenes can also proceed via a Diels-Alder-like... 

A particularly striking structural feature of the vanadium complex is the exceptional long distance (R) of 171 pm between the ring carbons closest to the metal compared to that in the Pt and Pd complexes (158 pm)315. This suggests that the vanadium complex approaches a metallacyclobutadiene structure, found in the analogous trigonal-bipyrami-dal rhenium complexes315 (Section V. A.2). 

A potential side reaction is the formation of a metallatetrahedrane complex by tau-tomerization of the metallacyclobutadiene intermediate or by its direct formation from the reactants756. 

By analogy with alkene metathesis, carbyne complexes might be expected to mediate the metathesis of alkynes, which indeed they do, but with some specific limitations. The basic mechanism parallels that for alkene metathesis, with the key intermediate being a metallacyclobutadiene which may break down in one of two possible directions (Figure 5.44). 

Molybdenum and tungsten carbyne (alkylidyne) complexes frequently undergo 2+2 cycloaddition reactions with alkynes to give the corresponding metallacyclobutadiene... 

Terminal acetylenes are not metathesized well. The main reason is that metallacyclobutadienes carrying a hydrogen atom at the central ring carbon atom are easily deprotonated. A few well-defined examples formed according to Eq. (205) from alkylidyne complexes and terminal acetylenes... 

A general approach to metallacyclobutadienes (90) is the reaction of carbyne complexes with alkynes. These four-membered rings are intermediates in the metathesis of acetylenes. Compounds 90 (M = W) offer a possi-... 

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)). 

Perhaps the most remarkable illustration of the ability of metals to activate alkynes comes from reactions in which complete scission of the carbon-carbon triple bond occurs. On the stoichiometric level these include examples in which carbyne complexes are produced from alkyne completes as in the melt-thermolysis of CpCo(PPh3)(RCsCR) [112] or from reactions of alkynes with unsaturated metal species (Scheme 4-34) [113]. The remarkable alkyne metathesis reaction (Scheme 4-35), which involves overall cleavage and regeneration of two o-and four rt-bonds, is conceptually related. A variety of functionalized alkynes can be tolerated as metathesis substrates [114] and especially effective catalysts for these reactions are Mo(VI)-and W(VI)-carbyne complexes. Metallacyclobutadienes 64, formed by the reaction of the alkyne with a metal-carbyne complex, appear to be central intermediates in these reactions and the equilibrium between metallacycle and alkyne/metal-carbyne is observable in some cases [115]. 

The metallacyclobutadiene formation from acetylene and molybdenum carbyne complex CI3M0CH was studied [45] along a postulated least-motion pathway as shown in Fig. 12. The CI3M0CH was brought together with the acetylene molecule such that the three carbon atoms and the metal center remained coplanar throughout the course of the reaction. It has been found... 

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