Metallacyclopentadiene

There are relatively few examples of C-C bond formation on solid surfaces under UHV conditions. There are virtually no examples of catalytic C-C bond formation under such conditions. Perhaps the closest precedent for the present studies on reduced Ti02 can be found in the studies of Lambert et al. on single crystal Pd surfaces. Early UHV studies demonstrated that acetylene could be trimerized to benzene on the Pd(lll) surface in both TPD and modulated molecular beam experiments [9,10]. Subsequent studies by the same group and others [11,12] demonstrated that this reaction could be catalyzed at atmospheric pressure both by palladium single crystals and supported palladium catalysts. While it is not clear that catalysis was achieved in UHV, these and subsequent studies have provided valuable insights into the mechanism of this reaction as catalyzed by metals, including spectroscopic evidence for the hypothesized metallacyclopentadiene intermediates [10,13,14]. 

An intriguing annulation has been observed upon treatment of a Zr(II)-borata-benzene complex with an alkyne (Scheme 23).38 This reaction is believed to proceed through generation of the normal metallacyclopentadiene intermediate, followed by migration of the Zr—C bond to the electrophilic boron, and then formation of the C—C bond. Both the starting Zr(II) complex and the annulation product have been crystallographically characterized. 

Rhodium-catalyzed reactions of diynes and an isonitrile give rise to iminocyclopentadienes (Equation (68)).421 Portionwise addition of the isonitrile (5 x0.2equiv.) was found to increase the yield. The reaction may proceed through formation of metallacyclopentadienes followed by insertion of an isonitrile molecule. 

Heteroatom transfer in metallacyclopentadienes was first developed in the context of cobalt chemistry in the mid-1970s [27]. Cobaltacyclopentadienes were converted into various five-membered heterocyclic compounds such as pyrrole and thiophene, and into six-mem-bered heterocyclic compounds such as pyridine and pyridone derivatives. In the case of zirconacydopentadienes, the heteroatom compound must bear at least two halide substituents, since the Cp2Zr moiety is re-converted to the stable Cp2ZrX2. Indeed, this is the driving force behind the heteroatom transfer of zirconacydopentadienes. 

Additions of metallacyclopentadienes to carbon—carbon triple bonds are rare, and only a few examples are known (Eq. 2.45) [37]. The 1,1-addition of zirconacyclopentadienes is quite different from other carbon—carbon bond-forming reactions described in this chapter. This reaction does not require transmetalation of zirconacyclopentadienes to other metals. Thus, in the absence of any added metal halide, zirconacyclopentadienes react with propynoates to give cyclopentadiene derivatives. This reaction requires the use of at least 2 equivalents of the propynoate (Eq. 2.46). 

Generally speaking, two mechanisms may be considered for the formation of benzene derivatives from metallacyclopentadienes. These are the concerted mechanism (Path A) and the insertion (addition) mechanism (Path B), as shown in Eq. 2.49. 

There are several examples of the concerted mechanism. However, no report of an insertion of a carbon—carbon triple bond into a metallacyclopentadiene had appeared prior to discovery of this reaction. At low temperatures, during the reaction of zirconacyclopentadienes with DMAD, the formation of trienes (79) is observed upon hydrolysis. This clearly indicates that the benzene formation involves the insertion (addition) reaction of DMAD. As shown in Eq. 2.50, the alkenyl copper moiety adds to the carbon—carbon triple bond of DMAD and elimination of Cu metal leads to the benzene derivatives 72. Indeed, a copper mirror is observed on the wall of the reaction vessel. 

Pyridine formation, by the reaction of a metallacyclopentadiene with a nitrile, has been extensively investigated in the case of cobalt [If]. When pyridine derivatives are prepared from two different alkynes and a nitrile, specific substituents are needed for the selective coupling reactions. In most cases, a mixture of two isomers (91 and 92) is obtained, the formation of which can be rationalized as shown in Eq. 2.61 [If,27a,44]. 

Terminal acetylenes and Ru3(CO)j2 yield complexes of the type [57] (9,190, 336), whereas internal acetylenes form either complexes [56] or acetylene-substituted RU4 complexes (229). Alternatively, two acetylene moieties are incorporated with formation of metallacyclopentadienes (229), a class of compounds more familiar in osmium cluster chemistry (cf. Chapter 3.4.). Instead of two acetylene molecules, one molecule of an arylbutadiene may be the precursor of the metallacycle (382). 

Incorporation of a third acetylene molecule takes place by CO replacement and without interference with the metallacyclopentadiene ring (170, 371, 379). In the cluster, then, the three acetylene ligands rearrange to a triacetylene ligand of unknown structure before the benzene is liberated (371, 379). 

It appears difficult to propose a unified mechanism to explain all experimental observations of the cyclotrimerization of acetylene. The most common pathway, studied mainly with cobalt complexes,72 73 involves a metallacyclopentadiene intermediate ... 

Metallacyclopentadienes undergo a range of synthetically versatile reactions which proceed with extrusion of the metal atom and attendant ligands. Thus, reactions with alkenes and alkynes afford cyclohexa-1,3-dienes and arenes (Scheme 6), and thiophenes, selena-cyclopentadienes, pyrroles and cyclopentadienones (indenones, fluorenones) can be obtained by treatment with sulfur, selenium, nitroso compounds and CO, respectively. The best studied substrates for such reactions are cobaltacyclopentadienes of the type (24a), which have been converted into a wide variety of arenes, cyclohexadienes and five-membered heterocycles, many of which would be very difficult to obtain by conventional organic procedures (74TL4549, 77JOM(139)169, 80JCS(P2)1344). 

Oxidative cyclization is another type of oxidative addition without bond cleavage. Two molecules of ethylene undergo transition metal-catalysed addition. The intermolecular reaction is initiated by 7i-complexation of the two double bonds, followed by cyclization to form the metallacyclopentane 12. This is called oxidative cyclization. The oxidative cyclization of the a,co-diene 13 affords the metallacyclopentane 14, which undergoes further transformations. Similarly, the oxidative cyclization of the a,co-enyne 15 affords the metallacyclopentene 16. Formation of the five-membered ring 18 occurs stepwise (12, 14 and 16 likewise) and can be understood by the formation of the metallacyclopropene or metallacyclopropane 17. Then the insertion of alkyne or alkene to the three-membered ring 17 produces the metallacyclopentadiene or metallacyclopentane 18. 

Many cyclization reactions via formation of metallacycles from alkynes and alkenes are known. Formally these reactions can be considered as oxidative cyclization (coupling) involving oxidation of the central metals. Although confusing, they are also called the reductive cyclization, because alkynes and alkenes are reduced to alkenes and alkanes by the metallacycle formation. Three basic patterns for the intermolecular oxidative coupling to give the metallacyclopentane 94, metallacyclopentene 95 and metallacyclopentadiene 96 are known. (For simplicity only ethylene and acetylene are used. The reaction can be extended to substituted alkenes and alkynes too). Formation of these metallacycles is not a one-step process, and is understood by initial formation of an tj2 complex, or metallacyclopropene 99, followed by insertion of the alkyne or alkene to generate the metallacycles 94-96, 100 and 101-103 (Scheme 7.1). 

Benzene and cyclooctatetraene (COT) derivatives are formed by [2+2+2] and [2+2+2+2] cycloadditions of alkynes. At first the metallacyclopropene 107 and metallacyclopentadiene 108 are formed. Benzene and COT (106) are formed by reductive elimination of the metallacycloheptatriene 109 and the metallacyclononate-traene 110. Formation of benzene by the [2+2+2] cycloaddition of acetylene is catalysed by several transition metals. Synthesis of benzene derivatives from... 

The metallacyclopentadienes 108 are converted to various cyclic compounds by insertion of several unsaturated bonds as summarized in Scheme 7.2. 

The reactions of the metallacyclopentadiene 108 with some unsaturated bonds, such as CO, CO2, alkenes and carbonyl groups, give rise to various five- and six-membered cyclic compounds, such as 142 and 143. 

There are other specific compounds that should be included in this section. One product from the reaction of Os3(CO)12 and diphenylacetylene is Os3(C4Ph4)(CO)9 (25) in which there is a metallacyclopentadiene ring connecting two of the three Os atoms (193). 

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