Of four-membered metallacycle

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... 

Cycloreversion of four-membered metallacycles is the most common method for the preparation of high-valent titanium [26,27,31,407,599-606] and zirconium [599,601] carbene complexes. These are usually very reactive, nucleophilic carbene complexes, with a strong tendency to undergo C-H insertion reactions or [2 -F 2] cycloadditions to alkenes or carbonyl compounds (see Section 3.2.3). Figure 3.31 shows examples of the generation of titanium and zirconium carbene complexes by [2 + 2] cycloreversion. 

Fig. 3.31. Generation of carbene complexes by [2 -i- 2] cycloreversion of four-membered metallacycles [26,27,605,607-609].

In this section, the synthesis of four-membered metallacycles by transformations of larger or smaller rings is discussed. The transmetallation of metallacyclobutene complexes is covered in Section 2.12.6.1.2. 

Syntheses of four-membered metallacyclic complexes with nitrosylruthenium and their ring-opening upon HCl addition... 

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 diazaphosphane or aminoiminophosphane ligands with a NPN framework are another subclass of cyclophosphazenes. These compounds with both phosphorus in oxidation state (111) [104-110] and (V) [111-112] have been employed in the synthesis of four membered heterocycles and coordination chemistry with group 13 derivatives. Several complexes of trivalent phosphorus derivatives with both aluminum halide and alkyls are known as illustrated for 48 in Scheme 21 [113-119]. The structure determination of 48 confirms the formation of a four membered metallacycle [116, 117],... 

Short nonbonded Si—Si distances have been observed in four membered metallacycles (Scheme 12b) with a Pt, Ir, W, or Nb atom [138-142] in place of one of the oxygen (nitrogen) atoms of 1,3-cyclodisilazanes (1,3-cyclodisilazanes) and in U-silylene-bridged dinuclear platinum complexes (Scheme 12c) [143, 144]. Electron donating occupied orbitals are expected to be on the platinum atoms like lone pair orbitals on the oxygen atoms in cyclodisiloxanes. 

Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 2] Cycloreversion can be induced thermally, chemically, or photochemically [49,591-595]. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 2] cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. 

Table 1 Computational investigations of four-membered ring metallacycles since 1993...

General crystallographic and spectroscopic features of a range of four-membered ring metallacycles have been discussed previously in CHEC-II(1996) <1996CHEC-II(lb)887>. 

The development of significant synthetic applications of four-membered ring metallacycles depends critically on the discovery of downstream conversions to elaborated organic products. 

The synthesis of four-membered ring metallacycles was initially driven by the recognition that low-valent transition metals could mediate cyclopropane carbon-carbon bond cleavage and by the extensive attention devoted to reactivity patterns thought to be relevant to olefin metathesis. More recently, however, organometallic and organic researchers... 

These early results, along with a vast number of other data [45], establish that ring-opening metathesis polymerisation proceeds via a chain process [scheme (4) in Chapter 2] in which the structures of the active species fluctuate between metal alkylidenes (carbenes) and four-membered metallacycles (metallacyclo-butane)s, a concept that was first introduced by Herisson and Chauvin [46]. 

Thus, the availability of the four-membered metallacycle in the catalytic cycle greatly enhances the rate of disproportionation of tetramethyl-disiloxane. To account for the catalytic activity leading to the oligomeric... 

With 1,3-diynes such as PhC=CC=CPh, complexes of types 359, 360, and 361 are formed, the diyne behaving as a phenylethynyl-alkyne. Minor amounts of 367, in which two diyne ligands have added to one carbon of the C2 ligand to give a branched-chain C10 ligand, and 368, in which one SMe group has inserted into an Ru-C bond to give a four-membered metallacycle, are also isolated.546... 

In subsequent work, Herrmann and co-workers isolated two closely related four-membered metallacycles, one of which was formally a metallaoxetane. 

Metallacycles have also been prepared from two molecules of alkenes or alkynes or from palladated o-alkyl- or aryl- substituted aryls by C-H activation. Metallacycles usually have five- or six-membered ring structures, and four-membered metallacycles have been shown to be intermediates in metathesis (Chapter 6). 

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