Four-membered metallacycles

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],... 

There are many four-membered metallacycles containing short metal—metal nonbonded distances. Cyclodisilazanes (Scheme 12a) isoelectronic to 1,3-cyclodis-iloxanes also have short Si—Si distances [136, 137]. 

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. 

In contrast to known structures of related metallacycles e.g. in W2( -CSiMe3)2(0-i-Pr)4> e four-membered ring in 8 is folded along the Mn-Mn1 bond with an angle of 18.45° (Fig. 5). 

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... 

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. 

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].

Figure 3 represents the only dimer formed in this series of metallacycles. The coordination number of the indium atom is increased by interaction with the nitrogen atom of a second molecule. The dimer 41 has a center of inversion and three edge-fused four-membered rings. This type of structure is very common in metallacycles that are derived from the ligand 25 (5(5). The central N2In2 ring is almost square-planar [In-N = 2.27(1) A] and the In-C dis-... 

Theoretical studies published since 1993 reporting computationally optimized structures for four-membered boracycles and metallacycles are listed in Table 1. Many of these investigations, however, maintain a specific focus on molecular transformations (i.e., reaction mechanisms) and no longer explicitly consider the details of metallacyclobutane structure. The most significant theoretical investigations of boracyclobutene derivatives were conducted sufficiently long ago to have been reviewed in CHEC-II(1996) <1996CHEC-II(lb)887> and are not discussed further. 

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... 

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. 

Oxidative Coupling Affording a Four-Membered Metallacycle... 

Matrix 5. Evaluation matrix for the oxidative coupling affording a four-membered metallacycle. For explanation, see Matrix 1. 

---