Metallacycle three membered

The transition-metal induced rearrangement of strained cyclopropanes is mostly caused by inserting metal atoms into a three-membered ring, thus producing metallacycles and/or rf- allyl metal complexes. Tipper reported the first example of the metallacycles obtained from [Pt(C2H4)Cl2]2 [3]. The stereospecific addition of cyclopropanes has been investigated from both mechanistic and synthetic view points [4],... 

Comparable three-membered metallacycles were also obtained by oxidative addition of C—H bonds. Reduction of TaCls by sodium sand in PMe3 as a solvent afforded [Ta(PMe3)3(r/2-CH2PMe2)(tj2-CHPMe2)] (39 7% yield), whose structure (Figure 31) shows that C(l) is bonded... 

Carbometallation is a term coined for describing chemical processes involving net addition of carbon-metal bonds to carbon-carbon Jt-bonds [1] (Scheme 4.1). It represents a class of insertion reactions. Whereas the term insertion per se does not imply anything chemical, the term carbometallation itself not only explicitly and clearly indicates carbon-metal bond addition but also is readily modifiable to generate many additional, more specific terms such as carboalumination, arylpalladation, and so on. In principle, carbometallation may involve addition of carbon-metal double and triple bonds, that is, carbene- and carbyne-metal bonds, as well as those of metallacycles. Inasmuch as alkene- and alkyne-metal Jt-complexes can also be represented as three-membered metallacycles, their ring expansion reactions via addition to alkenes and alkynes may also be viewed as carbometallation processes (Scheme 4.1). 

Insertion into C—H bonds of the ligands as well as formation of metallacycles (i.e., metallaoxiranes and 1,2,4-metalladioxolanes) according to Eqs. (2)-(4), depending on the coordination sphere and reaction conditions. All metallaoxiranes and 1,2,4-metalladioxolanes are listed in Tables VII and VIII. Three-membered rings have been synthesized by reaction of metal... 

At the outset of the subsection, it should be emphasized that distinction between metallacyclopropenes and q -alkyne-metal complexes or that between metallacyclopropanes and q -alkene-metal complexes is largely a matter of semantics or convenience. Most or perhaps all of these species should probably be viewed as hybrids of three-membered metallacycles and ir-complexes. 

These heterocycles contain at least one transition metal as part of a strained three-membered ring. Their structures therefore are partially described as heterocycles and partially as transition metal n complexes of heteroalkenes (Equation (1)). Evidence for metallacyclic structures is therefore be highlighted. A wide variety of transition metal complexes could be covered but this chapter is restricted to compounds where the ring carbon is tetravalent, bonded to hydrogen or carbon, and bonded to not more that two metal atoms. Related areas which have been reviewed include CO bridged bimetallics <84AOC(23)2i9> and C02 metal complexes <83AOC(22)l29>. 

Many of these three-membered rings have not been prepared by multiple synthetic routes. When a metallacycle is available by multiple routes, only the more efficient routes have been shown. 

Fig. 7.3 Dependance of the phosphorus resonance on the existence of a metal-metal bond (three-membered vs. four-membered metallacycle)...

As the structures have aU been assigned a metal-metal bond by the original authors and thus possess a three-membered M P metallacycle, we would expect the chemical shifts to be in the downfield region at around S = 150-300ppm for... 

Strain within the ZrP three-membered ring in [Cp°2Zr(Ti -P2Mes2)]. It is interesting to note that the effect of the. r-bonding in [Cp ZrCri -P R )] incidentally equals the S for the three- and four-membered metallacycles. 

Substitution of two carbonyl groups (four electrons) with a Cp-ligand (hve electrons) results in the disappearance of the Fe— Fe bond (see Fig. 7.12). In consequence, we expect a dramatic upheld shift as the Fe P three-membered metallacycle is replaced by a Fe Pj four-membered metallacycle. The phosphorus resonances in [CpFeCO(p-PPh2)]2 are observed at 4= 8.1ppm (cis) and 4= ll-6ppin (trans), respectively, some A<) =-125ppm upheld from the closely related complex [(CpFe),(p-CO)(p-PPh ),]. 

Substitution of a bridging phosphanide with a bridging hydride in [CpFe-CO ( j,-PPhj)]j (see Fig. 7.13) reintroduces the Fe— Fe bond, accompanied by a dramatic downfield shift of about A<) =200ppm due to the replacement of a four-membered metallacycle with a three-membered metallacycle. 

An alternative view of the bonding simation in [ Cr(CO)5 j( d-PBu )] would formulate a chromium-chronuum bond, placing the phosphorus atom into a three-membered metallacycle that would cause a substantial downfield shift. However, the chromium atoms would then no longer require a 7r-donor interaction involving the phosphorus lone pair, and the lack of. r-interactions on the phosphorus center makes the experimentally... 

The phosphorus resonances are found at 4.= 156 ppm and 257 ppm, respectively, and thus considerably upheld from where we might expect them, seeing that each phosphorus atom is embedded in four three-membered metallacycles. Obviously, this octahedron constitutes a special structure similar to the tetra-... 

The chemical shift value for [ CpMo(CO)2 2(fi-PMes )] at < p=687ppm is in accord with other similar compounds like [ Co(CO)3 2()t-PMes )] at < p=664ppm, and [ CpV(CO)2 2(p-PMes )] at < p=657ppm. The two major contributions to the downfleld shift are the three-membered metallacycle, and the 7r-donor interaction of the phosphorus lone pair with both metal centres. This is not surprising, as the PMes moiety is identical in all three compounds, and the metal fragments have 15 VE (Co and Mo) and 14 VE (V), respectively. They are isoelectronic with respect to the phosphorus moiety, and thus the vanadium centres are allowed to make up their formal electron deficiency by upgrading the metal-metal single bond to a double bond. 

A further upheld shift ofAty=-180ppm to a value of =220ppm is observed, if the r-donor interaction of the amino substituent on phosphorus is retained, but the three-membered metallacycle is removed (replacement of one of the CpMoCCO) groups by a chlorine atom). The P=Mo double bond is now focused onto one molybdenum atom, rather then delocalized over two molybdenum atoms. The phosphorus moiety is a phosphanide rather than a phosphinidene species, and the phosphorus resonance at < p=220ppm is well in keeping with those of terminal phosphanides. 

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