Metallacycles chair

Titanium-catalyzed cyclization/hydrosilylation of 6-hepten-2-one was proposed to occur via / -migratory insertion of the G=G bond into the titanium-carbon bond of the 77 -ketone olefin complex c/iatr-lj to form titanacycle cis-ll] (Scheme 16). cr-Bond metathesis of the Ti-O bond of cis- iij with the Si-H bond of the silane followed by G-H reductive elimination would release the silylated cyclopentanol and regenerate the Ti(0) catalyst. Under stoichiometric conditions, each of the steps that converts the enone to the titanacycle is reversible, leading to selective formation of the more stable m-fused metallacycle." For this reason, the diastereoselective cyclization of 6-hepten-2-one under catalytic conditions was proposed to occur via non-selective, reversible formation of 77 -ketotitanium olefin complexes chair-1) and boat-1), followed by preferential cyclization of chair-1) to form cis-11) (Scheme 16). 

The trithiastibocane RSb[(SCH2CH2)2S] (R = Ph, 4-Tol, 4-nitrophenyl) was prepared from dichloroarsine, dithiol and triethylamine. It contains an eight-membered metallacycle with boat-chair conformation . In the analogue with R = 4-nitrophenyl the transannular Sb-- S 1,5-interaction shows a distance of 319 pm while the inter-molecular Sb S interactions have values of 353 pm. An analogous bismuth complex is the oxadithiabismocane PhBi[(SCH2CH2)20] in chair-chair conformation (see 24). The eight-membered heterocycle contains two rather short Bi-S distances of 256.0 and 260.2 pm while the intermolecular Bi-S distances extend to 344.0 and 350.9 pm. The transannular Bi- -0 distance is 297 pm . ... 

Another example for a new titanium complex [Cp2Ti(As2Me4)]2[AsFg]4 contains a six-membered metallacycle. The temperature-dependent H NMR spectra indicate dynamic behaviour and a chair conformation (40). 

Two possible metallacycle intermediates, IV, and IV, also have to be considered (Fig. 5). The P-c/j methylene substituent in the chair conformation of (leading to the favored endo adduct) occupies the equatorial... 

In Rh(I) complexes with (—)-(25, 45)-2,4-bis(diphenylphosphino)pentane and diolefins as ancillary ligands the metallacycle is conformationally very flexible in solution [39]. The X-ray diffraction solid state structures show a chair conformation for the complex with NBD, and a skew conformation for the complex with COD, but in solution both complexes show fast conformational exchange and behave as having C2 symmetry. 

Figure 4.31 Structure of complexes [Rh(L-L)(COD)]+ with L L = diop and (HO-diop). Twist-chair (left), and boat (right) conformations of the Rh(L-L) metallacycle withL-L = dppb, R-diop and (HO-diop) are shown in the box. At the right, P variable temperature study on [Rh(diop)(COD)](Pp6) in CD2CI2/CHFCI2. R. Kadyrov et al. Ear. J. Inorg. Chem. 705 (1999) [87]. Spectrum reproduced by permission of Wiley-VCH.

---