Metallacycles eight-member

A novel chiral dissymmetric chelating Hgand, the non-stabiUzed phosphonium ylide of (R)-BINAP 44, allowed in presence of [Rh(cod)Cl]2 the synthesis of a new type of eight-membered metallacycle, the stable rhodium(I) complex 45, interesting for its potential catalytic properties (Scheme 19) [81]. In contrast to the reactions of stabihzed ylides with cyclooctadienyl palladium or platinum complexes (see Scheme 20), the cyclooctadiene is not attacked by the carbanionic center. Notice that the reactions of ester-stabilized phosphonium ylides of BINAP with rhodium(I) (and also with palladium(II)) complexes lead to the formation of the corresponding chelated compounds but this time with an equilibrium be-... 

As seen in Scheme 55, with the simple addition of a balloon of CO to the [5 + 2]-reaction of VCPs and alkynes or allenes, the [5 + 2]-reaction can be diverted via a nine-membered metallacycle to produce initially an eight-membered ring product. This Rh(l)-catalyzed three-component [5 + 2 + l]-reaction is mechanistically similar to the previously mentioned [6 + 2]-reaction (Section 10.13.2.5.2) differing only in regard to the insertion of CO in the pathway versus its incorporation in starting material (Scheme 56). 

A plausible mechanism that diverges at a common intermediate and may account for the product distributions is shown in Scheme 1. Reaction between the Ni catalyst and cyclopropylen-yne 1 would ultimately afford eight-membered metallacycle 6, which could result from either initial oxidative coupling between an alkene and alkyne (5a) [8-10] or initial isomerization of the VCP (5b). Ultimately, (i-hydride elimination from 6 and reductive elimi-... 

Reaction of CpM(CO)3R (R = Me, CH2Ph) with 2-butyne produces an alkenyl ketone metallacycle, thermally for Mo, photochemically for W (202). Addition of CNBu or PPh3 drives CO insertion into the metal position of the metallacycle, and lactone products form [Eq. (59)]. With trifluoromethyl as the original metal alkyl group multiple alkyne and CO insertions lead to an eight-membered oxymetallacycle ring [Eq. (60)]. 

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 . ... 

Finally, 69 served as starting material for a number of four-membered 1.8-naphthalenediyl metallacycles of Group 14 elements 92. 93 (w lth some of the corresponding dimeric eight-membered ring product) instead of 94, the dimer was obtained exclusively) [56], The analogous transition metal derivatives 95-99 were obtained in a similar fashion [57] (Scheme 11.28). 

Acceleration of the reaction has been achieved by the use of the polar solvent tri-fluoroethanol and also by the addition of silver triflat thus, it can be assumed that cationic rhodium complexes act as the active catalyst. Eight-membered metallacycles such as 9 are probably key intermediates. [6] Cyclopropyl-substituted five-membered metallacycles 8 and homoallyl complexes 10 can be considered as precursors of 9 [7] (Scheme 4). 

For an eight-membered metallacycle as reactive intermediate in the formation of a cyclo-heptadiene, see J. W. Herndon, G. Chatteijee,... 

The cationic eight-membered metallacyclic compound [Hfo( -dppm)2(DMSO)4] (CF3SOi )4 has been prepared in high yield and characterized using and HgNMR spectroscopy. For the first time a metal-metal spin-spin coupling in [M2(ju-dppm)2] rings without a formal metal-metal bond is reported (/( Hg- Hg) = 699-1061 Hz dependii on the solvent). ... 

Finally, the cycloaddition of 1,6-diynes wifli cyclobutanones provides a mechanistically intriguing entry to eight-membered ring products (Scheme 3-45). This sequence likely involves metallacycle formation involving addition of the cyclobutanone carbonyl, followed by ring expansion to a nine-membered metallacycle, and then C-C bond-forming reductive elimination. 

Conjugated dienesand allene react with carbon dioxide in the presence of nickel (o) catalysts to give eight- and five-membered ring metallacycles, respectively. Hydrolysis of the eight-membered ring metallacycle 28 with MeOH/HCl affords the c/x-dicarboxylic acid ester 29. 

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