Platinum metallacycles

On heating, the neopentyl Pt(PEt3)2(CH2CMe3)2 undergoes an intramolecular metallation elimination [108a] (Figure 3.59), which appears to involve initial phosphine loss affording a platinum(IV) metallacycle. 

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. 

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-... 

The platinum(0) complex [Pt(PhNO)(PPh3)2] reacts with C02 to afford the metallacyclic nitroso species [Pt 0N(Ph)C(0)0 (PPh3)2] (60), the first example of insertion of C02 into a Pt—N bond.186 Other unsaturated carbon compounds such as CS2 and electrophilic alkenes and alkynes react similarly. The diradical peril uoro-/V,/V -dimethylethane-l,2-bis(amino-oxyl) reacts readily by oxidative addition to the platinum(0) precursor Pt(PPh3)4 to afford the corresponding platinum(lI)-nitroso complex containing a seven-membered chelate ring (61). The resulting complex is stable in air for several days at room temperature.187... 

Cyclometallation of thiophene imine 110 with platinum complex 111 proceeded to give metallacycle 112 via an intramolecular C-H insertion on the thiophene ring . 

Louie J, Hartwig JF (1996) A route to PdO from Pdll metallacycles in amination and cross-coupling chemistry. Angew Chem Int Ed Engl 35 2359-2361 Mucalo MR, Coouey RP (1989) F.T.I.R. spectra of carbon monoxide adsorbed on platinum sols. J Chem Soc Chem Commun 94—95 Munoz MP, Martin-Matute B, Fernandez-Rivas C, Cardenas DJ, Echavarren AM (2001) Palladacycles as precatalysts in Heck and cross-coupling reactions. Adv Synth Catal 343 338-342... 

Attempt to prepare Jt-complexes of triafulvenes and related methylene cyclopro-parenes285,427 428 directly by ligand exchange reaction with transition metal complexes resulted in metal insertion into the sigma bond, forming metallacyclic complexes. Thus reaction of the electron-poor triafulvene l,2-diphenyl-3-dicyanomethylenecyclopropene with (ethylene)bis(triphenylphosphine)platinum in refluxing benzene gave two crystalline products whose platinacyclobutene structure was confirmed by X-ray structure analysis (equation 364)429. 

Reactions of the alkylidenecycloproparenes with rhodium(I) and platinum(O) reagents lead to metallacycles (Section VI.B.4). ... 

Selected structural data for 3-metallacyclobutanone complexes of palladium and platinum 5, obtained from both experimental and computational determinations, are provided in Table 5. The ab initio structures calculated at the self-consistent field (SCF) and Hartree-Fock (F1F) levels show good agreement with experimental values for the bond lengths of the metallacycle, but these computations deliver somewhat longer Pd-P bonds and smaller P-Pd-P... 

Pt(II) to the alkyne of the substrate likely triggers all these events. The cycloisomerization might undergo a metallacyclic intermediate that proceeds to eliminate /3-H. The formation of cyclopropanes is presumably succeeded via alkenyl platinum carbene followed by platina(IV)cyclobutane intermediates. The extension using formal metathesis of the enynes includes two transformations, the formation of 1,3-diene moieties and the stereoselective tetrasubstituted aUcene derivatives via O C allyl shift, both leading to diverse structural motifs and serving as the key step in the total synthesis of bioactive targets (Scheme 83). 

General evidence for that kind of mechanism comes from ab initio theoretical calculations performed on a related platinum complex [3, 44] and from the reactivity of four- and five-membered heterometallacyles [45] structurally close to some of the intermediates in postulated mechanisms (e.g.. Scheme 3). Moreover, related metallacycles have often been isolated from the reaction medium after nitroaromatic carbonylation, indicating that such species can easily be generated under typical carbonylation conditions [46, 47]. 

Yang and Younghave observed a novel Friedel-Crafts metallocyclization of halomethyl(arylphos-phine)platinum(II) complexes (30) in the presence of AgBF4 and a triarylphosphine to afford cationic metallacyclic species (32 equation 97). 

Nucleophilic attack by cyanate ion upon the coordinated ethylene in (48) leads to an A-ethyl-eneisocyanate ligand whieh adds electrophilically to the platinum (assisted by another cyanate) to form a metallaazacyclopentane (49) (Equation (8)), the structure of which was definitively established by x-ray crystallography <82JA7661>. Surprisingly, other nucleophiles which are known to be less reactive to ethylene and more towards Pt(II) did not lead to metallacycles <83CC40>. 

Pt(PPh3)2-(pcbd) pcbd = platinum-benzocyclo-pentene-1,2-dione metallacycle... 

It may be concluded that the data on multiple exchange indicate the intermediate formation of complexes with carbenes and metallacycles together with alkyl platinum complexes (Scheme VII.4). 

---