Metallacycle five membered

Low valent transition metal centers preferentially coordinate to the phosphorus in diazaphospholes. Accordingly, P-coordinated complexes of [l,2,3]diazapho-spholes with Cr, W, Fe, and Mn carbonyls were obtained as early as 1980 [1, 2,4], Later, Kraaijkamp et al. observed [108] both P- or -coordination modes in complexes of [l,2,3]diazaphospholes with MX2(PEt3) (M = Pt, Pd X = C1, Br). Methanolysis of these complexes led to the diazaphosphole ring opening and formation of five membered metallacyclic P,/V-chelates (103), incorporating P-bonded phosphonite and /V-coordinated hydrazone functionalities (Scheme 32) [109],... 

Metallacyclocumulenes Five-membered metallacyclocumulenes 20 are very unusual. They are the smallest discrete cyclocumulenes to be isolated and structurally characterized. The structures of titana- and zirconacyclocumulenes show an almost planar arrangement of the metallacycle, containing three C—C double bonds, of which the central one is elongated. This elongation is ascribed to the intramolecular interaction of this bond with the metal center, which stabilizes this type of complex [21]. 

Extended Hiickel theory (EHT) was applied to study the decomposition of the five-membered metallacycle intermediate proposed by Mimoun for the epoxidation by Mo bisperoxo complexes [44, 45]. Another EHT study [40] proposed the coordination of ethene to the metal center of an MoO(02)2 complex as the first step, followed by a slipping motion of ethene toward... 

The direct attack of the front-oxygen peroxo center yields the lowest activation barrier for all species considered. Due to repulsion of ethene from the complexes we failed [61] to localize intermediates with the olefin precoordinated to the metal center, proposed as a necessary first step of the epoxidation reaction via the insertion mechanism. Recently, Deubel et al. were able to find a local minimum corresponding to ethene coordinated to the complex MoO(02)2 OPH3 however, the formation of such an intermediate from isolated reagents was calculated to be endothermic [63, 64], The activation barriers for ethene insertion into an M-0 bond leading to the five-membered metallacycle intermediate are at least 5 kcal/mol higher than those of a direct front-side attack [61]. Moreover, the metallacycle intermediate leads to an aldehyde instead of an epoxide [63]. Based on these calculated data, the insertion mechanism of ethene epoxidation by d° TM peroxides can be ruled out. 

Density functional calculations reveal that epoxidation of olefins by peroxo complexes with TM d° electronic configuration preferentially proceeds as direct attack of the nucleophilic olefin on an electrophilic peroxo oxygen center via a TS of spiro structure (Sharpless mechanism). For the insertion mechanism much higher activation barriers have been calculated. Moreover, decomposition of the five-membered metallacycle intermediate occurring in the insertion mechanism leads rather to an aldehyde than to an epoxide [63]. 

Dissociation of Me3P from the 18-electron complex gives a 16-electron complex. Association of the carbonyl group gives a Ti(II) n complex that can also be described as a Ti(IV) metallaoxirane. Dissociation of the second Me3P, association of the alkene, and migratory insertion into the C2-Ti bond gives a five-membered metallacycle. 

Related ry -pentatrienyl compounds are also formed in the reactions of [OsClCp (=C=C=CPh2)(P -Pr3)] and [RuClCp (=C=C=CPh2) (P)- -Pr2PCH2C02Me ] with CH2=CHMgBr [197, 279]. In contrast, the insertion of the allenylidene ligand into the Os-C(alkenyl) bond of 69 has been reported to yield the five-membered metallacyclic compound 70 instead of the expected ry -pentatrienyl isomer (Scheme 23) [200]. 

In addition to the [3-1-2] and [2-I-2-I-1] carbocycHzations that facilitate the formation of five-membered rings, the [4-t-l] carbocycHzation also has merit. Several transition metals have been engaged in this transformation [37], wherein it was found that vinylallene 63 a reacts with 1 equiv of WiUdnson catalyst to afford the planar (T2-bonded (vinylallene)rhodium complex 64a upon simple ligand displacement (Scheme 11.16). The stmcture of 64a was confirmed unambiguously by X-ray crystallography, and represents the first structural characterization of a metallacycle intermediate [38 a]. 

Evans and co-workers simultaneously developed of the rhodium-catalyzed [4-+2 + 2] car-bocychzation reaction through the identification of a novel mode of reactivity for the five-membered metallacycle intermediate [14, 37]. Evans proposed that 1,3 dienes should be amenable to addition to this intermediate, which would provide a new reaction pathway. The tethered enyne 80 (Scheme 12.9), was expected to afford the metallacycle I, which, upon sequential incorporation of a butadiene and reductive ehmination, would afford the desired 5,8-fused product 81. Additionally it was rationahzed that this transformation should be highly diastereoselective, as outlined in Scheme 12.10. Treatment of the heteroatom-tethered enyne I under standard carbocyclization (Pauson-... 

The reactions of C02 with aliphatic aldehydes in the presence of Ni(bipy)(cod) have been studied. As the result of oxidative coupling of C02 and RCHO on the Ni(bipy) moiety, five-membered metallacycles are formed (equation 92).280,281... 

The C = C bond of several alkynes inserted into the Zr-C bond of the rj2-thioaldehyde zirconocene complexes 81 (see Scheme 21) to form the five-membered thiazirconacycles 144. The imine metallacycle obtained from 81 and butyronitrile tautomerized under the reaction conditions to cleanly give the enamine metallacycle 145 (Scheme 38).70... 

Parallel experiments provide ample evidence that formation of free ketenes is not responsible for the formation of 12. Rather, the five-membered metallacycles arise from an unprecedented [2+31-... 

An unusual rearrangement occurred during the reaction of CNBu with the thorium and uranium metallacycles[(Me3Si)2N]2MCH2Si(Me)2NSiMe3 to give 77. The five-membered ring is not fluxional in the H NMR of these... 

The carbonyl adduct 818 might be expected to react with PMe3 in the reversal of its formation reaction, but the product isolated after prolonged reaction at 50 °C is the five-membered metallacycle 822 (equation 283). 

Although there are few five-membered metallacycles reported for group 5, there are some examples of metallacyclopentane compounds for niobium and tantalum. The reaction of the labile vanadium naphthalene complex [CpV(Ci0H8)] with ethene provided the unusual binuclear complex [Cp2V2(p-C4H8)2] (34 [Eq. (13)], Fig. 9) in which the two vanadium centres are bridged by two butanediyl groups.38... 

Compound 63 has been structurally characterised by single crystal X-ray diffraction (Fig. 21a).52 53 The geometry of the five-membered metallacyclic ring is puckered with notably different Co C( ) distances. The C-C bonds are also observed to be significantly shorter than the standard (sp3)C-(sp3)C bond length. Compound 63 was found to be stable at room temperature under an N2 atmosphere and could be stored for months at —30 °C.67 However, in benzene solution, 63 decomposes at room temperature giving a mixture of 1-butene (11%), trans 2-butene (64%) and cis 2-butene (25%).68 Reaction of 63 with CO at 44 atm pressure afforded cyclopentanone as the product in 33% yield, while reaction with ethylene at 1 atm gave 1-hexene (65%) and 1-butene (11%) as the products.68... 

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