Vinyl complexes stability

Investigation of the kinetics of the reaction of 4-chloro-2-pentene, an allylic chloride model for the unstable moiety of polyfvinyl chloride), with several thermal stabilizers for the polymer has led to a better understanding of the stabilization mechanism. One general feature of the mechanism is complexing of the labile chlorine atom by the metal atom of the stabilizer. A second general feature is substitution of the complexed chlorine atom by a ligand (either carboxylate or mercaptide) bound to the metal. Stabilization requires that the new allylic substituent (ester or sulfide) be more thermally stable than the allylic chlorine. The isolation of products from stabilizer-model compound reactions supports the substitution hypothesis of poly(vinyl chloride) stabilization. 

The chemistry of osmium carbene and carbyne complexes is rich owing to the high Os-C bond strength that stabilizes these species. Alkyl and vinyl complexes can convert to carbene and vinylidene complexes. They can interconvert... 

Energy difference between the initial bis-o-vinyl complexes l Init and lA Init is only 0.6 kcal/mol, with the former being more stable. The relative order of the transition states (1 TS and 1A TS) reflects the stability of corresponding buta- 1,3-diene isomers in a free form. Thus, s-cis transition states are expected to lie higher in energy and we will continue the study following the s-trans pathway only. 

Stability of the Metal-Carbon-Halogen System I. The Preparation and Properties of Some Platinum(II) and Palladium(II)-Vinyl Complexes, B. F. G. Johnson, J. Lewis, J. D. Jones, and K. A. Taylor, J. C. S. Dalton, 1974, 34. 

Metal alkenyls have received relatively little attention beyond the ubiquitous vinyl complexes that are obtained either thermally or photochemically from simple alkene complexes of the type Tp M(t/ -C2H4)(L) (L = C2H4, PR3, NCMe, CO, Scheme 49, see also Section III-B.l). The reactivity of these species largely mirrors that of their 71-alkene precursors, in which their intermediacy has often been implicated. Most of this chemistry, and studies establishing the relative stability and accessibility of the Ti-alkene and hydridovinyl forms, are discussed in detail in Section III-B.l however, some merit further attention. 

Mossbauer studies show RaSnXa poly(vinyl chloride) stabilizers to be converted into RaSnXCl where X=/jo-octylthio-glycollate or -maleate, and not the dichloride. A range of organotin dithiocarbamates has been prepared, monoalkyl being 7-co-ordinate, with bis(2-thio-5-nitropyridine)-5 -di-n-butyltin, 6-co-ordinate, and a 6-mercaptopurine riboside(5-,A-)tri-n-butyltin 5-co-ordinate. Dithiophosphate complexes are similarly 5- and 6-co-ordinate, with... 

Heat stabilizers protect polymers from the chemical degrading effects of heat or uv irradiation. These additives include a wide variety of chemical substances, ranging from purely organic chemicals to metallic soaps to complex organometaUic compounds. By far the most common polymer requiring the use of heat stabilizers is poly(vinyl chloride) (PVC). However, copolymers of PVC, chlorinated poly(vinyl chloride) (CPVC), poly(vinyhdene chloride) (PVDC), and chlorinated polyethylene (CPE), also benefit from this technology. Without the use of heat stabilizers, PVC could not be the widely used polymer that it is, with worldwide production of nearly 16 million metric tons in 1991 alone (see Vinyl polymers). 

Poly(vinyl acetate) chains are also stabilized as aqueous-soluble anionic species by complexation with a surfactant. The charge on the water-soluble species prevents their absorption into the particle (114). 

The stereochemistry of addition is usually anti for alkyl-substituted alkynes, whereas die addition to aryl-substituted compounds is not stereospecific. This suggests a termo-iecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation mtermediate in the aryl case. Aryl-substituted alkynes can be shifted toward anti addition by including bromide salts in the reaction medium. Under these conditions, a species preceding the vinyl cation must be intercepted by bromide ion. This species can be presented as a complex of molecular bromine with the alkyne. An overall mechanistic summary is shown in the following scheme. 

Polymers in Schemes 12 and 13 were the first examples of the preparation of pyridinium and iminopyridinium ylide polymers. One of the more recent contributions of Kondo and his colleagues [16] deals with the sensitization effect of l-ethoxycarbonyliminopyridinium ylide (IPYY) (Scheme 14) on the photopolymerization of vinyl monomers. Only acrylic monomers such as MMA and methyl acrylate (MA) were photoinitiated by IPYY, while vinylacetate (VA), acrylonitrile (AN), and styrene were unaffected by the initiator used. A free radical mechanism was confirmed by a kinetic study. The complex of IPYY and MMA was defined as an exciplex that served as a precursor of the initiating radical. This ylide is unique in being stabilized by the participation of a... 

Part C of the present procedure illustrates a mild method for effecting the elimination of thiophenol from thioacetals and thioketals under essentially neutral conditions. The reaction of simple thioacetals and thioketals with bis[copper(I) trifluoro-methanesulfonate] benzene complex in benzene-tetrahydrofuran at room temperature affords vinyl sulfides in high yield (Table I). The reaction presumably occurs by coordination of the thiophilic copper(I) reagent with sulfur, heterolysis to a phenylthio-stabilized... 

The diamagnetic ylide complexes 34 have been obtained from the reaction of electron-deficient complexes [MoH(SR)3(PMePh2)] and alkynes (HC=CTol for the scheme), via the formal insertion of the latter into the Mo-P bond. The structural data show that 34 corresponds to two different resonance-stabilized ylides forms 34a (a-vinyl form) and 34b (carbene ylide form) (Scheme 17) [73]. Concerning the group 7 recent examples of cis ylide rhenium complexes 36 cis-Me-Re-Me) have been reported from the reaction of the corresponding trans cationic alkyne derivatives 35 with PR" via a nucleophilic attack of this phosphine at the alkyne carbon. 

The reaction of the stabilized yUde 46 (a-vinyl substituted) with the cycloocta-dienyl Pd(II) allows the synthesis of a novel complex, the (rj -allyl)palladium 47, in which the olefmic double bond participates in the coordination (Scheme 20) [83]. The coordination of the bis-yUde 48 with the same starting Cl2Pd(COD) leads to the formation of another new palladium complex 49 via COD exchange reactions. A C-coordination mode takes place between the carbanionic centers of the bis-ylide and the soft palladium and two stereogenic centers of the same configuration are thus created [83]. In contrast to the example described in Scheme 19, the Cl2M(COD) (M=Pd or Pt), in presence of a slightly different car-... 

An important contribution to silylium ion chemistry has been made by the group of Muller, who very recently published a series of papers describing the synthesis of intramolecularly stabilized silylium ions as well as silyl-substituted vinyl cations and arenium ions by the classical hydride transfer reactions with PhjC TPEPB in benzene. Thus, the transient 7-silanorbornadien-7-ylium ion 8 was stabilized and isolated in the form of its nitrile complex [8(N=C-CD3)]+ TPFPB (Scheme 2.15), whereas the free 8 was unstable and possibly rearranged at room temperature into the highly reactive [PhSi /tetraphenylnaphthalene] complex. ... 

When studying the free-radical copolymerization of methacrylic and acrylic acids with vinyl monomers, it was established that the addition of catalytic amounts of SnCl and (C6Hs)3SnH has a marked effect on the copolymer composition. It was found that complexes are formed by charge transfer between unsaturated acids and the above tin compounds. It has been suggested that the change in polymer composition is caused by the interaction of the tin compounds with a transition complex resulting in a decrease of the resonance stabilization of the latter 94,). 

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