Olefin complexes coupling

Olefin polymerization is not a simple single-step olefin insertion process it involves complex kinetics and multiple equilibria. Insertion is preceded by olefin complexation (Scheme 7.2a) or olefin complexation coupled with counterion displacement (Scheme 7.2b). In view of this complexity, how can the use of barrier differences be justified The use of barrier height differences arises from the Curtin-Hammett principle. The Curtin-Hammett principle is a kinetic analysis that describes product distributions for reactions involving a pair of equilibrating reactants or intermediates, each capable of forming a product. The stereodifferentiation of general isotactic polymerization can be... 

The use of tri-tert-butylphosphine has produced still higher selectivities, allowing near total control in the synthesis of (A)-vinylsilanes, including alkoxysilanes and disiloxanes.38,39 In the context of a total synthesis of an HMG-CoA reductase inhibitor, hydrosilylation with a chlorosilane catalyzed by a platinum(O) olefin complex, Pt2 [(CH2=CH)Me2Si]20 3 (also known as Karstadt s catalyst), followed by coupling with a 2,6-disubstituted aryl iodide forged a key intermediate shown in Scheme 6.38... 

Internal RCH=CHR 2-Hexene MeCH=CHC3H7 is not isomerized by complex 1 to 1- or 3-hexene, nor is its cis trans ratio changed. No olefin complexes or coupling products are obtained. The corresponding zirconocene complexes 2 likewise did not show any isomerization activity [15]. 

Examples of catalytic formation of C-C bonds from sp C-H bonds are even more scarce than from sp C-H bonds and, in general, are limited to C-H bonds adjacent to heteroatoms. A remarkable iridium-catalyzed example was reported by the group of Lin [116] the intermolecular oxidative coupling of methyl ethers with TBE to form olefin complexes in the presence of (P Pr3)2lrH5 (29). In their proposed mechanism, the reactive 14e species 38 undergoes oxidative addition of the methyl C-H bond in methyl ethers followed by olefin insertion to generate the intermediate 39. p-hydride elimination affords 35, which can isomerize to products 36 and 37 (Scheme 10). The reaction proceeds under mild condition (50°C) but suffers from poor selectivity as well as low yield (TON of 12 after 24 h). 

The title olefins form complexes with Ni(0) with equilibrium constants for formation decreasing in the order ethylene > styrene > propylene 1-hexene > disubstituted alkenes (28). With ethylene and styrene the (olefin)NiL2 complexes have been isolated with L = P(0-o-tolyl)3. Addition of HCN to solutions of the pure olefin complexes results in rapid and complete conversion to alkylnickel cyanide intermediates which are spectroscopically detectable subsequent C—C coupling gives the observed nitrile products propionitrile from ethylene and (predominantly) 2-phenylpropion-itrile from styrene (47). The same alkyl intermediates are formed when ethylene and styrene are added to HNiL3CN [L = p(0-o-tolyl)3]. 

We haved attempted carrying out similar coupling reactions using other olefine complexes. Reduction of MC14 (PMe3)3(M=Mo,W) with Na-Hg, under propylene, give yellow complexes of composition M(C3 H6) (PMe3)4. The appearance of an IR ab sorry---r "-------g—A" "" 1700 cm 1 and of a hydride... 

As previously discussed, the copolymers produced in the zinc chloride-free radical system are not necessarily random copolymers but are probably the result of the copolymerization of the acrylonitrile-complexed acrylonitrile complex with the olefin-complexed acrylonitrile complex. Further, the olefin-alkylaluminum halide complexed acrylonitrile complex only differs from the olefin—zinc chloride complexed acrylonitrile complex in degree rather than in kind—i.e., the former is an unstable charge transfer complex capable of spontaneous uncoupling of the diradical system followed by intermolecular diradical coupling, while the latter is a stable charge transfer complex requiring radical attack to uncouple the diradical system. 

Scheme 6.6 Coupling of chlorobenzene and styrene catalysed by Pd carbene-olefin complexes...

The similarities with Figure 28 are, i) the formation of a Pd-olefin complex ii) the coupling between the surface acetate and the vinyl group (equivalent to the nucleophilic attack depicted in Figure 29), and iii) the formation of a coordinated/adsorbed Pd-vinyl acetate species. 

Olefin-coupling reactions of Tj -allyliron complexes with a variety of cationic iron-olefin complexes (ethylene, propene, styrene, etc.) were utilized to give cationic bimetallic complexes with cr,7r-hydrocarbon bridges (80,81). The condensation of simple [FpColefin)]" substrates with Fp(allyl) precursors was extended to the reaction with Fp(l,3-butadi-ene)+. Initial attack at C-1 or C-4 leads to the formation of dinuclear complexes with cr-coordinated and 7r-coordinated Fp fragments, which by subsequent intramolecular condensation could give either cyclohexenyl or cyclopentenyl intermediates. Attack at C-2 yields a dinuclear complex incapable of further intramolecular reaction [Eqs. (6-8)]. 

One example of a bonafide bis(alkyne) complex has recently been prepared. Reaction of the in situ generated olefin complex prepared by alkylation of ( -CsHs ZrC 50 with the diaryl alkyne in Equation (7) yields 253.130 In this structure, C-C coupling has not occurred, presumably a result of the steric strain associated with the zirconacyclo-pentadienyl fragment (Equation (7)). The solid-state structure further establishes the compound as a bis(alkyne) complex. Computational studies suggest that a Zr(iv) resonance structure is the most suitable representation of the compound. However, reaction of 253 with iodine in THF yields ( -CsHs Zrle 254 and the dialkyne starting material, suggesting that the zirconium center can act as a source of Zr(n) (Equation (8)). 

The r)3-allylpalladium formate complex is considered as a model of the intermediate in a catalytic reductive cleavage of allylic formate or allylic acetate combined with formic acid to olefins. The r)3-allylpalladium formate was revealed to be decarboxylated to release olefins upon coupling of the produced palladium hydride with the r)3-allyl ligand (Eq. 7). 

During the past decade, considerable progress has been made in the area of transition metal-catalyzed cleavage and functionalization of the inert C-Cl bond in nonactivated chloroaromatic compounds. This new and important field of chemistry is reviewed in the present chapter, which describes both mechanistic and synthetic aspects of C-Cl activation. Oxidative addition reactions of chloroarenes to complexes of catalytic metals are discussed, along with their applications in a wide variety of reductive dechlorination, nucleophilic displacement, olefin arylation, coupling, and carbonylation reactions. 

Rearrangement of the olefin complexes L2PtCF2.CFBr and anion exchange gives a series of trifluorovinylplatinum complexes [267]. Parameters for two examples are given with the structure apart from the platinum-fluorine couplings, which were not observable even with CAT accumulation, the parameters for the other members of the series were very similar to those shown. (140) Parameters for the complexes... 

The stereochemical outcome was in agreement with a mechanism for the palladium-catalyzed cyclization/carboalkoxylation of a substituted alkene (Scheme 47) that involves outer-sphere attack of the indole on the palladium-olefin complex I which, coupled with loss of HCI, would form the alkylpalladium intermediate II. 1,1-Migratory insertion of CO into the Pd-C bond of II with retention of stereochemistry would form the acyl-palladium complex III, which could undergo methanolysis to release c/.v-product and form a palladium(0) complex. Oxidation with Cu(II) would then regenerate the active Pd(II) catalyst. 

Not all olefins react with Pt(II) complexes to form the expected product. Thus while allyl alcohol displaces chloride from K2[PtCl4] to form the expected olefin complex initially, this reacts further if held at 50°C for a couple of days to form a diolefin complex through a dehydration reaction ... 

The C—C bond formation in these complexes is reversible. Treatment of the butadiene or isoprene derivatives with molten triphenylphosphine leads to diene evolution, in moderate yields, but reductive coupling of the M—C bonds occurs when the complexes are reacted with CO at low T since 4-vinylcyclohexene is formed . An unstable olefin complex can be formed from divinylcyclobutane and (cyclododecatriene)Ni(tricyclohexylphosphine) that liberates divinylcyclobutane when... 

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