Olefin complexes intramolecular

A key question remains how is the olefin formed in the overall process Molecular tantalum complexes are known to undergo facile a- and transfer processes, leading to tantalumalkylidene and tantalum tt-olefin complexes, respectively (mechanism 9, Scheme 29) [98]. Moreover, olefin polymerization with tantalum complexes belongs to the rare case in which the Green-Rooney mechanism seems to operate (Eq. 10, Scheme 29) [102]. Finally, intramolecular H-transfer between perhydrocarbyl ligands has been exemplified (Eq. 11, Scheme 29) [103,104]. 

In order to rationalize the catalyst-dependent selectivity of cyclopropanation reaction with respect to the alkene, the ability of a transition metal for olefin coordination has been considered to be a key factor (see Sect. 2.2.1 and 2.2.2). It was proposed that palladium and certain copper catalysts promote cyclopropanation through intramolecular carbene transfer from a metal carbene to an alkene molecule coordinated to the same metal atom25,64. The preferential cyclopropanation of terminal olefins and the less hindered double bond in dienes spoke in favor of metal-olefin coordination. Furthermore, stable and metastable metal-carbene-olefin complexes are known, some of which undergo intramolecular cyclopropane formation, e.g. 426 - 427 415). 

The formation of acetaldehyde from the w-olefin complex was shown to involve intramolecular migration of a hydrogen atom from one carbon of the ethylene to the other, rather than 0H attack on a vinyl group generated by hydride abstraction with Pd (CH2=CH+ + OH -> CH2=CH0H) followed by rearrangement of the vinyl alcohol to acetaldehyde, since hydrolysis in DgO yielded acetaldehyde free of deuterium (59). 

Chatani s proposed mechanism bears some similarity to that of Jun s reaction (Scheme 9.12). They both begin with hydroamination of the C=C 7t-bond of a rhodium vinylidene. The resultant aminocarbene complexes (71 and 62) are each in equilibrium with two tautomers. The conversion of 71 to imidoyl-alkyne complex 74 involves an intramolecular olefin hydroalkynylation. Intramolecular syn-carbome-tallation of intermediate 74 is thought to be responsible for ring closure and the apparent stereospecificity of the overall reaction. In the light of the complexity of Chatani and coworkers mechanism, the levels of chemoselectivity that they achieved should be considered remarkable. For example, 5 -endo-cyclization of intermediate 72 was not observed, though it has been for more stabilized rhodium aminocarbenes bearing pendant olefins [27]. 

Yttrium-catalyzed cascade cyclization/hydrosilylation of 3-(3-butynyl)-l,5-hexadienes was stereospecific, and syn-19 (R =Gy, R = OGPh3) underwent cascade cyclization/hydrosilylation to form 80b (R = Gy, R = OGPh3) in 97% yield as a single diastereomer (Scheme 20). The regio- and stereoselective conversion of syn-19 to 80b was proposed to occur through an initial 5- x -intramolecular carbometallation via a chairlike transition state that resembles alkenyl olefin eomplex syn- m. followed by S-exo intramolecular carbometallation via a boatlike transition state that resembles alkyl olefin complex boat-llm. The second intramolecular carbometallation presumably occurs via a boatlike transition state to avoid the unfavorable 1,3-interaction present in the corresponding chairlike transition state (Scheme 20). 

When an appropriate chiral phosphine ligand and proper reaction conditions are chosen, high enantioselectivity is achieved. If a diphosphine ligand of C2 symmetry is used, two diastereomers of the enamide coordination complex can be produced because the olefin can interact with either the re face or the si face. This interaction leads to enantiomeric phenylalanine products via diastereomeric Rh(III) complexes. The initial substrate-Rh complex formation is reversible, but interconversion of the diastereomeric olefin complexes may occur by an intramolecular mechanism involving an olefin-dissociated, oxygen-coordinated species (18h). Under ordinary conditions, this step has higher activation enthalpies than the subsequent oxidative addition of H2, which is the first... 

Although metal-olefin complexation can be a source of enandoselection, reactions exploiting this mechanistic motif have not been developed much. Due to the facile enantioface interconversion process, the origin of the enantioselection often reverts back to Type C alkylation (Figure 8E, 1). To transfer chiral recognition of the coordination process to the ee of the product, kinetic trapping of the incipient 7t-allyl complex is required prior to any isomerization process. For this reason, few successful examples have come from the use of more reactive heteroatom nucleophiles (N, O and S) and/or intramolecular reactions. 

The model we have used for the description of the transition states implies that also the corresponding n-olefin complexes contain an asymmetric metal atom and that changes of configuration at the metal occur more slowly than the intramolecular transformation of the 7i-complex into the metal-alkyl complex. 

In this dissociative pathway (which is assumed to be the major one today) first a phosphine is displaced from the metal center to form an active 14-electron-intermediate 42. After alkene coordination cis to the alkylidene fragment the 16-electron-olefine-complex 43 undergoes [2 + 2]-cycloaddition to give a metallacylobutane 44. Compound 44 breaks down in a symmetric fashion to form carbene complex 45. The ethylene is replaced in the conversion to complex 46. In the next steps (they are not further discribed above), another intramolecular [2 + 2]-cycloaddition joins up the eight-membered ring 11 regenerating the catalyst 42. Each step of the reaction is thermodynamically controlled making the whole RCM reversible. With additional excess of phosphine added to the reaction mixture an associative mechanism is achieved, in which both phosphines remain bound. 

A common pathway in palladium-catalyzed oxidation reactions is that the 7r-olefin complex formed reacts with a nucleophile, either external or coordinated, and the new organometallic intermediate may then undergo a number of different reactions (Scheme l) (i) an intramolecular hydride shift leads to ketone formation (ii) a )6-elimination results in the formation of a vinyl functionalized olefin (iii) an oxidative cleavage of the palladium-carbon bond produces a 1,2-functionalized olefin and (iv) an insertion reaction, exemplified by insertion of an olefin, leads to formation of a new palladium-carbon bond, which may be cleaved according to one of the previous processes ()6-elimination or oxidative cleavage). In all cases palladium has removed 2 electrons from the organic molecule, which becomes oxidized. These electrons, which end up on Pd(0), are in turn transferred to the oxidant and Pd(II) is regenerated, in this way a palladium(II)-catalyzed oxidation is realized. 

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)]. 

If the alkyl ligand has hydrogen atoms in the p position with respect to the metal and the metal also has an empty coordination site (which might be attained by ligand dissociation), then intramolecular -hydrogen elimination can occur, e.g., by thermolysis, to give hydrido-olefin complexes ... 

The intramolecular dehydrogenation of alkyl groups in the o-alkyl metal complex molecule is well known to produce chelated 7t-olefin complexes [37] (examples see in Scheme IV-19). 

Attempts were also made to observe or isolate catalyticaUy relevant intermediates. Addition of one equivalent of PhCsCPh to ( PDI)Fe(N2)2 yielded a red paramagnetic solid identified as the iron Tj -alkyne complex, which was also characterized by X-ray diffraction. Treatment of the isolated product with 1 atm of H2 initially yielded stilbene, followed by 1,2-diphenylethane, establishing its catalytic competency (Scheme 4.8). Our laboratory has recently conducted experiments to elucidate the stability of related bis(imino)pyridine olefin complexes [87]. Treatment of C PDl)Fe(N2)2 vvith 1-hexene under a dinitrogen atmosphere resulted in slow conversion to n-hexane and the paramagnetic, NMR-silent intramolecular... 

Rapid Intramolecular Rearrangements in Pentacoordinate Transition Metal Complexes. VI. The Coupling of Olefin Rotation and Berry Pseudorotation in Tetracarbonyliron-Olefin Complexes, S. T. Wilson, N. J. Covllle, J. R. Shadely, and J. A. Osborne, J. Amer. Chem. Soc., 96,4038 (1974). 

Some kinetic parameters for restricted rotation in rhodium-olefin complexes were reported some time ago. These values have recently been revised, and parameters for related rhodium compounds determined. Activation parameters have also been determined for some platinum-olefin complexes. Here intramolecularity of mechanism is proved by the persistence of Pt- H coupling and the lack of change in the n.m.r. spectra of the non-olefinic ligands with varying temperature. The roles of p and d orbitals both in fixing the preferred orientation of the olefin perpendicular to the co-ordination square around the platinum and in the rotational process, are discussed. Wide-line n.m.r. spectra indicate some... 

The oxidative functionalization of olefins through ir-olefin complexes of palladium also has a long history, including the industrial production of acetaldehyde and vinyl acetate. Related reactions, including the conversion of olefins to vinyl ethers and enamines, have been studied in more recent times for fine chemical synthesis. These oxidative C-0 and C-N bond formations have been conducted with a variety of oxidants, including Oj, and have been studied as both intermolecular and intramolecular processes. 

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