Double bonds coordination

The most favorable coordination sites in thiophenes are the C2C3 and C4C5 double bonds ( -coordination, 38). This type of coordination greatly enhances the nucleophilic power of the sulfur atom, which then gives rise to two new modes of binding the metal atoms, as in the V, S-p.2-, 42, and V, S-p.3-species, 43. 

The presence of a remote C=C double bond in the alkyl halide was critical saturated analogs underwent nickel-catalyzed halogen-zinc exchange.248 It has been suggested that the double bond coordinates to the nickel atom. As a 7r-acceptor, the C=C bond removes some electron density from the metal, thus facilitating the reductive coupling (Scheme 154).246... 

The two stereoisomers of the bis(r 3-anti),A-trans intermediate 7b confirmed by NMR (References 20, 21) are distinguished by the Irons double bond coordination of opposite enantiofaces. Different from experiment, the major stereoisomer (established by NMR) with a parallel orientation of the trans double bond is predicted to be 0.6 kcal mol 1 (AG) less favorable than the minor stereoisomer with a nonparallel double bond arrangement. 

Salient features of the cationic pathway, which was introduced independently by Cabri [8] and Hayashi [9], are presented in Scheme 8G.20. Thus, subsequent to oxidative addition, a vacant coordination site is generated either by triflate dissociation or by halide abstraction by the Ag(I) salt in intermediate 20,4. This vacant coordination site facilitates double-bond coordination to form cationic intermediate 20.5, which ultimately forms the Heck product. 

The amine, by blocking one coordination point, prevents two-point coordination of the butadiene in the cis configuration, and monomer enters with only one double bond coordinated to the cobalt atom. It is presumed that in the active intermediate an organic group is in the bridge of the complex (II) (16), and we would represent the polymerization as indicated by III. 

An X-ray analysis 27) of the dicyclopentadiene complex CioHi2PtCl2 (104,182) has shown that the olefin has an endo configuration with both double bonds coordinated to platinum. The platinum-trigonal carbon atom distances are much the same as in the dipentene complex, but no indication is given of the orientation of the double bonds with respect to the platinum coordination plane. 

Platinum complexes incorporating an optically active amine have been employed for resolution of racemic mixtures of optically active olefins by reaction of the olefin with dichloro-platinum(II). The differing solubility of the diastereoisomers permits separation by fractional crystallization and the olefin can be recovered by reaction of the complex with aqueous alkali cyanide. Using either (-f)-l-phenyl-2-aminopropane (Dexedrine) or (-f)- or (—)-a-phenyl-ethylamine. Cope and co-workers have resolved the optical isomers of trans double bond coordinated and, with (—)-phenylethyl-amine)dichloroplatinum(II), a bridged complex with each double bond coordinated to a different platinum atom. 

It is known that (R3P)2NiBr2 readily releases a triphenylphosphine group in benzene 183). The next step in (41) is analogous to that of substitution of polyene ligands in the presence of strong electron donors. A released double bond coordinates the nickel atom to form an intermediate complex (X) so that an intramolecular transfer of the ligand is possible. 

Lithium halides (bromide or Iodide) may well modify the Lewis character of the zinc atom, probably via a zincate species [53], and prevent the efficient coordination of the zinc atom to the double bond, coordination which is required for the carbocyclization. Thus, in the Rieke method, it is essential to wash the active zinc thoroughly since the lithium naphthalenide reduction of zinc bromide also generates lithium bromide, which is detrimental to the success of the reaction. Indeed, the insertion of Rieke s zinc in the presence of LiBr leads to the linear organozinc iodide but not to the cyclic product [52]. 

This behavior can be rationalized by assuming that the remote double bond coordinates to the nickel center. Although a double bond coordinated to a metal center acts as a a-donor, it is also a JT-acceptor and therefore removes electron density from the metal center and facilitates... 

The general reaction model for the allylnickel complex-catalyzed 1,4-polymerization of butadiene is outlined in [26]. From the starting / -allylnickel(II) complex, which has a quasi-planar structure, two structurally different butadiene complexes are formed as the actual catalysts by successive ligand or anion substitution a monoligand allylnickel(II) complex, which may also contain the anion X instead of the neutral ligand L, with an coordinated butadiene, and a ligand-free complex with an t/ -cis coordinated butadiene. The concentration of these complexes, which is also limited by the double- bond coordination from the growing chain, and their reactivity determine the catalytic activity. 

In this case the stability difference between the transition states of the insertion step for the anti and the syn forms of the catalyst complexes e and f, which arises from the different steric conditions for double-bond coordination, is practically lost and, according to S n = A 2c/ 2t 1, an equibinary polybutadiene may... 

In contrast, copper(I)-catalysed intramolecular cycloaddition of the triene 111 affords l-vinyl-3-oxabicyclo[3.2.0]heptane (112) in >80% chemical yield.710 Only double bonds coordinated to the catalyst participate in the photocycloaddition the bridged arrangement of parallel constrained C=C bonds shown is preferred over all other possible modes of coordination (Scheme 6.47). 

Scarcely more complicated, in principle, than simple monoolefin complexes are those in which two unconjugated double bonds form independent linkages to a metal atom. Two of the more important complex-forming non-conjugated diolefins are 1,5-cyclooctadiene and norbornadiene, representative complexes of which are shown as (23-1) and (23-11), respectively. An interesting case of three unconjugated double bonds coordinated to one metal atom is shown in (23-III). 

C-C bond formation initiated from the square planar bis-o-vinyl complexes (6, 7 and 8) also proceeds through the three-centered transition state (Fig. 5). Upon metal-carbon o-bond breakage and reductive ehmination, the coordination vacancy becomes available at the metal center and a n-complexof buta-l,3-di-ene is formed. Double bond coordination (q -C=C) to the metal atom is preferred in this case, while the structure with central C-C bond coordination has an imaginary frequency corresponding to q -C-C—>q -C=C rearrangement. 

The transition state forms by a 1,4-dipolar addition to a polarized double bond. Coordination of the lithium atom to two oxygen atoms determines stereoregulation. Each new incoming monomer must approach from below the plane because the other side is blocked by an axial methyl group. This favors isotactic placement. There is doubt, however, whether it is correct to assume a rigid six-membered cyclic alkoxide structure for a propagating lithium enolate." ... 

Evidence for the intermediate formation of nitrone species during the carbonylation of nitroarenes in e s-cyclooctene as solvent catalysed by Ru3(CO)i2 have been obtained [14], Moreover, zerovalent palladium species with nitrogen donor ligands have been shown to be active catalysts in the reductive carbonylation of organic nitro derivatives [41]. The hypothesis that an intermediate having the olefinic double bond coordinated to the metal is formed during the catalytic cycle is supported by the steric effect that has been observed in the case of p,p -dimethyl-o-nitrostyrene (7f) as substrate. Moreover, such an intermediate could explain why a pentaatomic indole nucleus is preferentially formed, even when a conjugated double bond is present in the olefinic chain ... 

Oxidative addition to chelate catalyst-substrate complexes Since the chelating catalyst-substrate complexes with different mode of double bond coordination are diastereomers, they have different chemical properties, in particular, the rates of oxidative addition musf be different. Moreover, it is clear that hydrogen atoms must come from fhe side of rhodium. Hence, if the oxidative addition is irreversible and the double bond does not dissociate before, during, or affer fhe oxidative addition, the original mode of the double bond coordination could determine the stereochemical outcome of the reaction via the difference in reactivities of the corresponding catalyst-substrate complexes. This is the main idea of the so-called unsaturated mechanism of stereoselection (Scheme 1.13). - ... 

Computations showed that indeed in that case the oxidative addition of H2 to the re-coordinated complex, for example, 14, is the fastest mechanism of dihydrogen activation among all conceivable processes (hydrogenations of solvate complex, of nonchelating catalyst-substrate complexes and of sf-coordinated catalyst-substrate complexes are slower). Nevertheless, after formation of the dihydride intermediate 58, the double bond dissociation via the TS3 is much faster than migratory insertion via the TS2 (Scheme 1.17). As a result, the nonchelating dihydride intermediate 60 is formed, and (R)-57a is formed after stereoselective double bond coordination in a Rh(III) octahedral nonchelating complex vide infra). ° ... 

Thus, although in that case the coordination of the double lx)nd can be kept until the very late stage of the catalytic cycle, the reductive elimination is not possible within this pathway, and again the trail involving dissociation of the double bond must be taken. This leads to the loss of any chiral information acquired so far, and enantioselection must take place during the double bond coordination in the nonchelating Rh(III) dihydride intermediate 66. 

Hence, in all described cases, the handedness of the product will be determined by the mode of the double bond coordination in the nonchelating octahedral Rh(III) complex T (or its diastereomer, e.g., I, vide infra) and relative easiness of the following migratory insertion and reductive elimination steps. 

Scheme 1.30 Possible ways for the double bond coordination in I and 1. ...

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