Ir-allyl complexes

Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiCU (169) and the catalyst Cr -CjHsU/SiCU (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ir-allyl complexes (ir-allyl ligands being removed from the active center) (140) C14 of the...

The reactions are accelerated by bromide salts, which are thought to exchange for acetate in the ir-allylic complex. The reactions of acyclic compounds occur with minimal E Z isomerization. This result implies that the TT-allyl intermediate is captured by carbonylation faster than E Z isomerization occurs. 

These reaction conditions are applicable to primary chlorides, bromides, and tosylates. The active catalytic species appears to be a to-ir-allyl complex formed by dimerization of butadiene. 

When propylene chemisorbs to form this symmetric allylic species, the double-bond frequency occurs at 1545 cm-1, a value 107 cm-1 lower than that found for gaseous propylene hence, by the usual criteria, the propylene is 7r-bonded to the surface. For such a surface ir-allyl there should be gross similarities to known ir-allyl complexes of transition metals. Data for allyl complexes of manganese carbonyls (SI) show that for the cr-allyl species the double-bond frequency occurs at about 1620 cm-1 formation of the x-allyl species causes a much larger double-bond frequency shift to 1505 cm-1. The shift observed for adsorbed propylene is far too large to involve a simple o--complex, but is somewhat less than that observed for transition metal r-allyls. Since simple -complexes show a correlation of bond strength to double-bond frequency shift, it seems reasonable to suppose that the smaller shift observed for surface x-allyls implies a weaker bonding than that found for transition metal complexes. 

It is apparent from mechanistic considerations that an active species in the palladium-catalyzed dimerization of butadiene is a zero-valent palladium complex, which forms bis-ir-allylic complex 20. 

Two monomeric and dimeric 2-substituted ir-allylic complexes (548 and 549) are obtained by treatment of allene with PdCl2(PhCN)2. They are formed by the nucleophilic attack at the central carbon of allene[493, 494]. 

The compound ir-allyltricarbonylcobalt has also been made by treating allyl bromide with Na[Co(CO)4] 109, 111). If this reaction is carried out in an atmosphere of carbon monoxide then about one-half mol. of carbon monoxide is absorbed, and the infrared absorption spectrum of the product shows a band at about 1720 cm-1. This band is believed to be due to the presence of but-2-enoyltetraearbonylcobalt. On standing, carbon monoxide is evolved and the ir-allyl complex [Co(7r-C3H6)(CO)3] is formed. These reactions may be summarized as follows ... 

Allyl bromide reacts with nickel carbonyl in benzene to give the volatile ir-allyl complex [NiBr(ir-C3H6)]2 as a violet-red solid (75). 

Palladium forms the largest series of ir-allylic complexes of any metal, and many of these complexes are readily prepared. The majority are chloro-bridged complexes of the type [PdCl(all)]2, where all = an allylic radical. 

Diphenylacetylene and methylphenylacetylene react with palladium halides to give a very interesting series of compounds some of which are almost certainly ir-allylicpalladium complexes. Thus diphenylacetylene (ChHjo) reacts with palladium(II) chloride in ethanol to give hexaphenyl-benzene and a complex of the composition [PdCl(Ci4Hi0)2(OC2H6)] 148, 149) which we formulate as the ir-allylic complex (XLVI) (see Section... 

The bridging chlorine atoms in these ir-allylicpalladium complexes are readily replaced by bromine, iodine, or the thiocyanate group by treatment with the corresponding alkali metal salt in a suitable solvent such as acetone 105, 194). The chlorine bridge is split by amines for example, the ir-allyl complex [PdCl(7r-CsH6)]2, with p-toluidine, gives [PdCl(7r-C3H6) (p-tolui-dine)] as stable, pale yellow crystals (105). 

In contrast to palladium chloride, platinous chloride does not give a ir-allylic complex on heating with allyl alcohol instead, the diallyl ether complex [PtChi CsHsOCsHs)] is formed in good yield (105). 

It is likely that the mechanism involves reaction of the allylic compound with Ni(CO)4 to give one or more ir-allyl complexes, one of which may be 113, which can then lose CO to... 

The formation of metallacyclobutane through a ir-allyl complex without involvement of carbene species [Eq. (12.19)] was also suggested as the initiation step for systems where carbene formation from the alkene is difficult to occur for structural reasons 72-74... 

The reaction is believed to proceed via a c-allyl copper complex, in which the carbon-copper bond is formed at the "/-position, anti to the acetate leaving group. Reductive elimination of copper led to pure "/-substitution. With cyclic aliphatic allylic acetates, the selectivity is generally lower because the o-allyl copper complex can isomerize to the ir-allyl complex with loss of regioselectivity. 

The development of methods to effect nucleophilic addition to carbon-carbon double bonds by prior activation with metal cations has been applied, at least in a preliminary way, as a method of pyrrole ring closure. The conversion of butadienes to N-substituted pyrroles can be accomplished in two stages. In acetic add, 1,4-dienes react with PdnCl2 to give tr-allyl complexes with introduction of acetate at C-4. The ir-allyl complexes then react with amines to give a l-amino-4-acetoxy-2-butene (equation 70). When the addition of the amine is carried out in the presence of a silver salt and triphenylphosphine, a pyrrole is isolated, probably by cyclization of the amino-substituted allyl-Pd complex (equation 71) (81CC59). Although this procedure is attractive in terms of the simplicity of the... 

This C—H substitution process results in a Markovnikov orientation, with the H that is allylic to the more substituted end of the alkene preferentially abstracted. The stereochemistry of the resulting ir-allyl complex does not represent the stereochemistry of the starting alkene, as the complexes are capable of isomerization under the conditions in which they are formed. Typically, a thermodynamic mixture is obtained, with the syn form of the complex predominating over the anti form (equation 1). The syn form is more stable due to unfavorable steric interactions that the anti form encounters with the coordination sphere of the palladium. 

The coupling of alkenylzirconiums with ir-allyl complexes derived from the D-ring of steroids has been demonstrated to be regiospecific and to yield products which possess the natural configuration at C-20 (equation 39).155... 

Allenes insert into ir-allyl complexes so as to generate new ir-allyl species (equation 78).248 The insertion of 2-r-butyl-1,3-butadiene into ir-allylpalladium complexes proceeds normally, but is then followed by an unusual cyclization reaction, presumably due to the disposition of the butenyl frag-... 

Strained alkenes such as norbomene, norbomadiene and bicyclo[2.2.2]octenes readily insert into ir-allyl complexes with the less substituted allyl terminus linked to the alkene (equation 81).233-261 This reaction has been utilized to prepare interesting prostaglandin analogs.233 234... 

Intramolecular insertion of a simple alkene into a ir-allyl complex has also been demonstrated, resulting in synthetically useful cyclization methodologies (equations 82 and S3).262 263... 

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