Coordination double

For the catalyst system WCU-CsHbAICIs-CzHsOH, Calderon et al. (3, 22, 46) also proposed a kinetic scheme in which one metal atom, as the active center, is involved. According to this scheme, which was applied by Calderon to both acyclic and cyclic alkenes, the product molecules do not leave the complex in pairs. Rather, after each transalkylidenation step an exchange step occurs, in which one coordinated double bond is exchanged for the double bond of an incoming molecule. In this model the decomposition of the complex that is formed in the transalkylidenation step is specified, whereas in the models discussed earlier it is assumed that the decom-plexation steps, or the desorption steps, are kinetically not significant. 

Table 5 Tetra- and penta-coordinate double-bond compounds between heavier group 14 and 16 elements bearing nitrogen substituents...

All calculations were performed using Gaussian98 [77], which required making some additions and corrections to the UFF force field because Gaussian98 does not have atom types for square planar rhodium or coordinated double bonds. Details about these changes can be found in the supporting information of our original paper [72],... 

On the other hand, /J-alkoxyalkanoic esters are formed through a completely different mechanism, involving anti nucleophilic attack by R OH on the coordinated double bond followed by alkoxycarbonylation (Scheme 4). 

On the other hand, the methoxyester results from MeOH attack on coordinated double bond, followed by methoxycarbonylation (Scheme 11). In both cases, the formation of 7r-allylpalladium complexes directs the regio-chemistry of the process. By optimizing the reaction conditions, it has been possible to obtain the unsaturated diester selectively. The latter compound is particularly important, since it can be easily transformed after hydrolysis and hydrogenation into adipic acid [52-54], Selective alkoxy-alkoxycarbonylation of 1,3-dienes has also been achieved [55]. 

Serendipitous formation of bimetallic complexes 55, 56 with a /7 -coordi-nated phosphonio-benzophospholides was observed in the reaction of the zwitterion 26 with [M2(CO)io] (M=Mn, Re) [43]. Complexes 56a,b were characterised by spectroscopic data and single-crystal X-ray diffraction studies whose results pointed to a low degree of d(M) r (L) charge-transfer and thus low metallacycle character, suggesting that, similar as in the copper r-complex 51, the coordinated double bond acts predominantly as n-donor rather than an as n-acceptor [43]. 

These ligands all form 1 1 complexes with platinum(II) halides which are non-conductors, monomeric and, in the solid state, contain one coordinated and two free double bonds. Proton n.m.r. data indicate that all three double bonds are equivalent in solution due to a rapid equilibrium between bonded and non-bonded —CH=CH2 groups. The phosphine and arsine complexes PtBraL (L = tvpp, tvpa) react with two equivalents of bromine giving what are believed to be platinum(II) species containing one coordinated double bond and two —CHBr-CHaBr groups arising from the saturation by bromine of the two free double bonds. 

Aresta (54) has investigated the platinum complexes formed with o-allylphenol and o-allylthiophenol. The phenolic ligand reacts with the PtCl4 ion (in a suitable acetate buffer) to form the chelate complex shown in Fig. 40. The coordinated double bonds of this compound are successively replaced by two equivalents of pyridine. 

Aresta (54) and Deutscher (56) have also prepared chelate complexes by reacting the Giignard reagent formed from platinum bromide. The structure shown in Fig. 42 a has been attributed to the product of this reaction on the basis of its i.r. spectrum. Triphenyl phosphine displaces the coordinated double bonds to peld the complex... 

The chelate complex PtCl2(CH2=CHC6H4AsPh2-o) reacts with methoxide ion at th< coordinated double bond to give an alkyl complex (80 equation 238).690 The structures of boti compounds have been verified by X-ray crystallography. Analogous cationic complexes hav< been isolated, and their reactions with nucleophiles and electrophiles discussed.691... 

PdLX complex undergoing insertion of the coordinated double bond into the a-Pd-carbon bond to form a Pd-alkyl intermediate. With 1,4-penta-diene and 1,5-hexadiene, cyclic keto esters are formed in MeOH and a similar cyclic mechanism is suggested involving insertion of the coordinated double bond into the acyl Pd complex intermediate (16). Although CO pressures of 1000 atm were used, these cyclic ketones were produced also at 250 atm in the presence of p-toluene sulfonic acid, but no details were reported. 

Carbon disulfide complexes are included for comparison. The C=C bond length of free molecules ranges from 1.309 to 1.312 A. b Perpendicular refers to the molecular structure containing a coordinated double bond perpendicular to the molecular plane, and in-plane to that containing the double bond lying in the molecular plane. 

Both 1,4- and 1,5-dienes form stable complexes with Pd. For most 1,3-dienes, such as 1,3-butadiene, reaction with Pd° compounds leads to 7r-allyl formation. These reactions are described in Section 7. The coordinated double bonds in palladium diene complexes are reactive toward attack by many nucleophiles, and the resulting chelating alkene palladium alkyls are easily isolated. Many useful reactions of dienes were discovered by Jiro Tsuji in the 1960s and 1970s. These have been recently reviewed in a historical memoir. ... 

Another unusual behavior of NHC TM complex was recently reported by Nolan et al. and involves the insertion of an NHC into a platinum-olefin bond. In the course of preparation of new NHC-containing platinum complexes by reaction of equimolar amounts of [(l,5-hexadiene)PtCl2] and free NHC, the substitution product (286) in which one coordinated double bond was substituted by an NHC was isolated in good yield (Scheme 48). A by-product was also... 

Reaction of liquid 1,3-butadiene with solid CuCl (285) produces the complex (C4He)(CuCl)2. The infrared spectrum of this complex has bands at 1570 and 1507 cm which are considered to represent the C=C stretching frequencies of free and coordinated double bonds, respectively. Addition of 1,5-hexadiene to a solution of CuCl in concentrated HCl yields a white, rather unstable complex (C6Hio)(CuCl)2 (283). Complex formation results in a shift in the double-bond stretching frequency from 1640 to 1545 cm and the overall simplicity of the infrared spectrum suggests a trans configuration for the diene. 

---