Pd olefin complex

The acetaldehyde-forming step (eq. 7) involves nucleophihc attack by hydroxide or water on a coordinated Pd olefin complex followed by P-hydride elimination. 

The attack of Nu on the coordinated xr-allyl generates a Pd-olefin complex in the first instance which is often too unstable to be observable but can sometimes be identified.91... 

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. 

Carbon monoxide also reacts with olefins such as ethylene to produce high molecular weight polymers. The reaction of CO with ethylene can be initiated by an x-ray irradiator (62) or transition-metal cataly2ed reactions (63). The copolymeri2ation of ethylene with carbon monoxide is cataly2ed by cationic Pd (II) complexes such as Pd[P(CgH )2] (CH CN) (BF 2 where n = 1-3. With this catalyst, copolymeri2ation can be carried out at 25°C and pressures as low as 2.1 MPa. 

As was the case for the Ni (123) and Pd/C2H4 (140) systems, each of the binary olefin complexes isolated has associated with it a moderately intense, UV band, the bands for Pd complexes lying at higher energy than those of the nickel complexes in addition, for each olefin sys-... 

Pd(OAc)2 works well with strained double bonds as well as with styrene and its ring-substituted derivatives. Basic substituents cannot be tolerated, however, as the failures with 4-(dimethylamino)styrene, 4-vinylpyridine and 1 -vinylimidazole show. In contrast to Rh2(OAc)4, Pd(OAe)2 causes preferential cyclopropanation of the terminal or less hindered double bond in intermolecular competition experiments. These facts are in agreement with a mechanism in which olefin coordination to the metal is a determining factor but the reluctance or complete failure of Pd(II)-diene complexes to react with diazoesters sheds some doubt on the hypothesis of Pd-olefin-carbene complexes (see Sect. 11). 

Diboration provides another means of obtaining organoboranes. Studies [30-36] have been focused on the diboration reactions of alkynes and olefins with pinacol ester derivatives catalyzed by Pd(0) and Pt(0) metal complexes. Interestingly, it has been shown that Pt (0) complexes catalyze cis addition of the B-B bond in pinacol ester derivatives to alkynes but not to olefins. On the other hand, Pd (0) complexes do not catalyze diboration reactions, neither for alkynes nor for olefins. 

In the alkoxycarbonylation, the hydride mechanism initiates through the olefin insertion into a Pd - H bond, followed by the insertion of CO into the resulting Pd-alkyl bond with formation of an acyl intermediate, which undergoes nucleophilic attack of the alkanol to give the ester and the Pd - H+ species, which initiates the next catalytic cycle [35,40,57,118]. Alternatively, it has been proposed that a ketene intermediate forms from the acyl complex via /3-hydride elimination, followed by rapid addition of the alcohol [119]. In principle the alkyl intermediate may form also by protonation of the olefin coordinated to a Pd(0) complex [120,121]. 

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). 

In a quest for a more environment-friendly process it has been found that reaction 8.4 can be catalyzed by Pd(II) complexes of various nitrogen-donor ligands (Scheme 8.1) under not too harsh conditions (100 °C, air) without the need of copper chlorides [10,11]. Of the investigated ligands, sulfonated batophenanthroline proved to be the best. Higher olefins, such as 1-hexene or cyclooctene were similarly transformed by this catalyst. Very importantly, there was no isomerization to internal olefins and 2-hexanone was formed with higher than 99 % selectivity. This outstanding selectivity is probably due to the absence of acid and Cu-chlorides. 

III) complexes were able to hydrogenate olefins with high turnover frequencies(TOFs) (71). Scheme 15 shows the structure of these gold complexes. The isoelectronic Pd(II) complexes were also synthesized, having a similar activity as its gold analogs. 

Scheme 81 shows a highly enantioselective C—C bond formation in the BINAP-Pd(II) diacetate-catalyzed reaction of aryl triflate and 2,3-dihydiofuran (193). A BINAP-Pd(0) species generated by the action of a tertiary amine on the Pd(II) complex is the actual catalyst. Enhancement of the enantioselectivity through kinetic resolution of the intermediate is indicated by the double-bond isomer having opposite absolute configuration at the arylated carbon. / -Substituted 2-pyrrolines may also be used as olefinic substrates. 

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