Reaction mechanism bidentate ligands

Frankcombe, K. E., Cavell, K. J., Knott, R. B., Yates, B. F., 1997, Competing Reaction Mechanisms for the Carbonylation of Neutral Palladium(II) Complexes Containing Bidentate Ligands a Theoretical Study ,... 

The plausible mechanism of this ruthenium-catalyzed isomerization of allylic alcohols is shown in Scheme 15. This reaction proceeds via dehydrogenation of an allylic alcohol to the corresponding unsaturated carbonyl compound followed by re-addition of the metal hydride to the double bond. This mechanism involves dissociation of one phosphine ligand. Indeed, the replacement of two triphenylphosphines by various bidentate ligands led to a significant decrease in the reactivity.37... 

The formation of a six-membered ring is also feasible but is more limited, and the reaction is found to be more sensitive to the reaction conditions (Scheme 51). The difficulty for forming cyclohexanes is ascribed to the poorer ability of 1,7-enynes to function as bidentate ligands. This problem can be partially circumvented by introducing an alkene moiety (206 vs. 207) or a substituent that can coordinate to the metal, such as a free carboxylic acid, although in this case, the actual mechanism involves hydropalladation as the first step (see Section 10.07.4.1.3.(i).). 

Using a protocol for tandem carbonylation and cycloisomerization, Mandai et al.83 were able to synthesize cyclopentene and cyclohexene derivatives in high yield, including fused and 5/>/>0-bicycles (Scheme 25). The cyclohexene Alder-ene products were not isolable methanol addition across the exocyclic double bond (in MeOH/ toluene solvent) and olefin migration (in BuOH/toluene solvent) were observed. The mechanism of methanol addition under the mild reaction conditions is unknown. In contrast to many of the other Pd conditions developed for the Alder-ene reaction, Mandai found phosphine ligands essential additionally, bidentate ligands were more effective than triphenylphosphine. 

The reaction of planar Ni ([14]aneN4) + represented as shown in (2.10) with a number of bidentate ligands (XY) to produce c -octahedral Ni ([14]aneN4) XY + is first-order in nickel complex and [OH ] and independent of the concentration of XY.In the preferred mechanism, the folding of the macrocycle (base-catalyzed tmns — cis isomerization) is rate determining, and this is followed by rapid coordination of XY ... 

Figure 7 presents the overall, idealized reaction mechanism. The surface of MCM-48 contains 0.9 OH / nmJ, which react completely with DMDCS in the liquid phase, if NEt3 is used as a catalyst. The majority of the silanols react monofunctionally but a small fraction also reacts further, according to reaction (3) to yield inert, bidentate species. All chlorine functions on the surface are converted towards hydroxyls upon hydrolysis. The VO(acac)2 is reacted in a gas-phase reactor with this silylated, hydrolyzed surface. All recreated silanols react with the VO(acac)2 in a 1 1 stoichiometry, following a ligand-exchange mechanism. Upon calcination at 450°C, the acac ligands are decomposed but the methylsilyl functions remain intact. Most of the V-species are converted into isolated, tetrahedral VvOx species, as indicated in Figure 4. However, a small fraction clusters to form surface oligomers, hereby recreating a fraction of the silanols.

The d10 system, dominated by Ni(CO)4, sticks closely to the rule that substitution is dissociative. Very little had been done since the early days85-95-108 apart from the reaction between, inter alia, Ni(CO)4 and bidentate ligands where the interest was in the mechanism of chelation,109 and a volume of activation study110 that confirmed the assignment of mechanism. With ligands other than CO it is possible to include Pd(0) and Pt(0) reaction centres. M(PF3)4 (M = Ni, Pt)111 and M[P(OEt3)]4112 undergo dissociatively activated substitution. 

The domino carbonylation and Diels-Alder reaction proceed only as an intramolecular version. Attempted carbonylation and intermolecular Diels-Alder reaction of conjugated 2-yne-4-enyl carbonates 101 in the presence of various alkenes as dienophiles give entirely different carbocyclization products without undergoing the intermolecular Diels-Alder reaction. The 5-alkylidene-2-cyclopenten-4-onecarboxy-lates 102 were obtained unexpectedly by the incorporation of two molecules of CO in 82% yield from 101 at 50 °C under 1 atm [25], The use of bidentate ligands such as DPPP or DPPE is important. The following mechanism of the carbocyclization of 103 has been proposed. The formation of palladacyclopentene 105 from 104 (oxidative cyclization) is proposed as an intermediate of 108. Then CO insertion to the palladacycle 105 generates acylpalladium 106. Subsequent reductive elimination affords the cyclopentenone 107, which isomerizes to the cyclopentenone 108 as the final product. 

More recent works of this type have not focused so much on a particular complex as on a family of complexes with small variations in the ligands. In this case, the starting point is usually a complex of modest catalytic activity, and variations of it are analyzed in order to suggest which changes must be introduced to improve efficiency. This has been done on neutral salicy-laldiminato nickel (II) complexes [47] and zirconium bidentate non-cy-clopentadienyl complexes [48]. The detailed reaction mechanisms are always subtle and complicated, and bulky ligands are found in most cases to play a significant role in the relative energy barriers of the reaction paths. Most of the catalysts described above are cationic, and their efficiency is usually re-... 

The fact that the substitution reactions of the tungsten complex with monodentate ligands (see Table VII) are about 1000 times faster than those with bidentate ligands (2-picolinic acid) (214), whereas the kinetic results point to a dissociative mechanism (see Section IIIDl), is... 

The reaction of [MoO(H20)(CN)4] with 1,10-phenanthroline (211) and 2,2 -dipyridyl (215) has also been studied by means of reaction kinetics. It is clear that there are still uncertainties and questions regarding the mechanism of the reaction of these bidentate ligands For example, why is the reaction of the molybdenum complex faster at high pH values at which there is less of the aqua and more of the hydroxo) complex in solution (211) Scheme 5 was proposed for reaction with 2,2 -bipyridyl (215). 

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