Preventing Reductive Elimination

Several of the factors that appear to be important in preventing reductive elimination include the use of multidentate ligands, the employment of ancillary ligands that restrict the bite angle and the steric size of the substituents on the NHC and the electronics of these substituents. 

Cavell and co-workers elucidated that, overall, the nature of the substituent on the nitrogen of the NHC is the most important factor in determining stability, with the A-Mes containing complexes (56, 60) exhibiting the highest stability and the A-r-Bu systems (57, 61) being the least stable. This general pattern of stability could be extended to a wide number of complexes. 

It was found that ligands containing a methylene spacer between the NHC and the pyridine further increased the stability of the complexes, since they minimized the steric interaction between substituents within the plane. 

When these complexes were tested for catalytic activity, the presence or absence of the methylene spacer showed no influence on the catalytic activity. More interestingly, the least stable A-t-Bu complexes gave the highest activity. This is one of several examples in which a complex that decomposes under stoichiometric studies is highly active under catalytic conditions. 

DFT studies have shown that as P-Pd-P bite angle increases from 80° to 130°, the activation energy for reductive elimination decreases from 26.7 to 12.7KcalmoD Complexes bearing ancillary ligands with bite angles less than 110° will actually prefer to undergo oxidative addition of imidazolium in 

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Elsevier and co-workers have found that complexes of the type 4 (Scheme 13.5) were surprisingly resistant to reductive elimination [30]. However, this behaviour is not inconsistent with previous observations the A -substituent on the NHC is the bulky mesitylene, and whilst bulky substituents do not prevent decomposition they restrict approach of the a>methyl group, resulting in less effective interaction between the methyl and the NHC [5, 19]. The authors did, however, find that addition of CO to a dichloromethane solution of the complex led to rapid decomposition (R = Me) via reductive elimination to give the 2-acylimidazolium salt 5. 

USDA (2001) Analysis and Evaluation of Preventive Control Measures for the Control and Reduction/Elimination of Microbial Hazards on Fresh and Fresh-cut Produce. US Food and Drug Administration, Center for Food Safety and Applied Nutrition, September 30, 2001. www.cfsan.fda.gov... 

The isomerization barrier of 15.0-20.0 kcal mol-1 (AG ) can be considered to be large enough to allow isolation and characterization of bis(q3-<2 /),A- nms-dodecatrienediyl-Nin stereoisomers of 7b41 as reactive intermediates in the stoichiometric cyclotrimerization process. Furthermore, the trans orientation of the two allylic groups gives rise to an insurmountable barrier for reductive elimination for these cases, which prevents these species from readily leaving the thermodynamic sink via a facile reductive elimination. The isolated intermediates clearly constitute dead-end... 

In certain cases a reductive elimination is prevented due to the unfavorable formation of the antiaromatic benzocyclobutadiene (Equation (155)). Annulated fulvenes and pentalenes are accessible by this methodology, in excellent yields (Equation (156)).130 130a-130d... 

There are now a number of quite stable Pt(IV) alkyl hydride complexes known and the synthesis and characterization of many of these complexes were covered in a 2001 review on platinum(IV) hydride chemistry (69). These six-coordinate Pt(IV) complexes have one feature in common a ligand set wherein none of the ligands can easily dissociate from the metal. Thus it would appear that prevention of access to a five-coordinate Pt(IV) species contributes to the stability of Pt(IV) alkyl hydrides. The availability of Pt(IV) alkyl hydrides has recently allowed detailed studies of C-H reductive elimination from Pt(IV) to be carried out. These studies, as described below, also provide important insight into the mechanism of oxidative addition of C-H bonds to Pt(II). 

Another example of transient formation of a palladacycle is the Pd-mediated ortho-alkylation and ipso-vinylation of aryl iodides depicted in Scheme 8.23. In this multicomponent reaction the ability of norbomene to undergo reversible arylation and palladacycle formation is exploited. This reaction also illustrates that aryl halides undergo oxidative addition to Pd faster than do alkyl halides, and that aryl-alkyl bond-formation by reductive elimination also proceeds faster than alkyl-alkyl bond-formation. The large excess of alkyl iodide used in these reactions prevents the formation of biaryls. Benzocyclobutenes can also be formed in this reaction, in particular when the alkyl group on the aryl iodide is sterically demanding or when a secondary alkyl iodide is used [161]. 

The amination chemistry depends on preventing irreversible y -hydrogen elimination from the amido complexes before reductive elimination of the amine. At the early stages of the development of the amination chemistry, it was remarkable that the unknown reductive elimination of arylamines could be faster than the presumed rapid [71, 72] jff-hydrogen... 

Presumably, the use of a bidentate ligand such as ( )-BINAP or DPPF results in the occupation of a vacant coordination site, preventing -hydride elimination of the Pd (II) amide intermediate [53]. Dissociation of the imine and C-H bond reductive elimination results in formation of the reduced aryl bromide. If this -hydride elimination is rapid relative to reductive elimination and reversible, then significant erosion of the enantiomeric excess of optically active a-substituted amines maybe observed during the reaction (Scheme 3). 

The rationale behind the introduction of the ortto-methoxy group is its ability to behave as a hemilabile ligand, which is likely to stabilise the paUadium(O) catalyst species - after reductive elimination of the product in the catalytic cycle - by intermittently occupying the otherwise empty coordination sites preventing formation of palladium black. 

One role of the phosphite ligand (besides preventing catalyst deactivation) is to assist the reductive elimination step [17]. In fact, the olefin exists as an equilibrium mixture of isomers under the hydrocyanation conditions and, tlterefore, the regioselccUvity of the addition of HCN to a double bond is determined by the ulcfin insertion into the Ni—II bond and the relative rates... 

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