Association nucleophilic

Figs. 11 and 12 show typical mo diagrams for square planar and octahedral complexes. Inspection reveals that the metal orbital (z is the axial direction) in a square planar complex is involved in the n bonding system and available for a bonding in the transition state. This is a feature shared by nucleophilic substitution at square planar complexes with the spectacularly associative nucleophilic aromatic substitutions. The octahedral complexes discussed in this chapter... 

Figure 11 Scheme 3 — ligand-associated nucleophilic reaction. 

In terms of the dimer mechanism a term in [P]2 would also be expected in special systems according to the reacting scheme shown in Scheme 13. Actually, one molecule of pyridine would act forming the mixed associate nucleophile, and the second molecule... 

Alternatively, the rhodium dimer 30 may be cleaved by an amine nucleophile to give 34. Since amine-rhodium complexes are known to be stable, this interaction may sequester the catalyst from the productive catalytic cycle. Amine-rhodium complexes are also known to undergo a-oxidation to give hydridorhodium imine complexes 35, which may also be a source of catalyst poisoning. However, in the presence of protic and halide additives, the amine-rhodium complex 34 could react to give the dihalorhodate complex 36. This could occur by associative nucleophilic displacement of the amine by a halide anion. Dihalorhodate 36 could then reform the dimeric complex 30 by reaction with another rhodium monomer, or go on to react directly with another substrate molecule with loss of one of the halide ligands. It is important to note that the dihalorhodate 36 may become a new resting state for the catalyst under these conditions, in addition to or in place of the dimeric complex. 

Associative nucleophilic displacement at P(5) has been observed in other reactions of the phosphorane and nucleophiles described in Scheme II. 

All of these reactions have recently been reviewed (206), and it is clear that the combination of the properties of both trans-pairs of ligands can be as decisive in determining the geometry of concerted oxidative addition as they can for associative nucleophilic ligand replacement. 

Only for nickel(II) does the prospect of prereaction geometry change pose a potential complication and since more examples of faster associative nucleophilic ligand exchange at square-planar complexes (rather than their tetrahedral, 5-coordinate, or octahedral alterna-... 

Westheimer s guidelines for associative nucleophilic substitution at phosphorus 122... 

WESTHEIMER S GUIDELINES FOR ASSOCIATIVE NUCLEOPHILIC SUBSTITUTION AT PHOSPHORUS... 

In this section, we shall attempt to cover the literature to December 1987 as it relates to the guidelines for associative nucleophilic substitution at phosphorus. The queries that have been raised about these guidelines will be examined and conclusions about their future modification and predictive value will be drawn. Special consideration will be made of the effect of ring size reactivity. In Section 4 the influence of the guidelines on biological Sn2(P) mechanisms will be discussed, particularly in relation to the biologically important five- and six-membered cyclic phosphates 2, 3 -cNMP 3, 5 -cNMP and cIPj. ... 

Associative nucleophilic substitution at phosphorus contained in a six-membered ring is not a rapid process relative to the analogous reaction of acyclic compounds. Whereas associative reactions of five-mem bered sp>ecies are dominated by the influence of special strain effects, the same is not true for six-membered species. One must therefore bear in mind that other competitive processes may become significant in the reaction of six-membered ring phosphorus compounds with nucleophiles. For example, it has been suggested that the cAMP derivative [92] is hydrolysed by a dissociative Sf l mechanism (Scheme 32) (Bottka and Tomasz, 1985). 

Although direct information has come from studies on phosphate esters, studies on other tetrahedral and pentacoordinate phosphorus species have been required to elucidate principles such as pseudorotation, apicophilicity and the trigonal bipyramidal transition state. Application of the guidelines for associative nucleophilic substitution at phosphorus, discussed in the previous section of this review, has led to a good understanding of many biological mechanisms involving substitution at phosphorus. 

RhCl(cod)(PR3)(hexene)]. The fluxional nature of compounds (5) have been investigated by and NMR spectroscopy/ At room temperature the dangling and coordinated phosphines rapidly exchange positions. Retention of /rh-p above the high-temperature limit confirms this to be an intramolecular process. The iridium complex presumably behaves in the same way, but reaches coalescence at a lower temperature. The processes are probably analogous to ligand replacement by associative nucleophilic attack. 

Cobalt(i).—The main interest here is in cobaloximes and similar cobalt(i) chelate complexes as models for reduced forms of vitamin Big. Kinetic studies of alkylation at cobalt(i), as in the recent study of reaction of alkyl halides with, for example, cobaloximes and vitamin Bias, indicate an 5 n2 mechanism, with cobalt(i) acting as a very strong nucleophile. The 5 n2 mechanism has now been confirmed stereochemically by establishing that these reactions proceed with inversion of configuration at carbon. Reaction of vinyl halides with these cobalt(i) complexes also proceeds by an associative nucleophilic substitution mechanism, rather than via acetylenic intermediates. ... 

X- and y-rays Cell cycle effects, modulation of proliferation kinetics Increased activity of repair enzymes Enhanced expression of anti-apoptotic proteins Selection of apoptosis-resistant cells Elevated intracellular levels of glutathione and associated nucleophilic thiols Hall and Giaccia (2006)... 

Two groups of authors have suggested that there is a difference between the detailed mechanism of associative nucleophilic substitution at platinum(ii) or palladium(ii) and that at gold(iii). It is claimed that there are indications that bondbreaking and bond-making are asynchronous (A mechanism) at Pt and Pd, but synchronous (/a mechanism) at This distinction is hardly consistent with... 

Table 1 Kinetic parameters for associative nucleophilic attack at the [PtCy anion 10 k2/l mol s ...

So as to determine the optimum path of approach of a nucleophile to the nitroso group, model reactions of the associative nucleophilic substitution in the molecules of nitroxyl and nitrosyl flouride were studied by the MINDO/3 method [163] ... 

The general conclusions on the mechanism of associative nucleophilic substitution at pentacoordinate phosphorus come down to two points... 

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