Effect of Leaving Group

An early theoretical study suggested that better 0 donors are eliminated more easily from cA-MR(R )L2 type complexes [8]. However, this prospect is now known to be applicable only to a dialkyl complex series [e.g., Me Et Pr Bu]. Thus the rate of reductive elimination is rather sensitive to the other factors, especially to the orbital hybridization of hydrocarbyl ligands. In general, the reactivity decreases in the order [hydrido (s)alkenyl (sp ) aryl (sp ) 3 alkynyl (sp) alkyl (sp )]. While a Jt-allyl ligand ranks middle, its behavior in reductive elimination will be described separately in Section 9.4. 

Owing to the extremely high reactivity of hydrido ligand, direct observations of H-H and C-H reductive elimination have been limited to platinum complexes. H-H reductive elimination from c 5-PtH2L2 proceeds without notable activation barrier [13]. C-H reductive elimination is also a facile process cA-PtH(Me)(PPh3)2 and cA-PtH(CH2CF3)(PPh3)2 readily decompose at -40°C and room temperature, respectively [14]. These reactions involve a modest kinetic isotope effect ( h/ d = 

3) and a small activation entropy (2.0-7.8 eu) the kinetic data are consistent with a concerted mechanism involving a three-center transition state [15]. 

Lj = dppp ER3 = BEta, AIEts, BPhg. B(C6Fs)3, AlPhg 

Diorganoplatinum(II) complexes are fairly stable towards reductive elimination. On the other hand, c 5-PtR(SiR3)L2 type complexes have been shown to be reactive towards C-Si reductive elimination [25]. For example, cis-Pt(vinyl)(SiPh3)(PMe2Ph)2 undergoes reductive elimination in toluene- g at 25 °C with the first-order rate constant 1.2 x 10 s  

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To study generality of S Ar-based macrocyclizadon, effects of leaving groups and bases have been examined in synthesis of 14-membered macrocycles fBq. 9.8. Evidently, fluorides... 

Effect of Leaving Group. For both SeI and second-order mechanisms, the more polar the C—X bond, the easier it is for the electrofuge to cleave. For... 

The effects of leaving group, solvent, and nucleophile, on the kinetics of aminolysis of a series of substituted aryl diphenylphosphinates and their mono and dithio analogues have been investigated. ... 

In a series of recent papers, Komblum et at. have systematically investigated various factors of the reaction at saturated carbon, namely, the stereochemistry (Kornblum and Wade, 1987), the effect of light (Wade et ai, 1987), the effect of leaving groups (Kornblum et al., 1988) and the particular reactivity of the p-nitrocumyl system (Kornblum et al, 1987) as well as the conditions under which the cyano-group may replace the nitro-substituent in reactions involving a benzylic carbon (Kornblum and Fifold, 1989). 

Solvolysis of the R,R and R,S isomers of 2-bromo-9-(l-X-ethyl)fluorenes, X = Cl, Br, I, or OBs, in 25% (v/v) acetonitrile in water has been studied with respect to rates of formation of elimination products and of substitution products (X = OH or NHCOMe).142 The parent 9-(l-X-ethyl)fluorenes and the 2,2/-dibromo-9-(l-X-ethyl)-fluorenes were also studied. Various effects of leaving group and of the presence of nucleophiles on the competition between the reactions were observed and the Bronsted equation was applied to the results for the elimination reactions. A related study of solvolysis of 9-(X-methyl)fluorenes, X = I, Br, or Bs, was also carried out, in which the Swain-Scott equation was applied to nucleophilic selectivities in the S 2 reactions.143... 

It was argued that the products and the isomer distributions observed in reactions proposed to proceed by the S l mechanism differ from those obtained by independent generation of aryl radicals and that the effects of leaving groups, counterions and substituents are not in accord with the intermediacy of an aryl radical. 

Nonetheless, values for aryl glycosides hydrolysed by glycosidases continue to be measured the first such, for aryl p-glucosides hydrolysed by almond emulsin (which measured the effect of leaving group acidity from tabulated Hammett a constants, rather than directly from the leaving group pK ) dates... 

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