Substitution and Atom Abstraction Reactions

The dimer [CpCr(CO)3]2 has a weak metal-metal bond and dissociates in solution at room temperature to a measurable extent to generate the 17-electron [CpCr(CO)3]. This permitted a detailed kinetic study of CO substitution in the radical, which established a rapid associative mechanism. 15,16 A comparison of these results with reactivity data17-47 for [Cp W(CO)3] suggests that CO substitution by PPh3 in the 17-electron radicals is faster for tungsten than chromium by a factor of ca. 106. Photolysis of [CpFe(CO)2]2 in conjunction with time-resolved infrared spectroscopy was 

N-donor induced disproportionation of [Fe(CO)3(PR3)2]+ (R = Me, Bu, Cy, Ph) as well as halide induced disproportionation of [M(CO)3(PCy3)2]+ (M = Fe, Ru, Os) has been interpreted in terms of nucleophilic attack being rate determining.103 104 The rate data led to the conclusion that the reactivity of these 17-electron complexes is only weakly dependent on the metal, and the suggestion was made that periodic trends in 17-electron systems are generally attenuated in comparison to those for 18-electron analogues. However, it was noted previously that W Cr by ca. 106 1 for substitution in [CpM(CO)3]. A direct comparison of the rate of associative ligand substitution at a 17-electron center as a function of the metal for a complete triad (Cr, Mo, W) was reported for the reaction in Eq. (20).14 

In contrast to the behavior shown by 7+(Cr), the lack of any wave due to 7+(Mo, W) with excess P(OBu)3 present, even up to 100 V/s, indicated that the rate of Eq. (20) is much larger for the heavier metals. Nevertheless, it was possible to determine these rates by voltammetry with a deficiency of P(OBu)3 present. This procedure, developed by Parker,105 106 leads to 

There appears to be a fundamental difference in the way P-donors react with the chromium triad complexes [(arene)M(CO)3] and [(arene) M(CO)3]+ the former undergo arene displacement,108 109 whereas the latter prefer CO substitution, Eqs. (21) and (22). Equation (21) is known 

Elegant studies by Kochi and co-workers111112 established that rapid ligand substitution according to Eq. (23) occurs when a slight oxidizing 

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Polar factors can play an extremely important role in determining the overall reactivity and specificity of hotnolytic substitution.97 Theoretical studies on atom abstraction reactions support this view by showing that the transition state has a degree of charge separation.101 10 ... 

Along with free radical atom abstraction reactions, reactions of radical substitution are known where a free radical attacks the weak Y—Y bond and abstracts radical Y [56] ... 

There is a great diversity of initiating and propagation steps for radical substitution reactions. Bond homolyses, fragmentations, atom abstraction reactions, and addition reactions to C=C double bonds are among the possibilities. All of these reactions can be observed with substituted alkylmercury(II) hydrides as starting materials. For this reason, we will examine these reactions as the first radical reactions in Section 1.6. 

Detailed studies of H-atom abstraction reactions of cyclopropyl diketones U1), 2,2,5,5-tetramethyltetrahydrofuran-3-4-dione 130) cycloalkanediones57) and substituted benzils 121 , among others, have appeared. The cyclopropyl diketones reacted normally via semidione radicals without ring enlargement. 

Hydroxyl radicals ( OH) are powerful oxidants and participate in a number of reactions such as addition to the double bonds forming radical adducts, electron transfer reactions, and H-atom abstraction reaction. The rate constants for the reaction of OH radicals with organic substrates are mostly diffusion controlled (10 -10 ° M" s" ). When OH radical reacts with cellular organic molecules (RH) either by hydrogen abstraction [Eq. (4)] or by addition reaction, it leaves a radical site on the molecule (R ) and sometimes these radicals can add to the oxygen present in the cells, to be converted to peroxyl radicals [ROO, Eqs. (4) and (5)]. Rate constants for these reactions vary between 10 to diffusion-controlled limits depending on the nature and substitution on RH. °... 

It is clear that the ortho and para substituted diarylamines 16 and 17 are derived from capture of the singlet nitrene and the product of benzylic CH insertion 15 can be formed from either the triplet or singlet state of the nitrene. Decafluoroazobenzene is derived from a dimerization reaction of the triplet nitrene and pentafluoroaniline is formed by hydrogen atom abstraction reactions of triplet pentafluorophenyl nitrene, and possibly by some photoreduction of an excited state of the azide. It is clear from this data that the singlet and triplet nitrene are not rapidly interconverting, and there is no evidence for uphill intersystem crossing from the triplet to the singlet nitrene. 

Addition reactions of the 4-tm-butylcyclohexyl radical have been studied with a variety of alkenes and also in atom-abstraction reactions (see Section D.2.2.)3 58. While hydrogen or halide abstraction reactions yield preferentially the axial product, unselective addition occurs with terminally unsubstituted alkenes. In addition reactions of alkenes substituted by alkyl groups at the attacked olefinic center, the preference for axial attack decreases further and the equatorial addition product is formed. This influence of the size of the reaction partner on the selectivity is rationalized on the basis of the simultaneous presence of steric effects (hindering axial attack) and torsional effects (hindering equatorial attack), very similar to those discussed for nucleophilic addition reactions to cyclic ketones59. 

Atom abstraction reactions also occur with // -cyclopentadienyl substituted metal carbonyl complexes. Laser photolysis (460-490 nm) of [ / -cpM(CO)3]2 in the presence of organic halides (RX) leads to the formation of the halo compounds / -cpM(CO)3X (M = Mo, W). The reactivity trends RI > RBr > RCl, and benzyl > allyl > 3 > 2 > 1 > CH3, follow those expected for a free radical pathway. The reaction involves formation of the 17-electron intermediate / -cpM(CO)3, which then abstracts a halogen atom from the substrate (Scheme 6.9). The cpM(CO)3 radical is trapped by oxygen at a rate that is close to diffusion controlled. 

Substitution, addition, and group transfer reactions can occur intramolecularly. Intramolecular substitution reactions that involve hydrogen abstraction have some important synthetic applications, since they permit functionalization of carbon atoms relatively remote from the initial reaction site. ° The preference for a six-membered cyclic transition state in the hydrogen abstraction step imparts position selectivity to the process ... 

Atom or radical transfer reactions generally proceed by a SH2 mechanism (substitution, homolytie, bimolecular) that can be depicted as shown in Figure 1.6. This area has been the subject of a number of reviews.1 3 27 97 99 The present discussion is limited, in the main, to hydrogen atom abstraction from aliphatic substrates and the factors which influence rate and specificity of this reaction. 

Hydrogen-Atom Transfer. Many oxidation and reduction reactions are free-radical substitutions and involve the transfer of a hydrogen atom. For example, one of the two main propagation steps of 14-1 involves abstraction of... 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

The success of such reactions depends on the intramolecular hydrogen transfer being faster than hydrogen atom abstraction from the stannane reagent. In the example shown, hydrogen transfer is favored by the thermodynamic driving force of radical stabilization, by the intramolecular nature of the hydrogen transfer, and by the steric effects of the central quaternary carbon. This substitution pattern often favors intramolecular reactions as a result of conformational effects. 

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