Heterolytic addition

The next example for this rule may be the heterolytic addition of chlorine to the C=C bond. Fig. 4.3b indicates the partial valence-inactive population 60> of the 2pz AO of the /9-carbon in LU, calculated by the extended Htickel method. It is seen that this quantity, (c )2, largely increases according to the approach of the chlorine cation to the carbon atom at which the addition is to take place, so that the reactivity of the /9-position towards the second chlorine atom (anionic species) grows. Also Fig. 4.3a shows the decrease of the LU energy in the direction of the reaction path which has already been mentioned above. 

The simplest manifestation of substituent effects should be found by comparing the rates of addition of methyl radicals to branched olefins. As Table 1 shows, the effect of introducing methyl substituents adjacent to the double bond appears to be extremely small. The variation in rate is barely one power of ten compared with a variation of almost 106 for the heterolytic addition of bromine to ethylene and tetramethylethylene. 

We referred above to the large substituent effect on the heterolytic addition of bromine to olefins, which is a polar process actually involving intermediate ionic species. By contrast, the addition of alkyl radicals to olefins involves... 

Results of recent study, however, have been interpreted in terms of a homo-lytic process. Schechter and Conrad [49] have observed that the production of methyl-3-nitroacrylate and methyl-2-hydroxy-3-nitropropionate in the reaction between N204 and methyl acrylate could not be explained on the basis of heterolytic addition, but was to be expected if a homolytic process were occurring. Brown [80] has shown that olefin nitration under circumstances in which the nitronium ion (N02+) is the reactant has characteristics entirely different from those of the N204-olefin reaction. Brand and I. D. R. Stevens [81] also believed the reaction of addition of nitrogen dioxide to olefins to involve radicals. According to these authors the following experimental facts provide evidence for this ... 

This reaction has been utilized in the synthesis of azidosteroids ". Addition to 2-cholestene (224) occurs regioselectively and adducts 225 and 226 have been isolated in 37% and 27% yield respectively. By comparison, ionic addition leads to the /ranj-diaxial product (227). Such differing orientations of the products obtained from homolytic and heterolytic addition may be synthetically valuable. 

There are three general mechanisms for insertions concerted, free radical, and heterolytic addition. In the 1,2-insertion, the concerted mechanism proceeds via interaction of the 7t system of the unsaturated compound directly with the intact E-H bond, with each end of the n system directed at either the E or the H atom (Scheme 1). This interaction may or may not be preceded by precoordination of the unsaturated molecule to the element. The transition state for this reaction is considered to be four-centered, and yields products that are cis-substituted on the reduced unsaturated substrate. 

The concerted mechanism is very common for 1,1-additions, yet in this case it involves only one end of the n system interacting with the E-H bond (Scheme 2). Quite often the initial coordination of these substrates to the element is observed and characterized. Radical and heterolytic additions, also noted for these additions, proceed in a manner similar to the 1,2-addition. 

The final example of hgand-exchange processes to be treated in this chapter is the heterolytic addition of reagents [Til]. Here a substrate XY undergoes addition to the metal center without changing the formal oxidation state or coordination number of ftie metal center. The molecular fragments X or Y are bound to the metal center as shown schematically in Equation 2-13. 

In each case, hydrido metal compounds are formed as catalytically active complexes. Finally, it should be mentioned that in practice heterolytic addition can often not be distinguished from oxidative addition followed by reductive elimination, which is discussed later in this book. 

Ru retains the oxidation state +2 heterolytic addition of H2 the tertiary amine is a strong base that traps the protons and thus supports the reaction. 

In addition reactions, the substrate splits, and each fraction bonds separately with the catalyst (metal) center. These reactions can be classified as heterolytic addition, homolytic addition, and oxidative addition. They are distinguished by the nature of the change in the oxidation state upon the addition of a substrate to a metal center. 

The rate-limiting step is the initial heterolytic addition of H2 to give a monohydride and a solvated proton. This is followed by migratory insertion to give an alkyl complex that is protonated to form the product. The sources of the H atoms added are consistent with reaction (5.39) and, with a few numerical simplifications, the authors showed that the mechanism is consistent with the rate law. 

When the substrate is changed from a carboxylic acid to a species with no acidic protons, the system changes in a subtle but possibly important way. Addition of such a substrate to (BINAP)Ru(02CCHj)2 liberates acetate ion, and heterolytic addition of Hj will yield acetic acid rather than a solvated proton. The acetic acid may not be strong enough in methanol to bring about the protolysis of the Ru—C bond, shown as the last step in Scheme 5.32. 

Dinuclear ruthenium(ii) complexes 199, which have been used as catalysts in a large number of hydrogen-transfer reactions, are known to dissociate in solution into the mononuclear hydride complexes 200 and the coordinatively unsaturated dienone derivatives 201 (Scheme 15). Remarkably, hydrides 200 are selectively formed when THF solutions of 199 are heated under hydrogen atmosphere. This process involves the conversion of 201 into 200 via heterolytic addition of The related amino-cyclopentadienyl complexes 202 and 203 are also known, being... 

They reported 141) that (1) the complex 15 without any pendent base slowly oxidizes H2 with an overpotential of -0.73 V (using -0.14 V vx. Fc/Fc (ferrocene/ferrocenium) as the thermodynamic potential for H /H2 in CH3CN) (2) introduction of pendent N bases into the backbone of the diphosphine hgands (complex 16) causes a rapid heterolytic addition of H2 to form 17 and (3) the electrocatalytic ability of 16 is improved at low overpotentials (less than 0.08 V and a turnover frequency of H2 oxidation between 0.01 and 0.5 s under 1 atm of H2) presumably through an intermediate such as 18 as predicted by our DFT calculations. 

Addition at the double C=C bond occurs via various mechanisms ionic, radical, or molecular. In this Section we consider briefly the mechanisms of heterolytic addition. Electrophilic addition reactions Adg are widely abundant. Halogens and hydrogen halides often add according to this mechanism. The addition of HX to the 7i-C=C bond in a polar medium where HX is ionized occurs in two steps... 

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