Substitution Abstraction Reactions

The formation of the above anions ("enolate type) depend on equilibria between the carbon compounds, the base, and the solvent. To ensure a substantial concentration of the anionic synthons in solution the pA" of both the conjugated acid of the base and of the solvent must be higher than the pAT -value of the carbon compound. Alkali hydroxides in water (p/T, 16), alkoxides in the corresponding alcohols (pAT, 20), sodium amide in liquid ammonia (pATj 35), dimsyl sodium in dimethyl sulfoxide (pAT, = 35), sodium hydride, lithium amides, or lithium alkyls in ether or hydrocarbon solvents (pAT, > 40) are common combinations used in synthesis. Sometimes the bases (e.g. methoxides, amides, lithium alkyls) react as nucleophiles, in other words they do not abstract a proton, but their anion undergoes addition and substitution reactions with the carbon compound. If such is the case, sterically hindered bases are employed. A few examples are given below (H.O. House, 1972 I. Kuwajima, 1976). 

Substitution Reactions. Substitution reactions can occur on the methyl group by free-radical attack. The abstraction of an aHybc hydrogen is the most favored reaction, followed by addition to that position. 

Substitution Reactions. The chemistry at alpha positions hinges on the fact that an aHyUc hydrogen is easy to abstract because of the resonance stmctures that can be estabUshed with the neighboring double bond. The aHyUc proton is easier to abstract than one on a tertiary carbon these reactions are important in the formation of alkoxybutenes (ethers). 

Radical substitution reactions by iodine are not practical because the abstraction of hydrogen from hydrocarbons by iodine is endothermic, even for stable radicals. The enthalpy of the overall reaction is also slightly endothermic. Thus, because of both the kinetic problem excluding a chain reaction and an unfavorable equilibrium constant for substitution, iodination cannot proceed by a radical-chain mechanism. 

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 ... 

Photochemical substitution reactions of this type which involve selective hydrogen abstractions from intramolecular sites by the m.tt ketone oxygen, are reviewed in chapter 12. ... 

Monomeric thiazyl halides can be stabilized by coordination to transition metals and a large number of such complexes are known (Section 7.5). In addition, NSX monomers undergo several types of reactions that can be classified as follows (a) reactions involving the n-system of the N=S bond (b) reactions at the nitrogen centre (c) nucleophilic substitution reactions (d) halide abstraction, and (e) halide addition. Examples of each type of behaviour are illustrated below. 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

A radical is highly reactive because it contains an atom with an odd number of electrons (usually seven) in its valence shell, rather than a stable, noble-gas octet. A radical can achieve a valencC Shel octet in several wavs, for example, the radical might abstract an atom and one bonding electron from another reactant, leaving behind a new radical. The net result is a radical substitution reaction ... 

For a monograph on abstractions of divalent and higher valent atoms, see Ingold, K.U. Roberts, B.P. Free-Radical Substitution Reactions, Wiley NY, 1971. 

Allylic stannanes are an important class of compounds that undergo substitution reactions with alkyl radicals. The chain is propagated by elimination of the trialkyl -stannyl radical.315 The radical source must have some functional group that can be abstracted by trialkylstannyl radicals. In addition to halides, both thiono esters316 and selenides317 are reactive. 

Scheme 11.6 gives some examples of the various substitution reactions of aryl diazonium ions. Entries 1 to 6 are examples of reductive dediazonization. Entry 1 is an older procedure that uses hydrogen abstraction from ethanol for reduction. Entry 2 involves reduction by hypophosphorous acid. Entry 3 illustrates use of copper catalysis in conjunction with hypophosphorous acid. Entries 4 and 5 are DMF-mediated reductions, with ferrous catalysis in the latter case. Entry 6 involves reduction by NaBH4. 

The results presented in Section IV.D.l show that the net loss of the dioxorhenium(VII) species, and the ultimate formation of the phosphate R3PO, must occur in two stages because the rate of reaction (17) shows a direct first-order phosphine dependence. That said, the chemical mechanism is still open to discussion does the first step entail abstraction of an oxo oxygen or addition to it If the former, the cycle is completed by PR3 coordination to a four-coordinate rhenium intermediate if the latter, the addition step is then followed by yet another ligand substitution reaction. The alternatives are presented in Schemes 1 and 2. 

The presence of O2 clues you in that this is a free-radical mechanism, specifically a free-radical substitution. Because it is an intermolecular substitution reaction, it probably proceeds by a chain mechanism. As such it has three parts initiation, propagation, and termination. (We do not draw termination parts in this book.) The initiation part turns one of the stoichiometric starting materials into an odd-electron radical. This can be done here by abstraction of H- from C by 02. 

This is overall a substitution reaction — the C2-Brl and Snl4-H o bonds are swapped — so it is almost certainly a chain reaction. No initiator is listed, but it is likely that ambient air provides enough O2 to abstract H from Snl4. 

III). Furthermore, the progress of the radical substitution reaction does not depend on the abstraction of the hydrogen atom in the transition state by a second reagent. 

The reactivity of substituents has received little systematic study. However, with regard to electrophilic substitution reactions, it can be deduced from the products of monobenzo fused derivatives in the 1,4-series (Section 2.26.3.1.3) that the benzenoid ring is less reactive than the heteroring. The same conclusion applies to a phenyl group attached to 1,4-dithiin. In the dibenzo fused derivatives this type of competition is precluded and electrophilic attack occurs readily at the 2-position. The site of attack by a second incoming group is a little difficult to generalize upon and this is discussed in Section 2.26.3.2.2. Reactivity of the dibenzo fused compounds towards butyllithium has been well studied and proton abstraction occurs at C-l (or C-4 in phenoxathiin) (Section 2.26.3.2.3). 

Dibenzo[6,e][l,4]dioxin, phenoxathiin and thianthrene all react with butyllithium with proton abstraction from a benzene ring. Dibenzo[f ,e][l,4]dioxin and thianthrene are metallated at the 1-position (128), while lithiation of phenoxathiin occurs ortho to the C—O bond rather than the C—S bond, i.e. at C-4, (129). The lithiated products provide excellent intermediates for functionalizing the rings at these positions, usefully complementing the product distribution pattern in electrophilic substitution reactions. 

The significant changes imposed on the dithioaromatic ligands and complexes upon sulfur addition are illustrated in the structure of the Ni(p-/-PrPhDtaXp-(-PrPhDtaS) complex (Fig. 48) (Table XXII), determined by Fackler et al. (233, 257). The same workers explored the sulfur addition and abstraction reaction in depth (232) (see also Section IV). The rates and mechanisms of substitution reactions of square planar nickel(II) 1,1 -dithiolate complexes (502) is discussed in Section IV. 

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