Sigma adducts

In a subsequent study using diphenylnitrenium ion, several intermediates were detected. With 1,3,5-trimethoxybenzene or 1,3-dimethoxybenzene, the decay of the nitrenium ion occurred concurrently with the appearance of sigma adducts (141, Fig. 13.69). These were characterized on the basis of their absorption maxima and their behavior toward pyridine bases. On the other hand, when readily oxidized arenes, such as AW-dimethylanihne were employed, the characteristic ion radicals were detected (Fig. 13.70). ... 

The electrophilic nature of sulfuryl chloride is validated in a study on the chlorination of anthracene derivatives the observed substituent effects conform well with molecular orbital modeling859. Chlorination of ethyl 3,4,5-trialkylpyrrole-2-carboxylates results in a substantial amount of / -side-chain chlorinated products in addition to the expected a-side-chain chlorination860. This has been ascribed to the electrophilic nature of the reagent861 and the mechanism proposed involves an allylic migration of the halogen from the a-sigma adduct to the adjacent / -side chain. 

Study of the reaction between 6-chloro-3-methoxy-4-nitropyridazine 1-oxide or related compounds and methanolic ammonia or liquid ammonia to give the 3-amino compounds upon amino-demethoxylation, revealed the formation of an intermediate sigma-adduct. By H-NMR and C-NMR spectroscopy it could be established that the sigma-adduct involved addition at the 5-C-atom. ... 

Buncel E, Dust JM, and Terrier F, Rationalizing the regioselectivity in polynitroarene anionic sigma-adduct formation. Relevance to nucleophilic aromatic substitution, Chem. Rev., 95, 2261, 1995. 

Sigma adducts are ligands that are bound to a metal center via a cr-bond pair (an L-type ligand). The cr-adduct can be a reaction intermediate that arises prior to an oxidative addition step or subsequent to a reductive elimination step. In this way, a cr-adduct is a midpoint between a free molecule or substrate and an activated molecule that then serves as... 

The infrared spectra of adducts such as (5) formed between alkyl picrates and alkoxides show changes relative to the parent compounds that are consistent with a fully covalent structure. A series of strong bands between 1225 and 1040cm" is typical of ketals [118, 119]. Many sigma adducts are intensely colored, and attempts have been made to correlate UV-visible spectra with structural features [120]. However, these spectra do not generally provide such precise information as do NMR studies. Nevertheless, the absorption intensity usually obeys the Beer-Lambert law, so that absorption is directly proportional to concentration, and this is very useful in the determination of concentrations in kinetic and equilibrium studies. 

The H NMR spectra of sigma adducts were first reported in the 1960s [121, 122], and since then, the technique has proved invaluable in structural determinations. The method is... 

Figure 3 Formation of the sigma adduct [Cu NHC) o-catB-Bcat)r.

Substrates for EHC reactions consist of two electron-deficient alkenes tethered to one another. Reduction leads to the formation of an adduct wherein the )S-carbons are joined by a new sigma bond. As illustrated in Table 1, the methodology is exceptionally useful for the construction of three-, five-, and six-membered rings, but not for rings of sizes seven and eight. 

Keto ester 67 was converted to the unsaturated nitrile 70 in a routine manner. The latter proved to be an exceptionally useful intermediate. Concern that the significant steric demands which are associated with the formation of a sigma bond to the fully substituted beta carbon of the unsaturated nitrile would prevent reaction from occurring, were allayed by the discovery that the controlled potential reduction of 70 at —2.4 V in the presence of dimethyl malonate as the proton donor, alforded a 90% isolated yield of the requisite [3.2.1] adduct 71. This material was subsequently converted to enone 72 [42], a convergent point with an existing synthesis of quadrone (59). 

As mentioned above, the initial model (39) was chosen so as to break the enthalpy of adduct formation into electrostatic and covalent contributions, i.e., — AH =EaEb- -CaCb. The fact that more than one solution can exist enables us to attempt to break-up the enthalpy of adduct formation into a physically meaningful model other than the electrostatic and covalent model initially employed. One apparently obvious break-up of the enthalpy of adduct formation which is of chemical interest would be that of sigma, tr, and pi, n, contributions ... 

The LFP studies of the reaction of the A-methyl-A-4-biphenylylnitrenium ion with a series of arenes showed that no detectable intermediate formed in these reactions. The rate constants of these reactions correlated neither with the oxidation potentials of the traps (as would be expected were the initial step electron transfer) nor with the basicity of these traps (a proxy for their susceptibility toward direct formation of the sigma complex). Instead, a good correlation of these rate constants was found with the ability of the traps to form n complexes with picric acid (Fig. 13.68). On this basis, it was concluded the initial step in these reactions was the rapid formation of a ti complex (140) between the nitrenium ion (138) and the arene (139). This was followed by a-complex formation and tautomerization to give adducts, or a relatively slow homolytic dissociation to give (ultimately) the parent amine. 

The Me3Ge+ cation can also form adducts with arenes166 and quantitative gas-phase equilibrium measurements show that the stability of the adducts (—AG°) obeys the order 1,3-Me2C6H4 > h2o > MePh. The thermochemistry and reactivity of the [Me3Ge+.arene] adducts suggest that the most likely structure is that of a sigma complex. 

Tn previous work it has been shown that a competition exists during - ozonation of olefins between ozonolysis and epoxide formation (I). As steric hindrance increases around the double bond, the yield of epoxide or subsequent rearrangement products increases. This is illustrated with both old (1) and new examples in Table I for purely aliphatic olefins and in Table II for aryl substituted ethylenes. It was suggested that the initial attack of ozone on an olefinic double bond involves w (pi) complex formation for which there were two fates (a) entrance into 1,3-dipolar cycloaddition (to a 1,2,3-trioxolane adduct), resulting in ozonolysis products (b) conversion to a o- (sigma) complex followed by loss of molecular oxygen and epoxide formation (Scheme 1). As the bulk... 

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