Electrophilic addition, selectivity

Direct Chlorination of Ethylene. Direct chlorination of ethylene is generally conducted in Hquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the Hquid phase and combine in a homogeneous catalytic reaction to form EDC. Under typical process conditions, the reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor (77). Ferric chloride is a highly selective and efficient catalyst for this reaction, and is widely used commercially (78). Ferric chloride and sodium chloride [7647-14-5] mixtures have also been utilized for the catalyst (79), as have tetrachloroferrate compounds, eg, ammonium tetrachloroferrate [24411-12-9] NH FeCl (80). The reaction most likely proceeds through an electrophilic addition mechanism, in which the catalyst first polarizes chlorine, as shown in equation 5. The polarized chlorine molecule then acts as an electrophilic reagent to attack the double bond of ethylene, thereby faciHtating chlorine addition (eq. 6) ... 

Double bonds in a,/3-unsaturated keto steroids can be selectively oxidized with alkaline hydrogen peroxide to yield epoxy ketones. In contrast to the electrophilic addition mechanism of peracids, the mechanism of alkaline epoxidation involves nucleophilic attack of hydroperoxide ion on the con-... 

Electrophilic addition of HBr to 2-butene gives 2-butyl bromide. The product is chiral, and HBr addition may selectively form one enantiomer. 

Obtain the energy of each cation that might be generated by electrophilic addition of Br to biphenyl (biphenyl+Br+). Which one is most stable Are there others of comparable stability Examine the structure of the most stable cation(s), and draw all of the resonance contributors needed to describe this ion(s). Predict the product(s) of biphenyl bromination. Will the reaction be highly selective, moderately selective or unselective ... 

Many mechanistic results on this electrophilic addition are available but most of them deal with the first steps of the reaction in which the ionic intermediate is formed, rather than with the last steps in which the products are obtained by nucleophilic attack on this intermediate (ref. 2). The present paper reports results on the selectivity of olefin bromination, which have been obtained more or less systematically with a view to improving the existing rules which are too naive to be useful in synthesis (ref. 3). 

The regioselectivity and stereoselectivity of electrophilic additions to 2-benzyl-3-azabicyclo[2.2.1]hept-5-en-3-one are quite dependent on the specific electrophile. Discuss the factors that could influence the differing selectivity patterns that are observed. 

Whether nucleophilic addition of HY to C=0 proceeds stereo-selectively CIS or TRANS clearly has no meaning—unlike electrophilic addition of HY to C=C [(145)— (146) or (147)]—for with C=0 [(148) — (149) or (150)] the alternative products are identical, because of free rotation about their C—O bonds ... 

Table 18 Reactivity, log k,a and selectivities, ka.MJkH and VMe/ H> °f electrophilic additions to 1-ethoxyethylene.

It is concluded that the selectivities of electrophilic additions are not directly related to the reactivities but to the transition-state positions. Extensive comparison with similar data on the bromination and hydration of other ethylenic compounds bearing a conjugated group shows that this unexpected reactivity-selectivity behaviour can arise from an imbalance between polar and resonance effects (Ruasse, 1985). Increasing resonance in the ground state would make the transition state earlier and attenuate the kinetic selectivity more strongly than it enhances the reactivity. Hydration and halogenation probably respond differently to this imbalance. 

Facial selectivity in electrophilic additions (carbene addition, mercuration, epoxi-dation, and hydroboration) to 4-substituted 9-methylenenorsnoutanes (1) as model alkenes has been elucidated and the observed preference for yyn-attack (Table 1)... 

The origin of stereofacial selectivity in electrophilic additions to methylene-cyclohexanes (2) and 5-methylene-l,3-dioxane (3) has been elucidated experimentally (Table 2) and theoretically. Ab initio calculations suggest that two electronic factors contribute to the experimentally observed axial stereoselectivity for polarizable electrophiles (in epoxidation and diimide reduction) the spatial anisotropy of the HOMO (common to both molecules) and the anisotropy in the electrostatic potential field (in the case of methylenedioxane). The anisotropy of the HOMO arises from the important topological difference between the contributions made to the HOMO by the periplanar p C-H a-bonds and opposing p C—O or C—C cr-bonds. In contrast, catalytic reduction proceeds with equatorial face selectivity for both the cyclohexane and the dioxane systems and appears to be governed largely by steric effects. ... 

As already commented in the introduction of this chapter, regardless of its substitution pattern, the main trends of allenylidene reactivity are governed by the electron deficient character of the C and Cy carbon atoms of the cumulenic chain, the Cp being a nucleophilic center [9-15]. Thus, as occurs with their allcarbon substituted counterparts, electrophilic additions on 7i-donor-substituted allenylidene complexes are expected to take place selectively at Cp, while nucleophiles can add to both C and Cy atoms. However, the extensive 71-conjugation present in these molecules results in a reduced reactivity of the cumulenic chain and, in some cases, in marked differences in the regioselectivity of the nucleophilic additions when compared to the all-carbon substituted allenylidenes. In the following subsections updated reactivity studies on 7i-donor-substituted allenylidene complexes are presented by Periodic Group. 

It has already been mentioned (Section III) that the study of the diastereoselection in the electrophilic addition of singlet oxygen to the n face of chiral alkenes is of primary interest for the achievement of a selective oxyfunctionalization reaction. Zeolite confinement and cation- 7r interactions might be expected to affect significantly the diastereoselectivity in the photooxygenation of chiral alkenes. 

Fluorine substitution has greatly diminished the (negative) electrostatic potential for the internal double bond, but has had little effect on the potential for the external double bond. The change in selectivity (toward favoring addition onto the external double bond) is a direct consequence given that carbene addition is electrophilic addition. 

A 3-acyl-4,5-unsubstituted-2(3//)-oxazolone 157 smoothly undergoes electrophilic addition with Br2 (or NBS) and PhSeCl in methanol to give frani-5-bromo-4-methoxy- and frani-4-methoxy-5-phenylselenenyl-2-oxazolidinones 158, respectively, with full regio- and trans-selectivity (Fig. 5.39). Both substituents thus... 

The synthesis and chemistry of metal complexes of thiophenes have been reported including the electrophilic additions to osmium-thiophene complexes <9902988> and nucleophilic additions to ruthenium-thiophene complexes <99JOMC242>. The selectivity for the insertion of ruthenium into 3-substituted thiophenes was studied <99CC1793>. For example, treatment of 3-acetylthiophene (84) with Ru(cod)(cot) led to a regioselective 1,2-insertion of ruthenium giving thiaruthenacycle 85. 

In 1981 it was shown that rhodium(II) carboxylates smoothly catalyze the addition of ethyl diazoacetate to a variety of alkanes11. While some differentiation between possible sites of insertion was observed, selectivity is not as high for this carbenoid process as it is for the free radical process above. Rhodium-catalyzed intermolecular C-H insertion is thought to proceed via electrophilic addition of an intermediate rhodium carbene into the alkane C—IT bond. 

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