Unsaturated electrophiles

A betaine intermediate is formed, the fragmentation of which leads to formation of an oxirane ring by intramolecular substitution by the anionic oxygen (Eq. 89). The sulfonium ylides also react with other types of compounds containing an electrophilic unsaturated bond (C=C, C=N), giving cyclopropane derivatives and aziridines. 

The red metastable product, produced from P4 and KOH or KOEt, does not appear to be an intermediate in these, tertiary phosphine oxide syntheses, since when separately prepared, gave only minor amounts of phosphine oxide. This result indicates that the olefinic compounds must attack an earlier intermediate. The initial step was thought to involve nucleophilic attack of hydroxide ion on tetrahedral white phosphorus to give a phosphide ion that subsequently underwent a Michael-Addition to the electrophilic unsaturated compounds present (Rauhut, Bemheimer and Semsel). 

Hunger, K., Epoxidation of a,P-unsaturated phosphonic acid esters, Chem. Ber., 101, 3530, 1968. Kadyrov, A.A., Rokhlin, E.M., and Knunyants, LL., Epoxidation of highly electrophilic unsaturated fluorine-containing compounds by peracids, Izv. Akad. Nauk SSSR, Ser Khim., 2344,1982 Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl. Transl.), 2063, 1982. 

Nucleophilic intermediates formed in the white phosphorus-hydroxide reaction are also trapped by electrophilic unsaturated compounds such as acrylonitrile and acrylamide. Reaction of white phosphorus with potassium hydroxide and acrylonitrile in ethanol at 0-35 gave tris(2-cyanoethyl)phosphine oxide as substantially the only organophosphorus product (11). 

Unsaturated esters are good Michael acceptors because they are not very electrophilic. Unsaturated amides are even less electrophilic and (provided they are tertiary and have no acidic NH protons) will even give conjugate addition products with lithium enolates. 

Tetramethoxyallene adds to electrophilic unsaturated systems to give [2 + 2] adducts. With tetracyanoethylene the cyclobutane (339) is obtained, which is in equilibrium with the ring-opened dipolar form (340). This accounts for the n.m.r. spectrum which shows only a singlet (t6.23) at —95°C, three methoxy signals (ratio 1 1 2) become visible. Equivalence of the four methoxy-groups in (339) is believed to be a result of the equilibrium (339) (340) and not internal rotation in the allylic cation in (340). ... 

Trivalent carbenium ions are the key intermediates in electrophilic reactions of Tt-donor unsaturated hydrocarbons. At the same time, pen-tacoordinated carbonium ions are the key to electrophilic reactions of cr-donor saturated hydrocarbons through the ability of C-H or C-C single bonds to participate in carbonium ion formation. 

So far in this section we have combined enolate anions with other carbonyl compounds by direct attack at the carbonyl group. We can expand the scope of this reaction by using a,p-unsaturated carbonyl compounds as the electrophiles. This is the Michael reaction. Remind yourself of tliis by writing out the mechanism of a Michael reaction such as ... 

Aryl and vinylic bromides and iodides react with the least substituted and most electrophilic carbon atoms of activated olefins, e.g., styrenes, allylic alcohols, a,p-unsaturated esters and nitriles. 

The diminished rr electron density m the double bond makes a p unsaturated aide hydes and ketones less reactive than alkenes toward electrophilic addition Electrophilic reagents—bromine and peroxy acids for example—react more slowly with the carbon-carbon double bond of a p unsaturated carbonyl compounds than with simple alkenes... 

On the other hand the polarization of electron density map unsaturated carbonyl compounds makes their p carbon atoms rather electrophilic Some chemical conse quences of this enhanced electrophilicity are described m the following section... 

The boron atom in boron trifluoride is hybridized to the sp planar configuration and consequently is coordinatively unsaturated, ie, a Lewis acid. Its chemistry centers around satisfying this unsaturation by the formation with Lewis bases of adducts that are nearly tetrahedral sp [ The electrophilic properties (acid strengths) of the trihaloboranes have been found to increase in the order BF < BCl < BBr < BI (3,4). 

The quiaones have excellent redox properties and are thus important oxidants ia laboratory and biological synthons. The presence of an extensive array of conjugated systems, especially the a,P-unsaturated ketone arrangement, allows the quiaones to participate ia a variety of reactioas. Characteristics of quiaoae reactioas iaclude nucleophilic substitutioa electrophilic, radical, and cycloaddition reactions photochemistry and normal and unusual carbonyl chemistry. 

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