1.3- Alkadienes, electrophilic additions

Abstract This chapter deals with the methods for the syntheses of 2,5-dihydro-l,2-oxaphosphole-2-oxide derivatives, and some recent results of their biological activity testing. The electrophilic addition to 1,2-alkadiene- and alkatrienephospho-nate derivatives is one of the easiest and fruitful synthetic strategies for obtaining these compounds in preparative amounts. 

The submitted methods for preparation of target compounds based on the electrophilic addition reactions of 1,2-alkadiene- and alkatrienephosphonates show that they are very promising from synthetic point of view. 

T-n r nki iiir ATi-r Electrophilic addition reactions to nonconi mated alkadienes can occur at one or both double bonds. 

The properties of a compound with isolated double bonds, such as 1,4-pentadiene, generally are similar to those of simple alkenes because the double bonds are essentially isolated from one another by the intervening CH2 group. However, with a conjugated alkadiene, such as 1,3-pentadiene, or a cumulated alkadiene, such as 2,3-pentadiene, the properties are sufficiently different from those of simple alkenes (and from each other) to warrant separate discussion. Some aspects of the effects of conjugation already have been mentioned, such as the influence on spectroscopic properties (see Section 9-9B). The emphasis here will be on the effects of conjugation on chemical properties. The reactions of greatest interest are addition reactions, and this chapter will include various types of addition reactions electrophilic, radical, cycloaddition, and polymerization. 

The same transition metal systems which activate alkenes, alkadienes and alkynes to undergo nucleophilic attack by heteroatom nucleophiles also promote the reaction of carbon nucleophiles with these unsaturated compounds, and most of the chemistry in Scheme 1 in Section 3.1.2 of this volume is also applicable in these systems. However two additional problems which seriously limit the synthetic utility of these reactions are encountered with carbon nucleophiles. Most carbanions arc strong reducing agents, while many electrophilic metals such as palladium(II) are readily reduced. Thus, oxidative coupling of the carbanion, with concomitant reduction of the metal, is often encountered when carbon nucleophiles arc studied. In addition, catalytic cycles invariably require reoxidation of the metal used to activate the alkene [usually palladium(II)]. Since carbanions are more readily oxidized than are the metals used, catalysis of alkene, diene and alkyne alkylation has rarely been achieved. Thus, virtually all of the reactions discussed below require stoichiometric quantities of the transition metal, and are practical only when the ease of the transformation or the value of the product overcomes the inherent cost of using large amounts of often expensive transition metals. 

This review article deals primarily with addition reactions of nucleophiles, electrophiles, and neutral radicals to photochemically generated radical ions of organic compounds and some organometallic compounds. Photocyclodimerizations of electron-rich alkenes, photo-Diels-Alder reactions between alkenes and alkadienes via dimer or heterodimer radical cations, and photocycloadditions via triplexes are also included. 

As for many other nucleophiles, the nitrite anion undergoes addition to the iodonium ion generated by the reaction of alkenes and 1,3-alkadienes with electrophilic iodine reagents. Two procedures have been described bis(pyridine)iodine(I) tetrafluoroborate136,137 [prepared from mcrcury(II) oxide and tctrafluoroboric acid supported on silica gel and pyridine on dichloromethane] and copper(II) tetrafluoroborate [prepared from copper(II) oxide and te-trafluoroboric acid] and iodine138 139. trans Addition would be expected for all products from mechanistic considerations, however, only the cyclohexene adduct 1 has been shown to have trans configuration ( H-NMR spectroscopy)139. 

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