Reactions with Systems Containing Multiple Bonds

Reactions with Systems Containing Multiple Bonds... 

In spite of the general ambiphilicity of phosphonio-substituted phosphoHde derivatives, the aromaticity of the phosphoHde ring [10, 11] tends to reduce their electrophilicity while the intramolecular compensation of the negative charge by the phosphonio-substituents lowers at the same time their nucle-ophilicity [15, 16]. Bis-phosphonio-benzophospholides and -1,2,4-diaza-phospholides are therefore less reactive towards electrophiles and nucleophiles than other types of phosphorus containing multiple-bond systems and lack the notorious hydrolytic instabihty of many of these species [15, 16, 24]. Reactions are observed, however, with sufficiently strong electrophiles such as triflic acid or methyl triflate, or nucleophiles such as OH" or lithium alkyls, respectively. 

As far as substrates are concerned, while the usual 1,4-addition and 1,3-substi-tution (Sn2 ) reactions of simple unsaturated substrates have so far predominated, analogous transformations of ambident substrates with extended multiple bond systems (i.e., with two or more reactive positions) have come to attention only recently. Here, systematic investigations have shown that such 1,5-substitutions and even 1,6- and 1,8-addition reactions proceed highly regioselectively and ste-reoselectively, in particular when the substrate contains at least one triple bond besides one or more conjugated double bonds. These unusual reaction types not... 

Systemization of experimental data on the syntheses of heterocyclic compounds with perfluoroalkyl groups from perfluoroolefins is based on reactions with various 1,1-, 1,2-, 1,3-, and 1,4-binucleophilic reagents. While the main features of nucleophilic reactions are preserved, further transformations of the primary products (or adducts, or the products of substitution of the functional groups at the internal multiple bond) occur under the influence of the added functional group containing a heteroatom. Here one can expect dramatic differences in the effect of the nature of the nucleophilic reagent between cyclizations by new nucleophilic centers and centers already available in the molecule. Another important aspect is isomerization of the primary internal olefin into the terminal olefin or internal olefin with a different structure under the action of the nucleophilic agent. This may be critical to the structure of the heterocycle formed. 

The ionic hydrogenation of carbon-carbon multiple bonds has been demonstrated only for the reduction of carbon-carbon double bonds which on protonation result in the formation of tertiary, or aryl-substituted, carbocations. The ability to use this reduction reaction with functionally substituted alkenes will be limited to those systems that do not contain functional groups that would otherwise react with trifluo-roacetic acid or the organosilane. 

A review of lone pair effects involving multiple bonds between heavier main group elements contains much of relevance to pj -bonded phosphorus systems. The diphosphene (295) has been shown to undergo cycloaddition reactions with isocyanides, to give the iminodiphosphiranes (296)." A thirtyfive-fold excess of methyl triflate is needed to convert the diphosphene (297) to the salt (298), which is unstable in non-polar solvents. Experimental data show that the P=P bond becomes stronger on alkylation as is the case for N=N compounds. 

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