Involving electron-deficient heteroatoms

The only successful [2 + 2] cycloaddition reported so far involving an MCP derivative and a carbon-heteroatom double bond is the reaction of BCP (3) with chlorosulfonylisocyanate (CSI) (594) [13b, 143], CSI is a typical [2 + 2] cycloaddend with most alkenes, but has been demonstrated to be also involved in stepwise cycloadditions via polar intermediates [157], The reaction of BCP and CSI gives the [2 + 2] cycloadduct 596 only as a minor product, besides the major [3 + 2] adduct 599 (Scheme 83) [13b, 143], Therefore, it has been reasonably suggested that both products derive from a common dipolar intermediate 595, formed by nucleophilic attack of BCP on the electron-deficient carbon of CSI (Scheme 83) [13b]. 

On the other hand, however, these two areas of cationic polymerization are not completely separated fields. In spite of the differences, both processes proceed on electron-deficient active species cations or species with a partial positive charge. Thus, propagation in both cases involves attack of the nucleophile (double bond or heteroatom) on electrophilic active centers. Several basic principles will therefore hold for both vinyl and ring-opening cationic polymerization. 

Cyclopropanes adjacent to an electron-deficient centre X (carbon or heteroatom) undergo C3 - C4 ring enlargement into four-membered ring derivatives via routes involving cations, radicals or car bene intermediates. Furthermore, when the cyclopropanes also bear on the same carbon an electron-donor substituent Y they undergo a specific C3 - C4 ring expansion, important from the synthetic point of view, into cyclobutanone (or related) derivatives (equation 67). 

Although no cyclopropane formation was observed in the reactions of heteroatom-stabilized carbene complexes with simple alkenes such as cyclohexene or tetramethylethylene, cyclopropane formation has been observed both with electron-deficient a,) -unsaturated esters and with electron-rich vinyl ethers. The mechanisms involved in cyclopropane formation from these very different classes of alkenes may be substantially different. 

A stereoselective synthesis of novel spiroheterocycles in ionic liquids has been discovered by Rajesh et al. [84]. A multicomponenf reaction involving 1,3-dipolar cycloaddition is carried out at 100 °C in [bmim]Br for 1-2.5 h yielding fhe producf in 80-94% yield (Scheme 58). Ionic liquids play twin roles, as a solvent as well as a catalyst. H-atom of [bmim]+ being electron-deficient can form H-bonds with the heteroatoms, thereby catalyzing the reaction (Scheme 59). 

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