Cycle times 3 + 2 -Cycloaddition

Cycl[3.2.2]azines may also be obtained by the cycloaddition of a bifunctional three-carbon unit to a 3//-pyrrolizine. Vinamidinium salts have been used for this purpose <1984CB1649>, although the reactions require the use of a strong base (sodium hydride) and extended reaction times. They appear to proceed via a stepwise mechanism, since intermediates (the conjugated enamines 354 and 355) have been isolated in certain cases (Scheme 94). 

Now the new carbene can undergo [2 + 2] cycloaddition and reverse [2 + 2] cycloaddition again, this time with the alkene component, and out comes the diene, plus a Ru carbene ready to start the cycle again. 

The above—considered reactions of singlet carbenes involving cycloaddition and bond insertion processes by no means represent all the reaction diversity they are capable of At the same time, precisely these reactions exhibit all the principal steps of the most important intramolecular rearrangements of carbenes, such as the reactions of 1,2-shift, of cycle extension, etc. [82-84]. 

Imidazoles carrying a long alkyl chain as a ligand for the copper(I)-catalyzed azide-alkyne cycloaddition are also very efficient. An alkyl chain can readily change its conformation in a host molecule to adjust its volume to the cavity size and may have an appreciable steric effect [15]. The copper(I) isonitrile complex prepared upon treatment of isonitriles with CuCl in THF in 95% yield as [CuLCl] units extended in a polymeric chain is stable in air or water for several months. This complex catalyzes the reaction of azides and acetylenes in water at room temperature in 0.5-5 mol% loading to give excellent yields in very short times. The catalyst may be recycled through five cycles [16-18]. 

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