Cyclodimerization reversible

A reversible thermal solid-state [2-1-2] cyclodimerization which is stereospecific and proceeds in single crystals has been found with (q -C5H5)Co-(S2C6H4) (379). The dimer 380 forms with major conformational changes in... 

Unlike thermal [2 + 2] cycloadditions which normally do not proceed readily unless certain structural features are present (see Section 1.3.1.1.), metal-catalyzed [2 + 2] cycloadditions should be allowed according to orbital symmetry conservation rules. There is now evidence that most metal-catalyzed [2 + 2] cycloadditions proceed stepwise via metallacycloalkanes as intermediates and both their formation and transformation are believed to occur by concerted processes. In many instances such reactions occur with high regioselectivity. Another mode for [2 + 2] cyclodimerization and cycloadditions involves radical cation intermediates (hole-catalyzed) obtained from oxidation of alkcnes by strong electron acceptors such as triarylammini-um radical cation salts.1 These reactions are similar to photochemical electron transfer (PET) initiated [2 + 2] cyclodimerization and cycloadditions in which an electron acceptor is used in the irradiation process.2 Because of the reversibility of these processes there is very little stereoselectivity observed in the cyclobutanes formed. 

When the latter mixture is treated with the aminium salt at 0°C, the mixture isomerizes completely to the more stable trans,anti,trans isomer. Evidently, the cyclodimerization is reversible at the higher temperature. It is also noteworthy that even the reversal is stereospecific, since none of the other possible diaster-eoisomers are formed. This result is nicely interpreted in terms of the ionization of the trans,syn,trans cyclobutadimer to a long bond cation radical, followed by concerted fragmentation to trans-anethole and the trans-anethole cation radical, avoiding the formation of any cw-anethole. The cyclodimerization of phenyl vinyl ether gives both trans (or anti) and cis (or syn) isomers initially, but subsequent reaction induces some isomerization of the less stable cis isomer to the more stable trans isomer. This is believed to occur through the intervention of a distonic 1,4-butanediyl type cation radical. 

The reversibility of the cycloaddition or cyclodimerization via radical cation could be observed in certain cases. For example [209], with some indenones (Scheme 38), cyclodimerization into truxones may occur but the radical cation of the cyclodimer may lead to other kinds of cyclodimer. Note also that the open radical cation is strongly sensitive to the presence of residual water. Other induced [4 + 2]-cycloaddition reactions implying 2-vinylpyrroles and y6-accept or substituted enamines were described [210]. 

Ni(cod)2 or mixtures of Ni(cod)2 with an electron deficient alkenes (e.g. dialkyl fumarate or maleic anhydride) have been found to be the most efficient catalysts for the cyclodimerization of methylenecyclopropane and 2-methylmethylenecyclopropane no. i7i) Ni(cod)2 the combined yields of cyclodimerization products afe lower, but the ratio or four-membered to five-membered rings is higher. The reverse holds for the modified catalysts (Eq. 66). 

The kinetics of the ADMET reaction is not amenable to study by many traditional means, as these polymerizations are mostly conducted in bulk. The most effective way to measure the kinetics of the polymerization is to monitor the volume of evolved ethylene. This technique has been used to probe the difference in activity between [Mo] 2 and [Ru]l for ADMET polymerization of 1,9-decadiene [37]. At 26 °C in bulk monomer, [Mo] 2 promotes ADMET polymerization of 1,9-decadiene at a rate approximately 24 times that of [Ru]l. Additionally, [Mo] 2 polymerizes 1,5-hexadiene 1.7 times faster than 1,9-decadiene, while [Ru]l only cyclodimerizes 1,5-hexadiene to 1,5-cyclooctadiene. Monomers with coordinating functionality, specifically ethers and sulfides, were also investigated. Predictably, these monomers did not undergo polymerization as rapidly as hydrocarbon monomers however, this difference was dramatically more pronounced with [Ru]l than with [Mo]2. In fact, the dialkenyl sulfide monomers that were studied completely shut down the polymerization with [Ru]l, whereas the catalytic activity of [Mo]2 was only slightly lowered. This reduction in polymerization rate is most likely due to coordination of the heteroatom to the vacant coordination site of [Ru] 1, following phosphine dissociation. This reversible coordination of heteroatoms to the ruthenium complex likely occurs both intramolecularly and intermolecularly. Conversely, the steric bulk of the ligands in [Mo] 2 makes it less likely to intramolecularly form a coordinate complex, despite molybdenum being far more electrophilic than ruthenium. 

---