Cleavage/recombination

In contrast to the extensive body of work on the preparation of these zinc carbenoids, few investigations are on record concerning the mechanism of the Furu-kawa method for carbenoid formation. Two limiting mechanisms can be envisioned - a concerted metathesis via a four-centered transition structure or a stepwise radical cleavage-recombination (Scheme 3.11). 

Although the two competing intermediates, the hypothetical ketyl-aryl radical pair (4) and the oxaspirooctadienyllithium (3), are not the rate-determining transition states, they should lie at almost the same energetic level. The rearrangement is in accord with the intramolecular nucleophilic addition/elimination mechanism rather than with homolytic cleavage/recombination. 

An additional and noteworthy feature of the 3-allylcyclopropene photochemistry is side chain cleavage-recombination with retention of the three-membered ring. The photolysis of 246b delivers, in addition to the bicyclo[3.1.0]hexene 248b, the rearranged cyclopropenes 249 and 250. Homolytic cleavage of the cyclopropenyl-allyl bond and subsequent radical recombination at a phenyl-substituted centre accounts for these observations (equation 80). 

Fig. 2.5 Possible mechanisms for CTI via the heterolytic disruption of the conjugation of double bonds and pseudo double bonds (radical cleavage/recombination pathway was omitted there since it is not a true CTI process).

Photo-Fries migration has been reported for the sulphonate ester (316) to give (317) A homolytic cleavage-recombination mechanism akin to the photo-Fries rearrangement of sulphonamldes may... 

However, there may be another mechanism that is at least in competition with cleavage/recombination, as suggested by Schultz and Schlessinger (section IIIA, compound 48). This could involve inversion/racemization with little or no barrier in the excited state before otherwise nonproductive internal conversion down to the ground state (similar in principle to cis/trans isomerization... 

The products were the result of 3,5-, 1,5-, and 5,5-shift processes in roughly equal amounts. Furthermore, pyrolysis of a mixture of dideuterio and perproptio starting materials gave the 3,5- and 5,5-shift products with the monodeuterio isomer predominating, indicating a cleavage-recombination reaction path and not concerted, intramolecular reactions. 

This result is consistent with formation of the crossed ozonide via the cleavage-recombination mechanism ... 

If the driving force is insufficient, the unimolecular structural reorganization may be superseded by other processes. 2,2,3-Triphenylpropyllithium (325) does isomerize but by a cleavage/recombination rather than by a phenyl [l,2]-shift mechanism (Scheme 1-254). ° Cross-over experiments unequivocally establish the inter-molecularity of the benzyllithium readdition onto 1,1-diphenylethyIene. 

Scheme 1-254. 2,2,3-Triphenylpropyllithium (325) undergoing a cleavage/recombination process rather than a one-step phenyl [l,2]-migration.

Scheme 1-258. [3,2]-Sigmatropic rearrangement and homolytic cleavage/recombination affording isomers 331 and 332, respectively.

Scheme 1-259. Phenyl [l,2]-migration, allyl [3,2]-migration, or cleavage/recombination.

Radical cleavage recombination mechanisms are also known to occur in these reactions for example the 1,2-shift of the allyl group without inversion is not uncommon. The radical pathway generally appears to have a higher activation energy so that the occurrence of the concerted [2,3] shift depends upon structural features and on the reaction temperature. 

---