The cd Criteria and Cyclic Enediynes

it is 4.38 A at CCSD(T)/cc-pVTZ, and the microwave experimental value is 4.32 A. ) Their conclusion, the cd criteria, is that molecules having cd less than 3.2 A will cyclize spontaneously, while those with cd greater than 3.3 A are 

Stable. This argument implies that the critical feature that initiates (or precludes) Bergman cyclization is the distance between the ends of the two allqmes if they are close enough together, the reaction occurs, otherwise the enediyne is stable. 

Kraka and Cremer addressed the cd criteria by performing CCSD(T)/ 

Methyl substitntion at the termini of 38, giving (3Z)-3-heptene-l,5-diyne 48 and (4Z) -octene-2,6-diyne 49, only slightly increases the cd distance, yet the activation barrier increases by 3-4 kcal mol with each methyl addition (Table 4.11). 

TABLE 4.11 DFT-Computed cd Distances and Activation Enthalpies for Acyclic and Cyclic Enediynes  

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