Ring-opening dissociation

The elimination of two D atoms and two ring-opening dissociation channels were observed from the two-photon dissociation. 

Fig. 14. Energy diagram of the ring-opening dissociation C6D6 —> C5D3 + CD3 from ab initio results (Ref. 29). Note that there are many intermediates which can dissociate into C4D3 + C2D3 by a simple C-C bond fission.

In Scheme I, ki describes the cyclization rate and k-i, the ring opening dissociation rate. In polymer systems where the molecules have a finite polydispersity, the angle brackets on represent an ens id[>le average. The terms Ice and kn represent, respectively, the reciprocal lifetimes of the excimer (or exciplex) and the locally excited (monomer) state. 

An equilibrium involving reversible dissociation of the N-N bond in N-nitrobenzotriazole 299a has not been directly registered, although occurrence of the ring-opened form 299b was inferred on the basis of isolation of its derivative 300 (Scheme 112) [96CHEC-II(4)1]. 

The ring-opening mechanism was well supported by the snapshots and the overlap bond population obtained from TB-QCMD simulations, where the formation of new C-H and La-C bonds and the dissociation of La-H and proximal C-C bonds could be tracked. The obtained dynamic ring opening mechanism was similar to the static mechanism, however, a novel transition state was also proposed for insertion reaction of alkenes, with tetrahedral h4-coordination. This example perfectly illustrates the importance of mutual interplay between high-level first principle methodologies and simplified methodologies derived from ab initio quantum chemistry, massively applicable for real systems. 

Prior to the development of the aluminum initiators described above, a series of bimetallic /x-oxo alkoxides, such as (269) and (270) were examined as lactone polymerization initiators.811 812 At 10 °C, (269) polymerizes CL in a moderately controlled manner (60 equivalents, t1/2 = 23min), as shown by a linear DP vs. [M]0/[I]o plot and Mw/Mn values between 1.3 and 1.5. Mn data is consistent with only one of the terminal alkoxides initiating the ring opening, although in the presence of "BuOH, which is known to dissociate (269), all four alkoxides are active. 

Another piece of mechanistic evidence was reported by Snapper et al. [14], who describe a ruthenium catalyst caught in action . During studies on ring opening metathesis, these authors were able to isolate and characterize carbene 5 in which a tethered alkene group has replaced one of the phosphines originally present in Id. Control experiments have shown that compound 5 by itself is catalytically active, thus making sure that it is a true intermediate of a dissociative pathway rather than a dead-end product of a metathetic process. 

See Section IV.B.2 above for a recent example of the balance between dissociative (D) and associative mechanisms, in the case of ring opening of platinum(II) chelates of hemi-labile ligands. 

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