Intermolecular coupling stability

The proposed reaction mechanism (81) invokes a charge transfer complex which converts to a resonance stabilized diradical. The latter leads to a Diels-Alder adduct as a result of intramolecular coupling. However, when the diradical intermediate is attacked by a free radical, it is opened, and an alternating copolymer results from intermolecular coupling. 

Silicon substituents are known to stabilize both radical and carbanion centers. Thus, better and more consistent yields are obtained from intermolecular coupling reactions using trimethylsilyl substituted alkenes such as VII as substrates for reaction with carbonyl... 

Barton and Cohen 10) and Erdtman and Wachtmeister 11) have related the concept of free radical coupling of phenols to the biogenesis of natural products, and suggested that bisbenzylisoquinoline alkaloids are formed by this reaction from benzylisoquinoline units. The mechanism of the reaction consists of generation of the resonance-stabilized phenoxy free radical by one-electron oxidation of the phenoxy anion, followed by coupling and tautomerization to form hydroxylated diphenyls or diphenyl ethers. Intermolecular coupling may involve any... 

If the coupling to the substrate is weak (physisorption), as is the case for alkylsiloxanes on a SiO surface in the presence of a water layer, for example, the packing may also be mainly driven by intermolecular forces. Stability in this system is provided by crosslinking between the molecules (see below). 

In selected cases, the effect of solvation on the crystalline structure formed is, however, considerably more pronounced. For example, the observed packing in the crystal of 2,4,6-tris( 1,3-propylenediamine-N,N -)cyclotriphosphazene (4) dihydrate (Fig. 6) is due to strong intermolecular hydrogen bonds between molecules of water and suitable couples of N-H groups on the host moiety M). The HzO species form also continuous H-bonded layers of solvation around the cyclophosphazene derivatives, thus stabilizing the crystal lattice. 

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

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