DABCO intermolecular reactions

Some of the fastest photochemical processes occur on the ps time-scale, for instance electron transfer reactions. In the case of intermolecular electron transfers the actual reaction rate constants cannot be obtained when the diffusion of the reactants is the limiting factor. High concentrations must be used to ensure that encounters are faster than the reactions. Figure 8.7 shows the ps transient absorption spectra of the electron transfer between benzo-phenone and DABCO in acetonitrile. The triplet excited state of benzophe-none is seen to decay at 525 nm while the radical anion grows at about 700 nm to reach a maximum concentration after 1 ns. The decay and growth kinetics are shown in (b) of the same Figure. 

The rate and the conversion of the Baylis-Hillman-reaction was significantly improved when nucleophilic non-hindered bases, such as diaza[2.2.2]bicyclooctane (DABCO, 6), rather than simple tertiary amines were used. Further improvements were observed when 3-quinuclidinole (3-QDL, 7) was employed, due to stabilization of the zwitterionic intermediate 2 by formation of intramolecular hydrogen bonds [14a-c]. Similar effects were observed by the addition of methanol [14d] or acetic acid [14e] to the reaction mixture (formation of intermolecular hydrogen bonds) or by the presence of a hydroxy group in the acrylate [14f ]. The rate of the reaction was decreased by the presence of substituents in the a-position of tertiary amines. This was explained by the decrease of the rate of the addition of the catalyst onto the acrylate [15]. 

If these shift data really do represent the onset of an intermolecular electron transfer reaction in DABCO, ABCO, HMT clustered with amine, either, and aromatic solvents, one ought to be able to observe the reaction kinetics or dynamics. Consider the specific instance of DABCO. The singlet Rydberg state lifetime for DABCO (and all the other Rydberg molecules studied for this determination (Shang et al. 1993c, 1994a) is ca. 2 ps for the isolated molecule and ca. 1.2 ps for the nonpolar rare gas, hydrocarbon, and fluorocarbon solvents. This... 

The proton transfer reaction along a hydrogen bond between aliphatic dicarbox-ylic acids and DABCO, investigated by solid state NMR (Fig. 3.2.24), has shown that intramolecular O-H O and intermolecular N H-O hydrogen bonds are strong interactions, with proton chemical shifts of around 16 1.5 ppm, and N-0 and 0-0 bond lengths of around 2.55-2.60 A, while intermolecular N+-H- - O interactions are weaker and are characterised by a S1H of about 12.3 ppm and by an N-0 bond length of about 2.7 A [71, 72],... 

Intermolecular a-alkylation of aldehydes with 2-(bromomethyl)acrylates 18 were developed by the Palomo group [8]. The reaction was best catalyzed by diphenyl-prolinol TMS (6 )(Scheme 25.5) or dihexylprolinol triphenylsilyl ethers (Scheme 25.6), and required the presence of an acid scavenger, such as DABCO or DMAP. 

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