Open dimer

Fig. 4 Flavin amino acids FI and F2 used for the preparation of DNA PNA hybrid duplexes. These duplexes possess the flavin in the PNA strand and the backbone opened dimer in the DNA strand. a Indicates unstructured melting. M.p.= melting points of the constructs...

The relative abundance of each of these species can be followed for instance by infrared analysis (4, 5). Figure 3 shows the I.R. spectrum of acrylic acid in the 17 O O cm l region. The main peak at 1705 cm-1 is due to the vibration of the carbonyl group in the cyclodimeric form, whereas the shoulders at 1730 and 1740 cm-l correspond to "open dimers" and "linear oligomers". 

Upon dilution in solvents which may associate via hydrogen bonds (water, methanol, dioxane) the situation is more complex. I.R. and Raman spectroscopy indicate the formation of various monomer-solvent complexes (4, 6). The corresponding absorption bands are in the same range as the characteristic bands for open dimers and oligomers and the latter cannot therefore be determined quantitatively. However, the viscosity of carboxylic acids was found to rise upon addition of water or methanol (4, 7) suggesting that these solvents bind together "oligomers". The persis-... 

Scheme 9 Half-open dimers involved in epoxide opening by Zn-reduced Cp2TiCl2...

The central feature of the mechanism is the 3-cuprio(III) enolate Cpop, of an open, dimeric nature, as shown by comparison of theory with experimentation involving NMR and KIEs [80, 81]. This species serves as the direct precursor to the product (Scheme 10.5, top box). In this critical CPop complex, copper/olefin (soft/soft) and a lithium/carbonyl (hard/hard) interactions are present. The open complex may be formed directly, by way of an open cluster (bottom left of Scheme 10.5), or by complexation of a closed cluster with the enone (CPcl). Experiments have shown that the enone/lithium complex (top left of Scheme 10.11) is a deadend species [60, 74]. 

E. D. Becker (Bethesda) The results we reported yesterday are subject to considerable error, as I indicated in my talk. The heat of dimerization of methanol in CC14 is about 9 25 kcal. It is, of couise, possible that cyclic dimers exist in solution and open dimers in the vapour phase. There may even be both cyclic and open dimers in solution, but I will not take the time now to discuss the possibility. 

Figure 36 Closed dimer and open dimer of Me3AI. ...

Involvement of lithium amide open dimer Involvement of alkyllithium open dimer... 

Figure 37 Involvement of open dimer in deprotonation and alkylation reactions. ...

The importance of the reagent on open dimers was also pointed out in the proposed mechanism of deprotonation by lithium amides and alkylation by organolithiums in carbonyl and imine chemistry (Figure 37). This... 

The opposite sense of diastereoselectivity has been observed frequently, giving, for example, alcohol 29 or 30 exclusively in the MAD/MeM (M = Li, MgX) system (Equations (57) and (58)).232,233 For the deprotonation events of the ct-proton of ketones, a monomer pathway was proposed (Figure 39),234 but the open dimer mechanism cannot be ruled out entirely. The relatively bulky aluminum phenoxide may be involved in the monomer pathway due to its... 

Figure 38 Two different open dimers in alkylation of 4-But-cyclohexan-1 -one.

The dimer-based mechanistic pathways in the deprotonation of ketones and imines have been carefully studied by theoretical (MNDO) methods, particularly for LDA and pinacolone, microsolvated by various solvents (Scheme 6)47. The results suggest that, even if no general rule can be drawn, the open dimer mechanism is expected to dominate under all circumstances, and particularly with increasing amide and solvent steric... 

SCHEME 6. Monomer vs. open-dimer pathways in the aldol reaction (S = solvent)47... 

Actually, this system is extremely solvent-dependent, four different mechanisms being observed in pure THF (disolvated LDA monomer pathway), THF + HMPA (triple ion-based metallation), DMPU (mono- and disolvated LDA monomer) or f-butyl methyl ether (monosolvated LDA dimer)33. However, these mechanistic differences do not necessarily translate into rate differences. Note that in f-BuOMe, the complex between the ketone and the monosolvated LDA dimer evolves into an open dimer. In this reaction model, it was also shown, by multinuclear NMR, that the progressive formation of mixed aggregates... 

C-alkylation approach Monomer alkylation TS Open dimer alkylation TS... 

A novel oxidative ring-opening dimerization of tetrahydrofuran with acids was catalyzed by Re207 to produce unsymmetrical diesters, as represented by the following example <07JA1246>. This reaction, in which an oxygen atom of the rhenium oxide is transferred to the ester product, is unique to tetrahydrofuran itself. 

TABLE 9. Enthalpies of activation (AH ) for cyclic monomeric- and open dimeric-based transition structures (65 and 66) of ketone metalation... 

Amide/substrate/solvent A monomeric A open dimeric AAH ... 

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