Methyl dimerization

The hydrodimers D 3 are also oxidized enzymatically with the horseradish peroxid-ase-H202 system, the reactions being considerably more rapid in this case. Under these conditions, the methylated dimers D2 and D3 are quantitatively transformed to the parent monomers. In the case of dimer Dl5 however, formation of the monomer... 

A bridging methyl group may also show a large shift to high field. In the case of the ( 3-l,3-dimethylallyl)nickel methyl dimer (14), the structure of which has been confirmed by X-ray crystallography (26), the bridging methyl groups resonate at —20.3 ppm [ J(CH) = 118 Hz]. 

Cadmium tert.-Butyloxy-methyl-(dimer) XIII/2a. 908 f. 

Figure 17. (a) The H (500.1 MHz) MAS (vr = 35 kHz) spectra of the methyl (solid line) and ethyl (dashed line) benzoxazine dimers, 1. (b, c) The low-field regions of rotor-synchronized H DQ MAS spectra of (b) the methyl and (c) the ethyl dimers, (d, e) Schematic representations of the proposed arrangement of (d) the methyl dimers into pairs, and (e) the ethyl dimers into a chainlike structure. (Adapted with permission from Figures 1, 5, 6, 8, and 9 of ref 21, Copyright 1998 American Chemical Society.)... 

Diphenyl-CN-methyl dimer CN R= 1 Ph- Ph 26.2 109.6 Adsorption spectra 2004FRE/ALI... 

Figure 3A Illustrates a typical spectrum for a methylated dimer, "ApU, recorded at pD = 7.0 and 20"C. Except for differences In chemical shifts, the overall pattern of signal multiplets conforms to that observed for the corresponding parent dimer ApU, (Ezra et al., 1977) and related N6 methylated ApU derivatives (Altona, 1975). Similar correlations between parent and methylated derivatives are seen for the other compounds. Assignment of signals to specific protons followed procedures developed in earlier dimer studies (Kondo and Danyluk, 1976 Lee et al., 1976 Ezra et al., 1977). Eecause of the near equivalence in methyl shifts, no assignment is possible for these signals in "Ap A. Final best-fit values of chemical shifts and coupling constants are summarized in Tables I and II, respectively.

From the preceding discussion a generalized conformational model for methylated dimers emerges that is similar to that of normal dimers except for a higher population of extended forms. Both the shift and coupling data are consistent with a rapidly interconverting mixture of stacked right helical, loop stack, and extended forms (Kondo and... 

Figure 23 Structural mimicry in diarylethynylmethanol derivatives. (a) Generic chemical structures when Y = methyl, the molecule is bis(4-tolyl)ethynylmethanol, and when Y = chloride, the molecule is bis(4-chlorophenyl)ethynyhnethanol, (b) for Y = methyl, dimeric aggregate based on O-H- -ji interactions, and supramolecular chain based on ethynyl-C-H- -ji interactions, and (c) for Y = chloride, dimeric aggregate based on ethynyl-C-H- -n interactions, and supramolecular chain based on O-H- - -7T interactions. The O-H- - -n and C-H- - -ji interactions are shown as purple dashed lines.

Griep-Raming and Metzger [42] studied the thermal dissociation of the triphenyl-methyl dimer and of tetra-(p-anisyl) hydrazine, operating the ESI source as an electrolytic cell to ionize neutral species, for example, the triphenylmethyl radical. In this study, an electrospray ionization source able to heat the spray capillary was used. 

The relative ratio of the diastereomeric methylated dimers strongly depended on the ratio of monomer to initiator, the optical purity of the starting materials, and the temperature. " These results are consistent with the pro-R coordinated diastereomeric anions being more stable than the pro-S (Scheme 4), in which case there is a kinetic preference for the pro-R diastereomers at low temperature, depending on reactant stoichiometry and optical purity. However, the less stable isomers are more reactive and are in greater abundance at higher temperature where isomer interconversion is more rapid. 

---