Model compounds, reduction

In later publications we will describe some CO radical chemistry on alkaline earth oxides with reagents such as O2, H2, H2O, D2O, and organics. Also, we are testing MgO and CaO in model compound reduction sequences to determine if radical processes such as those described here are actually important in the CO-H2O reducing medium. 

One development involves the use of vitamin B 2 to cataly2e chemical, in addition to biochemical processes. Vitamin B 2 derivatives and B 2 model compounds (41,42) cataly2e the electrochemical reduction of alkyl haUdes and formation of C—C bonds (43,44), as well as the 2inc—acetic acid-promoted reduction of nitriles (45), alpha, beta-unsaturated nitriles (46), alpha, beta-unsaturated carbonyl derivatives and esters (47,48), and olefins (49). It is assumed that these reactions proceed through intermediates containing a Co—C bond which is then reductively cleaved. 

The reductions are effected in nature by ferredoxin (p. 1102). This behaviour can be reproduced surprisingly well by simpler, model compounds. Some of the best known of these are obtained by the addition of axial groups to the square-planar complexes of Co with Schiff bases, or substituted glyoximes (giving cobaloximes) as illustrated in Fig. 26.7. The reduced Co species of these, along with vitamin... 

The surface analyses of the Co/MgO catalyst for the steam reforming of naphthalene as a model compound of biomass tar were performed by TEM-EDS and XPS measurements. From TEM-EDS analysis, it was found that Co was supported on MgO not as particles but covering its surface in the case of 12 wt.% Co/MgO calcined at 873 K followed by reduction. XPS analysis results showed the existence of cobalt oxide on reduced catalyst, indicating that the reduction of Co/MgO by H2 was incomplete. In the steam reforming of naphthalene, film-like carbon and pyrolytic carbon were found to be deposited on the surface of catalyst by means of TPO and TEM-EDS analyses. 

Ralph, J. Helm, R. F. Rapid proton NMR method for determination of threo.erythro ratios in lignin model compounds and examination of reduction stereochemistry. J. Agric. Food Chem. 1991, 39, 705-709. 

Scheme 5.11 Sulfoxide-directed olefin reduction-model compounds.

The nitrated model compound, 9, proved even more resistant to reduction than the polymeric analog the dissolving metal technique used to reduce 15 failed on 9, but finally the amino model, 10, was produced by treatment of with a 25-fold excess of sodium borohydride. Compound 1 serves as a difunctional initiator for NCA polymerization. 

Finally the kinetic data are compared to the microstructures of the E-V copolymers obtained by (n-Bu)3SnH reduction of PVC to test the suitability of DCP and TCH as model compounds for PVC reduction. This is achieved by computer modeling the reduction of PVC to E-V copolymers with the aid of the kinetic parameters obtained from the study of DCP and TCH reduction, and then comparing the observed and modeled E-V microstructures. 

The 50.31 MHz 13C NMR spectra of the chlorinated alkanes were recorded on a Varian XL-200 NMR spectrometer. The temperature for all measurements was 50 ° C. It was necessary to record 10 scans at each sampling point as the reduction proceeded. A delay of 30 s was employed between each scan. In order to verify the quantitative nature of the NMR data, carbon-13 Tj data were recorded for all materials using the standard 1800 - r -90 ° inversion-recovery sequence. Relaxation data were obtained on (n-Bu)3SnH, (n-Bu)3SnCl, DCP, TCH, pentane, and heptane under the same solvent and temperature conditions used in the reduction experiments. In addition, relaxation measurements were carried out on partially reduced (70%) samples of DCP and TCH in order to obtain T data on 2-chloropentane, 2,4-dichloroheptane, 2,6-dichloroheptane, 4-chloroheptane, and 2-chloroheptane. The results of these measurements are presented in Table II. In the NMR analysis of the chloroalkane reductions, we measured the intensity of carbon nuclei with T values such that a delay time of 30 s represents at least 3 Tj. The only exception to this is heptane where the shortest T[ is 12.3 s (delay = 2.5 ). However, the error generated would be less than 10%, and, in addition, heptane concentration can also be obtained by product difference measurements in the TCH reduction. Measurements of the nuclear Overhauser enhancement (NOE) for carbon nuclei in the model compounds indicate uniform and full enhancements for those nuclei used in the quantitative measurements. Table II also contains the chemical... 

Our 13C NMR analysis (2) of the E-V copolymers obtained via the (n-Bu)3SnH reduction of PVC led to k /kr=1.31 0.1 in excellent agreement with the kinetics observed for the removal of chlorines from m- and r-DCP. We also found no W diads in those E-V copolymers made by removing more than 80% of the chlorines from PVC. This observation is confirmed in the (n-Bu)3SnH reduction of DCP where the chlorines in this PVC diad model compound were found to be 4 times easier to remove than the isolated chlorines in 2-chloropentane, 2-, and 4-chlorooctane. 

The excellent agreement between the simulated and observed reduction of PVC with (n-Bu)3SnH means that both DCP and TCH are appropriate model compounds for the study of PVC reduction. DCP is useful to obtain kinetic information on the relative reactivities of m- and r-diads and W and EV diads. Reduction of TCH yields the relative reactivities of the central and terminal chlorines in the VW triads. 

Whatever the reason may be behind the strict necessity to deprotonate the flavin donor, the reduced and deprotonated flavin was established in these model studies to be an efficient electron donor, able to reduce nucle-obases and oxetanes. In the model compounds 1 and 2 the pyrimidine dimer translates the electron transfer step into a rapidly detectable chemical cycloreversion reaction [47, 48], Incorporation of a flavin and of a cyclobutane pyrimidine dimer into DNA double strands was consequently performed in order to analyse the reductive electron transfer properties of DNA. 

Flavin-cyclobutane pyrimidine dimer and flavin-oxetane model compounds like 1-3 showed for the first time that a reduced and deprotonated flavin is a strong photo-reductant even outside a protein environment, able to transfer an extra electron to cyclobutane pyrimidine dimers and oxetanes. There then spontaneously perform either a [2n+2n cycloreversion or a retro-Paternd-Buchi reaction. In this sense, the model compounds mimic the electron transfer driven DNA repair process of CPD- and (6-4)-photolyases. 

In some model compound studies with the i-PrOH/KOH system we found that anthracene was converted to 9,10-dihydroanthracene in 64% yield. Benzyl phenyl ether was also studied and was converted to a polymeric material under the reaction conditions. There were no traces of phenol nor toluene, the expected reduction products. 

The model compound work for the three basic systems is summarized in Figure 1. A finding of no significant reduction in a system is designated by an x ed arrow. Our criterion of successful reduction requires that significant quantities of the starting... 

Figure 1. Summary of results for coal and selected model compounds in some strongly basic conversion systems. Arrows indicate that significant conversion to reduced products was observed. The X ed arrows indicate that no reduction was observed.

Table IV also contains results of UV absorption studies of hydroxylation effects on the DNA intercalative binding of ben-zo[a]pyrene metabolites and metabolite model compounds. The most important feature of these results is that hydrolysis of BPDE to BPT causes a four-fold reduction in the intercalation association constant. Of all the BP derivatives studied, the tetrol has the lowest binding constant for intercalation. The small binding constant of the tetrol compared with BPDE, coupled with the DNA catalyzed hydrolysis of BPDE to the tetrol may provide a detoxification pathway for removal of a portion of unreacted intercalated BPDE.

In conclusion, the use of homogeneous model compounds has enabled the discovery and elucidation of a new formyl reduction mechanism which merits serious consideration as a reaction pathway on certain CO reduction catalysts. Additional studies of the compounds described in this account are actively being pursued. 

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