ISSN:
0894-3230
Schlagwort(e):
Organic Chemistry
;
Physical Chemistry
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
,
Physik
Notizen:
A two-hydron transfer mechanism involving hydron transfers from carbon to nitrogen and from nitrogen to carbon was studied. The rearrangement of 1,3,3-triphenylpropene (1) into 1,1,3-triphenylpropene (2) catalyzed by 2,10-diazabicyclo[4.4.0]dec-1-ene (3) in benzene at 25·00°C was studied by 2H-labeling experiments and kinetic 2H-isotope effects. The synthesis and purification of [6,10-2H2]-2,10-diazabicyclo[4.4.0]dec-1-ene ([6,10-2H2]-3), [3-2H]-1,3,3-triphenylpropene ([3-2H]-1), [3-2H]-1,1,3-triphenylpropene ([3-2H]-2) and [3,3-2H2]-1,1,3-triphenylpropene ([3,3-2H2]-2) together with their precursors are reported. Partial reaction of [3-2H]-1 with [6,10-1H2]-3 gave 42% conversion into product 2, which was shown by 1H NMR to be composed of 88% [3-1H]-2 and 12% [3-2H]-2. Partial reaction of [3-1H]-1 with [6,10-2H2]-3 gave 43% of 2, composed of 73% [3-1H]-2 and 27% [3-2H]-2.These results clearly show that a substantial fraction of the reaction takes place in a bifunctional manner but isotope exchange and/or monofunctionally catalyzed reactions interfere. The following kinetic deuterium isotope effects on the rearrangement 1 → 2 were measured: kHH/kDH = 6·56; kHH/kHD = 1·19; kHH/kDD = 7·08; kHD/kDD = 5·94; and kDH/kDD = 1·08.On the basis of these results, a concerted two-hydron transfer mechanism is excluded. Instead, a stepwise mechanism is favored, in which at first the 3-hydron of 1 is abstracted by 3 yielding an ion pair(s), the carbanion of which in a separate step is then hydronated to yield the product 2.The abstraction of the 3-hydron from 1 might be hydrogen bond assisted. The two hydron transfer transition states are together rate limiting, although they limit the rate to different extents. A detailed mechanistic analysis is presented together with the results of an investigation of the nature of the catalyst. The dimerization constant for 3 was determined by 1H NMR to be 1·67 l mol-1 at 25·0°C. Isotopomer composition was measured by 1H NMR and GLC was used for the separation of the substrate and products. Computer-assisted capillary GLC was used for the kinetics.
Zusätzliches Material:
6 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/poc.610020602
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