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Bifunctionally catalyzed 1,3‐proton transfer of a propene by a sec‐amidine studied by deuterium isotope effects. A stepwise two‐proton transfer mechanism
Author(s) -
Ahlberg Per,
Janné Kjell,
Löfås Stefan,
Nettelblad Folke,
Swahn Lena
Publication year - 1989
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610020602
Subject(s) - chemistry , kinetic isotope effect , deuterium , catalysis , amidine , bifunctional , medicinal chemistry , proton , benzene , reaction mechanism , propene , organic chemistry , physics , quantum mechanics
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 2 H‐labeling experiments and kinetic 2 H‐isotope effects. The synthesis and purification of [6,10‐ 2 H 2 ]‐2,10‐diazabicyclo[4.4.0]dec‐1‐ene ([6,10‐ 2 H 2 ]‐ 3 ), [3‐ 2 H]‐1,3,3‐triphenylpropene ([3‐ 2 H]‐ 1 ), [3‐ 2 H]‐1,1,3‐triphenylpropene ([3‐ 2 H]‐ 2 ) and [3,3‐ 2 H 2 ]‐1,1,3‐triphenylpropene ([3,3‐ 2 H 2 ]‐ 2 ) together with their precursors are reported. Partial reaction of [3‐ 2 H]‐ 1 with [6,10‐ 1 H 2 ]‐ 3 gave 42% conversion into product 2 , which was shown by 1 H NMR to be composed of 88% [3‐ 1 H]‐ 2 and 12% [3‐ 2 H]‐ 2 . Partial reaction of [3‐ 1 H]‐ 1 with [6,10‐ 2 H 2 ]‐ 3 gave 43% of 2 , composed of 73% [3‐ 1 H]‐ 2 and 27% [3‐ 2 H]‐ 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: k HH / k DH = 6·56; k HH / k HD = 1·19; k HH / k DD = 7·08; k HD / k DD = 5·94; and k DH / k DD = 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 1 H NMR to be 1·67 l mol −1 at 25·0°C. Isotopomer composition was measured by 1 H NMR and GLC was used for the separation of the substrate and products. Computer‐assisted capillary GLC was used for the kinetics.