Mechanism, cyclopropane ring determination

Cyclopropane carboxylic acid is degraded via 3-hydroxybutyrate by both the bacterium Rhodococcus rhodochrous (Toraya et al. 2004) (Figure 7.33) and by fungi (Schiller and Chung 1970), although the mechanism for ring fission has not been determined. 

Linear correlations in the stoicheoimetric cyclopropanation by W(CHPh)(C0)5 with alkenes in a comparison with the catalytic reactions of Rh2(0Ac) suggest a unifying mechanism. Cyclopropanation stereochemistry is determined by interactions in the TT-complex and at the ring forming stage. 

Pd(OAc)2 works well with strained double bonds as well as with styrene and its ring-substituted derivatives. Basic substituents cannot be tolerated, however, as the failures with 4-(dimethylamino)styrene, 4-vinylpyridine and 1 -vinylimidazole show. In contrast to Rh2(OAc)4, Pd(OAe)2 causes preferential cyclopropanation of the terminal or less hindered double bond in intermolecular competition experiments. These facts are in agreement with a mechanism in which olefin coordination to the metal is a determining factor but the reluctance or complete failure of Pd(II)-diene complexes to react with diazoesters sheds some doubt on the hypothesis of Pd-olefin-carbene complexes (see Sect. 11). 

The cyclopropane acetals [10] and [11] also hydrolyze in aqueous sulfuric acid, but the reaction mechanisms for the two are not the same. Reaction of [10] involves pre-equilibrium oxygen protonation followed by rate-determining A1 ring opening, whereas that of [11] involves carbon protonation concurrent with... 

A carbon labelling study has elucidated the rearrangement mechanism for formation of chalcone (97) which accompanies formation of (91) by the expected vicinyl elimination of trimethylsilyl and benzotriazolyl groups from 2-benzotriazolyl-2-aryl-3-ketopropylsilanes, on reaction with fluoride ion in DMF. ° Thus, it has been possible to distinguish between the two alternative mechanisms depicted in Scheme 11 (via intermediates (93) or (95), respectively, by determining the fate of the labelled quaternary carbon of substrate (89). The results are consistent with the formation of a cyclopropane intermediate (95) which subsequently ring opens, with relief of strain, to form delocalized carbanion (96), from which the chalcone (97) is obtained (labelled... 

The mechanism of insertion of 2-alkylphenylnitrenes into a 1,5-related CH bond was studied by three methods 80 determination of isotope effects, stereochemistry, and radical clock. During the formation of indolines, a kn/kD of 12.6-14.7 was observed coupled with complete loss of stereochemical integrity at the CH carbon. When the CH insertion carbon bore a cyclopropane group, ring-opening products were observed. These observations suggest a mainly radical H-atom abstraction mechanism. The sensitivity of the isotope effects to solvent was taken to imply a small concerted nitrene insertion contribution. 

The cyclopropane derivative resulting from a symmetrical intermediate would give the 2-deuterocyclopropanecarboxylic acid 21 [ratio ( -H/a-H) 3], while an unsymmetrical intermediate would give an acid 22 unlabeled on the three-membered ring [ratio (j6-H/a-H) 4]. This ratio was readily determined from NMR spectra, and was consistent with the mechanism via an unsymmetrical intermediate. ... 

Deuterated cyclopropane has been brominated with bromine in order to determine the mechanism of electrophilic halogenation. The observed stereochemistry indicates initial edge halogenation with retention to form a four-membered bromonium ion, followed by nucleophilic ring opening with inversion194 l95. 

Ring synthesis by radical-radical coupling, as well as intramolecular radical substitution, is an alternative method for producing rings, which does not rely on the addition of a radical to an unsaturated bond. Cyclopropanes have been prepared from diiodides by the reaction of a catalytic amount of a fluorinated tin hydride with NaCNBHs in a fluorinated-organie solvent mixture. The rate constant for the reaction was determined using PhsSiH as a chain carrier and found to be 5 x 10 at 80 °C (Scheme 18). The cyclization of 2,2-diethylpropane-1,3-thiol and 4,4-diethyl-1,2-dithiolane in the presence of 2,5-dimethylhexa-2,4-diene and AIBN has been shown to proceed via a radical-chain mechanism (Scheme 19). ... 

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