Mechanism, cyclopropane ring opening

Karthikeyan S, Q Zhou, Z Zhao, C-L Kao, Z Tao, H Robinson, H-w Liu, H Zhang (2004) Structural analysis of Pseudomonas 1-aminocyclopropane-l-carboxylate deaminase complexes insight into the mechanism of a unique pyridoxal-5 -phosphate dependent cyclopropane ring-opening reaction. Biochemistry 43 13328-13339. 

The mechanism for bromination of semibullvalene has been proposed, based on quantum chemical calculations. The reaction pathway involves concerted bromine addition and cyclopropane ring opening to form an allylic cation, without the intermediacy of a bromonium or a cyclopropylcarbinyl cation.18... 

Stepwise radical mechanisms have been proposed for the apparent [2 + 2] cycloaddition of disilenes with ketones by Baines et a/.140,141 They have found that the reactions of tetramesityldisilene 1 with trara-2-phenylcyclopropane carbaldehyde (208a) and fra/M,fra/M-2-methoxy-3-phcnylcyclopropane carbaldehyde (208b), a mechanistic probe developed by Newcomb et al.,142 undergo characteristic cyclopropane ring-opening as shown in Eq. (99). 

With methyl 2-alkylcycloprop-2-enecarboxylates, on the other hand, only [2-1-1] cycloaddition to the tricyclic dimers of cyclopentadiene (18) occurs. Only the strained double bond in the norbornene substructure is cyclopropanated, and the exo-adducts are formed. The Z/E isomeric ratios (6 1 and 2.5 1) are comparable to the ratios obtained in the reactions with norbornadiene vide supra). A mechanism proposing ring opening of the cyclopropene and attack on the metal-coordinated alkene in a concerted way has been suggested in order to explain the stereoselectivity of the reaction. ... 

Totally intramolecular enyne metathesis Diels-Alder sequences have been demonstrated (e.g., the conversion of 244 into 246). Enyne RCM of substrate 247 affords stereochemically pure 248 due to kinetic resolution during metathesis.A process equivalent to enyne metathesis that proceeds through a non-carbene mechanism has been demonstrated using platinum chloride as a catalyst. In addition to routine observations, some other interesting side-reactions have been noted (Scheme 29). Competitive cross-metathesis with ethylene and intramolecular enyne metathesis were observed in the treatment of substrate 249 with catalyst 16 and ethylene.Cyclopropane ring opening was observed in the attempted enyne metathesis of substrate 253, resulting in alkylidenecyclopentene 254. 

Scheme 1.6 Unexpected cyclopropane ring-opening mechanism...

Organometallic catalysis insertion and ring opening mechanism of methylene-cyclopropane over Cp2LaH and Cp2LuH catalysts... 

Quinone methides 156 and 157 were generated on irradiation of the cis and trans forms of 155, respectively (Eqs. 1.42 and 1.43).99 100 The mechanism is believed to involve a radical ring opening of the cyclopropane ring, followed by hydrogen or methyl migration. 

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... 

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