The mechanism of the dirhodium tetracarboxylate catalyzed cyclopropanation of alkenes with both unsubstituted diazoacetates and vinyl- and phenyldiazoacetates was studied by a combination of (13)C kinetic isotope effects and density functional theory calculations. The cyclopropanation of styrene with methyl phenyldiazoacetate catalyzed by Rh(2)(octanoate)(4) exhibits a substantial (13)C isotope effect (1.024) at the terminal olefinic carbon and a smaller isotope effect (1.003-1.004) at the internal olefinic carbon. This is consistent with a highly asynchronous cyclopropanation process. Very similar isotope effects were observed in a bisrhodium tetrakis[(S)-N-(dodecylbenzenesulfonyl)prolinate] (Rh(2)(S-DOSP)(4) catalyzed reaction, suggesting that the chiral catalyst engages in a very similar cyclopropanation transition-state geometry. Cyclopropanation with ethyl diazoacetate was concluded to involve an earlier transition state, based on a smaller terminal olefinic isotope effect (1.012-1.015). Density functional theory calculations (B3LYP) predict a reaction pathway involving complexation of the diazoesters to rhodium, loss of N(2) to afford a rhodium carbenoid, and an asynchronous but concerted cyclopropanation transition state. The isotope effects predicted for reaction of a phenyl-substituted rhodium carbenoid with styrene match within the error of the experimental values, supporting the accuracy of the theoretical calculations and the rhodium carbenoid mechanism. The accuracy of the calculations is additionally supported by excellent predictions of reaction barriers, stereoselectivity, and reactivity trends. The nature of alkene selectivity and diastereoselectivity effects in these reactions is discussed, and a new model for enantioselectivity in Rh(2)(S-DOSP)(4)-catalyzed cyclopropanations is presented.

Allosteric proteins use energy derived from ligand binding to promote a global change in conformation. The "gating" equilibrium constant of acetylcholine receptor-channels (AChRs) is influenced by ligands, mutations, and membrane voltage. We engineered AChRs to have specific values of this constant by combining these perturbations, and then calculated the corresponding values for a reference condition. AChRs were designed to have specific rate and equilibrium constants simply by adding multiple, energetically independent mutations with known effects on gating. Mutations and depolarization (to remove channel block) changed the diliganded gating equilibrium constant only by changing the unliganded gating equilibrium constant (E 0 ) and did not alter the energy from ligand binding. All of the tested perturbations were approximately energetically independent. We conclude that naturally occurring mutations mainly adjust E 0 and cause human disease because they generate AChRs that have physiologically inappropriate values of this constant. The results suggest that the energy associated with a structural change of a side chain in the gating isomerization is dissipated locally and is mainly independent of rigid body or normal mode motions of the protein. Gating rate and equilibrium constants are estimated for seven different AChR agonists using a stepwise engineering approach.ion channel | nicotinic | allosteric | protein engineering S tructure and energy together define the mechanism of protein conformational change. The energy changes that occur when a protein changes shape are manifest in the rate and equilibrium constants of the process. Here, we describe a method of engineering the conformational change of an allosteric protein so that these constants can be measured easily, accurately, and precisely.Nicotinic acetylcholine receptor-channels (AChRs) are membrane proteins that spontaneously isomerize ("gate") between a resting, closed-channel conformation (R) and an active, openchannel conformation (R*) (1-4). Each of these five-subunit synaptic receptors has two ligand binding sites in the extracellular domain. The R ↔ R Ã gating equilibrium constant is influenced by the presence of small molecules at these sites. When devoid of ligands, neuromuscular AChRs almost always adopt the resting-closed shape, but when both sites are occupied by the neurotransmitter acetylcholine (ACh) they usually are, transiently, active-open. The driving force for the increase in the gating equilibrium constant with ACh at the binding sites is the higher affinity for the transmitter molecules in R* compared to R (5-7).The most accurate way to measure AChR gating rate and equilibrium constants is by using single-channel, patch-clamp electrophysiology. This method allows the separation of gating events from those associated with other reactions such as ligand binding and desensitization, and at an approximately 10 μs, single-molecule resolution. Typically, the diliganded gating equilibrium constant (E 2 ) is measured in the presence of ag...

The combined C-H activation/Cope rearrangement (CHCR) is an effective C-H functionalization process that has been used for the asymmetric synthesis of natural products and pharmaceutical building blocks. Up until now, a detailed understanding of this process was lacking. Herein, we describe a combination of theoretical and experimental studies that have resulted in a coherent description of the likely mechanism of the reaction. Density functional studies on the reactions of rhodium vinylcarbenoids at allylic C-H sites demonstrate that the CHCR proceeds through a concerted, but highly asynchronous, hydride-transfer/C-C bond-forming event. Even though most of the previously known examples of this process are highly diastereoselective, the calculations demonstrate that other transition-states and stereochemical outcomes might be possible by appropriate modifications of the reagents, and this was confirmed experimentally. The calculations also indicate that there is a potential energy surface bifurcation between CHCR and the competing direct C-H insertion.

Catalytic enantioselective methods for the generation of cyclopropanes has been of longstanding pharmaceutical interest. Chiral dirhodium(II) catalysts prove to be an effective means for the generation of diverse cyclopropane libraries. Rh2(R-DOSP)4 is generaally the most effective catalyst for asymmetric intermolecular cyclopropanation of methyl aryldiazoacetates with styrene. Rh2(S-PTAD)4 provides high levels of enantioinduction with ortho-substituted aryldiazoacetates. The less-established Rh2(R-BNP)4 plays a complementary role to Rh2(R-DOSP)4 and Rh2(S-PTAD)4 in catalyzing highly enantioselective cyclopropanation of 3- methoxy-substituted aryldiazoacetates. Substitution on the styrene has only moderate influence on the asymmetric induction of the cyclopropanation.