Interactions In Transition State Research Articles

Propellanes 100. The Nature of the Stereoelectronic Effect in the Highly Stereospecific Hydroxyl Elimination from syn, anti‐[4.3.3]Propellane‐8,11‐diol upon Isobutane Chemical Ionization

AbstractA stereospecific 18O‐labelling study shows that the syn‐hydroxy group is preferentially lost from the syn, anti‐title diol upon i‐C4H10 chemical ionization, indicating occurrence of a stereoelectronic effect in this process. A collision‐induced dissociation study of deuterium‐ and 18O‐labelled analogs shows that a symmetrical ether structure is not formed in the above process by an SNi mechanism. This result leads to the conclusion that the stereoelectronic assistance of the anti‐OH group in the loss of the syn‐hydroxyl is expressed either in a weak transition state interaction or by other processes such as β‐elimination via hydrogen transfer with concomitant double bond formation.

Alkylation of camphor imines of glycinates. Diastereoselectivity as a function of electronic factors in the alkylating agent

Alkylation of the (R)-camphor imine of tert-butyl glycinate with a variety of alkylating agents gave diastereoselectivities ranging from 0–100%. Simple alkyl halides larger than methyl give de's (diastereomeric excesses) of ca. 50% whereas those derived from allylic type systems afford de's of 75–100%. The results are best explained by invoking a transition state interaction between the π system of the alkylating agent and the imine which, for steric reasons, requires alkylation to occur from the pro-R face.

The mechanism of the Peterson reaction. Part 2. The effect of reaction conditions, and a new model for the addition of carbanions to carbonyl derivatives in the absence of chelation control

The stereochemistry of the Peterson olefination reaction between PhHSiMe3 and PhCHO has been studied under a wide variety of conditions with excellent reproducibility and extremely high yields. The effects of counterion, solvent, added salts, variation of the carbanion-forming base, and temperature have been investigated. The product stereochemistry is remarkably insensitive to the reaction conditions. The results lead to a new method of analysis of steric approach control based on the angle of approach as defined by Dunitz and Baldwin. The method involves analysis of ‘primary’ and ‘secondary’ transition-state interactions. This analysis should be generally applicable to carbanion additions in which chelation is insignificant.

Subunit interactions in enzyme transition states—Antagonism between substrate binding and reaction rate

The principles of structural kinetics as applied to polymeric enzymes have been reinvestigated in order to take account of the probable existence of subunit interactions in the enzyme transition states. On the basis of simple and plausible postulates, structural rate equations have been derived for dimeric enzymes and compared to substrate binding isotherms. It then becomes possible to understand how subunit interactions affect substrate affinity and enzyme reaction rate. There exists an antagonism between substrate binding to the enzyme and the steady state rate of product appearance. If subunit interactions increase the rate of product appearance, they decrease the fractional saturation of the enzyme by the substrate. Alternatively, if they decrease the reaction velocity they increase the fractional saturation. This seemingly paradoxical effect is the direct consequence of subunit interactions occurring in both the ground and the transition states.

Regioselektive dimetallierung von aromaten. Bequemer zugang zu 2,2′-disubstituierten biphenylderivaten

Lithiation of biphenyl with 2.4 mol of n-butyllithium in the presence of TMEDA led directly to the 2,2′-dilithio derivative (I) in modest, but preparatively useful yields. I, in turn, was converted to a variety of products. The activation of the 2′-position of 2-lithiobiphenyl was shown directly by a separate experiment. MNDO calculations indicate stabilization in I by double bridging and in 2-lithiobiphenyl by intramolecular π interaction of Li with the o-phenyl group. Similar interactions in substitution transition states rationalize the specificity of the reactions observed.

Concerning the relevance of ethane conformational analysis to acyclic aldol transition states

Molecular mechanics calculations on model compounds are used to evaluate the steric interactions in acyclic aldol and related transition states.

An analysis of the diastereomeric transition state interactions for stereoselective epoxidation of acyclic allylic alcohols with peroxy-acids

Consideration of the magnitude of A (1,2) and A (1,3) -strain in the transition state of the epoxidation of acyclic allylic alcohols, has powerful predictive value.

An analysis of the diastereomeric transition state interactions for the kinetic deprotonation of acyclic carbonyl derivatives with lithium diisopropylamide

Consideration of A (1,2) and A (1,3) -strain in the transition states for the deprotonation of carbonyl compounds using LDA as a base has powerful predictive value.

Evaluation of initial and transition state solvation effects in aprotic media by the competetive preferential solvation theory

A chemical reaction taking place in aprotic media is frequently submitted also to the influence of specific solvent effects. The problem is even more complicated in mixed aprotic solvents since preferential solvation may contribute to the observed effects. A recent proposed model [1] called the Competitive Preferential Solvation Theory (COPS theory) accounts quite naturally for these phenomena. Based essentially on five postulates, COPS theory allows the quantitative evaluation of solut—solvent interactions both in initial and transition states. The affinity constants obtained are perfectly transferable and the lead immediately to a new type of linear free energy relationship [2] connecting the interacting power of solvents with their catalytic activity. COPS theory accepts neither ‘inert’ solvents nor ‘free’ solutes and it implies a virtual microscopic partitioning in homogeneous media. Beside kinetics COPS theory have been successfully applied to NMR and UV spectroscopies, to nuclear relaxation, to fluorescence proving its general validity. The kinetic application to COPS theory is illustrated by two reactions (aminolysis of imides and 2 + 4 cycloaddition reaction) in about sixty different media. The results provide a kinetic confirmation of the existence of solvent classes discovered previously [3] in spectroscopy: halogen containing, n-donor and aromatic solvents. All these classes influence differently the chemical reactivity. No close bonding contacts are apparent between the cation and BPh 4 or CH 3CN. Upon heating the complex, as a solid or when dissolved in acetonitrile, we observe the loss of ethylene. Reaction of other donor ligands such as PMe 3, CO, O 2..are investigated and the results will be discussed.

Inversion of chirality at sulphinyl sulphur in a reaction not involving breaking of bonds of the chiral sulphur atom

Conversion of (+)-methyl p-tolyl sulphoxide into the corresponding (–)- or (+)-α-halogenomethyl derivatives in the presence or in the absence of silver nitrate, respectively, involves inversion of configuration at sulphur in one of the two reactions; the inversion was proved to occur without any breaking of bonds at the chiral centre or any diastereomeric interaction in the transition state.

Substitution at saturated carbon. Part II. Kinetics and mechanism of the electrophilic substitution of several tetra-alkyltins by mercuric iodide in solvent 96% methanol–4% water

The substitution of tetra-alkyltins by mercuric iodide in solvent 96% methanol–4% water proceeds by the rate-determining bimolecular reaction R4Sn + Hgl2→ RHgl + R3Snl, followed by the rapid reversible reaction R3Snl + Hgl2⇌ R3Sn++ Hgl3–. Second-order rate coefficients, at 25 and at 40°, of reaction (1) have been obtained for the substitution of tetramethyl-, tetra-n-propyl-, tetra-n-butyl-, tetraisobutyl-, and methyltri-n-butyl-tin, and the various activation parameters determined. The second-order rate coefficient at 40° has also been obtained for the substitution of tetra-isopropyltin.The rate coefficients decrease markedly along the series (for the symmetrical tetra-alkyltins): R = Me ≫ Et > Prn∼ Bun > Bui≫ Pri. Consideration of the rate coefficient for the substitution of the methyl group in methyltri-n-butyltin leads to the conclusion that the above sequence arises mainly from transition-state interactions of the alkyl group undergoing substitution with both the entering electrophile (Hgl2) and the leaving group (SnR3).It is suggested that these substitutions, in solvent 96% methanol–4% water, proceed by the SE2 (open) mechanism of electrophilic substitution at saturated carbon.