Transition state models for

The regioselectivity of 1,3-dipolar cycloadditions can also be analyzed by MO calculations on transition-state models. For example, there are two possible regioisomers from the reaction of diazomethane and methyl vinyl ether, but only the 3-methoxy isomer is formed. 

The idea of the activated complex was developed by, among others, Henry Eyring at Princeton in the 1930s. It forms the basis of the transition-state model for reaction rate, which assumes that the activated complex—... 

Transition state models for diastereoseleclive carbonyl additions ... 

Salahub, D. R., Chretien, S., Milet, A., Proynov, E. I., 1999, Performance of Density Functionals for Transition States in Transition State Modeling for Catalysis, Truhlar, D. G., Morokuma, K. (eds.), ACS Symp. Ser., 721, American Chemical Society, Washington, D. C. 

A. J. Mulholland, W. G. Richards, Modeling the Citrate Synthase Reaction. QM/MM and Small Model Calculations, in Transition State Modeling for Catalysis, D.G. Truhiar, K. Morokuma (eds), American... 

Scheme 12.19. Transition state models for cyclization based on the work of Spellmeyer and Houk.

Muller, C. Wang, L-H. Zipse, H. Enzymes, Abzymes, Chemzymes-Theozymes In Transition State Modeling for Catalysis Truhlar, D. G. Morokuma, K., Eds. ACS Symposium Series 721 American Chemical Society Washington, DC, 1999 pp 61-73. 

The transition-state model for these cyclizations (Scheme 34) differs fundamentally from the well-established Beckwith-Houk transition model for radical cyclizations [130,146-148]. Thus, while both models invoke chairlike transition states, without excluding the possibility of twist boatlike systems in some instances, the Beckwith-Houk model involves full conformational... 

Scheme 34 Chairlike transition-state model for cyclization in a contact ion pair...

Figure 11.5 Ball-and-stick transition state models for (i) the 5-dig-cyclization of the hexa-4,5-dien-l-yl radical (left) and the 5-exo-trig-cyclization of the hepta-5,6-dien-l-yl radical (right).

FIGURE 7. Transition state model for the reaction between cyclopentadiene and 17a catalyzed by 425c... 

Darzens reaction of (-)-8-phenylmethyl a-chloroacetate (and a-bromoacetate) with various ketones (Scheme 2) yields ctT-glycidic esters (28) with high geometric and diastereofacial selectivity which can be explained in terms of both open-chain or non-chelated antiperiplanar transition state models for the initial aldol-type reaction the ketone approaches the Si-f ce of the Z-enolate such that the phenyl ring of the chiral auxiliary and the enolate portion are face-to-face. Aza-Darzens condensation reaction of iV-benzylideneaniline has also been studied. Kinetically controlled base-promoted lithiation of 3,3-diphenylpropiomesitylene results in Z enolate ratios in the range 94 6 (lithium diisopropylamide) to 50 50 (BuLi), depending on the choice of solvent and temperature. ... 

The development of predictive transition state models for the interpretation of selectivity data pertaining to the use of cinchona alkaloid derivatives in all the processes described above is challenging due to the complex conformational behaviour of these natural scaffolds (for example, it is well known that 0-acylated quinidines undergo major conformational changes upon protonation) [223]. Consequently, hypotheses regarding the details of chirality transfer in these systems are notably absent. 

While we have had some success, we are aware of the hmitations inherent in a transition state model for rhodium-mediated C-H insertion that attempts to predict product ratios on the basis of Mechanics calculations. Arbitrary decisions limiting the several degrees of freedom possible in the transition state could lead one to a model for the point of commitmenf to cyclization that would be far from reahty. The work described herein is important because it offers experimental evidence for a key rotational degree of freedom in the dihedral angle between the ester carbonyl and the rhodium carbenoid. 

Scheme 6.167 Proposed transition-state models for the enantioselective Henry (nitroaldol) reaction in the presence of (S,S)-configured catalyst 183 TS 1 anti, anti conformation TS 2 gauche-onfi conformation TS 3 gauche-onfi conformation.

Figure 8.14. Chelating transition state model for addition of lithium amides to oxazolinylnaphthalenes.

The reaction is 100% stereoselective and affords 69 as a unique stereomer with the (/f)-configuration at C-6, assigned by X-ray analysis. Consequently, the configuration at C-6 results from the well-known chair transition state model for the (Z)-O-allyl enol 68 in which the aUyl unit is on the opposite side to the isopropylidene ketal at C3-C4 and reacts on the Si face of the trisubstituted carbon-carbon double bond. Such a transition state leads to the sole (/ )-configuration in 69. 

FIGURE 3. Transition state models for the Simmons-Smith cyclopropanation reaction... 

Scheme 4 Transition-state models for sugar hex-5-enyl radical cyclizations.7...

Figure 6B.1. Transition-state model for epoxidation with optically active peracids.

Figure 6B.2. Transition-state models for epoxidation with optically active oxaziridines and dioxiranes.

Our proposed transition state model for this catalytic enantioselective cyanosilylation of ketone is shown as 35.30a The titanium acts as a Lewis acid to activate the substrate ketone, while the phosphine oxide acts as a Lewis base to activate TMSCN. The intramolecular transfer of the activated cyanide to the activated ketone should give the ( )-cyanohydrin in high selectivity. The successful results described above clearly demonstrate the practicality of our asymmetric catalyst for cyanosilylation of ketones. 

Figure 9.1 Plausible transition-state model for nitroaldol reactions...

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