Zimmerman model

The correlation of metal enolate geometry and aldol product stereochemistry via diastereomeric chair-preferred transition states has been widely accepted (2,5,6,16). The observations that the steric bulk of the enolate ligand Rj and attendant aldol diastereoselection are directly coupled are consistent with the elaborated Zimmerman model illustrated in Scheme 3 for chair-preferred transition states. For example, for ( )-enoIates, transition state Q is predicted to be destabilized relative to Ci because of the Rj R3 variable steric... 

The SC descriptions of the electronic mechanisms of the three six-electron pericyclic gas-phase reactions discussed in this paper (namely, the Diels-Alder reaction between butadiene and ethene [11], the 1,3-dipolar cycloaddition offulminic acid to ethyne [12], and the disrotatory electrocyclic ring-opening of cyclohexadiene) take the theory much beyond the HMO and RHF levels employed in the formulation of the most popular MO-based treatments of pericyclic reactions, including the Woodward-Hoffmarm mles [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman model [4—6]. The SC wavefunction maintains near-CASSCF quality throughout the range of reaction coordinate studied for each reaction but, in contrast to its CASSCF counterpart, it is very much easier to interpret and to visualize directly. 

Figure 4. Prediction of stereochemistry of boron enolates 9 and 10 using chair-type six-membered transition state based on the Zimmerman model.

Zimmerman modeled and demonstrated an alMS design with low-cost fabrication by stacking together four inexpensive components. The ion swarm was narrow in width by fluid dynamics and by geometric constraints (i.e., a thin inlet aperture). The aperture for sample flow was about 10% that of the main mobility channel, and modeling showed some affects of unevenness in streams with joining two flows. Still,... 

Electi ocyclic reactions are examples of cases where ic-electiDn bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are fomred along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

The Zimmerman-Traxler like transition state model can involve either a chair or boat geometry. 

A mathematical analysis of the action in Kady and other colloid mills checks well with experimental performance [Turner and McCarthy, Am. Inst. Chem. Eng. J., 12(4), 784 (1966)], Various models of the Kady mill have been described, and capacities and costs given by Zimmerman and Lavine [Co.st Eng., 12(1), 4-8 (1967)]. Energy requirements differ so much with the materials involved that other devices are often used to obtain the same end. These include high-speed stirrers, turbine mixers, bead mills, and vibratoiy mills. In some cases, sonic devices are effec tive. 

Turner, D. B, Zimmerman, J. R., and Busse, A. D., An evaluation of some climatological dispersion models, in "Proceedings of the Third Meeting of the Expert Panel on Air Pollution Modeling." North Atlantic Treaty Organization Committee on the Challenges of Modem Society Pub. No. 14. Brussels, 1972. (National Technical Information Service PB 240-574.)... 

Busse, A. D., and Zimmerman, J. R., "User s Guide for the Climatological Dispersion Model." Environmental Monitoring Series EPA-R4-73-024. U.S. Environmental Protection Agency, Research Triangle Park, NC, 1973,... 

A Zimmerman-Traxler transition state model is postulated in order to rationalize the ul topicity of this aldol addition [i.e., the (S)-enolate preferentially attacks the 7 e-face of the aldehyde]33. In the two alternative transition states 3a [ul topicity (S)jRe] and 3b [Ik topicity (S)/Si, the substituents at the stereogenic center of the enolatc are oriented in such a way that... 

Although the chiral propanoates I and 5 are similar and the reaction conditions are almost identical, the stereochemical outcomes arc explained by completely different transition state models. Predominant attack of the ketene acetal 2 to the Ai-face of 2-methylpropanal is interpreted by assuming a Zimmerman-Traxler like model, which minimizes steric hindrance in a plausible way65. 

The preference of the (5, .S )-boron cnolatc to attack almost exclusively the Si-face of an aldehyde is rationalized by assuming the Zimmerman-Traxler transition state model. It is postulated that the methyl group of the propyl residue directs the 3-elhylpenlane-3-thiol group towards the borolane moiety, the chirality of which is thus effectively transferred34. 

Thus, jyn-adducts arise predominantly, as expected, according to the Zimmerman-Traxier model. Provided that either boron or zirconium is the enolate-metal atom, high syn selectivity is achieved. The total amount of anti-adducts is lower than 2% in the case of amides 1 and 2, and it approaches zero when the other reagents arc used94 . The induced stereoselectivities are impressive for the amides and remarkable in the case of the imides. 

The configurational course depends on the enolatc configuration and the metal ion which determines whether a cyclic (e.g., Zimmerman-Traxler type) or an acyclic transition state is traversed. At present the following transition state models have been proposed. 

Boyd DB, Palkowitz AD, Thrasher KJ, Hanser KL, Whitesitt CA, Reel JK, Simon RL, Pfeifer W, Lifer SL, Takenchi K, Vasudevan V, Kossoy AD, Deeter JB, Steinberg MI, Zimmerman KM, Wiest SA, Marshall WS. Molecnlar modeling and qnantitative strnctnre-activity relationship studies in pnrsnit of highly... 

A number of diflFerent animal models of uveitis have been developed) including that induced by organ-specific ocular antigens such as retinal S-antigen, rhodopsin and lens protein (Wacker et al., 1977 Rao et al., 1979). Other models are based on the injection of proteins foreign to the host, such as intravitreal injections of albumin or 7-globulin (Zimmerman and Silverstein, 1959 Kaplan etal., 1979). More recently, a third group of models has been developed based on the injection of inflammatory mediators such as interleukins-1 and 2, and tumour necrosis factor (Bhattacherjee and Henderson, 1987 ... 

TS, which is usually based on the chair (Zimmerman-Traxler) model. This pattern is particularly prevalent for the allylic borane reagents, where the Lewis acidity of boron promotes a tight cyclic TS, but at the same time limits the possibility of additional chelation. The dominant factors in these cases are the E- or Z-configuration of the allylic reagent and the conformational preferences of the reacting aldehyde (e.g., a Felkin-type preference.)... 

Zimmerman suggests that this bonding preference is consistent with approximate triplet energies of his isoconjugate models for bridge formation ... 

A proposed simplified mechanism for the conjugate addition/aldol cyclization, as depicted in Scheme 2.25, is based on detailed mechanistic studies performed on related Rh-catalyzed enone conjugate additions [45]. A model accounting for the observed relative stereochemistry invokes the intermediacy of a (Z)-enolate and a Zimmerman-Traxler-type transition state as shown in 2-110 to give 2-111. 

An explanation of these observations was attempted in terms of the existence of different states/compartmentalisation inspired by the model proposed by Zimmerman and Brittin,15 suggesting that a spin system can be made up of a fixed number of phases with each phase characterised by an inherent relaxation rate. The relaxation observed will then depend on the rate of exchange between phases, and three situations can be outlined (i) fast, it) slow, and (Hi) intermediate exchange. 

The Zimmerman (22) cyclic transition state model involving either the chair or the boat geometry adequately correlates existing data. 

However, despite their proven explanatory and predictive capabilities, all well-known MO models for the mechanisms of pericyclic reactions, including the Woodward-Hoffmann rules [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman treatment [4-6] share an inherent limitation They are based on nothing more than the simplest MO wavefunction, in the form of a single Slater determinant, often under the additional oversimplifying assumptions characteristic of the Hiickel molecular orbital (HMO) approach. It is now well established that the accurate description of the potential surface for a pericyclic reaction requires a much more complicated ab initio wavefunction, of a quality comparable to, or even better than, that of an appropriate complete-active-space self-consistent field (CASSCF) expansion. A wavefunction of this type typically involves a large number of configurations built from orthogonal orbitals, the most important of which i.e. those in the active space) have fractional occupation numbers. Its complexity renders the re-introduction of qualitative ideas similar to the Woodward-Hoffmann rules virtually impossible. 

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