Models, for carbonyls

The design of novel Lewis acidic reagents for additions to nonconjugated carbonyls, based on structural information, has attracted much attention in recent years. An excellent overview of this subject has recently appeared, in which the mechanistic aspects of and various models for carbonyl addition processes are Wghlighted. In particular, attention was directed towards the ab initio treatment of solvation effects in additions to the carbonyl group. 37 These calculations indicated that in the addition of water... 

The most intensely studied aldol addition mechanisms are those beUeved to proceed through closed transition structures, which are best understood within the Zimmerman-Traxler paradigm (Fig. 5) [Id]. Superposition of this construct on the Felkin-Ahn model for carbonyl addition reactions allows for the construction of transition-state models impressive in their abiUty to account for many of the stereochemical features of aldol additions [50a, 50b, 50c, 51]. Moreover, consideration of dipole effects along with remote non-bonding interactions in the transition-state have imparted additional sophistication to the analysis of this reaction and provide a bedrock of information that may be integrated into the further development and refinement of the corresponding catalytic processes [52a, 52b]. One of the most powerful features of the Zimmerman-Traxler model in its application to diastereoselective additions of chiral enolates to aldehydes is the correlation of enolate geometry (Z- versus E-) with simple di-astereoselectivity in the products syn versus anti). Consequently, the analyses of catalytic, enantioselective variants that display such stereospecificity often invoke closed, cyclic structures. Further studies of these systems are warranted, since it is not clear to what extent such models, which have evolved in the context of diastereoselective aldol additions via chiral auxiliary control, are applicable in the Lewis acid-catalyzed addition of enol silanes and aldehydes. 

Discuss the factors behind selection of the reactive conformation in the Felkin-Ahn model for conversion of 53 to 54. How would Cornforth have rationalized this stereochemical result [For an interesting system where the Cornforth and Felkin-Ahn models predict different stereochemical results see Evans, D. A. Siska, S. J. Cee, V. J. Resurrecting the Cornforth Model for Carbonyl Addition Studies on the Origin of 1,2-Asymmetric Induction in Enolate Additions to Heteroatom-Substituted Aldehydes Andrew. Chem. Int. Ed. 2003, 42, 1761-1765] (CJH-5)... 

Coulombic Terms. Coulombie energy of interaetion arises from permanent dipoles within the molecule to be modeled, for example, the partial - - and — charges within a carbonyl group... 

The stereoselectivity of an addition reaction is considerably lower when the reactions are conducted in polar solvents, complexing additives such as /V./V,A. A, -tetramethylethylenedi-arnine arc used, or when the stereogenic center carries a methoxy group instead of a hydroxy group. This behavior is explained as competition between the cyclic model and a dipolar model, proposed for carbonyl compounds bearing a polar substituent such as chlorine with a highly... 

In all of the models for 1,2-asymmetric induction described above, a perpendicular attack of the nucleophile (R20) is assumed. However, quantum mechanical calculations and crystallographic data show that the nucleophile approaches the carbonyl group at an angle ( ) of between 100° and 110°2,16,22 (Figure 7). 

Transition state models for diastereoseleclive carbonyl additions ... 

The carbonyl clusters provide an interesting set of models for the chemisorption of CO on nickel. It is very interesting that, for the Ni (CO) assignments, a plot of Pco versus 1/n for the three-center-bonded CO moieties extrapolates to 1950 cm for = oo (the "chemi-... 

If the substituents are nonpolar, such as an alkyl or aryl group, the control is exerted mainly by steric effects. In particular, for a-substituted aldehydes, the Felkin TS model can be taken as the starting point for analysis, in combination with the cyclic TS. (See Section 2.4.1.3, Part A to review the Felkin model.) The analysis and prediction of the direction of the preferred reaction depends on the same principles as for simple diastereoselectivity and are done by consideration of the attractive and repulsive interactions in the presumed TS. In the Felkin model for nucleophilic addition to carbonyl centers the larger a-substituent is aligned anti to the approaching enolate and yields the 3,4-syn product. If reaction occurs by an alternative approach, the stereochemistry is reversed, and this is called an anti-Felkin approach. 

By analogy to simple olefins, we propose that 0(3P) initially adds to the 1,4 or 1,2 double bonds in polybutadienes at ambient temperature. Since the rate constants for 0(3P) addition to cis-2-butene and 1-butene (as models for 1,4 and 1,2 double bonds, respectively) are in the ratio 4.2 1 at 298 K ( 6), preferential addition to the 1,4 double bonds is assumed to persist to very high vinyl contents (-8011). The biradical adducts then rearrange to epoxides and carbonyl compounds or give rise to chain rupture and/or crosslinking as a consequence of PIF, according to the scheme ... 

Large platinum carbonyl clusters have been investigated as models for the adsorption of carbon monoxide on platinum surfaces and on platinum electrodes. An issue is how large the clusters must be before they adopt the properties of the bulk metal. Teo et al. have investigated the magnetic properties of the clusters [Pt6(CO)12]2+, [Pt9(CO)18]2+, [Pt y(CO)22f+, and... 

The present economic and environmental incentives for the development of a viable one-step process for MIBK production provide an excellent opportunity for the application of catalytic distillation (CD) technology. Here, the use of CD technology for the synthesis of MIBK from acetone is described and recent progress on this process development is reported. Specifically, the results of a study on the liquid phase kinetics of the liquid phase hydrogenation of mesityl oxide (MO) in acetone are presented. Our preliminary spectroscopic results suggest that MO exists as a diadsorbed species with both the carbonyl and olefin groups coordinated to the catalyst. An empirical kinetic model was developed which will be incorporated into our three-phase non-equilibrium rate-based model for the simulation of yield and selectivity for the one step synthesis of MIBK via CD. 

As a simple model for the enzyme penicillinase, Tutt and Schwartz (1970, 1971) investigated the effect of cycloheptaamylose on the hydrolysis of a series of penicillins. As illustrated in Scheme III, the alkaline hydrolysis of penicillins is first-order in both substrate and hydroxide ion and proceeds with cleavage of the /3-lactam ring to produce penicilloic acid. In the presence of an excess of cycloheptaamylose, the rate of disappearance of penicillin follows saturation kinetics as the cycloheptaamylose concentration is varied. By analogy to the hydrolysis of the phenyl acetates, this saturation behavior may be explained by inclusion of the penicillin side chain (the R group) within the cycloheptaamylose cavity prior to nucleophilic attack by a cycloheptaamylose alkoxide ion at the /3-lactam carbonyl. The presence of a covalent intermediate on the reaction pathway, although not isolated, was implicated by the observation that the rate of disappearance of penicillin is always greater than the rate of appearance of free penicilloic acid. 

The development of comprehensive models for transition metal carbonyl photochemistry requires that three types of data be obtained. First, information on the dynamics of the photochemical event is needed. Which reactant electronic states are involved What is the role of radiationless transitions Second, what are the primary photoproducts Are they stable with respect to unimolecular decay Can the unsaturated species produced by photolysis be spectroscopically characterized in the absence of solvent Finally, we require thermochemical and kinetic data i.e. metal-ligand bond dissociation energies and association rate constants. We describe below how such data is being obtained in our laboratory. 

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