States controlling asymmetric

Figure 2. Models for the transition states controlling asymmetric induction in the hydrocarbonylation of olefins...

The fact that a model for the transition state controlling asymmetric induction based on steric interactions allows us to correctly predict the type of prevailing regio- and stereoisomer for about 85% of the asymmetric hydrocarbonylation experiments (including hydroformylation and hydrocarbalkoxylation) is an indication that asymmetric induction in these catalytic reactions is based mainly on steric interactions. The data obtained so far do not allow us to establish whether the more stable or the less stable 7r-olefin complex intermediate is the one that reacts preferentially. However, the regularities that we observed indicate that the kinetic features are the same, at least in most of the experiments. 

A Model for the Diastereomeric Transition States Controlling Asymmetric Induction in Hydroformylation... 

The prevalence of the (S) or (R) antipode of the product gives a qualitative idea of the relative energy of the transition states controlling asymmetric induction. However, the values of the enantiomeric excess do not necessarily indicate the extent but only a minimum value of the free energy differences between the above transition states (e.g. the reaction product could racemize after its formation). On this basis the already discussed model for the transition state controlling asymmetric induction has been formulated (Sect. 4.). 

Meerwein-Ponndorf-Verley reductions, unlike many asymmetric reductions, involve a reversible redox reaction. Hydride transfer from the asymmetric center is believed to take place within a six-membered cyclic transition state (A) or (B), Fig. 9]. The lower-energy transition state will be that having the larger groups trans [(B) in Fig. 9]. The enantiomer of the product carbinol which results from this lower-energy transition state will predominate in a kinetically controlled, asymmetric Meerwein-Ponndorf-Verley reduction. 

The enantiomers are obtained as a racemic mixture if no asymmetric induction becomes effective. The ratio of diastereomers depends on structural features of the reactants as well as the reaction conditions as outlined in the following. By using properly substituted preformed enolates, the diastereoselectivity of the aldol reaction can be controlled. Such enolates can show E-ot Z-configuration at the carbon-carbon double bond. With Z-enolates 9, the syn products are formed preferentially, while fi-enolates 12 lead mainly to anti products. This stereochemical outcome can be rationalized to arise from the more favored transition state 10 and 13 respectively ... 

The synthesis of key intermediate 6 begins with the asymmetric synthesis of the lactol subunit, intermediate 8 (see Scheme 3). Alkylation of the sodium enolate derived from carboximide 21 with allyl iodide furnishes intermediate 26 as a crystalline solid in 82 % yield and in >99 % diastereomeric purity after recrystallization. Guided by transition state allylic strain conformational control elements5d (see Scheme 4), the action of sodium bis(trimethylsilyl)amide on 21 affords chelated (Z)-enolate 25. Chelation of the type illustrated in 25 prevents rotation about the nitrogen-carbon bond and renders... 

The purpose of this work is to demonstrate that the techniques of quantum control, which were developed originally to study atoms and molecules, can be applied to the solid state. Previous work considered a simple example, the asymmetric double quantum well (ADQW). Results for this system showed that both the wave paeket dynamics and the THz emission can be controlled with simple, experimentally feasible laser pulses. This work extends the previous results to superlattices and chirped superlattices. These systems are considerably more complicated, because their dynamic phase space is much larger. They also have potential applications as solid-state devices, such as ultrafast switches or detectors. 

A yyw-selective asymmetric nitro-aldol reaction has been reported for structurally simple aldehydes using a new catalyst generated from 6,6-bis[(triethylsilyl)ethynyl]BINOL (g in Scheme 3.18).126 The syn selectivity in the nitro-aldol reaction can be explained by steric hindrance in the bicyclic transition state as can be seen in Newman projection. In the favored transition state, the catalyst acts as a Lewis acid and as a Lewis base at different sites. In contrast, the nonchelation-controlled transition state affords anti product with lower ee. This stereoselective nitro-aldol reaction has been applied to simple synthesis of t/ircodihydrosphingosine by the reduction of the nitro-aldol product with H2 and Pd-C (Eq. 3.79). 

Charge density wave, 54 Charge spreading, 127 Charge-resonant states, 13, 18 Charge-asymmetric dissociation, 9 Coherent control, 7... 

The diastereoselective alkylation reaction of endo-cyclic five-membered ring enolates exhibits good potential for both 1,3- and 1,2-asymmetric induction. In Scheme 2-6, the factor controlling the alkylation transition state is steric rather than stereoelectronic, leading to an auh-induction.13... 

The self-assembly of block polymers, in the bulk, thin film and solution states, produces uniformly sized nanostructured patterns that are very useful for nanofabrication. Optimal utilization of these nanoscopic patterns requires complete spatial and orientational control of the microdomains. However, the microdomains in the bulk state normally have grain sizes in the submicron range and have random orientations. In block copolymer thin films, the natural domain orientations are generally not desirable for nanofabrication. In particular, for composition-asymmetric cylindrical thin films, experimental... 

The modem concept of asymmetric induction is illustrated by the formulas in Fig. 1. As shown, the addition of hydrogen cyanide to the optically active aldehyde can lead to two diastereomers (1 and 2). If the process is under thermodynamic control, the formation of the more stable isomer will be favored that is, that isomer for which the non-bonded interactions between the newly formed cyano and the hydroxyl groups with the dissymmetric R group are weakest. On the other hand, the difference in the yields of 1 and 2 can be the result of kinetic control arising from a difference in the energies of the transition states—that state with the lower energy will form faster and lead to the product of higher yield. It is noteworthy that the tenets... 

Photodimerization of cinnamic acids and its derivatives generally proceeds with high efficiency in the crystal (176), but very inefficiently in fluid phases (177). This low efficiency in the latter phases is apparently due to the rapid deactivation of excited monomers in such phases. However, in systems in which pairs of molecules are constrained so that potentially reactive double bonds are close to one another, the reaction may proceed in reasonable yield even in fluid and disordered states. The major practical application has been for production of photoresists, that is, insoluble photoformed polymers used for image-transfer systems (printed circuits, lithography, etc.) (178). Another application, of more interest here, is the use that has been made of mono- and dicinnamates for asymmetric synthesis (179), in studies of molecular association (180), and in the mapping of the geometry of complex molecules in fluid phases (181). In all of these it is tacitly assumed that there is quasi-topochemical control in other words, that the stereochemistry of the cyclobutane dimer is related to the prereaction geometry of the monomers in the same way as for the solid-state processes. 

Despite the uncertainties of mechanism and of the identity of reactive species, attempts have been made to analyze stereochemical control in asymmetric reductions in terms of a model of the transition state in which steric or other interactions can be assessed. These models could prove useful in suggesting modifications for improving the design of selective reducing agents or for predictive purposes. However, it should be kept in mind that there are only two possible outcomes in the direction of asymmetric induction at a prochiral unit undergoing reaction, and confidence in the predictive usefulness of a given model can only be obtained after a considerable number of examples have been examined. 

These schemes have been frequently suggested [105-107] as possible mechanisms to achieve the chirally pure starting point for prebiotic molecular evolution toward our present homochiral biopolymers. Demonstrably successftd amplification mechanisms are the spontaneous resolution of enantiomeric mixtures under race-mizing conditions, [509 lattice-controlled solid-state asymmetric reactions, [108] and other autocatalytic processes. [103, 104] Other experimentally successful mechanisms that have been proposed for chirality amplification are those involving kinetic resolutions [109] enantioselective occlusions of enantiomers on opposite crystal faces, [110] and lyotropic liquid crystals. [Ill] These systems are interesting in themselves but are not of direct prebiotic relevance because of their limited scope and the specialized experimental conditions needed for their implementation. 

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