Houk control

In chapter 21 we emphasise that the most reliable method of controlling one chiral centre by another in open chain compounds is by Felkin-Anh orbital control of nucleophilic attack on a carbonyl group next to a chiral centre. We shall start this discussion with that method and follow with the next most reliable - Houk control of electrophilic attack on alkenes also next to a chiral centre. These are of course both 1,2-control and we shall deal with that before discussing 1,3-, 1,4- and remote control. 

Houk control concerns electrophilic attack on alkenes, enolates and the like. The alkylation of enolate 56 would be an example if it were not held in a ring by chelation. It can in fact be difficult to tell whether chelation is involved or not with many enolates and the outcome of the reaction may tell which. Chamberlin s asymmetric preparation of both pyrrolidine 2,3-dicarboxylic acids 141 and 142 from natural aspartic acid illustrates this perfectly. The key to the stereochemical control is the very large protecting group 9-phenylfluorenyl-143 introduced by Rapoport.22... 

Again the product 170 is converted into an aldehyde 171 for the third allyl silane 172 reaction. This adds two more chiral centres by Houk control 173 and completes all seven chiral centres in the fragment 165. The diene side chain is added but this involves no new chiral centres... 

J. Houk, M. J. Doa, M. Dezube, and J. M. Rovinski, "Evaluation of Dyes Submitted Under the Toxic Substance Control Act New Chemicals Programme," Colour Chemistry, Elsevier AppHed Science, London and New York, 1991. 

Hall, B.E. and Houk, S.G. "Fluid-Loss Control in the Naturally Fractured Buda Formation," SPE paper 12152, 1983 SPE Annual Technical Conference and Exhibition, San Francisco, October 5-8. 

Houk, K.N., Williams Jr., J.C., Mitchell, P.A. and Yamaguchi, K. (1981). Conformational control of reactivity and regioselectivity in singlet oxygen ene reactions Relationship to the rotational barriers of acyclic alkylethylenes. J. Am. Chem. Soc. 103, 949-951... 

Gouvemeur, V. E., Houk, K. N., Pascual-Teresa, B., Beno, B., Janda, K. D. and Lemer, R. A. Control ofthe exo and endo pathways ofthe Diels-Alder reaction by antibody catalysis, Science, 262 (1993), 204-208... 

K. N. Houk, K. Nakamura, C. Sheu, A. E. Keating, Gating as a Control Element in Constrictive Binding and Guest Release by Hemicarcerands , Science 1996, 273, 627-629. 

The calculations suggest that the well-documented kinetically controlled course of 5-exo cyclizations that has been manifested in the Beckwith-Houk... 

Extreme cases were reactions of the least stabilized, most reactive carbene (Y = CF3, X = Br) with the more reactive alkene (CH3)2C=C(CH3)2, and the most stabilized, least reactive carbene (Y = CH3O, X = F) with the less reactive alkene (1-hexene). The rate constants, as measured by LFP, were 1.7 x 10 and 5.0 X lO M s, respectively, spanning an interval of 34,000. In agreement with Houk s ideas,the reactions were entropy dominated (A5 —22 to —29e.u.). The AG barriers were 5.0 kcal/mol for the faster reaction and 11 kcal/ mol for the slower reaction, mainly because of entropic contributions the AH components were only —1.6 and +2.5 kcal/mol, respectively. Despite the dominance of entropy in these reactive carbene addition reactions, a kind of de facto enthalpic control operates. The entropies of activation are all very similar, so that in any comparison of the reactivities of alkene pairs (i.e., ferei)> the rate constant ratios reflect differences in AA//t, which ultimately appear in AAG. Thus, car-benic philicity, which is the pattern created by carbenic reactivity, behaves in accord with our qualitative ideas about structure-reactivity relations, as modulated by substiment effects in both the carbene and alkene partners of the addition reactions. " Finally, volumes of activation were measured for the additions of CgHsCCl to (CH3)2C=C(CH3)2 and frani-pentene in both methylcyclohexane and acetonitrile. The measured absolute rate constants increased with increasing pressure Ayf ranged from —10 to —18 cm /mol and were independent of solvent. These results were consistent with an early, and not very polar transition state for the addition reaction. 

K. N. Houk, N. G. Randan, and J. Mareda, Theoretical Studies of Halocarbene Cycloaddition Selectivities. A New Interpretation of Negative Activation Energies and Entropy Control of Selectivity, Tetrahedron, 1985, 41, 1555. 

Well-known "Beckwith transition states in which a maximum number of groups are in an equatorial orientation of a chair-like transition state provides a satisfactory rationalization for the formation of the products. A. L. J. Beckwith, C. J. Easton, T. Lawrence, and A. K. Serelis, Ausi. J. Chem. 36 545 (1983) see also D. C. Spellmeyer and K. N. Houk, A force field model for intramolecular radical additions, J. Org. Chem. 52 959 (1987) T. V. RajanBabu and T. Fukunaga, Stereochemical control in hex-5-enyl radical cyclizations Axial vs. equatorial 2-(but-3-enyl)cyclohexyl radicals, J. Am. Chem. Soc. Ill 296 (1989). 

Determining the underlying factors that control the cyclization reactions of hexenyl and related radicals has been the object of much study. The recent theoretical treatments of Beckwith and Schiesser,6 and Houk and Spellmeyer,7 relate the current state of understanding with many important references to the extensive body of prior work. 

Fatora III, F. C., Gochenour, G. B., Houk, B. G., and Kelly, D. N., "Closed-Loop Real-Time Optimization and Control of a World Scale Olefins Plant" Paper Presented at the National AIChE Meeting, New Orleans, Louisiana (April 1992). 

Houk, B. G. Kelly, D. N. Davenport, S. L. and Fatora HI, F. C., "Closed-Loop Plant-Wide Optimization and Advanced Control Technologies Capture Many Benefits", Paper Presented at the International Conference on Productivity and Quality in the Hydrocarbon Processing Industry (1992). 

From these various analyses, it is clear that dopamine regulation of the striatum does not simply control detailed movement, but is involved in the selection and initiation of appropriate goal directed actions (Dunnett and Robbins, 1992 Robbins and Everitt, 1992), as influenced by motor learning (i.e. the acquisition of skills and habits Mishkin et al., 1984 Jog et al., 1999), in the context of motivational information related to needs and rewards (Suri and Schultz, 1999). Theoretical formulations of this process have moved away from the neuropsychological theory, although still conceptually useful, to mathematical and neural network modeling (Houk et al., 1995 Servan-Schreiber et al., 1998), which is beyond the scope of the present review. 

Buda, A. B. Wang, Y. Houk, K. N. Acid-base-controlled torquoselectivity theoretical predictions of the stoeochanical course of the electrocycUc reactions of cyclobutene-3-carboxyUc acid and the conjugate base and acid, J. Org. Chem. 1989, 54, 2264-2266. 

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