Evans’ reaction

Although bases are not chirally modified (a stoichiometric amount of base was used in Evans reaction), three important contributions in catalytic enantioselec-tive carbon-carbon bond-forming reactions are described in this section ... 

The Evans reaction, involving f, y-unsaturated oxazolidinones and f),y-unsaturated aldehydes was unprecedented (but, according to the literature, Evans aldolizations of a-alkoxyaldehydes worked well).16... 

Allylic iodides with excess amounts of peroxy acids in a two-phase system in the presence of an inorganic base give the corresponding rearranged dlylic alcohols in good yields (Scheme 3). ° It has been demonstrated that the reaction proceeds under milder conditions than the Mislow-Evans reaction. 

Surprisingly, a methylenecyclohexane-derived carbazate akin to (154) gave negligibly more equatorial (55%) than axial attack when treated under the same conditions. > en the rearrangement was initiated with 10 mol % Cu(acac)2, equatorial attack amounted to 65%. The first observation is clearly inconsistent with Evans reaction (154) -> (155). The copper catalysis of the second process suggests the existence of a synthetically unexplored second rearrangement mechanism through a carbenoid. 

One approach to ent-4 -tetrahydrocannabinol, which is independent ofthe chiral pool, was developed by David Evans (reaction scheme see next page). [143] Via enantioselective Diels-Alder reaction on a copper-bis-(oxazoline) complex, there is obtained a cyclohexenecarboxamide, which after conversion into its benzyl ester and an exhaustive Grignard reaction gives ent-menth-l-ene-3,8-diol. Also this isomer can be converted in an analogous matmer stepwise into tetrahydrocannabinol. A -Tetrahydrocannabinol would be correspondingly accessible by use ofthe antipode ofthe catalyst. 

Evans M G and Polanyi M 1935 Some applications of the transition state method to the calculation of reaction velocities, especially in solution Trans. Faraday Soc. 31 875-94... 

Evans D FI 1991 Review of voltammetric methods for the study of electrode reactions Microelectrodes Theory and Applications (Nate ASI Series E vol 197) ed M I Montenegro, M A Queiros and J L Daschbach (Dordrecht Kluwer)... 

Although extraction of lipids from membranes can be induced in atomic force apparatus (Leckband et al., 1994) and biomembrane force probe (Evans et al., 1991) experiments, spontaneous dissociation of a lipid from a membrane occurs very rarely because it involves an energy barrier of about 20 kcal/mol (Cevc and Marsh, 1987). However, lipids are known to be extracted from membranes by various enzymes. One such enzyme is phospholipase A2 (PLA2), which complexes with membrane surfaces, destabilizes a phospholipid, extracts it from the membrane, and catalyzes the hydrolysis reaction of the srir2-acyl chain of the lipid, producing lysophospholipids and fatty acids (Slotboom et al., 1982 Dennis, 1983 Jain et al., 1995). SMD simulations were employed to investigate the extraction of a lipid molecule from a DLPE monolayer by human synovial PLA2 (see Eig. 6b), and to compare this process to the extraction of a lipid from a lipid monolayer into the aqueous phase (Stepaniants et al., 1997). 

Evans and co-workers investigated the effect of a number of -symmetric bis(oxazoline) ligands on the copper(II)-catalysed Diels-Alder reaction of an N-acyloxazolidinone with cyclopentadiene. Enantiomeric excesses of up to 99% have been reported (Scheme 3.4). Evans et al." suggested transition state assembly 3.7, with a square planar coordination environment around the central copper ion. In this scheme the dienophile should be coordinated predominantly in an cisoid fashion in... 

Ketones, in which one alkyl group R is sterically demanding, only give the trans-enolate on deprotonation with LDA at —12°C (W.A. Kleschick, 1977, see p. 60f.). Ketones also enolize regioseiectively towards the less substituted carbon, and stereoselectively to the trans-enolate, if the enolates are formed by a bulky base and trapped with dialkyl boron triflates, R2BOSO2CF3, at low temperatures (D A. Evans, 1979). Both types of trans-enolates can be applied in stereoselective aldol reactions (see p. 60f.). 

Chiral 2-oxazolidones are useful recyclable auxiliaries for carboxylic acids in highly enantioselective aldol type reactions via the boron enolates derived from N-propionyl-2-oxazolidones (D.A. Evans, 1981). Two reagents exhibiting opposite enantioselectivity ate prepared from (S)-valinol and from (lS,2R)-norephedrine by cyclization with COClj or diethyl carbonate and subsequent lithiation and acylation with propionyl chloride at — 78°C. En-olization with dibutylboryl triflate forms the (Z)-enolates (>99% Z) which react with aldehydes at low temperature. The pure (2S,3R) and (2R,3S) acids or methyl esters are isolated in a 70% yield after mild solvolysis. 

The two dashed lines in the upper left hand corner of the Evans diagram represent the electrochemical potential vs electrochemical reaction rate (expressed as current density) for the oxidation and the reduction form of the hydrogen reaction. At point A the two are equal, ie, at equiUbrium, and the potential is therefore the equiUbrium potential, for the specific conditions involved. Note that the reaction kinetics are linear on these axes. The change in potential for each decade of log current density is referred to as the Tafel slope (12). Electrochemical reactions often exhibit this behavior and a common Tafel slope for the analysis of corrosion problems is 100 millivolts per decade of log current (1). A more detailed treatment of Tafel slopes can be found elsewhere (4,13,14). 

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

A. K. Galwey, Reactions in the Sohd State, in Bamford and Tipper, eds.. Comprehensive Chemical Kinetics, vol. 22, Elsevier, 1980. Galwey, A. K., Chemistry of Solids, Chapman and Hall, 1967. Sohn, H. Y, and W. E. Wadsworth, eds.. Rate Frocesses of Extractive Metallurgy, Plenum Press, 1979. Szekely, J., J. W. Evans, and H. Y. Sohn, Gas-Solid Reactions, Academic Press, 1976. Uhmann, ed., Enzyklopaedie der technischen Chemie, Uncatalyzed Reactions with Solids, vol.. 3, 4th ed., Verlag Chemie, 1973, pp. 395-464. 

M. Tammaro, M. Sabella, J. W. Evans. Hybrid treatment of spatio-temporal behavior in surface reactions with coexisting immobile and highly mobile reactants. J Chem Phys 705 10277-10285, 1995. 

J. W. Evans, M. S. Miesch. Catalytic reaction kinetics near a first-order poisoning transition. Surf Sci 245 401-410, 1991. 

J. W. Evans. Kinetic phase transitions in catalytic reaction models. Langmuir 7 2514-2519, 1991. 

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