Cyclic kinetic control

The fundamental mechanistic concept by which the stereochemical course of the aldol addition under conditions of kinetic control has been analyzed involves a cyclic transition state in which both the carbonyl and enolate oxygens are coordinated to a Lewis... 

Optically active y-alkoxycyclopentenones have become popular in the diastereoselective synthesis of hms-3,4-disubstituted cyclopentanones. The Michael addition to these cyclic enones catalyzed by sodium ethoxide in ethanol277 or by potassium tm-butoxide278 279 proceeds under kinetic control trans with respect to the y-substituent. 

When the cyclic enone is unsubstituted, but the resulting enolate is quenched with an electrophile under conditions of kinetic control the irons adduct is formed exclusively303. Particularly successful is the sequential Michael addition/enolate alkylation in diastereoselective routes to frans-a,/j-difunctionalized cycloalkanones and lactones304-308. The key steps in the synthesis of methyl ( + )-jasmonate (3)309-310 (syn/anti diastereoselection) and (-)-khushimone (4) (syn/anti and induced diastereoselection) illustrate this sequence311 (see also Section D. 1.1.1.3.). 

In the construction of carbocycles, five-membered ring formation has been used for preparing fused cyclic compounds, such as functionalized diquinanes. ° The reaction of 36 with (TMSlsSiH furnished the expected product 37 in 80% yield and in a or.fi ratio of 82 18, as the result of a kinetic controlled reaction (Reaction 43). 

Cyclic dienes give mainly endo-products through kinetic control. 

Before the measurement of HOR activity, a pretreatment of the alloy electrode was carried out by potential sweeps (10 V s ) of 10 cycles between 0.05 and 1.20 V in N2-purged 0.1 M HCIO4. The cyclic voltammograms (CVs) at all the alloys resembled that of pure Pt. As described below, these alloy electrodes were electrochemically stabilized by the pretreatment. Hydrodynamic voltammograms for the HOR were then recorded in the potential range from 0 to 0.20 V with a sweep rate of 10 mV s in 0.1 M HCIO4 saturated with pure H2 or 100 ppm CO/H2 at room temperature. The kinetically controlled current 4 for the HOR at 0.02 V was determined from Levich-Koutecky plots [Bard and Faulkner, 1994]. 

Cycloaddition of the cyclic nitrone derived from proline benzyl ester with alkenes proceeds readily to give isoxazolidines with good regio-and stereoselectivity (Eq. 8.47).68 The reaction favors exo-mode addition. However, certain cycloadditions are reversible and therefore the product distribution may reflect thermodynamic rather than kinetic control. 

Representatives of the bridged sulfone system 70 have been subjected to ruthenium catalysed ring-closing metathesis reactions (Grubbs catalyst) and shown to afford, in low yields, a few selected cyclic dimers and trimers, of all the possibilities available. The diastereoselectivities observed were rationalised in terms of kinetic control involved with internal ruthenium/sulfonyl oxygen coordination . 

The synthesis and phase behavior of the model polydiethylsiloxane networks have also been studied. The networks were made by hydrosilylation of well-defined vinyl and allyl telechelic siloxanes obtained by kinetically controlled polymerization of cyclic trisiloxane.314 The effects of molecular weight between the cross-linkings on segment orientation in polydiethylsiloxane elastomers were studied.315... 

FIGURE 2.5. EC reaction scheme in cyclic voltammetry. Mixed kinetic control by an electron transfer obeying the Butler-Volmer law (with a = 0.5) and an irreversible follow-up reaction, a Variation of the peak potential with the scan rate, b Variation of the peak width with scan rate. Dots represent examples of experimental data points obtained over a six-order-of-magnitude variation of the scan rate. 

FIGURE 5.12. Cyclic voltammetry. Passage from kinetic control by enzymatic reaction (1) to control by substrate diffusion. From left to right log(I%k /yfDsFv/TIT) =6,5,4,3, 2,1,0, — 1. Adapted from Figure 4 of reference 20a, with permission from Elsevier. 

Among the oligomers, the cyclic trimer has been postulated to be uniquely stable [53, 54], This could be due to either a mechanism favouring the formation of trimer (kinetic control) or to the trimer having a lower energy than other oligomers (thermodynamic control), thus decreasing its rate of further reaction. 

A study of the stereoselectivity of reduction of 3,3,5-trimethylcyclohexanone (5) with TIBA in benzene showed that under kinetically controlled conditions (excess reagent and short reaction time) 96% of trans-3,3,5-trimethylcyclo-hexanol (trans-6) was formed (148). This high degree of stereoselectivity was explained by proposing a cyclic 6-center transition state with hydride transfer occurring preferentially from the less hindered side (Scheme 17). 

A large number of studies have addressed the condensation of cyclic ketones with both aliphatic and aromatic aldehydes under conditions that reflect both thermodynamic (cf. Table 2) and kinetic control of stereochemistry. The data for cyclohexanone enolates are summarized in Table 8. Except for the boryl enolates cited (6), the outcome of the kinetic aldol process for these enolates... 

Electrochemical reduction of camphor-and norcamphoroxime at a Hg cathode proceeds with a high degree of stereoselectivity to give products of opposite stereochemistry to those formed in the dissolving metal (Na-alcohol) reduction of the oximes. The electrolyses are proposed to proceed by a kinetically controlled attack by the electrode on each oxime from the less hindered side (Fig. 62) [348]. In contrast, the corresponding N-phenyl imines yield products of the same stereochemistry as those isolated from a dissolving metal reduction. Cyclic voltammetry and polarographic data point to RH and intermediates in this case that are proto-nated from the least hindered side [349]. 

The reductive cleavage of iodobenzene and 3-methyliodobenzene was studied by cyclic voltammetry in both DMF and acetonitrile at 21 and 56 °C at different scan rates and has shown that there is a transition between stepwise and concerted mechanisms at lower scan rates. 1-Iodonaphthalene undergoes a stepwise reductive cleavage with mixed kinetic control by electron transfer and follow-up bond breaking, whatever the scan rate. ... 

Benzylideneacetone reacts with lithiated phenylacetonitrile under kinetic control, in THF and media that favour association, to give 1,2- and 1,4-adducts in proportions which are directly related to concentrations of monomeric and dimeric ion pair species, respectively." An attempt has been made to explain the different regioselec-tivities towards a,-unsaturated carbonyl compounds, including cyclic a-enones and cinnamaldehyde, in terms of intermediate complex formation. 

The enolate formed from 2,2-dimethyl-3-pentanone under kinetically controlled conditions is the Z-isomer. When it reacts with benzaldehyde, only the syn aldol is formed.4 This stereochemical relationship is accounted for by a cyclic transition state with a chair-like conformation. The product stereochemistry is correctly predicted if the aldehyde is in a conformation such that the phenyl substituent occupies an equatorial position in the cyclic transition state. 

The enolates derived from cyclic ketones are necessarily. E-isomers. The enolate of cyclohexanone reacts with benzaldehyde to give both possible stereoisomeric products under kinetically controlled conditions. The stereochemistry can be raised to about 6 1 in favor of the anti isomer under optimum conditions.7 8... 

The extent to which cyclization occurs during polymerization depends on whether the polymerization proceeds under equilibrium control or kinetic control, the ring sizes of the possible cyclic products, and the specific reaction conditions. 

Kricheldorf and coworkers [2001a,b,c] have stressed that step polymerizations can proceed either with kinetic control or thermodynamic control. Polymerizations under thermodynamic control proceed with an equilibrium between cyclic and linear products. Polymerizations under kinetic control proceed without an equilibrium between cyclic and linear products. 

Enolates may be derived from a,/l-unsaturated ketones 16 by base-catalyzed proton abstraction. Under kinetic control the a -proton is abstracted and a cross-conjugated metal dienolate is formed, whereas under thermodynamic conditions the extended dienolate is the major product3,, l. Successful alkylations of dienolates derived from cyclic a,/l-unsaturated ketones have been performed (see Section 1.1.1.3.1.1.2.1.). The related a,/ -unsaturated ester systems have also been investigated22-24. Open-chain structures 16 pose a rather complicated... 

On the other hand, the favored formation (and the yields) of N-acetylglucofuranosylamines were explained on the basis of steric considerations.24,26 The formation of N-acylglycosylamines is a kinetically controlled, irreversible reaction, and the prevalence of the furanose over the pyranose structure has been shown in many irreversible, cyclization reactions of carbohydrates.73 This preponderance, as well as the favoring of a determined anomeric configuration, would be the result of a balance of relative ffee-energies of the transition states in the cyclization step, which would have virtually the same geometry as the final, cyclic structure.74 The five-membered... 

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