Spontaneous deracemizations is a challenging, multidisciplinary subject in current chirality research. In the absence of any chiral inductors, an achiral substance or a racemic composition is driven into an enantioenriched or even homochiral state through a selective energy input, e.g., chemical potential, photoirradiation, mechanical grinding, ultrasound waves, thermal gradients, etc. The most prominent examples of such transformations are the Soai reaction and the Viedma deracemization. In this review, we track the most recent developments in this topic and recall that many other deracemizations have been reported for solutions from mesophases to conglomerate crystallizations. A compiled set of simply available achiral organic, inorganic, organometallic, and MOF compounds, yielding conglomerate crystals, should give the impetus to realize new experiments on spontaneous deracemizations. Taking into account thermodynamic constraints, modeling efforts have shown that structural features alone are not sufficient to describe spontaneous deracemizations. As a guideline of this review, particular attention is paid to the physicochemical origin and symmetry requirements of such processes.

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The kinetics of spiropyran photochromic systems in solution in a stirred batch reactor continuously irradiated with monochromatic light was studied by UV/visible spectrophotometry. The plots of absorbance vs time were analyzed, and the desired parameters (quantum yields, UV/visible spectrum of the unstable photomerocyanine, ...) were extracted from an iterative computation which fitted the calculated curves to the experimental ones on the basis of a representative model of the reaction mechanism. The method was applied to spiro[benzothiazoline-2‘,2-benzopyran] whose corresponding photomerocyanine has a lifetime of 100 s in toluene; the two quantum yields of the direct process of photocoloration and the reverse reaction of photobleaching could be determined along with the spectrum of the corresponding photomerocyanine. To demonstrate the general nature of the photokinetic method, it was applied to the reaction of spiro[indoline-2‘,2-benzopyran], where the main photochromic process is accompanied by a photodegradation. Despite this interfering phenomenon, the photokinetic method could be used to extract the parameters of the main photochromic process. It also showed that the photodegradation products catalyzed the thermal back-isomerization. The order of magnitude of the rate constant of this catalytic process and the quantum yields of photodegradation were estimated.

The biphasic alkaline hydrolysis of ethyl caprylate (EC) exhibits highly nonlinear kinetics that has been attributed in the literature to “micellar autocatalysis” (Bachmann, P. A. et al. Nature 1992, 357, 57). New experimental results enable us to establish a macroscopic kinetic model quantitatively accounting for the dynamics of this reaction. According to the model, EC is carried from the organic to the aqueous phase by a transient micelle−EC complex (MEC). Curve fitting of the experimental kinetic data by inverse treatment shows that the formation of MEC is more favorable than that of the pure caprylate micelles (M) and occurs at a critical concentration that is smaller than the cmc associated with the formation of M. We demonstrate, in contrast to previous claims, that classical micellar catalysis is not involved in the overall reaction process, but the observed nonlinear kinetics is a consequence of the dynamics of the MEC-mediated phase-transfer reaction.

Kinetic modeling using nonlinear differential equations is proposed to analyze the spontaneous generation of enantiomeric excess in the autocatalytic addition of diisopropylzinc to prochiral pyrimidine carbaldehydes (Soai reaction). Our approach reproduces experimentally observed giant chiral amplification from an initial enantiomeric excess of <10 ؊6 % to >60%, high sensitivity and positive response to the presence of minute amounts of chiral initiator at concentrations <10 ؊14 M, and spontaneous absolute asymmetric synthesis from achiral starting conditions. From our numerical simulations using kinetic schemes derived from the Frank model, including stereospecific autocatalysis and mutual inhibition, we have shown that it is possible to reproduce the mirror-symmetry-breaking behavior of the Soai reaction under batch conditions leading to a bimodal enantiomeric product distribution. Mirror-symmetry breaking was found to be resistant to a loss of stereoselectivity up to 30%. While the mutual inhibition between enantiomers seems to originate from the presence of dimerization equilibria, the exact nature of the autocatalytic stereoselective process still remains to be revealed. From the kinetic viewpoint, simple autocatalysis involving monomers as the catalytic species is consistent with all reported experimental effects of the Soai reaction.alkylzinc addition ͉ autocatalysis ͉ chirality A symmetric synthesis usually requires the intervention of chiral chemical reagents or catalysts. Few examples are known in which the generation of enantiomerically enriched products occurs from achiral precursors without the involvement of such auxiliaries. These cases, known as absolute asymmetric synthesis (1, 2), have been mostly observed in the combination of spontaneous resolution and enantioselective catalysis (3) as well as by the influence of external chiral factors such as circularly polarized light (4) or vortex motion (5). Some cases were reported in which no obvious chiral inductor was used and, nevertheless, high enantiomeric excess (ee) has been obtained systematically. These examples display the signature of mirrorsymmetry breaking (6), i.e., a process in which a small random ee is greatly amplified, whereas the chirality sign remains unpredictable for each individual experiment. The generation of small random ee occurs practically in any chiral system for statistical reasons alone in which the ee is inversely proportional to the square root of the number of molecules, ee ϰ n Ϫ1/2 (7). However, this value remains negligible as long as a large number of molecules is involved, so an amplification mechanism is needed to increase such minute ee to a macroscopic level.Mirror-symmetry breaking in chemical systems is typically associated to autocatalytic kinetics, i.e., a feedback mechanism in which one or several reaction products directly increase the overall rate of the chemical reaction. Hence in the specific case of chiral autocatalysis, the enantiomeric product could act as an asymmetric catalyst of its proper forma...

The kinetic and structural behavior of a photochromic compound, 3-(2-fluorophenyl)-3-phenyl-3H-naphtho[2,1-b]pyran (F-Py), was investigated using 1H and 19F nuclear magnetic resonance (NMR) spectroscopy. Upon irradiation, the four theoretically predicted photomerocyanines appear along with a fifth form X, whose final structure has not been elucidated. This last form and two of the photomerocyanines are thermally labile, whereas the other two do not show any signs of decay. The system has been analyzed by NMR spectroscopy. This led to the structural assignment of each photomerocyanine. The kinetics of the thermal bleaching were monitored by directly and separately measuring the concentrations of each species at regular time intervals using 19F NMR spectroscopy. We therefore propose a plausible reaction mechanism. On the basis of this mechanism, the mathematical treatment and the study of the effects of temperature led to the determination of the kinetic and thermodynamic parameters (rate coefficients, enthalpy and entropy of activation) of this photochromic system. The leading role of the labile intermediate X on the formation of trans-transoid-cis (TTC) and cis-transoid-cis (CTC) photomerocyanines is pointed out.

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The photophysical and photochemical properties of four 3,3-diphenyl-3H-naphtho[2,1-b]pyrans substituted, via an acetylenic junction, to (thiophene) n oligomers (n = 0−3 units) were investigated by transient absorption in the femtosecond to microsecond time domain and by stationary absorption and fluorescence. The decay of the initially produced excited S1(ππ*) state is found to occur via three competing processes: fluorescence, intersystem crossing, and a ring-opening reaction leading to a colored merocyanine product, with relative yields varying drastically with n. Whereas ultrafast (sub-picosecond) reaction dynamics and high product quantum yield are observed for n = 0 and 1, the reaction is considerably slowed down on going to the n = 2 (105 ps) compound and does not occur for n = 3. A reaction scheme that accounts for this behavior is proposed and the effect of the oligothiophenic chain length on the photoinduced properties is discussed. It is suggested that increasing the chain length from 1 to 3 thiophene units stabilizes the S1(ππ*) state by π conjugation and induces an excited-state potential barrier along the reaction pathway.