Asymmetric photoreaction

Asymmetric Photoreactions of Conjugated Enones and Esters (Pete). . . Atmospheric Reactions Involving Hydrocarbons, ETIR Studies of (Niki and Maker). 

Pete, J.-P. (1996) Asymmetric photoreactions of conjugated enones and esters, in Advances in Photochemistry,... 

There are a couple of comprehensive reviews on general asymmetric photochemistry in solution [133,134] and also on asymmetric photosensitization [135,136]. An account on multidimensional control of asymmetric photoreaction by environmental factors has also appeared recently [137]. This reflects a keen interest in chiral photochemistry and photochemical asymmetric synthesis [29]. In this section, we will concentrate mostly on the asymmetric photosensitization of cycloalkenes. 

POWER OF ORGANIZED MEDIA ILLUSTRATED WITH THEIR INFLUENCE ON ASYMMETRIC PHOTOREACTIONS... 

Jorissen A, Cerf C (2002) Asymmetric photoreactions as the origin of biomolecular homochirality a critical review. Orig Life Evol Biosph 32 129-142... 

Direct asymmetric cpl-induced photoreactions are only observed if there are two enantiomeric ground state reactants presgntthat absorb different amounts of light. Thus the asymmetry of the cpl source is transformed into a different concentration of excited-state enantiomeric species, which becomes obvious in emitting systems in the circular polarization of luminescence [7]. These in turn react in a nonchiral environment with the same rate constants for the different deactivation channels. Thus asymmetric photoreactions are dependent or independent parallel reactions of the enantiomers with different net rates. 

Both the pioneers in understanding chirality, van t Hoff and Le Bel, pointed out that cpl might be used to induce asymmetry in chemical reactions [17,18]. In the following decades there were many vain efforts to realize this idea [19,20-22,24-27]. The failure, most probably, was due to the small effects caused by the small differences in Ae. The first successful asymmetric photoreactions were found by W. Kuhn, who published a series of papers in 1929 and 1930 [3,28-28] on the photolysis of racemic a-azido propionic dimethylamide 2 and a-bromopropionic ethyl ester 3 derivativeSv-Mkehell [33] reported in 1930 on humulene nitrosite (probably 4 ) and Tsuchida et al in 1935 on [Co(ox)3]3 Ca-malogue to 5) [98]. 

The kinetics of asymmetric photoreactions were developed by several groups [12,34,40]. General reviews of the kinetics of asymmetric thermal reactions, which can be adapted to photoreactions, were written by Straathoff and Jongejan in 1977 [41] and Kagan and Fiaud in 1998 [42]. In the following sections of this chapter the specialties of photoreaction kinetics are given in detail. 

Buchardt [43] introduced the tri-fold classification of asymmetric photoreactions ... 

Photoderacemization is the simplest case of direct asymmetric photoreactions induced by cpl. The enantiomers are interconverted, and the mixture becomes optically active. Reaction scheme 2 is a modification of Scheme 1 ground state racemization is excluded. The enantiomerization step R S was observed directly by Metcalf et al. [9] by means of the time-resolved circularly polarized luminescence of europium-tris(bipicolinate). By means of a cpl laser pulse, a difference in the excited state population is created, and the decay of circular... 

The advent of fs pulse lasers recently opened new perspectives for asymmetric photochemistry. The elaboration of this field still is in the theoretical realm. Pulse sequence [125,126] and coherence [127] scenarios are set up for chiral molecular products from achiral precursors. If, for example, phosphinothiotic acid H2PO(SH) molecules are preoriented, which can be effected by laser action, and a special sequence of cpl pulses is used, then the theoretical prediction is that the l enantiomer is transformed to the r enantiomer, but the reverse process is suppressed and vice versa for a different pulse sequence [125]. Chapter 2 of this book is dedicated to these coherent phenomena controlling asymmetric photoreactions. 

The cpl-induced asymmetry in photoreactions as described in Sec. B. of this chapter is not very pronounced. In order to obtain ees in excess of a few percent, photodestruction must be chosen and most of the reactant material must be sacrificed. Therefore amplification mechanisms for all types of cpl-induced asymmetric photoreactions would be highly desirable. Autocatalysis, i.e., an asymmetric synthesis where a chiral product acts as a catalyst for its own production [128], and autoinduction, i.e., the stimulation of a chiral catalyst by a chiral product [44,129], are options. Autocatalytic systems that will tilt to one enantiomeric side were introduced by Frank [130] and Seelig [131]. 

The purpose of this review is to examine the recent progress in the fi< of asymmetric photoreactions involving a chiral reactant, and to discuss the fact< that control the diastereodifferentiation. In the excited state as in the groi state, the level of asymmetric induction is strongly related to the conformation flexibility of the reactants. When carried out in the solid state, in a confin cavity of zeolites or in supramolecular scaffoldings, restrictions of the mobil occur, and photoreactions can become highly stereoselective. These aspects asymmetric induction will be covered in separate chapters, and this review v consider only diastereoselective photoreactions carried out in solution. ... 

Thus one can expect that the copper complexes with 2,2 -bipyridine, 1,10-phenanthroline, and their derivatives are successfully applied to asymmetric photoreactions, as with chiral ruthenium(II) complexes, if the optically active moiety is introduced to the ligand, as discussed above (see introduction). 

Since the copper complexes, [Cu(NN)2]+ and [Cu(NN)(PR3)2]+ (NN = 1,10-phenanthroline, 2,2 -bipyridine, and their derivatives) were applied to stoichiometric and catalytic photoreduction of cobalt(III) complexes [8a,b,e,9a,d], one can expect to perform the asymmetric photoreduction system with the similar copper(l) complexes if the optically active center is introduced into the copper(I) complex. To construct such an asymmetric photoreaction system, we need chiral copper(I) complex. Copper complex, however, takes a four-coordinate structure. This means that the molecular asymmetry around the metal center cannot exist in the copper complex, unlike in six-coordinate octahedral ruthenium(II) complexes. Thus we need to synthesize some chiral ligand in the copper complexes. 

Photochemistry also has drawbacks the excited-state interactions are weak and short-lived and are therefore difficult to control also the detection/observation of transient species and the subsequent elucidation of the reaction mechanism are in general more difficult [17]. Consequently, it has long been believed that the critical and precise control of asymmetric photoreactions is a hard task, and that the optical yields obtained therefrom are low. To overcome this, two strategies have been developed in the evolution of asymmetric photochemistry, or photo-... 

Pioneering work on the photochemical diastereocontrol in zeolite supercages was reported by Turro and coworkers in 1991 [48]. They investigated the diastereoselective photodecarbonylation of 2,4-diphenyl-3-pentanone (DPP) adsorbed in various cation-exchanged X and Y zeolites to find that the diastereo-selectivity of d9l- over mestf-2,3-diphenylbutane increases in the order LiX NaX < LiY NaY < KY. In 1996, Ramamurthy and coworkers reported the first example of photochemical asymmetric induction in chirally modified zeolites [49], where they employed the Norrish/Yang type II reaction of cis-4-tert-butyl-cyclohexyl aryl ketones to the corresponding cyclobutanols. Since then, a variety of asymmetric photoreactions in zeolite supercages have been reported as reviewed below. 

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