Chiral exciplexes

Many mechanisms had been proposed in the past to rationalize this selectivity (tri-oxanes, perepoxide, exciplex, dipolar or biradical intermediates) however, it is now generally accepted that the reaction proceeds through an intermediate exciplex which has the structural requirements of a perepoxide. This assumption is supported by (a) the lack of stereoselectivity in the reactions with chiral oxazolines and tiglic acid esters (b) the comparison of the diastereoselectivity of dialkyl substituted acrylic esters with structurally similar non-functionalized aUtenes (c) the intermolecular isotope effects in the photooxygenation of methyl tiglate and (d) the solvent effects on regioselectivity. ... 

FIGURE 5. Presumed geometry of the trans-stilbene chiral fumarate exciplex precursor of the p-truxinate cycloadduct. 

The dramatic switching of product chirality by temperature is entropic i origin, for which the different degrees of conformational changes induced by thj rotational relaxation to the enantiomeric twisted cyclooctene singlets (/ )- am (S)-lp within the exciplex are thought to be responsible [30,31]. This idea w supported experimentally by using chiral pyromellitate sensitizers 45g-j. C ing chiral auxiliaries with an electron-rich aromatic substituent as a donor moiel... 

The mechanism of the [3 + 2] cycloaddition is summarized in Scheme The first intermediate results from charge transfer interaction between the eli tronically excited aromatic compound at its singlet state S1 with the alkene w] leads to the formation of the exciplexes K. A more stable intermediate is generated by the formation of two C-C bonds, leading to the intermediates These intermediates have still singlet multiplicity and therefore possess zwii ionic mesomeric structures mainly of type M. In most cases and especially intramolecular reactions, chiral induction occurs during the formation of L. final products are then obtained by cyclopropane formation in the last step. 

This value is much larger than that observed in the previously reported deracemizations of [Cr(ox)3]3 and Cr(acac)3. In this reaction, the basic condition is necessary, and the addition of Hacac increases the enantiomer excess, for which the reason will be discussed below. The reaction mechanism shown in Scheme 17 was proposed. In the mechanism, the 3MLCT excited A- [Ru(( — )-men-bpy)3]2+ is oxidatively quenched by Co(acac)3 to form an exciplex with Co(acac)3 followed by electron transfer to Co(acac)3 from A- [Ru(( — )-menbpy)3]2 +, which leads to the formation of a successor complex, [A-Ruin(( — )-menbpy)33 + Con(acac)3 ]. This successor complex dissociates to A-[Rum(( — )-menbpy)3]3+, Co(acac)2, and acac. If the reducing reagent is absent or the reducing reagent does not effectively reduce the ruthenium(III) complex, Co(acac)2 reduces A-[Rum(( — )-menbpy)3]3+ to A-[Run(( — )-menbpy)3]2+ concomitantly with the formation of Co(acac)3. As discussed in Sec. II.A., the photoreduction of Co-(acac)3 occurs stereoselectively. In addition, the oxidation of Co(acac)2 to Co-(acac)3 occurs stereoselectively, because Co(acac)2 reacts with the chiral ruthen-... 

Chiral recognition techniques have been used in photoinduced electron transfer reactions and the differential formation of homochiral against heterochiral exciplexes. In the systems studied excited states of lanthanide complexes are quenched by transition metal complexes. The complexes with trigonal dihedral (D3) symmetry which can exist as enantiomers with either left handed (a) or right-handed (A) configurational chirality are chosen. The system consists of a mixture of racemic lanthanide complex and optically resolved... 

Another example of intramolecular CT complex formation is provided by trans-4-dimethvlamino-4 -(1-oxobutvl)stilbene Solvent effects on the spectrum give a value of 22D for the excited state dipole moment. The effect of electric field on the fluorescence of 4-(9-anthry1)-N.N.-2.3,5,G-hexamethy1-aniline shows this compound forms an excited state whose dipole moment does not change with solvent . Chiral discrimination in exciplex formation between 1-dipyrenylamine and chiral amines is very weak . In the probe molecule PRODAN (6-propionyl)-2-(dimethylamino)—naphthalene the initially formed excited state converts to a lower CT state as directly evidenced by time-resolved spectra in n-butanol. Rate constants for intramolecular electron transfer have been measured in both singlet and triplet states of covalently porphyrin-amide-quinone molecules . Intramolecular excimer formation occurs during the lifetime of the excited state of bis-(naphthalene)hydrazides which are used as photochemical deactivators of metals in polyethylene . ... 

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