Donor-acceptor substitution, role

The fundamental chemistry of donor-acceptor-substituted cyclopropanes is now well understood. This solid platform should allow many applications of known processes and exploration of new reaction types. A future challenge will be asymmetric syntheses which should be achievable, for instance, using Lewis acids containing enantiomerically pure ligands. Even more attractive might be cyclopropane formation under the influence of a suitable optically active catalyst. This intriguing approach could lead to enantioselective syntheses of many compounds in a most economical way. Finally it can be expected in the near future that transition metal induced reactions will also play an important role in this area of small ring chemistry. 

EFISH studies on push-pull phthalocyanines have been focused on determining the role of the donor-acceptor substitution pattern, the electron donor or acceptor... 

Zyss has established a relationship between various molecular stractures and optical nonUnearities by quantum mechairical calculations. Molecular hyperpolarizabiUty of monosubstituted benzenes and donor-acceptor-substituted phenyl-polyenes was evaluated from the finite field (FF) perturbed intermediate neglect of differential overlap (INDO) approach. From the calculations, the roles of charge transfer and chain length in NLO processes of conjugated molecules were confirmed. Oudar and Chemla su ested the two-state model for the design of NLO chromophores. In this model. 

The chiral catalyst 142 achieves selectivities through a double effect of intramolecular hydrogen binding interaction and attractive tt-tt donor-acceptor interactions in the transition state by a hydroxy aromatic group [88]. The exceptional results of some Diels-Alder reactions of cyclopentadiene with substituted acroleins catalyzed by (R)-142 are reported in Table 4.21. High enantio- and exo selectivity were always obtained. The coordination of a proton to the 2-hydroxyphenyl group with an oxygen of the adjacent B-0 bond in the nonhelical transition state should play an important role both in the exo-endo approach and in the si-re face differentiation of dienophile. 

Three research groups discovered almost at the same time that non-C2-symmetrical oxazolines of the type 32 can be even more effective ligands for asymmetric catalysis than type 4 ligands (Fig. 11). For the palladium-catalyzed allylic substitutions on 62, record selectivities could be reached using 32 (X = PPhj) [30]. It seems that not only steric but also electronic factors, which cause different donor/acceptor qualities at the coordination centers of the ligand, seem to play a role here [31]. The reaction products can subsequently be converted to interesting molecules, for example 63 (Nu = N-phthalyl) can be oxidized to the amino acid ester 64 [32]. 

Tetraalkylborates are mild and selective alkylation reagents [186, 187], and they are commonly considered as sources of nucleophilic alkyl groups (R ) just as borohy-drides are depicted as hydride (H ) sources. However, since organoborates represent excellent electron donors (see Table 5, Section 2.2.1), the question arises as to what role electron donor-acceptor interactions play in the nucleophilic alkyl transfer. Phenyl- and alkyl-substituted borate ions form highly colored charge-transfer salts with a variety of cationic pyridinium acceptors [65], which represent ideal substrates to probe the methyl-transfer mechanisms. Most pyridinium borate salts are quite stable in crystalline form (see for example Figure 5C), but decompose rapidly when dissolved in tetrahydrofuran to yield methylated hydropyridines (Eq. 65). 

Berlin, A., et al. 2004. New low-gap polymers from 3,4-ethylenedioxythiophene-bis-substituted electron-poor thiophenes. The roles of thiophene, donor-acceptor alternation, and copolymerization in intrinsic conductivity. Chem Mater 16 3667. 

These data can be explained in terms of the high stabilization energy resulting from solvation of the excited t state. High p values in these cases indicate that the polar solvent-solute intermolecular stabilization ofthe zwitterionic excited t state is very sensitive to intramolecular substituent effects. In contrast, there is no dependence of Stokes shifts on a-constants in cyclohexane, which is nonpolar aprotic solvent, where the vibrational relaxation ofthe Franck-Condon state plays a primary role in stabilizing the excited state. This implies that the vibrational relaxation is not sensitive to intramolecular donor-acceptor interactions. The observations showed that the ultrafast intra- and intermolecular electronic polarization plays a major role in determining the position of the Franck-Condon zwitterionic state and its sensitivity to the relaxation of polar-substituted stilbenes. 

In benzene solution, the Z- and E-stilbenes 42 form approximately equal amounts of the two regioi-somers of the fra s-substituted spiro-oxetanes (e.g., 43 from 4-Me-42) with CA in yields of 80 to 88%. These addition reactions do not occur in acetonitrile solution and Z- and -4-methoxystilbenes do not photoreact with CA in either solvent. However, high yields of the fra s-spiro-oxetanes 44 are formed from CA with Z- or E-ethyl cinnamate, ethyl 3-nitrocinnamate, benzalacetone, or chalcone in both benzene and acetonitrile. In contrast, allyl ethyl ether, methyl methacrylate, vinyl acetate, styrene, and a-chlorostyrene all yield cyclobutanes 45 with CA in benzene solution but under the same conditions, indene and E-(3-bromostyrene afford spiro-oxetanes. Such variation in reaction site continues to be discussed in terms of the electron-donor/acceptor characteristics of the addends and the role of single electron transfer in the reaction pathway. 

It has been suggested that when the trans ligand is a strong o-donor with no x-acidity the role of transition state stabilization is taken up by any biphilic species, such as thiourea.488 Such potential Jt-acceptors seem to be more capable than others in making use of the direct substitution pathway. 

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