Achiral arenes

Optically active Cr-complexed arylphosphines 57 can be prepared in three steps from achiral arene-Cr(CO)3 precursors 56. A successful addition of PPh2Li to the complex 56a with concomitant carbamate displacement has been suggested, giving rise to the desired phosphine complex 57a as an air-stable crystalline solid in 96 % yield (R = H) (Scheme 26). Therefore, efficient C-P bond construction can be achieved by reacting, for example, the biaryl complex 56b with PPh2Li, providing the optically active monophosphine ligand 57b in 87 % yield, which has been used in Pd-catalyzed allylic alkylation reactions (Scheme 26) [42]. 

Another stereochemical feature is that (achiral) arene ligands bearing two non-identical substituents in ortho or meta position give rise to chiral complexes (Fig. 2) which can be viewed as structures possessing a plane of chirality [12,... 

Due to the inherent unsymmetric arene substitution pattern the benzannulation reaction creates a plane of chirality in the resulting tricarbonyl chromium complex, and - under achiral conditions - produces a racemic mixture of arene Cr(CO)3 complexes. Since the resolution of planar chiral arene chromium complexes can be rather tedious, diastereoselective benzannulation approaches towards optically pure planar chiral products appear highly attractive. This strategy requires the incorporation of chiral information into the starting materials which may be based on one of three options a stereogenic element can be introduced in the alkyne side chain, in the carbene carbon side chain or - most general and most attractive - in the heteroatom carbene side chain (Scheme 20). 

To date, only few very recent gas-phase studies on this subject can be retrieved from the literature, i.e., (i) a gas-phase study on the displacement of several amino acids from the chiral amido esorcinarene 9 (Scheme 9) carried out by Speranza and coworkers using an electrospray-ionization Fourier-transform ion cyclotron resonance (ESl-FT-lCR) mass spectrometer," " and (ii) Lebrilla and coworkers study on the ability of the achiral calix[4]arene 7 and calix[6]arene 8 to form inclusion complexes with natural amino acids under matrix-assisted laser... 

In the aromatic-ring-annelated oxepin series the resonance effect is clearly the major influence dominating other factors (e.g. temperature, solvent, etc.) which affect the oxepin-arene oxide equilibrium. It is however very difficult to exclude the presence of a minor (spectroscopically undetectable) contribution from either tautomer at equilibrium. This problem has been investigated by the synthesis of chiral arene oxides from polycyclic aromatic hydrocarbons (PAHs). The presence of oxepin (26) in equilibrium with naphthalene 1,2-oxide has been excluded by the synthesis of the optically active arene oxide which showed no evidence of racemization in solution at ambient temperature via the achiral oxepin (26) <79JCS(Pl)2437>. 

Three types of reaction systems have been designed and applied for the enantioposition-selective asymmetric cross-coupling reactions so far. First example is asymmetric induction of planar chirality on chromium-arene complexes [7,8]. T vo chloro-suhstituents in a tricarhonyl("n6-o-dichlorobenzene)chromium are prochiral with respect to the planar chirality of the 7t-arene-metal moiety, thus an enantioposition-selective substitution at one of the two chloro substituents takes place to give a planar chiral monosubstitution product with a minor amount of the disubstitution product. A similar methodology of monosuhstitution can be applicable to the synthesis of axially chiral biaryl molecules from an achiral ditriflate in which the two tri-fluoromethanesulfonyloxy groups are enantiotopic [9-11]. The last example is intramolecular alkylation of alkenyl triflate with one of the enantiotopic alkylboranes, which leads to a chiral cyclic system [12], The structures of the three representative substrates are illustrated in Figure 8F.1. 

The synthesis of receptor 43 was based on calix[5]arene [73]. The NMR studies in tetrachloroethane/methanol 2 1 showed the ability of this receptor to bind achiral zwitterionic c-aminohexanoic acid. 

The multimeric assembly reported in Fig. 6 forms spontaneously when three calix[4]arene dimelamine units and six barbiturates or cyanurate are mixed in a 1 2 ratio in apolar solvents. These assemblies, made of nine achiral com-... 

Similar chiral supramolecular assemblies are also observed from achiral calix[4]arene dimelamine derivatives with chiral cyanurates thus leading to D3 symmetry species [21]. Analogously to chiral dimelamines, predominantly one of the two possible diastereomeric double-rosette assemblies was observed in most cases and it is noteworthy that the same (R) chiral moiety when mounted in the dimelamine is located on the left-hand side of the calix[4]arene and induces (M) chirality, while when implemented in the cya-nurate is placed on the right-hand side and therefore promotes P chirality (Fig. 8). 

Fig. 8 Schematic representation of highly diastereoselective non-covalent synthesis achiral dimelamine calix[4]arene with enantiopure chiral cyanurate components lead to chiral double rosettes with d.e. as high as 96% (R)-cyanurate induces complete (P) chirality, while (S) favors only (M) chirality...

Fig. 9 A Calix[4]arene tetraurea dimeric capsules with a particular head-to-tail hydrogen bond network. B Achiral tetraurea units form achiral homodimeric capsules, while C two different achiral units forms chiral heterodimeric capsules...

Fig. 11 Heterodimeric diastereoisomeric capsules arising from dimerization of achiral sulfonyl tetraurea and chiral tetraurea calix[4]arenes...

The geometry of achiral dimeric calix[4]arene endowed with urea functionalities offers further possibilities to obtain homodimeric chiral capsules. In fact, simply employing two different substituents A and B on the urea or on the ether part of the calix[4]arene aromatic rings and considering all the possible combinations depicted in Fig. 19, it was possible to obtain up... 

Fig. 19 Achiral calix[4]arenes with different substituents on the urea residues lead to diastereoisomeric chiral dimeric capsules...

Exploiting the same concept, Reinhoudt and Shinkai were able to obtain another example of enantioenriched chiral double-rosette made of achiral components. In this case achiral calix[4]arene bearing dimelamine moieties with pyridine functionalities were assembled with achiral cyanurate leading to racemic chiral double rosettes. The latter are perfect counterparts for chiral D-dibenzoyl tartaric acid via two-point hydrogen-bonding interactions thus... 

Finally, we mention a remarkable example of lateral resolution reported for supramolecular nanostructures on hopg [96]. Held together by 72 hydrogen bonds, the molecular nanostructure is formed from three melamine-substituted calix[4]arene units and twelve 5,5-diethylbarbiturate molecules (Fig. 31a). The nanostructure, basically a stack of four rosettes, has chiral symmetry. With its components all being achiral, both enantiomers are formed upon self-assembly in solution. Deposition of the tetrarosettes on hopg leaves this nanostructure intact and allows surface self-assembly. AFM studies revealed close-packed 2D lattices formed by the tetrarosettes on hopg... 

In the case of a-oxoamides 22, meta-substituted amides exhibit a relatively high tendency of crystallization with the chiral framework [19c]. If both observations that meta-substituted arenes prefer to crystallize in noncentrosymmetric space groups, in comparison to the ortho- and para-isomers, have some generality, then meta-substitution in an achiral compound could be an important structural... 

Alkylation of r-butylcalix[8]arene with l,2-bis-(bromomethyl)benzene led to the doubly bridged derivative 87 in 19% yield.167 In contrast to 86 it is achiral ( >M... 

The stereochemical properties of tetraurea dimers are intriguing and challenging as well. As already mentioned for the simplest case, the directionality of the C=0 groups makes each single calix[4]arene in the dimer chiral (C4-symmetry), while the dimer as a whole is an achiral meso form (S8-symmetry) composed of a pair of enantiomers. 

A spectacular activation of the chiral zirconium-BINOL Lewis acid complex was achieved by the addition of the (achiral ) r-butyl-calix[4]arene. Less than 2% of the catalyst were sufficient in the enantioselective allylation of various aldehydes by allyltributyltin to reach enantiomeric excesses of more than 90%, see Casolari, S. Cozzi, P. G. Orioli, P. Tagliavini, E. Umani-Ronchi, A. Chem. Commun. 1997, 2123-2124. 

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