No abstract

[structures: see text] The absolute configuration of 1,2-, 1,3-, 1,4-, and 1,5-diols formed by two secondary (chiral) hydroxy groups can be deduced by comparison of the NMR spectra of the corresponding bis-(R)- and bis-(S)-MPA esters. The correlation between the NMR spectra of the bis-ester derivatives and the absolute stereochemistry of the diol involves the comparison of the chemical shifts of the signals for substituents R1/R2 and for the hydrogens attached to the two chiral centers [H(alpha)(R1) and H(alpha)(R2)] in the bis-(R)- and the bis-(S)-ester and is expressed as delta deltaRS. Theoretical calculations [energy minimization by semiempirical (AM1), ab initio (HF), DFT (B3LYP), and Onsager methods, and aromatic shielding effect calculations] and experimental data (NMR and CD spectroscopy) indicate that in these bis-MPA esters, the experimental delta deltaRS values are the result of the contribution of the shielding/deshielding effects produced by the two MPA units that combine according to the actual stereochemistry of the diol. The reliability of these correlations is demonstrated with a wide range of diols of known absolute configuration derivatized with MPA and 9-AMA as auxiliary reagents. A simple graphical model that allows the simultaneous assignment of the two asymmetric carbons of a 1,n-diol by comparison of the NMR spectra (delta deltaRS signs) of its bis-(R)- and bis-(S)-AMAA ester derivatives is presented.

The elasticity (stretched/compressed) and helical sense (clockwise/anticlockwise) of a dynamic helical poly(phenylacetylene) (PPA), bearing (R)-a-methoxytrifluorophenyl acetic acid (MTPA) pendants [poly-(R)-1], can be selectively controlled by the donor and the polar character of the solvent. The basis for this effect lies on the presence in the pendants of two independently tuneable bonds:). An increase in polarity shifts the conformational equilibrium of the (O])C-C(-O) bond in favour of the most polar synperiplanar (sp) conformer, that is more sterically demanding, thus forcing the polymer to invert its helix. A donor solvent associates with the (H-)N-C(]O) bond favouring the shift to the cis form of the amide and inducing both the elongation of the polymer chain and the inversion of the helical sense (visualized as UV-Vis bathochromic shift and CD inversion respectively). This ability of poly-(R)-1 to respond to donor/polar properties occurs in solution as well as in film state. Poly-(R)-1 presents in CHCl 3 identical helical sense for both the internal (polyene backbone) and for the external (described by the pendants) helices (3/1 clockwise), while in THF their senses are opposite (internal anticlockwise; external 2/1 clockwise), as proven by DSC.

The controlled induction of a helix-sense bias in helical polymers [1] by external stimuli [1][2][3][4] -that is, the possibility of selecting a helix with a preferred handedness-has become a desirable goal [2,3] owing to the potential applications of these materials as chiral sensors, molecular devices, [5] chiro-optical switches, memory elements for information storage, chiral catalysts, and conductive materials, amongst others. [1][2][3] Since the pioneering work by Green et al. [6] on the amplification of chirality in polyisocyanates, helical polymers with chiral amplification properties have attracted much attention. [1][2][3]7] Herein we report the synthesis and evaluation of a new and highly dynamic poly(phenylacetylene) (PPA) derivative that bears chiral pendants. This polymer incorporates the two aforementioned features (selective helix induction and chiral amplification) to give a material that acts as a sensor for the valence of metal cations. In the amplification of chirality reported herein, the external stimulus-the trigger-is provided by the selective coordination of the pendants with mono-or divalent metal cations (achiral agents) in such a way that the valence of the metal determines the right-or lefthanded helical sense of the polymer and its chiroptical response. [8] a-Methoxyphenylacetic acid (MPA), connected to the phenylacetylene moiety through a (C 6 H 4 ) À NH À C( = O) amide bond, was the pendant of choice. This system was selected because: 1) the 1,2-(amide carbonyl-methoxy) moiety is an excellent chelating moiety for certain cations (as shown by CD and NMR spectroscopy studies) [9] and 2) metal chelation is known to switch the antiperiplanar/synperiplanar (ap/sp) conformational equilibrium of chiral MPA amides. [9] In this way, the (R)-and (S)-MPA phenylacetylene monomers 1 and 2 (Figure 1 a) ). The addition of divalent metal cations induced an inversion of the CD spectrum, indicating a conformational switch from the ap to the sp form. This inference was corroborated by optical rotation and by 1 H and Figure 1. a) Structures of 1, 2, poly-1, and poly-2. b) CD spectra of poly-2 in different solvents (0.1 mg mL À1). Poly-1 generates the corresponding mirror-image spectra. c) CD spectra of poly-1 with divalent metal cations in CHCl 3 .[14] d) CD spectra of poly-1 with monovalent metal cations in CHCl 3 .

The remarkable consequences in elongation, dynamic character, response to external stimuli (e.g., solvent effects, metal cations), and aggregation observed in helical poly(phenylacetylene)s (PPAs) when either the type of linkage with the pendant groups (i.e., anilide, benzamide) or the aromatic substitution pattern (i.e., ortho, meta, para) of the parent phenylacetylene monomer undergo modification are analyzed in depth. Two series of PPAs substituted at the phenyl ring in ortho, meta, and para with either (S)-α-methoxy-α-phenylacetic acid (MPA) or (S)-phenylglycine methyl ester (PGME) linked through anilide or benzamide bonds were prepared (i.e., o-, m-, p-poly-1 and poly-2 series) and characterized both in solution and in the solid state (CD, UV-vis, Raman, NMR, DSC, TGA, X-ray, AFM, SEM). Para-substituted polymers (p-poly-1 and p-poly-2) present the most compressed and dynamic helices, which respond easily to external stimuli. Meta-substituted PPAs (m-poly-1 and m-poly-2) exist as a mixture in equilibrium of two different helices (compressed and stretched), both less dynamic than the para counterparts and with a weak response to external stimuli. Moreover, in the solid state, m-poly-1 and m-poly-2 show separate fields for the compressed and for the stretched helices. For its part, the ortho-substituted PPA (o-poly-1) presents a highly stretched, almost planar and practically rigid helical structure, inert to external stimuli and prone to aggregate. These structural changes (elongation/dynamic behavior) are rationalized on the basis of the increasing difficulties imposed by the meta- and ortho-substitution on the accommodation of the pendants within the helical structure.