Long‐range effects of chirality in aromatic poly(isocyanide)s

Amabilino, David B.; Serrano, José Luís; Sierra, Teresa; Veciana, Jaume

doi:10.1002/pola.21366

Cited by 52 publications

(39 citation statements)

References 105 publications

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“…On the basis of the X-ray analysis of LC samples, L-20 was found to possess a rigid-rod 15/4 helix; this helical structure is similar in structure to that postulated for a long time for helical polyisocyanides (4/1 helix). 5,51,86,87 Polymerization of the same L-Ala-bound isocyanide monomer with the m-ethynediyl Pt-Pd catalyst promoted the living polymerization, 73 which, interestingly, simultaneously produced almost completely right-(L-20c) and left-handed (L-20d) helices with different molecular weights and narrow molecular weight distributions (Figure 10b). …”

Section: Synthesis and Structures Of Helical Polymers E Yashimamentioning

confidence: 99%

“…85 Given the success in visualizing the helical structures of dynamic helical poly(phenylacetylene)s by AFM, the same procedure has been applied to the L-Ala-bound helical polyisocyanide (L-20). The helical structures of polyisocyanides have been postulated to be a 4/1 helix (2), 5,51,86,87 but the exact helical structures and their absolute helical senses remain unknown except for h-27-Me. 69 We previously found that diastereomeric right-and left-handed helical polyisocyanides (L-20) were formed by an unprecedented helix-sense-controlled polymerization of an enantiomerically pure phenyl isocyanide bearing the same L-Ala residue with a long n-decyl chain as the pendant using an achiral nickel catalyst (NiCl 2 ) in different solvents, such as CCl 4 and toluene, at different temperatures (Figure 10a).…”

Section: Synthesis and Structures Of Helical Polymers E Yashimamentioning

confidence: 99%

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Synthesis and structure determination of helical polymers

Yashima

2010

Polym J

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During the last decade, remarkable progress in developing synthetic helical polymers with a controlled helical sense has been achieved. However, the exact helical structures of most of the already prepared synthetic helical polymers remain unsolved. In this review, the recent progress in the synthesis of helical polymers and their structural determination, including helical pitch and handedness based on X-ray diffraction and spectroscopic measurements, together with high-resolution atomic force microscopy, is described. Polymer Journal (2010) 42, 3-16; doi:10.1038/pj.2009 Keywords: atomic force microscopy; circular dichroism; helical polymer; helical structure; X-ray diffractionThe helix is a unique structure as observed in nature from microscopic to macroscopic levels and is inherently chiral. Inspired by biological helices, such as DNA and proteins, chemists have been challenged to synthesize helical polymers with a controlled helical sense that aims not only to mimic biological helical structures but also to develop chiral materials for the separation of enantiomers and asymmetric catalysis. [1][2][3][4][5][6][7][8][9][10][11][12] The history of synthetic helical polymers with optical activity extends back to the 1960s when Pino and Lorenzi 13 investigated the structural and chiroptical properties of isotactic vinyl polymers prepared by the polymerization of a-olefins bearing optically active substituents. Although helical polyolefins are totally dynamic in nature and consist of short helical segments separated by frequently occurring helical reversals among disordered, random coil conformations, 14 this study was significant in the field of synthetic helical polymers, from which a number of helical polymers have been synthesized.On the basis of the pioneering studies by Nolte et al., 15 Okamoto et al. 16 and Green et al., 17 the existing synthetic helical polymers that exhibit an optical activity solely because of their macromolecular helicity can be classified into two categories with respect to their helix inversion barriers, that is, static and dynamic helical polymers (Figure 1). 9,12 Optically active helical polymers, such as poly(triphenylmethyl methacrylate) (1), 16 poly(t-butyl isocyanide) (2) 15 and polychloral (3), 18 have a sufficiently high helix inversion barrier and belong to the family of static helical polymers. Therefore, these helical polymers with either a right-or left-handed helix can be prepared by the helix-sense-selective polymerization of the corresponding achiral monomers using chiral catalysts or initiators under kinetic control. On the other hand, dynamic helical polymers, such as polyisocyanates (8) 17 and polysilanes (9), 19 possess a very low helix inversion barrier.As a consequence, they consist of interconvertible right-and lefthanded helical segments separated by rarely occurring helical reversals, as demonstrated by Green et al.,6,20,21 so that a preferred-handed helical conformation can be induced in the presence of a small amount of chiral residues at the pendant 6 or ter...

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Section: Synthesis and Structures Of Helical Polymers E Yashimamentioning

confidence: 99%

Section: Synthesis and Structures Of Helical Polymers E Yashimamentioning

confidence: 99%

Synthesis and structure determination of helical polymers

Yashima

2010

Polym J

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“…The Ni ion (Ni1) is 0.042(6) A above the best plane through the coordinating C atoms, which has the carbene C (C4, 0.028 (6) ) and the one trans to it (C2, 0.027 (6)) out of the plane on the same side as the Ni, and the others, viz. the C of the isocyanide closest to the benzyl group (C1, -0.027 (6) The distances of the nitrogens to the coordinating carbon in the carbene, 1.22(1) A for the isocyanide nitrogen (N4) and 1.35(1) A for the benzylamine nitrogen (N5), are significantly longer than the C-N bond length in the coordinating isocyanide moieties (average 1.139(11) A), but shorter than the distances of N4 (1.48(1) A) and N5 (1.48(2) A) to sp -hybridized C in the benzyl and tert-pentyl units.…”

Section: ---Figure 2 ---mentioning

confidence: 99%

“…[5,6] Our approach for the XANES simulation does not involve any assumptions about the valence state of the Ni; the result is calculated relative to E0 and hence does not give an absolute energy. It is therefore not possible to draw conclusions with regard to possible oxidation/reduction processes in the activation step.…”

Section: ---Figure 3 ---mentioning

confidence: 99%

“…[5] The 'merry-go-round' mechanism introduced above has also been represented with structures of intermediates featuring 5-coordinated nickel. [6] In view of the uncertainty in both structure and valence of the intermediate, we have undertaken a study of the Ni-catalyzed polymerization of isocyanides by X-ray absorption spectroscopy (XAS) [7] at the Ni K edge. The EXAFS (extended X-ray absorption fine structure) part of the XAS spectrum provides information on distance, type, and number of the nearest ligands, whereas the XANES (X-ray absorption near edge structure) part is sensitive to the ligand geometry and the valence of the Ni ion.…”

Section: Introductionmentioning

confidence: 99%

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X‐Ray Spectroscopic and Diffraction Study of the Structure of the Active Species in the Ni^II‐Catalyzed Polymerization of Isocyanides

et al. 2007

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The structure of the active complex in the Ni-catalyzed polymerization of isocyanides to give polyisocyanides has been investigated. It is shown by X-ray absorption spectroscopy (XAS), including EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure), and single-crystal X-ray diffraction to contain a carbene-like ligand. This is the first structural characterization of a crucial intermediate in the so-called merry-go-round mechanism for Ni-catalyzed isocyanide polymerization.

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