Helical sense, control

Following up the previous work, the poly[Asp(OBzl)] 181 modified with different proportions of a diazobenzene moiety linked via an ester group was prepared by the co-polymerization of the respective NCAs of 179 (180) and L-Asp(OBzl) (Scheme 50)J92-94 Again, the idea was to use the well-known photoinduced cis-trans isomerization of the diazobenzene group to control the helical sense of the polypeptide framework. Impressive results obtained by CD showed that indeed the sense of the helix could be modulated by irradiation of the diazo group at 320-390 nm.[ l However, the changes observed are sensitive to the nature of the solvent. The photomodulation of the conformation was also observed with the para analogue of 181. 

Carbodiimide Polymers An optically active carbodiimide, (/ )-152 ([a]365 +7.6°), gives a polymer by polymerization using a titanium (IV) isopropoxide catalyst (Scheme 11.9) [203], The polymer showed optical activity essentially identical to the monomer however, on heating, the polymer indicated mutarotation and specific rotation reached a plateau value of [a]363 -157.5°, which is considered to be based on excess helical sense of the main chain. The mutarotation has been ascribed to a conformational transition from a kinetically controlled one to a thermodynamically controlled one. Excess single-handed helical conformation can be induced for polyfdi-/ -hexyl carbodiimide) by protonating the polymer with chiral camphorsul-fonic acid. 

The helical sense of polyisocyanates 88 and 89 can be controlled in terms of photoinduced isomerization of the side chain chromophores.165,170 For 88, pho-... 

The details of this mechanism are still unclear, and need to be clarified. However, this asymmetric polymerization system using a nickel catalyst with optically active amines seems to be unique, in that the chiral elements that become apart from the propagating termini control the helical sense of the entire polymer main chain. A similar, but more stereoselective system is discussed below for the Pd-mediated polymerization of diisocyanobenzenes, which is discussed later. 

Helix-sense-selective polymerization has been achieved for several other optically active isocyanide monomers. In the polymerization of l-105 with NiCl2, helix sense was controlled by solvent and temperature. The spectra of poly-105 obtained in nonpolar solvents such as CCI4 and toluene were almost mirror images of those of poly-105 obtained in polar THF, indicating that the polymers have opposite helical senses, that is, the polymers are diastereomeric helices. The poly-105s with opposite helical senses showed cholesteric LC phases with opposite twist senses. 

The helical sense of polyisocyanates 147 and 148 can be controlled in terms of photo-induced isomerization of the side-chain chromophores. For 147, photoirradiation causes cis-trans isomerization of the azo moiety, which induces a change in the helix population of the main chain. In the case of 148 having a chiral bicyclo[3.2.1] octan-3-one group in the side chain, photoirradiation results in rotation around the styryl double bond in the side chain.When (+)- or (-)-circularly polarized light (CPL) is used for irradiation, the chirality of the bicyclo... 

Switching of helicity of a Cu+ complex unit in a pseudomacrocycle 7 Cu was achieved by achiral Na binding (Scheme 1.3). Equilibrium between non-helical and helical forms effectively shifts to the helical form upon Na binding. This unique helicity control provides a new strategy for controlling helical sense because an aehi-ral guest Na regulates the chiral structures. 

The helical sense of the heteronuclear complexes can be modulated by incorporating a chiral auxiliary into a trisalamo chain. In solution, binaphthyl derivative 30 exclusively gives one helical ZujLa complex (Figure 1.12). The n-n interaction stabilizes the helical structure. A 1,2-diphenylethylene unit in 31 is not effective for controlling the helical sense in solution. " In contrast, the crystals consist of only the left-handed helical complex. 

Leiras S, Freire F, Seco JM, Quinoa E, Riguera R (2013) Controlled modulation of the helical sense and the elongation of poly(phenylacetylene)s by polar and donor effects. Chem Sci 4 (7) 2735-2743. doi 10.1039/C3SC50835H... 

The temperature measurements in range from —40 to 23 °C revealed the presence of conformational equilibrium between conformers of the opposite helicity (M- and P-helix). The barrier of 13 kcal/mol of the chemical exchange was estimated. It was shown that peripheral stereocenters control the absolute sense of helicity in the foldamers studied.103... 

Another significant cooperativity effect in preferential helical screw sense optically active copolymers is the majority rule phenomenon.18bl8q In this case, the screw sense of a helical main chain with unequal proportions of opposite chirality enantiopure chiral side groups is controlled by the enantiomeric excess only. Since this phenomenon was first reported from poly-a-olefins made of vinyl co-monomers bearing nonenantiopure chiral moieties by Green et al.8b and Pino et al.,16b this majority rule has been established in... 

Polymerization of optically active isonitiiles, 72, also leads to the formation of helical polymers with a preferential screw sense (219-222). Various factors distinguish this system from the preceding ones In the isonitrile case no new stereogenic atoms are formed during polymerization the helices are rigid and there is no indication of conformational equilibrium in the system the formation of a preferential screw sense is very probably a kinetically controlled process. 

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