Triphenylmethyl methacrylate , chiral

Okamoto and his colleagues60) described the interesting polymerization of tri-phenylmethyl methacrylate. The bulkiness of this group affects the reactivity and the mode of placement of this monomer. The anionic polymerization yields a highly isotactic polymer, whether the reaction proceeds in toluene or in THF. In fact, even radical polymerization of this monomer yields polymers of relatively high isotacticity. Anionic polymerization of triphenylmethyl methacrylate initiated by optically active initiators e.g. PhN(CH2Ph)Li, or the sparteine-BuLi complex, produces an optically active polymer 60). Its optical activity is attributed to the chirality of the helix structure maintained in solution. 

Polymerization of triphenylmethyl methacrylate in the presence of a chiral anion catalyst results in a polymer with a helical structure that can be coated onto macroporous silica [742,804). Enantioselectivity in this case results from insertion and fitting of the analyte into the helical cavity. Aromatic compounds and molecules with a rigid nonplanar structure are often well resolved on this phase. The triphenylmethyl methacrylate polymers are normally used with eluents containing methanol or mixtures of hexane and 2-propanol. The polymers are soluble in aromatic hydrocarbons, chlorinated hydrocarbons and tetrahydrofuran which, therefore, are not suitable eluents. 

Another interesting chiral chain end effect is exhibited by the helical polymer block co-polymer, poly(l,l-dimethyl-2,2-di-/z-hexylsilylene)- -poly(triphenylmethyl methacrylate), reported by Sanji and Sakurai (see Scheme 7) and prepared by the anionic polymerization of a masked disilene.333 The helical poly(triphenylmethyl methacrylate) block (PTrMA) is reported to induce a PSS of the same sign in the poly(di- -propylsilylene) block in THF below — 20 °C, and also in the solid state, by helicity transfer, as evidenced by the positive Cotton effect at 340 nm, coincident with a fairly narrow polysilane backbone UV absorption characteristic of an all-transoid-conformation. This phenomenon was termed helical programming. Above 20°C, the polysilane block loses its optical activity and the UV absorption shifts to 310 nm in a reversible, temperature-dependent effect, due to the disordering of the chain, as shown in Figure 45. 

The main limitation of these CSPs is their limited pressure stability, which makes them not very suitable for HPLC application. However, they have proved to be an excellent tool for the preparative separation of drugs by low-pressure HPLC. To make these CSPs accessible to HPLC, silica gel-based phases were developed. " This type of phase is available from Merck (Darmstadt, Germany) under the name Chiraspher. Polymer phases of different types have been developed by Okamoto s group. > They are prepared by the asymmetric polymerization of triphenylmethyl-methacrylate monomers. The original character of these polymers is that they do not possess any chiral centre and therefore their chirality is only due to their helicity. However, clear mechanisms have not been proposed... 

Another result of great importance—the conformational asymmetric polymerization of triphenylmethyl methacrylate realized in Osaka (223, 364, 365)— has already been discussed in Sect. IV-C. The polymerization was carried out in the presence of the complex butyllithium-sparteine or butyllithium-6-ben-zylsparteine. The use of benzylsparteine as cocatalyst leads to a completely soluble low molecular weight polymer with optical activity [a]o around 340° its structure was ascertained by conversion into (optically inactive) isotactic poly(methyl methacrylate). To the best of my knowledge this is the first example of an asymmetric synthesis in which the chirality of the product derives finom hindered rotation around carbon-carbon single bonds. 

The hydrocarbon 25 has been partially resolved by asymmetric complexation with Newman s reagent [TAPA ( )-a(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)prop-ionic acid] thereby establishing its chiral Z)2-structure 53). Similarity, the naphthaleno-phane 27b could be resolved by chromatography on silicagel coated with (—)-TAPA 49) and recently also by HPLC on optically active poly(triphenylmethyl methacrylate)49a) which also proved to be very useful for the optical resolution of many other axial and planarchiral aromatic compounds 49b>. 

FIGURE 9 Structures of (a) poly(triphenylmethyl methacrylate) [poly(TrMA)] and (b) diphenyl-2-pyridyl methacrylate [poly(D2pyMA)] chiral selectors used for the preparation... 

In spite of the development of more successful and reliable CSPs (Chaps. 2-8), these miscellaneous types of CSP have their role in the field of the chiral resolution also. The importance of these CSPs ties in the fact that they are readily available, inexpensive, and economic. Moreover, these CSPs can be used for some specific chiral resolution purpose. For example, the CSP based on the poly(triphenylmethyl methacrylate) polymer can be used for the chiral resolution of the racemic compounds which do not have any functional group. The CSPs based on the synthetic polymers are, generally, inert and, therefore, can be used with a variety of mobile phases. The development of CSPs based on the molecularly imprinted technique has resulted in various successful chiral resolutions. The importance and application of these imprinted CSPs lies in the fact that the chiral resolution can be predicted on these CSPs and, hence, the experimental conditions can be designed easily without greater efforts. Because of the ease of preparation and the inexpensive nature of these CSPs, they may be useful and effective CSPs for chiral resolution. Briefly, the future of these types of CSP, especially synthetic polymers and polymers prepared by the molecularly imprinted technique, is very bright and will increase in importance in the near future. 

Type II sorbents are based on an inclusion mechanism. Chiral recognition by optically active polymers is based solely on the helicity of that polymer. Optically active polymers can be prepared by the asymmetric polymerization of triphenylmethyl methacrylate using a chiral anionic initiator [264]. Helical polymers are unique from the previously discussed chromatographic approaches because polar functional groups are not required for resolution [265]. These commercially available sorbents have been used to resolve enantiomers of a-tocopherol [266]. The distinction between this group (lib) and the sorbents containing cavities is vague (Ila). 

In the early stage of helical polymer stereochemistry, a few polymers were known to retain a helical main chain with a predominantly single screw sense in solution at room temperature. For example, in cases of poly( f-bulyl isocyanides) [22], poly(triphenylmethyl methacrylate) [23], polyisocyanate [24], and poly-a-olefins [19], helical structures are kept through side group interactions. Since these pioneering works, many synthetic optically active polymers with a chromophoric main chain bearing chiral and/or bulky side... 

Triphenylmethyl methacrylate (TrMA) and azobenzene-modified methacrylates were randomly copolymerized in toluene at — 78°C with chiral catalysts to give optically active helical copolymers (19 in Fig. 6) [65]. The optical activity (optical rotation) of the copolymers decreased with the increasing content of the azobenzene-modified methacrylates in the copolymers. The single helical conformation of PTrMA is quite stable in solution, but the copolymers of TrMA with less bulky methacrylates cannot keep their helical structure and lose their optical activity during the polymerization or after the polymerization in solution, which is highly dependent on the bulkiness of the comonomers [22]. The copolymer (19 x = 2) containing 26 mol% azobenzene units, also lost its optical activity upon irradiation within 20 min. This change is due to the helix-to-coil transition of the copolymer and can occur in the dark. 

Enantiomeric purity of binaphthol is determined using chiral stationary phase HPLC Pirkle Type 1-A column (Regis Chemical Company) eluted with 20 1 hexane/2-propanol 1 or poly(triphenylmethyl)methacrylate on silica gel (Chiralpak OT, Daicel Chemical Industries, LTD) eluted with methanol. To determine enantiomeric purities >99% ee an HPLC trace of the unknown is compared to the HPLC trace of unknown containing 0.2% deliberately-added racemic material. 

In a similar manner, the coalescence temperature for the methyl groups of the tetramer was determined as 4°C. AG was calculated as 12.7 kcal/mol which was 3.7 kcal/mol smaller than that for the pentamer. The hexamer showed a total of seven signals with narrow linewidth due to two methyl groups and six methine protons even at 70°C, indicating that the rate of helix sense reversal is much slower than the rate for the pentamer. This suggests the possibility that the symmetrical oligomers over the pentamer level may be optically resolved at room temperature based entirely on conformational asymmetry. This was confirmed by the chiral HPLC technique using (+)poly(triphenylmethyl methacrylate) as a stationary phase.282... 

One of the most studied polymerization systems employs alkyllithium initiators that are modified by chiral amine ligands for the polymerization of sterically bulky methacrylates [8,38,39,40,41], acrylates [42],crotonates [43], and acrylamides [44]. A primary example is the reaction of triphenylmethyl methacrylate with an initiator derived from 9-fluorenyllithium and (-)-sparteine (3) at -78 °C (Scheme 4). The resultant isotactic polymer is optically active, and is postulated to adopt a right-handed helix as it departs from the polymerization site. This polymer has been particularly successful as a chiral stationary phase for the chromatographic resolution of atropisomers [8]. Many modifications of the or-ganolithium initiator/chiral ligand system have been explored. Recently, Okamo-to has applied enantiopure radical initiators for the enantioselective polymerization of bulky methacrylate monomers [45]. 

Synthetic polymer, poly(triphenylmethyl methacrylate) forms a helical structure similar to that of cellulose chirality arises from the twist of the helix with one enantiomer being formed because of the use of a chiral initiator in the polymerisation. 

Another type of specificity that can occur is the chirality. Isotactic poly(triphenylmethyl methacrylate) is the first known case in which the helicity of the polymer leads to chirality and optical activity [51,52]. A conformational analysis of this polymer has been reported by Cavallo et al. [53]. 

Triphenylmethyl methacrylate (TrMA) is a unique monomer, which affords a highly isotactic polymer (PTrMA) even by a radical process, and the polymer with more than a 95% triad isotacticity can be obtained by the anionic polymerization with butyllithium (BuLi) [10]. In 1979, we found that optically active PTrMA is formed during the anionic polymerization of TrMA with the complex of ( )-sparteine-n-BuLi at —78°C (Fig. 5) [11, 12]. This is the first example of helix-sense-selective polymerization preferentially producing a stable one-handed helical polymer through the polymerization process. The results clearly indicated that the existence of a stable helical polymer even in solution is possible on a vinyl polymer without chiral side groups. The helical structure is maintained due to the steric hindrance of the bulky triphenylmethyl groups. Therefore, when the ester groups are hydrolyzed with an acid, the optical activity of the polymer disappears. 

Yuki H, Okamoto Y, Okamoto 1 (1980) Resolution of racemic compounds by optically active poly(triphenylmethyl methacrylate). J Am Chem Soc 102 6356-6358 Cavazzini A, Dondi F, Marmai S, Minghini E, VHlani C, Rompietti R, Gasparrini F (2005) Adsorption equiUbria of benzodiazepines on a hybrid polymeric chiral stationary phase. Anal Chem 77 3113-3122... 

---