Acids chiral combined polymers

Scheme 2. Synthesis of chiral combined LC polymers by esterification of chiral acids with a preformed polyphenolic polymer. DCC, dicyclohexylcarbodi-imide.

Scheme 2.18 Domino Michael-iV-acyliminium cyclisation reaction catalysed by a combination of a chiral phosphoric acid and a polymer supported Schwesinger base.

Chiral (acyloxy)borane (CAB) is known as an effective chiral Lewis acid catalyst for enantioselective allylation of aldehydes. Marshall applied the CAB complex 1 to the addition of crotylstannane to achiral aldehydes and found that the CAB catalyst gives higher syn/anti selectivity than BINOL/Ti catalysts in the reaction [4]. CAB complex 2 was utilized in asymmetric synthesis of chiral polymers using a combination of dialdehyde and bis(allylsilane) [5] or monomers possessing both formyl and allyltrimethylsilyl groups [6]. 

Several methods promoted by a stoichiometric amount of chiral Lewis acid 38 [51] or chiral Lewis bases 39 [52, 53] and 40 [53] have been developed for enantioselective indium-mediated allylation of aldehydes and ketones by the Loh group. A combination of a chiral trimethylsilyl ether derived from norpseu-doephedrine and allyltrimethylsilane is also convenient for synthesis of enan-tiopure homoallylic alcohols from ketones [54,55]. Asymmetric carbonyl addition by chirally modified allylic metal reagents, to which chiral auxiliaries are covalently bonded, is also an efficient method to obtain enantiomerically enriched homoallylic alcohols and various excellent chiral allylating agents have been developed for example, (lS,2S)-pseudoephedrine- and (lF,2F)-cyclohex-ane-1,2-diamine-derived allylsilanes [56], polymer-supported chiral allylboron reagents [57], and a bisoxazoline-modified chiral allylzinc reagent [58]. An al-lyl transfer reaction from a chiral crotyl donor opened a way to highly enantioselective and a-selective crotylation of aldehydes [59-62]. Enzymatic routes to enantioselective allylation of carbonyl compounds have still not appeared. 

The chiral side chain polymers derived from asymmetric esters of terephthalic acid and hydroquinone can form (in a broad temperature range, including ambient temperature) an unusual mesophase (the isotropic smectic phase, IsoSm ) characterized by high transparency and optical isotropy within the visible wavelength range, combined with a hidden layered smectic ordering and some elements of helical superstructure at shorter dimensions of 10 to 250 nm. The short-pitch TGB A model seems to be the most adequate for the mesophase structure. 

Mechanistic considerations (e.g., the extensive work published on brush-type phases) or the practitioner s experience might help to select a chiral stationary phase (CSP) for initial work. Scouting for the best CSP/mobile phase combination can be automated by using automated solvent and column switching. More than 100 different CSPs have been reported in the literature to date. Stationary phases for chiral pSFC have been prepared from the chiral pool by modifying small molecules, like amino acids or alkaloids, by the deriva-tization of polymers such as carbohydrates, or by bonding of macrocycles. Also, synthetic selectors such as the brush-type ( Pirkle ) phases, helical poly(meth) acrylates, polysiloxanes and polysiloxane copolymers, and chiral selectors physically coated onto graphite surfaces have been used as stationary phases. 

Utilization of the single hydrogen bond between pyridine and benzoic acids in SLCP s has been a source of inspiration for other groups in the development of main-chain supramolecular polymers based on diacids and dipyridines.53-56 Supramolecular rod-coil polymers have been developed by assembly of 4,4 -bipyridines and telechelic polypropylene oxide with benzoic acid end-groups, which show highly ordered liquid crystalline phases.57 The use of tartaric acid derivatives in combination with bipyridine units resulted in the formation of hydrogen-bonded, chiral main-chain LCP s, as has been shown by circular dichroism measurements, optical microscopy, and X-ray data.58,59... 

Asymmetric epoxidation of unfunctionalized aUcenes catalyzed by chiral Mn(III)(salen) complexes has proven to be a useful solution-phase reaction [88]. To simplify product isolation and to avoid degradation of the Mn(salen) complex through formation of i-oxo-manganese(lV) dimers by spatial redistribution, the polymer-supported catalyst 112 was prepared by co-polymerization of complex 113, styrene 58, and divinylbenzene as a cross-linker (Scheme 20) [89]. As a stoichiometric oxidant, a combination of meta-chlor-operbenzoic acid (mCPBA) and N-methyl-morpholine N-oxide (NMO) in acetonitrile was used. Yields and rates of conversion were satisfactory for the epoxidation of styrene 58 and of methyl styrene, but only low enantioselectivities were obtained. Nevertheless, the catalyst retained its efficiency in terms of yields and enantioselectivities after repetitive use. Similar results have been described by other researchers [90]. 

Successful polymer supported stereoselective Diels-Alder reaction was performed using immobilized enantiopure 4-(3-hydroxy-4,4-dimethyl-2-oxopyrro-lidin-l-yl)benzoic acid 12 as a chiral auxiliary [15]. The corresponding resin-bound acrylate derivate has been applied as the dienophile 13. Preparation of the precursor started with the combination of pantolactone 10 and the sodium salt of 4-aminobenzoic acid. Conversion into the corresponding benzyl ester followed. The obtained racemate was esterified with (lS)-camphanic acid chloride to a dia-stereomeric mixture to gain the enantiopure compounds by chromatographic separation. After subsequent saponification of the camphanic acid moiety and hydrolysis of the benzyl ester the (R)-enantiomer 11 was coupled to Rink amide resin (Scheme 12.6). 

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