Polymeric boronate

A recent example of diasteromeric amplification with achiral guests and a racemic library can be seen in the work of Iwasawa and coworkers. The library members consisted of a racemic polyol and l,4-benzenedi(boronic acid) [2], When these components were mixed in an equimolar ratio in methanol, a precipitate formed, which was insoluble in other organic solvents and thought to be a polymeric boronate. However, when the library members were mixed in the presence of toluene or benzene, a precipitate again formed, but it was soluble in several (nonprotic) organic solvents where boronic ester exchange is slow. With toluene a [2 2] complex of the polyol and diboronic acid formed, as evidenced by NMR and FAB-MS data. X-ray crystallography confirmed that a cyclic structure formed with... 

The formation of polymeric boron ions in solution is well established. The significant increase in solubility in terms of B203 in aqueous mixtures of borax and boric acid relative to the individual components is explained by such polyborate formation. The comprehensive reviews by Nies (307) and Sprague (392) reveal that, despite the many investigations over the last 50 years, the identity of the borate ions involved has still not been completely resolved. 

Only in the case of X equals fluorine is the polymeric boron halide-cyanide sufficiently stable to allow isolation of a pure sample. The borane adducts, R3SiCNBH3, require elevated temperatures to yield the polymeric compound HjBCN. Di-n-butylboron chloride and trimethylsilyl cyanide... 

Boron trifluoride (BF3) is an excellent catalyst for cationic polymerization because it leaves no good nucleophile that might attack a carbocation intermediate and end the polymerization. Boron trifluoride is electron-deficient and a strong Lewis acid. It usually contains a trace of water that acts as a co-catalyst by adding to BF3 and then protonating the monomer. Protonation occurs at the less substituted end of... 

The above mentioned polymer-supported oxazaborolidines are prepared from polymeric amino alcohols and borane. Another preparation of polymer-supported oxazaborolidines is based on the reaction of polymeric boronic acid with chiral amino alcohol. This type of polymer can be prepared only by chemical modification. Lithiation of the polymeric bromide then successive treatment with trimethyl borate and hydrochloric acid furnished polymer beads containing arylboronic acid residues 31. Treatment of this polymer with (li ,2S)-(-)-norephedrine and removal of the water produced gave the polymer-supported oxazaborolidine 32 (Eq. 14) [41 3]. If a,a-diphenyl-2-pyrrolidinemetha-nol was used instead of norephedrine the oxazaborolidine polymer 33 was obtained. The 2-vinylthiophene-styrene-divinylbenzene copolymer, 34, has been used as an alternative to the polystyrene support, because the thiophene moiety is easily lithiated with n-butyl-lithium and can be further functionalized. The oxazaborolidinone polymer 37 was then obtained as shown in Sch. 2. Enantioselectivities obtained by use of these polymeric oxazaborolidines were similar to those obtained by use of the low-molecular-weight counterpart in solution. For instance, acetophenone was reduced enantioselectively to 1-phe-nylethanol with 98 % ee in the presence of 0.6 equiv. polymer 33. Partial elimination of... 

Table 6. Asymmetric cycloaddition reaction of cyclopentadiene and methacroleine in presence of 15 mol% polymeric boron catalyst, at -78°C...

Endoboron Compounds from CycloaU tboranes Cycloalkylboranes will also form heterocyclics by pyrolysis 24,67). Since cycloalkanes cleave readily at fairly low temperatures (ca. 180° C), cycliza-tion proceeds smoothly at 200° C. It is absolutely essential that the cyclic ring be large enough. Tricyclohexylborane does not form 7-borabicydo-[2.2.1]heptane instead, polymeric boron-containing products are formed and volatile materials evolved. 

Figure 12.3 Basicity dependence of polymeric boronate esterification. FE-SEM images and PXRD patterns of solid isolated from mixtures of BDBA and THB in THF at room temperature after addition of (a) pyridine, (b) 3-picoline, (c) 4-methoxypyridine, (d) 4-dimethylaminopyridine, and (e) triethylamine. The use of 3-cyanopyridine induced no aggregation. Conditions [BDBA] = [THB] = [base] = 1.0 x lo" M. The solids were isolated from the mixture by filtration at 15 min reaction time. (Reproduced with permission of Wiley-VCH Verlag GmbH from ref. 17.)...

Polystyrylboronic acid has similarly been used for the protection of hydroxyl groups in polyols. It has been claimed that the esters of the polymeric boronic acid are more stable to moisture and that the polymeric by-product is re-usable (Seymour and Frechet, 1976). Both the aldehyde polymer and boronic acid polymer have been used for simultaneous protection of C-4-0 and C-6-0 in derivatization reactions. A polymer incorporating a trityl group has also been used for protection of the C-6-0 group only (Frechet and Nuyens, 1976). 

Alternative process to prepare polymeric boronate was rhodium-catalyzed functionalization of polyolefins in the melt [9]. Polyethylethylene (12) was allowed to... 

Some procedures to prepare main-chain polymeric boron Lewis acids have been also developed. The most commonly used method for the synthesis of main-chain organoboron polymers is the hydroboration polymerization developed by Chujo [29, 30]. For example, hydroboration between (47) and (48) in TH F gave the organoboron... 

One attractive molecular species upon which polymeric boron nitride precursors could be based is the ring-compound borazine, BaNsHe The compound is easily made and has both the correct boron/nitrogen ratio and a hexagonal structure. Indeed, Paine and Paciorek have each demonstrated the use of linked polyborazine gels or polymers as precursors for boron nitride. 

Polymerization of Triglycerides by Cationic Polymerization Triglycerides can be made by cationic polymerization. Boron trifluoride diethyl etherate (BFj-OEtj), a Lewis acid, is commonly used initiator for the cationic polymerization of alkenes. 

Polymeric Boron Compounds.—number of polymeric or polymer-supported dihydroxyboron compounds have been recommended for the affinity chromato-... 

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