Lithiation of polystyrene

Farrall and Frechet recognized the possibility of forming polymer-bound boronic acids in 1976.66 Supported pnra-benzeneboronic acid groups were generated by direct lithiation of polystyrene, giving a para-lithio intermediate that could be used to generate a host of other resins also (Scheme 27). Conversion to the boronic acid was one of the more successful transformations. The purpose of this transformation was to allow the attachment of sugars to the solid phase via the boronate. 

Figure 4.1. Lithiation of polystyrene-bound arenes and heteroarenes [1,8-10],...

Lithiated polystyrene resins can be obtained either via convenient bromine-lithium exchange reaction using nBuLi starting from 4-bromo-substituted polystyrene 102,111-115 qj. jjy direct lithiation of polystyrene using nBuLi in cyclohexane in the presence of TMEDA 11L116 pjjjj method, however, yields a mixture of para- and meta isomers. The bromination of microporous resins in the presence of the Lewis-acid catalysts was carried out in the dark whereby the degree of functionalisation could conveniently be controlled by the amount of bromine used in the reaction.iii Macroreticular resins were brominated using Br and FeClj or stoichiometric amounts of thallium acetate as Lewis acid catalysts. 2 ... 

A study of methods for controlling the grafting sites of polystyrene polymers may serve as an example for the quantitative and structural analysis of the site of lithiation. Trimethylsilyl chloride serves as the quenching agent owing to the low incidence of side reactions besides metal replacement. Thus, on varying the conditions of metallation of low molecular weight polystyrene (409) and poly(4-methylstyrene) (410), aromatic and... 

Finally, the lithiation of a soluble chloromethylated polystyrene 16 (prepared by AIBN-indnced copolymerization of styrene and 4-chloromethylstyrene in a 3 1 molar ratio) with lithinm and a catalytic amonnt of DTBB (10%) in THF at —78 °C gave the expected... 

Figure 4.3. Preparation of polystyrene-bound organometallic compounds by transmetallation and direct lithiation [16,25,26,29],...

Thioethers have also been prepared on cross-linked polystyrene by radical addition of thiols to support-bound alkenes and by reaction of support-bound carbon radicals (generated by addition of carbon radicals to resin-bound acrylates) with esters of l-hydroxy-l,2-dihydro-2-pyridinethione ( Barton esters Entry 6, Table 8.5). Additional methods include the reaction of metallated supports with symmetric disulfides (Entries 7-9, Table 8.5) and the alkylation of polystyrene-bound, a-lithiated thioani-sole [65],... 

Farall MJ, Frechet JMJ, Bromination and lithiation two important steps in the functionalization of polystyrene resins, J. Org. Chem., 41 3877-3882, 1976. 

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... 

The lithiation of poly(styrene-DVB) can be carried out according to two reaction routes. The first involves the transformation of -Br groups into -Li groups by reaction of a brominated polystyrene with an excess of n-butyllithium (15,16). The second involves the direct lithiation of poly(styrene-DVB) by reacting the resin with n-butyllithium and N,N,N, N -tetramethyl-ethylenediamine (TMEDA)(15,17). 

As an extension to their work, the same group have prepared polymer-supported Zr di-Ind complex 251 from polystyrene (Scheme 89). Lithiation of the polystyrene, treatment with 1,4-dibromobenzene, and further lithiation gave 249 which was treated with PhSi(Ind)2Cl to give the supported ligand 250. The metal complex 251 was formed by treatment of this with ZrCU. The use of 251 for ethane polymerization was assessed and subsequently the mechanism of polyethylene growth on the surface of the supported catalyst beads has been studied. 

Leznoff and Fyles described bromination and lithiation of insoluble polystyrene, thus pioneering the synthesis of functionalised resins (Can. J. Chem. 1976,54, 935)... 

The widespread applications of polystyrene derived resins is due to the fact that styrene consists of a chemically inert aUcyl backbone carrying chemically reactive aryl side chains that can be easily modified. As discussed earlier, a wide range of different types of polystyrene resins exhibiting various different physical properties can be easily generated by modification of the crosslinking degree. In addition, many styrene derived monomers are commercially available and fairly cheap. Polystyrene is chemically stable to many reaction conditions while the benzene moiety, however, can be funtionalised in many ways by electrophilic aromatic substitutions or lithiations. As shown in Scheme 1.5.4.1 there are principally two different ways to obtain functionalised polystyrene/DVB-copolymers. 

The bromine-lithium exchange reaction on macroreticular PS-resins using nBuLi in THE could be driven to completion by repetitive lithiation as described by Frechet. The lithiation of highly loaded microporous resins, however, were successfully carried out in toluene or benzene. The direct lithiation reaction of microporous polystyrene-derived resins (2% DVB) using nBuLi in cyclohexane in the presence of TMEDA is much faster than that of macroreticular resins (20% DVB).l It is interesting to note that for this reaction THE and benzene are not the solvents of choice. Using cyclohexane as solvent allows the synthesis of resins with a low or medium degree... 

Jacobson, Mares, and Zambri report that the lithiation of 1% crosslinked polystyrene followed by treatment with triethoxyarsine and with hydrogen peroxide affords (Equation 5) the polymer-bound arsonic acid (30). 

The more effective methods of preparation of polymer-bound triarylphosphine for Wittig reactions are 1) lithiation of the polymer followed by reaction with chlorodi-phenylphosphine, and 2) reaction of brominated polystyrene with lithium or sodium diphenylphosphide as shown in Scheme 3. 

Involving a Suzuki reaction, Hodge prepared chemical robust polymer-supported diphenyl prolinol based catalysts 68 and 69 from 62 and 63 and polystyrene boronic acid prepared via a direct lithiation of 1% crosslinking gel-type polystyrene beads (2.21 mmg/mol of boronic acid residue) (Scheme 31) [59]. 

Polystyrene (PS) and polyisoprene (PI) form block copolymers with PA in the presence of Ti(OBu)4 catalysts [83-85]. The two copolymers are prepared similarly. For instance, styrene is first initiated by n-BuLi (typically 0.05 M) in an anionic polymerization. A lithiated polystyiyl anion can then displace one butoxy group from the titanium center to form a new Ti-C bond, which serves as the active site for the subsequent acetylene polymerization. However, before acetylene was added, this polymeric catalyst was often aged for 1 day (PS) or 2 days (PI). The acetylene polymerization was then carried out under dilute conditions so as to minimize side reactions. In this manner, acetylene can be polymerized through the Ti(UI) catalyst, forming an AB diblock copolymer. In the case of polystyrene, less than 20 wt% of PA in the copolymer renders the copolymer soluble. Gels that were not soluble could be pressed into thin films for characterization. 

Preparative Methods lithiated polystyrene can be prepared via direct lithiation of cross-linked polystyrene with (1 1) BuLi and TMEDA at 65 °C in cyclohexane (eq 1)T Subsequent addition of diisopropyl-dichlorosilane in benzene followed by washing with excess benzene and drying in vacuo provides the desired functionalized polymer. Loading can be determined by hydrolysis and acid-base titration. ... 

Spin-labelled polystyrenes were prepared by reacting the corresponding lithiated polymers with 2-methyl-2-nitrosopropane, either in para or in meta positions. In the latter case the lithiation results from the modification of polystyrene by BuLi complexed with N,N,N N -tetramethylethylenediamine the metallated polymer is reacted with 2-methyl-2-nitrosopropane (MNP), " as in reaction (18). 

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