BN‐ and BO‐Doped Inorganic–Organic Hybrid Polymers with Sulfoximine Core Units

Abstract While polysulfones constitute a class of well‐established, highly valuable applied materials, knowledge about polymers based on the related sulfoximine group is very limited. We have employed functionalized diaryl sulfoximines and a p‐phenylene bisborane as building blocks for unprecedented BN‐ and BO‐doped alternating inorganic–organic hybrid copolymers. While the former were accessed by a facile silicon/boron exchange protocol, the synthesis of polymers with main‐chain B–O linkages was achieved by salt elimination.

properties, which provedu seful in drug design and bioactivity adjustment. [10] Surprisingly,s ulfoximines have only once been applieda sb uilding blocks in polymers. [11] In that study,T akata et al. used Friedel-Crafts reactions to preparep olysulfoximines with molecular weights (M n )o fa pproximately 13 000. Herein, we describe the synthesis and characterization of the first inorganic-organic hybrid polysulfoximines.
In light of previous work, [12] sulfoximines 1 and 2 were identified as suitable organic starting materials. Both compounds were N-methylated, thereby confining the reactive anchors ites of the molecules to the free arylic amino and hydroxyl groups. With the vision to allow future variations of the N-substituent, phthalimid-and benzyl-protected NH-sulfoximines 4 and 7,r espectively,w ere targeted first. The synthetic sequences are shown in Scheme 1. The preparation of 1 startedf rom known diarylsulfide 3, [12] which was imidated and oxidized by adopting ap rotocol reported by Luisi, Bull,and others [13] to give 4 in 62 %y ield. Noteworthy,w ea pplieda queousa mmonia as an itrogen source insteado ft he originally suggested ammonium carbamate.
In the first experiment (trial 1), the mixture was kept in dichloromethane for 3days at ambient temperature. Trial 2i nvolved o-difluorobenzene (o-DFB) as the solventa nd heating the mixture to 80 8Cf or 24 h. The resulting alternating copolymers 11 were then purifiedb yp recipitation from concentrated solutionw ith hexane and subsequent drying in vacuo. The identities of copolymers 11,w hich were obtained as off-white solids,w ere unambiguously ascertained by NMR spectroscopy. Their molecular mass distributions were determined by gel permeation chromatography (GPC, Table 1). For both samples, the 1 H NMR spectrums howed as hift of the NH-Signal from d = 3.77 ppm in 9 to the aromatic region in 11 (d = 7.25 ppm), which was also observed in previously prepared related BN polymers. [3d] The GPC analyses revealed number average molecular weights of M n = 97 50 (trial 1) and 11 830 (trial 2), according to polymerization degrees of DP n = 13 and 15, respectively.T he polydispersity indicesw ere close to 2, as expected for stepgrowth polycondensation processes.
Next, copolymers with main-chain BÀOl inkages [17] were targeted. Hypothesizing that such products could be accessed by analogousS i/B exchange reactions as appliedb efore in the synthesis of 11,o rganic starting materials with silylatedp henolic hydroxyl groups became of interest.I no rder to get an estimate of the feasibility of such an approach, ap rior model reactionb etween 10 and trimethylsilylated phenol 12 was performed (Scheme 3). In the first experiment, the reactionw as run in dichloromethane at room temperature. As hypothesized, product 14 was indeed formed as revealed by 1 Ha nd 11 BNMR spectroscopy.T he initial presence of two doublets( d = 8.04 and 7.88 ppm) in the 1 HNMR spectrum suggested as tepwise formation of 14.H owever,t he entire process was very slow, and even after four weeks the conversion was not yet complete. As imilar outcome resulted when o-difluorobenzene was used as as olventa tareaction temperature of 80 8C. Also in this case, the conversion was slow,t aking five weeks in total. Althought hese resultss howedt hat aS i/B exchange could, in principle, be applied to accomplish aB ÀOb ond formation startingf rom 10,t he slow rate of this process provedu nfavorable for its application to co-polycondensation reactions. Therefore, we decided to investigate BÀOb ond formation between 10 andt he parentfree phenol (13).
While initial attempts without base remained unsatisfying, the presence of triethylamine had at remendously positive effect, leadingt oaclean andq uantitative coupling providing 14 within 24 hours at room temperature. Compound 14 was then isolated by filtration and characterizedb ym ultinuclear NMR spectroscopy.T he 11 B{ 1 H} NMR spectrum showed ar esonance at d = 47.3 ppm, which is in the expectedr ange fort he suggested constitution.
Encouraged by this result,t he aforementioned conditions were appliedinthe copolymerization of sulfoximine 2 with bisborane 10 (Scheme 4). Within three days in the presence of Et 3 N, the dichloromethane solution became highly viscous (trial 1). However,a fter work-up the GPC analysisr evealed that the product was of relatively low molecular weight (M n = 2400, DP n = 3; Ta ble 1). Consequently,i nt he next experiment (trial 2) the solvent was changed to o-DFB,a nd then the reaction temperature wasr aised to 80 8C. Pleasingly,i nt his manner, after 24 ht he molecular weight( M n )o fther esulting polymer 15 was increased to 5300, revealing an average chain length of DP n = 7( Ta ble1). [18] In summary,w eh ave prepared the first inorganic-organic hybrid sulfoximine-containing polymers as alternating copoly-Scheme2.Polycondensation reaction of sulfoximine 9 and bisborane 10 to give alternating copolymer 11.  [19] on the one hand, andt he well-established biomedical activity of many boron-containing polymers [20] on the other hand, we are currently exploring the biomedical potential of our novel sulfoximine-B=N/BÀOh ybrids in detail.