A new type of σ-sulfurane with a transannular S...N bond: structures of S-substituted N-methyl-6,7-dihydro-5H,12H +-dibenzo[b,g][1,5]thiazocinium salts and N-methyl-6,7-dihydro-5H-dibenzo[b,g][1,5]thiazocine S-oxide
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Cited by 14 publications
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Investigation of solid-state polymerization of crystalline monomers has attracted increasing attention over the past three decades.') Development of this subject relied on the use of ultraviolet(UV) light and ionizing radiation. The high penetration of y rays enables homogeneous initiation of the polymerization in whole crystals. It is also easy to determine the polymer yields per absorbed energy and t o compare inherent reactivities of monomer crystals. O n the other hand, the homogeneous initiation of the polymeri7ation with UV light is difficult unless extinction coefficients of monomers at irradiation wavelengths are sufficiently low. Reflection and scattering of U V light make it impossible to determine quantum yields of polymers. The advantage of U V light is selective excitation by choice of irradiating wavelengths.This chapter gives a comparison of the inherent reactivities of a series of crystalline diethynylbenzene derivatives in solid-state polymerization, which is initiated by radicals generated upon irradiation with y rays. Effects of the molecular packing in the crystals on the reactivities are described. The structures of the monomers are shown in Fig. 6. I , whereNumerous studies have been published on the solid-state polymerization of acetylenic compounds. Most of them examine diacetylenes having a wide variety of substituents.2' The polymeri7ation of the diacetylenes has been interpreted by a typical lattice-controlled mechanism. Crystalline 1,4-diethynylnaphthalene and its analogs including 1,4diethynylbenzene (1) are also known to polymerize when irradiated with UV light.') One of the ethynyl groups participates in the polymeriiation resulting in conjugated polyene backbones having ethynyl groups as pendants. 1,4-Diethynylbeni-ene is regarded as the simplest acetylenic compound that is crystalline at room temperature. Various crystalline diethynylbenzene derivatives, which are dimethyl-, dimethoxy-, or methylmethoxysubstituted bis(3-acetoxy-1 -propynyl)benienes (2-8), were synthesi~ed by procedures in the l i t e r a t~r e .~) The monomer crystals were sealed in Pyrex tubes under vacuum and irradiated with y rays from a h°Co source at room temperature. The product formation in the solid-state polymerization was correlated to the molecular packing in the crystals. When 0.5 M benzene solutions of the monomers were irradiated, there was no difference in the yields and the molecular weight distributions of the products among the monomers. R = CH20COCH1.
Investigation of solid-state polymerization of crystalline monomers has attracted increasing attention over the past three decades.') Development of this subject relied on the use of ultraviolet(UV) light and ionizing radiation. The high penetration of y rays enables homogeneous initiation of the polymerization in whole crystals. It is also easy to determine the polymer yields per absorbed energy and t o compare inherent reactivities of monomer crystals. O n the other hand, the homogeneous initiation of the polymeri7ation with UV light is difficult unless extinction coefficients of monomers at irradiation wavelengths are sufficiently low. Reflection and scattering of U V light make it impossible to determine quantum yields of polymers. The advantage of U V light is selective excitation by choice of irradiating wavelengths.This chapter gives a comparison of the inherent reactivities of a series of crystalline diethynylbenzene derivatives in solid-state polymerization, which is initiated by radicals generated upon irradiation with y rays. Effects of the molecular packing in the crystals on the reactivities are described. The structures of the monomers are shown in Fig. 6. I , whereNumerous studies have been published on the solid-state polymerization of acetylenic compounds. Most of them examine diacetylenes having a wide variety of substituents.2' The polymeri7ation of the diacetylenes has been interpreted by a typical lattice-controlled mechanism. Crystalline 1,4-diethynylnaphthalene and its analogs including 1,4diethynylbenzene (1) are also known to polymerize when irradiated with UV light.') One of the ethynyl groups participates in the polymeriiation resulting in conjugated polyene backbones having ethynyl groups as pendants. 1,4-Diethynylbeni-ene is regarded as the simplest acetylenic compound that is crystalline at room temperature. Various crystalline diethynylbenzene derivatives, which are dimethyl-, dimethoxy-, or methylmethoxysubstituted bis(3-acetoxy-1 -propynyl)benienes (2-8), were synthesi~ed by procedures in the l i t e r a t~r e .~) The monomer crystals were sealed in Pyrex tubes under vacuum and irradiated with y rays from a h°Co source at room temperature. The product formation in the solid-state polymerization was correlated to the molecular packing in the crystals. When 0.5 M benzene solutions of the monomers were irradiated, there was no difference in the yields and the molecular weight distributions of the products among the monomers. R = CH20COCH1.
Owing to the presence of low lying s* orbitals, sulfonium ions are inherently Lewis acidic and can interact with electron-rich substrates to form donor-acceptor complexes. Although this phenomenon has been documented, [1] efforts to use sulfonium ions as a binding site in Lewis acidic hosts have not been reported. As part of our fundamental interest in the chemistry of polydentate Lewis acidic boranes, [2] we have become interested in probing the synthesis and properties of anion receptors containing accessible sulfonium ions. As an added motivation for these studies, we anticipated that the anion binding properties of sulfonium boranes would also benefit from attractive Coulombic effects similar to those occurring in other cationic boron-based anion receptors. [3,4] To test the validity of the aforementioned concepts, we synthesized the cationic boranes [1] + and [2] + which feature adjacent sulfonium and boryl moieties connected by a 1,8-naphthalenediyl or o-phenylene linker, respectively (Scheme 1). The salt [1]OTf was obtained by reaction of the tetrakis(THF)lithium salt of dimesityl-1,8-naphthalenediylborate [5] with dimethyldisulfide and subsequent methylation of the resulting sulfide with MeOTf (Scheme 1). The salt [2]OTf could also be conveniently prepared in two steps by reaction of o-lithiothioanisole [6] with dimesitylboron fluoride and subsequent methylation with MeOTf.Both [1]OTf and [2]OTf have been isolated in an analytically pure form and characterized by multinuclear NMR spectroscopy, UV/Vis spectroscopy, and single-crystal X-ray diffraction. The detection of a 11 B NMR resonance near d = 70 ppm (d = 67 ppm for [1] + or d = 77 ppm for [2] + ) and the presence of a low energy UV/Vis absorption band at 340 nm in MeOH for both [1] + (e = 16 350 m À1 cm À1 ) and [2] + (e = 9300 m À1 cm À1 ) indicate the presence of a coordinatively unsaturated boron center, which mediates p conjugation of the aromatic ligands. [7] The resulting boron-centered chromophores are fluorescent and give rise to a broad emission band at 464 nm for [1] + (f = 0.02) and 450 nm for [2] + (f = 0.12) when excited at 350 and 340 nm in MeOH, respectively. As reported for other sulfonium salts, [8] [1] + and [2] + are sensitive to UV light and should therefore not be irradiated for extended periods of time. The crystal structures of these salts clearly show that: 1) the boron center adopts a trigonalplanar coordination geometry (S (C-B-C) = 359.68 for [1] + and 360.08 for [2] + ); 2) that the boron-sulfur separation in [1] + (3.07 ) is slightly shorter than in [2] + (3.12 ) (Figure 1). [9] Despite the similarity of this boron-sulfur separation, the two cationic boranes differ by the respective disposition of the boryl and sulfonio moieties which are oriented in a more convergent fashion in [1] + . A natural bond orbital (NBO) analysis carried out at the density functional theory (DFT) optimized geometry of [1] + indicates the presence of a lp(S)! p(B) donor-acceptor interaction (Figure 1) whose deletion leads to an increase of the ...
Owing to the presence of low lying s* orbitals, sulfonium ions are inherently Lewis acidic and can interact with electron-rich substrates to form donor-acceptor complexes. Although this phenomenon has been documented, [1] efforts to use sulfonium ions as a binding site in Lewis acidic hosts have not been reported. As part of our fundamental interest in the chemistry of polydentate Lewis acidic boranes, [2] we have become interested in probing the synthesis and properties of anion receptors containing accessible sulfonium ions. As an added motivation for these studies, we anticipated that the anion binding properties of sulfonium boranes would also benefit from attractive Coulombic effects similar to those occurring in other cationic boron-based anion receptors. [3,4] To test the validity of the aforementioned concepts, we synthesized the cationic boranes (Figure 1).[9]Despite the similarity of this boron-sulfur separation, the two cationic boranes differ by the respective disposition of the boryl and sulfonio moieties which are oriented in a more convergent fashion in [1] + . A natural bond orbital (NBO) analysis carried out at the density functional theory (DFT) optimized geometry of [1] + indicates the presence of a lp(S)! p(B) donor-acceptor interaction (Figure 1) whose deletion leads to an increase of the total energy of the molecule by E del = 6.8 kcal mol À1 . Therefore, in addition to being more sterically crowded, the boron center of [1] + experiences electron donation from the neighboring sulfur atom, a phenomenon which would be expected to reduce its electron deficiency. Analogous computations on [2] + show that this lp(S)!p(B) interaction (Figure 1), although present, is much weaker (E del = 2.3 kcal mol À1 ), which is in agreement with the more divergent orientation of the lp(S) and p(B) orbitals. Altogether, this analysis shows that [2] + features a more accessible and electron-deficient boron atom.Next, we decided to compare the Lewis acidity of these boranes by studying their behavior in aqueous solution as a function of pH. Since hydroxide binding to the boron center is expected to interrupt the p conjugation mediated by the vacant p-orbital on the boron atom, [10] we monitored the absorbance of the boron-centered chromophore as a function of pH in MeOH/H 2 O (5:95 v/v; see the Supporting Information) and observed that [1] + and [2] + are stable up to pH 9.5 and 7.0, respectively. These experiments indicate that [2] + is more acidic than [1] + by at least two orders of magnitudes.
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