Seong Jib Choi

Direct arylation polycondensation for the synthesis of bithiophene-based alternating copolymers

Direct arylation polycondensation reactions using a simple catalytic system gave eight kinds of bithiophene-based alternating copolymers. The conditions for the reactions of 3,3′,4,4′-tetramethylbithiophene with dibromoarylenes were optimized to obtain high-molecular-weight polymers without formation of cross-linked structures. In the reaction of a dibromoarylene containing a reactive C–H bond, a short reaction time (1.5 h) was suitable for preventing side reactions. In contrast, a long reaction time (6 h) gave high-molecular-weight polymers from dibromoarylene monomers without a reactive C–H bond. This polycondensation reaction enables the synthesis of polymers containing dye structures such as diketopyrrolopyrrole and isoindigo, which are applicable as materials for polymer solar cells.

Electrically conductive hydrogen-bond-based supramolecular polymer with a tetrathiafulvalene moiety: modulation of electrical conductivity and flexibility of film by external stimulus.

Nature uses weak intermolecular forces to form a self-assembled structure. Since the assembled architecture shows special biological and physical properties, many researchers have endeavored to mimic the system by constructing supramolecules with weak and noncovalent interactions. For example, hydrogen bonding, van der Waals forces, coordination bonds, and p–p stacking interactions have been used in supramolecular chemistry. In particular, hydrogen bonding is recognized as a useful tool in constructing a desired supramolecular motif owing to its directional property. Small molecules with multiple hydrogen bonds are assembled to form a supramolecular polymer which has similar properties to a conventional polymer in terms of rheology. The additional interaction to a hydrogen-bond-based supramolecular polymer gives rise to new chemical properties. In well-designed supramolecules, external stimuli change the strength of the interactions, resulting in a change in the shape of the material. For the construction of a new switchable material, the present work focuses on the hydrogen-bond-based supramolecular polymers with a p–p stacking interaction, the strength of which can be controlled by an external stimulus. A guanosine derivative is a good candidate for forming a supramolecular polymer because a guanosine unit possesses multiple hydrogen bonds that form a flexible and robust film. Since tetrathiafulvalene (TTF) has a stronger p–p stacking interaction in the oxidized state than that in the neutral state, a guanosine derivative with the TTF moiety is expected to form a supramolecular film with a flexibility that can be modulated by the oxidation of the TTF moiety. In addition, the film is also expected to be electrically conductive because TTF exhibits high electrical conductivity in the oxidized state. Herein, we report the investigation of the guanosine derivatives having a switchable capability in the electrical conductivity and the intermolecular interaction. For the syntheses of the desired guanosine derivatives, a silaketal group was used as the connecting moiety between 2-deoxyguanosine and the TTF moieties (Scheme 1). The products were obtained by the one-pot process through sequential additions of diisopropyldichlorosilane and 2-deoxyguanosine to the TTF derivatives with a primary alcohol moiety. The reaction proceeded smoothly without the need for a protective process on the guanine moiety (see Supporting Information). These structures were confirmed by NMR, ESI-mass, and elemental analyses. The electrochemical properties of the guanosine derivatives were investigated by cyclic voltammetry (CV). The CV of 1–3 exhibits two reversible peaks of the TTF moiety in acetonitrile (Figure S1 in the Supporting Information). The redox potentials of 3 are 0.05 and 0.44 V (vs Ag/Ag) corresponding to the formation of a radical cation and dication species. Since the redox potentials are similar to those of TTF (0.02 and 0.41 V), the alkyl chain and the guanosine moiety have a small effect on the redox processes on the TTF moieties (Table S1 in the Supporting Information). The hydrogen bonding of the guanosine derivatives was confirmed by H NMR spectroscopy. Figure 1 a–c show H NMR spectra of 3 in the concentrations of 7.5, 30, and 60 mm at room temperature in [D8]THF. The signals of NH and NH2 show an upfield shift with decreasing the concentration. The upfield shift of the signals was also observed for the elevated temperature in [D2]tetrachloroethane (Figure 1 d–f). A similar trend was observed in compounds 1 and 2. These results indicate the formation of a hydrogenbond network in the solution state. 2D nuclear Overhauser effect spectroscopy (NOESY) gives detailed information for the hydrogen-bond network. The spectrum exhibits an intermolecular interaction between imino NH and H8 of the gua[a] S. J. Choi, Dr. J. Kuwabara, Prof. T. Kanbara Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), Graduate School of Pure and Applied Sciences University of Tsukuba 1-1-1 Tennodai, Tsukuba 305-8573 (Japan) Fax: (+81) 853-4490 E-mail : kanbara@ims.tsukuba.ac.jp Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201000376.

Direct synthesis of tetraalkoxysilanes from silica and alcohols

A new simple and efficient process for synthesizing tetraalkoxysilanes (TROS) directly from silica and alcohols was developed using molecular sieves as dehydrating agents. Using this method, a variety of TROS (R = ethoxy, n-propoxy, or n-butoxy) were obtained over 70% yields within 6 h. We also employed various natural silica sources in this process for practical applications.

Halogen-Free Synthesis of Carbamates from CO2 and Amines Using Titanium Alkoxides

A direct synthesis of carbamates from amines and carbon dioxide in the presence of Ti(OR)4 (R=nBu (1), Me (2), Et (3), nPr (4)) was investigated. Aniline was reacted with titanium n-butoxide (1) in the presence of carbon dioxide (5 MPa) to give the corresponding n-butyl N-phenylcarbamate (BPC) in nearly quantitative yield (99 %) within 20 min. Furthermore, 1 could be regenerated upon reaction with n-butanol during water removal. The recovered 1 could then be reused in a subsequent reaction.