Construction of a unique structure of poly (ladder alkanes) from Gemini monomers

Ladderanes are a kind of trapezoidal hydrocarbons containing two or more fused Cyclobutanes These trapezoidal molecules and their derivatives have high ring tension and unique electronic properties, which have the potential to be developed into spacer, rigid rod and photoelectric devices In the natural biological system, there is also the presence of tyrane Scientists found that the membrane lipid of anammox contains trapezoidal structure, which is called trapezoidal membrane lipid Although the mechanism of biosynthesis of this structure is not clear, it still attracts a lot of researchers to explore Fig 1 Source of molecular structure of titane: the energy barrier and ring tension of angel W chem Int ed titane are very high, which makes its synthesis challenging There are many ways to construct the structure of titane, such as step-by-step addition, [2 + 2] halogenation addition and oligomerization However, it is not clear how to construct the polymer with the structure of ladder in the molecular skeleton Recently, Qian Li R Chu and his team from the University of North Dakota in the United States realized the first synthesis of poly (ladder alkanes) through the [2 + 2] Photocyclization addition reaction of conjugated dienes on the solid phase (angel Chem Int ed doi: 10.1002 / anie 201705937) Fig 2 The author of the paper, from left to right, from top to bottom: zhihanwang, Benjamin Miller, Jonathan Butz, Katelyn Randazzo, Zijun D Wang, and Qianli R Chu * source: the main challenge faced by the solid-phase reaction of University of North Dakota is how to organize the double bond in the desired reaction direction For the [2 + 2] Photocyclization addition reaction, the double bonds involved in the reaction must be aligned in parallel, and the distance between them should not exceed 4.2 μ In order to solve these problems, the author proposed a monomer design strategy: one end of the conjugated olefin is connected with an electron withdrawing group, such as ester carbonyl, and the other end is connected with an electron supplying group, such as methyl or aromatic ring The conjugated alkenes are arranged in a "head and tail" mode of electron complementary under the electrostatic action of two end groups, and enter a lattice (Fig 3a) When two conjugated alkenes are connected to a non reactive covalent spacer, the self-assembly of the Gemini monomers is arranged in a "head and tail" manner in the solid phase After [2 + 2] Photocyclization addition reaction, the target poly (t-alkane) is obtained (Fig 3b) Fig 3 Source of synthesis strategy: the synthesis path of angelw Chem Int ed is shown in FIG 4A Monomer I is prepared by esterification of two molecules of sorbic acid and one molecule of 1,4-butanediol In order to study the polymerization process, the single crystal of monomer I was obtained by recrystallization Single crystal structure analysis shows that the 1,3-diene in the monomer is arranged in parallel in the way of "head and tail" as envisaged (Fig 2b-d) The distance between 1,3-diene and 1,3-diene is about 3.80 μ m, which can be used for solid-phase photoreaction Fig 4 Synthesis of p-styrane IP and X-ray diffraction analysis of monomer I source: angelw Chem Int ed obtained the target monomer I, the author led the cyclization reaction with ultraviolet light, and monitored the reaction with FT-IR FT-IR spectrum showed that after 24 hours of reaction, the fingerprint area formed a broad peak, which means the completion of the light reaction and the formation of polymer The characteristic peak of C = O stretching vibration shifts from 1704cm-1 to 1723cm-1, which can be considered as the result of carbonyl de conjugation The disappearance of two C = C stretching vibration absorption peaks (164014 cm - 1) coincides with the completion of the [2 + 2] Photocyclization addition reaction The disappearance of the distorted C-H single bond absorption peak 1003cm-1 in the trans ch = ch unit can also confirm the completion of photopolymerization The final product of the polymer is soluble in common organic solvents, such as CH 3CL, MeOH, DMSO, CH 3CN, acetone and toluene Fig 5 The monitoring source of FT-IR spectrum for [2 + 2] Photocyclization addition reaction: angelw Chem Int ed monomer I has a relatively low melting point (69-70 ℃), but its reaction activity is very high at room temperature, which makes it difficult to realize the transformation from single crystal to single crystal (SCSC), and thus the direct method to determine the structure of poly (ladder alkane) is lost In order to solve this problem, the author introduced furan ring into the monomer to stabilize 1,3-diene and increase the melting point of the monomer (MP: 114-115 ℃) The synthesis method of monomer II is similar to that of I Single crystal X-ray diffraction analysis shows that the 1,3-diene part of monomer II is arranged in parallel with a distance of 3.54 Å (Fig 6D), while the distance of only two parallel double bonds on the chain is 3.84 Å (Fig 6e) The double bond distance of the two arrangements is within the required range of light reaction (< 4.2 μ), but the distance of Fig 6D is smaller In addition, the π orbital overlap of the adjacent C = C double bond in Fig 6D is higher The orbital overlap is very important for the position limited solid-state reaction, and the smallest atom movement can ensure the normal reaction Therefore, the photoreaction of monomer II is more likely to take place in the arrangement of Fig 6D, and the reaction results in a crystalline poly (stilane IIP) with three fused cyclobutane structures The Photocyclization of monomer II was also monitored by FT-IR After 36 hours of UV irradiation, the reaction was completed, and a part of the polymerized intermediate was obtained Then the single crystal to single crystal (SCSC) conversion analysis was carried out The results showed that 10% of the partially polymerized 1,3-diene IIA was converted into CIS, anti, cis - [3] - triane (Fig 6f-h) Through the structure and semi empirical calculation method of intermediate IIA, the author gives the best structure of poly (titane IIP) (Fig 6I), where the bond length of titane is consistent with the literature data Fig 6 Structure sources of monomer II, intermediate IIA and polymer IIP: other intermediates obtained in the solid-phase photo reaction of angelw Chem Int ed can also provide evidence for the structure visualization of poly (TNA) At the beginning of the reaction, dimer, trimer and tetramer were observed by MALDI analysis, and a small amount of dimer intermediate IIB was successfully separated by column chromatography Then, the author used HRMS, 1H and 13C NMR spectra (including cosy and Dept spectra) to characterize the intermediate IIB, and proved the CIS, anti, cis - [3] - decane structure Fig 7 Structural characterization source of IIB: angelw Chem Int ed finally, the author has carried out powder XRD test on monomers I and II, and the results show that these powders are actually microcrystalline This means that powder like monomers can also be used to generate poly (ladder alkanes), and the recrystallization step can be skipped Conclusion: a new polymer, poly (ladder alkane), was synthesized by solid-phase photo reaction with specially designed Gemini monomers The reaction conditions were mild and the starting materials were cheap The single crystal X-ray diffraction analysis and the structure characterization of the dimer show that the addition process of [2 + 2] Photocyclization and the formation of poly (TNA) The successful synthesis of poly (terylene) not only opens up the way to study its properties in the future, but also provides the possibility for its application in various fields.