Synthesis and Characterization of Cyanobutadiene Isomers—Molecules of Astrochemical Significance

Abstract

Four cyanobutadiene isomers of considerable interest to the organic chemistry, molecular spectroscopy, and astrochemistry communities were synthesized in good yields and isolated as pure compounds: (E)-1-cyano-1,3-butadiene ( E-1), (Z)-1-cyano-1,3-butadiene ( Z-1), 4-cyano-1,2-butadiene (2), and 2-cyano-1,3-butadiene (3). A diastereoselective synthesis was developed to generate (E)-1-cyano-1,3-butadiene (1) (10:1 E/Z) via tandem SN2 and E2′ reactions. The potential energy surfaces of the E2′ reactions leading to (E)- and (Z)-1-cyano-1,3-butadiene (1) were analyzed by density functional theory calculations, and the observed diastereoselectivity was rationalized in the context of the Curtin–Hammett principle. The preparation of pure samples of these reactive compounds enables measurement of their laboratory rotational spectra, which are the critical data needed to search for these species in space by radioastronomy.

Synthesis, Purification, and Rotational Spectroscopy of (Cyanomethylene)Cyclopropane—An Isomer of Pyridine

“…This molecule has been generated only twice previously, 19,20 and the characterization data are quite limited compared to other simple organic nitriles. As shown in Scheme 1a, Wilson first generated 5 in 1945 by pyrolysis of 2-cyanotetrahydrofuran (7) at 500−600 °C on silica with coproduction of tetrahydrofuran and dihydrofuran. 26 While effective at generating the desired compound, it is not an ideal synthetic approach for studying its rotational spectrum due to the generation of volatile byproducts with permanent dipole moments as sample contaminants.…”

“…Organic nitriles occupy a significant place in the field of astrochemistry and are well represented in the list of ∼220 species detected in space. , Their large molecular dipole moments, due to the cyano moiety, result in intense rotational transitions that facilitate identification of these species by radioastronomy. We have been keenly interested in measuring spectroscopic constants of organic nitriles in two groups: aryl nitriles/isonitriles − and nitrile-containing isomers of pyridine. − The former group was inspired by the potential of these aryl species to serve as tracers for their parent compounds ( e . g ., benzene, pyridine, pyridazine, pyrimidine, and pyrazine).…”

Rotational Spectra of Three Cyanobutadiene Isomers (C5H5N) of Relevance to Astrochemistry and Other Harsh Reaction Environments

“…The cyanobutadiene isomers were synthesized in our lab as described previously . Spectra were collected on a broadband spectrometer using 3–13 mTorr sample pressures in a flow system at 0 to −50 °C.…”

“…Liquid samples of all three cyanobutadiene isomers were kept in ice–water baths to minimize sample degradation. The (high) purity of each sample was established on the basis of 1 H NMR and mass spectra . No impurities were evident in the rotational spectra.…”

“…Herein, we present a detailed analysis of the rotational spectra of the ground vibrational states of E -1-cyano-1,3-butadiene, Z -1-cyano-1,3-butadiene, and 4-cyano-1,2-butadiene in the 130–375 GHz frequency range. These experimental studies are predicated on the availability of pure samples of the neat materials, which is not an inconsequential matter . These laboratory measurements provide the foundation for spectroscopic studies of astronomical and other harsh reaction environments.…”