Ru(0)-catalysed synthesis of borylated polyene building blocks by cross-dimerisation toward cross-coupling.

Conjugated and non-conjugated polyenes are important substructures and are often found in biologically active compounds and natural products. Their preparation often needs multiple steps or iterative reactions and as a result, they have poor step economies. In this feature article, we show a new methodology to prepare these substructures by combinations of cross-dimerisation giving borylated polyenes and subsequent cross-coupling reactions. This divergent reaction strategy allows for the opportunity to access many bioactive compounds and natural products as well as some electronic materials.

Direct synthesis of conjugated tetraenes from 1,3-enynes with 1,3-dienes.

New direct access to conjugated tetraenes has been achieved. A Ru(0)-catalysed reaction of 1,3-enynes with 1,3-dienes gives 1,3,5,7-octatetraene derivatives by formal regioselective insertion of the alkynyl group of 1,3-enynes into the terminal C-H bond in 1,3-dienes. With a silyl substituent on the alkynyl side in 1,3-enynes, the reaction regioselectively proceeds to give the linear cross-dimerisation product having the silyl group at the internal position. Stoichiometric and DFT calculations support the oxidative coupling mechanism for the linear cross-dimerisation. Methyl (2E,4E,6E,8E)-10-hydroxy-2,4,6,8-decatetraenoate, a versatile polyene intermediate, is accessed by this method as a formal synthesis of biologically active compounds.

Halogen-, oxidant- and directing group-free synthesis of donor-acceptor type conjugated polymers.

An intermolecular hydrogen migration from dibutadienylpyridine to dialkynylthiophene catalysed by [Ru(η6-naphthalene)(η4-oxa-bnd)] (oxa-bnd = 9-oxabicyclo[3.3.1]nona-2,6-diene) produces poly(dialkynylthiophene-alt-dibutadienylpyridine). The exact alternating copolymerisation nature is shown by MALDI-TOF MS.

Ru(0)-catalysed cross-dimerisation and -trimerisation of alkynyl- with butadienylheteroarenes.

Ru(0)-catalysed cross-dimerisation and -trimerisation give a series of di- and triheteroaryl compounds cross-linked by π-conjugated trienyl groups. Their photochemical behaviour is studied using UV-visible absorption spectra, fluorescence emission spectra, and TD-DFT calculations. The cross-trimer prepared from 2,5-dialkynylthiophene with 2 equiv. of 2-butadienylpyridine shows a longer wavelength shift in the absorption maximum than the cross-trimer prepared from dialkynylbenzene with 1-phenylbutadiene. The solvent effect and the TD-DFT calculations suggest that the planarity of the π-conjugated system contributes more than spontaneous polarization. Namely, in the 5-membered thiophene ring, the conjugated trienyl group extends in the same plane (dihedral angle: -4.0°) as the thienyl group, whereas in the 6-membered benzene ring, the planarity is reduced due to steric hindrance (dihedral angle: -24.1°). Thus, the cross-trimers with a 5-membered heteroaryl centre contribute to longer wavelengths of absorption and fluorescence emission due to the increased planarity of the conjugated trienyl groups.

Cobalt-Catalyzed Divergent Cycloadditions of Alkynes with Conjugated Dienes Yielding 3-Vinylcyclobutenes, Bicyclo[3.1.0]hexenes, and Cyclohexa-1,4-dienes.

A traditional cobalt catalyst system still contains undiscovered reactivity. Depending on the tertiary phosphines and substrates used, the catalytic system using CoBr2/tertiary phosphine/Zn/ZnI2 catalyzes divergent cycloadditions of internal alkynes with conjugated dienes, yielding 3-alkenylcyclobut-1-enes, bicyclo[3.1.0]hexenes, and cyclohexa-1,4-dienes. A [CoBr2(PPh3)2]/Zn/ZnI2-catalyzed reaction of 3-hexyne (1a) with 1-(4-methoxyphenyl)butadiene (2a) at room temperature in CH2Cl2 exclusively produces a [2 + 2] cycloaddition product (E)-2-(2,3-diethylcyclobut-2-ene-1-yl)vinyl-4-methoxybenzene (3aa). When [CoBr2(dppp)]/Zn/ZnI2 is used as a catalyst, a bicyclic compound 6-(4-methoxyphenyl)-2,3-diethylbicyclo[3.1.0]hex-2-ene (4aa) is dominantly formed in a 77% yield. The CoBr2/dppe/Zn/ZnI2 system can undergo a [2 + 4] cycloaddition to yield 3-(4-anisyl)-1,2-diethylcyclohexa-1,4-diene (5aa) as the dominant product in 38% yield. The bite angles of the ligands used contribute significantly to this catalytic diversity.

Ru(0)-Catalyzed Synthesis of Conjugated Iminotrienes and Subsequent Intramolecular Cyclization Giving Polysubstituted Pyrroles.

A reliable method for preparing polysubstituted pyrroles from conjugated iminohexatrienes has been discovered. Ru(0)-catalyzed cross-dimerization of iminoalkyne with conjugated dienes provides a series of conjugated iminohexatrienes. Subsequent treatment with a catalytic amount of acetic acid (7 mol %) leads to an unexpected cyclization yielding 2-alkenylpyrroles. The overall reaction can be considered as a formal (4 + 1) annulation that involves the formation of a conjugated iminohexatriene followed by an intramolecular aza-Michael-type 5-exo-trig cyclization and subsequent proton migration.

Catalytic cross-dimerisation giving reactive borylated polyenes toward cross-coupling.

A series of borylated conjugated trienes and skipped dienes is prepared by Ru-catalysed cross-dimerisation using alkynyl-, dienyl-, and vinyl boronates. These products are used as synthetic building blocks for polyene substructures by subsequent Pd-catalysed cross-coupling in a one-pot vessel without deprotection.