Intramolecular Transfer of Pd Catalyst on Carbon-Carbon Triple Bond and Nitrogen-Nitrogen Double Bond in Suzuki-Miyaura Coupling Reaction

Intramolecular transfer of t-Bu3P-ligated Pd catalyst on a carbon-carbon triple bond (C equivalent to C) and nitrogen-nitrogen double bond (N=N) was investigated and compared with the case of a carbon-carbon double bond (C=C), which is resistant to intramolecular transfer of the Pd catalyst. Suzuki-Miyaura coupling reaction of equimolar 4,4'-dibromotolan (1a) or 4,4'-dibromoazobenzene (1b) with 3-isobutoxyphenylboronic acid (2) was carried out in the presence of t-Bu3P-ligated Pd precatalyst 3 and KOH/18-crown-6 as a base at room temperature. In both cases, the diphenyl-substituted product was selectively obtained, indicating that the Pd catalyst walked from one benzene ring to the other through the C equivalent to C or N=N bond after the first substitution with 2. Taking advantage of this finding, we conducted unstoichiometric Suzuki-Miyaura polycondensation of 1.3 equiv. of 1 and 1.0 equiv. of phenylenediboronic acid (ester) 6 in the presence of 3 and CsF/18-crown-6 as a base, obtaining high-molecular-weight conjugated polymer with a boronic acid (ester) moiety at both ends, contrary to the Flory principle.