Intramolecular transfer of t-Bu 3 P-ligated Pd catalyst on a carbon-carbon triple bond (C≡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-Bu 3 P-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≡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.
We report Suzuki-Miyaura coupling polymerization of tetraalkoxy-substituted 4-bromostilbene-4 0 -boronic acid 1 with several t-Bu 3 P-ligated Pd initiators; this is the first example of catalyst-transfer condensation polymerization (CTCP) of a monomer containing a carbon-carbon double bond. When o-tolylPd( t Bu 3 P)Br was used as the initiator, the o-tolyl group was not introduced at the polymer end, but polymer with boronic acid at one end and bromine at the other was obtained. However, when we employed stilbenePd( t Bu 3 P)I generated in situ from iodostilbene and Pd( t Bu 3 P)G2 precatalyst, or isolated ArPd( t Bu 3 P)X (Ar, X 5 Ph, I; o-tolyl, I; and Ph, Br), the aryl group was introduced at the polymer end, indicating that CTCP of 1 proceeded. Therefore, the iodide and aryl group of the Pd initiator complex is crucial for CTCP of 1. However, the molecular weight distribution of the obtained polymer was broad, possibly because coordination of the carbon-carbon double bond of 1 to ArPd( t Bu 3 P)I resulted in slow initiation. V C double bond of 1 to ArPd( t Bu 3 P)I resulted in slow initiation.
Intramolecular catalyst transfer on benzoheterodiazoles was investigated in Suzuki–Miyaura coupling reactions and polymerization reactions with tBu3PPd precatalyst. In the coupling reactions of dibromobenzotriazole, dibromobenzoxazole, and dibromobenzothiadiazole with pinacol phenylboronate, the product ratios of monosubstituted product to disubstituted product were 0/100, 27/73, and 89/11, respectively, indicating that the Pd catalyst undergoes intramolecular catalyst transfer on dibromobenzotriazole, whereas intermolecular transfer occurs in part in the case of dibromobenzoxazole and is predominant for dibromobenzothiadiazole. The polycondensation of 1.3 equivalents of dibromobenzotriazole with 1.0 equivalent of para‐ and meta‐phenylenediboronates afforded high‐molecular‐weight polymer and cyclic polymer, respectively. In the case of dibromobenzoxazole, however, para‐ and meta‐phenylenediboronates afforded moderate‐molecular‐weight polymer with bromine at both ends and cyclic polymer, respectively. In the case of dibromobenzothiadiazole, they afforded low‐molecular‐weight polymers with bromine at both ends. Addition of benzothiadiazole derivatives interfered with catalyst transfer in the coupling reactions.
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