Heck Alkynylation (Copper-Free
Sonogashira Coupling)
of Aryl and Heteroaryl Chlorides, Using Pd Complexes of <i>t</i>‑Bu<sub>2</sub>(<i>p</i>‑NMe<sub>2</sub>C<sub>6</sub>H<sub>4</sub>)P: Understanding the Structure–Activity
Relationships and Copper Effects
- Publication date
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Abstract
L<sub>2</sub>Pd(0) and L<sub>2</sub>Pd(II) complexes,
where L= <i>t</i>-Bu<sub>2</sub>(<i>p</i>-NMe<sub>2</sub>C<sub>6</sub>H<sub>4</sub>)P, have been identified as efficient
catalyst
systems for the Heck alkynylation of a variety of aryl bromides (17
examples) and aryl/heteroaryl chlorides (31 examples) with a range
of aryl- and alkyl-acetylenes in excellent yields, under relatively
low Pd loadings. The single-crystal X-ray structure determination
of the presumably active catalytic species, L<sub>2</sub>Pd(0), was
carried out in this study to better understand the superior activity
of the current catalyst system from a structure–activity relationship
point of view. The P–Pd–P bond angle indicates that
the complex is bent (174.7°) in comparison to the perfectly linear
(180.0°) structure of the analogous Pd(<i>t</i>-Bu<sub>3</sub>P)<sub>2</sub>. Preliminary mechanistic studies on the negative
copper effect and substrate effect of aryl acetylenes were conducted
to better understand the cross-coupling pathway of Heck alkynylation
