Bench-and air-stable 1-methoxy-2,2,6,6-tetramethylpiperidine (TEMPO-Me) is relatively unreactive at ambient temperature in the absence of an electrochemical stimulus. In this report, we demonstrate that the one-electron electrochemical oxidation of TEMPO-Me produces a powerful electrophilic methylating agent in situ. Our computational and experimental studies are consistent with methylation proceeding via a SN2 mechanism, with a strength comparable to the trimethyloxonium cation. A protocol is developed for the electrochemical methylation of aromatic acids using TEMPO-Me.
Palladium(II) boronates are recognized as fundamental pre‐transmetalation intermediates in Suzuki–Miyaura cross‐couplings. While these typically transient species have been detected and studied spectroscopically, it is conspicuous that they have never been isolated since this important reaction was discovered over forty years ago. This study reports the synthesis of a family of unprecedented arylpalladium(II) boronates that are, by design, kinetically stable at ambient temperature, both in solution and in the solid state. These properties enabled unambiguous crystallographic confirmation of their structure for the first time and their chemical competence in a Suzuki–Miyaura reaction was demonstrated.
We have explored fundamental Pd-catalyzed C sp 2 − C sp 2 Suzuki−Miyaura cross-couplings of aryl iodides (Ar−I) employing "classical" Pd/PPh 3 catalyst systems. Surprisingly, we observed particularly inefficient couplings of these ostensibly reactive electrophiles in a range of conventional solvent mixtures at lower temperatures (∼50 °C), which was in stark contrast to analogous reactions featuring the equivalent aryl bromides. This feature of well-established Pd/PPh 3 -mediated Suzuki−Miyaura reactions has received scant attention in the literature. Most significantly, our studies suggest that the inefficient coupling of aryl iodides at lower temperatures derives from the unexpectedly poor turnover of the key on-cycle intermediate trans-[Pd(PPh 3 ) 2 (Ar)(I)] (or related Pd II −I species) in the presence of PPh 3 .
This two-part laboratory experiment
is concerned with investigating
key aspects of the Suzuki–Miyaura cross-coupling reaction.
The first part of this experiment utilizes high-throughput experimentation
techniques to reinforce, explore, and extend fundamental concepts
that are presented in the classroom. In contrast, the second part
employs guided-inquiry learning strategies to challenge students to
design, set up, and perform a preparative-scale Suzuki–Miyaura
reaction and suggest workup and isolation protocols. By design, this
experiment, which represents one of the final experiments performed
in the third-year undergraduate chemistry laboratory program, serves
as a capstone learning experience for students.
Covalently tethering a supporting ligand to a boron nucleophile
allowed typically elusive, kinetically unstable Suzuki–Miyaura
(SM) pretransmetalation species to be tamed. This strategy enabled
the synthesis and X-ray crystallographic characterization of an organonickel(II)
boronate pretransmetalation intermediate. Performing reactions in
the presence of carbon monoxide delivered pretransmetalation intermediates
in carbonylative SM reactions–aroylmetal boronates. This enabled
the X-ray crystal structure of an aroylnickel(II) boronate (and an
aroylpalladium(II) boronate) to be secured.
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