Borylation of Aryldiazonium Ions with N-Heterocyclic Carbene−Palladium Catalysts Formed without Added Base

Abstract

[reaction: see text] A highly efficient catalytic borylation process with aryldiazonium ions was developed using a carbene-palladium catalyst formed in situ to give arylpinacolatoborane products. An X-ray structure for the N-heterocyclic carbene-palladium complex, used as the catalyst formed from bis(2,6-diisopropylphenyl)-4,5-dihydroimidazolium chloride, was obtained without added base.

Palladium Complexes of N‐Heterocyclic Carbenes as Catalysts for Cross‐Coupling Reactions—A Synthetic Chemist's Perspective

“…[54,75,76,80] Furthermore, only low to moderate yields were often achieved when Pd-NHC complexes were synthesized under conditions similar to the conditions of the cross-coupling reactions. [50,83,85,86] In addition, coordination of NHCs through the backbone carbon atoms (with formation of "unusual" NHC complexes, Scheme 16) [87] and even incorporation of degradation products from the NHC precursor in the coordination sphere of Pd atom (Scheme 17) [88] were occasionally reported. The yields of the cross-coupling product observed when utilizing catalyst prepared in situ are cumulative of two distinct events: 1) the formation of a certain amount of active catalyst; 2) the intrinsic activity of this catalyst in the reaction of interest.…”

“…[89,114] Similar complexes can be prepared directly from the imidazolium salts, thus by-passing the cumbersome handling of free carbene: Andrus and coworkers prepared the SIPr-PdCl 2 dimer (111) in 37 % yield by simple heating Pd(OAc) 2 and 2.5 equivalents of SIPr·HCl (13) in THF (Scheme 36). [86] Despite the presence of a large excess of the NHC precursor, only a singly ligated Pd-NHC species was obtained. A mechanism for the formation of the complex, involving the attack of Pd(OAc) 2 on the 4,5-dihydroimidazolium cation, followed by ligand exchange with chloride and 1,2-elimination of acetic acid was proposed.…”

“…The actual catalyst was proposed to be [{(SIPr)PdCl 2 } 2 ] (111, Scheme 36). [86] Alkyl catechol [189] and pinacolboranes [191] were also used successfully by Andrus and co-workers for chemoselective cross-coupling with aryl diazonium salts by using the SIPr/Pd(OAc) 2 protocol in the presence of an alkyl bromide (Scheme 62). Especially noteworthy is the coupling of cyclohexylpinacolborane, the only example of the cross-coupling of a secondary alkyl nucleophile (product 259, Scheme 62) with a Pd-NHC catalyst to date.…”

Pd‐Catalyzed Aryl Amination Mediated by Well Defined, N‐Heterocyclic Carbene (NHC)–Pd Precatalysts, PEPPSI

“…This uncertainty surrounding the stoichiometry and composition of the active species is a major impediment to drawing mechanistic interpretation from the results, retarding both the understanding and further development of these processes. [18] Collectively, the aforementioned factors have slowed widespread adoption of NHC-based methodology. For these reasons, we [19] and others [16c, 20] have developed well-defined NHC-palladium precatalysts.…”

Aus der Sicht des Synthetikers: Palladiumkomplexe N‐heterocyclischer Carbene als Katalysatoren für Kreuzkupplungen

“…[54,75,76,80] Zudem sind die Ausbeuten oft nur moderat, wenn die Pd-NHC-Komplexe unter Bedingungen hergestellt werden, die den Bedingungen der Kreuzkupplung entsprechen. [50,83,85,86] Außerdem koordinieren NHCs gelegentlich über ihr Kohlenstoffrückgrat (unter Bildung "ungewöhnlicher" NHC-Komplexe, Schema 16), [87] und auch Abbauprodukte der NHC-Vorstufen können in die Koordinationssphäre des Palladiumatoms eingebunden werden (Schema 17). [88] [54] ¾hnliche Beobachtungen machten Fagnou et al, [89] als sie zu dem zuvor synthetisierten, [90] Der noch größere Komplex [(IAd) 2 Pd] (67) [91] war in der Suzuki-Miyaura-Kreuzkupplung von nichtaktivierten Chlorarenen bei Raumtemperatur hoch aktiv, der in situ hergestellte Katalysator (IAd/Pd 2:1) dagegen kaum.…”