Palladium-Catalyzed Alkylation of Aryl C−H Bonds with sp³ Organotin Reagents Using Benzoquinone as a Crucial Promoter

Abstract

The combination of directed C-H activation, batch-wise addition of tetraalkyltin reagents, and rate enhancement by benzoquinone and microwave irradiation provides a promising strategy for the direct coupling of C-H bonds with organometallic reagents. A variety of tetraalkyltins were coupled to C-H bonds to give the alkylated products in good yields by using 5 mol % Pd(OAc)2 as the catalyst. Benzoquinone was shown to be essential for the C-H activation when substrates containing non-pi-conjugated chelating groups are used. Monitoring the formation and reductive elimination of the Pd(Ar)(Me)L2 complex also revealed that benzoquinone promotes the reductive elimination step. Microwave irradiation enhances the reaction rate drastically. The versatility of this protocol was demonstrated by using substrates containing either oxazoline or pyridine as directing groups.

Palladium(II)‐katalysierte C‐H‐Aktivierung/C‐C‐Kreuzkupplung: Vielseitigkeit und Anwendbarkeit

“…[58] Die in unserer Arbeitsgruppe entwickelten Pd [59] Ein kurzer Vergleich der vorgeschlagenen C-H-Aktivierung/C-C-Kupplung mit den Pd 0 -katalysierten Kreuzkupplungen [60] [61] Es war bekannt, dass Benzochinon ein Oxidationsmittel für Pd 0 ist und die C-C-Bindungsbildung in verschiedenen Pdkatalysierten Reaktionen begünstigt. [62] Unsere Untersuchungen zur Bildung von cyclopalladierten Intermediaten und ihrer Folgereaktion mit Organozinnreagentien ließen jedoch eine zusätzliche Rolle für Benzochinon erkennen: die Begünstigung der C-H-Aktivierung.…”

Aliphatic and aromatic C–H activation of benzo[h]quinolines by Rh(I). Unique precursor dependent formation of mono-, di- and trinuclear complexes

“…The cyclometalated Pd dimer 2 was synthesized according to an improved procedure reported by Sanford and co-workers [16]. Complex 2 was treated with one equiv of Me 4 Sn and one equiv of p-benzoquinone, in analogy to a reported catalytic procedure for alkylation of aryl C-H bonds of oxazolinetype substrates [17] resulting in 89.6% yield of 3. 1 The same reaction in the absence of p-benzoquinone resulted in lower yield of 3.…”

CH Bond Alkylation (Including Hydroarylation of Alkenes)

“…This field, which has been far less explored than the parent C-H bond arylation, was pioneered by Yu and coworkers [59], who reported the ortho alkylation of arenes bearing an oxazoline directing group with organotin reagents (Scheme 19.37). The reaction proved applicable to different oxazolinecontaining systems and tetraalkyltin reagents.…”