Mechanistic Aspects of Metal‐Catalyzed C—C and C—X Bond‐Forming Reactions

Abstract

Cross-coupling reactions, such as the Stille reaction of organostannanes [1][2][3] and the Suzuki (or Suzuki-Miyaura) cross-coupling of organoboron compounds [4] have settled amongst the more general and selective palladium-catalyzed cross-coupling reactions [5] (Scheme 1-1). These reactions are closely related to other cross-couplings based on transmetallations of a variety of hard or soft organometallic nucleophiles [6] such as the Hiyama [7,8], Sonogashira [9], Kumada (or Kumada-Corriu), and other related couplings [10][11][12].Coupling reactions are somewhat related to the Heck alkenylation of organic electrophiles [13, 14], which is often referred to in the literature as a coupling process. However, although the first steps in both processes are identical, in the Heck reaction there is no transmetallation step. In the alkenylation reaction, the C-C bond is formed by an insertion process, which is followed by a b-hydride elimination to form the substituted alkene product. Cross-coupling (transmetallationbased) processes are a family of closely related catalytic processes that share most mechanistic aspects, although some differences exist on the activation of the organometallic nucleophile. So far, most of the detailed mechanistic studies have cen-

Effectiveness of the Suzuki−Miyaura Cross-Coupling Reaction for Solid-Phase Peptide Modification

“…In the Suzuki catalytic cycle, a base is used to generate a more reactive borate through coordination of a hydroxide ion with the boron atom that will react with a Pd complex in a transmetalation process . Previous results reported that the purity and selectivity of products obtained from a coupling reaction depend on the base and the catalyst used .…”

“…Furthermore, the base must be chosen according to the solvent used. Strong bases such as NaOH, KOH, or NaOCH 3 perform well in water or THF/H 2 O solvent systems, whereas weaker bases such as K 2 CO 3 , K 3 PO 4 , and Na 2 CO 3 are generally used in DMF …”