Transition Metal-Catalyzed Organometallic Reactions that Have Revolutionized Organic Synthesis

Abstract

Abstract The Pd- or Ni-catalyzed cross-coupling reactions of organometals containing Zn, Al, Zr, and B as well as related reactions of Mg and several other metals collectively represent the most widely applicable organic skeleton construction method discovered and developed over the past several decades, allowing the synthetic chemists to synthesize practically all types of organic compounds. Some of the seminal and critically important discoveries and early developments in the 1970s as well as their current scope (Tables 2 and 3) are briefly discussed. Some of the notable discoveries and developments include (1) identification of superior properties of Pd relative to Ni, (2) the broad scope of Pd- or Ni-catalyzed cross-coupling with respect to metal counter cations including Zn, Al, Zr, B, and Mg, (3) the hydrometallation–Pd-catalyzed cross-coupling tandem processes for selective syntheses of alkenes, dienes, oligoenes, and oligoenynes, (4) double metal catalysis involving Pd or Ni and added metal compounds containing Zn, In, Li, and others, and (5) realization of high turnover numbers (≥103–105) through the use of chelating phosphines, such as DPEphos and dppf. In these reactions, the metal counter cations in organometals and Pd or Ni are to work successively via transmetallation. The Zr-catalyzed alkyne carboalumination and the Zr-catalyzed asymmetric carboalumination of alkenes (ZACA reaction) have provided efficient and selective routes to methyl-branched (E)-trisubstituted alkenylalanes and 2-substituted chiral alkylalanes, respectively. These reactions provide two additional examples of prototypical transition metal-catalyzed organometallic reactions. Significantly, they can be readily combined with the Pd- or Ni-catalyzed cross-coupling for the synthesis of trisubstituted alkenes embracing a wide variety of natural products, such as terpenoids, carotenoids, and others, as well as various chiral organics including deoxypolypropionates and saturated terpenoids. The Zr-catalyzed alkyne carboalumination has been applied to the synthesis of well over 100 complex natural products (Table 4), while the ZACA reaction has been transformed from a mere scientific novelty to a full-fledged asymmetric synthetic method that is catalytic in both transition metal (Zr) and chiral auxiliaries through a series of breakthroughs (Schemes 12–17).