Discovery of ZACA reaction: Zr-catalyzed asymmetric carboalumination of Alkenes

Abstract

A single-stage, i.e., degree of polymerization of one, and enantioselective version of the Ziegler-Natta alkene polymerization was envisioned as a potentially significant and useful method for catalytic asymmetric C-C bond formation, shortly after the corresponding alkyne version involving Zr-catalyzed alkylalumination of alkynes was discovered in 1978. However, the discovery of such an asymmetric reaction proved to be a major challenge. At least three widely observable unwanted side reactions, i.e., (1) cyclic carbometalation, (2) β-H transfer hydrometalation, and (3) alkene polymerization, represented by the Ziegler-Natta polymerization, were noted and were to be avoided. With Zr as the metal at the catalytic center, we eventually came up with a notion that di- or multiple alkylation of Zr was to be avoided for achieving superior acyclic asymmetric carbometalation. This, in turn, led us to avoid the use of highly nucleophilic alkylmetals containing alkali metals and Mg. Aluminum used in Ziegler-Natta polymerization that can selectively monoalkylate Zr proved to be one of a very limited number of favorable metals. Even so, undesirable cyclic carbozirconation can occur in nonpolar solvents via intricate bimetallic routes to cyclic organozirconium species. The vastly different reactant-catalyst stoichiometry and the absence of polymerization promotors, such as methylaluminoxane, in our investigations permit us to ignore the Ziegler-Natta polymerization as a serious and unwanted side reaction, although use of limited amounts of promotors can in some cases accelerate otherwise sluggish ZACA reactions. Selection of the currently used Zr catalysts including (R)- and (S)-(NMI)2)ZrCl)2), dichlorobis(neomenthylindenyl) zirconium was made by screening 15 or so known chiral zirconocene derivatives. Systematic scientific design and screening of chiral ligands are clearly highly desirable, and efforts along this line are ongoing. Even at the current level of development, however, the ZACA reaction shows considerable promise as an efficient and selective method for catalytic asymmetric C-C bond formation, which has already been used to significantly modernize and improve syntheses of natural products including deoxypolypropionates and isoprenoids of biological and medicinal interest. © ARKAT-USA, Inc.