tailieunhanh - Ebook Advanced organic chemistry (Part A: Reactions and synthesis - 5th edition): Part 2

(BQ) Part 2 book "Advanced organic chemistry (Part A: Reactions and synthesis)" has contents: Reactions involving transition metals; reactions involving carbocations, carbenes, and radicals as reactive intermediates; aromatic substitution reactions; multistep syntheses,.and other contents. | 8 Reactions Involving Transition Metals Introduction In this chapter we discuss important synthetic reactions that involve transition metal compounds and intermediates. Reactions involving copper and palladium, the transition metals that have the widest applications in synthesis, are discussed in the first two sections. In the third section, we consider several other transition metals, including nickel, rhodium, and cobalt. In contrast to lithium, magnesium, and zinc, where the organometallic reagents are used in stoichiometric quantities, many of the transition metal reactions are catalytic processes. The mechanisms are described in terms of catalytic cycles that show the role of the catalytic species in the reaction and its regeneration. Another distinguishing feature of transition metal reactions is that they frequently involve changes in oxidation state at the metal atom. In the final two sections we deal with transition metal–catalyzed alkene exchange (metathesis) reactions and organometallic compounds that feature bonding of the organic component. . Organocopper Intermediates . Preparation and Structure of Organocopper Reagents The synthetic application of organocopper compounds received a major impetus from the study of the catalytic effect of copper salts on reactions of Grignard reagents with -unsaturated Although Grignard reagents normally add to such compounds to give the 1,2-addition product, the presence of catalytic amounts of Cu(I) results in conjugate addition. Mechanistic study pointed to a very fast reaction by an organocopper intermediate. 1 H. O. House, W. L. Respess, and G. M. Whitesides, J. Org. Chem., 31, 3128 (1966). 675 676 H2O CH3MgBr CHAPTER 8 H+ CH3CH CHC(CH3)2 OH O Reactions Involving Transition Metals CH3CH CHCCH3 Cul, CH3MgBr H2O (CH ) CHCH CCH 3 2 2 3 H+ O Subsequently, much of the development of organocopper chemistry focused on stoichiometric reagents prepared from organolithium compounds. Several types of .