Organoboranes transmetallation

Organoboranes undergo transmetallation. 1-Hexenylboronic acid (438) reacts with methyl acrylate via the transmetallation with Pd(OAc)2, giving methyl 2,4-nonadienoate (439)[399], The ( )-alkenylboranes 440, prepared by the hydroboration of terminal alkynes, are converted into the alkylated ( )-alkenes 441 by treatment with an equivalent amount of Pd(OAc)2 and triethylamine[400]. The ( )-octenylborane 442 reacts with CO in MeOH in the... 

Direct transmetalation of organoboranes to organocopper reagents is not a general reaction. Because of dieir similar bond energies and electronegativities, diis trans-nietalation is linided to die preparation of alkenylcopper and unfiinctionalized... 

Oxidative nickel-catalyzed coupling of aldehydes and alkynes to generate allylic alcohols. Intermolecular and intramolecular examples are both effective, and the transmetalating agent (MR" ) may be an organosilane, organoborane, organozinc, or alkenylzirconium. ... 

Palladium-catalyzed cross-coupling reaction of organoboranes with organic halides, triflates, etc. in the presence of a base (transmetalation is reluctant to occur without the activating effect of a base). For the catalytic cycle, see Kumada coupling on page 345. 

In equation 7.1. a 4-chloropyridine was coupled with diethyl(3-pyridyl)borane.3 The reaction was run in aqueous THF in the presence of potassium carbonate. The role of the base is to facilitate the transmetalation step through the formation of a borate ion, as organoboranes are usually not nucleophilic enough to transfer their organic moiety onto the palladium. An alternate function of the base is to increase the electrophilicity of the palladium through exchange of the halide to carbonate. 

Organoboranes are used for ketone synthesis under basic conditions. The cyclic ketone 482 is prepared from alkenyl iodode 479. Hydroboration of terminal double bond, followed by carbonylation generates 480, and the cyclic ketone 482 is formed by intramolecular transmetallation of 480 to afford 481 [236],... 

Although there are sevaral methods of preparing organoboranes such as by transmetallation, by ligand exchange reaction, and by Friedel-Crafts type reaction, the best and most convenient is the procedure via hydroboration, as mentioned previously. No detailed discussion of the hydroboration is attempted here because many authoritative and comprehensive books are available. 

A brief overview of some of the most important synthetic routes to organoboranes, that is, (1) transmetallation reactions, (2) the boration of unsaturated compounds, and (3) C-H activation wih be given. General aspects of these methodologies will be covered and illustrated with selected recent hterature examples. While a large number of transformations are known that convert one boron compound to another, the diversity of these methods does not allow for fiiU treatment in this context. Detailed information is given in several excellent comprehensive reviews that have been published. ... 

Transmetallation reactions represent an important method for the synthesis of organoboranes. Organometallic... 

As with organoboranes, there are also several examples of transmetallation in which ligand insertion occurs to give stable transition-metal organometallic complexes ... 

Initially, the metalation of (+)-2-carene was carried out with n-butyllithium and potassium /erf-butoxide. The metalation product was treated with trimethyl borate and the intermediate ate complex was hydrolyzed with dilute hydrochloric acid. Although the procedure worked well for the transformation of a- into (3-pinene, only a small amount of (+)-3(10)-carene in a mixture with (+)-2- and (+)-3-carene was obtained. Metalation-transmetalation under these conditions leads to organoborane intermediates which are hydrolyzed without isolation. Consequently, a substantial amount of unreacted (+)-2-ca-rene is present, rendering difficult the isolation of (+)-3(10)-carene. 

Chapter 3 by Jie Jack Li presents a collection of very interesting total syntheses of naturally occurring indole alkaloids where palladium chemistry plays a central role in the syntheses. Five different types of palladium-mediated reactions are treated (I) oxidative cyclization reactions promoted by palladium (II) species (2) transmetallation reactions with organoboranes, organoslannanes, and organozinc reagents (3) inter- and intramolecular Heck reactions (4) reactions with it-allylpalladium as the intermediate and (5) reactions using C-N bond formation as the key step for the synthesis. 

Three alkyl- or aryl groups can be attached to the boron atom via transmetallation of boron aikoxides with reactive organometaiiics (see also refs. 1-8, 5.3.2.4). Organoboranes were first synthesized in this way ... 

The most common methods of ring construction employ cyclic hydroboration, radical additions of H2Y to unsaturated organoboranes and transmetallation procedures. Specific examples of these processes are presented in the following sections. 

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