Boron compounds alkynylation reactions

The reactions described so far can be considered as alkylation, alkenylation, or alkynylation reactions. In principle all polar reactions in syntheses, which produce monofunctional carbon compounds, proceed in the same way a carbanion reacts with an electropositive carbon atom, and the activating groups (e.g. metals, boron, phosphorus) of the carbanion are lost in the work-up procedures. We now turn to reactions, in which the hetero atoms of both the acceptor and donor synthons are kept in a difunctional reaction produa. 

The boron cluster expansion reaction which occurs through the reaction of decaborane with alkynyl pyrimidine is in accord with two path ways of forming c/oso-compounds and m do-compounds. Acetonitrile accelerates decaborane to react and to take the role of a good nucleophile. 

The Suzuki coupling was developed by Professor Akira Suzuki of Hokkaido University. The Suzuki coupling uses a boron compound (R-BYj) and an alkenyl, aryl, or alkynyl halide or triflate (RX) as the carbon sources, with a palladium salt as the catalyst. Bromides and iodides are the most commonly used halides chlorides are less reactive. Alkyl halides can sometimes be used but are subject to elimination. A base is also required. The boron compound can be a borane (R jB), a borate ester (R B(OR)2), or a boric acid (R B(OH)2), where R is alkyl, alkenyl, or aryl. The general reaction is shown in the following scheme, where X is halide or triflate and Y is alkyl, alkoxyl, or OH. A list of the types of components that can be used is given in Table 24.1. This reaction is one of the principal methods now used to prepare biaryls. 

Palladium-catalyzed carbon-carbon bond forming reactions like the Suzuki reac-tion as well as the Heck reaction and the Stille reaction, have in recent years gained increased importance in synthetic organic chemistry. In case of the Suzuki reaction, an organoboron compound—usually a boronic acid—is reacted with an aryl (or alkenyl, or alkynyl) halide in the presence of a palladium catalyst. 

Not until 2005 were the first organoboron derivatives of pyridazines reported in the literature [57]. Harrity reported the preparation of pyridazin-4-ylboronic esters (99) via cycloaddition of tetrazines (98) with alkynyl boronic esters. Some of these boronic sters have been used in Suzuki reactions with iodobenzene. Although the organoboron compounds 99 are electron deficient (and some are also sterically hindered) protodeboronation usually proved no major problem. 

A similar reaction with B-alkynyl-9-BBN compounds gives 2-en-4-ynones. The elimination of the /3-alkoxy group may be assisted by the Lewis acid character of boron and may take place through a cyclic transition state. 

---