Carboboration reactions

Trialkylboranes generally show a low reactivity toward addition to alkenes and alkynes. Only trial-lylboranes react with alkynes under mild conditions. This reaction is often complicated by further intramolecular carboboration reactions and has found only limited synthetic application. 

Like alkenes, alkynes do not react readily with trialkylboranes. Under severe reaction conditions, only hydroboration products are obtained.116 Mikhailov113 showed that triallylboranes react with various alkynes (20 C) to afford syn addition products of type (78) which rapidly cyclize (40-60 C) to give the cyclic boranes of type (79 Scheme 42). In the case of trimethylsilylacetylene and ethoxyacetylene, the reaction affords compounds of type (78 R = MesSi or OEt) which do not cyclize further.113 Recently, a transition metal silylboration reaction has been described by Oshima and cowoikers.117 A formal syn carboboration reaction leading to a variety of alkenylboranes has been reported by Suzuki and cowoikers118 (Scheme 43). Hexamethyldistannylacetylene (80) reacts readily with various trialkylboranes119 to afford syn addition products of type (81 equation 25). 

Satoh, Y, Serizawa, H., Miyaura, N., Hara, S., Suzuki, A. 1988. Organic synthesis using haloboration reactions. 11. A formal carboboration reaction of 1-alkynes and its application to di- and trisubstituted alkene synthesis. Tetrahedron Lett. 29 1811—1814. 

It should be noted that a similar carboboration reaction has been carried out using a XantPhos-supported copper catalyst [80],... 

Hoveyda and co-workers also described the enantioselective hydroboration of styrene derivatives by NHC-Cu catalysts. Carboboration reactions of the same substrates were similarly performed. An electrophilic partner such as benzyl chloride or an in situ generated arylpalladium(ii) species could be used to react with the vinylcopper intermediate. 

Unlike the four-electron carboboration, some reactions of organoboranes proceed via concerted six-electron processes. Such reactions include (1) conjugate addition of organoboranes ... 

In the first two of these three reactions, organoboranes act as two-electron reactants, whereas they serve as four-electron reactants in the allylic and propargylic carboboration. 

Carboboration of carbon-carbon multiple bonds that realises simultaneous carbon-boron and carbon-carbon bond formations is achieved with B2(pin)2 and carbon electrophiles in the presence of a copper catalyst. The reaction of phenylacetylene with methyl iodide and B2(pin)2 in the presence of copper(i) chloride/Xantphos catalyst system gives the corresponding carboboration products (Scheme 15.3 8)." The carboboration of internal... 

Type of reaction C-B and C-C bond formation Reaction conditions Tetrahydrofuran, room temperature Synthetic strategy Regioselective carboboration of alkynes... 

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