Iridium-Catalyzed Boron-Addition

The remarkable stability of iridium-boryl complexes, as a function of the substituents on boron, is most likely responsible for the unique behavior of iridium in metal-promoted B-addition to unsaturated molecules. 

It is no coincidence that the first X-ray structure analysis of a transition metal-boryl complex [8] was made on the iridium complex jac-[IrH2(BC8Hi4)(PMe3)3] (2) [9] derived from the B—H activation of the 9-borabicyclo[3.3.1]-nonane (9-BBN) 

Iridium Complexes in Organic Synthesis. Edited by Luis A. Oro and Carmen Qaver Copyright 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim TSRN 978-VS27- 1996-1 

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Several reports have highlighted the ability of boronic adds to undergo rhodium-[443], ruthenium- [444], iridium- [445], or palladium-catalyzed [446] addition-dehydrogenation reactions on alkenes (Equation 81, Figure 1.39) [446c]. Similar couplings to terminal alkynes were reported [447]. 

In addition to arylboronic acids, arylboronates have also been successfully used in fluorination reactions (Scheme 7.56) [93]. These boron compounds are attractive substrates since they are typically more robust than other boron species and can often be stored for long periods of time. The catalyst system for this reaction was a copper(I) triflate species along with 2 equiv of silver fluoride. While several sources of electrophilic fluorine generated the aryl fluorides, N-fluoro-2,4,6-trimethylpyridinium hexafluorophosphate was the most effective. The chemistry displayed broad functional group tolerance with the lowest yields obtained with heteroarylboronates. The authors were also able to devise a one-pot borylation-fluorination reaction starting from the parent arene. The arene was converted into an arylboronate through an iridium-catalyzed borylation reaction in the first step of the reaction, while fluorination was achieved during the second step. This is particularly attractive since it facilitates the conversion of unfunctionalized substrates into aryl fluorides. 

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