Arylboronic acids, but not the corresponding deboronated arenes, recently have been found to be weakly mutagenic in microbial assays [1]. Hence arylboronic acids may be considered potentially genotoxic impurities, and controlling the levels of residual arylboronic acids in APIs could become a regulatory requirement. The issues should be decided by toxicology studies for the specific arylboronic acids in question.
Several approaches have been successful in removing boronic acids. Diethanolaminomethyl polystyrene (DEAM-PS) [2],[3] and immobilized catechol [4] have been used to scavenge boronic acids. Complex formation with diethanolamine may solubilize residual boronic acids in mother liquors. Since arylboronic acids ionize similarly to phenols, basic washes of an API solution may remove arylboronic acids. A selective crystallization can purge an arylboronic acid from the API.
The best means to control residual aryl boronic acids in APIs at the ppm level may be to decompose them through deboronation. Sterically hindered, electron-rich aryl boronates and heteroaromatic boronates are especially prone to deboronation [5],[6],[7]. Deboronation of a hindered difluorobenzeneboronic acid was accelerated in the presence of Pd, and increased in the presence of H2O [8]. Kuivila and co-workers found that protodeboronation of 2,6-dimethoxybenzeneboronic acid was slowest about pH 5, and rapid under more acidic or basic conditions [9]. Butters and co-workers found that protodeboronation occurred for an arylboronate anion, but not for the corresponding arylboronic acid [10]. Amgen researchers found that deboronation of an ortho-substituted arylboronic acid was fastest in the presence of K2CO3 [11]. The Snieckus group found that deboronation occurred readily when pinacol was added to 4-pyridylboronic acid [12]. Percec and co-workers found that deboronation of neopentylglycol boronates, especially an ortho-substituted arylboronic acid ester, was catalyzed by nickel species [13]. Kuivila and co-workers found that CuCl2 catalyzed the deboronation of 2,6-dimethoxybenzeneboronic acid and other arylboronic acids, with formation of the corresponding aryl chlorides [14]. Unfortunately, adding reagents to a reaction mixture increases the burdens of analysis and impurity removal, but additives such as these may accelerate deboronation in difficult cases. Simply extending the reaction conditions, which are generally basic for efficient Suzuki coupling, or heating with some amount of aqueous hydroxide are probably the preferred treatments to decompose an arylboronic acid. By knowing the kinetics of the decomposition of the arylboronic acid it may be possible to show by QbD that analyses for the residual arylboronic acid in an API are not necessary.
[1] O’Donovan, M. R.; Mee, C. D.; Fenner, S.; Teasdale, A.; Phillips, D. H. Mutat. Res.: Genet. Toxicol. Environ. Mutagen. 2011, 724(1-2), 1.
[2] Antonow, D.; Cooper, N.; Howard, P. W.; Thurston, D. E. J. Comb. Chem. 2007, 9, 437.
[3] Hall, D. G.; Tailor, J.; Gravel, M. Angew. Chem. Int. Ed. 1999, 38, 3064.
[4] Yang, W.; Gao, X.; Springsteen, G.; Wang, B. Tetrahedron Lett. 2002, 43, 6339.
[5] Goodson, F. E.; Wallow, T. I.; Novak, B. M. Organic Syntheses; Vol. 74; Smith, A. B., III, Ed.; Organic Syntheses, Inc.; 1997; p. 64.
[6] Baudoin, O.; Cesario, M.; Guénard, D.; Guéritte, F. J. Org. Chem. 2002, 67, 1199.
[7] Dai, Q.; Xu, D.; Lim, K.; Harvey, R. G. J. Org. Chem. 2007, 72, 4856.
[8] Cammidge, A. N.; Crépy, K. V. L. J. Org. Chem. 2003, 68, 6832.
[9] Kuivila, H. G.; Reuwer, J. F., Jr.; Mangranite, J. A. Can. J. Chem. 1963, 41, 3081.
[10] Butters, M.; Harvey, J. N.; Jover, J.; Lennox, A. J. J.; Lloyd-Jones, G. C.; Murray, P. M. Angew. Chem. Int. Ed. 2010, 49, 5156.
[11] Achmatowicz, M.; Thiel, O. R.; Wheeler, P.; Bernard, C.; Huang, J.; Larsen, R. D.; Faul, M. M. J. Org. Chem. 2009, 74, 795.
[12] Alessi, M.; Larkin, A. L.; Ogilvie, K. A.; Green, L. A.; Lai, S.; Lopez, S.; Snieckus, V. J. Org. Chem. 2007, 72, 1588.
[13] Moldoveanu, C.; Wilson, D. A.; Wilson, C. J.; Leowanawat, P.; Resmerita, A.-M.; Liu, C.; Rosen, B. M.; Percec, J. Org. Chem. 2010, 75, 5438.
[14] Kuivila, H. G.; Reuwer, J. F., Jr.; Mangravite, J. A. J. Am. Chem. Soc. 1964, 86, 2666.
