Treatment of benzofurans with bis(pinacolato)diboron and CsCO under nickel-NHC catalysis resulted in the insertion of a boron atom into the C2-O bond of benzofurans to afford the corresponding oxaborins. The scope of benzofuran substrates is wide, and the reactions proceeded without loss of functional groups such as fluoro, methoxy, and ester that are potentially reactive under nickel catalysis. The boron-inserted products proved to be useful building blocks and subsequently underwent a series of transformations, one of which led to the synthesis of fluorescent π-expanded oxaborins.
Among the plethora
of aromatic compounds, indoles represent a privileged
class of substructures that is ubiquitous in natural products and
pharmaceuticals. While numerous exocyclic functionalizations of indoles
have provided access to a variety of useful derivatives, endocyclic
transformations involving the cleavage of the C2–N bond remain
challenging due to the high aromaticity and strength of this bond
in indoles. Herein, we report the “aromatic metamorphosis”
of indoles into 1,2-benzazaborins via the insertion of boron into
the C2–N bond. This endocyclic insertion consists of a reductive
ring-opening using lithium metal and a subsequent trapping of the
resulting dianionic species with organoboronic esters. Considering
that 1,2-azaborins have attracted increasing academic and industrial
attention as BN isosteres of benzene, the counterintuitive aromatic
metamorphosis presented herein can feasibly be expected to substantially
advance the promising chemistry of 1,2-azaborins.
This paper describes the use of computational fluid dynamics for the calculation of the flow resistance through computer-generated models resembling silica monoliths. This study was undertaken to determine the effect of skeleton heterogeneity on the flow resistance and, more precisely, to test the hypothesis that increased skeleton heterogeneity decreases the flow resistance. To evaluate the proposed model, 24 real silica monoliths have been prepared using the same method, covering a wide range of skeleton sizes (2.2 microm < d(s) < 8 microm) and porosities (0.47 < epsilon < 0.66). The permeability of these monoliths was determined by pressure drop measurements, and structural information was obtained by image analysis of laser scanning confocal microscopy-generated 3D images of the skeleton structure. The results indicate that the presence of preferential flow paths due to an increased heterogeneity of the flow through pore space reduces the flow resistance of monolithic media. It is also shown that the pore size is hence a much better suited scaling dimension than the skeleton size to reduce the permeability of monolithic columns.
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