No abstract

An operationally convenient and general method for hydroboration of alkenes, aldehydes, and ketones employing Co(acac) 3 as a precatalyst is reported. The hydroboration of alkenes in the presence of HBpin, PPh 3 , and NaO t Bu affords good to excellent yields with high Markovnikov selectivity with up to 97:3 branched/linear selectivity. Moreover, Co(acac) 3 could be used effectively to hydroborate aldehydes and ketones in the absence of additives under mild reaction conditions. Inter-and intramolecular chemoselective reduction of the aldehyde group took place over the ketone functional group.

Stereoselective synthesis of trisubstituted alkenes is a long-standing challenge in organic chemistry, due to the small energy differences between E and Z isomers of trisubstituted alkenes (compared with 1,2-disubstituted alkenes). Transition metal-catalyzed isomerization of 1,1-disubstituted alkenes can serve as an alternative approach to trisubstituted alkenes, but it remains underdeveloped owing to issues relating to reaction efficiency and stereoselectivity. Here we show that a novel cobalt catalyst can overcome these challenges to provide an efficient and stereoselective access to a broad range of trisubstituted alkenes. This protocol is compatible with both mono- and dienes and exhibits a good functional group tolerance and scalability. Moreover, it has proven to be a useful tool to construct organic luminophores and a deuterated trisubstituted alkene. A preliminary study of the mechanism suggests that a cobalt-hydride pathway is involved in the reaction. The high stereoselectivity of the reaction is attributed to both a π–π stacking effect and the steric hindrance between substrate and catalyst.

An operationally convenient, room temperature synthesis of N,N-diborylated amines via hydroboration of nitriles using a commercially available catalyst.

The use of a nonconventional water resource for energy and industrial applications often requires extraction of low-level undesirable ions from matrices of benign dominant ions. In this study, the selective extraction of Ba2+ and Mg2+ from synthetic brine solutions was evaluated using strong acid cation-exchange membranes modified with the surface deposition of macrocyclic molecules including crown ethers and calixarenes. Compared to the bare membrane, vinylbenzo-18-crown-6 (VB18C6) and calix[4]­arene-amended membranes showed increased selectivity for Ba2+ and Mg2+ with respect to the dominant ion (Na+) by up to fourfolds, with the calix[4]­arene-modified membrane achieving more selective separation than VB18C6. Optimal selectivity was achieved at a moderate-to-high current density (3.1–6.3 mA/cm2), which was attributed to the alleviation of transport limitation in the boundary layer by the surface modification. Amendment of a calix[4]­arene derivative with a crown-6 “boot strap” and two carboxylic groups resulted in reduced selectivity due to the formation of strong complexes with divalent ions. These results show that surface deposition of ion sequestrants can be a versatile approach to improve the membrane’s selectivity; however, the performance is sensitive to feed water salinity, current loading, and the macrocycle-ion chemistry, where there is a trade-off between ion affinity and mobility.