Homogeneous catalysis, synthesis of liquid crystals to diboration of alkenes

Abstract

Palladium/copper catalyzed cross-coupling methodology involving terminal alkynes and arylhalides provided a general route to a series of rigid-rod 1,4-bis(p-R-phenylethynyl)benzenes (R-C6H4-C=C-C6H4-C=C-C6H4-R, BPEB) and their partially and fully fluorinated anzlogs. Liquid crystal (LC) studies have demonstrated that a change in terminal substituents or fluorination of various parts of the molecule can affect not only the melting and clearing points but also the type of liquid crystalline phases that are formed. Extensive single crystal X-ray diffraction studies have been conducted in order to attempt to correlate the solid state crystalline structures with their liquid crystalline phase behaviors. An analysis of the molecular packing in crystals of the four parent BPEBs containing 0,4,10, or 14 fluorines, and three related tolan (C6H5-C=C-C6H5) derivatives as well as several co-crystals is described, with particular emphasis being placed on the intermolecular interactions between phenyl and perfluorophenyl units.

In connection with the development of catalyst systems for the diboration of unsaturated organic substrates, a detailed study on the oxidative addition of the B-B bond in (RO)2B-B(OR)2 diboron compounds to low valent late transition metals Co(0), Ir(I)and Rh(I) has been carried out. Several interesting metal boryl [M-B(OR)2] complexes were synthesized and characterized by X-ray crystallography. The paramagnetic Co(II) bis(boryl) [cis-Co(PMe3)3(Bcat)2] (cat=1,2-O2C6H4) is the first well characterized Co bis(boryl) complex and the small B-Co-B bond angle indicates a possible remaining B-B interaction. Likewise, the novel Rh(I) mono(boryl) [Rh(PMe3)4(Bcat)] and Rh(III) tris(boryl) [fac-Rh(PMe3)3(Bcat)3] complexes were obtained by reactions of [Rh(PMe3)4Me] with one or two equivalents of B2cat2, respectively. The Ir(III) bis(boryl) [cis-fac-Ir(PMe3)3 Cl(Bcat)2] is the first bis(boryl) complex with two distinctive boryl environments.

It has also been found that Rh(I) complexes such as [Rh(PMe3)4Me] and [CpRh(PMe3)2] can react with B2cat3, a reagent which has always been considered to be an undesired side product in the metal catalyzed hydroboration and diboration processes. The rational design of reactions between B2cat3 and [(acac)RhL2] led exclusively to the formation of (acac)Bcat and a series of novel zwitterionic complexes [(n^6-Bcat2)RhL2](L=C2H4, COE, PPh2Me; L2=COD, dppm, dppe, dppp, dppb, dppf), some of which have been demonstrated to be extremely efficient hydroboration catalysts. This (acac)RhL2/B2cat3 system was readily applied to the catalytic addition of B2cat2 to 4-vinylanisole, and the reaction was complete within one hour with 89% of the desired 1,2-bis(boronate)ester product formed when 4 mol% (acac)Rh(dppm)/B2cat3 was employed. Moreover, diborations of unstrained internal alkenes such as trans-stilbene and trans-B-methylstyrene were realized for the first time and with excellent selectivities by applying the (acac)Rh(dppm)/B2cat3 catalyst system.