Direct (hetero)arylation: a new tool for polymer chemists.

The coupling of aryl halides with catalytically activated aryl C-H bonds provides a desirable and atom-economical alternative to standard cross-coupling reactions for the construction of new C-C bonds. The reaction, termed direct (hetero)arylation, is believed to follow a base-assisted, concerted metalation-deprotonation (CMD) pathway. During this process, carboxylate or carbonate anions coordinate to the metal center, typically palladium, in situ and assist in the deprotonation transition state. Researchers have employed this methodology with numerous arenes and heteroarenes, including substituted benzenes, perfluorinated benzenes, and thiophenes. Thiophene substrates have demonstrated high reactivity toward C-H bond activation when appropriately substituted with electron-rich and/or electron-deficient groups. Because of the pervasive use of thiophenes in materials for organic electronics, researchers have used this chemistry to modularly prepare conjugated small molecules and, more recently, conjugated polymers. Although optimization of reaction conditions such as solvent system, phosphine ligand, carboxylate additives, temperature, and time is necessary for efficient C-H bond reactivity of each monomer, direct (hetero)arylation polymerization (DHAP) can afford high yielding polymeric materials with elevated molecular weights. The properties of these materials often rival those of polymers prepared by traditional methods. Moreover, DHAP provides a facile means for the synthesis of polymers that were previously inaccessible or difficult to prepare due to the instability of organometallic monomers. The major downfall of direct (hetero)arylation, however, is the lack of C-H bond selectivity, particularly for thiophene substrates, which can result in cross-linked material during polymerization reactions. Further fine-tuning of reaction conditions such as temperature and reaction time may suppress these unwanted side reactions. Alternatively, new monomers can be designed where other reactive bonds are blocked, either sterically or by substitution with unreactive alkyl or halogen groups. In this Account, we illustrate these methods and present examples of DHAP reactions that involve the preparation of common homopolymers used in organic electronics (P3HT, PEDOT, PProDOT), copolymers formed by activation of electron-rich (bithiophene, fused bithiophenes) and electron-deficient monomers (TPD, 1,2,4,5-tetrafluorobenzene, 2,2'-bithiazole). Our group is optimizing these reactions and developing ways to make DHAP a common atom-economical synthetic tool for polymer chemists.

5,5-Dimethyl-2,8-diphenyl-5H-dibenzo[b,f]silepine: a synchrotron study.

The molecule of the title silepine compound, C28H24Si, lies on a crystallographic general position but has almost exact mirror symmetry. The seven-membered silepine ring has a boat conformation, with a fold angle of 99.99 (9)° between the C-C=C-C stern and the C-Si-C prow. The planes of the phenyl substituents are rotated by 32.23 (6) and 31.80 (7)° out of the planes of the respective fused benzene rings. Similar folded conformations are found for other dibenzo[b,f]silepines, while analogous dibenzo[b,f]borepines, in which the B atom can participate in conjugation, have a more flattened heterocyclic ring.

Dihydrogen activation with (t)Bu3P/B(C6F5)3: a chemically competent indirect mechanism via in situ-generated p-(t)Bu2P-C6F4-B(C6F5)2.

A chemically competent indirect pathway for the activation of dihydrogen by the nonmetal Lewis acid/Lewis base pair (t)Bu(3)P/B(C(6)F(5))(3) is described. The reaction between (t)Bu(3)P and B(C(6)F(5))(3) produces [(t)Bu(3)PH](+)[FB(C(6)F(5))(3)](-) and the known phosphinoborane p-(t)Bu(2)P-C(6)F(4)-B(C(6)F(5))(2) (1-(t)Bu) with elimination of isobutylene. At 1:1 stoichiometry, 1-(t)Bu is produced rapidly in detectable quantities and can act as a catalyst for the formation of [(t)Bu(3)PH](+)[HB(C(6)F(5))(3)](-) from (t)Bu(3)P and B(C(6)F(5))(3) in the presence of H(2). The extent to which this indirect path competes with the direct path is explored.

Mechanistic aspects of bond activation with perfluoroarylboranes.

In the mid-1990s, it was discovered that tris(pentafluorophenyl)borane, B(C(6)F(5))(3), was an effective catalyst for hydrosilylation of a variety of carbonyl and imine functions. Mechanistic studies revealed a counterintuitive path in which the function of the borane was to activate the silane rather than the organic substrate. This was the first example of what has come to be known as "frustrated Lewis pair" chemistry utilizing this remarkable class of electrophilic boranes. Subsequent discoveries by the groups of Stephan and Erker showed that this could be extended to the activation of dihydrogen, initiating an intense period of activity in this area in the past 5 years. This article describes the early hydrosilylation chemistry and its subsequent applications to a variety of transformations of importance to organic and inorganic chemists, drawing parallels with the more recent hydrogen activation chemistry. Here, we emphasize the current understanding of the mechanism of this process rather than focusing on the many and emerging applications of hydrogen activation by fluoroarylborane-based frustrated Lewis pair systems.

Dihydrogen activation by antiaromatic pentaarylboroles.

Facile metal-free splitting of molecular hydrogen (H(2)) is crucial for the utilization of H(2) without the need for toxic transition-metal-based catalysts. Frustrated Lewis pairs (FLPs) are a new class of hydrogen activators wherein interactions with both a Lewis acid and a Lewis base heterolytically disrupt the hydrogen-hydrogen bond. Here we describe the activation of hydrogen exclusively by a boron-based Lewis acid, perfluoropentaphenylborole. This antiaromatic compound reacts extremely rapidly with H(2) in both solution and the solid state to yield boracyclopent-3-ene products resulting from addition of hydrogen atoms to the carbons alpha to boron in the starting borole. The disruption of antiaromaticity upon reaction of the borole with H(2) provides a significant thermodynamic driving force for this new metal-free hydrogen-splitting reaction.

Sequential Rh(I)/Pd-catalyzed 1,4-addition/intramolecular allylation: stereocontrolled construction of gamma-butyrolactones and cyclopropanes.

The rhodium-catalyzed conjugate addition of functionalized vinyltin reagents to alkylidene Meldrum's acids, followed by Pd-catalyzed intramolecular allylation, is a direct entry into vinyl-substituted gamma-lactones via O-alkylation and vinylcyclopropanes via C-alkylation.

Acceptor-donor-acceptor oligomers containing dithieno[3,2-b:2',3'-d]pyrrole and thieno[2,3-c]pyrrole-4,6-dione units for solution-processed organic solar cells.

New acceptor-donor-acceptor (A-D-A) oligomers (1 and 2) containing a central dithieno[3,2-b:2',3'-d]pyrrole unit and end-capping thieno[2,3-c]pyrrole-4,6-dione groups have been synthesized and characterized. Bulk heterojunction solar cells were prepared together with PC61BM and PC71BM, and the best results were obtained for the latter acceptor using 1,8-diiodooctane as an additive. Photovoltaic devices containing these oligomers achieved high external quantum efficiencies up to 50%.