Warren E. Piers

Tandem Frustrated Lewis Pair/Tris(pentafluorophenyl)borane-Catalyzed Deoxygenative Hydrosilylation of Carbon Dioxide

The frustrated Lewis pair system consisting of 2 equiv of 2,2,6,6-tetramethylpiperidine (TMP) and tris(pentafluorophenyl)borane [B(C(6)F(5))(3)] activates carbon dioxide to form a boratocarbamate-TMPH ion pair. In the presence of triethylsilane, this species is converted to a silyl carbamate and the known ion pair [TMPH](+)[HB(C(6)F(5))(3)](-), which recently was shown to react with CO(2) via transfer of the hydride from the hydridoborate to form the formatoborate [TMPH](+)[HC(O)OB(C(6)F(5))(3)](-). In the presence of extra B(C(6)F(5))(3) (0.1-1.0 equiv) and excess triethylsilane, the formatoborate is rapidly hydrosilated to form a formatosilane and regenerate [TMPH](+)[HB(C(6)F(5))(3)](-). The formatosilane in turn is rapidly hydrosilated by the B(C(6)F(5))(3)/Et(3)SiH system to CH(4), with (Et(3)Si)(2)O as the byproduct. At low [Et(3)SiH], intermediate CO(2) reduction products are observed; addition of more CO(2)/Et(3)SiH results in resumed hydrosilylation, indicating that this is a robust, living tandem catalytic system for the deoxygenative reduction of CO(2) to CH(4).

Pentafluorophenylboranes: from obscurity to applications

Pentafluorophenyl substituted boranes and borates are important as co-catalysts in metallocene-based industrial processes for the homogeneous polymerization of olefins. Although first prepared in the early 1960s, the remarkable properties of tris(pentafluorophenyl)borane have only recently been exploited for applications in catalysis. Spurred by these developments, the related compounds bis(pentafluorophenyl)borane and salts of the tetrakis(pentafluorophenyl)borate anion have also found application in olefin polymerization and other fields. In this article, we trace the rise of pentafluorophenyl boron compounds from curiosities to important commodities.

Non-cyclopentadienyl ancillaries in organogroup 3 metal chemistry: a fine balance in ligand design

Abstract Over the past decade, several groups have targeted well-defined organometallic Group 3 metal complexes with ancillary ligand supporting environments alternative to the ubiquitous bis-Cp donor set. In addition to a desire to develop the fundamental organometallic chemistry of Group 3 bis-alkyl derivatives, these compounds are of interest as catalyst precursors for olefin and lactide polymerization processes, as well as olefin hydrosilation, amination and hydrogenation cycles. This review surveys the non-Cp organometallic chemistry of the ligands which have so far been explored for this purpose, commenting on the fine balance of steric and electronic properties necessary to stabilize monomeric, base-free organometallic compounds of these metals. While bona fide organometallic compounds have not been explicitly prepared in all cases, promising ligands in this regard are also included. The review covers the literature from about 1994 forward, encompassing 35 new ligand systems.

The Chemistry of Perfluoroaryl Boranes

Publisher Summary The chapter discusses methods for quantifying Lewis acidity; a survey of the chemistry of X 2 BAr F , XB(Ar F ) 2 and B(Ar F ) 3 compounds and their adducts; polyfunctional perfluoroaryl boranes; and applications that do not involve a-olefin polymerization by single-site catalysts via a coordination polymerization mechanism. The chapter highlights the chemistry of perfluoroaryl boranes. The related compound of perfluoroaryl boranes, that is, B(C 6 F 5 ) 3, is an effective co-catalyst for olefin- polymerization processes has led to the proverbial explosion of research activity in its chemistry and that of its derivatives. The unique properties of B(C 6 F 5 ) 3 , that is, thermal and hydrolytic stability coupled with strong Lewis acidity have led to extensive applications in widely varying areas of chemistry. The chapter focuses on applications as Lewis acids, but emerging applications such as anion transport additives in lithium batteries suggest an even broader utility for this remarkable class of compounds. The chapter discusses that as B(C 6 F 5 ) 3 and its derivatives become more broadly available, this should continue to be an active area of research.

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.

B(C6F5)3 catalyzed hydrosilation of enones and silyl enol ethers

Abstract The 1,4 hydrosilation of a variety of simple α,β-unsaturated enones as catalyzed by B(C 6 F 5 ) 3 (1–2%) is described. For substrates with no steric hindrance near the β-carbon, 1,4 addition of silane is very clean; in other instances, 1,2 hydrosilation is competitive. The reaction is facile with five commercially available silane reagents. For two examples, a novel hydrosilation of the resulting silylenol ethers was also observed. The net trans stereochemistry of H–Si addition to the silylenol ether C C bond was established and points to a stepwise mechanism for this reaction. This was supported by the observation and full spectroscopic characterization of a silylcarboxonium ion intermediate with an [HB(C 6 F 5 ) 3 ] − counteranion in the hydrosilation of the silylenol ether derived from 4,4-dimethyl-2-cyclohexen-1-one and PhMe 2 SiH.

Activation of Water, Ammonia, and Other Small Molecules by PCcarbeneP Nickel Pincer Complexes

Nickel complexes of a PC(carbene)P pincer ligand framework are described. Dehydrobromination of the precursor (PC(sp)(3)P)NiBr in the presence of a donor (PPh3 or NC(t)Bu) leads to the title complexes, which feature a rare nickel-carbene linkage as the pincer ligand anchor point. This strongly donating, nucleophilic carbene center engages in a variety of E-H bond activations (E = H, C, N, O), some of which are reversible. This represents a new mode of bond activation by ligand cooperativity in nickel pincer complexes.

Generation and Spectroscopic Characterization of Ruthenacyclobutane and Ruthenium Olefin Carbene Intermediates Relevant to Ring Closing Metathesis Catalysis

The reaction of phosphonium alkylidenes [(H2IMes)RuCl2=CHPR3]+[A]- (R = C6H11, A = OTf or B(C6F5)4, 1-Cy; R = i-C3H7, A = ClB(C6F5)3 or OTf, 1-iPr) with 1 equiv of ethylene at -78 degrees C, in the presence of 2-3 equiv of a trapping olefin substrate, yields intermediates relevant to olefin metathesis catalytic cycles. Dimethyl cyclopent-3-ene-1,1-dicarboxylate gives solutions of a substituted ruthenacyclobutane 3 of relevance to ring closing metathesis catalysis. 1H and 13C NMR data are fully consistent with its assignment as a ruthenacyclobutane, but 1JCC values of 23 Hz for the CalphaH2-Cbeta bond and 8.5 Hz for the CalphaH-Cbeta bond point to an unsymmetrical structure in which the latter bond is more activated than the former. In contrast, trapping with acenaphthylene leads to an olefin carbene complex (6) in which the putative ruthenacyclobutane has opened; this species was also fully characterized by NMR spectroscopy and compared to related species reported previously.

Reaction of pentaarylboroles with carbon monoxide: an isolable organoboron carbonyl complex

The highly Lewis acidic perfluoropentaphenylborole forms a stable, isolable adduct with the weak Lewis base carbon monoxide. A similar adduct with the unfluorinated borole is observed at low temperature, but this adduct undergoes reaction involving insertion into the B–C bonds due to the greater nucleophilicity of the α-carbons. Together these observations provide concrete chemical evidence for long held presumptions regarding the observed reactivity of organoboranes with carbon monoxide.

Benzo- and Naphthoborepins: Blue-Emitting Boron Analogues of Higher Acenes†

Unsaturated rings containing boron have been of fundamental and practical interest for many years. Five-membered borole rings I are isoelectronic to the antiaromatic cyclopentadienyl cation and have been important in addressing concepts of aromaticity. The six-membered borabenzene and boratabenzene compounds II (Nu = nucleophile), however, are aromatic and have found application as cyclopentadienide-like ligands with substantial tunability at the boron substituent. More recently, these cores have been tested as organic materials, finding application as fluoride sensors.