Robert D. J. Froese

Pd-PEPPSI-IPentCl: A Highly Effective Catalyst for the Selective Cross-Coupling of Secondary Organozinc Reagents†

No migration? No problem! A series of new N-heterocyclic carbene based Pd complexes has been created and evaluated in the Negishi cross-coupling of aryl and heteroaryl chlorides, bromides, and triflates with a variety of secondary alkylzinc reagents. The direct elimination product is nearly exclusively formed; in most examples there is no migratory insertion at all.

Amination with Pd–NHC Complexes: Rate and Computational Studies on the Effects of the Oxidative Addition Partner

Pd-PEPPSI-IPent, a recently-developed N-heterocyclic carbene (NHC) complex, has been evaluated in amination reactions with secondary amines and it has shown superb reactivity under the most mildly basic reaction conditions. Rate and computational studies were conducted to provide insight into the mechanism of the transformation. The IPent catalyst coordinates to the amine much more strongly than the IPr variant, thus favouring deprotonation with comparatively weak bases. Indeed the reaction is first order in base and only slightly more than zeroth order in amine.

Amination with PdNHC Complexes: Rate and Computational Studies Involving Substituted Aniline Substrates

The amination of aryl chlorides with various aniline derivatives using the N-heterocyclic carbene-based Pd complexes Pd-PEPPSI-IPr and Pd-PEPPSI-IPent (PEPPSI=pyridine, enhanced precatalyst, preparation, stabilization, and initiation; IPr=diisopropylphenylimidazolium derivative; IPent= diisopentylphenylimidazolium derivative) has been studied. Rate studies have shown a reliance on the aryl chloride to be electron poor, although oxidative addition is not rate limiting. Anilines couple best when they are electron rich, which would seem to discount deprotonation of the intermediate metal ammonium complex as being rate limiting in favour of reductive elimination. In previous studies with secondary amines using PEPPSI complexes, deprotonation was proposed to be the slow step in the cycle. These experimental findings relating to mechanism were corroborated by computation. Pd-PEPPSI-IPr and the more hindered Pd-PEPPSI-IPent catalysts were used to couple deactivated aryl chlorides with electron poor anilines; while the IPr catalysis was sluggish, the IPent catalyst performed extremely well, again showing the high reactivity of this broadly useful catalyst.

Selective Monoarylation of Primary Amines Using the Pd-PEPPSI-IPentClPrecatalyst

A single set of reaction conditions for the palladium-catalyzed amination of a wide variety of (hetero)aryl halides using primary alkyl amines has been developed. By combining the exceptionally high reactivity of the Pd-PEPPSI-IPent(Cl) catalyst (PEPPSI=pyridine enhanced precatalyst preparation, stabilization, and initiation) with the soluble and nonaggressive sodium salt of BHT (BHT=2,6-di-tert-butyl-hydroxytoluene), both six- and five-membered (hetero)aryl halides undergo efficient and selective amination.

The Selective Cross‐Coupling of Secondary Alkyl Zinc Reagents to Five‐Membered‐Ring Heterocycles Using Pd‐PEPPSI‐IHeptCl

The ability to cross-couple secondary alkyl centers is fraught with a number of problems, including difficult reductive elimination, which often leads to β-hydride elimination. Whereas catalysts have been reported that provide decent selectivity for the expected (non-rearranged) cross-coupled product with aryl or heteroaryl oxidative-addition partners, none have shown reliable selectivity with five-membered-ring heterocycles. In this report, a new, rationally designed catalyst, Pd-PEPPSI-IHept(Cl), is demonstrated to be effective in selective cross-coupling reactions with secondary alkyl reagents across an impressive variety of furans, thiophenes, and benzo-fused derivatives (e.g., indoles, benzofurans), in most instances producing clean products with minimal, if any, migratory insertion for the first time.

Nucleophilic deoxyfluorination of phenols via aryl fluorosulfonate intermediates

This report describes a method for the deoxyfluorination of phenols with sulfuryl fluoride (SO2F2) and tetramethylammonium fluoride (NMe4F) via aryl fluorosulfonate (ArOFs) intermediates. We first demonstrate that the reaction of ArOFs with NMe4F proceeds under mild conditions (often at room temperature) to afford a broad range of electronically diverse and functional group-rich aryl fluoride products. This transformation was then translated to a one-pot conversion of phenols to aryl fluorides using the combination of SO2F2 and NMe4F. Ab initio calculations suggest that carbon-fluorine bond formation proceeds via a concerted transition state rather than a discrete Meisenheimer intermediate.

2,2′-Azobis(2-methylpropionitrile)-Mediated Alkyne Hydrostannylation: Reaction Mechanism†

Not as radical as you think: The free-radical hydrostannylation of alkynes has been extensively studied and while every published mechanism involves solely radical intermediates, this appears not to be correct. Trace molecular oxygen is necessary for any radical-mediated hydrostannylation to occur with a wide selection of alkynes, thus leading to a proposed hybrid single-electron transfer/radical propagation mechanism. AIBN=2,2-azobis(2-methylpropionitrile).

Highly Stereo‐ and Regioselective Hydrostannylation of Internal Alkynes Promoted by Simple Boric Acid in Air

Facile hydrostannylation of alkynes in air: The ability to prepare vinylstannanes of high regio- and sterochemical fidelity in a safe manner employing a simple operational set up, especially on a large scale, has remained elusive. This study has shown that all of the autoxidation products of Et(3) B and boronic acids are capable of promoting hydrostannylation. Most importantly, simple boric acid itself can also promote the hydrostannylation of highly functionalized internal alkynes with complete selectivity under very mild conditions (RT to 80 °C) in air.

Molecular orientation, thermal behavior and density of electron and hole transport layers and the implication on device performance for OLEDs

Recent progress has shown that molecular orientation in vapor-deposited glasses can affect device performance. The deposition process can result in films where the molecular axis of the glass material is preferentially ordered to lie parallel to the plane of the substrate. Here, materials made within Dow’s Electronic Materials business showed enhanced performance when the orientation of the molecules, as measured by variable angle spectroscopic ellipsometry, was oriented in a more parallel fashion as compared to other materials. For one material, the anisotropic packing was observed in the as-deposited glass and was isotropic for solution-cast and annealed films. In addition, the density of an as-deposited N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)-2,2′-dimethylbenzidine (NPD) film was 0.8% greater than what was realized from slowly cooling the supercooled liquid. This enhanced density indicated that vapor-deposited molecules were packing more closely in addition to being anisotropic. Finally, upon heating the NPD film into the supercooled liquid state, both the density and anisotropic packing of the as-deposited glass was lost.

On the hydrostannylation of aryl propargylic alcohols and their derivatives: remarkable differences in both regio- and stereoselectivity in radical- and nonradical-mediated transformations.

Herein, we describe a highly regio- and stereoselective radical-mediated and molecular-oxygen (O2 )-dependent hydrostannylation of phenyl propargylic alcohols and their derivatives. There is a significant steric effect on the stereoselectivity of the tin-radical addition. Further, the uncatalyzed regio- and stereoselective hydrostannylation of aryl propargylic alcohols with nBu3 SnH and Ph3 SnH is also described and occurs with near titration kinetics. Although the uncatalyzed addition with nBu3 SnH gave a remarkable γ-regioselectivity irrespective of the electronic nature of the aryl moiety, addition with Ph3 SnH appears to be driven by the electronic nature of the aryl alkynes.