Troels Skrydstrup

Ex Situ Generation of Stoichiometric and Substoichiometric 12CO and 13CO and Its Efficient Incorporation in Palladium Catalyzed Aminocarbonylations

A new technique for the ex situ generation of carbon monoxide (CO) and its efficient incorporation in palladium catalyzed carbonylation reactions was achieved using a simple sealed two-chamber system. The ex situ generation of CO was derived by a palladium catalyzed decarbonylation of tertiary acid chlorides using a catalyst originating from Pd(dba)(2) and P(tBu)(3). Preliminary studies using pivaloyl chloride as the CO-precursor provided an alternative approach for the aminocarbonylation of 2-pyridyl tosylate derivatives using only 1.5 equiv of CO. Further design of the acid chloride CO-precursor led to the development of a new solid, stable, and easy to handle source of CO for chemical transformations. The synthesis of this CO-precursor also provided an entry point for the late installment of an isotopically carbon-labeled acid chloride for the subsequent release of gaseous [(13)C]CO. In combination with studies aimed toward application of CO as the limiting reagent, this method provided highly efficient palladium catalyzed aminocarbonylations with CO-incorporations up to 96%. The ex situ generated CO and the two-chamber system were tested in the synthesis of several compounds of pharmaceutical interest and all of them were labeled as their [(13)C]carbonyl counterparts in good to excellent yields based on limiting CO. Finally, palladium catalyzed decarbonylation at room temperature also allowed for a successful double carbonylation. This new protocol provides a facile and clean source of gaseous CO, which is safely handled and stored. Furthermore, since the CO is generated ex situ, excellent functional group tolerance is secured in the carbonylation chamber. Finally, CO is only generated and released in minute amounts, hence, eliminating the need for specialized equipment such as CO-detectors and equipment for running high pressure reactions.

Unique Identification of Supramolecular Structures in Amyloid Fibrils by Solid-State NMR Spectroscopy†

The fibril structure formed by the amyloidogenic fragment SNNFGAILSS of the human islet amyloid polypeptide (hIAPP) is determined with 0.52 A resolution. Symmetry information contained in the easily obtainable resonance assignments from solid-state NMR spectra (see picture), along with long-range constraints, can be applied to uniquely identify the supramolecular organization of fibrils.

Silacarboxylic acids as efficient carbon monoxide releasing molecules: synthesis and application in palladium-catalyzed carbonylation reactions.

Silacarboxylic acids have been demonstrated to be easy to handle, air-stable carbon monoxide precursors. Different silacarboxylic acids were synthesized from the corresponding chlorosilanes and carbon dioxide, and their decarbonylation, upon treatment with an array of activators, was evaluated. The release of CO from crystalline MePh(2)SiCO(2)H proved to be highly efficient, and it was successfully applied in a selection of palladium-catalyzed carbonylative couplings using near-stoichiometric quantities of carbon monoxide precursor. Finally, the synthesis of MePh(2)Si(13)CO(2)H and its application in carbonyl labeling of two bioactive compounds was demonstrated.

Access to 2,5-diamidopyrroles and 2,5-diamidofurans by au(i)-catalyzed double hydroamination or hydration of 1,3-diynes.

A Au(I)-catalyzed hydroamination or hydration of 1,3-diynes to access 2,5-diamidopyrroles and 2,5-diamidofurans has been developed. This method can also be expanded to 2,5-disubstituted furans and 1,2,5-trisubstituted pyrroles including the formation of deuterated heterocycles and (18)O-labeled furans.

The Development and Application of Two-Chamber Reactors and Carbon Monoxide Precursors for Safe Carbonylation Reactions

Low molecular weight gases (e.g., carbon monoxide, hydrogen, and ethylene) represent vital building blocks for the construction of a wide array of organic molecules. Whereas experimental organic chemists routinely handle solid and liquid reagents, the same is not the case for gaseous reagents. Synthetic transformations employing such reagents are commonly conducted under pressure in autoclaves or under atmospheric pressure with a balloon setup, which necessitates either specialized equipment or potentially hazardous and nonrecommended installations. Other safety concerns associated with gaseous reagents may include their toxicity and flammability and, with certain gases, their inability to be detected by human senses. Despite these significant drawbacks, industrial processes apply gaseous building blocks regularly due to their low cost and ready availability but nevertheless under a strictly controlled manner. Carbon monoxide (CO) fits with all the parameters for being a gas of immense industrial importance but with severe handling restrictions due to its inherent toxicity and flammability. In academia, as well as research and development laboratories, CO is often avoided because of these safety issues, which is a limitation for the development of new carbonylation reactions. With our desire to address the handling of CO in a laboratory setting, we designed and developed a two-chamber reactor (COware) for the controlled delivery and utilization of stoichiometric amounts of CO for Pd-catalyzed carbonylation reactions. In addition to COware, two stable and solid CO-releasing molecules (COgen and SilaCOgen) were developed, both of which release CO upon activation by either Pd catalysis or fluoride addition, respectively. The unique combination of COware with either COgen or SilaCOgen provides a simple reactor setup enabling synthetic chemists to easily perform safe carbonylation chemistry without the need for directly handling the gaseous reagent. With this technology, an array of low-pressure carbonylations were developed applying only near stoichiometric amounts of carbon monoxide. Importantly, carbon isotope variants of the CO precursors, such as (13)COgen, Sila(13)COgen, or even (14)COgen, provide a simple means for performing isotope-labeling syntheses. Finally, the COware applicability has been extended to reactions with other gases, such as hydrogen, CO2, and ethylene including their deuterium and (13)C-isotopically labeled versions where relevant. The COware system has been repeatedly demonstrated to be a valuable reactor for carrying out a wide number of transition metal-catalyzed transformations, and we believe this technology will have a significant place in many organic research laboratories.

In Situ Generated Bulky Palladium Hydride Complexes as Catalysts for the Efficient Isomerization of Olefins. Selective Transformation of Terminal Alkenes to 2-Alkenes

Application of an in situ generated bulky palladium(II) hydride catalyst obtained from a 1:1:1 mixture of Pd(dba)(2), P(tBu)(3), and isobutyryl chloride provides an efficient protocol for the isomerization and migration of a variety of olefins. In addition to the isomerization of (Z)- to (E)-olefins, the conjugative migration of allylbenzenes, allyl ethers, and amines was effectively achieved in near-quantitative yields and with excellent functional group tolerance. Catalyst loadings in the range of 0.5-1.0 mol % were typically applied, but even loadings as low as 0.25 mol % could be achieved when the reactions were performed under neat conditions. More interestingly, the investigated catalyst proved to be selective for converting terminal alkenes to 2-alkenes. This one-carbon migration process for monosubstituted olefins provides an alternative catalyst, which bridges the gap between the allylation and propenylation/vinylation protocols. Several substrates, including homoallylic alcohols and amines, were selectively transformed into their corresponding 2-alkenes, and examples using enantiomerically enriched substrates provided products without epimerization at the allylic stereogenic carbon centers. Finally, some mechanistic investigations were undertaken to understand the nature of the active in situ generated Pd-H catalyst. These studies revealed that the catalytic system is highly dependent on the large steric demand of the P(tBu)(3) ligand. The use of an alternative ligand, cataCXium PinCy, also proved effective for generating an active catalyst, and it was demonstrated in some cases to display better selectivity for the one-carbon shifts of terminal olefins. A possible intermediate involved in the preparation of the active catalyst was characterized by its single-crystal X-ray structure, which revealed a monomeric tricoordinated palladium(II) acyl complex, bearing a chloride ligand.

Highly regioselective Au(I)-catalyzed hydroamination of ynamides and propiolic acid derivatives with anilines.

A highly regioselective hydroamination of unsymmetrical electron-poor and electron-rich alkynes with anilines catalyzed by Au(I) under mild conditions is reported. In addition, applications toward indole syntheses are presented including an example of a one-pot synthesis from a nonfunctionalized aniline.

Direct vinylation and difluorovinylation of arylboronic acids using vinyl- and 2,2-difluorovinyl tosylates via the Suzuki-Miyaura cross coupling.

General reaction conditions were developed for the Pd(0)-catalyzed Suzuki-Miyaura coupling reaction of aryl boronic acids with a simple electrophilic vinylation reagent, vinyl tosylate, providing access to styrene derivatives in good yields. The easily accessible vinyl tosylate represents a stable and less toxic alternative to the vinyl halides and the triflate/nonaflate derivatives. Furthermore, this methodology was expanded to provide a facile and straightforward approach for the introduction of a gem-difluorovinyl substituent onto an aromatic ring using the similar and also readily available 2,2-difluorovinyl tosylate as the electrophilic complement.

Efficient Fluoride-Catalyzed Conversion of CO2 to CO at Room Temperature

A protocol for the efficient and selective reduction of carbon dioxide to carbon monoxide has been developed. Remarkably, this oxygen abstraction step can be performed with only the presence of catalytic cesium fluoride and a stoichiometric amount of a disilane in DMSO at room temperature. Rapid reduction of CO2 to CO could be achieved in only 2 h, which was observed by pressure measurements. To quantify the amount of CO produced, the reduction was coupled to an aminocarbonylation reaction using the two-chamber system, COware. The reduction was not limited to a specific disilane, since (Ph2MeSi)2 as well as (PhMe2Si)2 and (Me3Si)3SiH exhibited similar reactivity. Moreover, at a slightly elevated temperature, other fluoride salts were able to efficiently catalyze the CO2 to CO reduction. Employing a nonhygroscopic fluoride source, KHF2, omitted the need for an inert atmosphere. Substituting the disilane with silylborane, (pinacolato)BSiMe2Ph, maintained the high activity of the system, whereas the structurally related bis(pinacolato)diboron could not be activated with this fluoride methodology. Furthermore, this chemistry could be adapted to (13)C-isotope labeling of six pharmaceutically relevant compounds starting from Ba(13)CO3 in a newly developed three-chamber system.

Effective palladium-catalyzed hydroxycarbonylation of aryl halides with substoichiometric carbon monoxide.

A protocol for the Pd-catalyzed hydroxycarbonylation of aryl iodides, bromides, and chlorides has been developed using only 1-5 mol % of CO, corresponding to a p(CO) as low as 0.1 bar. Potassium formate is the only stoichiometric reagent, acting as a mildly basic nucleophile and a reservoir of CO. The substoichiometric CO could be delivered to the reaction from an acyl-Pd(II) precatalyst, which provides both the CO and an active catalyst, and thereby obviates the need for handling a toxic gas.