Tom Kinzel

A New Palladium Precatalyst Allows for the Fast Suzuki-Miyaura Coupling Reactions of Unstable Polyfluorophenyl and 2-Heteroaryl Boronic Acids

Boronic acids which quickly deboronate under basic conditions, such as polyfluorophenylboronic acid and five-membered 2-heteroaromatic boronic acids, are especially challenging coupling partners for Suzuki-Miyaura reactions. Nevertheless, being able to use these substrates is highly desirable for a number of applications. Having found that monodentate biarylphosphine ligands can promote these coupling processes, we developed a precatalyst that forms the catalytically active species under conditions where boronic acid decomposition is slow. With this precatalyst, Suzuki-Miyaura reactions of a wide range of (hetero)aryl chlorides, bromides, and triflates with polyfluorophenyl, 2-furan, 2-thiophene, and 2-pyrroleboronic acids and their analogues proceed at room temperature or 40 °C in short reaction times to give the desired products in excellent yields.

Evidence for in Situ Catalyst Modification during the Pd-Catalyzed Conversion of Aryl Triflates to Aryl Fluorides

A mechanistic investigation of the Pd-catalyzed conversion of aryl triflates to fluorides is presented. Studies reveal that C-F reductive elimination from a LPd(II)(aryl)F complex (L = t-BuBrettPhos or RockPhos) does not occur when the aryl group is electron rich. Evidence is presented that a modified phosphine, generated in situ, serves as the actual supporting ligand during catalysis with such substrates. A preliminary study of the reactivity of a LPd(II)(aryl)F complex based on this modified ligand is reported.

Combined jet relaxation and quantum chemical study of the pairing preferences of ethanol

The subtle trans-gauche equilibrium in the ethanol molecule is affected by hydrogen bonding. The resulting conformational complexity in ethanol dimer manifests itself in three hydrogen-bonded OH stretching bands of comparable infrared intensity in supersonic helium expansions. Admixture of argon or nitrogen promotes collisional relaxation and is shown to enhance the lowest frequency transition. Global and local harmonic frequency shift calculations at MP2 level indicate that this transition is due to a gauche-gauche dimer, but the predictions are sensitive to basis set and correlation level. Energetically, the homochiral gauche-gauche dimer is predicted to be the most stable ethanol dimer conformation. The harmonic MP2 predictions are corroborated by perturbative anharmonicity contributions and CCSD(T) energies. Thus, a consistent picture of the subtle hydrogen bond energetics and vibrational dynamics of the ethanol dimer is starting to emerge for the first time.

Studying Regioisomer Formation in the Pd-Catalyzed Fluorination of Aryl Triflates by Deuterium Labeling

Isotopic labeling has been used to determine that a portion of the desired product in the Pd-catalyzed fluorination of electron-rich, non-ortho-substituted aryl triflates results from direct C–F cross-coupling. In some cases, formation of a Pd-aryne intermediate is responsible for producing undesired regioisomers. The generation of the Pd-aryne intermediate occurs primarily via ortho-deprotonation of a L·Pd(Ar)OTf (L = biaryl monophosphine) species by CsF and thus competes directly with the transmetalation step of the catalytic cycle. Deuterium labeling studies were conducted with a variety of aryl triflates.

Synthesis of natural products and analogs using multiple Pd-catalyzed transformations

Palladium-catalyzed transformations are of great importance in modern synthetic organic chemistry. The vast number of reactions that can be catalyzed by Pd0- as well as Pd2+-complexes in combination with the relative stability of the intermediates offers the intriguing opportunity of carrying out multiple consecutive bond-forming processes. They can be even performed in a domino fashion and in the presence of chiral ligands to allow the efficient preparation of almost enantiopure compounds. In this article, the use of double Heck, Tsuji-Trost-Heck, and Wacker-Heck reactions for the total syntheses of estradiol, spinosyn A analogs, cephalotaxine, and vitamin E is described.

Determination of the origin of stereoselectivity in multiple-transition-state reactions using DFT calculations: enantioselective synthesis of homoallylic alcohols from aliphatic methyl ketones via an auxiliary-mediated allylation.

Computational investigations on the highly stereoselective allylation of butanone in the presence of a chiral norpseudoephedrine-derived auxiliary have been performed. They suggest an SN1-type mechanism via the attack of allyltrimethylsilane to an intermediately formed oxocarbenium ion. The identification of preferred transition states (TSs) leads to a straightforward rationalization of the observed selectivity which can be extended to analogues of the auxiliary. A screening process has been devised to select 61 potentially relevant TSs from a total of almost 300 theoretically possible TSs. Final results were obtained from gas-phase calculations employing the B3LYP/6-31+G(d) level of theory as well as in dichloromethane solution using the B3LYP/6-311++G(2d,p)//B3LYP/6-31+G(d) level of theory in combination with polarizable continuum model and the UAKS set of radii. The agreement of theoretically predicted and experimentally observed selectivities is very good in both cases. However, the relative energy differences for several relevant TSs differ significantly when going from gas phase to solution, thus illustrating the necessity of performing calculations in solution to draw correct conclusions.

Asymmetric allylation of methyl ketones by using chiral phenyl carbinols.

Novel chiral auxiliaries for the stereoselective allylation of aliphatic methyl ketones with allyltrimethylsilane and their use in the synthesis of homoallylic ethers are described. In a multicomponent domino process catalyzed by trifluoromethanesulfonic acid, the allyl moiety and the auxiliary are transferred onto the substrate to yield tertiary homoallylic ethers. The most useful auxiliary for a general application turned out to be the trimethylsilyl ether of phenyl benzyl carbinol with an induced diastereoselectivity of 90:10 using ethyl methyl ketone and 94:6 using isopropyl methyl ketone as substrates. The transferred substituted benzyl moiety has good protecting properties in subsequent transformations and can easily be removed under reductive conditions to provide the corresponding homoallylic alcohol. The origin of the high selectivity could be elucidated by identifying the relevant transition states using quantum-chemical calculations. An excellent agreement between calculated and experimentally observed selectivities was obtained assuming an oxocarbenium ion as intermediate.

Stereoselective Allylation of Ketones: Explanation for the Unusual Inversion of the Induced Stereochemistry in the Auxiliary‐Mediated Crotylation and Pentenylation of Butanone by DFT Calculations

Auxiliary-mediated domino crotylations and pentenylations of butanone yield homoallylic ethers with two newly formed stereogenic centers. With our norpseudoephedrine-derived auxiliary, we observed the formation of anti isomers exclusively, and the nature of the major isomer was independent of the substrate double bond geometry. Interestingly, there is a switch in induced selectivity when going from crotylation to pentenylation. Here, we present the computational rationalization for this behavior by identification of the relevant transition states (TSs), the energies of which were determined by using the B3LYP/6-31+G(d) level of theory in combination with the PCM/UAKS method to include the effects exerted by the solvent dichloromethane. To quickly narrow down the number of potentially relevant TSs from the whole set of 288 and 864 TSs for the crotylation and pentenylation, respectively, we employed a screening process based on B3LYP//AM1 energies. The predicted selectivities are in good agreement with experimentally determined ones. Furthermore, the obtained results also facilitate an explanation of the selectivities obtained in hexenylations and heptenylations. Finally, activation energies were determined that account for the significantly longer reaction times than those for the domino allylation with unsubstituted trimethylallylsilane.