Ewan R. Clark

The Hydride‐Ion Affinity of Borenium Cations and Their Propensity to Activate H2 in Frustrated Lewis Pairs

A range of frustrated Lewis pairs (FLPs) containing borenium cations have been synthesised. The catechol (Cat)-ligated borenium cation [CatB(PtBu(3))](+) has a lower hydride-ion affinity (HIA) than B(C(6)F(5))(3). This resulted in H(2) activation being energetically unfavourable in a FLP with the strong base PtBu(3). However, ligand disproportionation of CatBH(PtBu(3)) at 100 °C enabled trapping of H(2) activation products. DFT calculations at the M06-2X/6-311G(d,p)/PCM (CH(2)Cl(2)) level revealed that replacing catechol with chlorides significantly increases the chloride-ion affinity (CIA) and HIA. Dichloro-borenium cations, [Cl(2)B(amine)](+), were calculated to have considerably greater HIA than B(C(6)F(5))(3). Control reactions confirmed that the HIA calculations can be used to successfully predict hydride-transfer reactivity between borenium cations and neutral boranes. The borenium cations [Y(Cl)B(2,6-lutidine)](+) (Y = Cl or Ph) form FLPs with P(mesityl)(3) that undergo slow deprotonation of an ortho-methyl of lutidine at 20 °C to form the four-membered boracycles [(CH(2){NC(5)H(3)Me})B(Cl)Y] and [HPMes(3)](+). When equimolar [Y(Cl)B(2,6-lutidine)](+)/P(mesityl)(3) was heated under H(2) (4 atm), heterolytic cleavage of dihydrogen was competitive with boracycle formation.

Haloboration of Internal Alkynes with Boronium and Borenium Cations as a Route to Tetrasubstituted Alkenes

Hail boration! 2-Dimethylaminopyridine-ligated dihaloborocations [X2B(2-DMAP)](+) with a strained four-membered boracycle were used for the haloboration of terminal and dialkyl internal alkynes (see scheme). Esterification then provided vinyl boronate esters as useful precursors to tetrasubstituted alkenes. Following mechanistic studies, the scope of the haloboration was expanded simply by variation of the amine. Pin = 2,3-dimethyl-2,3-butanedioxy.

N-Methylacridinium Salts: Carbon Lewis Acids in Frustrated Lewis Pairs for σ-Bond Activation and Catalytic Reductions†

N-methylacridinium salts are Lewis acids with high hydride ion affinity but low oxophilicity. The cation forms a Lewis adduct with 4-(N,N-dimethylamino)pyridine but a frustrated Lewis pair (FLP) with the weaker base 2,6-lutidine which activates H2, even in the presence of H2O. Anion effects dominate reactivity, with both solubility and rate of H2 cleavage showing marked anion dependency. With the optimal anion, a N-methylacridinium salt catalyzes the reductive transfer hydrogenation and hydrosilylation of aldimines through amine-boranes and silanes, respectively. Furthermore, the same salt is active for the catalytic dehydrosilylation of alcohols (primary, secondary, tertiary, and ArOH) by silanes with no observable over-reduction to the alkanes.

Direct C(sp2)C(sp3) Cross‐Coupling of Diaryl Zinc Reagents with Benzylic, Primary, Secondary, and Tertiary Alkyl Halides

The direct C(sp(2) )-C(sp(3) ) cross-coupling of diaryl zinc reagents with benzylic, primary, secondary, and tertiary alkyl halides proceeded in the absence of coordinating ethereal solvents at ambient temperature without the addition of a catalyst. The C(sp(2) )-C(sp(3) ) cross-coupling showed excellent functional-group tolerance, and products were isolated in high yields, generally without the requirement for purification by chromatography. This process represents an expedient, operationally simple method for the construction of new C(sp(2) )-C(sp(3) ) bonds.

E–H (E = R3Si or H) bond activation by B(C6F5)3 and heteroarenes; competitive dehydrosilylation, hydrosilylation and hydrogenation

In the presence of B(C6F5)3 five-membered heteroarenes undergo dehydrosilylation and hydrosilylation with silanes. The former, favoured on addition of a weak base, produces H2 as a by-product making the process catalytic in B(C6F5)3 but also enabling competitive heteroarene hydrogenation.

Tricationic analogues of boroxines and polyborate anions.

Addition of E(2)O (E = Me(3)Si or H) to [(pyridyl)BX(2)][AlX(4)] (X = Cl or Br) and subsequent heating produced the unprecedented trications [(2,6-lutidine)(4)B(5)O(6)](3+) and [(pyridine)(4)B(3)O(3)](3+).

Complete reductive cleavage of CO facilitated by highly electrophilic borocations

The addition of CO to [((R3N)BH2)2(μ-H)][B(C6F5)4] leads to formation of trimethylboroxine ((MeBO)3) and [(R3N)2BH2][B(C6F5)4]. When R = Et, [(Et3N)H2B(μ-O)B(CH3)NEt3][B(C6F5)4], is isolated and demonstrated to be an intermediate in the formation of (MeBO)3.

Syntheses of, and structural studies on, benzo-fused 1,2,4-thiadiazines

The syntheses of nine benzo-fused-1,2,4-thiadizines are reported. The use of microwave synthesis has been shown to afford high yields and short reaction times in several key reaction steps. The molecular geometries of these heterocycles are discussed and their solid state packing motifs reveal a strong tendency for the N–H group to form hydrogen bonded chains.

Highly nucleophilic dipropanolamine chelated boron reagents for aryl-transmetallation to iron complexes

New aryl- and heteroarylboronate esters chelated by dipropanolamine are synthesised directly from boronic acids. The corresponding anionic borates are readily accessible by deprotonation and demonstrate an increase in hydrocarbyl nucleophilicity in comparison to other common borates. The new borates proved competent for magnesium or zinc additive-free, direct boron-to-iron hydrocarbyl transmetallations with well-defined iron(II) (pre)catalysts. The application of the new borate reagents in representative Csp(2)-Csp(3) cross-coupling led to almost exclusive homocoupling unless coupling is performed in the presence of a zinc additive.

Decontamination and Remediation of the Sulfur Mustard Simulant CEES with “Off-the-Shelf” Reagents in Solution and Gel States: A Proof-of-Concept Study

Abstract The decontamination and remediation of sulfur mustard chemical warfare agents remains an ongoing challenge. Herein, we report the use of “off‐the‐shelf” metal salts alongside commercially available peroxides to catalyze the degradation of the simulant 2‐chloroethyl ethyl sulfide (CEES) in solution and encapsulated within a supramolecular gel.