Jan J. Weigand

Mechanism of Pd(NHC)-Catalyzed Transfer Hydrogenation of Alkynes

The transfer semihydrogenation of alkynes to (Z)-alkenes shows excellent chemo- and stereoselectivity when using a zerovalent palladium(NHC)(maleic anhydride)-complex as precatalyst and triethylammonium formate as hydrogen donor. Studies on the kinetics under reaction conditions showed a broken positive order in substrate and first order in catalyst and hydrogen donor. Deuterium-labeling studies on the hydrogen donor showed that both hydrogens of formic acid display a primary kinetic isotope effect, indicating that proton and hydride transfers are separate rate-determining steps. By monitoring the reaction with NMR, we observed the presence of a coordinated formate anion and found that part of the maleic anhydride remains coordinated during the reaction. From these observations, we propose a mechanism in which hydrogen transfer from coordinated formate anion to zerovalent palladium(NHC)(MA)(alkyne)-complex is followed by migratory insertion of hydride, after which the product alkene is liberated by proton transfer from the triethylammonium cation. The explanation for the high selectivity observed lies in the competition between strongly coordinating solvent and alkyne for a Pd(alkene)-intermediate.

Bistetrazolylamines—synthesis and characterization

The acid-catalyzed cyclization reaction of sodium dicyanamide and sodium azide in the ratio of 1 : 2 afforded 5,5′-bis(1H-tetrazolyl)amine (H2bta, 2) in high yield (88%) as the monohydrate. Dehydration of 2·H2O at elevated temperature and reduced pressure gave anhydrous 2, while recrystallization from DMSO yielded 2·H2O·DMSO. 2 was converted into 5,5′-bis(2-methyltetrazolyl)methylamine (Me3bta, 6) in two steps. In the first step, 2 was twice deprotonated with sodium hydroxide and alkylated with MeI producing 5,5′-bis(2-methyltetrazolyl)amine (Me2bta, 5) in moderate yield (55%). The second step involved the alkylation of 5 with dimethyl sulfate in alkaline solution (72%). In all cases, the obtained colorless, crystalline compounds were fully characterized by vibrational (IR, Raman) spectroscopy, multinuclear NMR spectroscopy, mass spectrometry, elemental analysis, X-ray structure determination, and initial safety testing (impact, friction and electrical spark sensitivity). According to the UN Recommendations for the “Transport of Dangerous Goods”, compounds 2·H2O, 5, and 6 are classified as “insensitive” while 2 is described as “sensitive”. The thermal behaviors were investigated using differential scanning calorimetry (DSC). The heats of formation (ΔfHm° (2*H2O) = 203 kJ mol−1, ΔfHm° (2) = 633 kJ mol−1, ΔfHm° (5) = 350 kJ mol−1, ΔfHm° (6) = 583kJ mol−1) were calculated using heats of combustion (Δcomb.H (2·H2O) = −1714 kJ mol−1, Δcomb.H (2) = −1858 kJ mol−1, Δcomb.H (5) = −2932 kJ mol−1, Δcomb.H (6) = −3843 kJ mol−1) obtained from oxygen bomb calorimetry. In addition, explosion parameters such as the detonation velocity (D(2·H2O) = 7792 m s−1, D(2) = 9120 m s−1, D(5) = 7291 m s−1, D(6) = 7851 m s−1) and detonation pressure (P(2·H2O) = 220 kbar, P(2) = 343 kbar, P(5) = 172 kbar, P(6) = 205 kbar) were calculated using the program EXPLO5. “Koenen tests” were successfully performed for compound 2 using critical diameters of 8 mm and 10 mm.

Preparation of the [(DippNP)2(P4)2]2+-Dication by the Reaction of [DippNPCl]2 and a Lewis Acid with P4

The controlled activation of white phosphorus, P(4), provides a key entry point into many aspects of phosphorus chemistry. The activation and functionalization of P(4) by N-heterocyclic carbenes and carbene-like main group element fragments is of considerable current interest. In this communication, we report on the first use of a disguised bifunctional Lewis acid [DippNP](2)(2+) obtained from the cyclo-1,3-diphospha-2,4-diazane [DippNPCl](2) for P(4) functionalization. This has enabled the targeted preparation of novel mono- and dicationic phosphorus-rich clusters [(DippNP)(2)(P(4))Cl](+) and [(DippNP)(2)(P(4))(2)](+). The utilization of such a bifunctional phosphenium cation represents a rational and potentially versatile synthetic method for the assembly of large clusters using P(4) as a building block.

New Synthetic Procedures to Catena-Phosphorus Cations: Preparation and Dissociation of the First cyclo-Phosphino-halophosphonium Salts

Chlorination of 1,2,3,4-tetracyclohexyl-cyclo-tetraphosphine (2) by PhICl(2) or PCl(5) in the presence of Me(3)SiOTf or GaCl(3) provides a stepwise approach to salts of the first cyclo-phosphino-chlorophosphonium cations [Cy(4)P(4)Cl](+) ([19](+)) and [Cy(4)P(4)Cl(2)](2+) ([20](2+)). The analogous iodo derivative [Cy(4)P(4)I](+) ([17](+)) is obtained as the tetraiodogallate salt from reaction of 2 with I(2) in the presence of GaI(3). Reactions of the dication [20](2+) with PMe(3) or dmpe effect a dissociation of the cyclic framework resulting in the formation of salts containing [Me(3)PPCyPCyPMe(3)](2+) ([27](2+)), [dmpeCyP](2+) ([29](2+)), and [dmpeCyPCyP](2+) ([30](2+)), respectively. The new cations represent phosphine complexes of the [PCy](2+) and [P(2)Cy(2)](2+) cationic fragments from [20](2+), demonstrating the coordinate nature of the phosphinophosphonium bonds in cyclo-phosphino-halophosphonium cations. The compounds have been characterized by NMR spectroscopy, single crystal X-ray crystallography, and Raman spectroscopy.

Self-assembly of an imidazolate-bridged FeIII/CuII heterometallic cage

A rare, discrete, mixed-valent, heterometallic Fe(III)/Cu(II) cage, [Cu6Fe8L8](ClO4)12·χsolvent (H3L = tris{[2-{(imidazole-4-yl)methylidene}amino]ethyl}amine), was designed and synthesized via metal-ion-directed self-assembly with neutral tripodal metalloligands. The formation of this coordination cage was demonstrated by X-ray crystallography, ESI mass spectrometry, FT-IR, and UV-vis-NIR spectroscopy.

Formation of cationic [RP5Cl](+)-cages via insertion of [RPCl](+)-cations into a P-P bond of the P4 tetrahedron.

Fluorobenzene solutions of RPCl(2) and a Lewis acid such as ECl(3) (E = Al, Ga) in a 1:1 ratio are used as reactive sources of chlorophosphenium cations [RPCl](+), which insert into P-P bonds of dissolved P(4). This general protocol represents a powerful strategy for the synthesis of new cationic chloro-substituted organophosphorus [RP(5)Cl](+)-cages as illustrated by the isolation of several monocations (21a-g(+)) in good to excellent yields. For singular reaction two possible reaction mechanisms are proposed on the basis of quantum chemical calculations. The intriguing NMR spectra and structures of the obtained cationic [RP(5)Cl](+)-cages are discussed. Furthermore, the reactions of dichlorophosphanes and the Lewis acid GaCl(3) in various stoichiometries are investigated to obtain a deeper understanding of the species involved in these reactions. The formation of intermediates such as RPCl(2)·GaCl(3) (14) adducts, dichlorophosphanylchlorophosphonium cations [RPCl(2)-RPCl](+) (16(+)) and [RPCl(2)-RPCl-GaCl(3)](+) (17(+)) in reaction mixtures of RPCl(2) and GaCl(3) in fluorobenzene strongly depends on the basicity of the dichlorophosphane RPCl(2) (R = tBu, Cy, iPr, Et, Me, Ph, C(6)F(5)) and the reaction stoichiometry.

Preparation of Ligand‐Stabilized [P4O4]2+ by Controlled Hydrolysis of a Janus Head Type Diphosphorus Trication

A door to new opportunities: The stepwise hydrolysis of a diphosphorus trication is an efficient method for the preparation of an unusual ligand-stabilized dication that contains a novel cationic [P4O4]2+ framework (see Scheme; gray C, blue N, red O, orange P). This approach demonstrates the potential of the diphosphorus trication as a source for phosphorus building blocks to be used in the construction of novel cationic ring and cluster systems.

The chemistry of cationic polyphosphorus cages – syntheses, structure and reactivity

The aim of this review is to provide a comprehensive view of the chemistry of cationic polyphosphorus cages.

[3+2] Fragmentation of an [RP5Cl]+ Cage Cation Induced by an N‐Heterocyclic Carbene

The cage compound [DippP5 Cl][GaCl4 ] (Dipp=2,6-diisopropylphenyl) reacts with an NHC (N-heterocyclic carbene) by an unprecedented [3+2] fragmentation of the P5 (+) core. This yields an imidazoliumyl-substituted P3 species featuring a triphosphaallyl anion motif and a neutral P2 compound. The mechanism of the fragmentation reaction was elucidated by means of experimental and quantum chemical methods.

Versatile Reagent Ph3As(OTf)2: One-Pot Synthesis of [P7(AsPh3)3][OTf]3 from PCl3

Compound Ph3 As(OTf)2 as a pentacoordinated As(V) Lewis acid readily forms dicationic Lewis acid/base adducts upon addition of various Lewis bases. It also represents a stronger chloride-abstracting agent than Me3 SiOTf and facilitates the reductive coupling of PCl3 in the presence of AsPh3 to the unprecedented cation [P7 (AsPh3 )3](3+) as triflate salt. This crystallographically characterized nortricyclane-type cation represents a P7 R3 -derivative with the most electron-withdrawing substituents, resulting in a pronounced effect on the structural parameters of the P7 core.