Synthesis and characterization of novel carbene complexes of phosphorus(V) fluorides with potential liquid-crystalline properties

Silylimidazolium Hexafluorophosphate Salts as Synthetic Precursors to N-Heterocyclic Carbene Pentafluorophosphorus Adducts

N-Heterocyclic carbene pentafluorophosphorus (NHC-PF5) adducts are six-coordinate phosphorus(v) compounds with emerging applications but poor synthetic accessibility. We have developed a simple and high yielding protocol for synthesizing imidazolylidene NHC-PF5 adducts from silylimidazolium hexafluorophosphate salts. Using this methodology, we have prepared a series of NHC-PF5 adducts in high yields, including new NHC-PF5 building blocks amenable to subsequent synthetic diversification. We also demonstrate that a similar approach enables access to analogous, synthetically elusive NHC-BF3 and NHC-SbF5 adducts.

Synthesis and Photolysis Properties of a New Chloroquine Photoaffinity Probe

A new chloroquine-derived photoaffinity probe has been prepared by a convergent synthesis from derivative of 4,7-dichloroquinoline and N1,N1-diethyl-N4-methylpentane. The features of this probe are a unique 3-azido photolabel, the pyridine ring of the quinoline, and the presence of a secondary amine at the 4-position of the quinoline. These features, particularly the 4-amino methylation, prevent triazole formation through combination of the 3-azide and the 4-amine. This undergoes facile cleavage with exposure to a medium-pressure mercury lamp with a 254 nm excitation wavelength. Trapping of the nitrene byproduct is accomplished with its reaction with N-phenylmaleimide as its cycloazidation product. The structure of a ring-opened DBU amine has been structurally characterized.

Stretching the P-C Bond. Variations on Carbenes and Phosphanes

The stability and the structure of adducts formed between four substituted phosphanes (PX3, X:H, F, Cl, and NMe2) and 11 different carbenes have been investigated by DFT calculations. In most cases, the structure of the adducts depends strongly on the stability of the carbene itself, exhibiting a linear correlation with the increasing dissociation energy of the adduct. Carbenes of low stability form phosphorus ylides (F), which can be described as phosphane → carbene adducts supported with some back-bonding. The most stable carbenes, which have high energy lone pair, do not form stable F-type structures but carbene → phosphane adducts (E-type structure), utilizing the low-lying lowest unoccupied molecular orbital (LUMO) of the phosphane (with electronegative substituents), benefiting also from the carbene-pnictogen interaction. Especially noteworthy is the case of PCl3, which has an extremely low energy LUMO in its T-shaped form. Although this PCl3 structure is a transition state of rather high energy, the large stabilization energy of the complex makes this carbene-phosphane adduct stable. Most interestingly, in case of carbenes with medium stability both F- and E-type structures could be optimized, giving rise to bond-stretch isomerism. Likewise, for phosphorus ylides (F), the stability of the adducts G formed from carbenes with hypovalent phosphorus (PX-phosphinidene) is in a linear relationship with the stabilization of the carbene. Adducts of carbenes with hypervalent phosphorus (PX5) are the most stable when X is electronegative, and the carbene is highly nucleophilic.

Recent Advances in the Catalytic Synthesis of Imidazolidin-2-ones and Benzimidazolidin-2-ones

2-Imidazolidinone and its analogues are omnipresent structural motifs of pharmaceuticals, natural products, chiral auxiliaries, and intermediates in organic syntheses. Over the years, continuous efforts have been addressed to the development of sustainable and more efficient protocols for the synthesis of these heterocycles. This review gives a summary of the catalytic strategies to access imidazolidin-2-ones and benzimidazolidin-2-ones that have appeared in the literature from 2010 to 2018. Particularly important contributions beyond the timespan will be mentioned. The review is organized in four main chapters that identify the most common approaches to imidazolidin-2-one derivatives: (1) the direct incorporation of the carbonyl group into 1,2-diamines, (2) the diamination of olefins, (3) the intramolecular hydroamination of linear urea derivatives and (4) aziridine ring expansion. Methods not included in this classification will be addressed in the miscellaneous section.

Metal free oxidation of vinamidine derivatives: a simple synthesis of α-keto-β-diimine ligands

Oxidation of vinamidinium salts with meta-chloroperbenzoic acid is the key synthetic step towards new persistent 1,3-di(amino)oxyallyl radical cations. When applied to parent vinamidines, this protocol allows for a simple straightforward synthesis of α-keto-β-diimine ligands, for which no convenient synthesis was previously available.

Oxidation of vinamidinium salts with meta-chloroperbenzoic acid not only provides access to new persistent 1,3-di(amino)oxyallyl radical cations, but also to α-keto-β-diimine ligands, for which no convenient synthesis was previously available.

Sequential Electrophilic Substitution Reactions of Tungsten-Coordinated Phosphenium Ions and Phosphine Triflates

ion of chloride from [W(CO)₅{PPhCl₂}] with AgOSO₂CF₃ leads to the phosphine triflate complex [W(CO)₅{PPhCl(OSO₂CF₃)}] which undergoes electrophilic substitution reactions with N,N-diethylaniline, anisole, N,N-dimethyl-p-toluidine, toluene, biphenyl, naphthalene, 2,7,9,9-tetramethyl xanthene, and allyltrimethylsilane to form the chlorophosphine complexes [W(CO)₅{PPhClR}], where R = p-diethylanilinyl, p-anisyl, 2-(N,N-dimethyl-4-methylphenyl), p-tolyl, p-phenylphenyl, 1-naphthyl, 4-(2,7,9,9-tetramethylxanthyl), and allyl. Abstraction of the second chloride with AgOSO₂CF₃ leads, in most cases, to the respective phosphine triflates [W(CO)₅{PPhR(OSO₂CF₃)}], which react with ferrocene to form the ferrocenyl phosphine complexes [W(CO)₅{PPhR(C₁₀H₉Fe)}]. The W(CO)₅ unit can be removed via photolysis in the presence of bis(diphenylphosphino)ethane to form metal-free phosphines.