Origin of the stereo- and regioselectivities in the Diels-Alder reactions of azaphospholes: a DFT investigation

Computations of the Diels-Alder (DA) reactions of azaphosphole representative namely, thiazolo[3,2-d][1,4,2]diazaphosphole with 1,3-butadiene and isoprene at the density functional theory level reveal concerted mechanisms via asynchronous transition states. The activation energies (B3LYP/6-311++G**// B3LYP/6-311G**), 16-19 kcal mol⁻¹, are much smaller than the value (32.57 kcal mol⁻¹) calculated for the DA reaction of the non-phosphorus analogue, imidazo[2,1-b]thiazole with 1,3-butadiene. An electron-withdrawing group at the 3-position of the dienophile enhances both stereo- and regioselectivities, which agree nicely with the experimental values. Inclusion of solvent effect (PCM model) reveals that the stereo- and regioselectivities are not affected appreciably. The relative stabilities of the transition structures corresponding to the endo/exo stereoisomers and meta (P/Me, 1:3)/para (P/Me, 1:4) regioisomers have been rationalized on the basis of the secondary molecular orbital interactions.     

Modern Aspects of Pseudohalogen Chemistry: News from CN‐ and PN‐Chemistry

The pseudohalogen concept still provides a powerful tool to understand the correlation between chemical properties, structure and bonding of pseudohalogen species. Starting from a modern version of the pseudohalogen concept, this research report gives an overview on different subjects of CN‐ and PN‐chemistry dealt with in our group. One of our fundamental interests lies in the synthesis and study of structure and bonding of resonance stabilized pseudohalides such as methanides, amides or binary PN‐pseudohalides. The first chapter describes new syntheses of alkaline dinitroso‐ and nitro(nitroso)cyanomethanides, methanide‐based ionic liquids and their chemical properties. The second chapter deals with azaphospholes which can be considered (i) as resonance stabilized dimers of P‐analogues of covalent azides in case of the triazadiphospholes or (ii) as [3+2] cycloaddition product of a hidden PNN 1,3 dipole (P‐analogue of a covalent azide) with dipolarophiles possessing NN or PN moieties. Starting from a [3+2] synthetic tool kit, new PN‐heterocycles (triazadiphospholes and tetrazaphospholes) and a new synthetic approach called GaCl₃‐assisted [3+2] cycloaddition are discussed.     

GaCl3-assisted [3+2] cycloaddition: A route to new binary PN-heterocycles

Starting from a [3+2] synthetic tool kit, new PN-heterocycles may be isolated according to a new synthetic approach called GaCl₃-assisted [3+2] cycloaddition. According to this concept the dipolarophile Mes^∗-NP-Cl (Mes^∗ = 2,4,6-tri-tert-butylphenyl) was reacted with the 1,3-dipole (TMS)₂N-N(TMS)-PCl₂ (TMS = trimethylsilyl) yielding the kinetically stabilized [3+2] cycloaddition product triazadiphosphole, Mes^∗N₃P₂, stabilized as GaCl₃ adduct. Synthesis, structure and bonding are discussed.     

Structural Variability of R2C Adducts of 3a,6a‐Diaza‐1,4‐diphosphapentalene: Tuning the N→P Bonding

3a,6a‐Diaza‐1,4‐diphosphapentalene (DDP) reacts with hexachlorocyclopentadiene to form a stable adduct (ClC)₄C=DDP ( 4 ). The phosphorus atom involved into coordination has a pyramidal arrangement but retains partial double bonding with carbon [1.752(3) Å]. At the same time, the P–N bond remains covalent [1.824(3) Å]. The adduct 4 is better described as a zwitterionic compound with strongly delocalized positive and negative charges. A similar zwitterionic adduct DDP=C(CN)₂ was prepared by the reactions of dichloro‐DDP ( 7 ) with malononitrile in the presence of Et₃N. DFT calculations showed that related structures are formed in the case of the substituents (ClC)₄C=, (HC)₄C=, (NC)₂C=, and (MeCO)₂C=, possessing electron‐delocalizing properties. Compounds with other R₂C groups (R = Ph, Me, C₆F₅, Cl), possessing electronegative properties as well, but insufficient e‐delocalization, demonstrate the noncovalent P–N bonding and a little shorter R₂C–P bond lengths (ca. 1.70 Å).     

Diastereo- and regioselectivity in Diels-Alder reaction of [1,4,2]diazaphospholo[4,5-a]pyridines

[1,4,2]Diazaphospholo[4,5-a]pyridines undergo diastereoselective Diels-Alder reaction at the >CP- functionality with 2,3-dimethylbutadiene and isoprene in the presence of sulfur or selenium. The reaction with isoprene occurs regioselectively. On carrying out the reaction with diene in presence of methyl iodide, the initially formed [2+4] cycloadduct is methylated regioselectively at the σ²,λ³-nitrogen. The results of the DFT calculations of the Diels-Alder reaction with isoprene are in accord with the observed regioselectivity. The relative stabilities of the two transition structures have been explained on the basis of NBO analysis.     

The structural and theoretical study of 1H-3,5-di-phenyl-1,2,4-diazaphosphole in the solid state

The N-, P-containing five-membered heterocyclic compound 1H-3,5-di-phenyl-1,2,4-diazaphosphole (1) was prepared in good yield and has been structurally characterized. 1H-3,5-Di-phenyl-1,2,4-diazaphosphole (1), crystallizing in two unexpected cyclic dimers with N-H?N hydrogen bonds, presents in the solid state a dynamic proton disorder implying a dynamic equilibrium within both dimers. The conformations of the phenyl rings, the disorder of the NH protons, and the intermolecular hydrogen bond of several 1,2,4-diazaphospholes (1-5) in the solid state have been rationalized by DFT [B3LYP/6-311++G(d,p)] calculations.