Pn Ring Size Dependence on NHC-Induced Ring Contraction Reactions of [CoCp‴(η4-P4)], [Ta(CO)2Cp″(η4-P4)], and [FeCp*(η5-P5)]: A DFT Study

The facile ring contraction of [CoCp‴(η⁴-P₄)] and [Ta(CO)₂Cp″(η⁴-P₄)] to [CoCp‴(η³-P₃)][(^MeNHC)₂P] and [Ta(CO)₂Cp″(η³-P₃)] [(^MeNHC)₂P] induced by ^MeNHC and the absence of the ring contraction of [FeCp*(η⁵-P₅)] under the same conditions are studied by density functional theory (DFT) computations. The latter is estimated to be thermodynamically the least favorable reaction and also has a very high energy barrier. The similar strain energies of P₃ and P₄ rings and the lower strain energy of the P₅ ring play a decisive role in the ring contraction capability of these [TM-cyclo-P_n] complexes. Theoretical approaches involving NBO and IBO analysis have been employed to provide a qualitative picture of the overall reactions. The role of substituents and the nature of transition metals in determining the energetics of these reactions has also been studied and an isolobal perspective on these systems affords a simplified picture.

An Iron‐Catalyzed Route to Dewar 1,3,5‐Triphosphabenzene and Subsequent Reactivity

The application of an alkyne cyclotrimerization regime with an [Fe(salen)]₂‐μ‐oxo (1) catalyst to triphenylmethylphosphaalkyne (2) yields gram‐scale quantities of 2,4,6‐tris(triphenylmethyl)‐Dewar‐1,3,5‐triphosphabenzene (3). Bulky lithium salt LiHMDS facilitates a rearrangement of 3 to the 1,3,5‐triphosphabenzene valence isomer (3′), which subsequently undergoes an intriguing phosphorus migration reaction to form the ring‐contracted species (3′′). Density functional theory calculations provide a plausible mechanism for this rearrangement. Given the stability of 3, a diverse array of unprecedented transformations was investigated. We report novel crystallographically characterized products of successful nucleophilic/electrophilic addition and protonation/oxidation reactions.

Unprecedented solvent induced inter-conversion between monomeric and dimeric silylene–zinc iodide adducts

The reaction of silylene [PhC(NtBu)₂SiN(SiMe₃)₂] with ZnI₂leads to both monomeric [PhC(NtBu)₂Si{N(SiMe₃)₂} → ZnI₂]·THF (1) and dimeric [PhC(NtBu)₂Si{N(SiMe₃)₂} → ZnI₂]₂(2) adducts which are inter-convertible.

Reaction of N-heterocyclic silylenes with thioketone: formation of silicon-sulfur three (Si-C-S)- and five (Si-C-C-C-S)-membered ring systems

Three- and five-membered rings that bear the (Si-C-S) and (Si-C-C-C-S) unit have been synthesized by the reactions of LSiCl (1; L=PhC(NtBu)2 ) and L'Si (2; L'=CH{(CCH2 )(CMe)(2,6-iPr2 C6 H3 N)2 }) with the thioketone 4,4'-bis(dimethylamino)thiobenzophenone. Treatment of 4,4'-bis(dimethylamino)thiobenzophenone with LSiCl at room temperature furnished the [1+2]-cycloaddition product silathiacyclopropane 3. However, reaction of 4,4'-bis(dimethylamino)thiobenzophenone with L'Si at low temperature afforded a [1+4]-cycloaddition to yield the five-membered ring product 4. Compounds 3 and 4 were characterized by NMR spectroscopy, EIMS, and elemental analysis. The molecular structures of 3 and 4 were unambiguously established by single-crystal X-ray structural analysis. The room-temperature reaction of 4,4'-bis(dimethylamino)thiobenzophenone with L'Si resulted in products 4 and 5, in which 4 is the dearomatized product and 5 is formed under the 1,3-migration of a hydrogen atom from the aromatic phenyl ring to the carbon atom of the CS unit. Furthermore, the optimized structures of probable products were investigated by using DFT calculations.