Synthesis and Characterization of Primary Cyclopentadienylphosphines and Cyclopentadienylarsines

Abstract

Primary cyclopentadienylphosphines and cyclopentadienylarsines have been prepared by reduction of the corresponding dichloro derivatives and characterized by NMR and photoelectron spectroscopy and mass spectrometry. Their fluxional behavior has been established by both low-temperature NMR experiments and theoretical (DFT) calculations. The structure and the possible pathways to the circumambulatory rearrangement have been determined by quantum chemical computations. The high rate of the 1,2-rearrangement which favors retention of configuration at the migrating atom is explained by a low barrier due to the aromaticity of the transition states. The NMR and photoelectron spectra were assigned by making use of HF-GIAO and OVGF computations, respectively. The observed splitting of the photoelectron bands, compared to those of cyclopentadiene and EX 3 (E ) P, As; X ) H, Cl), was attributed to a hyperconjugative interaction between the cyclopentadienyl ring and the carbon-element bond.

Towards Spontaneous Heterolysis of the Homonuclear PP Bond in Diphosphines: The Case of Diazaphospholeniumtriphospholides

“…Since the energy differences between PP and PC bonded isomers in 6 H – 11 H are less than 10 kcal mol −1 , and the barrier for the diazaphospholene shift is significantly smaller (5–6 kcal mol −1 for 11 H and 12 H at B3LYP/6‐311+G**, see Supporting Information) than for the shift of a PH 2 group on cyclopentadiene (20.8 kcal mol −1 , compare reference 18), both findings may be taken as hints towards a substantial ionic bonding contribution. This idea is further supported by comparison of calculated bond lengths and Wiberg bond indexes (WBI) for the central PP bonds in the most stable conformers of 5 H – 11 H (Table 1), which discloses a general bond lengthening and concomitant decrease in covalent bond order with an increasing number of phosphorus atoms in the polyphosphole ring ( 10 H being the only exception from a strictly monotonous sequence).…”

Stability trends and tautomerization of chalcocyclopentadienes. The role of aromaticity

“…13,23,[25][26][27][37][38][39][40][41] Another aspect which renders the chemistry of cyclopentadiene particularly rich is that it can be functionalized by introducing different kinds of substituents containing main group elements to its C 5 perimeter. 8,[42][43][44][45][46][47][48][49][50] Depending on the range of electronegativities of the main element the bonding characteristics of the system and its structure may vary dramatically from an almost purely ionic interaction to a covalent one. These differences in the bonding are clearly mirrored in the structure exhibited by the corresponding derivative.…”

“…Hence, while cyclopentadienylberyllium hydride (CpBeH), in which the interaction is highly ionic, is a highly symmetric C 5v structure, 51,52 the amino- 53 or the hydroxy-derivative presents low or no-symmetry. Among the three possible isomers which can be formed by substitution of one of the five ring-carbon atoms, namely 1-, 2-and 5-substituted 1,3-cyclopentadienes (see Scheme 1), the latter have received particular attention because: (i) structurally speaking most of these compounds can be considered highly Scheme 1 fluxional systems, 6,32,36,39,46,[54][55][56] which very often easily undergo degenerate 1,5-suprafacial shifts, usually known as circumambulatory rearrangements, (ii) in Diels-Alder reactions they show a facial selectivity by adding the dienophiles preferentially syn to the heteroatom when substituted at C5 with nitrogen, oxygen or fluorine, [57][58][59][60][61][62][63][64] (iii) for substituents of groups 13, 14, and 15 (CpX, X = BH 2 , AlH 2 , GaH 2 , CH 3 , SiH 3 , GeH 3 , PH 2 , AsH 2 ) they exhibit a rather enhanced acidity with respect to the unsubstituted parent compound, 53,56,65 associated to changes in the aromatization of the systems and to significant anionic hyperconjugation effects, which reinforced the C-X bond in the anionic species. The alkaline-earth derivatives (CpX, X = BeH, MgH, CaH) are an exception, 52 as they are predicted to be weaker acids than the unsubstituted parent compound.…”

On the Origin of the Enhanced Acidity of Chalcocyclopentadienes (Cyclopentadiene Chalcogenols) in the Gas Phase

“…[12][13][14] However, the origin of this acidity enhancement seems to be not always the same, and although the high acidity of cyclopentadiene itself is a direct consequence of the aromaticity of its deprotonated form, deprotonation of many of its derivatives leads to anions which are less aromatic than C 5 H 5 À . This is the case, for instance, for cyclopentadienylphosphine and cyclopentadienylarsine, synthesized for the first time in 2001, [15] which are stronger acids than cyclopentadiene, [12] although their anions are less aromatic than the cyclopentadienyl anion. A much greater acidity enhancement was reported for cyclopentadienyl derivatives in which the substituents at all the positions of the ring are strongly electron withdrawing groups such as cyano or nitro.…”