Thiaphosphiranes and Their Complexes: Systematic Study on Ring Strain and Ring Cleavage Reactions

Abstract

A computational study on energies and geometries of a representative set of thiaphosphirane derivatives 1a-e and their W(CO) (W) and BH (B) complexes is reported. A particular focus was put on ring-opening reactions of κP- (2) and κS-complex isomers (3). Concerning the ring strain energy, a general trend was observed for compounds 1a,d, 2Wa,d, and 2Ba,d: (i) substituted rings are less strained than the parent compounds, and (ii) κP-complexation with a W(CO) group (2Wa,d) significantly increases the ring strain (5.63 and 4.38 kcal/mol) which is exceeded in the case of κP-BH complexation (2Ba,d) (7.14 and 7.22 kcal/mol). To unveil the thermal endocyclic bond weakness, a variety of bond strength related descriptors such as bond distance, relaxed force constants k, Bader's quantitative theory of atoms-in-molecules parameters such as the electron density ρ(r) and its Laplacian at bond critical points, and several bond order quantities (Wiberg bond index, Mayer bond order, and Löwdin bond order) were calculated. Heterolytic ring-opening reactions were investigated, revealing some general trends: (i) the strongest donor substituent at carbon significantly lowers relative energies for both the P-C and C-S bond cleavage products as well as the corresponding transition states, (ii) κP-complexes are more stable than the corresponding κS-complexes, for cyclic and acyclic species, and (iii) P-to-S haptotropic shifts in P-C bond cleavage products are disfavored processes, whereas it is more favored for C-S bond cleavage products. Other rearrangement products, being within energetic reach, were located on the potential energy surface. Two deserve particular mention as one stems from a combined H elimination and C-S bond cleavage of 2Bb and the other represents a first case of peribicyclic reaction leading to 7B'.

Comparative Computational Study on the Reaction of Chloroacetone with Trimethylphosphite: Perkow versus Michaelis–Arbuzov Reaction Paths

“…Local correlation schemes of type LPNO (local pair natural orbital) for high level single reference methods, , such as CEPA (coupled electron-pair approximation), here the slightly modified NCEPA/1 version implemented in ORCA was used, were computed for comparison and constituted the reference level for obtaining vertical ionization potentials and electron affinities. Very close values between these two levels has been systematically reported for close-shell systems with rmsd (root-mean-square deviation) typically below 0.2 kcal/mol, − and for this particular case amounts to 0.21 kcal/mol (see the Supporting Information) for gas-phase calculations. Also SCS-MP2 , and PWPB95-D3 , methods were checked for comparison, with rmsd values of 0.38 and 0.87 kcal/mol, respectively.…”

A Computational Study on the Stability of Oxaphosphirane Rings towards Closed‐Shell Valence Isomerization

“…Almost all C–P bond‐cleavage reactions proceed as the reverse process of a concerted four‐electron (2n O + 2σ C–P ) electrocyclization, according to the observed conrotatory stereochemistry and in agreement with the Woodward–Hoffmann orbital‐symmetry rules . The same conrotatory stereochemistry was recently reported for the C–P bond cleavage in the sulfur‐analogous thiaphosphiranes and their κ P ‐tungsten complexes . As a case in point, the transition state (TS) for the conrotatory electrocyclic C–P ring cleavage in 1a displays a HOMO showing one node between the C‐ and O‐atoms (Figure ); this feature lends aromatic character to a four‐electron (Möbius) TS, hence favouring this particular stereochemical path.…”

“…Very close values between these two levels has been systematically reported for close‐shell systems with rmsd (root mean square deviation) typically below 0.2 kcal mol –1 . [14c], , , Also, SCS‐MP2 and PWPB95‐D3 methods were checked for comparison. LPNO‐NCEPA1 and PWPB95‐D3 were found to be the most accurate methods for a set of 110 energy values, as shown by the low rmsd (root mean square deviation) values (0.20 and 0.21 kcal mol –1 , respectively), computed taking DLPNO‐CCSD(T) as the reference, using the def2‐TZVPP basis set.…”

Quantum Chemical Calculations on CHOP Derivatives—Spanning the Chemical Space of Phosphinidenes, Phosphaketenes, Oxaphosphirenes, and COP− Isomers

“…Also, properties derived from the topological analysis of the electron density, in line with Bader’s AIM (atoms-in-molecules) theory [43,44,45], in particular the electron density itself (ρ) and the Lagrangian kinetic energy ( G ) computed at the bond critical point (BCP), were calculated. The latter was only recently used in characterizing bond strength in phosphorus-containing three-membered heterocycles [46] and, soon afterwards, the Lagrangian kinetic energy density per electron ( G /ρ) was reported as a remarkable bond-strength property [47] which correlates with ring strain energies (within a series) when computed at the ring critical points [48].…”

“…As double bonds normally entail enlarged bond angles, their incorporation into small cyclic systems should be expected to result in an increase in the RSE, as occurs, for instance, in moving from cyclopropane to cyclopropene (RSE = 29.0 and 54.5 kcal/mol, respectively [52]). In the case of the parent oxaphosphirene ring system 1a , the RSE was computed by evaluating the energetics of appropriate homodesmotic reactions (Scheme 5), similar to those used for other three- [46,48,51,53,54,55,56,57] or four-membered [47] saturated phosphorus heterocycles. In such reactions, for the cleavage of every endocyclic X–Y (or X=Y) bond, a H n X–YH m (or H n X=YH m ) reagent is used, the valences of X and Y being completed with H atoms.…”