Density Functional Theory Study on Dimerizations of Phospholes

Abstract

Dimerization reactions through [4 + 2] cycloadditions on 1H-, 2H-, and 3H-phospholes and cyclopentadiene were examined at the B3LYP level using 6-31G* and 6-311+G** basis sets. Activation energies for all the structures indicate that endo transition states are lower by about 1−4.5 kcal/mol compared to the corresponding exo transition states. While the transition-state stabilizations are controlled by secondary orbital interactions, the dimer stabilities are controlled by a mix of secondary orbital interactions and steric factors. Among the transition-state structures, 1-TS-endo, syn -1P-TS-endo, 2P-TS-endo4, and 3P-TS-endo2 are akin to bis-pericyclic transition-state structures and the rest are regular [4 + 2] cycloaddition transition states. Computational prediction of the facile dimerization of 2H-phospholes involving P···P bond formation is in agreement with Mathey's experimental observation. In comparison to the dimerization of cyclopentadiene, phosphole dimerizations involving the conversion of CP to C−P are stabilized by about 10 kcal/mol. However, only when conversion of CC is involved in the case of 1H-phosphole dimerizations, the activation energies are slightly higher. Further reactivities of dimers in the direction of cage formation or oligomerization are explored. The cage formation seems to be very feasible in 3H-phosphole dimers and very unlikely for 1H-phosphole dimers. Similarly, 1H- and 2H-phospholes are not expected to oligomerize further, and 3H-phospholes have a very high probability of oligomerization.

The Continuing Development of the Chemistry of Phospholes

“…This mechanism has now been studied by the computational approach, which has given support to the concept of the initial double bond rearrangement by the sigmatropic shift mechanism [111]. The trapping of the 2H-phosphole shift product in a DielsAlder reaction has also been examined in this computational study, as well as in another [115,116], giving agreement with the experimental structure Experimentally the Diels-Alder reaction of phospholes with dienophiles gives a 7-phosphanorbornene derivative with endo ring fusion and the P-substituent anti (Section 4). Calculations [60] of the relative energies of the four possible a d d u c t s f r o m m a l e i m i d e a n d 2 , 6 -d iisopropylphenylphosphole, with and without a 3-methyl on the phosphole, showed that the lowest energy product would indeed have the anti, endo structure, as generally observed (Section 4).…”

“…An important feature of 113 is that there are trimethylsilyl groups present at the 2,5-positions; they play a useful role, since the silyl groups can be transferred to the carboxylate ions formed at these positions to generate a new phospholide (114). After one carboxylate has been attached to the 2-position, a second series of reactions occurs to attach a carboxylate to the 5-position (115). The sequence of reactions for achieving the mono-carboxylate 114 is shown in Equation 38; the same reactions then lead to the di-carboxylate (115).…”

“…After one carboxylate has been attached to the 2-position, a second series of reactions occurs to attach a carboxylate to the 5-position (115). The sequence of reactions for achieving the mono-carboxylate 114 is shown in Equation 38; the same reactions then lead to the di-carboxylate (115). The rapidity of the [1,5] rearrangement, if truly in operation, is quite remarkable, requiring only 2 minutes at room temperature.…”

Bifurcations on Potential Energy Surfaces of Organic Reactions

“…Figure 6 a shows a model contour plot of a PES with two competing cycloaddition transition states, TS a and TS b, a second-order saddle point (SOSP) that connects them, and the Cope TS (TS c), corresponding to isomerization of the cycloadducts. In the diene dimerizations studied by Caramella and others, [55] TS a and TS b have merged into a single "bis-pericyclic" transition state, TS1 (Figure 6 b). On such a surface, the second-order saddle point is lost and a VRI point intervenes between the bis-pericyclic TS and the Cope transition state, TS2.…”

Phosphines and Related Tervalent Phosphorus Systems

“…808 Dimerisation of 1H-, 2H-and 3H-phospholes through [4þ2] cycloaddition reactions have been studied by density functional theory, and compared to the related dimerisation of cyclopentadiene. 809 Treatment of the [4þ2]-dimer (289) of 2,5-diphenyl-5H-phosphole with iodomethane gives a monophosphonium salt, which, with thallous ethoxide, is converted into the new chelating bis(phospholene), (290) via cleavage of the P-P bond. 810 New tricyclic phosphines, e.g., (291), have been obtained via intramolecular Diels-Alder reactions of phospholes bearing an allyloxy, allylamino or 3-buten-1-yl substituent at phosphorus.…”