Synthesis of Dicyclopenta[a,e]pentalenes via a Molybdenum Carbonyl Mediated Tandem Allenic Pauson−Khand Reaction and the X-ray Crystal Structure of a Planar Dicyclopenta[a,e]pentalene

Abstract

Two 14 pi cross-linked annulenes which belong to the family of dicyclopenta[a,e]pentalenes have been synthesized, 14 pi bis enol triflate ester 27 and the 3,7-diisopropylsilyl substituted 14 pi dicyclopenta[a,e]pentalene 30. The new allenic tandem Pauson-Khand reaction mediated by Mo(CO)(6) was employed as the key process to construct the core of the tetracycles. The two linear dicyclopenta[a,e]pentalenes 27 and 30 underwent significant electronic delocalization, perhaps even aromaticity, as revealed by the X-ray structure of 27. The tetracyclic rings in 27 assumed a flat geometry (Figure 4); the bond lengths of the tetracycle in 27 also fit well into the criteria for aromatic compounds. A comparison of the NMR and UV spectra of both 27 and 30 demonstrated that they both exhibited similar electronic properties, therefore, the purple colored 14 pi cross linked annulene 30 is planar as well as delocalized.

Thermal [2+2] Intramolecular Cycloadditions of Fuller‐1,6‐enynes

“…The structure of this TS points to a concerted but very asynchronous addition, as suggested in previous work on similar chemical processes. [17,18] The energy barrier and the structure of this TS bear some resemblance to those found for the addition of ethylene to ketene and isocyanate [17] and also for the addition of ethylene to ethyne. In this latter case, the energy barrier computed at the UB3LYP/6-31G* level is about 7 kcal mol À1 higher than that of the corresponding 10 a!11 a reaction, as expected from the fact that the double bond in C 60 is more activated toward cycloaddition reactions than that of ethylene.…”

Exploiting [2+2] cycloaddition chemistry: achievements with allenes

“…97 Thus, when compound 201 was heated in toluene at 100 1C in the presence of a slight excess of Mo(CO) 6 (2.2 equiv.) and 10 equiv.…”

New promoters for the molybdenum hexacarbonyl- mediated Pauson–Khand reaction

“…The dissociation of the CO from the molybdenum was obviously inhibited by the presence of carbon monoxide, which retarded the generation of a vacant coordination site and of the reaction, as recently described. 12 Further exploration of "NHC" and phosphine oxide ligands will be made to achieve possible applications in asymmetric-and/or catalytic PKR.…”