Tricyclo[2.1.0.0^2,5]pent-3-ylidene: Stereoelectronic Control of Bridge-Flapping within a Nonclassical Nucleophilic Carbene

Abstract

Tricyclo[2.1.0.0 2,5 ]pent-3-ylidene is a carbene foreseen to rearrange to pyramidane ( c -C 4 H 4 )C, a highly strained molecule featuring an inverted C atom. Modeling of the carbene, using the (U)MPWB1K/cc-pVTZ//(U)MPWB1K/6-311G(d) theoretical model, indicated a large singlet–triplet energy gap (Δ E S–T = −45 kcal/mol), a high gas-phase proton affinity (PA = 258 kcal/mol), and a preference for electron-poor alkenes. These properties are consistent with those of nucleophilic carbenes. Structural differences between the C s -symmetric singlet (ω flap = ±44 deg) and C 2 v -symmetric triplet (ω flap = 0 deg) stem from nonclassical electron delocalization in the former and the lack thereof in the latter. Degenerate bridge-flapping of the singlet’s main bridge, which comprises the reactive divalent C3 atom, is computed to be slow due to a high activation barrier of the C 2 v -symmetric transition state (TS) ( E a = 17 kcal/mol). The position of the conformeric equilibrium is subject to stereoelectronic control. 1-Substituted derivatives of the carbene (R ≠ H) are sensitive to σ inductive effects. A proximal conformation is preferred when R is electron-donating and a distal one is favored when R is electron-withdrawing. Finally, carbene rearrangements via 1,2-C atom shift or enyne fragmentation were computed. The C 2 v -symmetric bridge-flapping TS has the proper geometry to initiate enyne fragmentation.

Pyramidanes: newcomers to the anti-van’t Hoff–Le Bel family

“…(2) practically planar C 4 -bases with negligible folding of 0.711 (for 11a), 0.581 (for 11b), and 1.081 (for 11c); 2.440(5)-2.453(4) Å (Pb-C in 11c), and these values were remarkably greater than the sum of the single bond covalent radii of 1.96 Å (r Ge + r C ), 81 2.15 Å (r Sn + r C ), 81 and 2.19 Å (r Pb + r C ). 81 Quite surprisingly, 1 H-, 13 C-, and 29 Si NMR spectra of all pyramidanes 11a-c were strikingly similar to each other, thus testifying for the little extent of the apex(E)-to-base[C 4 (SiMe 3 ) 4 ] bonding interaction, and accordingly to the weakness of the apical E-C bonds. The signals of the cyclic carbon atoms were found at 102.9 ppm (11a), 106.5 ppm (11b), and 114.8 ppm (11c), and these values well agree with those of the cyclobutadiene dianion dialkali and alkaline-earth metal salts of 104.1 ppm (10 2À Á[Li + (thf)] 2 ) 34 and 106.4 ppm (10 2À Á[Mg 2+ (thf) 3 ]).…”

“…9, right structure) exhibited NMR spectral and X-ray crystallographic features that were quite comparable to those of their phosphorus Like the (chlorophospha)housene 14 (Fig. 8), the (fluorostiba)housene 21 (Scheme 19, X = F) revealed a facile ''merry-go-round'' migration of the (F-Sb)-apex over the C 4 -base on the NMR timescale, visualized in its simple NMR spectra ( 1 H, 13 C, and 29 Si) showing only one set of resonances. 75 This was again realized in terms of the extraordinarily low energy barrier for this migration of only 5.1 kcal mol À1 [B3LYP/Def2TZVP level, H-substituted model] 75 (cf.…”

“…28 Remarkably, the activation barriers on going from B to A and from C to A were only 3.5 and 16.3 kcal mol À1 [at the CCSD(T)/DZP level of theory], respectively, thus making both B and C suitable immediate precursors for the synthesis of pyramidanes. 28,29 In contrast to the steric stabilization approach of the tetracoordinate pyramidal carbon employed by Radom and co-workers in their hemispiroalkaplanes, 20 pyramidane is stabilized by purely electronic factors. This stabilization is mostly caused by the formation of doubly degenerate bonding 1e-orbitals upon interaction of the p x , p y -orbitals of the apical C atom with the e g -orbitals of the cyclobutadiene base (Fig.…”

“…At first, they prepared a (sila)pyramidane precursor, namely, cyclobutene 41 featuring an exocyclic SiQC bond, unexpectedly formed upon the reaction of the tetrakis(trimethylsilyl)cyclobutadiene with Roesky's silylene PhC(N t Bu) 2 SiCl (Scheme 28). 98 Being an intramolecularly donor-stabilzied silene, 41 exhibited a high-field 29 Si NMR signal observed at À35.3 ppm. The SiQC double bond in 41 of 1.736(2) Å was in the range expected for amidinate-supported silenes.…”

“…The (sila)pyramidane 42 reacted with Fe 2 (CO) 9 to form the corresponding iron tetracarbonyl complex 43 featuring the Fe(CO) 4 unit at the Si apex (Scheme 29). 98 Upon complexation, the 29 Si NMR resonance shifted to the lower-fields: from À448.3 ppm (in 42) to À291.6 ppm (in 43).…”