Some Pericyclic Reactions of Carbenes

Many fundamental properties of carbenes, particularly basicity, remain poorly understood. Herein, an experimental and computational examination of the deprotonation of a series of benzhydryl cations has been undertaken. These studies represent the first attempt at providing experimental values for diarylcarbene basicities. Pathways to deprotonation, including whether the singlet or triplet carbene is formed, are probed. Because diarylcarbenes are expected to be among the strongest organic bases known, assessing the energetics of protonation of these species is of fundamental importance for a wide range of chemical processes.

Section: Resultsmentioning

confidence: 99%

Gas-Phase Deprotonation of Benzhydryl Cations: Carbene Basicity, Multiplicity, and Rearrangements

Mieres‐Perez

Sánchez-García

et al. 2019

“…33,34 Direct reaction of 1 to 2 has been posited to occur either by 1,3-C−H bond insertion via TS(1/2) a (Scheme 1, path a) 5 or by C−C bond insertion via TS(1/2) b or TS(1/2) c (Scheme 1, paths b and c). 5,35 Pericyclic TS(1/2) b is unlikely (Scheme 1, path b), 22 however, because a cheletropic reaction within 1 between its C3 atom and C1−C5 bond suffers from orbital misalignment. 22,36 In contrast, the elementary step 1 → 2 via zwitterionic TS(1/2) c (Scheme 1, path c), 3,5 which stems from C−C bond heterolysis, is the likely mechanism because TS(1/2) c is lower in energy than TS(1/2) a .…”

Section: ■ Introductionmentioning

confidence: 99%

“…5,35 Pericyclic TS(1/2) b is unlikely (Scheme 1, path b), 22 however, because a cheletropic reaction within 1 between its C3 atom and C1−C5 bond suffers from orbital misalignment. 22,36 In contrast, the elementary step 1 → 2 via zwitterionic TS(1/2) c (Scheme 1, path c), 3,5 which stems from C−C bond heterolysis, is the likely mechanism because TS(1/2) c is lower in energy than TS(1/2) a .…”

Section: ■ Introductionmentioning

confidence: 99%

“…Tricyclo[2.1.0.0 2,5 ]pent-3-ylidene ( 1 ) − is an enigmatic carbene − accessible only, as yet, via computational modeling . The reactive intermediate has garnered much interest as a possible precursor to pyramidane ( 2 ), − ,, an elusive target featuring an inverted C atom. , Direct reaction of 1 to 2 has been posited to occur either by 1,3-C–H bond insertion via TS( 1 / 2 ) a (Scheme , path a) or by C–C bond insertion via TS( 1 / 2 ) b or TS( 1 / 2 ) c (Scheme , paths b and c). , Pericyclic TS( 1 / 2 ) b is unlikely (Scheme , path b), however, because a cheletropic reaction within 1 between its C3 atom and C1–C5 bond suffers from orbital misalignment. , In contrast, the elementary step 1 → 2 via zwitterionic TS( 1 / 2 ) c (Scheme , path c), , which stems from C–C bond heterolysis, is the likely mechanism because TS( 1 / 2 ) c is lower in energy than TS( 1 / 2 ) a .…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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

Rosenberg

Brinker

2020

Self Cite

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.

“…Carbenes are neutral molecules that feature a di valent C atom. − Stabilizing tetra valency is restored when the hypovalent C atom obtains a Lewis octet. If possible, this occurs after fast isomerization(s) . Computational chemistry is now widely used to supplement the experimental results of these short-lived, high-energy reaction intermediates .…”

Section: Introductionmentioning

confidence: 99%

Competitive 1,2-C Atom Shifts in the Strained Carbene Spiro[3.3]hept-1-ylidene Explained by Distinct Ring-Puckered Conformers

Rosenberg

Schrievers²,

Brinker

2016

Self Cite

Spiro[3.3]hept-1-ylidene is a markedly strained carbene reaction intermediate that was generated by high-vacuum flash pyrolysis (HVFP) of the corresponding p-tosylhydrazone sodium salt. Five hydrocarbons were produced from the Bamford-Stevens reactant in 82% overall yield. The carbene undergoes two [1,2]-sigmatropic rearrangements via competing 1,2-C atom shifts. Ring-contraction yields cyclopropylidenecyclobutane, while ring-expansion affords bicyclo[3.2.0]hept-1(5)-ene. The ring contraction is regiospecific despite the formation of some 1-methylenespiro[2.3]hexane. It does not originate from the carbene under HVFP conditions. Instead, it comes from a methylenecyclopropane-type rearrangement of chemically activated cyclopropylidenecyclobutane. Similarly, some chemically activated bicyclo[3.2.0]hept-1(5)-ene rearranges to 1,2-dimethylenecyclopentane via electrocyclic ring-opening. Accounting for the conversion of primary products to secondary ones, relative yields indicate that ring-contraction within the carbene prevails over ring-expansion by a factor of 6.7:1. Computational chemistry was used to assess the structures, conformations, energies, strain energies, transition states, and activation energies of these rearrangements with the goal of explaining product selectivities. The dual-ringed carbene is predicted to assume four distinct geometric conformations that have a bearing on transition-state selection. The reactive cyclobutylidene units of two conformers are significantly puckered, like cyclobutylidene itself, while those of the other two are flatter. The selectivity of the title carbene is compared with that of spiro[2.3]hex-4-ylidene.