Kinetics of intramolecular carbene reactions

Merrer, Dina C.; Moss, Robert A.

doi:10.1016/s1079-350x(01)80004-6

Cited by 37 publications

(34 citation statements)

References 129 publications

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“…5 kcal/mol . Triplet carbenes react orders of magnitude slower than singlet carbenes do. , Thus, 3 1 is expected to react orders of magnitude slower than 1 1 . Of course, isc cannot be ignored if the gas-phase Δ E S–T is low, although a small condensed-phase Δ E S–T tends to suggest that isc from 1 1 to 3 1 , and vice-versa, will actually be slow. ,, …”

Section: Resultsmentioning

confidence: 99%

“…Absolute rate constants ( k T )s for the elementary steps of “invisible” carbene reaction intermediates can be measured by laser flash photolysis kinetics experiments if pyridine is used to form traceable ylides. ,− The results for cyclobutyl(fluoro)carbene ( 8 ) at T = 246–313 K show that ring expansion to cyclopentene 9 is noticeably slow ( k 296 = 1.8 × 10 6 s –1 ) despite ring-strain relief (Scheme ). , The prevailing model of a conjugative intramolecular e u –p π MO interaction in a C s -symmetric (cyclobutyl)carbinyl system was used to explain this phenomenon (Figure ; cf. endo - 3 1b HOMO{−2} in Chart ).…”

Section: Resultsmentioning

confidence: 99%

“… − Carbene 11 was chosen instead of 1 to study 1,2-C atom shifts for two main reasons. The CH 3 -group on its C1′ atom avoids a potentially competitive 1,2-R group shift; CH 3 -groups have a low migratory aptitude. , Additionally, the F atom lowers Δ E S–T (i.e., Δ E S–T ≪ 0 kcal/mol), so unwanted chemistry from higher-energy 3 11 is avoided. Regrettably, no evidence for these precautions against using unfettered 1 was provided .…”

Section: Resultsmentioning

confidence: 99%

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Bent Singlet Cyclobutylcarbene: Computed Geometry, Properties, and Product Selectivity of a Nonclassical Carbene

Rosenberg

Brinker

2019

J. Org. Chem.

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Ab initio computations of cyclobutylcarbene (c-C 4 H 7 CH) were performed using the UMP4(fc)/6-311++G(2df,2p)//UMP2(full)/6-311++G(d,p) theoretical model. The carbene's most striking feature is its :CH-group. It is markedly bent toward the elongated C1′−C2′ bond in the singlet ground-state but not in the triplet state, which is at least 1.1 kcal/mol higher in energy. Nonclassical 3C2E bonding among the C1, C1′, and C2′ atoms is prominent in the HOMO{−1}. The electron-donating ability of the nonbonding HOMO is thereby enhanced. The intensified nucleophilicity of the singlet carbene is manifested in quantifiable ways. For example, its hard and soft acid and base (HSAB) hardness, HSAB absolute electronegativity, and gas-phase proton affinity rival those of ylide-stabilized Nheterocyclic carbenes. It is computed to act as a nucleophile toward alkenes with higher HSAB hardness values. Transition states from singlet cyclobutylcarbene to bicyclo[2.1.0]pentane, cyclopentene, and methylenecyclobutane were computed and confirmed by intrinsic reaction coordinate calculations. Activation energies depend on the singlet's conformation with regard to c-C 4 H 7 ring-puckering, :CH-group rotation, and :CHgroup bending. The singlet's bent :CH-group favors bicyclo[2.1.0]pentane and cyclopentene formation.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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Bent Singlet Cyclobutylcarbene: Computed Geometry, Properties, and Product Selectivity of a Nonclassical Carbene

Rosenberg

Brinker

2019

J. Org. Chem.

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“…(Figure 9.19 ). 46 Both incarcerated antiBredt olefi ns are stable at room temperature in the absence of oxygen. In aerated solution, they add O 2 to yield ketoaldehydes 68 and 69 .…”

Section: Anti -Bredt Bridgehead Olefi Nsmentioning

confidence: 99%

Reactions Inside Carcerands

Warmuth

2010

Molecular Encapsulation

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“…Dialkylcarbenes undergo rapid 1,2-H atom shift reactions if a suitable α-C–H bond is present. − This would give tricyclo[2.1.0.0 2,5 ]pent-2-ene ( 3 ) in the case of 1 (Scheme a) and adamantene ( 5 ) − in the case of adamantylidene ( 4 ) − (Scheme b). Carbene 4 does not yield 5 , however, because the bridgehead C–C double bond violates Bredt’s rule. − Strain within 5 , caused by the severely distorted π bond, impedes 4 → 5 .…”

Section: Introductionmentioning

confidence: 99%

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

Rosenberg

Brinker

2020

J. Org. Chem.

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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.

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Kinetics of intramolecular carbene reactions

Cited by 37 publications

References 129 publications

Bent Singlet Cyclobutylcarbene: Computed Geometry, Properties, and Product Selectivity of a Nonclassical Carbene

Bent Singlet Cyclobutylcarbene: Computed Geometry, Properties, and Product Selectivity of a Nonclassical Carbene

Reactions Inside Carcerands

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

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Kinetics of intramolecular carbene reactions

Cited by 37 publications

References 129 publications

Bent Singlet Cyclobutylcarbene: Computed Geometry, Properties, and Product Selectivity of a Nonclassical Carbene

Bent Singlet Cyclobutylcarbene: Computed Geometry, Properties, and Product Selectivity of a Nonclassical Carbene

Reactions Inside Carcerands

Tricyclo[2.1.0.02,5]pent-3-ylidene: Stereoelectronic Control of Bridge-Flapping within a Nonclassical Nucleophilic Carbene

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