Kinetics of a carbene rearrangement: the 1,2-carbon migration of cyclopropylchlorocarbene

Ho, Guo Jie; Krogh‐Jespersen, Karsten; Moss, Robert A.; Shen, S.; Sheridan, Robert S.; Subramanian, R.

doi:10.1021/ja00199a077

Cited by 47 publications

(30 citation statements)

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“…15,[18][19][20][21] They nicely complement and expand the well-established reactivity patterns of many other donor-substituted carbenes. [22][23][24][25][26][27][28][29] For instance, matrix-isolated tert-butylchlorocarbene (7, Scheme 1) undergoes a [1,3]CH-insertion reaction to 8 by a tunneling mechanism within 0.5-6 h at 11 K. 30 Similarly, phenylhydroxycarbene exclusively gives benzaldehyde through facile H-tunneling within 2.5 h. 31 As neither carbene is an ideal substrate to examine competing reaction paths as for 1, we had initially hoped that o-methoxyphenylhydroxycarbene (4) would provide evidence for both the intramolecular CH-insertion of the intermediate hydroxycarbene and a competing [1,2]H-shi driven by tunneling. However, 4 cannot be observed directly and only the insertion product was detected.…”

Section: Introductionmentioning

confidence: 99%

Tunneling control of chemical reactions: C–H insertion versus H-tunneling in tert-butylhydroxycarbene

2013

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Elusive tert-butylhydroxycarbene was generated in the gas phase via high-vacuum flash pyrolysis of tert-butylglyoxylic acid at 960 C. The pyrolysis products were subsequently matrix isolated in solid Ar at 11 K and characterized by means of IR spectroscopy. While still being exposed to the harsh pyrolysis conditions, the hydroxycarbene undergoes CH-insertion to dimethylcyclopropanol, as well as a CCinsertion to novel methylbutenol, with activation barriers of 23.8 and 31.0 kcal mol À1 , respectively. Once embedded in the cold Ar matrix, the carbene transforms to its isomer pivaldehyde not only by photolysis, but it also cuts through the barrier of 27.3 kcal mol À1 by quantum mechanical tunneling.The temperature independent half-life is measured as 1.7 h; the tunneling pathway was entirely blocked upon O-deuteration. The experimental half-life of tert-butylhydroxycarbene was verified by tunneling computations applying the Wentzel-Kramers-Brillouin formalism on the minimum energy path evaluated at the computationally feasible M06-2X/6-311++G(d,p) level of theory. Our experimental findings are supported by relative energy computations at the CCSD(T)/cc-pVDZ level of theory.Scheme 1 Competing pathways of [1,2]H-tunneling shifts vs. [1,n]CH-insertions in hydroxycarbenes 1, 4, and 9 as well as tert-butylchlorocarbene (7).

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Section: Introductionmentioning

confidence: 99%

Tunneling control of chemical reactions: C–H insertion versus H-tunneling in tert-butylhydroxycarbene

2013

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“…570 nm in tert-butylchlorocarbene, 22 540 nm in adamantylchlorocarbene, 16 and 460 nm in cyclopropylchlorocarbene. 23 In the present case of the 1-naphthyl system 9, the analogous weak visible absorption is at the limits of our detection, but we speculate that the strong UV band system (probably ππ*) is now moved out to the 320-426 nm region. We have found previously that CIS calculations, however, are only marginally successful in modeling the absolute wavelengths of these transitions in phenylchlorocarbene (4), 21 with excitations calculated at 531 and 233 nm vs. the observed 750 nm and 310 nm.…”

Section: Matrix Isolation Photochemistry Of 3-chloro-3-(1-naphthyl)di...mentioning

confidence: 39%

“…In closing, we note the very recent work by several groups on the related 2-naphthyl(carbomethoxy)carbene (23). 26 Interestingly, in that system both a surprisingly long-lived singlet excited state and a triplet ground state of the carbene could be observed both under matrix isolation conditions 26a and at room temperature in laser time-resolved IR experiments.…”

Section: Methodsmentioning

confidence: 63%

Matrix isolation and photochemistry of 1- and 2-naphthylchlorocarbene

Rempała¹,

Sheridan²

1999

J. Chem. Soc., Perkin Trans. 2

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We have for the first time characterized the 1-and 2-naphthylchlorocarbenes by IR and UV/vis spectroscopy in N 2 matrices at 10 K. Although evidence suggests the presence of predominantly only one geometric isomer in the case of the 1-naphthylchlorocarbene, the IR and UV/vis spectra of the 2-naphthylchlorocarbene indicate two distinct conformations. With selective irradiation, the longer-wavelength absorbing s-Z-isomer of 2-naphthylchlorocarbene can be photochemically driven to the alternate s-E conformation. Irradiation of the 1-and 2-naphthylchlorocarbenes induces cyclization to 7-chloro-4,5-benzobicyclo[4.1.0]hepta-2,4,6-triene and 7-chloro-2,3-benzobicyclo[4.1.0]hepta-2,4,6-triene, respectively. The cyclization can be reversed by irradiation at shorter wavelengths.Our results on the singlet naphthylchlorocarbenes photochemical ring-closures parallel observations of Chapman, McMahon, and co-workers on the parent triplet 1-and 2-naphthylcarbenes (

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“…However, substituents directly attached to the carbenic carbon can have a dramatic influence on the activation barriers for the 1,2-H shift to this centre (7,8). Thus, substitution of a methoxy group at the carbenic carbon of an alkylcarbene suppresses the characteristic 1,2-H migration (9) and the direct spectroscopic observation of methylmethoxycarbene in solution has been reported (10).…”

Section: Resultsmentioning

confidence: 99%

Intramolecular 1,2-alkyl shifts in unsymmetric dialkoxycarbenes studied by very low vapour pressure (VLVP) pyroiysis – mass spectrometry

Suh¹,

Pole²,

Warkentin³

et al. 1996

Can. J. Chem.

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Methoxy-(2,2,2-trifluoroethoxy)carbene radical cations, CH,O-C-OCH,CF,'+, I", are cleanly generated by the dissociative electron ionization (EI) of 2-methoxy-5,5-dimethyl-2-(2,2,2-trifluoroethoxy)-~-1 ,3,4-oxadiazoline I. Neutralization-reionization (NR) mass spectrometry of the neutral carbene 1, generated by one-electron reduction of I'+, shows no recovery ion signal and thus 1 is not a viable species within the ks time scale of the experiment. Very low vapour pressure (VLVP) pyrolysis -mass spectrometry of I in conjunction with (multiple) collision experiments shows that 1 completely isomerizes, via a 1,2-trifluoroethyl shift, into methyl 3,3,3-trifluoropropionate, CF,CH,C(=O)OCH,, l a . This technique was also used to study the related dialkoxycarbenes C,H,O-C-OCH,CF,, 2, CH,O-C-0C2H,, 3, and CH,O-C-OCH(CH,),, 4, generated from the corresponding 2,2-dialkoxy-5,5-dimethyl-A3-1 ,3,4-oxadiazolines. The pyrolytically generated carbene 2 behaves analogously to 1 and completely isomerizes to ethyl 3,3,3-trifluoropropionate, 2a. The neutral carbenes 3 and 4 undergo only a partial isomerization via 1,2-alkyl shifts in which the ethyl and isopropyl groups show a slightly greater migratory aptitude, respectively, than the methyl group. The differences in migratory aptitude are explained in terms of a transition state model similar to that of a 1,2-H shift in carbenes, with development of negative charge in the migrating group. The greater migratory aptitude of CF,CH,, as compared to CH, and CH,CH2, is attributed to the stabilization of negative charge in the transition state by strongly electron-withdrawing p-fluorines whereas the differences in migratory aptitude between the alkyl groups in 3 and 4 are largely due to the greater polarizability of isopropyl and ethyl groups, as compared to the methyl group.Key words: dialkoxycarbenes, pyrolysis, tandem mass spectrometry.Resume : On peut gCnCrer proprement les cations radicaux mtthoxy-(2.2.2-trifluoroCthoxy)carbknes, CH30-C-0CH2CF,'+, 1'+, en procCdant ? I une ionisation Clectronique (IE) dissociative de la 2-mCthoxy-5,5-dim~thyl-2-(2,2,2-trifluoroCthoxy)-~3-1 ,3,4-oxadiazoline, I. La spectroscopie par neutralisation-rkionisation (NR) du carbkne neutre, 1, gCnCrC par une rCduction un electron du 1". ne prksente pas de signal correspondant ii de la rCcupCration d'ion et le composC 1 n'est donc pas une espkce approprike ii I'tchelle de temps, ks, de I'expCrience: La spectromCtrie de masse du composC I avec pyrolyse ii trks basse tension de vapeur (VLVP), rCalisCe de concert avec des expkriences de (multiples) collisions ont montrC que le composC 1 s'isomkrise complktement, par le biais d'un dCplacement-1,2 d'un groupe trifluoroCthyle, en un 3,3,3-trifluoropropionate de mCthyle, CF,CH,C(=O)OCH,, la. On a aussi utilisk cette technique pour Ctudier des dialkoxycarbknes apparentis : C2HjO-C-OCH,CF,, 2, CH,O-C-OC,H,, 3, et CH,O-C-OCH(CH,),, 4, gCnCrCs a parti des 2,2-dialko~~-5,5-dirnCth~l-~~-1,3,4-oxadiazolines correspondantes. Le carbkne 2 prCparC d'une f a~o n py...

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Kinetics of a carbene rearrangement: the 1,2-carbon migration of cyclopropylchlorocarbene

Cited by 47 publications

References 0 publications

Tunneling control of chemical reactions: C–H insertion versus H-tunneling in tert-butylhydroxycarbene

Tunneling control of chemical reactions: C–H insertion versus H-tunneling in tert-butylhydroxycarbene

Matrix isolation and photochemistry of 1- and 2-naphthylchlorocarbene

Intramolecular 1,2-alkyl shifts in unsymmetric dialkoxycarbenes studied by very low vapour pressure (VLVP) pyroiysis – mass spectrometry

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