Rotationsbarriere in 1,2‐Dienen; Resonanzenergie des Allyl‐Radikals

Roth, Wolfgang R.; Ruf, G.; Ford, Phillip W.

doi:10.1002/cber.19741070106

Cited by 81 publications

(39 citation statements)

References 20 publications

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“…This value is much higher than the 46.2 and 46.9 kcal mol -I barriers for the thermal racemization of optically active 2,3-pentadiene and 1,3-di-tert-butylallene, respectively, a process for which planar allenes were suggested transition state species [83]. Consistent with this result is a calculated [18] barrier to racemization of S0.1 kcal tool -I (Section II.C) corresponding to a bent, planar allene singlet diradical as the transition state.…”

Section: Planar Allenesmentioning

confidence: 83%

Reactive intermediates in the interconversions of C3H4 hydrocarbons

1984

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Section: Planar Allenesmentioning

confidence: 83%

Reactive intermediates in the interconversions of C3H4 hydrocarbons

1984

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“…20,21 When the resulting 6a 1 orbital is occupied, the energies of the C 2V structures can be lower than that of the linear structures.…”

Section: Table 4: Absolute Energies (In Au) Number Of Imaginary Freqmentioning

confidence: 97%

“…At his best level of theory, the barrier is 45.1 kcal mol -1 , 7a,c in agreement with the experimental barriers for 1,3-dimethyl-and 1,3-di-tert-butylallene (46.2 and 46.9 kcal mol -1 , respectively). 20 Somewhat higher activation barriers (which would be expected for the unsubstituted species) were obtained by Seeger 21 (50.1 kcal mol -1 ) and by Rauk et al 15 (48.0 kcal mol -1 , MP3/6-31G**//RHF/3-21G). As Roth 20 suggested and Seeger 21 et al confirmed computationally, the TS is a planar bent C 2V symmetric species corresponding to a 1 A 2 singlet biradical.…”

Section: Introductionmentioning

confidence: 95%

Ring Opening of Cyclopropylidene and Internal Rotation of Allene

et al. 1996

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Density functional theory and ab initio quantum mechanical techniques were employed to study the ring opening of singlet cyclopropylidene 1-S and the internal rotation of singlet allene 2-S. The B3LYP, CISD, and CASSCF(4,4) geometry optimizations used a TZP basis set. Single-point TZ2P energies were evaluated at CCSD(T) and multireference configuration interaction with single and double excitations (MR-CISD) levels. Employing the four most important configurations at the MR-CISD/TZ2P//UB3LYP/TZP level gave singlettriplet separations for 1 of 11.1 kcal mol -1 and for 2 48.9 kcal mol -1 . Asymmetric transition structures for the ring opening of 1-S were obtained at B3LYP/TZP and CISD/TZP. The barriers are 4.2-4.5 kcal mol -1 at CCSD(T)/TZ2P with these geometries. In contrast, only one C s symmetric TS was located at CASSCF-(4,4), and the ring opening barrier is only 3.6 and 3.1 kcal mol -1 at CCSD(T)/TZ2P and MR-CISD/TZ2P, respectively. At CCSD(T)/6-31G* an asymmetric structure for the TS results confirming the predictions of the B3LYP and CISD methods. The B3LYP/6-31G* intrinsic reaction coordinate (IRC) delineated the complex modes of rotation of the methylene groups during the ring opening. Although the overall rotation in going from 1-S to 2-S must be conrotatory, the ring opening of 1-S starts with a disrotatory motion of both methylene groups. However, before the C 1 TS is reached, the sense of rotation of one of the methylene groups changes. At the B3LYP and CASSCF(4,4) levels, the transition state for the internal rotation of allene is a planar bent (C 2V ) open-shell singlet ( 1 A 2 ). The D 2h open-shell singlet is a higher lying second-order stationary point. All D 2h and C 2V closed-shell species are significantly higher in energy. MR-CISD single-point activation barriers obtained from the B3LYP and CASSCF(4,4) geometries were 44.6 and 45.5 kcal mol -1 , respectively. The B3LYP/TZP ring opening (4.8 kcal mol -1 ) and allene internal rotation (44.6 kcal mol -1 ) barriers are in remarkably good agreement with the CCSD(T)/TZ2P//B3LYP/TZP value (4.5 kcal mol -1 ) on the barrier and with the MR-CISD/TZ2P//B3LYP/TZP barrier for the latter (44.6 kcal mol -1 ).

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“…Die Rotationsbarriere der C=C-Bindung in 1,3-Dimethylallen wurde aus der in der Gasphase ermittelten Racemisierungsgeschwindigkeit zu 46,2 kcal/mol bestimmt [19]. Unter (a und b)4) formuliert werden kann (vgl.…”

Section: Synthese Derunclassified

Eine thermische, intramolekulare [2 + 2]‐Cycloaddition eines Allenyl‐benzols; Synthese von Allenylbenzolen durch säurekatalysierte Dienol‐Benzol‐Umlagerung

Summematter

Heimgartner

1984

Helvetica Chimica Acta

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SummaryA few years ago, it has been shown that the acid-catalyzed dienol-benzene rearrangement of 2-propinyl-substituted cyclohexadienols is a convenient synthesis for allenyl-substituted benzene derivatives. The cyclohexadienols 20 and 21 were prepared via C-alkylation of the corresponding phenols with 2-propinylbromide (Scheme 3), Sollowed by reduction of the cyclohexadienones 13 and 17 with LiAIH,. Treatment of 20 and 21 with p-toluenesulfonic acid in ether at -15" yielded the desired allenyl benzenes 8 and 9, respectively, via [3,4]-sigmatropic rearrangements (Scheme 4 ) . The 2-propinylbenzenes 22-24, formed via [ 1,2]-sigmatropic shift of the 2-propinylgroup, were found as by-products. Thermolysis of allenyl benzene 8 in decdne yielded two bicyclic (25 and 26) and two tricyclic products (27 and 28; Scheme 5). For the formation of 25 and 26, a pericyclic reaction mechanism (Scheme 6) as well as a mechanism via biradical intermediates (Scheme 7 ) is discussed. A [2 + 21-cycloaddition of the a,p-allenic and the allylic C,C-double bond of 8 led to the tricyclic products 27 and 28 (Scheme 8). In this case too, a mechanism via biradical intermediates can be formulated (Scheme 9). All attempts to realize a [1,7]-sigmatropic H-shift in the allene 9 failed so far, and the starting material underwent a rapid polymerisation.

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Rotationsbarriere in 1,2‐Dienen; Resonanzenergie des Allyl‐Radikals

Abstract: Die Rotationsbarriere der C--C-Bindungen im 1,3-Dimethyl-(la) und 1,3-Di-tert-butylallen (1 b) wurde aus den in der Gasphase crrnittelten Racemisierungsgeschwindigkeiten zu 46

Cited by 81 publications

References 20 publications

Reactive intermediates in the interconversions of C3H4 hydrocarbons

Reactive intermediates in the interconversions of C3H4 hydrocarbons

Ring Opening of Cyclopropylidene and Internal Rotation of Allene

Eine thermische, intramolekulare [2 + 2]‐Cycloaddition eines Allenyl‐benzols; Synthese von Allenylbenzolen durch säurekatalysierte Dienol‐Benzol‐Umlagerung

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