Resonance energies via direct interaction method

Helal, Mustafa R.

doi:10.1016/s0166-1280(99)00500-x

Cited by 13 publications

References 6 publications

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Calenzanane Sesquiterpenes from the Red Seaweed Laurencia microcladia from the Bay of Calenzana, Elba Island: Acid‐Catalyzed Stereospecific Conversion of Calenzanol into Indene‐ and Guaiazulene‐Type Sesquiterpenes

Guella¹,

Skropeta²,

Mancini³

et al. 2003

Chemistry A European J

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It is shown here that calenzanane sesquiterpenes (1 and 6) can be isolated from organic extracts from the red seaweed Laurencia microcladia Kützing from the Bay of Calenzana, Elba Island, provided contact with acidic media is minimized. Such contact induces rearrangements of 1 in dry solvents to indene-type 5 and the blue-colored guaiazulenium-type ion 17, via spectrometrically (NMR) characterized indene-type transient intermediates 10, 14, and 12. Addition of NEt(3) to the reaction mixture at appropriate stages allowed the isolation of 12 (and 8 on workup on SiO(2)), and guaiazulene (18). Prolonged contact with silica gel led to complete degradation of 1, giving calenzanane-type epimeric enones 20 a/20 b as well as indene-type epimeric carbinols 22 a/22 b and fulvene 7. The latter was also formed during silica-gel flash chromatography of the algal extracts. A unifying mechanistic view of these branching and cascade transformations may have both heuristic value, suggesting possible artefact origin of azulenoids, and synthetic applications.

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Calenzanane Sesquiterpenes from the Red Seaweed Laurencia microcladia from the Bay of Calenzana, Elba Island: Acid‐Catalyzed Stereospecific Conversion of Calenzanol into Indene‐ and Guaiazulene‐Type Sesquiterpenes

Guella¹,

Skropeta²,

Mancini³

et al. 2003

Chemistry A European J

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Revision of the dual substituent parameter treatment using the DFT‐calculated reaction energies

Exner

Böhm

2007

J of Physical Organic Chem

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The dual substituent parameter (DSP) data treatment is a broadly used procedure correlating the reaction energies or other physical quantities with two sets of substituent constants, inductive (s I ) and resonance (s R ). It was here revised using the most extensive sets of experimental reactivities available in the literature and two sets of reaction energies calculated at the level B3LYP/6-311þG(d,p): acidities of 4-substituted benzoic acids and 5-(E)-substituted penta-2,4diene-(E)-acids with 19 or 15 common substituents. The latter two series enabled us to investigate the substituent effects more systematically than it was ever possible with the experimental data; this means in particular separate treatment of the undissociated acid molecules and of their anions, further separation of donor and acceptor substituents. In addition, the standard statistical treatment was improved when testing the significance of the resonance term. The DSP treatment is not valid generally, this applies both to the standard reference series and to the series commonly investigated. At best, DSP may be considered to hold for donor substituents but the effects of acceptors are much less variable and do not depend on the constants s R nor on any other measure of resonance. The small efficiency of acceptor substituents is due by the fact that the constant functional group (COOH in the standard series) is itself an acceptor. A correct treatment would be to investigate the donor and acceptor substituents separately, donors with an acceptor functional groups, and vice versa; substituents with weak resonance effect should be not included. The popularity and apparent success of the DSP treatment can be attributed to several grounds, most important has been the unbalanced choice of substituents.

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Conjugation of two functional groups through an unsaturated system

Exner

Böhm

2005

J of Physical Organic Chem

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Energies of 51 1‐(E),4‐(E)‐disubstituted 1,3‐butadienes (1), 36 1,4‐disubstituted benzenes (2) and 36 (E)‐1,2‐disubstituted ethenes (3) with dipolar substituents were calculated at the B3LYP/6–311 + G(d,p) level and evaluated in terms of isodesmic reactions expressing the interaction of substituents through the conjugated system. The energy of interaction reaches up to 40 kJ mol−1, it is roughly similar in the three series and most regular in the series 1. While its correlation within the framework of dual substituent parameter analysis lacks physical meaning, it is possible to separate the conjugative (resonance) component by subtracting the inductive component with reference to 1,4‐disubstituted bicyclo[2.2.2]octanes 4. The conjugative interaction is strongly stabilizing for the combination acceptor–donor and destabilizing for two donors; in these cases it is parallel to changes of geometry as they are predicted by the common resonance equations. Interaction of two acceptors is weak; in addition, there are groups that cannot be classified either as donors or as acceptors. Therefore, one can construct a scale of the resonance ability of donors in conjugation with an acceptor and vice versa, but it is not possible to express the interaction of two donors or of two acceptors on a unified scale for all substituents. The resonance description is certainly appropriate for the typical examples (interaction of NO2 and NH2) but should not be generalized to all possible structures. Copyright © 2005 John Wiley & Sons, Ltd.

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Resonance energies via direct interaction method

Cited by 13 publications

References 6 publications

Calenzanane Sesquiterpenes from the Red Seaweed Laurencia microcladia from the Bay of Calenzana, Elba Island: Acid‐Catalyzed Stereospecific Conversion of Calenzanol into Indene‐ and Guaiazulene‐Type Sesquiterpenes

Calenzanane Sesquiterpenes from the Red Seaweed Laurencia microcladia from the Bay of Calenzana, Elba Island: Acid‐Catalyzed Stereospecific Conversion of Calenzanol into Indene‐ and Guaiazulene‐Type Sesquiterpenes

Revision of the dual substituent parameter treatment using the DFT‐calculated reaction energies

Conjugation of two functional groups through an unsaturated system

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