On the Mechanism of Ring Inversion in Cycloheptene and Its Derivatives

1981

The conformational and dynamic properties of 1,5-dioxa-6,7-benzocycloheptene (2; 1,5-benzodioxepin) and its dimethyl derivative (7) have been investigated by dynamic 1H and 13C nmr methods. Analysis of the spectra at low temperatures (below coalescence) indicates that the most stable seven-membered ring conformations detected for solutions in CHF2Cl are a mixture of the chair (C, 80%) and the twist-boat (TB, 20%) for 2 and a mixture of TB (64%) and C (36%) for 7. Free energy barriers were determined for the chair inversion of 2 (6.5 kcal/mol) and 7 (7.5 kcal/mol) as well as for the twist-boat pseudorotation of 7 (6.8 kcal/mol). The conformational properties of these seven-membered heterocycles are discussed and compared to those known for 2,4-benzodioxepins.

Section: Conformational Averaging Processesmentioning

confidence: 72%

Conformational dynamic investigation of 1,5-benzodioxepin and its dimethyl derivative by ¹H and ¹³C nmr

Menard¹,

1981

“…The conformational inversion of the chair form of benzocycloheptene and its derivatives is known to involve a rate-determining chair-to-boat step (1,22,23). Consequently the AG* values reported i l l Table 1 characterize this interconversion process for compounds 7 to 9. Thus it is seen that the substitution of a gem-dimethyl group on C-2 affects AG* only slightly as was observed to be the case for analogous substitution on 1,3-dithiane (24).…”

Section: Conformational Dynamicsmentioning

confidence: 99%

Proton and carbon-13 nuclear magnetic resonance studies of the conformational properties of seven-membered heterocycles. 2,4-Benzodithiepin and its derivatives

Sauriol-Lord¹,

1979

F. S~L R I O L -L O R Dand hf. SF-JACQUES. Can. J . Chern. 57.3221 (1979). The conformational properties of 1,3-dithia-5,6-benzocycloheptene (7) (2,4-benzodithiepin), three 2,2-dialkyl derivatives, and two monosubstituted derivatives (2-CH, and 2-OCH,) have been investigated by 'H and I3C dnmr methods. It is reported that the most stable conformation of the seven-membered ring is a chair on which, in the case of the monosubstituted compounds, the 2-CH, group adopts an equatorial position while the 2-OCH, group takes the axial position. Only for the 2,2-pentamethylene derivative (10) was a minor amount (13%) of TB form detected. These results are markedly different from those found previously for the oxygen analogs and the conformational effect of the cyclic heteroatonis will be discussed in terms of the anomeric and steric effects. Chern. 57,3221 (1979). On a etudie les proprietes conformationnelles du dithia-1,3 benzo-5,6 cycloheptene (benzo-2,4 dithikpine) (71, de trois dkrives dialkyl-2,2 et de deux derives monosubstitues (CH3-2 et OCH,-2) en utilisant les methodes de la rmn dynamique du 'H et du I3C. Dans le cas des coniposCs monosubstitues on note que la conformation la plus stable des cycles a 7 est la forme chaise dans laquelle le groupe CH, en position 2 occupe la position Cquatoriale tandis que le groupe OCH, est axial. On a detecte de faibles quantites (13%) de forme bateau tordu seulement dans le cas du derive pentamethylene-2,2 (10). Ces resultats sont tres differents de ceux obtenus anttrieurement pour les analogues oxygenes et I'effet conformationnel des heteroatomes dans ces molkcules sera discute en termes d'effets anomkriques et stiriques.[ Traduit par le journal]Recently results from 'H and 13C nmr studies (1) of seven-membered heterocyclic compounds containing the -0-CR2-0-and -0-CHR-Omoieties such as in 2,4-benzodioxepin, its monosubstituted, and its disubstituted derivatives (i.e. 1-6), have revealed that the ring conformations were determined by the nature and pattern of substitution. Thus were observed: a mixture of chair (C) and twistboat (TB) forms for 1, a TB conformation for the dialkyl derivatives 2, 3, and 4, a C form for the 2-methyl derivative 5, and a TB form for the 2-methoxy derivative 6; that is, two different ring conformations were observed for the monosubstituted compounds.It is of obvious interest to extend such observations to ring systems containing other heteroatoms, such as S and N, in order to delineate all of the factors governing the conformational features of sevenmembered heterocycles.The current interest in the properties of 1,3-dithianes (2) suggests that a study of seven-membered heterocycles containing the -S-CR2-Sand -S-CHR-Sfragments such as for compounds 7-62 ought to yield valuable complementary information useful in characterizing the fundamental differences between the conformational properties of similar families of six and seven-membered heterocycles.Compounds 7-12 were consequently prepared and studied by 'H and 13C nmr undcr experimental conditions s...

“…In fact, it was only recently that the conformation of benzocycloheptene (1) was identified and fully characterized through a p.m.r, study (7) at -120"; that work provided accurate vicinal coupling constants for this basic molecule from which reliable dihedral angles were determined for all vicinal protons about the C,-C, (and C,-C,) bond. Our continuing systematic investigation of the cycloheptene-benzocycloheptene system (7)(8)(9) has led us to study the effect of oxygen on both the static and dynamic conformational properties of this system and possible inferences on the future stereochemical applications of the Karplus theory (10,11) to derivatives containing heteroatoms. Although much has been written about the effect of heteroatoms on vicinal coupling constants (11,12) and significant improvements have been made to account for deviations caused by oxygen (13)(14)(15), it now appears that generalizations are limited to well-defined sets of compounds and cannot be transplanted confidently from one set to another.…”

mentioning

confidence: 99%

Stereochemical Implications from the Conformational Analysis of the Seven-membered Ring in 5-Oxabenzocycloheptene

Canuel¹,

1974

Self Cite

LOUIS CANUEL and MAURICE ST-JACQUES. Can. J. Chem. 52, 3581 (1974). The computer analysis (LAOCN-3 program) of the p.m.r. spectrum of 5-oxabenzocycloheptene (3) at -105' has provided values for all coupling constants about the C3-C4 bond, while the complete line-shape simulation (DNMR program) has provided the following activation parameters: AG* = 9.5 kcal/rnol (at -57"), AH' = 9.9 kcal~mol, and AS* = 1.5 e.u. A novcl long distance (transannular) deuterium isotope effect has been discovered during the analysis of the spectrum of a partially deuterated derivative of 3. A comparison of the p.m.r. parameters for 5-oxabenzocycloheptene and benzocycloheptene provides the basic data from which stereochemical and conformational inferences can be made for more complex derivatives in this series.LOUIS CANUEL et MAURICE ST-JACQUES. Can. J. Chem. 52, 3581 (1974). La simulation a l'ordinateur (programme LAOCN-3) du spectre r.m.p. de I'oxa-5 benzocycloheptene (3) a -105 ' C a permis d'obtenir toutes les valeurs des constantes de couplage pour la liaison C3-C,; de p!us la simulation des formes raies (programme DNMR) a permis d'obtenir les parametres d'activation suivants: AG' = 9.5 kcal/mol (a -57'C), AH* = 9.9 kcal/mol et AS* = 1.5 e.u. Un nouvel effet isotopique (transannulaire) a ete mis en evidence durant l'analyse du spectre d'un derive partiellement deuterie de 3. Une cornparaison des parametres r.ni.p. entre I'oxa-5 benzocycloheptene et le benzocycloheptene etablit les donnees de base permettant de faire des deductions stereochimiques et confortnationnelles pour des derives plus complexes dans cette serie.[Traduit par le journal]Nuclear magnetic resonance spectroscopy is a particularly well-suited technique to investigate the conformational and stereochemical properties of cyclic organic molecules. Whereas five-and six-membered rings have been investigated extensively in the last 10 years (1-3), relatively little information is available on basic seven-membered cyclic compounds (2-6). In fact, it was only recently that the conformation of benzocycloheptene (1) was identified and fully characterized through a p.m.r, study (7) at -120"; that work provided accurate vicinal coupling constants for this basic molecule from which reliable dihedral angles were determined for all vicinal protons about the C,-C, (and C,-C,) bond. Our continuing systematic investigation of the cycloheptene-benzocycloheptene system (7-9) has led us to study the effect of oxygen on both the static and dynamic conformational properties of this system and possible inferences on the future stereochemical applications of the Karplus theory (10, 11) to derivatives containing heteroatoms. Although much has been written about the effect of heteroatoms on vicinal coupling constants (11, 12) and significant improvements have been made to account for deviations caused by oxygen (13)(14)(15), it now appears that generalizations are limited to well-defined sets of compounds and cannot be transplanted confidently from one set to another.Our first objective the...