A proton magnetic resonance study of the conformation of the seven‐membered ring in benzocycloheptene

Abstract

The computer analysis of the PMR spectra of several partially deuterated benzocycloheptene derivatives at -120" provides values for all the coupling constants about the C,-C,bond. An interpretation using the Karplus equation shows unambiguously that the seven-membered ring exists as a chair conformation. The relationship between the coupling constants determined shows that the form of the Karplus equation established empirically for cyclohexane is applicable to this seven-membered ring. A comparison of coupling constants shows that the benzocycloheptene chair is more puckered than the cyclohexane chair.

NMR study of π–σ* stereoelectronic interactions in 3-substituted benzocycloheptene derivatives

“…To explain the results obtained for the polar substituted derivatives (5)(6)(7)(8)(9)(10)(11)(12)(13)(14), it is useful to divide them into two subgroups: the first one includes mainly the halogen compounds (5-8) while the second contains the oxy derivatives (8)(9)(10)(11)(12)(13)(14). The 0CH3 substituent (8) is included in both subgroups for reference purposes, as will be explained below.…”

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

“…Discussion about the C,-C, and C,-C, bonds as sugConformational Interconversion gested from a comparison with simple models It has been clearly established that the ring such ether and Propane for which conformation of benzocycloheptene is a chair barriers to methyl rotation are 2.5 and 3.3 (7) and consequently the stable conformation kcal/mol, respectively (25,261. The barrier reducof 5-oxabenzocycloheptene is undoubtedly also tion of 1.2kcal/mol is thus best explained by a a chair.…”

“…Although promising, the DAERM method is particularly limited when electronegative substituents are bonded directly to the molecular segment under consideration and the error introduced is expected to be maximal when the electronegative substituent is antiperiplanar to one of the hydrogen nuclei under consideration. It is therefore clear that these quantitative treatments are not able to define the precise geometry of 3 for comparison to that of 1 which has been determined accurately (7).…”

“…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)(3)(4)(5)(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)(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.…”

“…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.…”