Geminal coupling constants in methylene groups—III

Cahill, R.; Cookson, R. C.; Crabb, Trevor A.

doi:10.1016/s0040-4020(01)83013-1

Cited by 33 publications

(3 citation statements)

References 241 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Nevertheless, the large geminal coupling constant for the Si-H protons observed in the 1 H NMR spectrum of 17a ( 2 J ¼ 16 Hz; 1,2-addition across the Nb-oxo moiety) contrasted with the relatively small coupling constant observed for 3a ( 2 J ¼ 9.1 Hz; 1,2-addition across the Nb-imido moiety) is indicative of different electronic environments for these diastereotopic silyl groups. 96 While our attempts to thoroughly characterize compounds 17a-c were met with limited success, we were able to provide further support for our connectivity assignment by studying their reactivity. We found that siloxide compound 17c reacted with tert-butylisocyanate via insertion into the Nb-H bond to give the Nb k 1 -imidate 18 (Scheme 4).…”

Section: Toward Nb Oxo Imido and Suldo Imido Complexes: [2 + 2] Cyclmentioning

confidence: 86%

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

et al. 2020

View full text Add to dashboard Cite

show abstract

Section: Toward Nb Oxo Imido and Suldo Imido Complexes: [2 + 2] Cyclmentioning

confidence: 86%

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

et al. 2020

View full text Add to dashboard Cite

show abstract

“…The protons of the CH, group appear as an AB quartet (J = -15.0 Hz) centered at 2.80 ppm, very close to the chemical shift of the methylene group hydrogens in 2,2-dimethylindane, 2.75 ppm. The geminal coupling constant is assumed to have a negative sign (19). The CH signal is a singlet at 3.07 ppm, with a I3c satellite 61.0 Hz to higher frequency.…”

Section: Racemic Dimer (1) -Nmr Resultsmentioning

confidence: 99%

Conformational behaviour of highly hindered ethanes: meso- and racemic-2,2,2′,2′-tetramethyl-1,1′-biindanyl

Grindley¹,

MacLeod²,

Pincock³

et al. 1996

Can. J. Chem.

View full text Add to dashboard Cite

MM3(94), AMI, and STO-3G calculations on racemic 2,2,2',2'-tetramethyl-I, 1 '-biindanyl ( I ) show that seven minima are present on its conformational potential energy surface. 1 crystallizes in the space group Pbcn (no. 60) with cell parameters a = 1 1.190(7), b = 1 1.679(7), c = 13.19( 1) A, V = 1724(3) A3, Z = 4, R = 0.0357, R,, = 0.0362 in the conformation predicted to be the global minimum by STO-3G and AM 1, but not by MM3(94). Application of the Karplus equation and observation of a deshielded proton signal indicate that the same conformation is the major species present in solution, perhaps along with another conformation. The nleso isomer 2 shows evidence of slow exchange between conformers in its room temperature NMR spectra. Barriers to exchange were obtained from I3c NMR coalescence experiments and from 'H NMR line shape analysis of the exchanging AB pattern. The potential energy surface for 2 was calculated as for 1 and used to determine the nature of the process observed. The slow exchange was calculated to be between two enantiomeric conformers. The populated conformers for both 1 and 2 had the benzene rings from the two halves of the dimer stacked in an offset manner and the favourable interactions associated with this stacking may be a factor in the relative stabilities of these conformations.Key words: MM3, molecular mechanics, hindered rotation, dialkyldiphenylethanes, X-ray crystal structure. On a determine les bari-ieres pour 1'Cchange h partir d'expCriences de coalescence des spectres RMN du I3c et d'une analyse de la forme des raies des spectres RMN du 'H du patron AB des atomes qui s'tchangent. Comme dans le cas du produit 1, on a dCtermin6 la surface de I'Cnergie potentielle du composC 2 et on I'a utilisCe pour determiner la nature du processus observC. On a calculC que I'Cchange lent se fait entre les deux conformkres Cnantiomkres. Les conformkres les plus populeux des composCs 1 ainsi que 2 comportent des noyaux benzkniques des deux parties du dimkre empilees de f a~o n dCcalCe; il est possible que les interactions favorables associCes h cet empilement soient un facteur dans les stabilitCs relatives de ces conformations.

show abstract

“…The general trends in A values resemble those vicinal coupling in the 6-methyltetrahydro-1,4-oxazin-2-ones (4)' (J 5ax-6ax = 8.3 to 8. 6 Hz, however, corresponding to a dihedral angle 8 < 180"), than in hexahydro- This suggests that the conformation of the lactone ring in 12 resembles a classical half chair with a pseudoaxial C1 proton rather than the conformation shown in 3 adopted by the methyl substituted hexahydropyrido [2, I-c] [ 1,4]oxazin-3(4H)-ones where H9a and Hlax are trans-diaxial. The smaller geminal coupling between the C4 protons in 12 (-17.9 Hz, compared with -17.5 Hz in l), indicative of a slightly different orientation of the lactone group with respect to the C4 methylene, supports a slightly altered stereochemistry for the lactone ring in 12.…”

Section: Cis-89a-h-8-methylhexahydropyridomentioning

confidence: 97%

Proton magnetic resonance studies of compounds with bridgehead nitrogen atoms—XX: The NMR spectra and stereochemistry of some hexahydropyrido[2, 1‐c] [1, 4] oxazin‐3(4H)‐ones and related compounds

Cahill

Crabb

1972

Org. Magn. Reson.

Self Cite

View full text Add to dashboard Cite

A series of substituted hexahydropyrido[2,1-c][l,4]oxazin-3(4H)-ones has been synthesised, and the configurations of these bicyclic lactones assigned utilising chemical and spectral data. All the compounds adopt frans-fused conformations and the conformation of the lactone ring is discussed with reference to the magnitude of the geminal coupling constant of the N-CH,-C(0)-0 protons, and the vicinal couplings between the angular proton and the methylene protons adjacent to the ring oxygen atom. The lactone ring conformation is shown to differ slightly from the half chair conformation described for some monocyclic 6-lactones. The synthesis and NMR spectra of some related compounds possessing the bridgehead N-CH,-C(0)-0 system are discussed and these compounds arc also shown to adopt a trans-fused ring conformation.A VALENCE-BOND approach to the calculation of adjacent n-electron effects on the geminal coupling constant (J,,,)? between methylene group protons has produced2 a relationship dependent upon the dihedral angle (#) between the z-bond and the C-H bond. This J -C $ relationship has been tested3 for a variety of cyclic compounds possessing a CH, adjacent to a keto-group or C=C and the results are generally in accord with the prediction^.^ to Most published applications of the Barfield-Grant J -# relationship to conformational analysis refer to cyclic and very little work has been described dealing with systems possessing a modified carbonyl group (e.g. COOR, CONH,). Accordingly, some substituted hexahydropyrido-[2,1-~][1,4]oxazin-3(4H)-ones (1) and related systems were chosen for study in order to provide data on the magnitude of Jgem for methylene grosp protons adjacent to lactone carbonyl. RESULTS A N D DISCUSSION Methyl substituted hexahydropyrido[2,1-c] [1,4]oxazin-3(4H)-onesThe presence of the conformationally mobile nitrogen atom in hexahydropyrido-[2,1-c] [ 1,4]oxazin-3(4H)-one (1) permits its existence as an equilibrium mixture of trans-fused and cis-fused ring conformations. In addition, various conformations of the perhydrooxazinone ring in these structures are possible (seen in a Dreiding model by movement of H4ax' and Hleq' towards or away from each other). NMR studies on some 6-methyItetrahydr0-1,4-oxazin-2-ones,~ some substituted valerolactones8 and some steroidal lactoness are consistent with a half chair conformation for the lactone ring, with the planar C-C(0)-0-C grouping demonstrated in jacobine bromodilactone and themonoterpenoid iridomyrmecin by crystal structureanalysis.loS1l * For Part XIX, see Ref. 1. t S, , , is assumed to be negative throughout this paper. 509

show abstract

Geminal coupling constants in methylene groups—III

Cited by 33 publications

References 241 publications

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

Conformational behaviour of highly hindered ethanes: meso- and racemic-2,2,2′,2′-tetramethyl-1,1′-biindanyl

Proton magnetic resonance studies of compounds with bridgehead nitrogen atoms—XX: The NMR spectra and stereochemistry of some hexahydropyrido[2, 1‐c] [1, 4] oxazin‐3(4H)‐ones and related compounds

Contact Info

Product

Resources

About