N.M.R. methyl shifts and conformational equilibria of protonated methylcyclohexanones

✍ Scribed by T.D.J. D'Silva; Howard J. Ringold

• Publisher: Elsevier Science
• Year: 1967
• Tongue: French
• Weight: 277 KB
• Volume: 8
• Category: Article
• ISSN: 0040-4039
• DOI: 10.1016/s0040-4039(00)90990-0

✦ Synopsis

The effectofadjacentcarboniumion centers on the chemical shifts ofmethylgroups in the n.m.r. spectrum has been the subject of numerous studies (1,2). However, alkyl shifts in ., systems where partial. carbonic ion character has been Introduced by protonatlon of a carbowl function (e.g. I) have not been previously investigated. The present paper deals with the n.m.r. spectra of various alkyl cyclchexanoues in 97.B sulfuric acid and illustrates that significantmethylshiftsmaybe induced in2-and3-methylcyclchexanones. Further, we report a solvent cage effect which permits the~fomation and trapping of the "less stable" transisaner of 2,6-dimethylcyclckxamne in concentrations far greater than are present in a free1 equilibratedmixture. lkble I lists the centers of the n.m.r. methyl resonance peaks in deutcrochloroform and in Sulfuric acid solutiou2. In each instance the methyl group appeared farther damfield iu sulpuric acid, A representing the shift in c.p.6. The CR3 substituents of k-t-butyl-and k-isopropyl-cyclohex, which exhibited A values of 6.5 c.p.s., are sufficiently removed fran the protonated carbowl. center to reflect the shift due to solvent alone3. 'Iherefore shifts greater thau 6.5 c.p.s. in C-2 and C-3 substituted cyclohexanones reflect deshielding 1 Supported by Grant ~-185, American Cancer Society. 2Bk,r. spectra were obtained with a Varian m/DA-6C spectmncter at a frequency of 60 Me/S. Tetxametl@_silane at0 c.p.s. was the internal standard in CDC13 while tetramethylsnmonim chloride at 192.0 c.p.8. was the standard in sulfuric acid (R.E. Rcavili, J. Chcm. Sot., 519 (1964). 3 It may be noted that virtwlly identical deshielding has been f&ml (T. D. J. D'Silv~ and II. J. Riugold, unpublished results) for the lg-methyl group of 3-desaxy steroid ami the 18-methyl. group of lV-deoxy steroids.