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17O NMR spectroscopic study of steric hindrance in phthalic anhydrides and phthalides

✍ Scribed by A.L. Baumstark; P. Balakrishnan; D.W. Boykin

Publisher
Elsevier Science
Year
1986
Tongue
French
Weight
278 KB
Volume
27
Category
Article
ISSN
0040-4039

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✦ Synopsis

170 NMR data for sterically hindered phthalic anhydrides and phthalides showed unusual deshielding effects which were correlated with in-plane bond angle distortions (X-ray results and MM2 calculations); indicative of van der Waals repulsions. Table I. 170 NMR Data (21 ppm) for For both lb and lc, two well-defined Phthalic Anhydrides (la-c) and I70 signals for the sterically dif-Phthalides (2-3) in CH3CN at 75". ferent carbonyl groups, separated by 11 and 29 ppm, respectively, were Anhydrides Lactones observed. The single bond oxygen for Cmpd. s(c=o)a a(-o-)b Cmpd. a(C=O)C a(-o-)d all the anhydrides appeared at 263kl ----ppm. The I70 data for the two t-butyl la 374 263 2a 320 170 phthalides (2c, 3) allow the assignlb 383 264 2b 332 170 ment of the downfield carbonyl signal 372e in the substituted anhydrides to the lc 396 262 2c 346 168 carbonyl adjacent to the substituent 367e Note that the "carbonyl" signal for

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