Tautomeric Equilibria in Solutions of 2-Phenacylbenzimidazoles

Table 1

Selected ¹H and ¹³C NMR chemical shifts for 2-phenacylbenzimidazoles [2-(1H-benzo[d]imidazol-2-yl)-1-phenylethan-1-ones] K and (Z)-2-(1H-benzo[d]imidazol-2-yl)-1-phenylethen-1-ol O (solutions in DMSO-d₆, regular characters) and ¹³C CPMAS spectral data .

Comp.	Taut.	R =	H10	H1 and H3	C2	C10	C11	R
2a	K	p-OCH3	4.66	12.23	137.44	39.85	193.89	55.71,
								56.06
2a	O	p-OCH3	6.06	12.39	154.34	79.58	169.62	d
2b	K	p-CH3	4.64	12.25	137.58	40.03	195.08	21.41
								21.65
2b	O	p-CH3	6.07	12.35	154.21	80.51	168.96	e
					154.7	77.5	177.2	22.4
2c	K	m-CH3	4.66	12.33	136.57	40.01	195.67	21.33,
								21.56
2c	O	m-CH3	6.10	12.26	154.14	81.30	169.42	e
2d	K	H	4.68	12.34	136.51	40.12	195.60	-
2d	O	H	6.10	12.27	154.11	80.83	169.94	-
2e	K	p-F	4.67	12.33	135.97	40.09	194.24	-
2e	O	p-F	6.00	12.22	153.99	79.12	171.04	-
2f	K	p-Cl	4.67	12.33	135.20	39.67	194.06	-
2f	O	p-Cl	6.03	12.24	153.28	78.89	170.45	-
2g	K	p-Br	4.67	12.34	135.84	39.80	194.90	-
2g	O	p-Br	6.03	12.26	153.84	79.38	171.03
2h	K	m-F	4.70	12.34	135.64	40.26	194.66	-
2h	O	m-F	6.05	12.27	153.81	79.54	170.98	-
2i	K	p-NO2	4.78	12.39	f	f	f	-
2i	O	p-NO2	6.13	f	153.45	80.33	171.03	-
						80.3	171.1
					152.5	76.5	175.3

Literature data collected from DMSO-d₆ solutions. Literature data: 4.6 ppm - 4.8 ppm for K and 6.0 ppm - 6.25 ppm for E [4–6, 8]. Since this signal is overlapped by the solvent absorptions, DEPT technique was used to determine its position in the spectrum. Due to their comparable intensities, the signals cannot be referred to the definite tautomers ([K] ≈ [O], see Table 2). The more intense signal was assigned to the O tautomer ([K] < 50 %, see Table 2). Due to low amount of the K tautomer, these signals were not observed. Literature data [4].