Preparation of Baylis-Hillman adducts
Scheme 25. Illustration of rotameric options for the benzamide derivatives (117-119)
Since the rotamers exist due to exchange through an intramolecular process (Scheme 25), their NMR spectra reflect the differences in the resonance frequencies of their nuclei and the rate of exchange. The rate of exchange (rotation) is generally low at lower temperatures often resulting broad signals for particular nuclei. At high temperature, the exchange is fast and, assuming a singlet, a single sharp peak corresponding to the average NMR frequencies of the nuclei under study is observed. At intermediate and coalescence temperatures, the signals broaden and merge to give a broad signal. In 13C NMR spectra, the broadening may be sufficient to result in the apparent disappearance of the signal(s).
Above the coalescence temperature, the rate of exchange of rotamers to the point where their magnetic environments are time-averaged resulting in the detection of one set of spins with a life-time determined by spin-lattice and spin-spin relaxation mechanism. These studies are usually conducted using 1H NMR because of its sensitivity. Moreover, carbon signals tend to broaden intensely thereby disappearing into the noise.208 Figure 26 below shows dynamic 1H NMR studies of N,N-dimethylacetamide reported elsewhere,205, 206 focusing on the resonance frequencies of the N-methyl groups which become apparently chemically equivalent on rapid internal rotation at higher temperatures.
60 Figure 26. Intramolecular exchange of chemically equivalent on NMR line shapes.205
2.1.3.1.Structural elucidation of the benzamides (117-119).
Interpretation of the NMR spectra and the assignment of signals was facilitated by running spectra of the benzamides (117-119) in DMSO-d6 at different temperatures between 298 and 373 K. The rate of rotation around the N-(CO) bond at various temperatures was not determined as this study was mainly concerned with establishing a synthetic route to the target benzamides and their unambiguous characterisation. For example, Figure 27 shows the variable temperature 1H NMR spectra of 3-[(N-cyclohexylbenzamido)methyl)]-6-methoxy-(1H)-2- quinolone 117e from 313 to 373K to demonstrate facilitation of interpretation resulting from raising the temperature.
Slow exchange
Intermediate
Coalescence
Fast exchange
High T.
Low T.
νA νB
∆ν νA
∆ν
∆ν = νA – νB
61 Figure 27. Variable temperature 1H NMR spectra of compound 117e.
On comparing the spectra of compound 117e at different temperatures, it is apparent that the spectrum becomes more clearly resolved as the temperature is increased. Varying the temperature provided particular details which, when combined with the use of 2-D spectra permitted detailed structural elucidation of the benzamides (117-119). The apparent absence of the expected 1H- and/or 13C NMR N-methylene and/or N-methine signals in certain spectra is attributed to site-exchange line-broadening effects. The presence of these nuclei in such cases is, however, supported by the HRMS and, in the case of 3-[(N-cyclohexylbenzamido)methyl)]- (1H)-2-quinolone 117a, an HSQC experiment (Figure 28), conducted at 298 K, revealed a peak correlating to an N-methylene 13C signal with the observed 1H signal overlapping with the DMSO-d6 signal. The cross peak (circled in blue) corresponding to the N-methylene of the minor rotamer does confirm correlation to an apparently absent N-methylene 13C signal.
62 Figure 28. HSQC spectrum of 117a in DMSO-d6 at 298 K.
Even with a better resolved 1H NMR spectrum that shows the limited effect of signal- broadening of the methylene and the methine signals, the HSQC spectrum of 117a at 373 K reveals the apparent absence of the methylene, methine and cyclohexyl carbon signals but shows correlation of cyclohexyl carbons with corresponding proton signals (Figure 29). This analysis exemplifies the usefulness of NMR experiments obtained at different temperatures for satisfactory structural elucidation – as evidenced by the observation that at 373 K a clearer 1H NMR spectrum was obtained whilst at 298 K, a better 13C NMR was obtained.
The effect of site-exchange in the NMR analysis of compound 117a is evidenced in the HSQC spectrum at 373 K (Figure 29) by the absence of the N-methine carbon as well as the cross peaks due to signal broadening of the corresponding N-methine proton, and the absence of cyclohexyl methylene carbons. However, at 298 K, the DEPT-135 spectrum (Figure 30) showed both the methine and the methylene signals at 58.2 and 39.2 ppm, respectively as well as the three cyclohexyl methylene carbons at 24.1, 25.0 and 30.4 ppm.
CH2N
N-methine 13H /13C correlation N-methylene 13H /13C correlation
NCH N
63 Figure 29. HSQC spectrum of 117a at 373 K in DMSO-d6.
Figure 30. DEPT-135 NMR spectrum of 117a at 298 K in DMSO-d6.
In addition to the use of IR and HRMS analysis, the inherent intramolecular exchange required the use of variable temperature 1D- and 2D-NMR analyses in the characterisation of all nine benzamides (117-119) synthesised. The 1H NMR spectrum of 117a at normal probe temperature, 298 K, (Figure 31) demonstrates the complexity associated with hindered rotation in the aromatic region and reduced ring flipping in the aliphatic cyclohexyl ring. In the
Ar-C
NCH
NCH2
Cyclohexyl CH2 CH2N correlation
64 corresponding 13C NMR spectrum, a clear spectrum was obtained at 298 K as shown in the HSQC spectrum (Figure 28). The singlet at 4.42 ppm corresponds to the N-methylene protons (CH2N) whilst the signal at 3.54 ppm corresponds to the methine proton (NCH). In contrast, at 373 K, the spectrum (Figure 32) is comprised of relatively sharp, time-averaged signals resulting from fast rotation and ring flipping.
Figure 31. The 1H NMR spectrum of 117a at 298 K in DMSO-d6.
65 Figure 32. The 1H NMR spectrum of 117a at 373 K in DMSO-d6.
Figure 33 shows the 1H NMR spectrum of 3-[(N-cyclopentylbenzamido)methyl)]-6-methoxy- 2(1H)-quinolone 118e obtained at 313 K reflecting an intermediate site-exchange process.
Unlike the 1H NMR spectrum of 117a in Figure 32, and instead of a singlet representing the two N-methylene (CH2) protons, the spectrum obtained at 313 K showed a number of signals corresponding to the geminal methylene protons (CH2N) and, the methine proton (NCH) that resonate as broad signals at ca. 3.55, 4.44 and 5.26 ppm due to the intermediate exchange process. Notable, in Figure 33 are the multiple signals between 10.0 and 12.0 ppm corresponding to the quinolone NH proton, but this cannot be employed as a reliable indication of the presence of rotamers as they are isotopically exchangeable with H2O in DMSO-d6. The methoxy protons resonated at 3.79 and 3.92 ppm instead of a singlet and this was seldom encountered.
These observations signify slow intramolecular site-exchange and hence the presence of rotamers at 313 K. Although the aromatic and cyclohexyl proton signals were resolved as the temperature was raised to 333 K and 353 K, negligible improvements in the corresponding 13C NMR spectra were observed. A significant transition to fast site-exchange, beyond coalescence, for this molecule, occurs somewhere between 353 and 373 K. Such transitions
66 could be clearly observed when the temperature was increased in small increments; in our case, the temperature was raised at 20 K increments.
Figure 33. 1H NMR spectrum of 118e in DMSO-d6 at 313 K.
Figure 34 shows a stack-plot of the 13C NMR spectra of 118a which reveals the presence of methine and methylene carbon signals at 313 K which subsequently broaden and disappear into the noise as the temperature is increased from 333 K to 373 K. In the carbonyl region, the appearance of three carbonyl signals instead of two is associated with hindered rotation. The effect of hindered rotation is also evident in the resolution of aromatic carbon signals as the temperature is increased in 20 K increments. The two signals corresponding to the cyclopentyl methylene carbons were, however, not affected signifying that their chemical environment is essentially constant regardless of apparent cyclopentyl ring flipping, mainly between the envelope and the half-chair conformation.209 The IR spectrum, as expected, exhibits absorption bands at 1656 and 1621 cm-1 corresponding to the two carbonyl groups whilst the ESI mass spectrum of compound 118a reveals the pseudo-molecular ion peak at m/z = 347.1764.
67 Figure 34. A stack-plot of 13C NMR spectra of 118a at different temperatures.
Figure 35. IR spectrum of 118a.
313 K
373 K 333 K K 353 K K
68 Figure 36. ESI mass spectrum of 118a.
In light of the NMR site-exchange effects discussed above and the realisation that changing the temperature brings about significant changes in the spectrum, we reported NMR data for each of the benzamides (117-119) at a temperature at which the best NMR spectrum was obtained.
For example, the 1H NMR spectrum of 119a obtained at 333 K shows the expected ten aromatic protons. The signal at 5.27 ppm corresponds to the methylene protons, the signal at ca. 1.2 ppm the cyclopropyl methylene protons, and the signal at 11.8 ppm to the quinolone NH proton.
Figure 37. 1H NMR spectrum of 119a at 333K in DMSO-d6..
KS2
m/z
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150
%
0 100
MS_Direct_151116_19 29 (0.136) Cm (29:32-9:12) 1: TOF MS ES+
1.91e5 347.1764
158.0618
221.5656 273.1606
348.1798
504.2290 369.1570
391.2034
715.3253 693.3420
505.2332 716.3317
1061.4923 1039.5054
69 The corresponding 13C NMR spectrum (Figure 38) in conjunction with the HSQC spectrum (Figure 39), reveals the presence of four aromatic quaternary carbons at 118.6, 127.5, 129.5 and 138.3 ppm. And the signals at 160.7 and 165.4 ppm correspond to the carbonyl carbons.
The cyclopropyl methine (NCH) methylene carbons resonate at 61.6 and 23.2 ppm, respectively.
Figure 38. 13C NMR spectrum of 119a at 333K in DMSO-d6.
Figure 39. HSQC spectrum of 119a at 333 K in DMSO-d6.
*
*
*
*
* = quarternary carbon
70