phenylacetaldimine.JPG

Objective

To study the formation of an imine from phenylacetaldehyde and tert-butylamine in deuterated chloroform to confirm its formation in the Ugi reaction.

Procedure

CDCl3 solutions of phenylacetaldehyde (240 ul in 2 mL, 0.88 M) and t-butylamine (208.5 ul in 2 mL, 0.89 M) are prepared in separate 1 dram vials. 800uL of the solutions are used to obtain initial H and C NMR spectra. 400uL of each solution is transfered to a clean nmr tube and shaken vigorously for a few seconds, the reaction is monited by H and C NMR.

Results

t-butylamine solution (BA)

HMR (methyls at 1.15 ppm and amine at 1.20 )
CMR

phenylacetaldehyde (39B) solution (PA)

HMR aldehyde H at 9.70 (t)
CMR It is very impure 50% approx based on integration in the 2-3 ppm region

44A (BA + PA)

HMR (t= 1 min) triplet ( J = 5.25 Hz) at 7.64 ppm shows up and 9.73 ppm goes down (81% conversion to imine)

44B

CMR (t= 4 min)

44C

HMR (t= 13 min) based on 9.73/7.64 triplets 98% conversion

44D

CMR ( t= 25 min)

44E

HMR (t= 41 min) based on 9.73/7.64 triplets 99% conversion

44F

HMR (t= 52 min) based on 9.73/7.64 triplets 98.6% conversion

44G

CMR (t= 61 min)

44H

HMR (t= 76 min) based on 9.73/7.64 triplets 98.55% conversion

44I

CMR (t= 84 min)

44J

CMR (t= 120 min)

44K

HMR (t = 130 min) based on 9.73/7.64 triplets 99.4% conversion

44L

CMR (t= 168 min)

44M

CMR (t= 209 min)

44N

HMR (t= 221 min) based on 9.73/7.64 triplets 99.5% conversion

44O

CMR (t= 258 min)

44P

HMR (t= 272 min) based on 9.73/7.64 triplets 99.55% conversion
044kinetics2.JPG

K = 8.527 /M*min
Excel Data Sheet

Discussion

This experiment is difficult to interpret because of the impurity of the phenylacetaldehyde used. Effectively there is an excess of amine, although it should kept in mind that the nature of the impurites in the aldehyde is not known. Aldol condensation products seem likely.
Notwithstanding this uncertainty, some observations can be made about the formation of the imine. We assign the triplet at 7.63 ppm (J = 5.3 Hz) as the imine H and the doublet at 3.56ppm (J = 5.3 Hz) as the methylene with which it couples.This is close to the repoted value of 5.1Hz doublet at 3.44 ppm [1] Using the appearance of the triplet at 7.66 ppm and disappearance of the triplet at 9.73 ppm, the conversion of the aldehyde to imine is 81% after 1 min then stabilizes to about 98-99% after 13 min, presumably because equilibrium has been reached. Using a starting concentration of 0.5M and 81% conversion of the aldehyde at 1 min provides a second order rate of about 10/M*min. The rate is likely higher, given the lower concentration of the aldehyde on account of its impurity. In order to obtain an accurate rate the reaction would have to be redone at much lower concentrations and with pure starting materials. Nevertheless, we can estimate that the rate of imine formation for this aliphatic aldehyde is at least three orders of magnitude faster than for the aromatic veratraldehyde (EXP045). Find ref to compare the aliphatic/aromatic rates

Conclusion

Phenylacetaldehyde must be purified by distillation before use. Imine formation between phenylacetaldehyde and t-butylamine is at least 3 orders of magnitude faster than for aromatic aldehydes.

Log

2006-12-14

1. 15:37] A 0.88 M solution of phenylacetaldeyde [fraction 39B] was prepared by adding 240uL of phenylacetaldehye in 2ml of CDCl3 in vial. Another 0.89M (assuming the volumes are additive) solution of tert-butylamine was made by adding 208.5uL of tert-butylamine in 2ml of CDCl3.
2. 15:45] A HMR of CDCL3 as a blank sample was obtained.
3. 16:00] A CMR of CDCl3 as blank sample was obtained.
4. 16:10] A HMR of tert-butylamine was obtained.
5. 16:15] A CMR of ter-butylamine was obtained.
6. 16:33] A HMR of phenylacetaldehyde was obtained.
7. 16:39] A CMR of phenylacetaldehyde was obtained
6. 16:58] 400 uL of 0.88M phenylacetaldehyde solution (made previously) was added to 400 uLof 0.89 M tert-butylamine to obtain 44A.
7. 16:59] A HMR of 44A was obtained; -- 1 min
8. 17:02] A CMR was obtained; -- 4 min
9. 17:11] A HMR was obtained; -- 13 min
10.17:23] A CMR was obtained; -- 25 min
11.17:39] A HMR was obtained; -- 41 min
12.17:50] A HMR was obtained; -- 52 min
13.17:59] A CMR was obtained; -- 61min
13.18:14] A HMR was obtained; -- 76 min
14.18:22] A CMR was obtained; -- 84 min
15.18:58] A CMR was obtained; -- 120 min
16.19:08] A HMR was obtained; -- 130 min
17.19:46] A CMR was obtained; -- 168 min
18.20:27] A CMR was obtained; -- 209 min
19.20:39] A HMR was obtained; -- 221 min
20.21:16] A CMR was obtained; -- 258 min
21.21:30] A HMR was obtained; -- 272 min

References

1. Miguel A. Esteruelas et al. Organometallics; 2005; 24(21) pp 5084 - 5094

Tags

Phenylacetaldehyde InChI=1/C12H17N/c1-12(2,3)13-10-9-11-7-5-4-6-8-11/h4-8,10H,9H2,1-3H3/b13-10-
tert-butylamine InChI=1/C4H11N/c1-4(2,3)5/h5H2,1-3H3
imine InChI=1/C8H8O/c9-7-6-8-4-2-1-3-5-8/h1-5,7H,6H2