In the Laboratory
Use of 15N Label in Organic Synthesis and Spectroscopy. Part I: Preparation of 15N-Labeled tert-Butylamine Erach R. Talaty,* Christopher A. Boese, Sanni M. Adewale, Mohammed S. Ismail, Frank A. Provenzano, and Melissa J. Utz Department of Chemistry, Wichita State University, Wichita, KS 67260-0051; *
[email protected] In connection with another project involving a study of selectivity in nucleophilic ring-opening of aziridinones, it became necessary to obtain 15N-labeled tert-butylamine, an unknown compound. The obvious approach to this problem is to survey known methods for the preparation of tert-butylamine (t-Bu) and focus on those that can be adapted to afford the corresponding 15N-labeled compound. If an undergraduate student were to be asked to list several methods for preparing this amine, the answer would probably be the Hofmann degradation of pivalic amide or reaction of tert-butyl chloride with ammonia. In fact, both of these methods have been investigated, but the yields are extremely small (1). Other methods that would not be found in undergraduate textbooks are catalytic hydrogenation of 2,2-dimethylethylenimine (2), reaction of tert-butylmagnesium chloride with monochloramine (3) or with O-methylhydroxylamine (4), hydrolysis of N-tert-butylformamide produced from a cyanide salt by the Ritter reaction (5), and the industrial process involving catalytic addition of ammonia to isobutylene (6 ). Although the reported yields in these methods range from moderate to good, their adaptation to the preparation of 15N-labeled amine is limited by the availability of 15N-labeled reagents. A catalog of a supplier of such reagents (Cambridge Isotope Laboratories) lists ammonia and sodium or potassium cyanide bearing this isotopic label. Since working with a solid rather than a gas was deemed more convenient, we chose to study the Ritter reaction in detail. Thus, the purpose of this experiment is for senior-level students (who have completed the usual two-semester course in organic chemistry) to gain experience with the use of 15N in organic synthesis and to study the effect of 15N on NMR and IR spectra as an advanced synthesis/spectroscopy project. Since only experienced senior-level students will be involved, the risk of using a highly toxic cyanide salt is probably reasonable. However, the person in charge of this experiment must give a well-considered briefing in advance of the hands-on part of the experiment and must be instructed to closely monitor the usage of the cyanide reagent. Loss of this material from the laboratory would pose a significant risk to the community. Experimental Procedure
Preparation of tert-butyl[15N]formamide (1) A 25-mL, round-bottomed flask containing a magnetic stirring bar was equipped with a dropping funnel and a reflux condenser protected from moisture by a calcium chloridefilled drying tube. The flask, surrounded by a water bath, was charged with 2.00 g (0.031 mol) of KC15N, 5 mL of acetic acid, and 2.29 g (0.031 mol) of tert-butyl alcohol. Into this mixture was added very slowly (over 30 min) a solution of 9 g of concentrated sulfuric acid in 5 mL of acetic acid from the dropping funnel. The dropping funnel was replaced
with a stopper, the reaction vessel was allowed to stand overnight, and the reaction mixture was then poured slowly, with ice cooling, into 60 mL of water; this was followed by gradual neutralization with 50% NaOH at 0 °C. The formamide was extracted with seven 30-mL portions of ether and distilled in vacuo after drying (potassium carbonate) and removal of the ether to yield 2.66 g (85%) bp 60–61 °C/0.2 mm (lit. [5] 202 °C/atmos. pressure). IR cm᎑1: 3282, 1677, 1539; 1H NMR (CDCl3) δ: 1.34 (d, 3H, 3J N᎑Me = 2.4), 1.38 (d, 3H, 3J N᎑Me = 2.4), 5.55 (dd, 1H, 1JN᎑H = 88.2, 3JH,H = 2.2), 6.39 (dd, 1H, 1 JN᎑H = 86.4, 3JH,H = 12.4), 8.03 (dd, 1H, 2J N᎑H = 14.8, 3JH᎑H = 2.2), 8.27 (dd, 1H, 2J15N-H = 13.6, 3JH-H = 12.4); 13C NMR (CDCl3) δ: 28.8 (d, 2J = 2.9), 30.8 (d, 2J = 4.3), 50.3 (d, 1J = 8.6), 51.3 (d, 1J = 7.2), 160.5 (d, 1J = 13.0), 160.6 (d, 1J = 11.5), 163 (d, 1J = 14.4). 15
15
15
Preparation of tert-Butyl[15N]amine Hydrochloride (3) A mixture of 1.01 g (0.010 mol) of the formamide 1 and 5 mL of concentrated hydrochloric acid was heated under reflux overnight and the solution was evaporated to dryness to furnish 1.04 g (95%) of the salt 3: mp 312 °C (lit. [5] 310 °C); IR (KBr) cm᎑1: 2789, 2693, 2581, 2486 (N–H stretches), 2046 (overtone), 1607 and 1504 (asymmetric and symmetric NH3 bend); 1H NMR (D2O) δ (Me3Si CD2CD2CO2Na as reference): 1.374 (d, J = 3.0); 13C NMR (D2O) δ: 29.3, 54.7 ( J = 3.3). CAS Registry Number for All Chemicals Potassium [15N]cyanide (5297-01-8); acetic acid (64-19-7); tert-butyl alcohol (75-65-0); sulfuric acid (7664-93-9); sodium hydroxide (1310-73-2); anhydrous potassium carbonate (58408-7); hydrochloric acid (7647-01-0); ether (60-29-7); chlorobenzene (108-40-7). Hazards Concentrated sulfuric acid and concentrated hydrochloric acid are corrosive and should be handled with care. Glacial acetic acid is an irritant and causes burns. The Ritter reaction in this experiment involves hydrogen cyanide, which is highly toxic (7), and hence the whole experiment should be conducted in a good fume hood with the sash kept as low as possible for optimum fume capture. Potassium cyanide has an oral lethal dose of 2.8 mg/kg in humans. Poisoning can occur by ingestion, absorption through injured skin, or inhalation (7). Hence, it should be transferred from the container in the hood and weighed in a closed weighing bottle. Gloves and safety glasses are essential during this process. To ensure minimum handling of this substance, we recommend that only an approximate amount of KCN be transferred to the weighing bottle and the stoichiometry of the other ingredients be adjusted accordingly after the weight of the weighing bottle is recorded, rather than vice versa. This
JChemEd.chem.wisc.edu • Vol. 79 No. 2 February 2002 • Journal of Chemical Education
221
In the Laboratory
chemical should always be stored in a sealed container. When empty, the container should be placed in a bag, sealed, and held for waste disposal. Results and Discussion The Ritter reaction in the present case employs a solution of tert-butyl alcohol in acetic acid solution to which one equivalent of sodium or potassium cyanide is added (eq 1). O
CH3 HC
N + CH3
AcOH, H2SO4
OH
C
H 2O
CH3
H C NH C CH3
CH3
CH3
(1)
1
The reaction, which occurs spontaneously when sulfuric acid is added, involves the generation of the tert-butyl carbocation and its in situ trapping by the nucleophilic nitrogen of hydrogen cyanide. Although Ritter depicted the reaction as proceeding through tert-butyl hydrogen sulfate, a better representation of the mechanism should show a discrete carbocation (Scheme I). CH3 C+
H 3C
N CH
CH3 H O CH3 H 3C
H
H
O CH3
+
H3C
N CH
C
C
H
+
N CH
CH3
CH3
Figure 1. 1H NMR (300 MHz) spectra of HCONH–t -Bu. Upper spectrum: after treatment with D 2O.
H O H
H +
CH3 H 3C
O
C N C
H
H CH3
H
H3C
CH3
O
C N C H CH3
H
+
O
H
H +
CH3 H3C
H
O
C N C H H CH3
H O H
CH3 H 3C
O
C N C H H CH3
H
+
+
O
H
H
Scheme I
Ritter apparently adopted a standard set of conditions for the preparation of several tertiary carbinamines, without attempting to optimize the yields in each case. Hence, we decided to vary the conditions to see if the yield of the formamide 1 could be improved from the reported 50%, using first the unlabeled potassium cyanide. Ritter added the sulfuric acid portionwise, keeping the temperature at 50–60 °C in an open vessel. Under similar conditions, our yield was 58%. Mixing the components slowly at 0 °C or working up the reaction mixture after it had stood for three hours instead of overnight did not change the yield significantly (46–61%). However, mixing the components gradually in a closed system with water-bath cooling, followed by workup after overnight standing, improved the yield of 1 to 85%. Working in a closed system has the added advantage of minimum escape of HCN, even in the fume hood. 222
Hydrolysis of the formamide 1 with aqueous NaOH, as described by Ritter (5), and capture of the amine 2 in a trap containing HCl furnished the hydrochloride salt 3 of tert-butylamine in 80% yield. Hydrolysis of 1 with concentrated hydrochloric acid furnished 3 more directly and in better yield (95%). Treatment of the hydrochloride salt 3 with the least amount of aqueous NaOH or sodium carbonate at 0 °C, followed by repeated extraction with cold ether, afforded an ether solution of 15N-labeled tert-butylamine, starting with KCN bearing the same label. This solution was used immediately for a study of the mode of ring-opening of aziridinones, since the amine is almost as volatile as ether. When the pure amine is desired, the extraction is performed with a high-boiling solvent (cold chlorobenzene), followed by separation of the two liquids by fractional distillation. It is instructive to study the NMR spectra of the compounds prepared—especially the influence of 15N, an isotope not normally encountered in undergraduate classes. In contrast to 14N, which has a spin quantum number (I ) of unity, 15 N has I = 1⁄2 and hence no quadrupole moment. However, because of its low natural abundance (0.37%), coupling of 15 N to other nuclei is observed only in samples enriched in this isotope. Comparison of the NMR spectra of the formamide 1 without and with 15N enrichment is a lesson in restricted rotation around the nitrogen–carbonyl single bond and enables a comparison of the 1H and 13C spectra of the compound containing 14N with the same spectra of the 15N
Journal of Chemical Education • Vol. 79 No. 2 February 2002 • JChemEd.chem.wisc.edu
In the Laboratory
Figure 2. 1H NMR (300 MHz) spectra of H–CO– 15NH– t -Bu (composite of expansions of various portions). Left spectra: after treatment with D2O.
compound. In the unenriched case (Fig. 1), there are double signals for all proton resonances (CH3, HCO, NH), indicating the presence of syn and anti forms (see structures below) in approximately equal amounts at room temperature. The NH resonances are broad because of the rapid relaxation caused by the quadrupole moment of 14N. However, the signal of the formyl proton consists of four lines owing to coupling between CH and NH in each form, the larger of the two couplings (12 Hz and 2 Hz) probably indicating the anti form (δ 8.27) and the smaller one the syn form (δ 8.03) (simplified by shaking with D2O). A copy of all spectra of unenriched 1 has been published (8). H
+
C
t-Bu
H
N
᎑
t-Bu
O
anti
H
N
᎑
H
O
+
C
syn 1
Incorporation of 15N into 1 causes two striking changes in the appearance of the 1H NMR spectrum: (i) the two broad 14 N–H signals (syn and anti forms) now become sharp signals on account of the presence of 15N, each consisting of four lines (coupling of the amide hydrogen to the formyl proton as well as to 15N), and (ii) the other proton signals are split further into doublets by virtue of coupling to 15N (Fig. 2). Shaking the sample with D2O changes the formyl signals virtually to doublets; but appearance of a small triplet in the most downfield doublets may be tentatively interpreted as a doublet (2JNH = 13.6) of 1:1:1 triplets (H–D coupling, 3J = 1.9) attributed to the deuterated anti form, with a tiny amount of overlapping unexchanged signals (3JH᎑H = 12.4) (even after 24 hours) which would distort the relative intensities of the 1:1:1 triplet. The H–H coupling constant is normally 6.6 times the H–D coupling constant. Two sets of resonances due to syn and anti forms are also observed for all carbons in the unenriched 13C NMR spectrum of 1 (Fig. 3). Incorporation of 15N leads to further
splitting of the signal of the adjacent carbon atoms (7–14 Hz) as well as that of the methyl carbons, albeit to a lesser extent (Fig. 4). An interesting observation is that although the 14N compound exhibits two signals for the carbonyl carbon in its 13C NMR spectrum (δ 161 and 163), the 15N compound reveals an extra signal that almost overlaps the one at δ 161 (all being doublets). This extra resonance may be simply due to the “keto” form H–CO–NH–t-Bu (in addition to syn and anti forms), revealed by an isotope effect on chemical shift of carbon (9), which otherwise overlaps the signals of one of the geometrical isomers. The extent of NMR coupling between 15N and the hydrogen atoms and carbon atoms in (CH3)3C–15N+H3Cl᎑, dissolved in D2O, is in accord with similar couplings in other quaternary ammonium salts. Thus, in the 1H NMR spectrum of 3, the three-bond 15N–H coupling constant across two tetracoordinate carbon atoms is 3.0 Hz, similar to that in 15 + N (CH2CH3)4 (2.5 Hz) and in (CH3CH2) 15N+(CH3)3 (3 Hz) (10). In the 13C NMR spectrum of salt 3, the one-bond 15 N–13C coupling constant is 3.3 Hz, comparable to that in 13 CH315N+H3 (
