Symmetry Loss in Piperidine and Morpholine by Nitrogen Coordination Angelina Flores-Parra, Gregorio Cadenas-Pliego, Rosallnda Contreras, Noe Zuniga-Villarreal and Maria de 10s Angeles PazSandoval Centro de lnvestigacion y de Estudios Avanzados del lnstituto Politecnico National, Departamento de Quimica, Apartado Postal 14-740, Mexico D. F. 07000, Mexico
We describe herein different degrees of asymmetry in four N-adducts 3-6 obtained by reaction of morpholine 1and piperidine 2 with borane-tetrahydrofurane (BHTTHF) 3 and 4 or with pentadienyltricarbonylmanganese 5 and 6 as evidenced by both the 'H and 13C nuclear magnetic resonance spectra (NMR). The ability of both Lewis acids to coordinate to the nitrogen atom in piperidine and morpholine serves to illustrate certain of symmetry, confOrmationd andvsis. and NMR suectroscoov. " .
."
Nuclear Magnetic Resonance NMR ( I ) is an analytical tool that allows one to
Table 2.
IH NMR Chemical Shifts (6, ppm) and Coupling Constants (J, Hz) of Heterocyclic Ring from Compounds 1-6
Hz, H2ax 3.62' 3b 4g 5 6 am,
Haq Haax 2.8Za
2.77' 1.5Za 3.09~ 2.7gd 3.76'3.60' 3 . 2 6 ' 2 . 5 ~ ~ 1.80a 1.55= 1.95' 2.8ga 2.52e2.12' 1.15a1,258
H4eq Hex -
Hseq Hsax 2.82'
Hseq Hsax 3.6Za
1.5Za 1SO 1.37 0.780.63
1.5Za 3 . X e 3.60' 1.55'1 .80a 2.55a 1.15a0.96a
2.77= 309' 2.7gd 2.5Zd3.26' 0.90a 1.50a1.00a
' ~ ( z e q , 2ax)=(6eq, 6ax)=13. 5; (3eq, 3ax)=(5eq, 5ax)=(2ax, NH)= (6ax, NH)=
establish not the molecular structure but also (2ax,3ax)=(~~,spx)=12.5; (2ax.3eq)=(6ax.5eq)=3.5:(2eq.3%)=(6eq,5%)=2.3. *d its dynamic behivior. Because the rate of the dy, , $d r.x ,u namic processes can be related to the time scale of gJ(2eq,2%)=(2ax,NH)=(2ax,3ax).ll.o: (2eq,NH)= (~eq,3eq)=l .A; (2ax,3eq)=3.0. the NMR, this technique may be used to obtain information concerninrr the enerev of the bonds involved in these changes. ~ n a l ~ s i sthe b f 'H and 13C cules having a plane perpendicular to the ring plane passNMR spectra of compounds 1-6 (Figs. 1and 2; Tables 1 ing through the heteroatoms in morpholine or through the and 2) clearly reveals the different degrees of symmetry N and C 4 atoms in piperidine, as a result atoms in posishown by these six-membered ring heterocycles. Moreover, tions 2 and 6 as well as 3 and 5 are equivalent (Fig. 3). the two-dimensional HETCOR spectra allows one to wrreSymmetry is important in spectroscopical analyses, for late the 13Cand 'H NMR spectra and to assign each of the example. In NMR the atoms or groups that can be exsignals unambiguously. changed by a symmetry operation also have a mametic equivalence and appear together in the spectrum T3, 4). Thus, in morpholine and piperidine methylene groups in Symmetry position 2 and 6 and 3 and 5 appear together in both 13C Symmetry (2)is a property of the shape of objects and and 'H spectra (Figs. la and 2a). molecules that determines if some parts of them are equivalent through some symmetry operation, for example, reConformational Analysis flectionthrough a center or a plane, or rotation through an axis. Morpholine and piperidine are both symmetric moleThis field concerns the study ofmobile systems known as conformers that are obtained by rotation of one or several Table 1. I3cNMR Chemical shifts (8, ppm, J, Hz) bonds (5)of a given molecule. The most stable conformer of of Heterocyclic Ring of 1-6 six-membered rings is the chair-form. In the case of morpholine and piperidine two equivalent chair conformers (A C2 C3 C4 C5 C6 and B, Fig. 31, are interconverting through low energy barriers, calculated from the 'H NMR as being 41.44 KJImole 1 45.21 66.62 66.62 45.21 for morpholine and 43.53 KJmole for piperidine (6).Nitrogen atom substitution of an H by a CH3 group raises ring 2 46.35 26.11 24.10 26.11 46.35 inversion energy barrier about 6.5 KJImole, (AG* for N3 51.95 65.67 65.67 51.95 methyl morpholine is 48.14 and for N-methylpiperidine 4 53.34 25.25 22.50 25.25 53.34 49.81 KJmole (6). Thus, the ring inversion lock in the substituted heterocycle is observed a t higher temperature 5 61.60 65.30 62.90 44.40 than i n the N-H parent compound (6). N-coordination 6 64.10 26.50 23.30 23.50 45.90 plays the same role than substitution. Also, the latter data U"
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shows t h a t morpholine has a smaller energy barrier than piperidine for ring inversion (L2 KJImole) (6). This difference can be attributed to the associated inversion of the oxygen atom. RHT ffi m In addition, nitrogen inversion E6 50 45 gives rise to two possible coniigurational isomers on the nitrogen having either an axial or an equatorial hydrogen atom, the last one being most stable (Fig. 3). The inversion barrier for the nitrogen atom was estimated a s 25.53 KJImole for piperidine (7). BeI 0 cause the energy barrier for the 2 nitrogen atom inversion is lower 1 I than the ring inversion energy, the former process is a faster phenomenon. Depending on the time scale of the NMR, the observation I frequency of a given element, and the interconversion energy it is possible to observe rigid& mobile systems (8).Thus, the 270 MHz 'H NMR spectra of morphoH-6a~ line or piperidine obtained a t H-3eq ~-3ax H-6eq H-Sax H-Zeqroom temperature evidences the C r existence of mobile systems, the 53 fast ring interconversion renders equatorial and axial protons equivalents (Figs. l a and 2a). Upon cooling the compounds, the NMR spectra of both morpholine or piperidme show first of all that the ring freezes and later at lower temperature that the nitrogen atom configuration is blocked. When a n atom i n t h e sixmembered ring bears two different substituents, including a nitrogen atom that has a hydrogen atom and a lone pair of electr&, the two faces of the ring become differentiated, and two classes of hydrogen atoms (cis and trans) appear (Fig. 3). However, in the case of morpholine or piperidine the low nitmgen inversion energy precludes differentiation of one of the two ring faces. Since the ring conformation exchange is faster than the NMR time scale only the average of these two conformations are observed. On the other hand, if the nitrogen atom inversion is blocked by N-coordination, and the dissociation enerw of the adducts is higher than thTenergy of ring inversion. onlv rine inversion would be observed. ~ L r t h e r more, if one of the substituents of the ring is bulky, only one frozen or anchored conformer would be observed. All these events were observed in 1 and 2 uDon nitmgen coordination. Thus, recent work in our laboratories afforded Figure 1. Two-dimensional NMR spectra (HETCOR)of: (a) free morpholine 1; (b) N-BH, morpholine wmplex 5 (10).The acidic N-H proton adduct 3 (9);(c)I-morpholyl-q3-penteny~tricarbon~l-mange the morpholine and piperidine and the BH, hydrides do not appear in the spectra 28 and Zb. 4 borane adducts (9) and the 1-
L
a
M
B
d
-
b
~~~~~
'
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Figure 4. Perspective view of the structure of wmplex 6 (10). Acknowledgment We are indebted to R. L. Santillan for helpful discussions and comments. Figure 3. Interconversion of conformers in morpholine 1 or piperidine 2 due to ring (I) and nitrogen (11) inversion. The free energy (KJlmol) for both interwnversions in piperidine 2 is known: AG' (I) = 43.53 (Q and DG' (11) = 25.53 (7). complex 6 is completely rigid in benzene solution, contrasting with complex 5. For both compounds 5 and 6 the wnfiguration of the nitrogen is fixed, thus resulting in the formation of a prochiral center. Hence, it is interesting to observe how NMR speetroscopy is giving us evidence of the structure, symmetry, and conformation of these six compounds in which as expected the decrease in symmetry, is evident in the complexity of the spectra.
Literature Cited 1. Demme,A. E. M a b m N M R TPehnlgues/arCh~misfry&~rch:OrganicChemistly Seriap.Vol6, Pergaman Reas, 1988. 2. Orehi", M.: Jaffe, H.H.S y m W , Orbit& and Spfftm (S. 0.81: Wlw-Jntersrienee, 1911. 3. Rothchild, R.;Venlratasubban,K S.Educ. in Chem IOK3. 26.85. 4. Colborn, R. E. J Ckm. Edue lsBO.67.438. 5. ~ l i e l E. , L. Sfomochemiafry of Carbon &om; MeGraw-Hill: Kogekuhs. LTD., Tobo, 1962.
7. Aoet, F A. L.;Ysvari, I. J.Am. Chem Soe. 1917,99,%794 8. (a)Oki,M.ApplimtionaofDy~micNMRSp~I~m~ to O r p i c C h i s f r y : VCH Publishers:Florida, 1982. (b1 Anet, F. A L.;Anet, R. In D y ~ m i eNMR Sprctmsmpy;Jackman L. M ; C o r n , F. A. Eda; Academic Reer:NewYork, 1975, Chapter 14. 9. FlarebParra, A; Farfan, N.; Hernandez-Bautiata, A. I.; Fernandee Sanchez, L. ; Contmros, R. nfmhedmn 1891,47,6903. lo. Zunip. Villmal, N.; PsrSandomI. M. A,: JosepLNathan, P.; Eaquivel, R. 0. Or ganometollies 1981,10,2616.
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