Intramolecular dynamics in 4- to 6-membered saturated azacycles: a MM3 study

THEO

CHEM ELSEVIER

Journal of Molecular

Intramolecular

Structure (Theochem)

429 (1998) 265-273

dynamics in 4- to 6-membered saturated azacycles: a MM3 study’

Anatoly M. Belostotskii *, Pinchas Aped, Alfred Hassner Chemistry Department, Bar-llan Universit.v,Ramat-Gan, 52900, Israel Received 3 July 1997; revised 16 September

1997; accepted

16 September

1997

Abstract We show that the MM3 force field provides a reasonable modeling of the transition states of dynamic processes related to ring inversion for azetidines, pyrrolidines and piperidines. These involve isolated ring inversion (RI) and concerted ring inversion-nitrogen inversion-C-N rotation (RINIR). Pseudorotation and concerted pseudorotation-nitrogen inversion are additional processes for piperidines. The schemes of conformational transformations (including a relationship of conformers with corresponding transition states) were established for these amines using a normal mode vibrational analysis. Pyramidal

nitrogen inversion was found to occur only as the RINIR process for azetidine and pyrrolidine compounds, while nitrogen inversion as well as RINIR are inherent to N-Me piperidine. Only the Cs mode was attributed to interconversion of the piperidine ring (differing from the known cyclohexane case). The designed conformational schemes allowed the reassignment of the previously measured barriers (NMR) for these cyclic amines. 0 1998 Elsevier Science B.V. Keywords: Ring inversion; Nitrogen inversion; Molecular mechanics; Cyclic amines

1. Introduction Intramolecular dynamics for azetidine [ 11, pyrrolidine [2] and piperidine [3] derivatives is usually considered in terms of nitrogen inversion [isolated nitrogen inversion (MI) for NH-containing heterocycles and nitrogen inversion-C-N rotation (NIR) for N-alkylated compounds [4-611 and ring inversion (RI). In other words, a typical scheme of conformational transformations, e.g. for N-alkyl azacycles, consists of four consecutive steps NIR-RI-NIR-RI (see for example Fig. 1 for piperidine 1 [7]). * Corresponding author. ’ This paper is dedicated to the memory of Prof. Gerrit Lib15

the experimental Therefore, barriers were assigned to these intramolecular motions and concerted processes, e.g. ring inversion-nitrogen inversion (RINI) or ring inversion-nitrogen inversion-C-N rotation (RINIR), were rarely considered [1-3,8-l 11. Only in a few cases the possibility of concerted processes was suggested in discussion of DNMR of piperidine derivatives [3,10,11]. However, no evidence has been presented to support or exclude this mechanism. For instance, isochronism of the geminal ring protons in the NMR spectra was observed for N-tert-Bu piperidine and di-N-tert-Bu piperazine [3]. Nevertheless, unfavourable axial orientation of the tert-Bu substituent (this orientation appears in

0166-1280/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SO1 66- 1280(97)00368-O

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RI #

--;3

11 NIR

Fig. 2. Interchange of the geminal protons via one-step conformational transformation (RINIR) in N-tert-Bu piperidine. Asterisk is a formal label.

I ~:::_.,R,N~R,,:::::+ -. -. .*_‘.*‘<, *..l.

+:-

_,-:.,>

. -.

-.:.. I. *. --_:+

RI #

Fig. 1. Simplified piperidine 1.

scheme of conformational

transformations

for

the pathways which include separated RI and NIR) was considered [3] only as an insufficient argument in favour of the one-step RINIR pathway for these N-tert-Bu derivatives (see Fig. 2). Other authors [8,9] assumed arbitrarily the barriers of concerted processes in piperidines to be higher than the barriers for the isolated processes. A concerted process (actually RINIR) was included [ 1 l] in the simplest four step conformational scheme for piperidine derivatives (see Fig. 1). However, the discussion recognized the inability to distinguish between isolated and concerted processes in these compounds based on NMR-data alone [I I]. Thus, reinvestigation of the dynamic conformational behavior of saturated azacycles is desirable. It was shown recently that MM3 force field can represent quite accurately the experimental nitrogen inversion barriers at room temperature for tertiary amines excluding the ‘bicyclic effect’ systems such as azanorbornanes [5,12]. We note here that no principal difficulties exist for the MM3-based modeling of the RI- and pseudorotation-related transition states, since the possible change in hybridization of the N-atom is already successfully handled in the NIR case [5,12,13]. We have found in the literature only one example of the use of MM3 methodology for the study of ring dynamics, on perhydroazepine [ 131. However, it is impossible to estimate the accuracy of this MM3-based methodology because no experimental barriers have been measured for azacycloheptane. Furthermore, the widely used MM2-based methodology [ 13- 161 for the study of the ring dynamics may insert an inaccuracy due to restriction

of the motion of the atoms (i.e. fixation of the geometry) during the minimization procedure for the transition states. Full matrix minimization, a MM3 component, permits the geometry optimization without such restriction. Therefore it is attractive to apply this simple but powerful tool for the study of intramolecular dynamics of rings. The present study deals with classical objects of conformational analysis, namely, methyl substituted azetidines, pyrrolidines and piperidines. Experimental values for the barriers of intramolecular dynamic processes for these compounds are known [ l-3,9,17]. We have designed the schemes of conformational transformations for these azacycles using MM3 and checked the previous assignment of the experimental barriers for certain types of intramolecular motion. Since many of these heterocycles are of pharmacological interest, often associated for amines with inversional flexibility of the amino fragment [18], an understanding of conformational transformations for these azacycles is important.

2. Experimental The 1994 version of the MM3 program [19-211 was used for molecular mechanics calculations. Stochastic search followed by full matrix NewtonRaphson minimization (option 9) was used for locating the transition states. Stable conformations were derived from transition states and independently by stochastic search followed by block diagonal minimization (option 10). The vibplot program was used for normal mode vibrational analysis. Coordinates derived from the eigen vector (produced by option 5) of vibrational modes with imaginary frequency were employed as starting coordinates for minimization in the establishment of the formal relationship between conformers and transition states.

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3. Results and discussion 3.1. Schemes of conformational transformations azetidines, pyrrolidines and piperidines We performed amines 2-7

HLI 2

5

MM3 calculations

/ND 3

for

for the cyclic

/d4

7

6

using stochastic search for the stable conformations as well as for the transition states (for details see Section 2). All stable conformers as well as transition states are included into the conformational schemes for N-Me amines 3,s and 7 (see Figs. 3-5) excluding two cases: (a) most of the the transition states for rotation of Me-substituent; and (b) the second and higher order transition states which possess an

267

appreciably higher energy (more than 2 kcal mol-’ above the highest energy first order transition state). In order to establish the relationship between conformers and transition states for each amine 3,5 and 7 intermediate structures along the conformertransition state-conformer pathway were generated using the eigen vectors related to the imaginary vibrational mode of the saddle points found by the stochastic search. Energy minimization led to the corresponding stable conformers. We were able to utilize this approach [ 13,161 using only the MM3 package (MM3 and vibplot programs) since all the needed calculation components of this methodology (full matrix minimization, normal mode vibrational analysis and calculations of the eigen vector) exist in MM3. The conformational energies (kcal mol-‘) for the optimized structures of the stable conformations or the transition states of amines 3,5 and 7 are shown in Figs. 3-5. Transition states (first order saddle points) for RI, RINIR, methyl group(-s) rotation, pseudorotation (PR) and concerted pseudorotation-nitrogen inversion-C-N rotation (PRNIR) were identified for amines 2-7 (for azetidines and pyrrolidines RI and RINIR belong to the PR type processes). Second order transition states, which are no more than 2 kcal mol-’ above the highest energy first order transition state for the corresponding compound, are

TRANSITION STATE of RI TRANSITION STATE of RINIR

Fig. 3. Scheme of conformational transformations for azetidine 3 [energies (kcal mol-‘) are relative to the lowest energy conformer; and the relative energies for the transition states are in bold]. Asterisk is a formal label.

the names

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et al.iJournal of Molecular Structure (Theochem) 429 (1998) 265-273

0.0

: * TRANSITION

STATEof

I

:

RINIR

--

5.3

TRANSlTION STATE of

RINIR

\

ON 2.1

. . . . . . . . . .._.._____ .I.&

.I \

,,

;;rx

{ 3.2

TRANSITION SlATEofT

~““TNRolJCHn

)_ 0.0

1

NMWOTA~~

:

TRANSlllON

STATE

\>d

Fig. 4. Scheme of conformational transformations for pyrrolidine 5 [energies (kcal molt’) are relative to the lowest energy conformer; the names and the relative energies for the transition states are in bold]. Asterisk is a formal label; # depicts second order transition states.

also included in the conformational schemes. For instance, for pyrrolidine 5 the one step transformation envelope form-envelope form (lowest energy conformer with pseudoequatorial NMe substituent) is possible via two transition states of different geometry with 0.3 kcal mall’ energy difference (Fig. 4). The first order RINIR transition state of a bent (twist) geometry is the lowest energy pathway and the second order RINIR transition state with a flattened ring is an alternative pathway in this transformation which leads to isochronism of the geminal ring substituents. For piperidine 7 the presence of the NIR transition state was also established. Surprisingly, no NI or NIR transition states were found for azetidines 2 and 3,4, respectively (and also for pyrrolidines 5 and 6; see Figs. 3 and 4). Pyramidal atomic inversion in these compounds occurs only as concerted RINIR process (and as RINI for the parent NH compound 2). The planar geometry of the RINI and RINIR transition states for 2 and 3,4, respectively, resembles the structure that was calculated [22] ab initio for 2, but has been attributed to an IN1 transition state. Intramolecular dynamics for azetidines 2 and 4 is basically the same as for 3. Additional transition states, which correspond to isolated as well as concerted rotation (second order transition state) of the

methyl groups and another RI - N-Me rotation transition state (second order transition state) were found in 4 (see Fig. 6 for the calculated barrier values). The 1,4-half-chair transition state (i.e. the C2 mode transformation) with a planar amino fragment was assumed [ 171 as the rate-determining transition state for inversional processes in 7, i.e. similar to that of cyclohexane derivatives [ 10,23-251. Our MM3 calculations, performed during this study, indeed confirm the accepted conformational scheme for cyclohexane [23-251, including a first order CZ transition structure and a slightly higher energy second order Cs one. The stereodynamics of six-membered ring 7 is found now to be different from the cyclohexane case. The pseudorotation cycle (Fig. 5) is the same for the carboand aza-analogs. The ‘entry’ into this cycle of transformations is of the Cz mode (through the half-chair transition state) and of the Cs mode (through the sofa transition states) for the hydrocarbon compound (with a C2 mode preference) [23] but only of the Cs mode for the azacompound. It is remarkable that this ‘entry’ for 7 takes place through 1,4-boat and not through the potential 2,5-boat forms (the corresponding 2- and 3-sofa or 2,5-half-chair transition states are absent). However, extrapolation of these conclusions to the more crowded piperidines (e.g. to

A.M. Belostotskii

TRANSITION _......-. STATE of RI

TRANSITION STATE of RINIR

4-sofa

1-sofa \

0.0

269

et al.lJournal of Molecular Structure (Theochem) 429 (1998) 265-273

12.3

9x

4.1

0.0

chair

2,5-@&t

2,Stwist

TRANSITION

TRANSITION STATE of PR

STATEof PR

II

1,4-twist (planar N)

1,4-h&t

1 ,Chrvist 8.3

8.3 \

fl N--. /

11

. 9.6 2.6~boot TRANSITION STATEof PR

TRANSITION STATEof NIR

‘Qi.a/ // L-/

1

u

-==e

8.7 2,5-twist

TRANSITION STATE of PR

8.7

10.0

2.5~twist

II .

TRANSITION STATE of PR

chair (planar N) 9.1

I

* throughthe Me rotation transitionstate

/

chair 2.4

1-sofa 11.9

li TRANSITION STATE of RI

Fig. 5. Scheme of conformational transformations for piperidine 7 [energies (kcal mol-‘) are relative to the lowest energy conformer; and the relative energies for the transition states are in bold]. Asterisk is a formal label.

1) is not straightforward. Indeed, our preliminary calculation results for 1,2,2,6,6_pentamethylpiperidine 8 and related compounds lead to a more complicated scheme of conformational transformations

the names

involving the half-chair (i.e. the C2 mode transformation) and 2- and 3-sofa transition states. In addition, both PRNIR and NIR are inherent to stereodynamics of amine 7. The PRNIR process

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A / +~~_~~N~~__~~&+ Fc/ I+--*

TRANSITION STATE o‘R,~nd,SR

(______ ___._ _ . .._..... 5.4#

*

.

2.2

5.3

.i

I

l

0.0

2.3

‘,,, i

L-

7:.9#

TRANSITION STATEof RI

8.1 TRANSITION STATE of RINIR

0.0

L

is

Fig. 6. Simplified conformational scheme for azetidine 4. Dotted arrows join the values of the rotation barriers and the corresponding groups; asterisk is a formal label; # depicts second order transition states.

occurs for 7 ‘within’ the pseudorotation cycle (through 2,5-boat and I,4 boat with a planar amino fragment) while RINIR occurs ‘outside’ this transformation cycle (through the 4-sofa transition state). 3.2. Assignment

of the NMR-measured

barriers

Each step in the designed conformational schemes is characterized quantitatively. The relative energies of the conformers and the transition states of amines 2-7 (relative to the minimal energy conformer) are given in Figs. 3-6. These data permit a revision of the NMR-based assignment of the experimental barriers for azetidines 2-4, pyrrolidines 5,6 and piperidine 7 (reviewed in Refs [l-3]). It may be also noted that we compare experimental AC’ values (extrapolated to 298 K) with MM3-calculated AE values (for explanation of this approach see Ref [S]). One has to bear in mind that when comparing calculated barriers with experimental ones, the highest energy transition state along the lowest energy conformational pathway corresponds to the NMR-measured barrier. One RINIR step, as well as two consecutive NIR and RI steps, lead to the isochronism of the geminal substituents in 3-7. This isochronism was observed in the NMR spectra of these compounds at ambient temperature (see Refs [l-3,17,22,26,27] and references therein). Since RINIR excludes the NIR process for azetidines 3,4 and pyrrolidines 5 and 6 (see Section l),

methyl

the NMR-measured barriers for these compounds should belong to RINIR and not to NIR as was previously accepted [ 1,2]. In the case of NH compound 2 experimental barrier should be refered to RINI and not to INI. Two conclusions are derived from this statement: (a) The decrease of the experimental barriers in the sequence aziridines > azetidines > pyrrolidines is often used (see for example Ref. [3]) as an illustration of the influence of ring strain on the value of the NIR barrier (or on the IN1 barrier for the NH compounds). However, one can conclude now that this widespread comparison is not relevant. The measured barriers belong to different processes: while the inversion process in aziridines is only NIR, this process for azetidine and pyrrolidine compounds is a concerted RINIR (RINI for the secondary azacycles). (b) Unusually high NIR barriers for 7azabicyclo[2.2. llheptanes 9 [5,26-291 (see Fig. 7; N-alkyl substituent is not N-tert-Bu [5]) relative to those of the monocyclic components of the bicyclic system as well as barriers for related bicyclic systems were analyzed using different theoretical approaches [5,27,30,3 11. MM3 calculations of the NIR barrier for azacycle 10a (15.3 kcal mol-‘) [12] permit the attribution of amines 10 also to the ‘high NIR barrier’ systems. For comparison, while the experimental NIR barrier for 9a is

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R--N

R---N

HIGHNIR BARRIER BICXCLIC SYSTEMS

g .

Lfzl

10

R---N

A

14

IOa: R = Me

9a:R=Me

9,10,14: R = Alk or Hal

13.77 kcal mol-I 1271, those for 2-azabicyclo[2.2.1]heptane 11 [30] or the N-bridged systems, 8-azabicyclo[3.2.l]octane 12 [32] and 9azabicyclo[3.3.l]nonane 13 [33], are 7.2, 9.2 and 8.1 kcal mall’, respectively. Thus, an increase of the NIR barriers for systems 9 and 10 is inherent to two azetidine or pyrrolidine rings which are rigid due to their inclusion into a nitrogen-bridged bicyclic backbone. Such ring systems can not achieve the RINIR transition state during inversion due to the rigidity of the bicyclic skeleton (in contrast to monocyclic azetidines or pyrrolidines). To achieve some flexibility (e.g. observed by NMR for 9 [5,26-29]), these systems must pass a much higher NIR transition state. In other words, if the N-alkyl substituent is not N-tert-Bu, the presence of two N-bridged azetidine or pyrrolidine rings in polycyclic system (e.g. as for lo-azatetracyclo[6.3.0.0.[4,13]0.[5,lO]]undecanes [29]) is a structural criterion indicating a ‘high NIR barrier’ system for polycyclic amines. For instance, 6-azabicyclo[2.1. llhexanes 14 should possess increased NIR barriers. Indeed, calculated barrier of NIR for amine 14a is 12.9 kcal mol-‘. Even among more flexible systems 12 and 13 such a barrier is remarkably higher for tropane 12. Excluding pyrrolidine compounds [ 121, MM3 reproduces the measured values for the amines studied quite accurately (see Table 1). Unfortunately, experimental data for azetidines 2 and 4 and

J3 14a:R=Me

Alk # fert-Bu

pyrrolidines 5 and 6 are not reliable since these values were obtained by the less accurate coalescence method [2,34]. Two different barriers were measured for piperidine 7 [3,17]. An accurate line shape analysis of DNMR spectra was performed for piperidine 7 [17] for determination of the higher barrier. This barrier (12.0 kcal mol-‘; gas phase NMR experiments [ 171) was assigned to RI and the lower barrier (8.7 kcal mol-‘; ultrasonic techniques [3]) was assigned to NIR. The present study is in very good agreement with this assignment. The calculated barrier for 7 (9.1 kcal mol-‘, the NIR transition state) is very close to the lower experimental value. The higher experimental barrier corresponds well to the highest points on the two possible pathways (11.9 and 12.3 kcal mol-‘) which provide the Table I The reported experimental barrier values (at 298 K),a MM3 calculations results and assignment of these barriers for amines 2-8 Entry

2 3 4 5 6 7

AEm,, (kcal mol-‘, assignment) 8.0 8.7 9.1 5.3 4.9 9.1 1 I .9 12.3

“For the methods Ref. [5].

(RINI) (RINIR) (RINIR) (RINIR) (RINIR) (NIR) (RI) or (RINIR)

of extrapolation

AG” (kcal mole’, exp.) [Ref.] 8.2 9.1 8.9 7.2 6.7 8.7 12.0

[22] [I] [I] [2,27] [26] [3] [3,17]

of AGU values to 298 K see

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NMR-observed isochronism [ 171 of the geminal substituents. The lowest energy pathway consists of the following sequence (see Fig. 5): chair (equatorial N-Me group)- 1-sofa (equatorial N-Me group)- 1,Cboat (equatorial N-Me group)-pseudorotation to 1,4boat (axial N-Me group)-l-sofa (axial N-Me group)-chair (axial N-Me group). The highest energy point (the rate-determining transition state) on this pathway corresponds to the l-sofa conformation with an axial N-Me substituent (with a relative energy of 11.9 kcal mol-‘) differing from the halfchair form in the case of cyclohexane [23-251. An alternative pathway [chair (equatorial N-Me group)- 1-sofa (equatorial N-Me group)- 1,4-boat4-sofa-chair (equatorial N-Me group)] is a RINIR process with a high energy point (Csofa) of 12.3 kcal mol-‘. The difference (0.4 kcal mol-‘) in the MM3-calculated barriers is too small to decide between the two pathways, RINIR or RI. We may conclude only that the experimental barrier belongs to both ring reversal-related processes. How relevant the designed conformational schemes are is undoubtedly the most important question. In our opinion, the success of the proposed modeling is demonstrated by an excellent fit between experimental and those calculated barriers for azetidines 2-4 and piperidine 7 (see above) which lie in the ‘isochronism-determining’ itineraries in the resulted schemes. Thus, the common intramolecular motions in organic molecules, rotation [35], nitrogen inversion [5,12,13], ring inversion and pseudorotation (this work and Ref. [ 131) including concerted processes (this work) may be quantitatively described by MM3. We conclude, that these economic calculations may serve as an effective method for establishment of the sequence of conformational transformations for cyclic compounds as well as for estimation of the barriers for these processes.

4. Conclusions Schemes of conformational transformations were designed for the simple azetidines, pyrrolidines and N-methylpiperidine using MM3-derived methodology. An important role of concerted processes was

429 (1998) 265-273

established for these compounds. Concerted ring inversion-nitrogen inversion - C-N rotation is identified as the process whose rate was measured previously by NMR for N-methyl derivatives in azetidine and pyrrolidine series. The experimental barrier for N-methylpiperidine (from the literature data) is attributed to this process and also to ring inversion. Thus, the old concepts of intramolecular dynamics in 4-6-membered azacycles are revised. In addition, a structural criterion for amine systems, which possess unusually high nitrogen inversion - C-N rotation barrier, is established.

Acknowledgements A grant to Dr A.M. Belostotskii from the Giladi State Commission and the Absorption Ministry is gratefully acknowledged. We are grateful for partial support by Bar-Ilan University.

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