Study of alkyl adamantanes by NMR spectroscopy using 13C nuclei

STUDY OF A L K Y L A D A M A N T A N E S BY NMR SPECTROSCOPY USING x'C NUCLEI* A. S. MURAKHOVSKAYA, A. U. STEPAI~YAI~TS, Y~.. I. BAGRII, T. Yu. FRID, K. I. ZIMII~A and P. I. SAI~I~ Institute of Chemical Physics, U.S.S.R. Academy of Sciences A. V. Topehiev Institute of Petrochemical Synthesis, U.S.S.R. Academy of Sciences (Received 7 May 1973)

THE comparatively high reactivity of adamante derivatives [1], the increased stability of adamantyl carbonium ions [1, 2] and the interesting spectroscopic properties of compounds [3, 4] account for the wide range of investigation of adamantane derivatives. Alkyl adamantanes as models for the study of the direction of inductive effect of alkyl substituents in saturated systems [5-8], are of considerable interest. The use of the NMR-1H method in studies of alkyl adamantane structure is difficult owing to slight differences in chemical shifts of non-equivalent protons. NMR-1H spectra have only been examined for a few alkyl adamanTABLE

Alkyl substituents and their position in the adamantane nucleus 1-Ethyl 1-n-Propyl 1-n-Butyl 1-Isobutyl 2-Ethyl 2-n-Propyl

1. C l s - C x a A L K Y L ADAM-A.NTANES

n~°

ity, %wt.

Alkyl substituents and their position in the adamantane nucleus

1.4945 1.4902/ 1.49071 1.48901 1-5005 1"4965

99.5 98.0 97.5 97.0 98.5 99-0

1,3-Dimethyl '201.5 1"4768 99"5 1,3-n-Dipropyl 275~f 1.4882 99"5 1-Ethyl-3-methyl 226 1.4850 99"0 1-Ethyl-3,5-dimethyl 228 / 1.4805 99"0 1,3,5-Trimethyl 205 1.4748 99"5 1,3,5,7 -Tetramethyl / 208-51 99'0

PurS.p.~*

°C 223 242 254¢ 248~

Fur -

B.p.,* °C

n~O

ity, %wt.

* Boiling points were determined chromatographically from relative retention times on squalane, the s t a t i o n a r y phase. ? Apiezon L was the stationary phase in these cases.

tanes [9-11]. The use of lanthanide displacement reagents now widely used to interpret complex proton resonanse spectra of adamantanes with functional groups [12, 16] offers no prospect for alkyl adamantanes because of the absence of polar groups. * l~eftekhimiya 14, No. 2, 220-227, 1974. 39

40

A.S. Mv~,x~ovs~YA et al.

Pehk and Lippmaa [17] demonstrated the undoubted advantages of NMR-13C spectroscopy using adamantanes with functional groups in order to examine functional derivatives of adamantane. The NMR-;3C spectrum, however, has only been reported for a single alkyl derivative of adamantane. We carried out a systematic investigation of NMR-~sC spectra of monoand poly-alkyladamantanes with substituents on the tertiary and secondary carbon atoms of the adamantane nucleus. EXPERIMENTAL

NMR spectra for 13C nuclei were obtained at a frequency of 25-15 Mc/s using a J-NM-PFT-100 spectrometer with complete hetero-nuclear proton separation, Fourier transform and field development. The specimens were studied in eight milimeter ampoules in CC14solutions of 10-20% concentration. To improve the signal to noise ratio, the spectrum is stored in a IEC-6 device. Chemical shifts of x3C were measured in the spectra in relation to the CC14 signal taken as internal standard (chemical shift of CC14 in relation to tetramethylsilane ~T~S-96:4 p.p.m.) with accuracy of ±0.1 p.p.m. To identify peaks, the spectra of an unadjusted hetero-nuelear double resonance were examined obtained under conditions of incomplete hereto-nuclear separation of carbon atoms from protons. The mult~iplicity of peaks (M) in these spectra is determined by the number of protons (n) added to a given carbon atom by the formula M = n + 1. Thus, peaks were simply attributed to carbon atoms of--CH3,~CH2,~/CH,~/C--groups based on spectra of double unadjusted heteronuclear resonance. Further, the choice between different types of carbon with the same number of protons added was made by comparing chemical shifts of peaks in spectra of various alkyl adamantanes, their intensities and considering the effect of alkyl substituents according to the law of additivity. Characteristics of the alkyl adamantanes studied are shown in Table 1. The preparation and properties of these compounds have been described previously [10, 18]. RESULTS

A study of the symmetry of the adamantane nucleus (Fig. la) indicates that in the adamantane structure there are only two types of non-equivalent carbon atoms, ~ and • contained in four methine and six methylene groups. Accordingly, the carbon spectrum of unsubstituted adamantane. [17] consists of two peaks of methine and methylene carbons. -~ Transition from the structure of unsubstituted adamantane to the structure of substituted adamantane in position 1 reduced the symmetry of the adamantune nucleus and complicates the carbon spectrum of the compound. Figure lb shows the molecular symmetry of substituted adamantane in position 1. The non-equivalence of the position of the substituent in relation to carbon

Study of alkyl adamantanes by NMR spectroscopy

4t

atoms of a and ~ methine and fl and 5 methylene groups results in ~ substituted adamantane molecule in position 1 of four non-equivalent types of carbon atom of the adamantane nucleus. Figure 2 shows the NMR-laC spectrum of 1-isobutyladamar~tane. The overall number of peaks in the spectra is equal to the total number of nonequivalent types of carbon atoms of the adamantane nucleus and the hydrocarbon radical. Table 2 shows chemical shifts for carbon atoms of the adamantane nucleus and alkyl radicals and the effect of various alkyl substituents on the shifts of individual types of carbon atom of the adamantane nucleus. It follows from Table 2 that, independent of the chain length, the alkyl radical introduced causes a displacement of peaks on a, fl and y-carbons in the direction of weak field. At the same time, a comparison of chemical shifts of alkyl carbon atoms indicates that a strong de-screening action exists for the adamantane nucleus on the a-carbon ~atom of the side chain. Thus, the methyl carbon atom of 1-methyladamantane undergoes resonance at 31.1 p.p.m. [17]; the addition of a methylene group between adamantyl and methyl causes a reduction in de-screening of the latter b y the adamantane nculeus and a displacement of the peak of the methyl group of 1-ethyladamantane to 7.1 p.p.m, in the direction strong field. The same effect is obsecved on comparing the chemical shifts of carbon of the ethyl methylene group (36.9 p.p.m.) and fl-methylene group of the propyl radical (15-8 p.p.m.) and the a-methylene group of the propyl radical (47.6 p.p.m.) and the fl-methylene group of n-butyl (25.0 p.p.m.) Tables 1, 2, 3, 4 and 5 and Figures 1, 2 and 3 here. Pehk and Lippmaa [17] observed the effect of 1,4-interaction between fl-carbons of the adamantane nucleus and substituents in position 1 consisting of two or several atoms heavier than hydrogen. This interaction could be expected even in the alkyl adamantane system starting with the substituent CH2--CH 3, as follows from Table 2. The addition of a substituent in position 2 of adamantane forms another type of molecular symmetry, compared with adamantane substituted in position 1. This fact is simply recorded in the carbon spectrum and may be used to identify the position of the substituent in adamantane derivatives. Molecular symmetry of the substituted adamantane in position 2 is shown in Figure lc. Figure 3 shows the spectrum of 2-ethyladamantane. The nonequivalence of the position of substituent in relation to ~-sym and ?-anticarbon atoms of methylene groups and 5-syn and g-anti-carbon atoms of met hine groups results in the formation of seven non-equivalent types of carbon atoms of the adamantane nucleus. The overall number of peaks in ~MR-laC spectra of the substituted adamantanes examined in position 2 does not correspond to the overall number of non-equivalent types of carbon atom of the adamantane nucleus and hydrocarbon radical. Only 8 peaks were observed instead of 9 in the spectrum of 2-ethyladamantane, which

/

- -

- -

CH3

.

33.3 37.5

37-6

36.9 37.6 37.6

54.4

44.8

36.9 47-6 25-0

15.8

CH~

23.9

CH~

26.0

14.3

31.1 7.1 15.4

CHa

22.8

CH

4-7

3.8

0 1.3 3.8 4.0

CH~--CHs CH2--CH~--CHs

Substituent

[ fl

JTMS, p . p . m .

ly-syn[y.antils.synIJ_anti[ e

I CH,]CI-Is

44.4

32.0

32.0

39.6

28-3

28.5

35.0/ 39.8

CHs , a 14"119"8 20.9 14-7 6.4

,78 330j330,07 r297f300139.91209j

a

4"4 3.4

fl

0.4

0.5

0 0.3 0.3 0.4

Lp.m.

2-7 1.6

--0.3

1.1

--0.1

1"4I

dJ, p.p.m. l y - s y n l y - a n t i [ J-syn[J-anti[ --5.0 --6"0

--0"5

--0"4

0 --1.1 --0-4 --0.4

1"9 --2.7

2-ALKYL ADAMANTANES

5.3

4-8

0 6"6 4.3 5.4

P

~St,

IN 1-ALKYL ADAMANTANES

TABLE 3. CHEMICAL SHIFTS OF 18C (JT~S) AND ADDITIVE EFFECTS OF ALlryL SUBSTITUENTS ( A ~ ) I N

t N e g a t i v e values o f J 5 correspond to a d i s p l a c e m e n t o f the p e a k v a l u e in the direction o f a s t r o n g field.

29.0

29.1

42.8

43.3

28.9 28.9 29.0

* Results g i v e n in a p r e v i o u s p a p e r [17].

\

--CH~--CH

C~H3

--H 28.6 --CHe* 29.9 --CH~--CHs 32.4 - - C H ~ - - C H s - - C H 8 32-6 --CHs--CH~-- - C H 2- - C H 3 32.4

GH2

JTMS, p . p . m

13C (~TMS) AND ADDITIVE EFFECTS OF ALKYL SUBSTITUENTS ( ~ )

38.0 44.6 42.3 43.4

2. CHEMICAL SHIFTS OF

Substituent

TABLE

O

:n

Study of alkyl adamantanes by NMR spectroscopy

43

would be expected on the basis of molecular symmetry, while 9 peaks were observed instead of 10 in the spectrum of 2-n-propyladamantane. This discrepancy is due to the equality of chemicM shifts of two non-equivalent types of carbon atom (fl and 5-syn), which is confirmed by an abnormally high

of

o(

af a

c

X

X

/~~pp

pd~pP'

d

e

x

x

x

f

g

h

:Fro. 1. Types of non-equivalent carbon atoms in molecules of non-substituted and substituted a d a m a n t a n e s : a - - n o n - s u b s t i t u t e d adamantane; substituted a d a m a n t a n e in position: b - - l ; c--2; d - - l , 3 - - w i t h the same substituents; e - - l , 3 with different substituents; f--1,3,5 with the same substituents; g - - l , 3 , 5 - - w i t h two types of substituents; h--1,3,5,7--with the same substituents.

peak intensity (6T~s=33.0 p.p.m, for 2-ethyladamantane and 6TMS=320 p.p.m, for 2-n-propyladamantane) and the separation of this peak into a complex multiplet with components of doublet and triplet in the spectrum of unadjusted double hetero-nuclear resonance (Figure 3b). Chemical shifts of carbon atoms of substituted adamantanes in position 2 and the values calculated of the effect of alkyl substituents on chemical shifts of 13C of an ~damantane nucleus are shown in Table 3.

A. S. MURAKHOVSKAYAet al.

44

I t is n o t e d [17] t h a t the s y m m e t r y of s u b s t i t u t e d a d a m a n t a n e s in position 2 is such t h a t v e r y strong i n t e r a c t i o n m u s t t a k e b e t w e e n the s u b s t i t u e n t a n d 7-syn-carbon atoms. This effect is also observed in the c a r b o n s p e c t r a of t h e

/3 H20CHz

HSC\ cH/ CH3

%'ell

I

a

Z

HsR CHaR

O~

/3 ;"

i

sqn I

OHR

HMDS ~,OHz anfi ~.t~ ~

I

J ##.z/

I

I

~O.J 375 2.,9-0

I

(~TM8

/3

cH:cH3

CZ

O p.p.m.

rh

I

I

I

!

#78 ~0.7 &Y'O

N/

L

O P.Fm.

"~Ms

rh

b

k.w FIG. 2

FIG. 3

FIG. 2. NMR zsC spectrum of 1-isobutyladamantane obtained under conditions of complete hetero-nuclear separation of protons (a) and by unadjusted double hetero-nnclcar resonance (b). FIG. 3. I~MR 13C spectrum of 2-ethyladamantane obtained under conditions of complete hetero-nuclear spearation of protons (a); spectrum fragment (peak at 6T~s= 33-0 p.p.m.) obtained under conditions of unadjusted double hereto-nuclear resonance (b).

Study of alkyl adamantanes by NMR spectroscopy

45

2-ethyl- and 2-n-propyladamantanes studied. I t is expressed b y the displacement of the ?-syn-carbon atoms b y 5.0-6.0 p.p.m, in the direction of strong field, compared with the chemical shift of unsubstituted adamantane. The addition of second and third substitutents to the adamantane molecule causes further reduction in symmetry and a complication of the carbon spectrum of the compound. T A B L E 4. CHEIV[ICAL SHIFTS OF 13C II~ 1 , 3 - D I A L K Y L ADA]YIANTA.I~ES W I T H T H E SAME ALKY'L S U B S T I T U E N T S (~TMS)

JTMSp.p.m. 7? ~

Substituent s

I CI-I~

CHs

CHs

/

--CH8 --CH2--CH~--CH3

30.85 (30-3) 33.3 (33.0)

52.05 (48.8)

44.1 (43.5) 42.5 (43.0)

29"5 36"4 !I -(29.2) (35.8) 1 29.4 37.3 / 47.3 (29.4) (37.2)

31"3

16.0

15"3

_Wore. Chemical shifts calculated from the law of additlvity are given in Tables 4 and 5 in brackets.

There are five non-equivalent types of carbon atom in the molecule of a di-substituted adamantane in positions 1 and 3 with the same substituent (Fig. ld). Therefore, the spectrum of 1,3-di-n-propyladamantane consists of 8 peaks: five-for the carbon atoms of the adamantane nucleus and three for the carbon atoms of propyl substituents, while the spectrum of 1,3-dimcthyladamantane has six peaks: five--for carbon of the adamantane nucleus and the peak for the methyl substituent. Chemical shifts of 1,3-dialkylaziamantanes with the same substituents are shown in Table 4. In the adamantane nucleus of di-substituted adamantane in positions 1, 3 with different substituents there are 7 non-equivalent types of carbon atom (Figure lb). Accordingly, the spectrum of 1-ethyl-3-methyladamantane contains 10 peaks: seven for carbon atoms of the adamantane nucleus, two for carbon atoms of the ethyl radical and one for the carbon of the methyl group. A similar pattern is observed for tri-substituted adamantane in positions, 1, 3, 5 with two types of substituent (Figure lg). Therefore, the spectrum of 1-ethyl-3,5-dimethyladamantane also has 10 lines. Chemical shifts of 1-ethyl3-methyl and 1-ehtyl-3,5-dimethyladamantanes are shown in Table 5. According to the four types of carbon atoms in the tri-substituted adamantane nucleus in positions 1, 3, 5 with the same substituents (Figure l f) 5 peaks are observed in the spectrum of 1, 3, 5-trimethyladamantane with the following chemical shifts (ST~S): 31.6 (30.5)*

51.4 (50.1)

43.4 (42.4)

30.2 (29.5)

* Calculated values of chemical shifts are given in brackets.

31.0

- -

GHa

-- C H s

R1

--

-- GH~

CH~

-- CI-I 3

R,

-- CH~--

--H

CH,

1%8

lubstituents

o~'77'

o~y'7' 31.5 (30.5)

30.8 (30-2)

33"3 (32.8) 34"2 (33.0)

~'r

~7'

48.8 (47.8)

PB',~

49'5 (48'9)

fl/~'

40.9 (40.1)

/~'~'

41.6 (41.2)

/~6'

51.7 (50"8)

p'p'6

44'6 (44"2)

/~'6

OT~S, p.p.m.

30-0 (29.5)

7Y'7'

29"5 (29.2)

r7'

43"8 (43"8)

36'6 (36"5)

66'

TABLE 5. CHEMICAL SHIFTS OF lsC IN 1,3-DI AND 1,3,5-TRIALKYL ADAMANTANES WITH DIFFERENT ALKY~L SUBSTITUENTS (~TMS)

36'0

36"8

CH,

7.2

7.3

CtI 3

30.9

31.6

Ctt,

),

o

[m

.>

Study of alkyl adamantanes by NMR spectroscopy

47

The spectrum of 1,3,5,7-tetramethyladamantane consists of three peaks, two of which correspond to two types of carbon atom of the adamantane nucleus (Figure lh) and the third, to carbons of methyl radicals. Chemical shifts are as follows: a???

/~/~

CH~

32.5 (30.8)

50.8 (49.0)

30.6

A simplification of spectra for polyalkyladamantanes with the same substituents in nodal positions is due to an increase in molecular symmetry of these compounds. For all d i - a n d polyalkyl-substituted adamantanes chemical shifts were calculated from additivity values. Results concerning the effect of ethyl and propyl substituents (additive effects Ag) obtained in this study and the effect of a methyl substituent determined previously [17] were used in calculation. A comparison of calculated and experimental chemical shifts clearly indicates additivity of 18C of chemical shifts of alkyl adamantane systems. In some cases calculation of chemical shifts appears to be the only reliable criterion for identifying spectroscopic peaks. Calculated results alone, in particular, enable us to determine simply the chemical shifts of carbons of different types of methylene groups in the adamantane nucleus of 1-ethyl-3-methyl and 1-ethyl-3,5-dimethyladamantanes, of which the peaks have the same intensity in spectra with proton separation and the same multiplicity in spectra of unadjusted double hetero-nuclear resonance. $

SUMMARY

1. laC N M R spectra of twelve alkyl adamantanes were studied and their structures correlated with carbon chemical displacement. It was shown that ~aC NMR spectroscopy is a very effective method for investigating alkyl adamantane structures and at the same time enables the type of substitution in the adamantane nucleus to be determined. 2. It was established that the effect of alkyl substituents on chemical displacements of 13C in di- and polyalkyl adamantancs is additive. 3. It was found that alkyl and adamantyl radicals have a mutual de-screening effect in the alkyl adamantane system showing a displacement to the weak field of peaks of the a-carbon atom of alkyl and nearest carbon atoms of adamantane nucleus. 4. I t was established for 2-alkyl adamantancs that ?-syn-carbon atoms of the adamantane nucleus interact with alkyl substituents: this is shown b y the difference of chemical shifts of ?-syn- and ?-anti-carbon atoms of the adamantane nucleus.

48

A . S . MUR.~HOVS~¥A et al.

REFERENCES 1. R. C. FORT and Jr. P.' VON E. SCItLEYER, Chem. Revs. 64, 277, 1964 2. H. FUJIMOTO, U. KITAGAWA, H. HAO and K. FUKUI, Bull. Chem. See. J a p a n 43, 52, 1970 3. K. W. BOWERS, G. J. NOLFI and Jr. E. D. GREEN, J. Amer. Chem. Soc. 85, 3707, 1964 4. P. VON R. SCH.LEYER, It. C. FORT, JR., W. E. WATTS, M. B. COMMISAROW and G. A. OLAH, J. Amer. Chem. See, 86, 4195, 1964 5. P. VON R. SCHLEYER and C. W. WOODWORTH, J. Amor. Chem. Soc. 90, 6528, 1968 6. R. C. FORT, Jr., and P. VON R. SCHLEYER, J. Amer. Chem. See. 86, 4194, 1964 7. G. H. WAHL, Jr. and M. R. PETERSON, Jr., J. Amer. Chem. Soc. 92, 7238, 1970 8. I. B. M A Z H ~ , I. S. YANKOVSKAYA and Ya. Yu. POLIS, Zh. obsheh, khimii 16, 1633, 1971 9. R. C. FORT and P. VON R. SCHLEYER, J. Organ. Chem. 30, 789, 1965 10. Ye. I. BAGRII, L. A. FEDOROV, G. G. KAKABEKOV, T. Yu. FRID and P. I. SANIN, Dokl. AN SSSR 199, 342, 1971 11. T. Yu. FRID, Ye. I. BAGRII, A. Yu. KOSKEVNIK, M. V. SHISHKINA and P. I. SANIN, l~eftekhimiya 12, 511, 1972 12. G. WAHL and M. PETERSON, Chem. Commun., 1167, 1970 13. A. G. YURCHENrKO and S. D. ISAYEV, Zh. organ, khimii 7, 2628, 1971 14. R. V. AM~ON and R. D. FISCHER, Angew. Chemic 84, 737, 1972 15. M. HAJEK, L. VODICKA, Z. KSANDR and S. LANDA, Tetrahedron Letters 28, 4103, 1972 16. P. KRISTIANSEN and T. LEDAAL, Tetrahodron Letters 27, 4457, 1971 17. T. P E H K and Ye. LIPPMAA, Organ. Magn. Resonance 8, 783, 1971 18. Ye. I. BAGRH, T. Yu. FRID and P. I. SANiN, Neftekhimiya 12, 511, 1972