Chemistry and Physics of Lipids 18 (1977) 1 3 0 - 1 4 4 © Elsevier/North-Holland Scientific Publishers Ltd.

13C-NMR OF D O U B L E A N D TRIPLE B O N D C A R B O N ATOMS OF UNSATURATED FATTY ACID METHYL ESTERS Jan BUS, Izagk SIES and Marcel S.F. LIE KEN JIE * Unilever Research, Vlaardingen, The Netherlands Received July 5th, 1976

accepted August 5th, 1976

Fhe carbon magnetic resonance spectra of m a n y fatty acid methyl esters with cis and trans double bonds and triple bonds at various positions and in m a n y different combinations have been investigated. The influence of the ester group on double and triple bonds in the fatty acid chain depends strongly on the positions of these bonds. For a given position the influence is constant, even if one or more other double or triple bonds are present. Together with the evaluated chemical shift parameters for the effects o f double and triple bonds on each other, complete assignments are possible and spectra of various types of unsaturated esters can be predicted with high accuracy (± 0.1 ppm).

I. Introduction The synthesis and isolation programmes in relation to investigations on essential fatty acids and prostaglandins in Unilever Research by Van Dorp and coworkers [1 ] have stimulated us to exploit NMR methods more fully for the structure elucidation of various types of unsaturated fatty acids. Proton magnetic resonance (IH-NMR) spectra at 220 MHz have been investigated by Frost and Gunstone [2,3], but in that study a number of structural differences could be determined from the 1H-NMR spectra only after addition of lanthanide shift reagents [4]. Carbon-13 magnetic resonance (13C-NMR) spectrometry [5,6] has proven to be very sensitive to subtle structural differences and applications in the field of unsaturated fatty acids have given promising results. Batchelor et al. [7,8] and Tulloch and Mazurek [9] described 13C-NMR spectra of a number of unsaturated fatty acids; Barton has measured unsaturated phospholipids [10], whilst in our laboratory the work in this field has also made considerable progress [1 I - 1 3 ] . The carbon chemical shifts of the methyl, methylene and methoxycarbonyl groups in the methyl esters of unsaturated fatty acids could be described accurately by a set

* Department of Chemistry, University of Hong Kong. 130

J. Bus et al., 13C-NMRof unsaturated fatty acid methyl esters

131

of empirical chemical shift parameters [ 13]. The scope of the present paper is to discuss the results for double and triple bond carbon atoms.

lI. Experimental A11 l aC-NMR measurements were made on a Bruker WH90 Fourier Transform NMR spectrometer operating at 22.63 MHz with proton noise decoupling. The Nicolet B-NC 12 computer with a 20 K data memory was used for the 16 K or 8 K free-induction decay. The resulting accuracy of the carbon chemical shifts is + 0.05 or -+0.1 ppm respectively. The spectra (3000-10,000 accumulations; 1 5 - 4 0 ° pulses; 28°C) were obtained from solutions in CDCI3 ( 0 . 2 - 0 . 5 mol/1) which also served as an internal deuterium lock. Chemical shifts are given as 6-values in ppm downfield from the internal TMS-13C signal and are rounded off to the nearest 0.05 ppm. The lanthanide shift reagents were used at molar ratios (reagent/substrate) of 0 - 0 . 2 .

III. Materials The greater part of the compounds were synthesized or isolated in Unilever Research. The C11 esters and the ethylene-interrupted cis, cis-octadecadienoates and diynoates were synthesized in the Department of Chemistry of the University o f Hong Kong. Methyl cis-9,trans-11-octadecadienoate was a gift from C.R. Scholfield, Northern Regional Research Laboratory, Peoria, USA. Eu(fod)3-d3o and Pr(fod)3d3o were purchased from the Norell Chemical Company, Landing, New Jersey. The following methyl esters of unsaturated fatty acids were the basis of this study. They are denoted as follows: mc stands for a eis double bond at position m in the carbon chain with respect to the methoxycarbonyl group, t for trans, a for triple bond and Cn for a fatty acid chain of n carbon atoms.

Alkenoates: C8 : 3t; C10 : 4c; C11 : ( 2 - 9 ) c ; C12 : 2t; 6c; C14 : 5c; 8c; C16 : 7c; 10c; C17 : 11c;C18 : ( 5 - 1 2 ) e ; 6 t ; ( 8 - 1 2 ) t ; C 2 0 : 1 lc; 17c. Alkadienoates: C14 : 5e8c; C16 : 7c 10c; C18 : 2c6c; 3c7c; 4c8c; 5c9c; 5t9e; 6c9c; 7cl lc; 8c12c; 8t12c; 9cl lt; 9cl 2c; 9c12t; 9tl 2c; 9t12t; 9c13c; 9c13t; 10cl2t; 10tl2c; 10tl2t; 10c14c; 11c15c; 1 2 c 1 6 e ; C 2 0 : 1 1 e 1 4 c ; C 2 2 : 13c16c. Alkatrienoates: C18 : 2t9c12c; 3t9c12c; 5c8tl lc; 5t9c12c; 6c9c12c; 8cl lel4c. Alkatetraenoates: C19 : 4c7c10c13e; C20 : 4c7ci lc14c; 5c8cl lc14c; 5c8cl l c l 4 t ; 5t8cl lel4c; C21 : 5c8cl lc14c. Alkynoates: C8 : 7a; C10 : 9a; C11 : ( 2 - 1 0 ) a ; C22 : 1 la. Alkadiynoates: C16 : 7al0a; C18 : 2a6a; 3a7a; 4a8a; 5a9a; 6al0a; 7al la; 8a12a; 9a13a; 10al4a; 1 lal5a; 12a16a; 13a17a. Alkatriynoate: C20 : 5a 8a 1 la. Alkenynoates: C13 : 4t7a; C15 : 6t9a; C18 : 8t12a; 9c12a; 9t12a; 9a12t; 9a13t. Alkenediynoate: C15 : 5a8tl la.

132

J. Bus et al., 13C-NMR o f unsaturated f a t t y acid m e t h y l esters

IV. Results and discussion The carbon chemical shifts of the monoene esters are given in table 1, the dienoates in table 2 and the tri- and tetraenoates in table 3. Table 4 lists the carbon chemical shifts of the ethynic carbon atoms and table 5 includes the esters with both double and triple bonds. The very good additivity, found previously for the methylene carbon atoms in these esters [13], encouraged us to investigate whether the chemical shifts of ethylenic and ethynic carbon atoms could also be described on the basis of additivity principles. A systematic study of the data in tables 1 - 5 has resulted in the assignments of all the carbon chemical shifts to the various carbon atoms. The way these assignments have been obtained will be discussed in further detail in the following sections.

A. Effect of terminal methyl group The additivity of the influence of the methyl group and the methoxycarbonyl group on a double bond was found by comparing the methyl c i s - u n d e c e n o a t e isomers with methyl c i s - o c t a d e c e n o a t e s which have the same double bond position with respect to the methoxycarbonyl group and with the corresponding decenes [14]. For

Table 1 Carbon chemical shifts (6/ppm) of ethylenic carbon atoms of methyl cis- and trans-alkenoates in CDC13. Compound

6C(n)

6C(n + 1)

Compound

6C(n)

6C(n + 1)

e-2-C11 t-2-C12 c-3-C 11 t-3-C8 c-4-C 10 c-4-C11 c-5-C11 c-5-C 14 c-5-C18 c-6-C11 c-6-C12 e-6-C18 t-6-C 18 c-7-C11 c-7-C16 c-7-C18 c-8-C I 1

119.25 121.00 120.75 121.55 127.45 127.35 128.40 128.40 128.40 129.20 129.20 129.15 129.65 129.75 129.55 129.50 129.20

151.05 149.95 133.70 135.00 131.75 131.75 131.30 131.30 131.30 130.55 130.65 130.55 131.10 130.00 130.35 130.25 131.80

c-8-C14 c-8-C18 t-8-C18 c-9-C 11 c-9-C 18 t-9-C18 c-1 O-C16 c-1 O-C18 t-lO-C18 c-11-C17 c-11-C 18 c-11-C20 t -11 -C 18 c-I 2-C18 t -12-C 18 c-17-C20

129.70 129.70 130.20 130.85 129.80 130.30 129.85 129.85 130.40 130.00 129.90 129.90 130.35 130.00 130.45 129.45

130.10 130.15 130.65 123.75 130.10 130.55 130.00 130.00 130.55 130.05 130.00 129.95 130.45 130.00 130.45 13 1.60

Internal reference TMS, n = double bond position, c = cis, t = trans.

J. Bus et al., 13C-NMR o f unsaturated fatty acid methyl esters

133

Table 2 Carbon chemical shifts (6/ppm) of ethylenic carbon atoms of methyl alkadienoates in CDC13. Compound

6/ppm

c-5, c - 8 - C 1 4 c-7, c-I 0-C][6 c-9, c-12-C18 c-11, c-14-C20 c-13, c-16-C22 c-2, c - 6 - C 1 8 c-3, c - 7 - C 1 8 c-4, c - 8 - C 1 8 c-5, c - 9 - C 1 8 c-6, c-10-C18 c-7, c-11-C18 c-8, c-12-C18 e-9, c-13-C18

128.70(5) 129.85(7) 130.05(9) 130.20(11) 130.20(13) 119.70(2) 121.25(3) 127.85(4) 128.90(5) 129.65(6) 130.05(7) 130.20(8) 130.30(9) 130.35(10) 130145(11) 130.45(12) 129.40(5) 130.65(8) 130.15(9) 130.40(9) 130.80(9) 131.00(9) 130.00(9) 130.00(10) 134.55(10) 132.30(10)

c-10, c-14-C18

c-ll, c-15-C18 c-12, c-16-C18 t-5, c - 9 - C 1 8 t-8, c-12-C18 c-9, t-13-C18 c-9, t-12-C18 t-9, c-12-C18 t-9, t-12-C18 c-9, t-ll-C18 c-10, t-12-C18 t-10, c-12-C18 t-10, t-12-C18

129.40(6) 128.40(8) 128.25(10) 128.10(12) 128.05(14) 150.05(3) 132.95(4) 130.95(5) 130.55(6) * 129.85(7) 129..60(8) 129.45(9) 129.35(10) 129.30(11) 129.30(12) 129.25(13) 131.15(6) 129.95(9) 129.35(10) 127.95(10) 128.60(10) 128.85(10) 125.75(10) 125.75(11) 128.80(11) 130.60(11)

127.75(8) 127.95(10) 128.10(12) 128.10(14) 128.05(16) 128.30(6) 128.70(7) 128.95(8) 129.05(9) 129.15(10) 129.20(11) 129.20(12) 129.25(13) 129.45(14) 128.70(15) 130.25(16) 129.10(9) 129.20(12) 129.80(13) 128.40(12) 127.85(12) 128.70(12) 128.90(11) 128.80(12) 125.90(12) 130.55(12)

130.45(9) 130.35(11) 130.20(13) 130.20(15) 130.20(17) 131.30(7) 130.90(8) 130.65(9) 130.65(10)* 130.55(11) 130.55(12) 130.45(13) 130.40(14) 130.20(15) 132.05(16) 124.15(17) 139.45(10) 130.30(13) 130.85(14) 130.90(13) 130.55(13) 131.20(13) 134.80(12) 134.70(13) 130.10(13) 132.45(13)

* Confirmed with Pr(fod)3-d3o. Internal reference TMS. Assignments to Cn are indicated by (n); c = cis, t = trans.

e x a m p l e , in cis-8-C18 ester: H I CH3(CH2) m -

H I

C ~--- C(CH2)n_ 2 - - COOCH 3 n+l n

(1)

(1; n = 8; m = 8) 8 C ( 9 ) - 8 C ( 8 ) = 0 . 4 5 (table 1). In cis-8-C11 ester (1 ; n = 8; m = 1) 6 C ( 9 ) - 6 C ( 8 ) = 2.60. In cis-3-ClO alkene (2; m = 1) 8 C ( 3 ) - 6 C ( 4 ) = 2.21 [14]. H I

H 1

CH3(CH2) m - - C = C ( C H 2 ) 6 _ m - - CH 3

(2}

It is reasonable to e x p e c t that in the C18 ester the double b o n d is affected only

121.00(2) 121.75(3) 129.15(5) 129.60(5) 129.05(5) 129.60(6) 130.25(8) 127.95(4) 127.85(4) 129.00(5) 129.00(5) 129.05(5) 129.60(5)

t-2,c-9, c-12-C18 t-3,c-9,c-12-C18 c-5,t-8, c-11-C18 t-5,c-9,c-12-C18 c-5,c-9,c-12-C18 c-6, c-9,c-12-C18 c-8, c-11,c-14-C18 e.4, c-7,c-lO, e-13-C19 c-4,c-7,c-11,c-14-C20 c-5,c-8, c-11,c-14-C20 e-5,c-8, c-11,c-14-C21 e-5, c-8, c-I 1, t-14-C20 t-5,c-8, c-11,c-14-C20 149.65(3) 134.75(4) 128.95(6) 131.00(6) 130.40(6) 128.35"(7) 128.05(9) 129.45(5) 129.70"(5) 129.00(6) 129.00(6) 129.05(6) 129.45(6)

129.90(9) 129.95(9) 128.65(8) 129.40(9) 129.35(9) 128.15(9) 128.35(11) 128.10(7) 128.20(7) 128.30"(8) 128.30"(8) 128.20"(8) 128.00(8)

* Ambiguity remains. Internal reference TMS. Assignments to Cn are indicated by (n).

8/ppm

Compound 128.40(10) 128.30(10) 129.15(9) 128.60(10) 128.70(10) 128.50"(10) 128.45(12) 128.45(8) 129.80"(8) 128.25"(9) 128.25"(9) 128.35(9) 128.60(9)

127.95(12) 128.05(12) 127.60(11) 128.05(12) 127.95(12) 127.70(12) 128.05(14) 127.95(10) 129.35(11) 127.95(11) 128.00(11) 128.25 *(11) 128.00(11)

130.35(13) 130.25(13) 130.75(12) 130.25(13) 130.40(13) 130.45(13) 130.25(15) 128.70(11) 128.75(12) 128.70(12) 128.70(12) 128.35(12) 128.60(12)

Table 3 Carbon chemical shifts (8/ppm) of ethylenic carbon atoms of methyl alkatrienoates and alkatetraenoates in CDCI3.

127.65(13) 127.95(14) 127.65(14) 127.70(14) 128.05(14) 127.70(14)

130.55(14) 130.40(15) 130.55(15) 130.55(15) 131.15(15) 130.50(15)

Table 4 Carbon chemical shifts (6/ppm) of ethynic carbon atoms of methyl alkynoates, alkadiynoates and -triynoates in CDC13. Compound

6/ppm

2-yne-Cll 3-yne-Cll 4-yne-Cll 5-yne-Cll 6-yne-Cll 7-yne-Cll 8-yne-Cll 9-yne-Cll

72.95(2) 71.35(3) 78.10(4) 78.75(5) 79.45(6) 80.00(7) 79.40(8) 79.30(9)

90.00(3) 84.00(4) 81.25(5) 81.40(6) 80.75(7) 80.35(8) 81.80(9) 75.40(10)

7-yne-C8 9-yne-C10 10-yne-C11

84.35(7) 84.60(9) 84.70U0)

68.40(8) 68.25(10) 68.15(11)

2,6-diyne-C18 3, 7-diyne-C18 4, 8-diyne-C18 5, 9-diyne-C18 6, 10-diyne-C18 7, 11-diyne-C18 8, 12-diyne-C18 9, 13-diyne-C18 10,14-diyne-C18 11, 15-diyne-C18 12, 16-diyne-C18 13, 17-diyne-C18

73.50(2) 72.30(3) 79.05(4) 79.80(5) 80.40(6) 80.80(7) 80.95(8) 81.10(9) 81.10(10) 81.15(11) 81.25(12) 81.65(13)

87.95(3) 82.45(4) 79.80(5) 79.95(6) 79.35(7) 79.15(8) 79.00(9) 78.95(10) 78.90(11) 78.85(12) 78.80(13) 78.15(14)

77.30(6) 78.50(7) 78.65(8) 78.75(9) 78.75(10) 78.80(11) 78.80(12) 78.85(13) 79.00(14) 78.15(15) 78.00(16) 83.15(17)

82.35(7) 81.45(8) 81.35(9) 81.35(10) 81.25(11) 81.25(12) 81.25(13) 81.15(14) 81.00(15) 82.50(16) 76.35(17) 68.90(18)

7, 10-diyne-C16

80.10(7)

74.90(8)

74.50(10)

80.60(11)

5, 8, 11-triyne-C20

75.45(5)

74.95(6)

74.65(8)

75.10(9)

73.75(11)

80.95(12)

Internal reference TMS. Assignments to Cn are indicated by (n).

Table 5 Carbon chemical shifts (6/ppm) of ethylenic and ethynic carbon atoms of methyl alkenynoates in CDCI3. Compound

~/ppm

t4,7-yne-C13 t-6,9-yne-C15 t-9, 12-yne-C18 cO,12~ne-C18 9-yne, t-12-C18 t-8, 12-yne-C18 9-yne, t-13-C18 5-yne, t-8,11-yne-C15

129.45(4) 131.15(6) 131.75(9) 131.20(9) 81.95(9) 131.50(8) 80.50(9) 80.95(5)

126.45(5) 125.50(7) 125.05(10) 125.25(10) 77.85(10) 128.85(9) 79.90(10) 78.20(6)

77.20(7) 77.55(9) 77.70(12) 78.40(12) 124.80(12) 79.75(12) 128.65(13) 126.20(8)

Internal reference TMS. Assignments to Cn indicated by (n).

82.40(8) 82.30(10) 82.10(13) 80.05(13) 131.95(13) 80.60(13) 131.70(14) 126.55(9)

77.15(11)

82.30(12)

136

J. Bus et al., 13C-NMR of unsaturated fatty acid methyl esters

by the methoxycarbonyl group and in the decene only by the nearer methyl group. In cis-8-C11 ester, where both effects are expected to play a part, a good additivity is found: 2.21 + 0.45 = 2.66 (experimental: 2.60). A similar good additivity is found for cis-9-C 18 ester (1 ; n = 9; m = 7), cis-9-C 11 e s t e r ( 1 ; n = 9 ; m = 0) and cis-2-decene (2; m = 0): 0.30 - 7 . 4 4 ~ - 7 . 1 0 . For the combination of cis-7-C 18 ester, cis-7-C 11 ester and cis-4-decene, a good fit is only obtained, however, if the assignments for the alkene double bond carbon atoms made by De Haan et al. [14] are reversed which thus leads to 0.75 - 0 . 5 3 ~ 0.25. If this reversal is correct, their assignment for cis-4-nonene [14] must be reversed as well. The influence of CH 3 on 8C(n + 1) in (1) can be described with the additive parameters ~ (for m = 0),/3 (for m = 1), etc. and on 8C(n) by c~' (m = 0),/3'(m = 1), etc. The actual values for these parameters are given in table 8.

B. Effect o f methoxycarbonyl group The electric field of the dipolar methoxycarbonyl group induces a charge separation in the polarizable rr-electrons of a double or a triple bond [8]. The relatively positive carbon atoms are deshielded (shifted to lower field) and the other ones are shielded (3). H

H

I

I

CH 3 .... C ~ C .... C - ~ C n .... COOCH 3 . 8+ 88+ 8 -

(3)

The carbon chemical shifts of the ethylenic carbon atoms for monoenoates have been expressed by Batchelor et al. [7] by equation (4). 8 = A-+ 0 . 5 8 .

(4)

This description suggests the two lines to be equidistant to the basic value A. If we consider the double bonds in methyl alkenoates between C(n) and C(n + 1), the chemical shift contributions for 8Cn and 8C(n + 1) can be developed by comparison with homologous compounds in which the influence of the methoxycarbonyl group has become too small to be measurable (n/> 12). This contribution does not depend on chain length, as can be seen by comparing cis-5-C11 with cis-5-C18 or cis-8-C 14 with cis-8-C 18 (table 1), provided that the terminal methyl group has no influence ( 1 ; m >1 4). The contributions obtained from the monoenoates referred to above, appear to be applicable to di-, tri-, and tetranoates as well. The sets of chemical shift contributions for double and for triple bonds, that fulfill the requirements for all compounds in the best possible way, are given in table 7. These sets clearly indicate that the upfield shifts for C(n) are always larger than the downfield shifts for C(n + 1), except for position 2, which illustrates the inadequacy of equation (4). The contribations listed in table 7 can be applied to any non-conju-

J. Bus et aI., 13C-NMR of unsaturated fatty acid methyl esters

137

Table 6 Basic carbon chemical shifts (6/ppm) of the carbon atoms of double and triple bonds in various unsaturated fatty acid m e t h y l esters. H I

1t I

H I C~=C

C=C

C=C

130.00

I H 130.45

H H I 1 C=C

HC~C

80.20

68.10

--

84.75

H H H I I I C = C - (CH2)n-C = C L

H H I I (CH2)n-C = C

a

b

H d

c

a

b

n

a

b

n

a

b

c

d

1 2

130.20 130.50

128.05 129.25

0 1 2

130.05 130.55 130.35

125.80 127.85 129.25

128.80 128.45 129.80

134.70 130.95 130.95

H I

H I

H I C =C-(CH2) n-C I H a b

C =C I H a b

=C I H

(CH2)n

C ~-C

c

d

n

a

b

n

a

b

c

d

0 1

132.35 131.15

130.55 128.70

1 2

131.95 131.75

124.80 128.65

77.65 79.75

82.10 80.60

C ~ C

a

H H I I C =C-(CH2) n-C

(CH2) n - C ~- C

a

b

b

~C c

d

n

a

b

n

a

b

c

d

1 2

80.50 81.20

74.60 78.80

1

131.40

125.10

78.40

80.05

H

H H H I I I C=C-(CH2)n-C =C-CH a

b

c

H I

H I =C

e

f

2-C

d

(continued on pp. 138 and 139)

J. Bus et al., 13C-NMR of unsaturated fatty acid methyl esters

138

Table 6 (continued)

n

a

b

c

d

e

f

l 2

130.50 130.60

127.80 129.10

128.30 129.55

128.55

128.00

130.35

H H H H H I I I I I C =C-(CH2) n-C =C-CH 2-C =C I a

H b

n

a

b

c

d

e

f

2 4

131.10 130.70

129.65 130.20

129.55 130.15

128.40 128.15

128.00 128.05

130.30 130.30

c

d

e

f

r

H H H H H H I I I I I I C =C-CH 2-C =C-(CH2)n-C=C-CH a

b

c

a

b

e

d

1 2

130.55 130.40

127.70 127.95

128.60 128.70

128.05 129.40

H H H H I I I I C =C-CH 2-C =C-CH 2

H H I I C =C-CH

a

e

c

2-C

H [ =C

d

n

b

H t

d

H I 2-C

f

=C I H g h

a

b

c

d

e

f

g

h

130.55

127.70

128.55

128.10

128.35

128.30

128.10

131.15

H H I 1 C =C-CH

a

H I 2-C

b

H I =C-CIt 2-C I H

c

a

b

c

130.70

127.65

128.95

H I =C

J. Bus et al., 13C-NMR o f unsaturated fatty acid methyl esters

139

Table 6 (continued) H I

C ~C-CH

2-C

=C-CH

2-C

~C

I

H a

b

c

a

b

c

82.45

77.00

126.45

C ~ C - CH 2 - C ~ C a

b

- CH 2 - C

~ C

c

a

b

c

80.90

73.75

74.85

Solvent CDC13. I n t e r n a l r e f e r e n c e TMS.

Table 7 Effects (/x6) o f m e t h o x y c a r b o n y l g r o u p o n c a r b o n c h e m i c a l shifts o f C(n) a n d C(n + 1) o f d o u b l e a n d triple b o n d s ( l o c a n t n) in f a t t y acid esters. n

2 3 4 5 6 7 8 9 10 11 12

C=C

C~=C

AaC(n)

AOC(n + 1)"

z~aC(n)

A a C ( n + 1)

-10.75(-9.40) - 9.20(-8.90) - 2.55 - 1.55 - 0.85 - 0.50 - 0.30 - 0.20 - 0.10 - 0.05 0

+21.05(+19.45) + 3.70(+ 4 . 6 0 ) + 1.75 + 1.30 + 0.65 + 0.40 + 0.20 + 0.15 + 0.10 + 0.05 0

-7.25 * -8.90 -2.15 -1.45 -0.80" -0.40 -0.20 -0.10 -0.05 -0.05 0

+9.80 * +3.70 +1.00 +1.20 +0.60" +0.30 +0.25 +0.15 +0.05 0 0

* D e v i a t i n g values w e r e f o u n d for m e t h y l 2, 6 - o c t a d e c a d i n y o a t e ( - 7 . 7 5 ; + 9 . 1 5 ; - 1 . 5 0 ; +1.10). The values in this table m a y , t h e r e f o r e , not b e valid for 2 - a l k y n o a t e s if a s e c o n d u n s a t u r a t e d b o n d is present. V a l u e s for cis a n d trans are the same, e x c e p t for n = 2 and n = 3 (trans b e t w e e n b r a c k e t s ) .

140

J. Bus et al., 13C-NMR of unsaturated fatty acid methyl esters

gated double or triple bond system given in table 6 which summarizes the basic chemical shifts for all systems studied. The above results lead to the important outcome that the influence of the methoxycarbonyl group on a double or triple bond is constant, i.e. it is not modified by the presence of other double or triple bonds. Table 7 reveals also that the electric field splitting is the same for cis and trans double bonds except for the positions 2 and 3. The deviation for the latter two positions may well be caused by differences in through-bond effects exerted by the methoxycarbonyl group. Moreover, apart from the 2 - , 3 - , and 4-positions of the triple bond, the effects of the methoxycarbonyl group are also nearly equal to those on the triple bond which are only slightly smaller. Only three compounds with a terminal triple bond were available, viz. at positions 7, 9 and 10 (table 4). l f i t is assumed that the effect of the ester group is equal to that for disubstituted alkynes and a correction for the effect of the methoxycarbonyl group is made according to table 7, we find the basic values given in table 6. However, it is not certain that the polarizing effects remain the same if the terminal triple bond is closer to the methoxycarbonyl group. The following example illustrates the calculation of the chemical shifts. H

I CH3(CH2)2C ~ C C H z C :-- CCH2C - - C(CH2) 3 C O O C H 3

I H

Table

C(12)

COl)

C(9)

C(8)

C(6)

C(5)

Basic COOCtI 3 CH 3

6 7 8

82.45 0.0 - 0.20

77.00 - 0.05 + 0.20

126.45 + 0.20 0.0

126.45 - 0.30 0.0

77.00 + 1.20 0.0

82.45 - 1.45 0.0

Calculated l£xperimental

5

82.25 82.30

77.15 77.15

126.65 126.55

126.15 126.20

78.20 78.20

81.00 80.95

In this, and all other cases of non-conjugated unsaturated methyl esters, the calculated and experimental chemical shifts differ 0.1 ppm or less. Since not enough positional isomers of the conjugated dienoates were available, we could not find whether the combination of tables 6 and 7 will lead to correct shifts for these compounds. Batchelor et al. have suggested [7] that for a trans, transconjugated system the electric field effects are smaller than for isolated double bonds. We have made the assignments for the conjugated double bond carbon atoms by a comparison with 2, 4-hexadienes [14] and by taking into account the a and c~' methyl effects. For the trans-lO, trans-12-C18 ester it is assumed, moreover, that

Table 8 C a r b o n c h e m i c a l s h i f t p a r a m e t e r s d e s c r i b i n g t h e e f f e c t s o f a single m e t h y l , e t h y l e n i c o r e t h y n i c g r o u p o n d o u b l e o r t r i p l e b o n d C - a t o m s in l i n e a r e s t e r s . H

H

I CH3onC

I = C

CH 3 on C ~ C

a = -6.40

a' = + 1.00

a = -4.85

/3=+1.60

/3'= - 0 . 5 5

/3 = + 1 . 3 0

3" = - - 0 . 3 5 5 = -0.15

3" = + 0 . 1 5 5' = -0.05

3' = - 0 . 2 0 ,5 = 0 . 0 5

H

H

H

H

H

I

I

1

I

I

C=C

on C=C

a' t3' 3" 5'

= = = =

-0.80 -0.65 +0.20 +0.05

H

I

C=C

on C=C

E

I

H

H

/3= - 1 . 9 0

t3' = + 0 . 2 5

~ = +0.10

a' =+1.90

3" = - 0 . 7 0

3" = + 0 . 4 0

/3= - 1 . 7 5

/3' = + 0 . 7 0

H

H

H

H

H

H

I

I

I

I

I

[

C----C on C----C

C----C on C----C

I

L

H

H

a = -1.65

a' = +4.25

a = -4.20

c~' = + 0 . 0 5

/3 = - 2 . 0 0 3" = - 0 . 6 0

/3' = + 0 . 5 0 3" = + 0 . 5 0

/3 = - 2 . 1 5 3' = - 0 . 7 0 e = -0.05

~' = + 0 . 5 5 3" = + 0 . 3 5 e' = + 0 . 1 0

H

H

I

L

C=C

on C~C

C~C

on C=C

1

I

H

H

/3 = - 2 . 5 5

9' = + 1 . 9 0

/3 = - 5 . 5 5

/3' = + 1 . 5 5

3' = - 0 . 4 5

3,'=+0.40

3"= - l . 8 0

3"=+1.30

H

H

H

H

I

I

I

I

C~-C

on CzC

/3 = - 1 . 8 0

/3' = - 0 . 1 5

CzC

on C=C

/3 = - 4 . 9 0

t3' = + 1 . 4 0

C------C o n C------C

C---=C o n C ~ C H

/3 = - 5 . 6 0

/3' = + 0 . 3 0

3" = - 1 . 6 0

3" = - 1 . 4 0

3" = + 1 . 0 0 HCzC 3" = - 2 . 0 5

3" = + 0 . 8 0

on C~C 3/ = + 1.45

Basic v a l u e s a r e 1 3 0 . 0 0 (cis); 1 3 0 . 4 5 ( t r a n s ) a n d 8 0 . 2 0 (C ~- C). c~,/3, ... f o r t h e e f f e c t o n t h e n e a r e r C - a t o m , a ' , / 3 ' , ... f o r t h e r e m o t e o n e .

J. Bus et al., 13C-NMR of unsaturated fatty acid methyl esters

142

the polarization of ~-electrons causes C(13) and C(11) to absorb at a lower field than C(10) and C(12) respectively. Only very few other unsaturated esters have chemical shifts differing more than 0.10 ppm from those calculated as outlined above. In these cases, the double or triple bond is conjugated with the ester group. For cis-2, cis-6-C18 the shifts 6C(3) and 6 C(7) deviate more than 0.10 (0.25 and 0.15 respectively) from 6(calc.); for 2, 6-diyne-C18 the deviations are even larger (see footnote to table 7). In the latter case, it may be better to treat the 2-ynoate structure as one group with its own polarizing properties. The above examples suggest that this empirical calculation of chemical shift might give erroneous results if double or triple bonds are conjugated with a methoxycarbonyl group or with each other. The very few remaining ambiguities in the assignments (table 3, asterisks) concern pairs of carbon atoms, the chemical shifts of which show only very small differences.

C Influences o f double and triple bonds on each other To describe the effect of a double bond on the chemical shifts of a second double bond, we use a notation similar to that outlined for the methyl group. H

H

H

I

I

I (5J

C = C(CH2)nCA = C B

I

14 The effect of the trans double bond on C A is expressed by c~ (for n = 0),/3 (n = 1), 7 (n = 2) .... and on CB by c~' (n = 0),/3', 7', .... From the basic chemical shifts (table 6) the parameters for the effects of the various types of unsaturations on the carbon chemical shifts o f double or triple bonds can be found. These parameters were evaluated by starting with a single, isolated double bond (table 6). The methylene-interrupted cis, trans-diene system for instance, (5; n = 1), has the chemical shifts: H

H

H

I

I

I

C=C--CH

z - C -

C •

(6)

I 14 130.95 128.45

127.85 130.55

The basic shift for a trans double bond is 130.45. Hence, we find 128.45 - 130.45 = - 2 . 0 0 for the 13effect of a cis double bond on a trans double bond and 130.95 130.45 = 0.50 for the corresponding/3' parameter. If a second cis double bond separated from the trans double bond by one methylene group is added to this system, the

J. Bus et al., 13C.NMR of unsaturated fatty acid methyl esters

143

central trans double bond in this triene system has the basic shift of 128.95 (table 6). This value is obtained by adding this ~3parameter to 130.95 ( - 2 . 0 0 = 128.95) or the /3' parameter to 128.45 (+0.50 = 128.95). By comparing all the systems in table 6 with each other, we have evaluated a set of parameters (table 8) describing the effect of one double or triple bond on the carbon atoms of a second one separated from the first by (CH2) n for n = 0, 1 .... However, the influence of unsaturation beyond another double or triple bond is not inincluded in these data. The presence of such an effect is significantly shown by the following example in which we give the carbon chemical shifts of a cis double bond to which a second, a third and a fourth double bond are connected, each time separated by a methylene group (7).

n

H

H

H

L

I

I

(CH2C = C)nCH2C

0 1 2 3

130.00 128.05 127.80 127.70

H I

CCH2 130.00 130.20 130.50 130.55

More examples of this phenomenon can be found from the data in table 6. D. General

Table 8 shows that the effect of a trans double bond on a cis double bond is felt beyond at least 5 single carbon-carbon bonds (e-effect). A triple bond may have an even stronger long-range effect, because the parameters are at least twice as large as those for double bonds. If we recall that double or triple bonds will probably not have any dipole moment component in the direction of the carbon chain, it is quite remarkable that an apolar triple bond or double bond has an effect on a second (nonconjugated) unsaturated bond (same sign, but different magnitude) comparable to that of a methoxycarbonyl group. This may mean that the effect of the latter group is more complicated than that due to only an electric field effect, even at a longer distance. A study of the solvent effects of 13C.NM R spectra of alkadienes or alkadiynes may clarify the character of the interactions of double or triple bonds.

V. Conclusion The carbon chemical shifts of the carbon atoms of double and triple bonds of the methyl esters of unsaturated fatty acids can be treated by additive chemical shift parameters. The additivity is valid for all the relevant groups, e.g. methoxycarbonyl, terminal methyl-, cis- or trans-ethylenic and unconjugated ethynic groups. The chemical shifts can be predicted for many types of unsaturated esters with a high accuracy (-+ 0.1 ppm).

144

J. Bus et al., 13C.NMR of unsaturated fatty acid methyl esters

Acknowledgement We t h a n k H.J.J. P a b o n a n d J.B.A. S t r o i n k a n d t h e i r co-workers o f U n i l e v e r Research V l a a r d i n g e n and C.H. L a m o f the University o f H o n g K o n g for s u p p l y i n g i n o s t of the esters. M e t h y l cis-9, trans-11-octadecadienoate was kindly supplied b y C.R. Scholfield o f t h e N o r t h e r n Regional R e s e a r c h L a b o r a t o r y , Peoria, II1., U.S.A.

References [1] D.A. van Dorp and E.J. Christ, Recl. Trav. Chim. Pays-Bas 94 (1975) 247, and references cited therein [2] D.J. Frost, Thesis, Amsterdam (1974) [3] D.J. Frost and F.D. Gunstone, Chem. Phys. Lipids 15 (1975) 53 ]4] D.J. Frost and I. Sies, Chem. Phys. Lipids 13 (1974) 173 [5] J.B. Stothers Carbon-13 NMR Spectroscopy Academic Press, New York (1972) . . ' and W. Voelter, 13C-NMR Spectroscopy, ' . Wemhclm . • (1974) [6] E. Breltmaler Verlag Chemle, [7] J.G. Batchelor, R.J. Cushley and J.H. Prestegard, J. Org. Chem. 39 (1974) 1698 [8] J.G. Batchelor, J.H. Prestegard, R.J. Cushley and S.R. Lipsky, J. A. Chem. Soc. 95 (1973) 6358 [9] A.P. Tulloch and M. Mazurek, Lipids 11 (1976) 228 [10] P.G. Barton, Chem. Phys. Lipids 14 (1975) 336 [11 ] J. Bus and D.J. Frost, Recl. Trav. Chim. Pays-Bas 93 (1974) 213 [12] J. Bus and D.J. Frost, in: R. Paoletti, G. Jacini and R. Porcellati (Eds.), Lipids, Vol. 2: Technology, Raven Press, New York (1976) p. 343 [13] J. Bus, I. Sies and M.S.F. Lie Ken Jie, Chem. Phys. Lipids 17 (1976) 501 [14] J.W. de Haan and L.J.M. van de Ven, Org. Magn. Reson. 5 (1973) 147