'"c

1.2.4

a

b

C

NMR

x

H

OCH3 C l

Examination of Some morpholine-Bromine Adducts

N=Thioaro yl=

Ugo Chiacchio and Antonino Corsaro" Istituto Dipartimentale di Chimica dell'Universit8, Wale A. Doria 8, 95125 Catania, Italy

Francesco A. Bottino Istituto di Chimica della Facolt5 di Ingegneria, Universita di Catania, Wale A. Doria 8, 95125 Catania, Italy

The 13C NMR chemical shifts of three N-thioaroylmorpholine-bromine adducts, previously used as synthetic intermediates, are compared and discussed in the light of the corresponding shifts of the parent thioamides and suitable model compounds. Chemical shift changes are interpreted in terms of the electronic perturbation resulting from bromine coordination at sulphur.

We have recently emphasized the versatility of N- thioaroylmorpholine-bromine adducts la-c as synthe- tic intermediates in the incorporation of a thioaroyl moiety.1-3 Because of their low stability in the solid state2 they could not be purified satisfactorily, and were used for a subsequent step without being anal- ysed. We assigned the sulphurane structure 1 to these adducts, however, on the basis of their reactivity. This was characterized by the tendency for carbon and sulphur nu~leophiles'-~ to give a bromine displace- ment reaction at sulphur through the cation 2, in a similar manner to other sulphurane~.~~'

Evidence for sulphur-coordinated adducts of di- methylaminothioacetamide or tetramethylthiourea with iodine has been reported? The adduct 3 of 1,3- dimethylimidazolethione with bromine has been iso- lated as a crystalline solid of unique stability, and described as a tricoordinate sulphurane on the basis of x-ray diffra~t ion.~ Compound 3 was successfully used in a synthetic approach to stable 'push-pull' substi- tuted thione ylides, owing to its ability to accept carbon nucleophiles at ~ u l p h u r . ~ The lower thermal stability of 1 compared with that of 3 was ascribed by us to the lower ability of the carbon ligand to stabilize the positive charge. Undoubtedly the imidazole ring, with its six rr-electrons, provides a higher positive charge stabilization owing to its greater conjugative ability. We report here an attempt to substantiate the assumed sulphurane structure for thc three ad- ducts la-c, which were most frequently used in our synthetic work, by the use of 13C NMR spectroscopy. This technique was considered to be very suitable, since significant changes were expected in the chemical shifts of N-thioaroylmorpholines 4a-q from which la-c are prepared, because of the sulphur coordina- tion effect. The charges were expected to be compara- ble to those induced by the protonation shown to occur at sulphur in the case of thiobenzamide.'

* Author to whom correspondence should be addressed.

I

CH3

3

Br I

- s - 1 Br

("7 L N

'4 ' C 0 _ -

X

+

- -!I Br

2

\ c= s

X / d 4

RESULTS AND DISCUSSION

The "C chemical shifts for bromine adducts la-c are listed in Table 1. They are compared with and discus- sed in the light of the chemical shifts of the corres- ponding carbons in the parent molecules 4a-c. The

Ccc-0030-4921/84/0022-0724$01.50

724 ORGANIC MAGNETIC RESONANCE, VOL. 22, NO. 11, 1984 @ Wiley Heyden Ltd, 1984

13C NMR EXAMINATION OF SOME N-THIOAROYLMORPHOLINE-BROMINE ADDUCTS

Table 1. I3C Chemical shifts for adducts la-c and their parent thioamides 4a-c

Com- pound C-I C-2.6 C-3.5 C-4 C-7 C-8 C-9.11 C-12 OCH,

l a 131.2 127.6 129.7 132.1 172.6 59.2 65.4 60.4 4a 142.8 126.1 128.7 129.0 201.2 49.6 66.7 52.5 I b 125.0 133.7 115.3 165.4 170.7 59.8 65.8 60.5 56.0 4b 135.2 128.3 113.9 160.6 201.6 51.0 66.7 53.9 55.4 I c 131.8 130.9 130.2 139.1 171.4 59.3 65.5 59.9 4c 141.1 127.7 129.0 135.2 200.0 49.7 66.7 52.6

signal assignments for la-c followed from the applica- tion of various techniques such as chemical shift ranges, multiplicity in the off -resonance proton de- coupled spectra, signal intensity and comparison with the spectra of appropriate compounds and their parent thioamides 4a-c. The 13C values for the N-thioaroyl- morpholines 4a and 4b were very close to the values reported by Piccinni-Leopardi et aE.,9 whose carbon numbering, shown in canonical structure I, was used for convenience. The chemical shifts of 4c were as- signed in the same way. Compared with 4a-c, the resonance of C-7 in la-c is shifted upfield to a region of the spectrum (170-173 ppm) which is quite distinct from that of aromatic or aliphatic carbons, and it could thus be unequivocally assigned. The location at 170- 173 ppm corresponds well to the chemical shifts of carbons attached to sulphur in protonated thio- benzamide and N-thiobenzoylmorpholine (Table 2), thus indicating the expected similarity of the local electron density at the C-sp2 nuclei in these systems. The C-7 chemical shifts of la-c are also very close to the chemical shift (174.5 ppm) reported for the carbon bonded to sulphur of S-methyltetramethylthiouronium iodide.*' This large shielding at C-7 is consistent with a resonance effect in which the paired electrons of the nitrogen atom to a greater extent, and the six r r - electrons of the phenyl ring to a minor extent, are involved (canonical structures I-V). The high con- tribution of canonical structure I1 results in a marked increase in electron density and, thus, shielding at C-7. The importance of structure 11 for la-c was, moreover,

c; / Y r c -s -

~~ ~ ~

Table 2. -C Chemical shifts of the thiocarbonyl and imino carbons for thiobenzamide, N-thiobenzoylmorpholine and N-methyl- benzaldimine with and without methane- sulphonic acid (MSA)

Compound With MSA Without MSA

Thiobenzamide 168.8 202.6 N-Thiobenzoylmorpholine 170.8 201.2 N-Methylbenzaldimine 172.9 162.6

supported by the observation that the chemical shift of the imino carbon of N-methylbenzaldimine moved from 162.6 to 172.9ppm (Table 2) on addition of an equimolecular amount of methanesulphonic acid. The value of 172.9 ppm is unequivocally attributable to an iminium carbon.

Compared with 4a-c, adducts la-c exhibited C-1 signals at higher fields and C-2,6 and C-4 signals at lower fields, while those of C-3,5 remained almost unchanged. These shifts are in good agreement with the known rr-electron acceptor substituent effects in substituted benzenes." Solvent interactions can be excluded, since all spectra were recorded in the same solvent (CDCl,). The intense shielding at C-1 and the deshielding at C-2,6 and C-4 can thus be attributed to the increase in the positive character of C-7 induced by sulphur coordination.

The C-8 and C-12 signals of la-c were shifted downfield by -9.2 and -7.2 ppm, respectively, from the corresponding carbons of their parent compounds, and those of C-9,11 were slightly upfield. Further, a smaller difference (A8 6 1.2 ppm) was found between the C-12 and C-8 chemical shifts in la-c than in 4a-c (AS = 2.9 ppm). The downfield shifts at C-8 and C-12 again appear to support the highest contribution from structure I1 in the resonance of la-c, as the deshield- ing can be attributed to the inductive effect of the positive charge on nitrogen. Downfield shifts have been observed for the a-carbons in aryl cyclic carbo- cations on increasing the positive charge at the cationic carbon centre,'* and for the a -N-methylene

C

X $$ B r

I _ I

- S

B r

X' I l l

V

ORGANIC MAGNETIC RESONANCE, VOL. 22, NO. 11, 1984 725

V. CHIACCHIO, A. CORSARO AND F. A. BOTITNO

carbons in cyclic amines on protonation. l3 Assuming, on the basis of theoretically derived and observed tetra~ovalent'~ and tricovalent7 sulphurane structures, that la-c have an axial three-centre geometry, with the BrSBr bonds almost collinear and the N-C-C bond-

S ing system planar in an equatorial plane, the decrease in At3 C-12/C-8 in la-c with respect to 4a-c could be explained in terms of a lower barrier to rotation around the C-N bond, or in terms of a changed anisotropic effect at C-8 because of a decrease in the double bond character of the C-S bond. The variable-temperature 'H NMR spectra of 4c and its bromine adduct l c supported the hypothesis of a lower anisotropic effect at C-8, since a higher barrier was found for l c than for 4c. Thioamide 4c, chosen because of its lower . coalescence temperature (T, = 355 K), has a A G f of 74.00 kJ mol-' (Au = 40 Hz), while l c did not coalesce until its decomposition temp- erature (Td = 370 K), thus indicating A G f > 79.44 kJ mol-l.

Although a nitrogen coordination could also ac- count for the downfield shifts in the a-N-methylene carbons, the marked upfield shift at C-7 and the increase in the barrier around the C-N bond are expected only as a result of a sulphur coordination in adducts la-c.'

ported.' Thiobenzamide was commercially available. N-Methylbenzaldimine was obtained according to the 1iterat~re.l~ Bromine adducts la-c were prepared di- rectly in an NMR tube by mixing equal volumes (1 ml) of 1~ solutions of the thioamide and bromine in deuteriochloroform. The purification of bromine was achieved according to a literature procedure. l6

The 13C NMR spectra were obtained on a Bruker WP80 FT spectrometer operating at 20.11 MHZ and 300 + 2 K. Chemical shifts are relative to tetramethyl- silane with CDC1, as solvent and deuterium lock. Typical I T conditions were: spectral width, 5 kHz; data points, 16 K; pulse width, 1.0 p s (15"); decoupler frequency, 6 kHz; decoupler power, 2 W; pulse repeti- tion time, 0.82s; number of pulses, 500-6000. The accuracy of the I3C chemical shift measurements was *0.1 ppm.

C NMR spectra of protonated compounds were obtained by adding an equimolar amount of methane- sulphonic acid to CDCl, solutions of the parent. 13C NMR spectral data for 4a and 4b agreed with those of Ref. 9. Temperature-dependent 'H NMR spectra were measured at 80MHz on the same instrument equipped with suitable accessories for variable- temperature operation. The accuracy of the tempera- ture values was *1 "C. Approximately 0.5 M solutions of the thioamide 4c or adduct l c in nitrobenzene-d, containing 1% UIV of tetramethylsilane, as an internal reference, were employed.

13

EXPERIMENTAL

The preparation and physical and analytical data for N - thioaroylmorpholines 4a-c have been previously re-

Acknowledgements

This work was supported by the Italian M.P.I.

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Received 21 March 1984; accepted 30 May 1984

726 ORGANIC MAGNETIC RESONANCE, VOL. 22. NO. 11, 1984