Jan., 1961
NOTES
187
Using methods identical in all respects 00 the cases unexpected spin-spin coupling of the protons in the above a value of 105.87 for E’ccC7is found. The ar-positions of the two heterocyclic rings proved to calculations necessitate the use of 99.50 kcal./mole be the key to the interpretation of the spectra; alfor ECHin methane, 80.88 kcal./mole for E ’ ’ c c C 7 though these protons are separated by six bonds, in cycloheptatriene and cis-cycloheptene, and the the coupling constant amounts to about 1 C.P.S. standard heats of formation reported in the litera- On the basis of the spectra of representative comture or computed from heats of h y d r o g e n a t i ~ n . ~ J ~ pounds !~l in the series, we wish to report here the interpretation of the spectrum of N-benzylthienoConciusions [3,2-b]pyrrole (I), the synthesis of whichis reported To summarize then, we have these bond energies e1sewhe1-e.~ The relative magnitudes of the crossfor Cspz-CSp2 single bonds ring couplings in this compound can be explained in terms of a a-electron mechanism. in butadiene 106.47kcal./mole The a-proton of a 5-membered aromatic heteroin cycloheptatriene 105.87 kcal./mole cycle generally absorbs at a lower magnetic field in cyclooctatetraene 103.73 kcal./mole than a corresponding P-proton.KsBI n the spectra in cyclooctatriene 103.54kcal./mole of thieno [3,2-b]pyrroles the peak assigned to the It is still impossible to attribute all of the difference 5-proton occurs between -3.19 and -2.18 p.p.m. in energy of ca. 2.8 kcal./mole between the single (relative to water) and that to the 6-proton between bond in butadiene and the C8 cyclopolyenes t o - 1.84 and - 1.31 p.p.m., all spectra showing good resonance or an;y other single phenomenon. The peak separation a t both 40 and 60 Me.’ I n the difference in bond energy is, however, in good agree- spectra of several 2,3-disubstituted pyrroles (11) ment with the irotational barrier in butadiene as and 2,3-disubstituted (111) and 2,3,4-trisubstituted calculated by Millliken and Parr12 and later refined (IV) thienopyrroles the value of the coupling conby M ~ l l i k e n . ~Turner has derived a value of 2.4 stant3r5 JaB(Js6 in the thienopyrroles) is in the kcal./mole for the stabilizat,ion energy of cycloocta- range 3.0 f 0.5 C.P.S. (In the NH series, there is t,etraene as referred to a cyclohexene-cyclooctatriene further splitting of the a and P protons by the NH model.’& If this 2.4 kcal./mole is due mostly to proton, and, since J45 % J.IB J 5 6 , a pair of triplets trans-annu’lar a-bond interaction, then half of it is observed3.) The J 5 6 values of several 2,3,4-trimust be added t o the 2.8 kcal. difference in bond substituted thienopyrroles are included in Table I . energy between t,he coplanar and 90” rotated model to get 4.0 A 0.3 kcal./mole for the rotational barrier TABLE I in butadiene, in excellent agreement with Mulliken. PROTON-PROTON COUPLING CONSTANTS IN C.P.S. OBSERVED The conclusioin we then draw is that there is THIENO [3,2-b]significant st,abilization energy in butadiene which I N THE N.M.R. SPECTRA O F SUBSTITUTED PYRROLES (Iv) cannot be attributed to a pure CSp~-Csp~ bond Coupling alone. Even if the use of average bond energies ,.Substituents constantsa Rz Ra Itr RB J66 Ja does not subsequently prove to be exact (ie., if OH COzEt H 3.7 .. E’CHin cyclooctatetraene is different from E‘CHin COzEt OH Et H 2.7 .. ethylene), we would see an error in the E’cc cal- COzEt OCHi COzEt H 3.2 .. culated in the models used but probably very little COzEt OCHI Et H 3.0 .. error reflected in. the differences between the E’CC C02Et COzEt OAC COzEt H 3.5 .. values.
-
(10) R. B. Turner and W. R. Meador, ibid.. 79, 4133 (1957). (11) J. B. Conn, G. 13. Kistiakowsky and E. A. Smith, ihid.. 61, 1868 (1939). (12) R. S. Mullikm and R. G. Parr, J . Chem. Phys., 19, 1271 (1951).
H OAC COCH, H COCH, OAC COCH, H H OAC COCH, CHzCsHs The values given are the average splitting8 60 Mc. with moderate resolution, uncorrected order effects.
3.5 3.5
1.2
.. 1.3 observed at for second-
..
NUCLEAR MAGNETIC RESONANCE SPECTRUM OF N-BENZYLTHIENO [3,2-b1PYRROLE‘
With the knowledge of the approximate line positions of the 5 and 6 protons and of the value for J56, the spectrum of 3-acetoxy-4-acetylthienoBY R. J. TVITE,HI. R. SNYDER,A. L. PORTE A N D H. S. [3,2-b]pyrrole (V) can be explained. I n addition to GUTOWSKY the expected coupling of the 5-proton a t -2.36 N o y e s Chemical Laboratory, University of Illinois, L’rbana, Illinois pap.m.with the 6-proton a t -1.70 p.p.m. (JSSS Received July $6,1960 3.5 c.P.s.), there is a further splitting of the -2.36 We have found that the position of nuclear sub- p.p.m. peak by the 2-proton, which appears a t 1.2 c.P.s.). The latter stitution in thieno [3,2-b]pyrroles2p3usually can be -2.00 p.p.m. ( J 2 5 deduced directly from n.m.r. spectral data. An cross-ring splitting is also observed in the spectrum of VI (J25 1.3c.p.s.).4 (1) This investigation was supported in part by the Officeof Naval
Research and in part by a grant IC3969(Cl)Bio] from the National Cancer Institute, Public Health Service. Also, grateful acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of part of the work. (2) D. S. Matteson and H. R. Snyder, J . Am. Chem. Soc., 79, 3610 (1957). (3) W.R. Carpenter and H, R. Snyder, ibid.. 81,2582 (1960).
=
(4) 9. D. Josey, R. J. Tuite and H. R. Snyder, ibid., 83,1597 (1960). ( 5 ) R. Abraham and H. Bernstein, Canad. J . Chem., 37, 1056
(1959). (6) E. J. Corey, G . Slomp, S. Dev, S. Tobinaga and E. R. Glazier, J . A m . Chem. Soc., 80, 1204 (1958). (7) The spectra were determined with a Varian Associates V-4300-B high resolution n.m.r. spectrophotometer on 20% solutions in deuterioohlorofwm with methylene chloride aa an external stsndsrd.
KOTES
188
I11
The values of J66 and J26 now can be used in interpreting the spectrum of I. In addition to the partially resolved benzenoid multiplet, the aromatic region contains a series of two quartets and two doublets, under conditions of moderate resolution. Each component of the two doublets is further resolvable into a close doublet, giving the series of four quartets shown in part A of Fig. 1. From
H= ,
Vol. 65
The values cited for J26 and J a are surprisingly large when one considers that, six bonds separate the nuclei which are coupled. To our knowledge, there are no previously reported cases of spin-spin interaction of appreciable magnitude between two protons which are separated by more than five bonds. However, coupling via five bonds has been reported in several cases, including 1 C.P.S. between the aldehydic proton and the 5-proton in 2-thiophene carboxaldehyde,1° 0.9 C.P.S.between the corresponding protons in furfural," and 0.15 to 0.45 C.P.S.between the para protons in derivatives of benzene. 1 2 ~ 3 The observed long-range cross-ring couplings in I must be transmitted through the a-electronic system, for they are too large to be accounted for by transmission through the a-framework. There must, therefore, be significant interaction between the u- and the a-electronic systems in these fivemembered rings. The relative magnitudes of couplings can be accounted for, qualitatively, by eIectronic interactions shown schematically in I a and Ib.
4
I CH2.C,H, ZIH
5H
3,H
R
Ia
R
Ib
u
C IO 20 (CYCLES) Fig. L-The: spectrum of the heterocyclic ring protons in N-benzylthieno[3,2-b]pyrrole,observed at 60 Mc. with the magnetic field increasing from left to right (A). The line spectrum (B) was computed ocith an electronic digital computer (11lia)c) using exact expressions for the four-spin system and the parameters, all in c.p.8.: R = 0; v3 = 16.3; Y6 8.9; V I 33.8; J z ~ 4.9; J z ~ 1.3; J ~ =B 0; J j s = 0; J ~ iI= 0.5; and JW = 2.9. 3
their positions and coupling constants, these quartets are assigned (from lower to higher field) to the 2-, 5-, 3- and 6-protons1 respectively. We have made a full analysis of the spectrum of the heterocyclic ring protons, the results of which are summarized in Fig. 1, part B. The value of 4.9 C.P.S. found for J9!3agrees well with that reported for J d in mononuclear thiophenes.*m9 The absence of further resolved splittings indicates that Jza and J s are ~ less than ca. 0.3 c.P.s., which is the approximate limit of resolution of the instrument a t 60 Mc. (8) 9. Gronowita and R. A. Hoffman, Arkiu. Kemi, 13, 279 (1958). (9) R. Abraham and H. Bermkin, Canad. J . Chem., 37, 2095 (1969).
u-a Interaction at carbon-2 couples the spin of proton-2 to the a-electronic system, and the mesomeric effect then transmits this coupling to the remaining protons of the system via u-a interactions at other sites in the molecule. However, as shown in (Ia), the mesomeric effect couples electronic perturbations at carbon-2 preferentially with carbon-5 rather than with carbon-6, ie., J26 is greater than J26. Similarly, in (Ib), the mesomeric effect couples proton-3 with proton-6 rather than with proton-5, Le., J36 is greater than J35. Furthermore, the contribution of structure I b to the total ?r-electron wave function for the molecule is less than that of and so JZsis greater than Jse. It is to be expected that mutual interactions between substituents will be transmitted across the ring system in a manner similar to the coupling between protons. For example, chemical effects between substituents in the 2- and 5- positions will be greater than those between the same substituents in the 2- and 6-positions. (10) S. Gronowitz and R. A. Hoffman, Acta Chem. Scand., 13, 1687 (1959). (11) J. B. Leane and R. E. Richards, Trans. Faraday Soc., SS, 618 (1959). (12) J. P. Heeschen and H. 9. Gutowsky, unpubliahed work. (13) Coupling constants via five bonds have beon reported and reviewed by F. A. L. Anet, J . Chsm. Phys., 89, 1274 (19601,who reports ornabring coupling of 0.8 C.P.E.between the 4- and %hydrogens, wbioh are separated by five bonds, in two substituted quinolines. (14) V. Schomsker and L. Pauling, J . Am. Chem. Soc., 61, 1779 (1939).
