摘要:
NUMBER 20, 1965 483 The Structures of Three Naphthalene Tetrachlorides By P. B. D. DE LA MARE, M. D. JOHNSON, J. S. LOMAS, and V. SANCHEZ DEL OLMO (Chemistry Department, Bedford College, Regent’s Park, London, N . W. 1, and William Ramsay and Ralph Forster Laboratories, University College London, Gower Street, London, W.C. 1) OF the six possible geometric isomers of naphthalene 1 2,3,4-tetrachloride, several are known, but only one has been assigned a structure unambiguously. This is the a-isomer, m.p. 182”, which has the configuration indicated diagram- matically at (I), and the conformation in the solid state indicated at (II).l The y-isomer, m.p. 134O, has been assigned by Schiitz and Hahnfeld2 to one of the structures (111) and (IV) on the basis of its rate of alkaline dehydrochlorination.We have now examined its proton magnetic resonance spectrum. This shows that the environments of H, and H, (and likewise of H2 and H3) are the same (an A2X2 spectrum). Only four of the six possible naphtha- lene tetrachlorides should have this symmetry, and the coupling constants for the pairs of aliphatic protons provide further information. All three are large (Jl,2 = J3,* = J2,3 = 8.0). Karplus’ relationship3 tells us that there must be a large* angle between each pair.? The only possible conformation, then, is that having all four hydrogens axial or pseudo-axial, and hence all four chlorines equatorial or pseudo-equatorial. Structure (111) is therefore correct, and (V) is the predominant conformation in solution. Schiitz and Hahnfeld2 assigned to the %isomer, m.p.9 6 O , one or other of the structures (VI) and (VII) . Proton magnetic resonance spectroscopy again allows a decision between these two possi- bilities. This isomer has a very unsymmetrical ABCD spectrum (with JAc = Jan = J,, = 0), as is consistent with either (VI) or (VII), but with no other configuration. The coupling constant between the 2- and 3-hydrogen atoms, J2,3, is very large (10.9). These two hydrogen atoms must therefore both be axial; (VI) must be the structure, and (VIII) the preferred conformation in solution. The coupling constants Jl,2 and J3,4 are consistent with this structure. From reaction of naphthalene with chlorine in carbon tetrachloride in the sunlight, a new naphthalene tetrachloride, which we designate the €-isomer, is formed, together with the more insoluble y-isomer.We have isolated the new compound by fractional crystallisation of the mixed tetrachlorides; it has m.p. 85-87’. Its p.m.r. spectrum shows that, like the 01- and * The possibility of a very small angle between the G-H bonds can be excluded by examination of models. t We assume, following other writers,* that the relatively unstrained “pseudo-chair” form of the cyclohexene-like system, exemplified for the =-isomer in (11), is favoured in compounds of this type. 1 M. A. Lasheen, Acta Cryst., 1952, 5, 593. F. Schiitz and K. Hahnfeld, Chew. Ber., 1952, 85, 131. M. Karplus, J . Chem. Phys., 1959, 30, 11. cf. W. G. Dauben and K. S. Pitzer, “Conformational Analysis”, in “Steric Effects in Organic Chemistry”, ed.. M. S. Newman, Wiley, N.Y., 1956.484 CHEMICAL COMMUNICATIONS c1 c1 + c1 c1 H c+P c1 H (1') (Naphthalene y- t e t r achlor ide) Compound M.p. . . . . Spectrum type : 7-Values (p.p.m.) : Aromatic : l-H . . .. 2-H .. .. 3-H . . .. 4-H . . .. (Naphthalene a-tetrachloride) c1. c1 c1 c1 c1 c1 c1 FE (VI) (W (VIII) (Naphthalene S- te trachloride) c1 c1 c1 - 61 (XI (Naphthalene e t e t ~-,ichloride) TABLE Proton magnetic resonance spectra of naphthalene 1,2,3,4-tetrachlorides a-Tetrachloride .. 182" .. - 4 2 x 2 2-48 (single) .. 4-28 .. 4-93 .. 4.93 .. 4.2 8 y-Tetrachloride 134' A&, 2-26 (single) 4.48 5.42 5-42 4.48 &Tetrachloride 96" ABCD 2-2-2.7 (unsymm.) 4.56 5.53 5.08 4.60 Coupling constants (c. /sec.) : Ji,, . . .. .. 3.5 0.4 8.0 f 0.4 3.2 & 0.4 J2,3 .. .. . . 11.0 f 0.6* 8.0 f 0.6 10.9 f 0.4 J3,4 . . .. .. 3.5 f 0-4 8-0 f 0.4 7-6 f 0.4 Chlorine positions in solution . . . . a'eea'* e'eee' a'eee' €-Tetrachloride 85-87' A2X2 (A2X2) 2-4 4.53 4.96 4.96 4.53 5.8 f_ 0.4 5.8 f 0.4 > 3-5 a'aee' + e'eaa' * Tetrahydrofuran or dioxan as solvent; we have not been able to determine J2,3 for this compound in deutero- chloroform, because of its low solubility and the resulting very low intensity of the relevant outer peaks of the A,X2 spectrum.NUMBER 20, 1965 485 y-isomers, the environments of H, and H, (and likewise of H, and HJ, are magnetically equiva- ‘lent. Of the two remaining possibilities having this symmetry, viz. (IV) and (IX), the latter should, in either of its possible conformations, have low coupling constants for each pair of protons.This does not accord with the experi- mental data (see Table). The former, on the other hand, should be an equilibrium mixture containing the enantiomorphic conformations (X) and (XI) in equal amounts. Each of these should have a large coupling constant (note the large angle between the C,H and C,H b o d s in (X) for one of the outer pairs of protons and a small coupling constant for the other. The rapidly equilibrating mixture should have coupling con- stants, J1,2 and Ja,a, equal and intermediate between a high value of about 8 or greater and a low value of about 3. The observed value, 5-8, is consistent with this and only this structure. The other naphthalene tetrachlorides were prepared essentially as is described in the litera- ture; (cf. reference 2). In isolating the &isomer, chromatography on silica gel proved helpful. The melting points agreed satisfactorily with those recorded by other workers ; satisfactory analyses were obtained for all the isomers, which had characteristic infrared spectra. Their p.m.r. spectra (determined on a Perkin-Elmer 60 Mc./ sec. spectrometer, in deuterochloroform, with tetramethylsilane as reference, except where otherwise noted) did not show the presence of any isomeric material. The Table shows the 7-values and coupling constants. The results will be discussed in more detail elsewhere, as will be the mechanistic implications of the modes of formation of these compound^.^ (Received, August 23rd, 1965; Corn. 530.) P. B. D. de la Mare, R. Koenigsberger, 84, 109. J. S. Lomas, V. Sanchez del Olmo, and A. Sexton, Rec. Trav. chim., 1965,
ISSN:0009-241X
DOI:10.1039/C19650000483
出版商:RSC
年代:1965
数据来源: RSC