3558
homogeneous, and the volume of the solution was reduced to ca. When cooled to On, the mixture deposited dark brown needles, mp 186-189” (sealed tube). Anal. Calcd for C2JIz0N4: C, 79.09; H, 5.53. Found: C, 78.86; H, 5.33. Determination of Equilibrium Constants. The Benesi-Hildebrand treatment was used to determine the molar extinction coefficients and equilibrium constants. In a typical determination four concentrations were prepared, with the mole fraction of the
donor varying from about 0.010 to about 0.033, while the concentration of the TCNE was held between 0.007 and 0.011 M. Good straight lines were obtained using the four points for each complex except for [3.3]- and [9]paracyclophane-TCNE salts whose K’s were determined only from least-squares analyses. The recorded values of the K‘s for the other complexes were calculated both by least-squares analyses and Benesi-Hildebrand plots, and the values from each were in good agreement.
45 ml.
Cyclopropyl Aromatic Chemistry. I. Ultraviolet Spectra of Certain Cyclopropyl Aromatic Systems Roger C. Hahn, Philip H. Howard, Sum-Man Kong, George A. Lorenzo,’a and Niels L. Millerlb Contribution f r o m the Departments of Chemistry, Syracuse Unicjersity, Syracuse, New York 13210, and University of South Dakota, Vermillion, South Dakota 57069. Receiued January 15, 1969
Abstract: The ultraviolet spectra of a- and P-cyclopropylnaphthalenes (I and 11) resemble those of the corresponding methylnaphthalenes much more closely than those of a- and P-alkenylnaphthalenes. Data are tabulated to support the suggestion that a minimal electron-attracting ability of the naphthalene T system evokes a minimal conjugative response from the cyclopropyl group. To answer the question of the dependence of cyclopropane conjugation on geometry in arylcyclopropanes, uv spectra were obtained of 3- and 4-nitro-l ,la,6,6a-tetrahydrocycloprop[ulindenes 0-and /3’-N02-IV),6’- and 5 ’-nitrospiro[cyclopropane-1,l’-indans] @- and P’-N02-V), 5-nitro-la,2,3,7btetrahydro-1H-cyclopropa[a]naphthalene@ ’-NOz-VI), 6 ’-nitrospiro[cyclopropane-1,1 ‘-tetralin] @ ’-NOz-VII), and 7- and 6-nitro-l,la,2,3,4,8b-hexahydrobenzo[a]cyclopropa[c]cycloheptenes@- and P ‘-NO*-VIII). Values for s’,A,, (1L&band, 95% EtOH) are: P-nitroindan, 282 mp; P-NOz-IV and P-NOZ-V,282; P-NO2-VII1, 280 (nitro meta to cyclopropyl in P series); P’-N@-VIII, 285; @‘-N02-IV,301 ; @‘-N02-VIand P’-N02-VII,306; P’-NO2-V, 313 (nitro para to cyclopropyl in p’ series), The absence of noticeable differences in the maxima of P-nitroindan, P-N02-IV,and P-NO2-Vis attributed to an absence of inductive donation by the cyclopropyl group to the nitro-
aromatic chromophore. The bathochromic A,, shifts from P-NOz-IV to P’-NOZ-IV and from P-NOp-V to P’NOz-V are seen as results of cyclopropane conjugative interaction which is possible only in the P’-nitro systems. The bathochromic A,, shifts in the P’ series from P’-N02-VIII to P’-N02-V are related to increasingly favorable cyclopropane geometries for conjugation. It is concluded that cyclopropane conjugation in cyclopropyl aromatic systems is a spectroscopically detectable function of cyclopropane geometry if the interacting chromophore is sufficiently electron attracting.
I
t is widely accepted that the functioning of a cyclopropane ring as a ground-state electron donor, as in conjugation with neighboring unsaturated systems, 2 , in neighboring group interaction with carbonium ions, and in interaction with various electrophilic reagents,j involves specific steric requirements for maximum effect. Increasingly, attention has been directed also to the effects of cyclopropyl substitution on the electronic spectra of T-electron systems.6- l o Results of (1) (a) National Science Foundation Trainee, 1965-1968 (Syracuse University) ; (b) National Science Foundation Academic Year Institute Participant, 1962-1963 (University of South Dakota, Vermillion). This work was supported in part by a grant from the University of South Dakota Research Fund. (2) H. C. Brown and J. C. Cleveland, J . A m . Chem. Soc., 88, 2051 (1966). (3) T. Sharpe and J. C. Martin, ibid.,88, 1817 (1966). (4) H. Tanida, T. Tsuji, and T. hie, ibid., 89, 1953 (1967); M. A. Battiste, C. L. Deyrup, R. E. Pincock, and J. Haywood-Farmer, ibid., 89, 1954 (1967). ( 5 ) L. Joris, P. von R. Schleyer, and R. Gleiter, ibid., 90, 327 (19681, and references therein. (6) For a review, see J.-P. Pete, Bull. SOC.Chim. France, 357 (1967). (7) W. G. Dauben and G. H. Berezin, J . Am. Chem. Soc., 89, 3449 (1967). (8) R. E. Pincock and J. Haywood-Farmer, Tetrahedron Letters, 4759 (1967). (9) C. H. Heathcock and S. R. Poulter, J . Am. Chem. Soc., 90, 3766 (1968), and references therein.
most of these studies indicate that cyclopropane geometry has a n important effect on electronic spectral characteristics. However, for vinylcyclopropane~~ and arylcyclopropanes, geometric factors have been reported to be unimportant. I n an attempt to begin to accommodate this apparent divergence in the spectral behavior of cyclopropylsubstituted K systems, we have carried out a study of the ultraviolet spectra of a series of cyclopropyl aromatic compounds. The data presented here allow further evaluation of resonance and induction factors, and of conformation effects in cyclopropyl aromatic systems. We also discuss the role of chromophore electronegativity in affecting the auxochromic character of cyclopropanes. Results and Discussion Differences in ultraviolet absorption characteristics of certain a- and P-alkenylnaphthalenes have been attributed to steric interaction of a substituents with the naphthalene peri hydrogen, which can force the (IO) M. J. Jorgenson and T. Leung, ibid., 90, 3769 (1968), and references therein. (11) A. L. Goodman and R. H. Eastman, ibid., 86,908 (1964).
Journal of the American Chemical Society 1 91:13 / June 18, 1969
3559 a-alkenyl group away from the coplanar conformation needed for maximum conjugative interaction. l 2 These phenomena prompted us to look for similar effects in a- and P-cyclopropylnaphthalenes (I and II), syntheses of which are outlined in Chart I.
Table I. Uv Absorption Maxima for Some Substituted Naphthalenes
-Naphthalene substituent p-Methyla,b p-CyclopropyP p-Isopropenyla d p-( 1-Cyclohexeny1)eJ a-Methylasc a-Cyclopropyl" a-Propenyla a-(l-Cyclohexenyl)eJ sc
Chart I.
I
RCH C , H2C H20H
HBI, H2SO4 @
RCH,CH,CH,Br
1 NBS
( R = a- OR 8-NAPHTHYL)
](PhCO2Ip
R CH Br C H2CH2Br
I
Zn-Cu
DMF
h
IC
J
IBb
A,, mp (log E ~ ~ ~ ) - - - - - 'L, band band
224 (4.94) 227.5 (4.90) 238 (4.70) 247 (4.61) 224 (4.99) 224.5 (4.86) 228 (4.73) 225-6 (4.93)
275 (3.68) 277 (3.72) 283 (4.05) 286 (4.08) 282 (3.69) 283 (3.82) 296 (3.97) 282 (4.07)
EtOH. * Data from R. A. Friedel and M. Orchin, "Ultraa 95 violet Spectra of Aromatic Compounds," John Wiley & Sons, Inc., Data from J. Jacques, New York, N. Y., 1951. This work. M. Legrand, and J. Bourdon, Bull. SOC.Chim. France, 362 (1954). Cyclohexane. f Data from ref 12. 0 Data from R. N. Beale and E. M. F. Roe,J. Chem. SOC.,2888 (1951).
a position,13 any exercise of the electron-donating
I
II
Models indicate that the a-cyclopropyl group does not have free rotation around its bond to the naphthalene carbon, because of interaction with the peri hydrogen. Preferred conformations of I thus may have
~H
A
(e= O o )
the cyclopropane ring swiveled away from the bisected geometry normally considered to be needed for maximum electronic interaction with an adjacent T system.2,3 This geometry is exemplified in structure A, in which 8 is defined as the angle made by the cyclopropyl methine carbon-hydrogen bond with the plane of the benzene ring." The cyclopropane ring is thus perpendicular to the plane of the aromatic system in this conformation. To the extent that such electronic interaction occurs, and if there is a geometry requirement for cyclopropyl conjugation with the naphthalene T system, one might expect to see effects of steric hindrance in the electronic spectra of I and 11. Selected data from the uv spectra of some naphthalene derivatives are assembled in Table I. The a- and p methylnaphthalenes have been chosen as comparison standards. We will consider the /3 system first. The listed A, values for @-isopropenylnaphthalene and 0-(1-cyclohexeny1)naphthalene show substantial bathochromic shifts for both bands, compared to the corresponding values for @-methylnaphthalene. It has been suggested'* that even in the 0-cyclohexenyl system, with the largest of the substituents included here, there is minimal hindrance to achievement of the coplanarity needed for maximum orbital overlap of the vinyl T system and the naphthalene T system. The A,, values for P-cyclopropylnaphthalene (11) are most similar to those for @-methylnaphthalene. Since steric hindrance is negligible in the P position, and the electron-attracting ability of naphthalene is greater here than at the (12) L. H. Klemm, H. Ziffer, J. W. Sprague, and W. Hodes, J . Org. Chem., 20, 190 (1955).
capacity of the cyclopropane ring should be evident here. However, there is only a very minimal bathochromic shift for 11, relative to @-methylnaphthalene. It is concluded that the naphthalene system, particularly in the excited electronic states involved here, does not have sufficient electron-attracting power to make any great demand on the conjugative ability of the p-cyclopropyl group. What little conjugative electron donation is provided by the cyclopropane endocyclic orbital(s) appears to be approximately counterbalanced by the inductive electronegativity of the cyclopropane exocyclic orbital relative to that of an ordinary alkyl group (e.g., methyl).I4 Of the a systems in Table I, only the a-propenylnaphthalene maxima are bathochromically shifted, relative to those of a-methylnaphthalene. The lack of conjugative effect in a-(1-cyclohexenyl)naphthalene has been attributed to steric interaction of the cyclohexene ring and the naphthalene peri hydrogen. l 2 Steric hindrance is also present in I, as discussed previously. The low-field peri-proton absorption in the nmr spectrum of I (see Experimental Section) may be consistent with preferred conformations in which the cyclopropane ring does not achieve bisected geometry. 15 The uv spectrum of I shows absorption maxima very similar to those of a-methylnaphthalene. Again, this similarity may be ascribed to a balance of conjugative and inductive effects of the cyclopropane ring, made possible by the minimal electron demand of the naphthalene ring. However, this rationalization, to the extent that it admits any significant conjugative effect, now requires either that the steric effect of the naphthalene peri hydrogen is minimal in preventing achievement of optimum cyclopropane geometry, or that the cyclopropane ring has no geometric requirements for conjugative interaction. If conjugative interaction is minimal, no conclusions can be drawn concerning the importance of cyclopropane geometry. While the above work was in progress, Goodman and Eastmanl' reported that the ultraviolet absorption spectra of 1,la,6,6a-tetrahydrocycloprop[a]indene(IV), (13) Y . Okamoto and H. C. Brown, J . Am. Chem. Soc., 79, 1903
(1957).
(14) For a summary of theories of bonding of cyclopropanes see W. A . Bernett, J . Chem. Educ., 44,17 (1967), and references therein. (15) (a) K. Tori and K. Kitahonoki, J . Am. Chem. Soc., 87, 386 (1965), and references therein; (b) V. Balasubramaniyan, Chem. Rev., 66, 567 (1966), and references therein.
Hahn, et al.
Uv Spectra of Cyclopropyl Aromatic Systems
3560
spiro[cyclopropane-I, 1'-indan] (V), and la,2,3,7b-tetrahydro-1H-cyclopropa[a]naphthalene (VI) are almost superimposable, despite the different geometries of the cyclopropane rings in these compounds. They concluded that there is no preferred geometry for conjugation of a cyclopropane ring with a benzene ring. I
4'
PI
3'
(e=10*)
It occurred to us that the concept of minimal cyclopropane conjugation in the presence of minimal electron demand might apply to rationalization of the spectral features noted for IV, V, and VI. This concept has the additional appeal of not conflicting with the mass of evidence attesting to the importance of geometry in systems where cyclopropane participation is more strongly evoked. *--I1 A survey of spectroscopic data in the literature provides evidence pertaining to the magnitude of cyclopropane substituent effects on the uv spectra of i~ systems of varying electronegativity. In Table 11, reported A, values for some substituted uv chromophores are listed in rough order of increasing electronattracting ability; for each chromophore the effects of alkyl, cyclopropyl, and alkenyl substitution are given where available. Table 11. Substituent Effects on Ultraviolet Absorption Maxima
Compound Indan and 1,l '-dimethylindan IV and V Indene and I-methyleneindan Tetralin VI 1,2-Dihydronaphthalene Ethylbenzene Cyclopropylbenzene (111) Styrene Isopropyl methyl ketone (XX) Cyclopropyl methyl ketone ( Y Y ) Methyl vinyl ketone (ZZ) p-Nitroethylbenzene p-NO*-I11 p-Nitrostyrene 2,4-Dinitrophenylhydrazone (DNPH) of XX 2,4-DNPH of YY 2,4-DNPH of'ZZ
, , , ,A
mp
-2 18= -225a -250a -2 1 -229 -260a 206a 220Q 245.9 193a 20% 2194 274, 5a 297a 3OOb 36@ 379 374c
Ah,,,, re1 to alkyl group Ref -7 -32 -7 -42 14 39.5 15 26
11 11 11 11 11 11 d d d d d d e
22.5 25.5
f g
7 6
g g
a In 95% ethanol. In methanol. In methylene chloride. M. T. Rogers, J . Am. Chem. Soc., 69, 2544 (1947). e W. M. J. R. Schubert, J. Robins, and J. L. Haun, ibid.,79,910 (1957). M. F. Hawthorne, Joy and M. Orchin, ibid., 81, 305 (1959). J . Org. Chem., 21, 1523 (1956). d
It can be noted from Table I1 that the cyclopropyl group in all cases causes a greater bathochromic shift than does an alkyl group, and that the magnitude of
the shift approaches that of an alkenyl group as the electronegativity of the chromophore increases. A conclusion similar to that previously reachedL6is that the cyclopropyl group makes a conjugative electron contribution commensurate with the demands of the chromophore. Except for the present reported naphthalene systems (Table I), compounds IV-VI in Table I1 show the smallest bathochromic shifts for cyclopropyl substitution, and therefore appear to be relatively insensitive models for detecting geometry effects on cyclopropane conjugation. The broadness of the 'La band maxima prevents confident assignments of A, values for indan, tetralin, and cyclopropane derivatives IV-VI (Table 11). However, it was reported'' that for the relatively sharp 'Lb band maxima near 270 mp, there are small A, and emax increases in the order VI < IV < V. These changes were attributed to increasing ground-state strain perturbations in the same order. We have confirmed these results, but noticed that for indan and tetralin 'Lb transitions (0-0 band), A, and emax changes go in opposite directions. Comparison of, e.g., indan and IV, or tetralin and VI is similarly inconsistent. Strain factors alone therefore cannot provide a common rationale for both energy and intensity changes in these systems. An earlier study" of 'Lb 0-0 band characteristics of the benzocycloalkene series benzocyclobutene through benzocyclooctene showed no simple relation between A, and E,, changes. To gain further insight into the roles of cyclopropane geometries and strain perturbations in the uv spectra of cyclopropyl aromatic systems, it was decided to prepare spiro[cyclopropane- 1,l 'detralin] (VII) and I,la,2, 3,4, Sb - hexahydrobenzo[a]cyclopropa[c]cycloheptene (VIII). System VI11 is of particular interest because the value of 0 as estimated from the most stable conformation based on consideration of molecular models1* is very close to 90". This angle corresponds to a 3
0 5 404
geometry which normally totally precludes conjugative interaction of the cyclopropane ring with an adjacent T system. The 'Lb 0-0 band A, and E,, values for IV-VIII, the corresponding benzocycloalkenes, and other benzene hydrocarbon derivatives of interest are listed in Table 111. Inspection of Table I11 leads to the conclusion that absorption intensities may indeed increase with increasing ground-state strain. However, such strain appears to be produced in a variety of sometimes subtle ways, and while intensity variations in series of closely J.
(16) R. Fuchs, C. A. Kaplan, J. J. Bloomfield, and L. F. Hatch, Or;. Chem., 27,733 (1962); (17) W. R. Moore, E. Marcus, S. E. Fenton, and R. T. Arnold,
Tetrahedron, 5, 179 (,959).
Journal of the American Chemical Society / 91:13 / June 18,1969
(18) Preferred conformations of IV, v, and VI are similarly based."
3561
the same A, have emaxrs at the lower, middle, and upper levels of the intensity range covered in Table 111. The reported l 1 bathochromic effect of introducing a cyclopropane ring into a benzocycloalkene system is confirmed and extended, and though the AA values are small, appears to be fairly well correlated with cyclopropane geometry in these compounds. However, it was felt that more responsive systems were needed to answer more unequivocally the question of geometry effects on conjugation in cyclopropyl aromatics. The powerful influence of nitro substitution on electronic spectra (previous observationsz0 and data in Table 11) suggested that such structures could be prepared simply by introduction of a nitro group para to the cyclopropyl group in IV-VIII. As strain-free reference compounds, m- and p-nitrocyclopropylbenzene (m- and p-N02-III) were used. To help evaluate effects of ring size on spectral characteristics, P-nitroindan, p-nitrotetralin, and 0-nitrobenzosuberan were employed. To determine the importance of relative positions of the nitro and cyclopropyl group, P-NO2-IV, P-N02-V, and /3-N02-VIII (nitro meta to cyclopropyl) were prepared. Syntheses of previously unknown compounds are outlined in Chart 11. The classical sequence for converting, e.g., a p-nitro com-
Table 111. Ultraviolet Absorption, , ,A and emax Values (’Lb Transition, 0-0 Band) for Some Benzene Derivatives . , , ,A
Comoound Isoprop ylbenzene” Cyclopropylbenzene“ Styrene“ o-Diethylbenzenl Benzocyclopropenec Benzocyclobutened Indane IVe V6
Benzonorborneneg Benzonorbornadieneh Tetraline VI6 VIIL 1,4-Dihydronaphthalenei Benzosuberanc VI116
mu
AA
Emax
268 275 29 1 271 275 271.5 273 (273)’ 278 28 1 271 276 274 (274)’ 277 279 274 271 (271)j 272
140 380
7 23
600 200, 208 1900 1860 1390 (1410)’ 1300 1790 1120 500 623 (631)’ 427 917 850 269 (277)i 307
5
8
3 5
1
a Cyclohexane; data from ref 19c. * Isooctane; API Research Project, 44, I1 (1951). c Cyclohexane; E. Vogel, W. Grimme, and S. Korte, Tetrahedron Letters, 3625 (1965). d Ethanol; M. P. Cava and D. R. Napier, J . Amer. Chem. Soc., 80, 2255 (1958). e Hexane; this work. Isooctane; data from ref 19a. 0 n-Heptane; data from ref 19a. h Cyclohexane; G . Wittig and E. Knauss, Chem. Ber., 91,895 (1958). * Hexane; R. A. Morton and A. J. A. de Gouveia, J . Chem. Soc., 916 (1934). 3 Isooctane; ref 17
’
C h a r t I[. S y n t h e s e s
of JII - Pm
6
and
H2-Pf; Ac20; HN03
Nitro D e r i v a t i v e s
- Ac20;
*
Hydrol; HN02; H3PO2
NO2
&NO2
(hprocess)
-~ m N 0 2 - a
-B’-NO2-IP
or X or 8 - o n d !‘-NO,-SUU
(+isomers)
d
(CH2)n-4
n =5,6
+(PhI3 P C H 2
+-+
or (CH312SCH2 I1
d2
( c H Z ) ~ .(0s ~2 steps) obve*
0
&2)n-4
p
r
E
s
F
d
02N
B ‘-NO 2 -Y ( n = 5 )
P-NO2-X
or 8 ’ - N 0 2 -pII(n.6) ( + isomers)
related structures may be r a t i ~ n a l i z a b l e ’ ~no ~ ’ ~single unifying concept is apparent. Absorption maxima clearly are not governed by the same structural features affecting intensities. For example, the three compounds o-diethylbenzene, benzocyclobutene, and benzonorbornene, with essentially (19) (a) H. Tanida and R. Muneyuki, J . Am. Chem. Soc., 87, 4794 (1965); (b) M. J. S . Dewar, “Hyperconjugation,” TheRonald Press Co., New York, N. Y., 1962; (c) W. W. Robertson, J. F. Music, and F. A . Matsen, J . Am. Chem. Soc., 72,5260 (1950).
pound to a m-nitro compound has been termed the “is0 process” for brevity.
Nitrations Under the conditions given in Chart 11, nitration of hydrocarbons IV-VI1 afforded the P’-nitro isomer (nitro para to cyclopropyl) as the major product. It (20) H. H. Jaffe and M. Orchin, “Theory and Applications of Ultraviolet Spectroscopy,” John Wiley & Sons, Inc., New York, N. Y., 1962.
Hahn, et al. / UGSpectra of Cyclopropyl Aromatic Systems
3562 Table IV. Chemical Shifts (7,CDC4) of Nitrocyclopropyl Aromatics f
12
0 0 09-
-
I
(Y
060
0
::
D
4 03-
was expected from consideration of u+ values for freely rotating p-cyclopropyl and p-ethyl (-0.46 and -0.29, respectively) and from estimation of the preferred cyclopropane conformations in IV-VI1 that the cyclopropyl group would dominate directive effects in these systems. In contrast, nitration of hydrocarbon VI11 gave more P- than P'-nitro isomer, and considerable amounts of &'-nitro isomer, while no a-nitro isomer was isolated. This result constitutes the second reported example of electrophilic attack on a cyclopropyl aromatic hydrocarbon occurring predominantly meta to the cyclopropyl group; Brown and Cleveland2 noted that acetylation of 2,6-dimethylcyclopropylbenzene takes place 8 5 % at the 3 position. Significantly, the cyclopropane ring in their system also is though to be predominantly in the worst possible conformation for conjugative interaction. In such a situation, the observed domination of directive effects by ordinary alkyl groups is compatible with the previously mentioned relative electronegativity of cyclopropane exocyclic orbitals. l 4 Structure Assignments The pmr T values of aromatic protons selected to assign structures of the isomeric nitrocyclopropyl aromatic systems described in this study are listed in Table IV. Coupling constants in all cases were observed to be ca. 2.5 cps for Jaband 8-9 cps for Jbc(partial structure
Compound
H,
Hb
H,
P-Nitroindan P-NO%-IV '-NOz-IV P-NO%-V p '-NOg-V P-Nitrotetralin p '-NOz-VI p '-NOrVII /3-Nitrobenzosuberan 0-NOz-VI11 P'-NOrVIII
2.09 1.97 2.12 2.62 2.00 2.30 2.13 2.10 1.99 1.83 2.10
2.05 2.11 2.05 2.12 2.04 2.32 2.07 2.15 2.07 2.02 2.01
2.74 2.83 2.68 2.80 3.31 2.95 2.65 3.28 2.77 2.82 2.58
and P'-N02-VIII (2.68 and 2.58) are shifted downfield by essentially the same amounts (-0.15 and -0.24 ppm) relative to H, values listed for P-N02-IV and P-NO2-VI1I. The H, value for P'-N02-VI is assumed to be similarly affected by cyclopropane deshielding. Strong cyclopropane anisotropic shielding effects, arising when a proton is near a cyclopropane face,22 are reflected in the H, values for P'-NO2-V and 0'N02-VII, and in the Ha value for P-N02-V. A recent report of unusual anisotropic carbonyl deshielding effects in the pmr spectra of ortho-substituted a ~ y l a n i l i n e s ~prompts ~ mention of nitroacetanilides C and D, isolated in the course of the corresponding "is0 processes" (Chart 11). Cyclopropane anisotropy effects on the a proton in these two compounds (deshielding in C, shielding in D) confirm the aromatic proton resonance assignments ( T values shown), since the a' proton should have an essentially constant value, All of the cyclopropane anisotropy
I
H
1.53
CH
