J,Org. Chem. 1982,47, 5232-5234
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starting stannane. Trifluoroacetolysis of cyclohexyltriisopropylstannanes thus proceeds stereospecifically with retention of configuration. (We estimate that 4.5.
0022-3263/82/1947-5232$01.25/0
inolysis than for protonolysis. Rahm and Pereyre2 opined that the crucial feature favoring inversion was front-side steric bulk on the basis that retention was favored for sec-butyltriisopropylstannane,in contrast to overall inversion for the trineopentyl derivative. Our results for the trans-triisopropylstannanes,Le., predominant inversion, suggests the more cautious view’ that other factors may be just as important as front-side steric bulk. Further work is needed to delineate these factors and their relative importance. Entries 7-10 demonstrate a statistical mixture of cis- and trans-cyclohexyl bromides, a result appropriate for bromine atom transfer to a 4-alkylcyclohexyl free radical. Brominolysis of (4-alkylcyclohexyl)mercuricbromides in nonpolar solvents proceeds ~imilarly.~ We attempted to observe the result for electrophilic destannylation in a nonnucleophilic s01vent’~~ (entry 11)by suppressing the radical route by hydroquinone and air, but a statistical distribution of bromides WM still obtained. While this type of result has been attributed3 to competing inversion and retention pathways, it may simply indicate a failurelo to suppress the radical route. The pmsibility that increasing front-side steric bulk may result in more specific electrophilic bromodestannylation (inversion) is being explored for cyclohexylstannanes and full details of this work will be presented at a later date. Acknowledgment. We are grateful to the Australian Research Grants Scheme for partial funding of this work. (10) See footnote 17 in ref 2 also.
Henry A. Olszowy, William Kitching* Department of Chemistry University of Queemland Brisbane 4067, Queensland, Australia Received August 16, 1982
Substituent Effects in l,fi-Methano[ 101annulene: Carbon-13 Nuclear Magnetic Resonance Spectra of 2and 3-Substituted Derivatives Summary: The carbon-13 nuclear magnetic resonance spectra of a series of 2(a)- and 3(P)-substituted 1,6methano[ 101annulenes have been obtained for relatively dilute (0.2 M) solutions in deuteriochloroform. The spectra have been assigned and substituent chemical shifts calculated for all ring positions. The substituent shifts at nonproximate sites have been analyzed by the dual substituent parameter treatment and appropriate comparisons have been made with the corresponding positions in the isoelectronicnaphthalene systems. Blends of inductive (pI) and resonance (pR) effects are remarkably similar for corresponding substituent-probe dispositions in the two systems. Sir: 1,6-Methano[lOIannulene, fust synthesized’ by Vogel in 1964, satisfies the chemical and spectroscopic criteria for aromaticity, and if the 1,6 bridge is neglected, I is the second member of the (4n + 2)7r annulene series with a neutral (4n + 2) carbon f r a m e w ~ r k . ~Fundamental ,~ to (1) Vogel, E.; Roth, H. D. Angew. Chem., Znt. Ed. Engl. 1964,3, 228. (2) Reviews of the synthesis, chemistry, and spectra of the 1,6methano[lO]anndene system are available. For example, see: Vogel, E. In ‘Aromaticity: An International Symposium”;The Chemical Society: London, 1967; p 113 and references therein. Vogel, E. R o c . Robert A. Welch Found. Conf. Chem. Res. 1968, 12, 215.
0 1982 American Chemical Society
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Table I. Substituent Chemical Shiftsa in 2- or 3-Substituted 1,6-Methano[lO]annulenes carbon number
X
1
2
3
4
5
6
7
8
9
10
11
2.98 3.39 2.94 3.55 3.83 5.62 -0.15 1.68 0.02
1.86 2.09 0.74 2.12 0.51 2.45 -1.84 1.82 -1.05
0.80 1.00 -0.29 0.91 0.65 -1.00 -2.51 -0.76 0.34
Y
COOCH, COOH CN COCH, CHO NO2 OCH, Br CH 3
-0.94 -0.35 -0.08 -1.45 -2.14 -8.15 -8.81 -3.93 -0.57
0.19 -0.98 -19.46 9.89 8.73 16.14 29.72 -10.76 9.52
5.24 6.32 6.16 4.80 9.51 -0.96 -20.38 2.70 0.55
-0.94 -0.91 0.31 -1.75 -1.49 -0.18 1.43 1.60 0.17
5.25 6.33 5.28 6.06 1.68 8.15 -4.91 -0.18 -2.08
2.13 2.22 2.16 2.14 1.34 5.16 3.38 2.57 1.38
-0.31 -0.19 -0.16 -0.74 -0.54 0.65 -2.42 0.18 -1.28
0.85 1.02 1.82 0.92 0.88 2.36 1.53 1.60 0.24
@ ;x 7
5
0.49 0.43 3.62 -0.10 1.28 0.20 1.33 -0.11 coocI-€, -1.76 3.67 0.76 0.35 1.51 -0.05 COOH 0.66 4.29 -0.02 -0.12 0.86 1.62 -1.66 4.87 1.62 3.21 1.48 0.97 -0.61 0.45 1.96 2.26 CN -1.99 5.55 -17.60 1.89 4.94 CHO -0.62 0.73 1.19 0.04 -0.22 8.57 1.99 -0.82 7.80 1.74 5.94 1.57 -4.96 0.4 5 19.94 2.67 2.10 -0.07 -3.47 -1.57 NO 2 OCH, 1.82 -2.94 -8.42 -0.40 -0.96 -18.46 -1.23 0.87 -0.45 1.19 33.30 1.84 -0.57 -1.18 1.65 3.96 1.14 0.43 -0.05 Br 0.58 0.14 -6.37 0.16 -2.06 -0.11 -0.40 0.46 1.96 -0.57 -0.30 9.35 -0.47 -1.37 CH3 Defined as the difference (ppm) between the chemical shift of the designated carbon atom and that of the appropriate carbon in the parent hydrocarbon. For X = H, the 13Cshifts used were as follows: C , = 114.93, C2 = 128.75, C, = Not included in DSP analysis. 126.16, C,, = 34.92 ppm from Me,Si.
an understanding of aromatic chemistry is an appreciation of how substituents perturb the electron-density patterns and hence influence the spectroscopic and reactivity characteristics. In this respect, comparisons of derivatives of I with those of the iso-10-?r-electronicnaphthalene are of obvious importance. Provided the proper comparisons are made, 13C NMR chemical shifts provide powerful insights into r-electron fluctuations (particularly) in aromatic systems) and we herein report the substituent chemical shifts (SCS) for a range of 2(a)- and 3(@)-substituted1,6-methano[101annulenes I1 and 111, respectively, their analysis by the dual substituent parameter (DSP) treatment, and comparisons with the corresponding l(a)-and 2(@)-substitutednaphthalene data.5 X
I
I1
I11
The compounds were synthesized by reported procedures2 or were gifts from Professor E. Vogel. The 13C NMR spectra were assigned by consideration of some or all of the following: chemical shifts, signal intensities, specific (a)deuteration, “-coupled spectra, coherent off-resonance decoupled spectra, and the spectra of certain disubstituted compounds. Full details can be found (3) (a) Boschi, R.; Schmidt, W.; Gfeller, J . C. Tetrahedron Lett. 1972, 4107. (b) Grunewald, G. L.; Uwaydah, I. M.; Christoffersen, R. E.; Spangler, D. Idib. 1975, 933. (4) For example, see Shorter, J. In “Correlation Analysis in Chemistry: Recent Advances”; Shorter,J., Chapman, N. B., Eds.; Plenum, New York, 1978; Chapter 4. See also Craik, D. J.; Brownlee, R. T. C.; Sadek, M. J. Org. Chern. 1982, 47, 658 and citations therein. (5) Kitching, W.; Bullpitt, M.; Gartahore, D.; Adcock, W.; Khor, T. C.; Doddrell, D.; Rae, I. D. J.Org. Chern. 1977,42,2411. Web,P. R.; Arnold, D. P.; Doddrell, D. J . Chern. Soc. Perkin Trans. 2 1974, 1745.
el~ewhere.~~’ The substituent chemical shifts for I1 and I11 are presented in Table I. (These data when combined with the 13Cshifts for I (Table I) permit calculation of the chemical shifts for I1 and 111.) The observed 13C SCS (Table I) have been correlated by the dual substituent parameter (DSP) approach: which employs a linear combination of the inductive and resonance substituent constants uI and uR, viz., 13CSCS = pIuI + ~ R u R . The pI and pR values provide a measure of the relative transmission of inductive and resonance effects. The results of this analysis and comparison with the appropriate data from the naphthalene system5are presented in Table 11. Overall, the precision of fit of the correlations (“f’ values in Table 11) is only moderate and generally poorer than for the corresponding naphthalene dispositions. Nevertheless, the following aspects deserve emphasis. 1. Regarding the 2- (or e-)substituted annulenes, resonance transmission to C5 is strong (pR = 19.19), resembling that in naphthalene (PR = 19.98) with similar X values (2.36, 3.38). This is in accord with electrophilic substitution proceeding predominantly at C5 in 2-methoxy or 2methyl-1,6-methano[10]annulenes.2~9Significant resonance effects at the formally conjugated Cg are indicated (pR = 5.82) but overshadowed by the polar contribution (PI= 6.711, a situation that applies also to the analogous (C,) position in a-substituted naphthalenes. C7in I1 is also formally conjugated, but the precision of fit is hopeless, a situation that applies to the analogous (C,) position in the a-substituted naphthalenes. Moreover, scrutiny of the (6) D’Arcy, B. D.; MSc. Thesis, University of Queensland, Brisbane, Australia, 1982. (7) Olszowy, H.; Kitching, W., unpublished results. (8) Ehrenson, S.; Brownlee, R. T . C.; Taft, R. W. Prog. Phys. Org. Chern. 1973, I O , 1. Wells, P. R.; Ehrenson, S.; Taft, R. W. Ibid. 1968,6, 147. (9) Effenberger, F.; Klenk, H. Chern. Ber. 1976, 109, 769. Klenk, H. Dissertation, University of Stuttgart, Stuttgart, Germany, 1974.
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Table 11. Best Fit Parameters of DSP Equation for ''C NMR Shielding Effects at Nonproximate Positions in 1,6-Methano[lO]annulenes
no.a type
pT
PR
hb
fd
ne
2-Substituted Compounds. System I1 4 O R O ~ 0.55 -4.83 -8.75 0.73 0.63 8 (3) (-1.80) (-1.63) (0.91) (0.44) (0.49) (9) 5 ORO 8.12 19.19 2.36 1.18 0.21 (4) (5.92) (19.98) (3.38) (0.66) (0.12) (9) 6 aRo 6.92 -2.48 -0.36 1.13 0.41 8 (10) (1.36) (-1.66) (-1.22) (0.40) (0.46) (9) OR0 -0.74 3.10 -4.18 0.96 0.91 8 7 (5) (0.82) (0.59) (0.72) (0.46) (0.80) (9) 8 OR0 3.74 -1.03 -0.27 0.21 0.15 8 (6) (2.23) (0.41) (0.18) (0.17) (0.18) (9) 6.71 5.82 0.87 9 ORO 1.01 0.32 8 (7) (4.10) (3.89) (0.95) (0.27) (0.15) ( 9 ) 3-Substituted Compounds. System I11 5 ORO 3.57 -1.46 -0.41 0.63 0.41 7 (4) (2.95) (-2.00) (-0.68) (0.31) (0.21) (9) ORO 5.29 12.54 2.37 0.92 0.25 7 6 (10) (0.41) (11.23) (27.11) (0.38) (0.12) (9) ORO 0.34 0.67 1.98 0.29 0.90 7 7 (5) (-0.04) (0.36) (-9.70) (0.13) (0.78) (9) E aRo 3.05 4.54 1.49 0.30 0.18 7 (6) (4.01) (7.74) (1.93) (0.14) (0.06) ( 9 ) 9 ORO 2.53 -0.76 -0.30 0.32 0.31 7 (7) (2.85) (0.37) (0.13) (0.19) (0.16) (9) 10 O R O 2.49 2.95 1.19 0.46 0.35 7 (8) (1.28) (4.32) (3.39) (0.52) (0.40) ( 9 ) 1 ORO -4.20 -0.59 0.14 0.63 0.36 7 (9) (-1.30) (-3.80) (2.91) (0.34) (0.30) (9) a Values in parentheses refer to the corresponding positions in the naphthalene system, e.g., C, in the 2-substituted-l,6-methano[l0]annulenesis analogous to C, in the 1-substituted naphthalenes. h = p ~ / p ~ .The standard deviation of the fit. The fit parameter, f = SD/ RMS where RMS is the root mean square of the data points. e Number of substituents in the data set. Correlations with OR+ were extremely similar.
SCS a t C, (Table I) reveals no obvious connection with substituent type (although OCH3and CH3SCS are clearly negative). There are clearly some undetermined nonresonance factors influencing the 13Cshifts,a conclusion that has also been drawn for the a-naphthyl case as well.5 2. Although c6 in system 111is sensitive to resonance phenomena (pR = 12.54; p = 11.23 for the analogous Clo in @-substitutednaphthalenes), a major difference is apparent for pI (5.29 vs. 0.41). This certainly reflects the role of the bridging methano bonds, causing the vectorial s u m
of electric-field components along the bonds about c6 to be substantial as opposed to the near cancellation that is the case at Clo in the planar @-substitutednaphthalenes.5J0 The precision of the fit at the formally conjugated C8 in I11 is good (f = 0.18) with a dominating resonance term (pR = 4.54, X = 1.49), very similar to the situation at c6 in @-naphthalenes( p =~ 7.74, X = 1.93). The greater p~ values for naphthalene may be associated with the nonplanarity of the annulene perimeter," with resultant diminution in a overlap through the C1 and ce regionsa3 Similar reasoning may explain the differences for Clo in I11 and C8 in the @-naphthalenes(A = 1.19 and 3.39, respectively). 3. The present data permit reasonable predictions concerning preferred sites of electrophilic (and other) substitution, but little detailed reactivity or isomer distribution data is available, particularly for system 111. It would be of considerable interest to examine the 19FSCS of selected 2- and 3-fluoro-l,6-methano[lO]annulenes, as comparisons of 13Cand 19FSCS can provide insight into the subtleties of polar effe~ts.~.'~ However, the synthetic aspects of this are a ~ e s 0 m e . l ~ Acknowledgment. Some of the authors (University of Queensland) are grateful to the Australian Research Grants Scheme for partial funding of this work, and for providing access to the National NMR Center, Canberra. (Officerin-Charge: Dr. Alan Jones.) Professor E. Vogd (University of Koln) was most generous with samples and enthusiasm. (10) Kitching, W.; Alberta, V.; Adcock, W.; Cox,D. P. J . Org. Chern. 1978,43,4662. (11) Dobler, M.; Dunitz, J. D. Helo. Chirn. Acta 1966,48, 1429. (12) Adcock, W.; Cox, D. P. J. Org. Chern. 1979,44,3004. (13) Diazotization of 2-amino-1,6-methano[10]annuleneis not
straightforward and synthetically useful2 Prof. Filler (Illinois Institute of Technology), at our request, kindly attempted direct (XeFJ fluorination of I, but this was not successful.
Bruce R. D'Arcy, William Kitching* Henry A. Olszowy, Peter R. Wells* Department of Chemistry University of Queensland Brisbane, Australia 4067 William Adcock, Gaik B. Kok School of Physical Sciences The Flinders University of South Australia Bedford Park, S.A. 5042, Australia Received August 16, 1982
