J . Am. Chem. SOC.1990, 112, 8120-8126
8120 IC
L e
4.53 4.34
+0.07
4.47
-0.47
-0.08
1
I
increased electron-attracting power of the phenyl compared with the ethyl group; there is little steric requirement at phosphorus in the diphenyl series compared to that in the ethyl series. The reactivity difference, which is relatively small on an energy scale, is unlikely to result from a change in mechanism to a stepwise process in the diethyl case. The conclusions of this paper refer to symmetrical and unsymmetrical displacements where at least one of the nucleophiles is weakly basic. Kinetically observable pentacoordinate phosphorus species from neutral phosphoacyl anions have always involved strong nucleophiles.2 Addition intermediates observed in acidcatalyzed phosphoryl group transferiPare neutral and thus possess no driving force to expel a ligand, and they can, thus, exist as observable entities. General-Base Catalysis. The sterically hindered nucleophiles (2,4,6-trichlorophenolate ion, 2,6-dichlorophenolate ion, and ’
(23) Phenoxide ion attack on the 4-nitrophenyl ester of diethyl and diphenyl hosphoric acid has bimolecular rate constants 6.7 X lo-‘ M-s-’ at M-’s-I at 25 OC, respectively, with 32% MeOH/ 39 OCIPand 4.3 X water l6 H droxide ion attack on these esters has the rate constants 8.6 X lo-’ M-I smYz4 and 0.26 M-I s-’ l6 at 25 O C in water. (24) Ba-Saif, S. A.; Williams, A. J . Org. Chem. 1988, 53, 2204.
2,6-dimethylphenolate ion) and the data for acetate ion have reactivities well below those predicted from the regression line (eq 4) and may include some general-base component. The reactivity of the imidazole species, which comes close to that of a phenolate ion of similar pKa, confirms previous conclusions that this reagent acts as a nucleophile.12 Otherwise, the reactivity would be at least below that of the above sterically hindered phenolate anions. Imidazole reacts with the hindered diphenylphosphinyl esters as a general base12and with the less hindered dimethylphosphinyl group as a n ~ c l e o p h i l e .Our ~ ~ studies indicate that the oxyanion from 2-iodosobenzoic acidIh” is some 3.9 X 105-foldmore reactive against 4-nitrophenyl diphenyl phosphate than a phenoxide ion of similar pKa; this is consistent with the nucleophile possessing the enhanced reactivity of an ‘0-nucleophile”.
Acknowledgment. We thank the SERC for partial support of this work (Grant GR/E06558), and S.A.B.3. thanks the government of Saudi Arabia for a studentship. Appendix The parameters &‘,p,”, and p, refer to the Br~nsted-type parameter for the plot of the logarithm of the equilibrium constants for steps k , and k2 and for the overall reaction against the pKa of the appropriate phenol. The simple relationship Pq = + &,” follows from the relationship between the three equilibrium constants. Thus, &” = 6, -’0, and &” must, therefore, be greater than 0.9 if &’ C 0.5 and 3f, = 1.4.
eq’
Supplementary Material Available: Table of reactivity data of 4-nitrophenyl diphenyl phosphate with buffers at different pH’s containing phenol (1 page). Ordering information is given on any current masthead page.
The ,8 Effect of Silicon in the Synperiplanar Geometry Joseph B. Lambert*.’’ and Erik C. Cheliuslb Contribution from the Department of Chemistry, Northwestern University, Euanston, Illinois 60208-31 13. Received April 20, 1990
Abstract: The effect of silicon on the development of @ positive charge has been measured for the synperiplanar geometry of the Six-C-X fragment in endo-3-(trimethylsilyl)-endo-2-norbornylesters (3). Solvent effects were used to demonstrate that solvolysis took place by a carbocation mechanism. When the leaving group X was mesylate (3-OMS) and the solvent was 97% trifluoroethanol, the 0 effect was found to be about IO5 by comparison of the solvolysis rate with the analogous structure lacking the trimethylsilyl group, endo-Znorbornyl mesylate (5-OMS). Thus, the synperiplanar 9, effect is much smaller than the antiperiplanar 0 effect (about loi2under similar conditions). The effect may be smaller because of poorer vertical overlap in the synperiplanar geometry or because the syn leaving group prevents optimal vertical overlap. Alternatively, the antiperiplanar effect may be larger because of contributions from nonvertical participation.
One of the most important electronic properties of silicon is its substantial ability to stabilize positive charge on a B atom, the sc-called j3 effect2 Positive charge may be produced salvolytically in solution or by various reactions in the gas phase. The presence of a ,8 silicon atom can produce an acceleration (compared with hydrogen at the same 0 position) of up to 1OI2 (about 18 kcal mol-’) for secondary systems in ~ o l u t i o n . ~Mass spectrometric
studies have assessed the /3 effect for primary systems (Me3SiCH2CH2+)at 39 or 48 kcal the difference resulting from the chosen value of the heat of formation of Me3Si+. Li and Stone found the effect to be about 28 kcal mol-’ for secondary ~ y s t e m s . By ~ a b initio calculation, Jorgensen and co-worker@ found the B, effect to be about 38 kcal mol-’ for the primary case, 22 kcal mol-’ for the secondary case, and 16 kcal mol-’ for the
( I ) (a) This work was supported by the National Science Foundation (Grant No. CHE-8910841) and by the donors of the Petroleum Research Fund, administered by the American Chemical Society. (b) Fellowship recipient from the Division of Organic Chemistry of the American Chemical Society, sponsored by the Rohm & Haas Co. We acknowledge helpful discussions with Professor Xavier Creary of Notre Dame University. (2) For a comprehensive review, see: Lambert, J. B. Terrahedron 1990, 46, 2677-2689.
(3) Lambert, J. B.; Wang, G.-t.;Finzel, R. B.; Teramura, D. H. J . Am. Chem. Soc. 1987, 109, 7838-7845. (4) Hajdesz, D.; Squires, R. J . Chem. Soc., Chem. Commun. 1988, 1212-1214. (5) Li, X.; Stone, J. A. J . Am. Chem. Soc. 1989, 1 1 1 , 5586-5592. (6) Wierschke, S. G.; Chandrasekhar, J.; Jorgensen, W. L. J . Am. Chem. Soc. 1985, 107, 1496-1500. Ibrahim, M. R.; Jorgensen, W. L. Ibid. 1989, 1 1 1 , 819-824.
0002-7863/90/ 1 5 12-8 120$02.50/0
0 1990 American Chemical Society
p Effect of Silicon in Synperiplanar Geometry
J . Am. Chem. SOC.,Vol. 112, No. 22, 1990 8121
tertiary case (in these theoretical studies the silicon-containing group was SiH& Several mechanisms may contribute to the p effect. (1) Silicon is more electropositive than carbon or hydrogen and may cause an acceleration due to charge induction. In a secondary ~ y s t e m , ~ we calculated an inductive contribution of about 102 (about 3 kcal mol-I) based on an analogy to p H / D kinetic isotope effects. Jorgensen and co-workers6 calculated an inductive effect of about 9 kcal mol-' for the primary case but nil for the secondary case. Thus, the exact level of inductive contribution, if any, is unclear. (2) Silicon may serve as a classical nucleophilic neighboring group that displaces the p nucleofuge to form a three-membered ring, 1. Traylor' has termed this process nonvertical because nuclear R, +SI
'-
H2C
'CHR'
motion of silicon takes place. ( 3 ) The very high polarizability of the carbonsilicon bond makes it particularly able to stabilize positive charge by hyperconjugation (a-?r overlap or doublebond-no-bond resonance, 2 and 2'). Traylor called this mode vertical because movement of the silicon nucleus is not necessary.
45
00
135
180
Dlhedrel Angle (dep.)
Figure 1. Qualitative dihedral dependences for the inductive, vertical, and nonvertical modes of B stabilization.
Scheme I
Although a clear-cut distinction has not been made, the general weight of evidence seems to favor vertical stabilization (hyperconjugation) as providing the most important share. Jorgensen's calculations6 found the bridged form 1 to be about 2.4 kcal m o P more stable than the open form 2 for the primary case (R, R' = H), and the open form was not a minimum. In the secondary case (R = H, R' = Me), however, the open form was found to be about 4 kcal mol-' more stable than the bridged form, and in this case the bridged form was not an energy minimum. Thus, direct calculation did not provide a clear answer. Moreover, theory addresses only the fully developed carbation, whereas the actual transition state has only a partial positive charge and an incompletely departed leaving group. Experiment also has been ambiguous. Traylor and co-workers7 found that the frequency of the charge-transfer band of Me,SiCH,Ph agreed with a vertical mechanism of stabilization, although admittedly this neutral system is not closely related to those carrying positive charge. Eaborn and co-workers*concluded that vertical stabilization is insufficient to explain all of the kinetic enhancement by analysis of u values. Davis and Jacocks9 compared the effect of one sitcon with that of two and found similar levels of acceleration for the first and second silicons. Because only one silyl group can engage in the bridging process to form 1, this observation strongly favors vertical Stabilization. Li and StoneS found in the gas phase that the effect was determined primarily by the substitution pattern at the nucleofuge, as expected for the vertical mechanism but not for the nonvertical mechanism (although fortuitous adding can permit the bridged form in symmetrical cases). Our own approach has been stereochemical, as each mechanism has a distinct dependence on the dihedral angle of the Si-C-C-X fragment (X is the nucleofuge). The inductive effect should be relatively independent of dihedral angle, as it occurs primarily through bonds. The nonvertical process leading to a bridged species by backside attack on the bond to the nucleofuge should occur uniquely at the antiperiplanar geometry (a dihedral angle of about 180') with a rapid and monotonic dropoff at smaller angles. Vertical stabilization or hyperconjugation will follow a +
Br
&OH
cosine-squared dependence on the dihedral angle, much like the Karplus curve for vicinal coupling constants (H-C-C-H) and the dependence of H / D secondary kinetic isotope effects on the H(D)-C-C-X dihedral angle. By this mechanism, there should be maxima at 0 and 180' and a minimum at 90'. These different dependencies are shown graphically in Figure 1, in which t h e y coordinates are not meant to be quantitative. In our earlier study' we used cyclohexane systems to measure the p effect of silicon (necessarily with secondary leaving groups) at dihedral angles of about 60 and 180'. We found a lo'* acceleration at 180' but still a substantial acceleration of 10" at 60'. The latter large value at a synclinal (skew or gauche) geometry is consistent with the cosine-squared dependence for vertical stabilization. It is inadmissable for the nonvertical mechanism, unless the entire factor of lo4 results from induction. Studies in five-membered rings with dihedral angles of about 55 and 150' also were consistent with the vertical model.IO Because the large value at 60' can result either from vertical stabilization or from a substantial inductive effect (the large effects at 180' then being due entirely to nonvertical stabilization), we have sought to explore additional geometries. We report herein the synthesis and solvolysis of the first systems with the synperiplanar ( -Oo) and anticlinal ( - 120') geometries.
Results Synthesis. The successfully prepared molecule with a dihedral angle of about Oo was endo-3-(trimethylsilyl)-endo-2-norborneol (3-OH). That with a dihedral angle of about 120' was exo-
x (7) Hanstein, W.; Berwin, H. J.; Traylor, T. G.J . Am. Chem. SOC.1970, 92, 829-836. (8) Cook,M. A,; Eaborn, C.; Walton, D. R. M. J . Orgunomel. Chem. 1970, 24, 301-306. (9) Davis, D. D.; Jacocks, H . M., 111, J . Orgunomet. Chem. 1981, 206, 33-47.
x
x
3-( trimethylsilyl)-endo-2-norborneol(4-OH). The leaving group (IO) Lambert, J. B.; Wang. G.-t. J . Phys. Org. Chem. 1988, I, 169-178. Wang, G.-t.; Li, D.; Chelius, E. C.; Lambert, J . B. J . Chem. SOC.,Perkin Trans. 2 1990, 331-334.
8122 J . A m . Chem. SOC.,Vol. 112, No. 22, 1990 Scheme I1
&OH
X is endo in both 3 and 4. Considerable effort was expended without success to obtain the exo,exo isomer. The preparation of S O H began with norcamphor in a sequence of procedures similar to those used by Kowalsky et al." (Scheme I). The (Y bromination, enolization, 0-silylation, C-silylation, and hydrolysis proceeded in an overall yield of 62%. Hydride reduction (67%) occurs from the exo side to give the endo alcohol. The synperiplanar geometry of the product was confirmed by the large JZ3(10.5 Hz). The COSY spectrum of 3-OH helped verify the structure and stereochemistry.12 The preparation of the exo,endo isomer 4-OH was not possible by the epimerization of the silyl group in the silyl ketone of Scheme 1. The successful, albeit low-yield synthesis is shown in Scheme 11. The silylated hydrazone was prepared in 60% yield from norcamphor. Removal of the hydrazone group by ozonolysis occurred in very low yield (1 1%). The major products were devoid of silicon. Because alternative methods were even less successful, the ozonolysis reaction was repeated several times. Hydride reduction occurred in good yield (73%) to give the desired exo,endo isomer, whose stereochemistry was confirmed by the small value of J Z 3(5.8Hz) and by the COSY spectrum.I2 Both the trifluoroacetate (3-TFA) and the mesylate (3-OMS) of the diendo isomer was prepared in high yield. The former was stable at room temperature, but the latter decomposed to norbornene at room temperature and had to be stored at -35 "C. The trifluoroacetate of the exo,endo isomer (4-TFA) was prepared in high yield and found to be stable, but the corresponding mesylate was too reactive to be isolated (only norbornene was recovered). The trifluoroacetate of endo-2-norborneol (STFA) was prepared as a kinetic model. It has no silyl group, but the nucleofuge is in the same endo stereochemistry as for 3 and 4. 2,2,2-TrifluoroethanoI (TFE) was chosen as the reaction medium because of its high ionizing power (Y= 1.83 for 97% TFE) and low nucleophilicity ( N = -2.79). These properties are optimal for rate-limiting formation of a carbocation (k,) and minimize competition from nucleophilic attack by solvent (ks). First-order rate constants were measured for 4-TFA, 4-OMs, and 5-TFA in TFE with varying amounts of water present (Table I). Rate constants for 5-OMs, which had been measured earlier,13are also included in Table I. Disappointingly and surprisingly, the exo,endo trifluoroacetate (4-TFA) was insoluble in TFE and ethanol at all levels of water. Kinetic measurements were carried out in aqueous acetone, which, however, has low ionizing power ( Y = 0.66 for 60% acetone) and high nucleophilicity ( N = -0.41). Rate constants for 4-TFA, as well as for 3-TFA and 5-TFA, in aqueous acetone are given in Table 11. Acid-catalyzed eliminations were carried out for 3-OH and 4-OH in 0.12 M DCI in 91% acetone-d6. Rate constants were measured by NMR according to the method of Wang14 and may be found in Table 111. I n order to determine whether the reactions of 3-X, 4-X, and 5-X occur by carbocation mechanisms ( k , ) or by nucleophilic reactions ( k s ) , it is necessary to measure rate constants as a function of solvent ionizing power and nucleophilicity.ls A k , ( I I ) Kowalski, C. J.; O'Dowd, M . L.; Burke, M . C.; Fields, K. W. J . Am. Chem. SOC.1980, 102, 5411, 5412. ( I 2) Chelius, E. C. Ph.D. Dissertation, Northwatern University, Evanston, IL,1989. (13) Allard, B.; Casadevall, E. Nouu. J . Chem. 1983, 7, 569-574. (14) Wang. G.-t. Ph.D. Dissertation, Northwestern University, Evanston, IL, 1988.
Lambert and Chelius Table 1. Rate Measurements of Norbornyl Systems and Cyclohexyl Mesylate in Aqueous Trifluoroethanol Solvents leaving % temp, SVSt RD TFE" OC r k . s-I 3 (endo,endo) TFA 60 60.1' 0.9991 4.1 x 10-5 50.2' 0.99990 1 . 4 x 10-5 25.0' 6.6 X IO-' 69.9' 0.991 80 1.8 X IO-' 3.8 X IO-$ 6 0 . p 0.998 50.2' 0.9993 2.45 X IO-' 25.0' 1 . 1 x IO" 97 7 0 . p 0.994 1.24 X IO-' 65.0' 0.98 8.8 X 5.2 x 10-5 60.2' 0.994 25.OC 1.5 X 10" OMS 80 -4.8' 0.997 1.14 X -12.2' 0.9991 5.0 X IO-' 25.OC 2.06 X IO-1 90 -2.9' 0.994 1.35 X IO-* -8.5' 0.9992 8 . 1 x 10-3 25.0' 1.27 X IO-' 97 -5.8' 0.9995 1.6 X -12.2' 0.995 7.75 x 10-3 25.0' 3.35 x 10-1 5 (endo) TFA 60 79.8 0.99995 4.30 x 10-5 70.1 0.99998 2.83 x 10-5 25.OC 2.85 X IO" 82.9 0.9998 80 2.67 x 10-5 75.4 0.99996 1.59 x 10-5 25.OC 2.37 X IO-' 25.v 97 7.6 X endo-norbornyl OMS 97 3.56 X IO" 25.0' 25.OC 4.90 X IO" 90 25.0' 7.62 X IO" 80 9.46 X IOd 25.0' 50 OMS 97 1.53 X IO-' 68.0 0.997 cyclohexyl 7.21 x 10-5 58.6 0.9991 25.OC 3.30 X IO" "Percent by weight. 'Average of two or more runs; worst correlation coefficient reported. cExtrapolated from other temperatures. dCalculated from the OMs/TFA ratio of 4.7 X IO3 found for cyclohexyl. CTaken from ref 13. Table 11. Rate Measurements of Norbornyl Systems of Aqueous Acetone Solvents leaving % temp, syst gp acetone' 'C r k , s-I 4 (exo,endo) TFA 60 65.1 0.996 3.35 X IO-' 60.2 0.998 2.24 X IO-' 55.2 0.9997 1.43 X IO-' 25.0' 7.77 x 104 80 53.2c 0.996 4.7 X 45.2c 0.995 2.8 X 25.0' 6.1 X IO" 3 (endo, endo) TFA 60 75.0 0.99998 5.35 x 70.1 0.99998 3.53 X 65.1 0.99998 1.93 X 25.0' 1.59 X IO-' 5 (endo) TFA 60 72.0 0.99997 5.12 X 64.1 0.99997 4.06 X 25.0' 1.09 x 10-5 " Percent by volume. Extrapolated from other temperatures. cAverage of two or more runs; worst correlation coefficient reported.
'
Table Ill. Rate Measurements of the Acid-Catalyzed Elimination of Norbornyl Systems' syst temp! O c r k , s-l 3-OH 70.9 0.998 2.47 x 10-5 66.0 0.9995 1.43 x 10-5 25.0' 7.24 X IO-'' 4-OH 70.9 0.99 1 2.96 x 10-5 65.8 0.996 1.75 X IO-' 25.OC 1.26 x 10-7 a Measured in 0. I2 M DCI in 9 1 vol % acetone-d6 and 9 vol % D20. *Measured with an anhydrous ethylene glycol standard. Extrapolated from other temperatures.
p Effect of Silicon in Synperiplanar Geometry
J . Am. Chem. SOC.,Vol. 112, No. 22, 1990 8123
97% 70%
m 80% 60% eTFE
* M
eTFE
EloH
1, m
7
-8-
-
.7
1 m,
-
-9
.10
-I .7
-6
-5
-log(k)
.4
Figure 2. Raber-Harris plot for endo-2-norbornyl trifluoroacetate (5TFA).
reaction rate will be insensitive to nucleophilicity, whereas a k, reaction rate will be insensitive to ionizing power. The ionizing power of aqueous TFE changes little with variation of the water content, and the nucleophilicity of aqueous ethanol is essentially constant with variation of the water content. Therefore, it is useful to compare rates in TFE and ethanol. Rate constants were measured in aqueous ethanol for 3-TFA, 3-OMs, 5-TFA, and 5-OMsI3 (Table IV). As mentioned earlier 4-TFA is insoluble in aqueous ethanol. Product Studies. Products were identified in 97% TFE, 80% TFE, and 60% acetone. Solvolysis of 5-TFA produced only endo-2-norborneol (>98%) in 80% TFE and 60% acetone. Solvolysis of 5-OMS in 97% TFE gave exo-2-norborneol (>95%). Solvolysis of 3-TFA gave only exo-2-norborneol (>98%) in 97% TFE, whereas solvolysis of $OMS in the same solvent gave >75% norbornene,
