J. Am. Chem. SOC.1992,114, 11010-11011
11010
nm (acetonitrile) and was not further changed on recooling to 12 K. This is the first UV spectrum to be recorded for a two-coordinate phosphoryl compound. INDO/S-CI computations'O suggest that the transient bands are due to transitions of A A* nature; a more detailed analysis of the spectrum is in progress. We have also observed the 31PNMR spectrum of phosphenite 3. This was accomplished by condensing the gases from the pyrolysis of trimer 2 on a vacuum line (300-350 'C, 0.1 mm) into an N M R tube sealed on the line and chilled by liquid nitrogen, into which ethylene dichloride had been condensed to serve later as the solvent. The sample was thawed (about -30 "C), and the tube was rapidly inserted in the probe of a Varian 300 MHz NMR spectrometer. The spectrum consisted mainly of signals for the dimer (6 176.5) and trimer (6 120.0 (d, J = 10 Hz), 127.9 (t, J = 10 Hz), whose parameters matched those reported.4~~ However, a small signal (about 4 4 % of total intensities) was present at 6 238. It disappeared after a few hours, behavior that suggested it could arise from the monomeric phosphenite 3. Nearly the same 31PNMR spectrum,containing the weak transient signal at 6 238, was obtained when phosphenite 3 was generated by a different method, the thermal fragmentation (250 OC, 0.03 mm) of the 7-phosphanorbomene derivative 6.l' Since this is the first report
Thermal Interconvenion of a Pair of Diastereomeric Cyclopropanones. An Upper Limit for a Cyclopropanone-Oxyalyl Energy Separation
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6 of the 31PNMR shift of any two-coordinate phosphoryl compound, we felt it necessary to validate our assignment by computational methods. The simpler model PhO--P=O was held in fixed rotational conformations for these calculations.I2 Values (f30) calculated for the fully coplanar conformations 7 (syn) and 8 (anti) were 6 247.9 and 6 265.0, respectively. With phenyl perpendicular H
I
'H
7 (sun)
..
Matthew H. J. Cordes and Jerome A. Berson* Department of Chemistry, Yale University New Haven, Connecticut 0651 I Received September 14, 1992 The racemization of enantiomerically enriched tranr-2,3-ditert-butylcyclopropanonela,b in various solvents at 80 OC, described in a pioneering paper by Greene and mo-workers,'is the only prior case in which a thermal stereomutation experiment has been used to estimate the energy separation between a cyclopropanone and its corresponding oxyallyl. The authors' postulated a disrotatory2 ring opening of the cyclopropanone to a planar oxyallyl intermediate or transition state 2,in which one tert-butyl group would occupy a hindered position on the allylic moiety (Scheme I). To evaluate the stereomutation barrier in a less hindered case and thereby provide a bridge to the simple cyclopropanones amenable to computational study,jq4 we have investigated the diastereomeric spirocyclopropanones 3 and 4. Because of the decreased hindrance in the cognate oxyallyls 5 and 6 (Scheme 11) relative to that in 2,we expected AG*values for 3 and 4 to be much lower than those for la,b. Compounds 3a and 4a were obtained5 in a 1.65:l ratio from the addition of diazomethane to the corresponding ketene, 2(oxomethylene)bicyclo[2.2.llheptane. Aliquots of diethyl ether solutions of this mixture of 3a and 4a were stored for a series of times at several temperatures over the range 235-256 K. As monitored by direct capillary gas chromatography, quenching with methanol-ether gave two hemiketal products from each cyclopropanone, in addition to negligible amounts of Favorski esters. The hemiketal ratio changed with time, and a t 283.5 K, for example, reached an equilibrium l:L value (1 /Kq)of 0.82. By this technique (GC method), the rate constant kOMfor approach to the equilibrium composition could be obtained. It corresponds to the sum of the forward and reverse rate constants, kfand k, (eq 1).
Q"=" H
8 (ant0
to the plane of the 0-P=O moiety, the anti conformer had 6 280.2, the syn 6 258.1. If an equal contribution from all conformers is arbitrarily assumed, an averaged shift for phenyl phosphenite is 6 263 f 30. The experimental value of 6 238 is within this range, and we believe the assignment of this shift to the more highly substituted derivative 3 is supported by the theoretical computations.
Acknowledgmeot. Support by grants from the Army Research Office and the Petroleum Research Fund of the American Chemical Society is gratefully acknowledged. D.B.C acknowledges CPU time provided by North Carolina Supercomputing Center. (10) Ridley, J. E.;Zerner, M.C. Theor. Chim. .4cra 1973, 32, 11; 1976, 42, 233. ( 1 1) Qu!n, L. D.; Jankowski, S.; Sommese, A. G.;Wu, X.P., manuscript
in preparation. (12) These are based on Ditchfield's GIAO perturbed Hartree-Fock scheme" using the Pople [4s,3p,d] basis for H, C. and 0 and the McLeanChandler [6s,5p,2d] basis for hosphorus, an ap,. Jach that yields phosphorus shifts accurate to *30 ppm.Ig Shifts are reported as 6 values referunced to the theoretically determined phosphorus shift i: the pk. sphate anion ( u = 332.9 ppm c a l c ~ l a t e d ,u' ~= 328.4 ppm observed).Is (13) Ditchfield, R. Mol. Phys. 1974, 27, 789. (14) Chesnut, D. B.; Rusiloski, B. E. Chem. Phys. 1991, 157, 105. (15) Jameson, C. J.; De Dim, A.; Jameson, A. K. Chem. Phys. Leu.1990, 167, 575.
0002-7863/92/ 1514-1 10 10$03.00/0
The reaction also could be monitored (229-247 K) by direct ZH N M R observation (Figure 1) of 3b and 4b in a high-field spectrometer (1 1.74 T, rf 500 MHz for protons), which for each diastereomer showed only the two CD2 signals. The isotopically labeled compounds were synthesized from 2-(oxomethy1ene)bicyclo[2.2. llheptane and dry6 dia~omethane-d,.~~~ Rate and equilibrium constants determined by the GC and N M R methods were in agreement to within 20% and 5%, respectively. Scheme I1 illustrates a hypothetical mechanism for stereomutation via oxyallyls 5 and 6. The phenomenological rate (1) (a) Camp, R. L.; Greene, F. D. J. Am. Chem. Soc. 1968,90,7349. (b) Sclove, D. B.; Pazos, J. F.; Camp, R.L.; Greene, F. D. J. Am. Chem. Soc. 1970, 92, 7488. (2) Hoffmann, R. J . Am. Chem. Soc. 1968, 90, 1475. (3) For a recent theoretical treatment, see: Coolidge, M. B.; Yamashita, K.; Morokuma, K.;Borden, W. T. J . Am. Chem. Soc. 1990,112, 1751. See also ref 4. (4) Ichimura, A.; Lahti, P. M.;M a t h , A. J . Am. Chem. Soc. 1990, 112, 2868. (5) (a) The procedures were adapted from the cyclopropanone syntheses
developed by Turro and H a m m ~ n d .(b) ~ ~Turro, N. J.; Hammond, W. B. Tetrahedron 1968, 24,6017. (c) Details of syntheses and characterizations are given in the supplementary material. (6) Transferred using a modification of the apparatus reported by Bush, M. T.; Sanders-Bush, E. Anal. Biochem. 1980, 351. (7) Markey, S. P.; Shaw, G. J. J . Org. Chem. 1978, 43, 33. (8) Damtoft, S.; Jensen, S. R.; Nielsen, B. J. J . Chem. Soc.,Perkin Trans. I 1983, 1943.
0 1992 American Chemical Society
Computer Software Reviews
J . Am. Chem.Soc., Vol. 114, No. 27, 1992 11011
Figure 1. 2HN M R spectra showing the equilibration of 3b (outer pair of peaks) and 46 (inner pair). Each spectrum spans the range 1.8-0.9 PPm. Scheme I
la
2
lb
Scheme 11
value of the free energy separation at 246.6 K between the reactant cyclopropanone 3 and the oxyallyl on the favored pathway for its stereomutation is AG' = -RT[ln (k42) -In (RT/Nh)] I 19.1 kcal/mol in diethyl ether (NMR method). The present finding that cyclopropanone-oxyallyl interconversion can occur at low temperature carries mechanistic implications for the nature of the intermediates in the Favorski rearrangement.1° We have found a strong solvent dependence of the rate of stereomutation. Anticipating the completion of a detailed study, we offer here comparative half-lives for the kOMprocess of about 80 min at 244 K in diethyl ether and at 195 K in dichloromethane. Arrhenius activation parameters (values by G C and N M R methods) for the equation log kf = log A - (Ea/2.3RT) in diethyl ether were E, = 16.3 f 1.3 and 15.3 f 1.4 kcal/mol and log A = 10.4 f 1.4 and 9.6 f 1.4 (A in s-l). The origin of the low A value is under study. At 353 K, AG* for the stereomutation of trans-2,3-di-tertbutylcyclopropanone, 27.4-29.2 kcal/mol in five solvents,l*lI is 7-9 kcal/mol greater than that for 3. Differences in the structure and position of the substituents may contribute electronic and/or bond angle strain components to this increment, but it seems likely that the dominant factor is steric strain in the transition state. The putatively disrotatory thermal ring opening creates a large 1,3-allylic interaction in the oxyallyl of the di-tert-butyl system. The present experiments thus suggest that the barrier for opening of a relatively unhindered dialkylcyclopropanone is remarkably low. Its magnitude provides a calibration point for future theoretical studies. Acknowledgment. We thank the National Science Foundation for partial support of this work and for a graduate fellowship to M.H.J.C. This work was also supported in part by the National Institute of General Medical Sciences.
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Supplementary Material Available: Listings of details of syntheses and characterizations (10 pages). Ordering information is given on any current masthead page. (9) (a) We find that the cycloaddition of 3 and 4 to conjugated dienes (cyclopentadiene,furan, 6,6-dimethylfulvene)requires temperatures at least as high as those required for 3 4 stereomutation. This is consistent with, but insufficient to prove,9w the intermediacy of oxyallyl intermediates in the addition reaction. (b) Cf. Turro, N. J.; Hammond, W. B. Tetrahedron 1968, 24, 6017. (c) Turro, N. J.; Edelson, S. S.; Williams, J. R.; Darling, T. R.; Hammond, W. B. J. Am. Chem. SOC.1969, 91, 2283. (d) Edelson, S. S.; Turro, N. J. J. Am. Chem. SOC.1970, 92, 2770. (10) Reviews: (a) Hunter, D. H.; Stothers, J. B.; Warnhoff, E. W. In Rearrangements in Ground and Excired States; de Mayo, P., Ed.; Academic Press: New York, 1980; Vol. 1, pp 437 ff. (b) March, J. Aduanced Organic Chemistry, Reactions, Mechanisms, and Strucrure, 2nd ed.; McGraw-Hill: New York, 1977; pp 991 ff. (1 1) Since the rate constant for enantiomerization of 1 is k,,,/2, and since the symmetry number of the reactant (u = 2) differs from that of its transition state (u = 1). we have calculated"b AG* for enantiomerization as -RT[ln (kJ(2 X 2)) - In ( R T I N h ) ] . (b) Pollak, E.; Pechukas, P. J . Am. Chem. SOC.1918, 100, 2984.
-
6
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constant for the forward reaction 3 4 can be defined in terms of the forward rate constants for the two component pathways as kf = k5 k6. Although the present experiments do not permit a separation of k5 and k6,the minimum value for the faster of the two is kf/2. Whether the oxyallyls 5 and 6 are assumed to be transition states or metastable intermediate^,^ the maximum
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Computer Software Reviews MacFormulP, Version 3.11. JED Software: 3857 MacGregor Common, Livermore, California 94550. List Price: $29.00 plus $3.00 shipping in US, 85.00 foreign. (Upgrade from prior versions available by returning disk with $3.00 for shipping and handling.) MacFormula is a very simple program for the Macintosh for calculating molecular weights, stoichiometry, and elemental analysis data. Once one gets past the title screen, the main window is shown. It has exactly two boxes to enter data. The first is for any molecular formula. The box is highlighted with the message 'Enter Formula Here" at the start. One types any formula, using element symbols and numbers (which are not subscripted). Then clicking in the second box (or using the tab key, a useful feature for mouse phobes) lets one enter a number of milligrams. The average mass, the exact mass, and the number of millimoles are displayed, as well as percentage composition of each ele-
ment present. If one clicks on the word 'milligrams" it toggles to millimoles. One has to click on the 'calculate" button or hit "Enter" to get the new calculation. Data for the entire periodic table are programmed in, and one can enter some user-chosen values (Le. for isotopes). Extra molecular fragments (such as waters of hydration) and fractional molecular formulas can also be accommodated and common abbreviations for molecular fragments, such as Ph = phenyl, can be used. The entire dictionary of elements and fragments that are recognized can be printed to a printer (it would be useful to be able to print them on screen also). MacFormula runs on any Macintosh and can be run in the background with system 7. It is a simple but very useful program, especially to have on a computer in the lab. John N. Marx, Texas Tech Uniuersity
