7153
J . Am. Chem. SOC.1988, 110, 7153-7159
The Claisen Rearrangement of Allyl Phenyl Ether: Heavy-Atom Kinetic Isotope Effects and Bond Orders in the Transition Structure Lidia Kupczyk-Subotkowska,' William H. Saunders, Jr.,*,2 and Henry J. Shine*i3 Contribution from the Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, and the Department of Chemistry, University of Rochester, River Station, Rochester, New York 14627. Received February 12, 1988
Abstract: Kinetic isotope effects (KIE) were measured for the rearrangement at 220 OC of allyl phenyl ether (1) which was labeled successively at its oxygen atom (['80]-1), a-carbon atom ( [a-I4C]-1),y-carbon atom ([y-I4C]-1), and ortho-carbon atom ([2-I4C]-1). The KIE were 1.0297 (I8O), 1.0306 (a-I4C),1.0362 (y-I4C), and 1.0375 (2-I4C). KIE were determined from isotopic abundances in the product, o-allylphenol (2), which were measured by multiscan mass spectrometry ( lSO)and
scintillation counting (14C). A model of the transition structure was then developed, whose bond orders gave calculated KIE in reasonably good agreement not only with the experimentally determined heavy-atom KIE but also with the deuterium KIE reported earlier by McMichael and Korver.I3 A loose transition structure was thus found for the rearrangement of 1 into 2. In this transition structure the C,-O bond is 5 M O % broken while the Cy-Cortho bond is only 10-20% complete. The results thus define this concerted, nonsynchronous rearrangement more explicitly than hitherto possible.
The Claisen rearrangement of allyl phenyl ether and its homologues is probably the most thoroughly studied and best documented thermal, sigmatropic rearrangement known.4 Even before the clarifying classification of pericyclic reactions was made, according to the principle of the conservation of orbital symmethe concerted nature of the aromatic Claisen rearrangement had been deduced. The isotopic labeling and kinetic and stereochemical studies that led to this deduction have been summarized in a number of reviews.*-I0 Much interest, furthermore, has been shown in the nature and geometry of the transition structure" of the rearrangement. In broad terms, a chair-like transition structure (4) is now well-accepted for the rearrangement of allyl phenyl ether (1) into the dienone precursor (3) of the final product, o-allylphenol (2), as well as for analogous, structurally uncomplicated rearrangement^.^,^^ Interest has also been shown in a more explicit description of the transition state, particularly as to the timing of the concerted breaking and forming of the u bonds. In this respect, McMichael and Korver13 measured the deuterium kinetic isotope effects (KIE) for rearrangements of a-dideuterio- and y-dideuterioallyl phenyl ether. They concluded that in the transition state of this rearrangement breaking of the C,-0 bond was advanced as compared with forming of the Cv-Conha bond. Specifically, the bond orders of these bonds in the transition structure were calculated to be respectively -0.23-0.43 and 0.22-0.62. This picture, then, is of a non(1) Present address: Institute of Organic and Physical Chemistry, Technical University, Wroclaw, Poland. (2) University of Rochester. (3) Texas Tech University. (4) The term Claisen rearrangement is often taken to mean the rearrangement of allyl aryl ethers and aryl ethers containing extended unsaturated groups such as pentadienyl. More recently, the term has been applied also to allyl vinyl ether and its derivatives. The rearrangement of allyl vinyl ether is sometimes also called an oxa-Cope rearrangement. In this paper, therefore, when it is useful or necessary to use general terminology, we will call the rearrangement of allyl aryl ethers the aromatic Claisen rearrangement. (5) Woodward, R. B.; Hoffmann, R. J . Am. Chem. SOC.1985, 87, 395, 2511. (6) Hoffmann, R.; Woodward, R. B. J . Am. Chem. SOC.1965, 87, 2046. (7) Fukui, K.; Fujimoto, H. In Mechanisms of Molecular Migrations; Thyagarajan, B. S., Ed.; Interscience: New York, 1969; Vol. 2, pp 117-190. (8) Rhoads, S. J.; Raulins, R. N. Org. Reacf. 1975, 22, 1-252. (9) Hansen, H.-J.; Schmid, H. Tetrahedron 1974, 30, 1959. (10) Shine, H. J. Aromatic Rearrangements; Elsevier: New York, 1967; pp 88-1 19. (1 1) We use the term fransifionStructure to describe the structure of the reacting system as it passes through the transition stare, which is the highest point on the minimum-energy path between reactant and product. (12) Hansen, H.-J.; Schmid, H. Chem. &if. 1969, 5 , 111. (13) McMichael, K. D.; Korver, G. L. J . Am. Chem. SOC.1979, ]Of,2746.
0002-7863/88/1510-7153$01.50/0
OH
I
2
3
4
synchronous rearrangement with an unsymmetrical transition structure. Nonsynchronous rearrangement of allyl vinyl ether into 5-pentenal was deduced by Gajewski and Conrad, also on the basis of deuterium KIE.14 The use of deuterium KIE in such rearrangements (the Claisen and Cope) has been discounted by Dewar as being incapable of throwing light on their synchroni~ity.~~ On the other hand, Dewar and Healy16-17have characterized the rearrangements of allyl vinyl ether and some of its derivatives as concerted but nonsynchronous'* on the basis of M N D O calculations. Thus, the nature of the transition structures in these rearrangements commands a lively interest.'O Substitution of a heavy isotope for one or both of the atoms between which a bond is being broken or formed may give rise to a heavy-atom KIE. In that case, the magnitude of the KIE reflects the magnitudes of the vibrational frequencies, and hence the bond orders, in the transition structure relative to the reactant. With this in mind we have set out to obtain a measure of the bond order of the u bond which is breaking and of the one which is (14) Gajewski, J. J.; Conrad, N. D. J . Am. Chem. SOC.1979,101, 2747. (15) Dewar, M. J. S. J . Am. Chem. Soc. 1984, 106, 209. (16) Dewar, M. J. S.; Healy, E. F. J . Am. Chem. SOC.1984, 106, 7127. (17) Dewar, M. J. S.; Healy, E. F.; Stewart, J. P. J . Chem. SOC.,Faraday Trans. 2 1984, 80, 227. ( 1 8 ) Concerted, nonsynchronous reactions, also called two-stage (as distinct
from two-step) reactions, are defined by Dewarls and by Dewar and Chantranupong.19 (19) Dewar, M. J. S.; Chantranupong, L. J . Am. Chem. SOC.1983, 105, 7152. ..-.
(20) See also: Gajewski, J. J. In Isotopes in Organic Chemistry: Buncel, E., Lee, C. C., Eds.: Elsevier: New York, 1987; Vol. 7, pp 115-176.
0 1988 American Chemical Society
Kupczyk-Subotkowska et al.
7154 J . Am. Chem. SOC.,Vol. 1!0, No. 21, 1988 Table I Kinetic lsotope Effects (KIE) for the Thermal Rearrangement of Allyl Phenyl Ether (1) to o-Allylphenol at 220 OC in Diohenvl Ether run no. isotope conv. KIE" 0.1 1 1.0315 f 0.0041 1 180 0.14 1.0300 f 0.0022 2 180 0.15 1.0298 f 0.0018 3 180 1.0282 f 0.0027 0.16 4 180 1.0272 f 0.0022 0.165 5 180 1.03 16 f 0.0024 0.26 6 180 1.0350 f 0.0006 7 d4C 0.009 1.0274 i 0.0024 0.15 8 ff-14c 1.0288 f 0.0007 0.16 9 aJ4C 0.18 1.0348 f 0.0016 10 a-'4C 0.19 1.0236 f 0.0019 11 aJ4C 0.25 12 a-'4C 1.0341 f 0.0021 0.157 13 yJ4C 1.0351 f 0.0006 0.158 14 XJ4C 1.0390 f 0.0014 0.165 1.0358 f 0.0013 15 yJ4C 0.165 1.0342 f 0.0014 16 y-"C 0.184 17 yJ4C 1.0355 f 0.0005 0.221 18 yJ4C 1.0374 f 0.0028 1.0421 f 0.0023 0.08 19 2-14c 1.0358 f 0.0025 0.13 20 2-14c 0.13 1.0389 f 0.0015 21 2-14c 0.16 1.0352 f 0.0026 22 2-14c 0.24 1.0342 f 0.0038 23 2-14c 0.25 24 2-14c 1.0385 f 0.0023 a KIE for [2I4C]-1 have been corrected for intramolecular competition between !*C and I4C. ~~
~~
~
forming in the transition structure of the aromatic Claisen rearrangement. For this purpose we have chosen the classical rearrangement of allyl phenyl ether (1) itself into o-allylphenol (2) and have measured KIE for substitution at each of the four atoms of the u bonds involved. That is, we have prepared 1 labeled with I8O and with I4C at its a-C, y-C, and Corthopositions. We refer to the isotopically labeled compounds as [*80]-1,[a-I4C]-1, [-y-I4C]-1,and [2-14C]-1,respectively. We have then constructed models of the transition structure and have computed bond orders that accommodate not only the heavy-atom but also the earlier deuterium KIE.
Results and Discussion Kinetic Isotope Effects. We have described the syntheses of [a-14C]-1 and [yI4C]-1 earlier.21 Syntheses of [180]-1 and [ 2-I4C]-1 were from 4-nitroaniline and from 2-nitro[ 1-l4C]aniline by adaptation of routine methods. Initially, we tried to carry out rearrangements of 1 in carbitol, but we were unable to isolate 2 cleanly for isotopic assay. Rearrangements, therefore, were carried out in diphenyl ether solution at 220 OC. The rate constant at 197 OC ( k = 3.86 X s-l) and activation energy ( E , = 31.6 kcal/mol) for rearrangement of 1 in this solvent have been reported by Goering and Jacobson,22 S-I) for rearfrom which the rate constant ( k = 1.87 X rangement at 220 "C was calculated. Timing for stopping rearrangement, by cooling, for specified low conversions was calculated from this rate constant. However, the extent of conversion needed for use in calculating KIE was measured in the diphenyl ether solution by 'H N M R spectroscopy before beginning isolatioii of 2. 100% conversions were also confirmed with 'H N M R before beginning isolation of 2. Product 2 was not itself used for measuring isotopic abundances. For I4C KIE measurements by scintillation counting, 2 was converted into its phenylurethane, for better handling and for purifying by sublimation. For l 8 0 KIE measurements by multiscan mass spectrometry, 2 was first methylated and next oxidized, ending in c-methoxybenzoic acid. The reason for this was that 2 itself, and its methyl ether, had a very low parent-ion mass-spectrometric abundance. In contrast, o-methoxybenzoic acid gave a substantial parent ion and a neg(21) Kupczyk-Subotkowska, L.; Shine, H. J. J. Labld. Compd. Radiopharm. 1987, 24, 303. (22) Goering, H. L.; Jacobson, R. R. J. Am. Chem. SOC.1958,80,3277.
Table 11. Averaged KIE isotoue '80
aJ4C yJ4C 2-14c
KIE 1.0297 1.0306 1.0362 1.0375
Table 111. Assumed Force Constants value, type mdyn type C-H str 4.5 H-C-O bd C-0 str 5.0 H-C(C)Z OOPbd C-H str 4.1 O-C(C)Z OOP bd 0.67 C - C U C tors C-C-H bd H-C-H bd 0.55 C-C-C-O tors 0.83 CC-C-H tors C-C-C bd 0.7 C-C-C-C tors c-Qc bd 0.7 C-C-O bd
value, mdyn A-l 0.67 0.2 0.2 0.044 0.066 0.09 0.071
ligible M - 1 ion abundance, thus making calculation of KIE simple. Rearrangements were clean, giving rise, however, to small amounts of higher (assumed) molecular weight products from which 2 was separated by column chromatography. In the total of eight 100% conversions that were carried out, the yield of 2 isolated by column chromatography averaged 93.8%, while byproducts, isolated in two successive chromatographic elutions (see Experimental Section), averaged a total of 6.1%. In this connection, small amounts of higher molecular weight product(s) have been reported by others, e.g., in the rearrangement of allyl ptolyl ether.23 KIE are listed in detail in Table I and are summarized in Table 11. They are substantial for all of the four positions and are consistent with the long-held view of a concerted process in the aromatic Claisen rearrangement. Calculations of Bond Orders in the Transition Structure. The Claisen rearrangement presents a considerable challenge to isotope effect calculations. There are few reactions in the literature for which isotope effects at so many positions are available. Even though the number of disposable parameters in this complex system still exceeds greatly the number of experimental data, it proved difficult to arrive at models that predicted all of the observed isotope effects. The models do not include all the atoms of the complete reactant (1) and transition structure (4) but follow the Wolfsberg-Stern cutoff procedures, in which atoms more than two bonds from the site@)of isotopic substitution are ~ m i t t e d .These ~ ~ ~procedures ~~ reduce considerably the number of parameters that must be assigned, and the resulting models have been shown to give results differing little from those obtained with full models. Initially we worked with four different cutoff models so as to explore most economically the influence of force constants and bond orders on the various calculated isotope effects. In the final calculations 6 for the reported here, we still use two different models, 5 160/180, a-12C/14C,and a-H2/D2 effects, and 7 8 for the y-'2C/14C, y-H2/D2, and oJ2C/l4C effects. These models were chosen in preference to a single model for all isotope effects so as to avoid the problems encountered with redundancies when cyclic transition structures are involved.26 The calculations on these models were performed by using the Bebovib-IV p r ~ g r a m . ~Standard ~ , ~ ~ single-bond lengths (rco = 1.43 A, rcc = 1.54 A, rCH = 1.09 A) were used and adjusted by
--
(23) Kincaid, J. F.; Tarbell, D. S . J. Am. Chem. SOC.1939, 61, 3085. Rearrangement of the neat ether gave 4% of nonvolatile product. (24) Wolfsberg, M.; Stern, M. J . Pure Appl. Chem. 1964, 8, 225-242. (25) Stern, M.; Wolfsberg, M. J . Chem. Phys. 1966, 45, 4105-4124. (26) Keller, J. H.; Yankwich, P. E. J . Am. Chem. SOC. 1974, 96, 2303-23 14. (27) Sims, L. B.; Burton, G. W.; Lewis, D. E.BEBOVIB-IV, QCPE No. 337, Department of Chemistry, Indiana University, Bloomington, Indiana. (28) Sims, L. B.; Lewis, D. E. In Isotopes in Organic Chemistry; Buncel, E., Lee, C. C., Eds.; Elsevier: New York, 1984; Vol. 6, pp 161-259.
J . Am. Chem. SOC.,Vol. 110, No. 21, 1988 1155
The Claisen Rearrangement of Allyl Phenyl Ether the program according to the bond orders in a particular model. Geometry was taken as tetrahedral for sp3 and trigonal for sp2 atoms. At first the geometry of atoms that change hybridization in the reaction was varied for different transition structures. This variation was found to have little influence on the isotope effects, however, so fixed intermediate geometries were adopted for such atoms in the transition structures 6 and 8. The force constants used are listed in Table 111. They are taken from or estimated from standard compilations2’31 and are adjusted by the program according to the bond orders in particular models.
Table IV. Calculated Isotope Effects for Model 5 Off-Diagonal F Matrix Elements
C
’C
c‘ 5
6. The Role of
klightlkbcavy for ( Y - ~ 2 / ~ 2 d rr-12c14 c
dncca
amnb
IQ/l*cy
0.910.1 0.7jo.3 0.5/0.5 0.310.7 0.1 /0.9
a12
1.0130 1.0126 1.0130 1.0141 1.0165
1.0341 1.0842 1.1388 1.1983 1.2622
I C 1.0070 1.0085 1.01 10 1.0143 1.0180
1.0536 1.0516 1.0478 1.0418 1.0320
1.0305 1.0851 1.1459 1.2132 1.2871
1.0100 1.0158 1.0228 1.0307 1.0384
0.910.1 0.7/0.3 0.510.5 0.310.7 0.110.9
H\/c
-
a23
0.910.1 0.710.3 0.5/0.5 0.310.7 0.110.9
a34
1.0057 1.0071 1.0091 1.0120 1.0160
1.0600 1.1131 1.1704 1.2323 1.2978
1.0425 1.045 1 1.0475 1.0491 1.0484
0.910.1 0.710.3 0.510.5 0.310.7 0.1/0.9
a45
1.0040 1.0038 1.0041 1.0051 1.0066
1.0364 1.0863 1.1406 1.1999 1.2651
1.0144 1.0156 1.0176 1.0206 1.0247
exptl 1.0297 1.18 1.0306 “All bond changes synchronous as follows (bond, bond order change): 1, 1.5 1.0; 2, 1.0 2.0; 3, 1.0 0.0; 4, 1.0 2.0; 5, 2.0 1.0. C-O refers to C,-0 and C-C to C,-C,,h,. *See eq 1 and 2. The designated a,,,,, = 1.025. All other a,,,,, = 0. ‘At 220 O C . dAt 180 OC.
H
-
-
-
-
-
117.
a value of ca. -0.05, which gave imaginary reaction coordinate frequencies of a few hundred cm-l. Force constants of reacting bonds in the transition structures were assumed to be directly proportional to the bond order, n, of the reacting bonds. Tunnel corrections from the first term of the Bell equatiod3 are included in all calculated isotope effects, but the corrections are small enough to be within the experimental errors of the observed isotope effects. Until the present work, the only isotope effects available for the Claisen rearrangement of allyl phenyl ether were for the a-d2 (1.18 f 0.02) and 7-d2(0.95 f 0.02) species in the temperature range 170-195 O C . 1 3 They were interpreted as resulting from a concerted reaction with a transition structure in which bond breaking had proceeded to a greater extent than bond m a l ~ i n g . ~ ~ * ~ * One of the aims of our calculations was to determine whether such a transition structure also fits the heavy-atom isotope effects. The most obvious feature of the observed I2C/l4Cand I60/I8Oeffects is that they are fairly large and all in the normal (light faster than heavy) direction. Simple qualitative reasoning based on zero-point energy effects would make one expect quite small normal or even inverse heavy-atom isotope effects in a concerted rearrangement, for new bonds are being made to the isotopic atoms at the same time as old bonds are being broken. This should result in little net change in bonding to these atoms, and in zero-point energies associated with the vibrations involving them. Our first task was to decide what factors were most important in determining the magnitudes of the various primary and secondary isotope effects. We tried a synchronous concerted model in which no change in total bond order to any atom was allowed: the C,-C, and C,-C, bond orders of the allyl group were respectively 1.1, 1.9; 1.3, 1.7; 1.5, 1.5; 1.7, 1.3; 1.9, 1.1, and bond orders varied analogously for the other reacting bonds. In order to explore the role of the off-diagonal F matrix elements, we carried out four separate sets of calculations in each of which one pair (Fmnand Fnm,eq 1 and 2 ) was assigned a finite value and the other three pairs were zero. The results are shown in Tables IV and V.
(31) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acra 1965, 21, 169. (32) Melander, L.; Saunders, W. H., Jr. Reaction Rates of Isoropic Molecules; Wiley-Interscience: New York, 1980; pp 64-66.
(33) Bell, R. P. The Tunnel Effect in Chemistry; Chapman and Hall: New York, 1980; pp 60-63.
/\
C
7
8
H
(TS)
Diagonal force fields were used for the reactants. In the transition structures 6 and 8, off-diagonal F matrix elements for stretches of the reacting bonds were introduced by the usual procedure,32 where (eq 1)
and the amnvalues are given (eq 2) by
+
+-
1 - a l z 2- a23* - a342- a452 a122a342 a122a452 +-
a232a452 = D (2) where the subscripts indicate the bonds denoted by the corresponding numbers in the structures. The parameters am,,measure the strength of the coupling between the stretching motions of bonds m and n. A positive value signifies that bond n contracts as m stretches (and vice versa) in the motion along the reaction coordinate. The curvature parameter, D, was generally assigned ~~
(29) Wilson, E. B., Jr.; Decius, J. C.; Cross, J. Molecular Vibrations; McGraw-Hill: New York, 1955. (30) Schachtschneider, J. H.; Snyder, R. G. Spectrochim. Acta 1963, 19,
Kupczyk-Subotkowska et al.
7156 J. Am. Chem. SOC.,Vol. 110, No. 21. 1988 Table V. Calculated Isotope Effects for Model 7
Off-Diagonal F Matrix Elements
-
8. The Role of
Table VI. Calculated Isotope Effects for Models 6 with Equal Coupling of Bond Changes' ncol
0.910.1 0.710.3 0.510.5 0.310.7 0.1/0.9
0.9960 0.9916 0.9880 0.9854 0.9837
0.9273 0.8836 0.8432 0.8058 0.77 12
1.0098 1.0085 1.0080 1.008 1 1.0087
0.910.1 0.710.3 0.5/0.5 0.310.7 0.110.9
1.0183 1.0100 1.003 1 0.9978 0.9938
0.9492 0.8994 0.8546 0.8140 0.7770
1.01 17 1.0160 1.0180 1.0195 1.0209
0.910.1 0.710.3 0.510.5 0.310.7 0.110.9
1.0297 1.0305 1.0285 1.0255 1.0218
0.9580 0.9148 0.8726 0.8326 0.7948
1.0062 1.0059 1.0048 1.0036 1.0028
0.910.1 0.710.3 0.5/0.5 0.310.7 0.110.9
1.0045 0.9999 0.9964 0.9937 0.9918
0.931 1 0.8870 0.8463 0.8088 0.7740
0.9982 0.9971 0.9965 0.9965 0.9970
1.0375 exDtl 1.0362 0.95 'All bond changes synchronous as follows (bond, bond order change): 1, 1.5 1.0; 2, 1.0 2.0; 3, 1.0 0.0; 4, 1.0 2.0; 5, 2.0 1.0. C-O refers to C,-0 and C-C to C,-Cd, bSee eq 1 and 2. The designated a,,,,, = 1.025. All other a,,,,, = 0. CAt220 OC. dAt 180 OC.
-
-
-
-
-
The secondary deuterium isotope effects are most easily interpreted. They show little dependence, barely beyond experimental error, on the magnitudes of the off-diagonal F matrix elements. In addition, they vary steadily with progress from reactant-like to product-like transition structures and support the earlier c o n c l ~ s i o nthat ' ~ ~breaking ~~ of the C,-0 bond has proceeded farther than formation of the c,-co&, bond in the transition state. The calculated heavy-atom isotope effects present a more complex picture. They appear to be of little use as measures of progress along the reaction coordinate; some decrease, and some do not even change monotonically. The most striking feature is that the effects tend to be small or even inverse unless the isotopically substituted atom is between two bonds whose stretches are coupled by off-diagonal F matrix elements. It has previously been noted that isotope effects can be significantly enhanced over those expected from zero-point energy effects if motion of the isotopically substituted atom contributes to the motion along the reaction ~ o o r d i n a t e . ~The ~ fairly large normal values of the experimental heavy-atom isotope effects thus mean that the process is concerted and the bonding changes are rather strongly coupled. None of the models of Tables IV and V, however, satisfactorily reproduces all of the experimental isotope effects. The next logical step was to go to models in which all bonding changes are strongly and equally coupled. Table VI reports the results of calculations in which all a,,,,, = 0.6 for both models. The first five entries refer to completely synchronous processes in which the total bond order to each atom remains constant from the most reactant-like to the most product-like transition structure. In the remaining entries, the formation of the Cy
