J . Am. Chem. SOC.1987, 109, 219-225 Table X. Atom Coordinates and U , for Cp*,ScCH, (2) atom X' Y Z Ueqb 495 (1) 1131 (1) 53 (0.3) sc 2029 (2) -913 (8) C1 249 (8) 579 (6) 72 (3) -602 (IO) C2 1362 (17) 175 (4) 78 (3) C3 2718 (14) -1018 (11) 353 (6) 94 (4) 867 (7) 100 (4) C4 2615 (17) -1597 (9) C5 1060 (19) -1602 (9) 1061 ( 5 ) 90 (4) -856 (14) 576 (10) 313 (11) C6 -1500 (11) 100 (10) -404 (5) 334 (10) 1113 (29) C7 -79 (8) 241 (7) -919 (13) C8 4244 (19) 315 (10) C9 4009 (22) -2203 (13) 1168 (11) 367 (11) C10 451 (38) -2361 (14) 1537 (6) 1859 (10) 1569 (5) c11 65 (11) 82 (3) 2489 (7) C12 1416 (13) 1568 (4) 72 (3) C13 2458 (10) 1959 (12) 1982 (5) 84 (4) 951 (10) 2239 (4) C14 1604 (17) 93 (4) 977 (9) C15 169 (15) 1965 (6) 92 (4) 155 (4) 2274 (10) 1200 (6) C16 -1359 (12) 1190 (5) 151 (4) 3624 (8) C17 1792 (14) 218 (6) 2394 (15) 2178 (6) C18 4001 (14) 284 (9) 151 (12) 2729 (4) C19 2175 (26) 183 (5) 263 (11) 2165 (6) C20 -1291 (15) 757 (51 156 (41 1293 (91 C21 4259 (131 O x , y , and z have been multiplied by lo4. bU, = ' / 3 ( U , l + U,, U j 3 )X lo3;uU, = 6-'/2(uUj,/U,,)U,.
1859 had F, > 0 and 1143 had (F:) > 3u(F,Z). Three check reflections monitored every 97 reflections showed linear decay of 1.5% in F over the 230 h required to collect the data. The data were corrected for this decay and reduced to F:; variances were assigned as described above. The least-squares refinement used all 2076 data and minimized the quantity Z w ( F 2 - FC2),,where w = 1/g2(F:). No absorption correction was made because of the small value of krmax.A Patterson map gave scandium coordinates, and successive structure factor-Fourier calculations located the remaining atoms. Hydrogen atoms on the Cp* methyl groups were introduced, based on difference maps calculated in the planes where they were expected, and included in the subsequent calculations as constant contributions to the structure factors, with isotroplc B of 10.0A2. Six cycles of full matrix least squares, the hydrogen atoms being repositioned two times, concluded the refinement; the final R = 0.1 13 for all the data with F: > 0 and 0.072 for data with F: > 3u(F?). The goodness of fit was S = 2.98,where n = number of data = 2076 and p = number of parameters. In the final refinement no parameter shifted more than one-half its standard deviation. A final difference map showed no excursion greater than f0.44 e/A3. Crystal data are given in Table VI11 and atom coordinates in Table X. Calculations were done by using the programs of the CRYRM crystallographic computing system64on a VAX 11/780computer; the final drawing was done by using ORTEP.65
~
~~
~
219
+
dinates in Table XIII, and bond lengths and angles in Table XV. (Tables XI, XIII, XV are found in the Supplementary Material.) The final value of the secondary extinction parameter is 1.55 (16) X 10". The atoms numbered 1 and 2 are the pyridyl atoms bound to scandium. Atoms C3-C6 are the remaining atoms (carbons) of the pyridyl ring. C11-Cl5 and C31-C35 are the ($-C5Me5) inner ring carbons, and C21-C25 and C41-C45 are the methyl groups associated with them, respectively. Structure Determination for (qs-C,Me5)2ScCH3.A crystal approximately 0.3X 0.3 X 0.2 mm, that appeared satisfactory from an oscillation photograph, was centered on a Syntex P2, diffractometer equipped with graphite-monochromated Mo K a radiation. An orthorhombic cell was found, and cell dimensions were obtained from a least-squares fit to the setting angles of 15 reflections (various forms of six independent reflections) with 20" < 28 < 26'. Systematic absences observed in the data of hOO, h = 2n + 1, OkO, k = 2n + 1, and 001,l = 2n + 1 are-mique for the sea_cegroup no. 19,P2,2,2,.Five octants of data ( h k l , hkl, hkl, hkl, and h k l ) were collected with 4O < 28 < 40' and one octant, hkl, with 40° < 28 < 50°, a total of 6684 reflections. The data were merged (goodness of fit = 1.04)to give 2076 independent reflections, of which
Acknowledgment. W e thank Dr. Patricia Watson of the DuPont Company for her assistance in portions of this work as well a s for helpful discussions. J.E.B. and M . E . T . thank the DuPont Company for inviting M.E.T. to DuPont to carry out some of the experiments. W e thank Pamela Shapiro for contributing to the preparation of this manuscript. Discussions with Dr. Kim Shugart and Professor William A. Goddard 111 were very helpful. This work was supported by the National Science Foundation (Grant No. CHE-8303735) by the U S D O E Office of Energy Research, Office of Basic Energy Sciences ( G r a n t N o . D E - F G 0 3 85ER13431), and by Shell Company Foundation which a r e gratefully acknowledged. T h e use of the Southern California Regional NMR Facility, supported by National Science Foundation Grant No. CHE 7916324, is also gratefully acknowledged. J.E.B. and M.E.T. thank the Atlantic Richfield Company for a graduate fellowship to M.E.T. Supplementary Material Available: U,'s in Table X I and XII, hydrogen atom coordinates in Tables XI11 and XIV, and bond lengths and angles in the Tables XV and XVI (6 pages); structure factor tables (16 pages). Ordering information is given on any current masthead page.
Kinetic and Thermodynamic Studies of the Thermal Electrocyclic Interconversions of Perfluorinated Dienes and Cyclobutenes William R. Dolbier, Jr.,*+ Henryk Koroniak,+ Donald J. Burton,*$ Pamela L. Heinze,' A. R. Bailey,' G . S. Shaw,' and S. W. Hansenl Contribution from the Department of Chemistry, University of Florida. Gainesuille, Florida 3261 1 , and the Department of Chemistry, University of Iowa. Iowa City, Iowa 52242. Received August 14, 1986 Abstract: Detailed kinetic and thermodynamic analyses were carried out for the thermal interconversions of ( Z , Z ) - and (E,E)-perfluoro-2,4-hexadienes with rrans-perfluoro-3,4-dimethylcyclobutene,(E,Z)-perfluoro-2,4-hexadienewith perfluoro-cis-3,4-dimethylcyclobutene, (Z,Z)- and (E,E)-perfluoro-3,5-octadieneswith trans-perfluoro-3,4-diethylcyclobutene, and (Z)and (E)-perfluoro-l,3-~entadiene with perfluoro-3-methylcyclobutene. In each case the (Z,Z)- or (Z)-dienes exhibited substantial kinetic advantage over the (E,E)- or (,!?)-dienes in their cyclization processes.
T h e elegant stereochemical studies of Criegee in 1959 on the ring opening of cis- and trans-l,2,3,4-tetramethylcyclobutenes' were the first to demonstrate unambiguously the conrotatory 'University of Florida. *Universityof Iowa.
nature of the cyclobutene-butadiene electrocyclic interconversion. It wasn't until 1965, of course, that the classic papers of Woodward (1) Criegee, R.; Noll,K. Liebigs Ann. Chem. 1959, 627, 1 ; Chem. Ber. 1965, 98, 2339.
0002-7863/87/ 1509-0219$01.50/0
0 1987 American Chemical Society
220
J . Am. Chem. SOC.,Vol. 109, No. 1, 1987
Dolbier et al.
Table I. Activation Parameters for the Perfluoro-2,4-hexadiene System kEE ~ - E E
kzz k-ZZ kEZ k-EZ (I
In kcal/mol.
E,' 50.6 f 0.9 43.6 f 0.9 29.1 f 0.5 23.5 f 0.5 38.1 f 0.7 32.7 f 0.7
loa A 15.4 f 0.4 12.5 f 0.4 12.1 f 0.3 10.4 f 0.3 13.9 f 0.3 11.1 f 0.3 In cal/deg.
and Hoffmann rationalized such empirical observations in terms of orbital symmetry.*
mean temD. OC 257.8 257.8 93.8 93.8 207.8 207.8
AG*" 45.8 44.9 30.3 27.6 36.2 36.8
7.0 -4.5 -5.4 -13.2 2.0 -10.5
thermodynamic parameters for the reversible ring opening of three perfluorinated cyclobutene systems, the cis- and trans-3,4-dimethylcyclobutene system, 17 and 6, the trans-3,4-diethylcyclobutene system, 7, and the 3-methylcyclobutene system, 8.
Syntheses
2
I
A P
AH*a 49.5 42.5 28.3 22.8 37.1 31.8
The perfluoro-2,4-hexadiene and -3,5-octadiene systems were approached synthetically via the preparation of the respective dienes using methodology developed within the Burton group.6
3
4
5
The question of why none of the isomeric (2,Z)-diene, also able to be formed via conrotatory opening, had been observed in the thermolysis of 3 was not addressed by early workers except in terms of a probable steric rationale. Later, when results began to appear which were obviously inconsistent with the steric arthere gument Le., the ring opening of 3-ethyl-3-methylcyclobutene,3
9
2.1 : I
arose a need for an alternative approach to the problem. Curry and Stevens proposed that since methyl is the poorest donor of the alkyls, the residual negative charge about methyl will experience more repulsion of C, upon inward rotation than will occur for better donor g r o u p s 3 The need for better and more quantitative understanding of the factors involved in determining the mode of rotation in cyclobutene ring opening was dramatized by our recent preliminary report of the kinetic behavior of the isomeric perfluoro-2,4-hexadienes in their reversible cyclobutene f ~ r m a t i o n work ,~ which has been expanded and more fully interpreted in the present report. Further related examples of the effect of chloro, methoxy, and acetoxy substituents upon cyclobutene ring opening were reported by Kirmse et al. shortly thereafter,5 thus indicating that such effects were not unique to fluorine. These authors also presented a theoretical rationale for their and our results which was consistent with the kind of dramatic kinetic effects we have observed and report further upon in this paper. In order to better understand the effect of substituents upon the kinetics and stereochemistry of the cyclobutene-butadiene electrocyclic process, we have examined in detail the kinetic and
R2'R4:CF,,
II 12
RI=R4:CF3,R2:R3:F
13
R ~ = R ~ = C ~ ~ , R I : R ~ ' F
RI
i
Rl'R3.F
R ~ Z C ~ RFp :~ Rq:F ,
14 R ~ R= ~
f
I
R I = R 3 = CF3, R2= R4= F
10
: c ~ F ~ , R ~ : R ~ = F
The perfluoro-1,3-pentadienes15 and 16 were synthesized via a modification of a procedure of Heinze,' wherein the isomeric perfluoropropenylzinc reagents were coupled with trifluorovinyl F
F
I
Rp
RI
ZnX
15 16
R,=F, R ~ Z C F ~ R1=CF3,R2=F
iodide in the presence of tetrakis(tripheny1phosphine)palladium. Triglyme was used as the solvent in order to facilitate the isolation of the dienes. No reaction occurred a t room temperature, in contrast to related work of Normant.' The isomeric dienes were fully characterized spectroscopically. Of key significance were the vicinal vinyl F-F coupling constants which allowed the distinguishing of the Z and E isomers. For the Z , Z isomers 10 and 13, these coupling constants were found to be 5.1 and 95% pure by I9FN M R for an 80% yield. 1R (gas cell, room temperature, 20 mmHg): 1776.2 (CF= CF2, w), 1408.8 (w), 1346.1 (w), 1313.8 (s), 1228.5 (s), 1183.7 ( s ) , 1141.1 (w). 1077.6 (w), 860 cm-l. Mass spectrum: CsF8+,212, (15.7%); C2F7+,193, (24.9%);C4Fs+, 143, (77.3%); C4F4+,124, (22.4%); C,F,+, 93, (100%); CF3+,69, (21.8%). High resolution mass spectrum gave a parent ion of 21 1.9872. See Tables VIII-X for I9F N M R data. When this same reaction was conducted at 75 OC for 6 h, the yield was 55%. Preparation of (Z)-CF3CF=CFCF=CF2 from (E)-CF,CF=CFZnI and CF,=CFI. (E)-CF,CF=CFZnI was prepared from 11.4 g of (E)-CF,CF=CFI (44 mmol) and 4.0 g of activated zinc (61 mmol) in 20 mL of TG in a 50-mL flask equipped with a water-cooled condenser topped with a glass tee. The apparatus was maintained under an inert atmosphere, and the mixture was stirred with a magnetic stirring bar for 3 h at room temperature. The zinc reagent was Schlenk filtered under argon into a dry, lOO-rnL, round-bottomed flask equipped with stirring bar and a dry ice/isopropyl alcohol condenser. To the zinc reagent, 8.2 of CF,=CFI (39 mmol) and 1.0 g of Pd(PPh3)4(2.3 mol %) were added, and the mixture was stirred at 75 OC for 3 h. The pot contents were distilled at 50 mmHg at room temperature to give 4.2 g of >95% pure (I9F NMR analysis) product for a 50% yield. IR (gas cell, 5 mmHg): 1780 (m), 1720 (m), 1370 (s), 1345 (s), 1225 (s), 1185 (s), 1060 (s), 920 (m), 750 cm-l (w). Mass spectrum: C5F8+,212, (11.0%);C2F,+, 193, (12.1%); C4F6+, 162, (18.5%); C4F5+, 143, (79.6%); C4F4+, 124, (18.9%); C3F3+,93, (100.0%); C3F2+,74, (18.0%); CF,', 69, (28.2%). See Tables VIII-X for I9F NMR data. Thermal Isomerizations and Kinetics. General Procedure. All thermal isomerizations were carried out in highly conditioned, spherical, Pyrex vessels of 150-250-mL capacity. These vessels were submerged in a stirred, thermostated, molten salt bath (eutectic mixture, 5C-50 by weight of NaNO, and KNO,, mp 158 "C). Temperature was measured via a Chromel-Alumel thermocouple in coordination with a Tinsley Type
-
e
f
75.5 (dm, (Jds = 5.5 Hz) 74.1 (m) 82.2 (m, CF,) 118.2, 127.1 (AB,m, CF,) 75.7 (dd, J,, = I O , J b e = I O HZ)
75.5 (dm, Jcf= 5.5 Hz) 74.1 (m) 82.2 (m, CF,) 118.2, 127.1 (AB, m,CF2) 117.6 (AB, dm, J d f = 197 HZ)
Table XIII. Rate Data for the 6 + 9 temp, OC 1O7k,b,d Ka 272.5 45.4 f 1.0 0.30 268.0 29.0 f 0.8 0.29 263.0 19.8 f 0.5 0.28 256.25 10.7f0.1 0.26 247.25 5.18f0.1 0.24 240.75 2 . 5 4 f 0 . 0 7 0.22 'K = 10/6. bkEE = kabsd(l K).
Interconversion 1o6k-EE 11.0 f 0.3 7.38 f 0.2 5.29 f 0.16 3.06f0.03 1.63f0.03 0.86f0.05
107kEEb 59.2 f 1.0 37.5 f 0.8 25.3 f 0.5 13.4f0.1 6.41f0.1 3.11f0.1
Table XIV. Equilibrium Ratios for Perfluoro-2,4-hexadiene System temp, OC 6 10 17 11 9 405.75 5.73 4.76 1 10.34 4.45 397.25 5.80 4.58 1 9.83 4.21 389.20 5.88 4.41 1 9.37 3.99 381.75 5.96 4.26 1 8.95 3.80 370.40 6.07 4.03 1 8.32 3.52 355.75 6.24 3.74 1 7.55 3.18 349.75 6.31 3.62 1 7.25 3.04 343.50 6.38 3.50 1 6.94 2.91 337.75 6.45 3.39 1 6.66 2.79 332.75 6.52 3.30 1 6.42 2.69 Table XV. Rate Data for 7 + 13 Interconversion temp, OC 1OSkzz 1OSk-Zz 69.6 0.53 f 0.05 1.75 f 0.05 3.60 f 0.05 79.0 1.27 f 0.05 88.5 3.08 f 0.1 7.63 f 0.10 97.5 9.12 f 0.3 20.0 f 0.3 28.9 f 0.5 101.5 13.9 f 0.5 52.9 f 0.6 107.25 27.4 f 0.06 Table XVI. temp, "C 327.0 319.75 314.25 302.6 296.5 292
Rate Data for 7 12 Interconversion 104kEE,SKI 1o4kobsdr s-I KO 4.27 f 0.02 4.78 f 0.02 2.98 3.28f0.02 3.66f0.02 2.87 2.48 f 0.02 2.22 f 0.02 2.79 0.99 f 0.01 1.10 f 0.01 2.62 0.716 f 0.005 0.79 f 0.005 2.53 0.515 f 0.003 0.570 f 0.003 2.47 " K = 1317. bkEE = kobsd(l + K ) .
1 04kEE,b s-I
19.0 f 0.1 14.18f0.1 9.39 f 0.1 3.98 f 0.05 2.81 f 0.02 1.98 f 0.01
Table XVII. Equilibrium Ratios for the Perfluoro-3,5-octadiene System temp, OC 13 7 14 12 327.0 2.98 1 7.77 4.45 319.75 2.87 1 7.50 4.33 314.25 2.79 1 7.30 4.23 1 6.87 4.03 302.6 2.62 2.53 1 6.66 3.92 296.5 3.84 2.47 1 6.50 292.0 Table XVIII. Rate Data for 8 + 15 Interconversion temp, OC 107k, 1 0 4 ~ 131.0 0.607 f 0.003 0.559 f 0.003 150.5 4.74 f 0.02 2.79 f 0.01 157.25 9.02 f 0.05 4.55 f 0.03 165.25 18.9 f 0.1 8.02 f 0.05 168.25 29.0 f 0.2 11.55 f 0.07 175.0 55.9 f 0.3 19.4 f 0.1
J . A m . Chem. Soc. 1987, 109, 225-228 Table XIX. Rate Data for (8 + 15) G= 16 Interconversion temp, "C 105k-, 219.0 0.883 f 0.007 257.25 14.9 f 0.1 269.0 35.9 f 0.2 283.75 91.5 f 0.3 288.25 124.3 f 0.5 293.25 164.7 f 0.5 O K = 15/8. *kE = kobsd(l
107kobsd 0.462 f 0.004 16.7 f 0.1 49.8 f 0.3 163.8 f 0.5 239.9 f 0.9 345.2 f 1.0
225
KO
0.0065 0.0119 0.0141 0.0173 0.0183 0.0195
I 07kE* 0.0465 f 0.004 16.9 f 0.1 50.5 f 0.3 166.6 f 0.5 244.3 f 0.9 352. f 1.0
+a.
Table XX. Equilibrium Ratios for the Perfluoro-1,3-pentadiene System temp, "C 8 15 16 294.2 1.38 1 200.1 209.6 235.7 1.31 1 188.9 1.25 1 219.5 156.0 1.20 1 228.4 88.0 1.05 1 257.25 71.0 1 .oo 1 269.0 54.9 0.95 1 283.75 51.0 0.93 1 288.25 46.9 0.91 1 293.25 3387E potentiometer. The thermocouple was immersed in a well which was placed midway between the center of the two pyrolysis vessels. The temperature of the bath was controlled by using a Hallikainen (now Totco Instrument). Thermocontrol Proportional Controller with a platinum resistance probe. For low-temperature runs (below 160 "C), a similar apparatus was used: the pyrolysis vessels were immersed in silicon oil bath, with the Omega Proportioning Thermocontroller Model 49 with platinum resistance probe. The temperature was constant to 0.1 "C. Thermocouple-derived temperatures were calibrated with thermometers (Brooklyn Thermometer Co.). The thermocouple and thermometers temperatures were found to be close enough that any possible systematic error in the precision in reading of the temperature was found to contribute a smaller error than the standard deviation in the activation parameters. The starting materials (perfluorodienes or/and perfluorocyclobutenes) were introduced into the pyrolysis vessels by expansion from the contiguous vacuum line. Initial pressure of kinetic runs varied between 4 and 12 mmHg of a starting material. The test runs at higher pressure of a starting material (25-30 mmHg) did not show any significant differences in obtained kinetic parameters. Each kinetic pyrolysis run was sampled at least 6-8 times by removing a small fraction of the pyrolysis mixture by expansion into a small section of the vacuum line into a vessel, diluting with argon (total pressure -40@500 mmHg), and removing to make multiple GLPC injections via a gas sampling valve. As indicated above, a Hewlett-Packard 5710A gas chromatograph in conjunction with a Hewlett-Packard 3380s integrator
was used for all these analyses. Base-line resolution of peaks was observed under all GLPC quantitative studies. Each point in a rate constant is an average of at least three GLPC runs. Each rate constant plot contains at least six points, while all reported rate constants and activation parameters were derived by a linear least-square analysis of the experimental data, with each such analysis yielding a correlation coefficient of at least 0.999. Rate constants for the 2,4-hexadiene system are given in Tables XI-XIII, the 3,Soctadiene system in Tables XV and XVI, and the 1,3-pentadiene system in Tables XVIII and XIX. To determine the thermodynamic parameters, the thermal equilibria of investigated systems have been studied. The starting material was introduced to the reaction vessel and pyrolyzed for at least 10 half-lives, and then at least 3 independent samples of reaction mixture (within next 2 half-lives) were taken and analyzed. The equilibrium data were found to be identical despite the starting materials which have been used (perfluorodienes and appropriate perfluorocyclobutene). To determine the thermodynamic parameters for the above-described systems, the data at at least six different temperatures have been taken into consideration and derived by a linear least-squares analysis, yielding a correlation coefficient of at least 0.997. Equilibrium data for the three systems are provided in Tables XIV, XVII, and XX. The gas sampling technique utilized in all of the above-described studies introduced multiple pressure variations per run. The fact that good unimolecular behavior was nevertheless always observed for over 5 half-lives indicates clearly the lack of significant surface effect problems. The kinetic apparatus as well as a technique utilized in determining rate constants is modeled after the apparatus and technique of Dr. H. M. Frey, University of Reading, England.
Acknowledgment. W e gratefully acknowledge financial support of this research in part by t h e National Science Foundation (W.R.D., D.J.B.), t h e Air Force Office of Scientific Research, (D.J.B.), and the Petroleum Research Fund, administered by the American Chemical Society, (W.R.D.).
Registry No. 6, 89031-88-9; 7, 10531 1-62-4; 8, 10531 1-66-8; 9, 83168-67-6; 10, 83168-65-4; 11, 83168-66-5; 12, 10531 1-63-5; 13, 105311-61-3; 14, 105311-64-6; 15, 10531 1-65-7; 16, 105311-67-9; 17, 89031-87-8; F,C=CFI, 359-37-5; (E)-F,CCF=CFI, 102682-82-6; (Z)-F,CCF=CFI, 102682-81-5.
Anti, Vicinal Hydrogen-Hydrogen Interactions. A Fundamental Shift Effect in 13CNMR Spectroscopy James K. Whitesell* and Mark A. Minton Contribution f r o m the Department of Chemistry, University of Texas at Austin, Austin, Texas 78712. Received June 30, 1986
Abstract: The spatial disposition of two vicinal hydrogens in an anti stereochemical relationship contributes to a downfield shift for each of the carbon atoms involved. Applications of this shift effect to both configurational and conformational stereochemical analyses are discussed.
T h e dependence of I3C NMR spectral shifts on t h e spatial position of y substituents (the y effect) represents one of the most powerful features of carbon NMR spectroscopy. Following the lead of key researchers in this area,'-3 we have recently correlated 0002-7863/87/1509-0225$01.50/0
in a highly precise fashion stereochemical a n d conformational properties of molecules with I3C spectral These results ( I ) Smith, D. H.; Jurs, P. C.J . Am. Chem. SOC.1978, 100, 3316.
0 1987 American Chemical Society
