J. Phys. Chem. 1984, 88, 5251-5254
concentration of Hg12 could exist in the vicinity of the electrode without reacting with mercury (reaction 5 ) . Thus the results of these coulometric and chronocoulometric experiments are in excellent agreement with the conclusions made in the previous paper discussing polarographic behavior of halide ions.2 The most interesting general finding is the tenacity of the adsorbed monolayer film of HgX3-. Formation of HgX,-(ads) up to one monolayer coverage is the most favored anodic process, regardless of potential, and the cathodic removal of the film is slow on voltammetric time scales at overpotentials as large as 0.5 V. There is no evidence of removal of the film on the diffusion plateau for the more anodic process. The overall behavior is strikingly different from that observed in aqueous solution.
If we assume that all HgX3- diffusing to the electrode during the forward pulse (time T ) was forming Hg,X2 which remained on the surface of the electrode, the following relation would be obeyed nFI' = 2 t i F C ( D ~ / ? r ) ' / ~
5251
(11)
where C and D are for the HgX3- ions. The charge density calculated from eq 11 is plotted in Figure 6 against r1I2(curves 1-3) together with the experimental values of OQr - OQ normalized to unit concentration of HgX3-. Only for higher concentrations of HgCl; or HgBrY are the experimental lines close to the theoretical ones. Data for the lower concentrations of HgX,- (X = C1 or Br) show that only a small fraction of the charge transferred during T is preserved on the surface in the form of Hg2X2. These results are reasonable when one considers the equilibrium concentrations of HgX, in contact with Hg and Hg2X2(s). The case of Hg13- is substantially different. Formation of insoluble Hg,12 was hardly detected even for higher concentrations of Hg13- (Figure 5c and the values of OQ, in Table 111). The slope S , is much larger for higher CHg13-, which indicates that a higher
Acknowledgment. Unpublished materials by Joseph H. Christie assisted greatly in treating the chronocoulometric results. This work was supported by the National Science Foundation under Grants CHE7917543 and CHE8305748. Registry No. HgC12, 7487-94-7; HgBr2, 7789-47-1; Hg12, 7774-29-0; HgCI3-, 14988-07-9; HgBr,-, 21388-05-6; HgI), 19964-1 1-5; C1-, 16887-00-6; Br-, 24959-67-9; I-, 20461-54-5; Hg,7439-97-6.
Structures of Dimesitylcarbene and Related Compounds' A. S. Nazran, F. L. Lee, E. J. Gabe, Y. Lepage, D. J. Northcott, J. M. Park, and D. Griller* Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada KIA OR6 (Received: January 18, 1984)
The structure of triplet dimesitylcarbene was investigated by use of single crystals of dimesityldiazomethane and a series of organic glasses as hosts. Electron paramagnetic resonance spectroscopy showed that the carbene had a structure which was significantly less bent than that of diphenylcarbene. Moreover, in soft matrices, the central C-C-C angle of the carbene expanded irreversibly on annealing. The electronic consequences of these structural differences are discussed in terms of the chemistry of the carbene.
reaction involving a triplet biradical which must persist until triplet to singlet intersystem crossing can lead to the final product, eq
Introduction The singlet and triplet states of dimesitylcarbene2-4 show quite distinct chemistries which follow the principles of spin selection as delineated by Skell and Woodworth.s The singlet state of the carbene reacts rapidly with methanol or 1-propanol to give the expected "insertion product", with rate constants in excess of 2 X lo7 M-' s-I, reaction 1,3,4 By contrast, the triplet ground state
-
+
(Mes),C: ROH (Mes),CHOR Mes = 2,4,6-trimethylphenyl; R = Me or n-Pr
4.3,4
(1)
The examples discussed above show that it is the spin state of dimesitylcarbene which dictates its mode of reaction. However, at first glance, this appears not to be the case for the majority of diarylcarbenes that have been investigated in detail, all of which have triplet ground states. For example, of the optical absorption spectra that have been investigated in detail, triplet fluorenylidene6 and diphenylcarbene'-" are quenched by methanol, a substrate which, according to the spin selection rules,* should quench only the singlet state. Similarly, the triplet states of all these carbenes are quenched by their parent diazo compounds, a process which
of dimesitylcarbene is unreactive toward alcohols but undergoes self-reaction.2-4 This process is spin allowed since there is a '/9th probability that the triplet-triplet encounter will lead to the olefin as product in its singlet state, reaction 2. Similarly, the reaction 2(Mes),C:
-
(Mes),C=C(Mes),
(2)
with oxygen proceeds smoothly to form ketone following a similar spin-allowed mechanism, reaction 3.334 ( M ~ S ) ~ C+: O2
-
(Mes),C=O
(3)
The reaction of triplet dimesitylcarbene with cis-2-pentene gives cyclopropanes with loss of stereospecificity as expected for a (1) (2) 2149. (3) (4) (5)
Issued as NRCC publication no. 23643. Zimmerman, H.E.; Paskovich, D. H. J . A m . Chem. SOC.1964, 86, Nazran, A. S.; Griller, D.J . Chem. SOC.,Chem. Commun. 1983,850. Nazran, A. S.; Griller, D.J . A m . Chem. SOC.1984, 106, 543. Woodworth, R. C.; Skell, P. S . J. Am. Chem. SOC.1959, 81, 3383.
0022-3654/84/2088-5251$01.50/0
(6) Brauer, B.-E.; Grasse, P. B.; Kaufmann, J.; Schuster, G. B. J. Am. Chem. SOC.1982, 104, 6814. (7) Closs, G. L.; Rabinow, B. E. J . A m . Chem. SOC.1976, 98, 8190. (8) Eisenthal, K. B.; Turro, N. J.; Aikawa, M.; Butcher, J. A,, Jr.; D u h y , C.; Hefferon, G.; Hetherington, W.; Korenowski, G. M.; McAuliffe, M. J. J. Am. Chem. SOC.1980, 102, 6563. (9) Turro, N. J. Tetrahedron 1982, 38, 809. (10) Griller, D.;Nazran, A. S.; Scaiano, J. C. J . Am. Chem. SOC.1984, 106. - - ,-198. --
(1 1) For the origin of this proposal see: Bethell, D.; Stevens, G.; Tickle, P. J . Chem. SQC.D 1970, 792.
Published 1984 American Chemical Society
5252
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984
-
is formally spin forbidden for the same reasons, reaction 5. Ar2C: + Ar2CN2 Ar2C=N-N=CAr2 (5) Ar = aryl These apparent anomalies have been explained with the proposal that the singlet and triplet states of these carbenes are in thermal equilibrium under the reaction conditions, eq 6 . Although the Ar2C: (singlet)
Ar2C: (triplet)
(6)
details of this hypothesis have been challenged,1° the results nevertheless imply that the singlet-triplet energy gap in dimesitylcarbene must be substantially greater than those for the other diarylcarbenes which have been inve~tigated.~-~ The most obvious inference which can be drawn is that the steric influence of the ortho-methyl groups is responsible for the distinctive chemistry of dimesitylcarbene. In an attempt to discover structural differences between dimesitylcarbene and other carbenes, we have carried out an electron paramagnetic resonance (EPR) study of the carbene using hydrocarbon glasses and a single crystal of its parent diazo compound as inert matrices. To support this investigation, we have also determined the crystal structures of several related compounds.
Experimental Section Materials. Dimesityldiazomethane was synthesized by the method of Zimmerman and Paskovich.z Dimesitylmethane and tetramesitylethylene were generated by photolyzing dimesityldiazomethane (0.04 M) in cyclopentane at 25 OC. They were isolated by preparative thin layer chromatography on silica gel using a hexane:ether mixture (90:lO v/v) as eluent and were recrystallized from hexane! The NMR and mass spectra of these materials have been described e l ~ e w h e r e . ~ EPR Experiments. EPR measurements were made on a Variao E12 X-band spectrometer using a microwave power of 30 mW and a modulation amplitude of 4 G. Sample temperatures were controlled by an Air Products Ltd. 3-1 10 nitrogen gas flow system. In a typical experiment, a solution of dimesityldiazomethane (10-3-10-5 M) in an appropriate solvent was deoxygenated by at least three freeze-pumpthaw cycles. It was then quickly frozen to form a glass in the cavity of the spectrometer and was irradiated briefly, using mercury-xenon lamp, in order to generate the carbene. The field positions of the spectral lines were measured with a Varian N M R gaussmeter and the microwave frequency was determined with a Hewlett Packard 5246L frequency counter with a 5255A plug-in unit. The structure of the carbene was also investigated with single crystals of dimesityldiazomethane as matrices. Measurements were made on two separate crystals which were oriented by X-ray diffraction. The crystals were rotated in three planes with a two-circle goniometer and the field positions of the resonance lines were recorded at 5 O or loo intervals in each of the planes. For most orientations only one lhMsl= 1 transition was visible, but in a few orientations both high-field lAM,I = 1 transitions were observed. The low-field transitions (nominally IAMJ = 2) were detectable over a wide range of angles but because of their relative insensitivity to orientation and of the difficulty in measuring fields below 1000 G, they were not included in the data to be fitted. Data were least-squares fitted by the program L S F ' ~ to the usual spin Hamiltonian, eq 7 . N = gPH*S+ S.D*S (7) The spectral line widths (ca. 40 G) were too large to allow an accurate determination of the g tensor. Accordingly, the g values were assumed to be isotropic and equal to 2.003 based on data for diphenyl~arbene.'~No attempt was made to refine the g value so that the only fiting parameters were the five independent elements of the D tensor. (12) Dalal, N. S.;Dickinson, J. R.; McDowell, C. A. J. Chem. Phys. 1972, 57, 4254. (13) Hutchison, C. A,, Jr.; Kohler, B. E. J . Chem. Phys. 1969, 51, 3327. Doetschman, D. C.; Hutchison, C. A,; Jr. J . Chem. Phys. 1972, 56, 3964.
Nazran et al. TABLE 1: Details of Space Group, Cell Parameters, Data Collection, and Structure Refinement for Dimesityldiazomethane, Dimesitylmethane, and Tetramesitylethylene dimesityldiazodimesityltetramesitylmethane methane ethylene P21IC P2dn I4,lacd space group a, 8, b, A c, 8,
P> A radiation; X K q , A cell parameter reflections intensity data reflections unique reflections significant reflections; a limit R , = ClIF,I -
8.5135 (14) 9.6065 (14) 18.185 (3) 98.38 (1) Cu; 1.5406 46; 28 > 1000 2423; 28 S 120 2188 1880; 2.5
8.5817 (8) 8.8175 (6) 22.076 (2) 91.45 ( 1 ) Mo; 0.70932 32; 28 > 40° 3659; 28 6 45 2185 1514; 3.0
17.578 (2)
Cu; 1.5406 60; 28 > 67O 4481; 28 S 120 1138 653; 2.5
0.042
0.043
0.073
0.049 0.041
0.110 0.029
0.122 0.042
0.041
0.030
0.043
19.709 (2)
-l~clr2l~ol
(incl insignificant) R2 = X W ( F , ~
F,)
2TxwF:
(incl insignificant)
X-ray Crystallography. The structures of dimesityldiazomethane, dimesitylmethane, and tetramesitylethylene were determined by X-ray diffraction and details are given in Table I. Cell parameters were obtained from a least-squares treatment of the aligned orientation angles for high 20 reflection as shown in Table I. Intensity data were collected on a Picker four-circle diffractometer using monochromatized Kcr radiation. The 0/20 scan technique was used with profile a n a l y ~ i s . ' ~Lorentz and polarization correction^'^ were applied, but absorption corrections were not considered necessary. Crystals of tetramesitylethylene were not of high quality and led to a relatively high fraction of reflections which were considered insignificant. This gave rise to a rather high final R value. The structures were solved with a variant of MULTAN'~and were refined by a block-diagonal least-squares procedure using weights based on counting statistics. Hydrogen atoms were located from difference maps and their positional and isotropic thermal parameters were refined for dimesityldiazomethane and dimesitylmethane. Atomic coordinates, thermal parameters, and structure factors are included as supplementary material. (See paragraph at end of text regarding supplementary material.) All calculations were carried out with the NRC PDP-8e system of program^'^ and scattering factors were taken from the "International Tables for X-ray Crystallography".'8
Results and Discussion One of the most striking features of triplet dimesitylcarbene is that it will dimerize to form tetramesitylethylene, reaction 2, and yet will not react with its parent diazo compound to form azine, reaction 4. The ethylene can be formed directly from the triplet carbene, but the reaction to form azine in one step requires triplet-to-singlet intersystem crossing before the final product can be formed. This observation led Zimmerman and Paskovich2 to the conclusion that the triplet-singlet energy gap in dimesitylcarbene was substantially larger than those for other diarylcarbenes where azine formation is particularly facile. The above results are all the more surprising since the olefin is a sterically congested compound. Its structure is shown in Figure 1 and is given in detail as supplementary material. The olefin (14) Grant, D. F.; Gabe, E. J. J . Appl. Crystallogr. 1978, 11, 114. (15) LePage, Y . ;Gabe, E. J.; Calvert, L. D. J . Appl. Crystallogr. 1979, 12, 25. (16) Germain, G.; Main, P.; Woolfson, M. M. Acta Crystallogr.,Sect. A 1971, A27, 368. (17) Larson, A. C.; Gabe, E. J. "Computing in Crystallography"; Schenk, H., Olthof-Hazekamp, R., van Koningsveld, H., Bossi, J. C., Eds.; Delft University Press: Delft, Holland, 1978. ( 1 8 ) "International Tables for X-ray Crystallography" Kynoch Press: Birmingham, 1974; Vol. IV, Tables 2-2B.
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 5253
Structures of Dimesitylcarbene
TABLE II: Principal Values and Eigenvectors of the X, Y , and 2 ComDonents of the D Tensor for Dimesitvlcarhene component principal value
X Y Z
MHZ
-3320 & 50 -3940 & 50 7260 & 50
eigenvector -0.3440 & 0.371 1 0.7648 & 0.4225 0.5453 & 0.8268
(a,b,c*)a 0.8625 0.4864 -0.1382
aEigenvectors are referred to crystal axes a,b,c* where E = a X b; & signs chosen consistently relate one distinguishable site from the other.
A
8
c'9
Figure 1. Structure of tetramesitylethylene: (A) stereodrawing; (B) bond angles (degrees) and bond lengths (A).
has the crystal symmetry 222 at the point (0, 1/4, 1/8) in the center of its double bond. This means that the four aryl rings are arranged as a propellor so as to minimize the steric interactions between their o-methyl groups. The congestion suggests that the rings cannot be free to rotate. As the figure shows, the C2-Cl-C, angle is compressed to 114.2' so as to alleviate some of the interaction between the cis-aryl groups. The chemical evidence described above points to an unusually large triplet-singlet energy separation for dimesitylcarbene. Accordingly, we carried out an EPR study of the triplet carbene to see if its structure reflected this possibility. Single-Crystal EPR Spectra. The EPR spectra of carbenes are characterized by two parameters D and E.I9 The D value is related to the separation between the unpaired electrons. The value of E when weighted by D is a measure of the deviation of the carbene from axial symmetry or, more plainly, it described the extent to which the molecule is bent. Both D and E can be obtained from the spectra of randomly oriented carbenes trapped in glassy mat rice^.'^-^^ However, if the spectra are measured in properly oriented single crystals it becomes possible to define the , ' ~ to relate these to directional properties of the D t e n ~ o r ' ~and the basic structure of the carbene. Dimesityldiazomethane was chosen as a host matrix for the carbene and its crystallographic parameters are given in summary in Table I and in detail as supplmentary material along with those of dimesitylmethane. From the point of view of this study, the most important aspects of its structure, apart from its symmetry, are that the dihedral angle between the aryl groups was 90' and that the central C-C-C angle was 130'. Single crystals of dimesityldiazomethane were mounted in known orientations on a two-circle goniometer which was placed (19) Bolton, J. R.; Wertz, J. E. 'Electron Spin Resonance"; McGraw-Hill: New York, 1972; Chapter IO, p 223. (20) Wasserman, E.; Snyder, L. C.; Yager, W. A. J . Chem. Phys. 1964, 41, 1763.
in the cavity of an EPR spectrometer. The samples were cooled to 77 K and a few seconds of irradiation with an argon laser were sufficient to generate the spectrum due to the carbene. The spectrum of the carbene was quite persistent up to at least 200 K and the spectral parameters were essentially temperature independent. For convenience, 100 K was used for all of the detailed measurements. As expected from the monoclinic crystal symmetry, the EPR spectra were characteristic of carbene centers in two magnetically distinguishable sites. In addition to the main signals, at least two other related but weaker signals were observed. These satellites followed the angular variation of the main lines quite closely (not deviating by more than 300 G between 1500 and 6500 G ) , but their intensities had significantly different temperature dependencies than those of the main lines. The most likely explanation for these observations is that the satellite lines are due to carbenes trapped in slightly different environments than those of the main species. These orientations may represent slightly different sites in a somewhat imperfect crystal. However, a further possibility is that all of the carbenes are formed in similar sites but that the steric compression between the products of photolysis, namely, the carbene and nitrogen, causes a proportion of the cabenes to skew to a different orientation or to adopt slightly different structures to those which give rise to the main signal (vide infra). The X, Y, and Z values of the D tensor of the main species are given in Table I1 along with their eigenvectors, referred to the a, b, and c* axes of the crystal. Comparison with the X-ray crystal structure shows that the directions measured for the carbene correspond closely to characteristic directions of the host dimesityldiazomethane molecule. Specifically, the Z value corresponds to a vector joining the centers of the two aryl rings. The X and Y directions which are expected to represent the principal axes of the orbitals containing the unpaired electrons correspond to the C-N-N direction (Y)and to a vector normal to the central C-C-C. plane (X). These results imply that when the carbene is formed in the crystal of the diazo compound it maintains a position in the lattice which is almost identical with that of its diazo precursor and therefore presumably maintains the orthogonality of the aryl rings. The slight differences between the directions for the carbene and those of dimesityldiazomethane are probably outside the experimental errors and correspond to a rotation of about 6' in the Y-Z plane as the molecule opens out. The values of D and E were calculated from the values of X , Y,and Z by using the relationship^'^ X = D/3-E
(8)
Y = D/3 + E
(9)
Z = -2D/3
(10)
and were found to be 10890 f 70 and 310 f 20 MHz, respectvely. Endor studies on diphenyl~arbene'~ show that the central C-C-C angle is 148'. The D and E values for this and essentially all substituted diphenylcarbenes are D N 12000 MHz and E = 580 MHz implying that they all have similar structure^.'^*^^,^* For dianthrylcarbene, in which the aryl groups are orthogonal, D = 9020 MHz, indicating that there is extensive delocalization of the (21) Trozzolo, A. M.; Wasserman, E. In "Carbenes"; Moss, R. A.; Jones, M., Jr., Eds.; Wiley: New York, 1973; Vol. 11, Chapter 5. (22) Higuchi, J. J . Chem. Phys. 1963, 39, 1339.
5254
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984
TABLE III: Values of D and E for Dimesitylcarbene in Various Glasses matrix
temp, K
D, M H z
before annealing isopentane/ether 1,I-diphenylethylene 1,I-diphenylethylene n-octane
6 6 77 77
10675 f 10600f 10540f 10550f
after annealing isopentane/ether 1,l-diphenylethylene 2-methyltetrahydrofuran
77 77 77
10380 f 60 10480 i 50 10460 f 30
100 100 60 30
E, MHz 375 348 355 345
f 30 et 30 f 20 f 10
245 f 20 235 f 15 250 f 10
unpaired electrons. Moreover, E = 0 which shows that the carbene has a linear s t r u c t ~ r e . ~ Clearly, ~ * ~ ~ the D and E values for dimesitylcarbene lie in between these extremes and suggest that its structure is closer to linearity than those of unhindered diarylcarbenes. The expansion of the central C-C-C angle observed for dimesitylcarbene presumably results from the steric influence of the o-methyl groups. In addition, the methyl groups presumably maintain the orthogonality of the aryl rings thus allowing more effective conjugation with the orbitals containing the unpaired electrons which leads in turn to a reduction in D. EPR Spectra in Glasses. Values of D and E for dimesitylcarbene were also determined with glasses made form organic solvents as matrices (Table 111) and showed unusual variations when these hosts were annealed.% For example, when the carbene was generated in 1,l-diphenylethylene at 77 K, D and E values of ca. 1 0 5 4 0 and 350 MHz were observed which are similar to those measured in the single-crystal study. However, as the samples were warmed to 188 K in 10 K increments, the x and y lines of the spectrum moved closer together, indicating that a significant reduction in E had occurred. This change was not reversed when the samples were cooled. Similar effects were observed for isopentane/ether glasses when these were warmed from 6 to 77 K. Effects of this kind but not of this magnitude have been observed for dianthryl~arbene.~~ They show that when the carbene is formed at low temperatures or in its diazo precursor the rigidity of the matrix prevents it from assuming its minimum energy geometry. However, when the matrix is softened on annealing, the carbene relaxes to a structure which is closer to linear as evidenced by the substantial reduction in E. The small reductions in D which were observed are also consistent with this picture since they indicate that the unpaired electrons are more efficiently delocalized in the relaxed geometry. In contrast with dimesitylcarbene, E values for diphenylcarbene, measured in a variety of glasses, were essentially insensitive to the matrix:s suggesting that the carbene had achieved its relaxed geometry and that it was not perturbed by the environment. For diphenylcarbene the central C-C-C angle is 148' .l3l2' Theory suggests that opening this angle to 180' requires very little additional and we therefore conclude that the triplet state (23) Waserman, E.; Kuck, V. J.; Yager, W. A.; Hutton, R. S.; Green, F. D.; Abegg, V. P.; Weinshenter, N . M. J . Am. Chem. Sor. 1971, 93, 6335. (24) For a preliminary report of this work see: Nazran, A. S.; Gabe, E. J.; LePage, Y.; Northcott, D. J.; Park, J. M.; Griller, D. J . Am. Chem. SOC. 1983, 105, 2912. (25) Trozzolo, A. M.; Wasserman, E. Yager, W. A. J . Chim. Phys. Phys.-Chim. Biol. 1964, 61, 1663.
Nazran et al. of dimesitylcarbene is not significantly destabilized with respect to diphenylcarbene. The fact that dimesitylcarbene is relatively unreactive when compared with diphenylcarbene must reflect the steric protection which the o-methyl groups afford the reactive center, i.e., the triplet carbene is persistent rather than stabilized.28 By contrast, theory s ~ g g e s t s *that ~ * ~opening ~ the central C-C< angle strongly destabilizes the singlet state in which the two electrons are localized in the same orbital, a prediction which is in complete accord with chemical studies. However, there is another available singlet state which will be stabilized by such an angular expansion. This is the state which is similar to the triplet, but which has its electrons in different orbitals and with antiparallel spins. The reactions of the singlet state in question should resemble those of the triplet. However, if it is low lying and close in energy to the triplet state so that it can be populated thermally to a significant extent, then its presence should be revealed by deviations from a Curie-Law p10t.l~ Studies of the temperature dependence of the spectrum in 1,l -diphenylethylene as the matrix showed that the intensity of the EPR signals followed Curie's law over the temperature range 4-189 K. This confirms that the ground state for the carbene is indeed a triplet and that the singlet state in question cannot be thermally populated to a significant extent. Dimesitylcarbene showed no sign of decay over several hours at 77 K in all of the glasses in which it was investigated even at concentration of diazo compound as low as M. This behavior contrasts sharply with that of diphenylcarbene and fluorenylidene which are persistent only when generated in glasses containing high concentrations of diazo compound, ca. 0.1 M. The effect presumably arises because carbenes are not sterically protected and are persistent only when formed in microcrystallites of their parent diazo compounds, or that only a very small proportion of the matrix sites are capable of sustaining a highly reactive carbene.
Summary The o-methyl groups of dimesitylcarbene have a profound effect on both its structure and chemistry. When formed in matrices from its parent diazo compound, the carbene has a structure which is closer to linearity than the structures observed for most other diarylcarbenes and in which the aryl groups are orthogonal. In very soft matrices or an annealing of harder matrices, the carbene undergoes a further and irreversible expansion of the central C-C-C angle to gain even greater relief from steric compression. Theory suggests that for such a structure there will be a large energy separation between the triplet ground state and the singlet state in which both electrons are localized in the same orbital. Chemical studies are in complete accord with these predictions. Registry No. Dimesitylcarbene, 85236-86-8; dimesityldiazomethane, 61080-14-6; dimesitylmethane, 733-07-3; tetramesitylethylene, 3857532-5.
Supplementary Material Available: Tables of atomic coordinates, thermal parameters, and structure factors for dimesitylcarbene and related compounds (47 pages). ordering information is available on any current masthead page. (26) Metcalfe, J.; Halevi, E. A. J . Chem. SOC.,Perkin Trans. 2 1977, 364. (27) Hoffman, R.; Zeiss, G. D.; Van Dine, G. W. J . Am. Chem. SOC.1968, 90, 1485. (28) Griller, D.; Ingold, K. U.Arc. Chem. Res. 1976, 8, 13.
