Cyclobutanediyls: a new class of localized biradicals. Synthesis and

1356

J . Am. Chem. SOC.1988,110, 1356-1366

Cyclobutanediyls: A New Class of Localized Biradicals. Synthesis and EPR Spectroscopy Rakesh Jain, Michael B. Sponsler,'' Frank D. Coms,lband Dennis A. Dougherty*lC Contribution No. 7651 from the Arnold and Mabel Beckman Laboratory of Chemical Synthesis, California Institute of Technology, Pasadena, California 91 125. Received August 17, 1987

Abstract: Nine triplet cyclobutanediyls (1) have been synthesized and observed by matrix isolation EPR spectroscopy. The zero-field splitting (zfs) parameters provide detailed information on the spin densities in these structures. The observed zfs values can be quantitatively modeled by using a straightforward semiempirical scheme, as long as one explicitly incorporates the spin polarization effects known to be important in radicals such as allyl and benzyl. In addition, interpretable hyperfine coupling (hfc) has been observed in many cyclobutanediyls. Spectral simulation produces the hfc constants, which provide further information on spin distribution and indicate that the four-membered ring in 1 is planar. Several new procedures have been developed for the synthesis of the azoalkane precursors, 3. which are described in detail.

A wide variety of biradicals and related structures have been directly observed by matrix isolation EPR spectroscopy in the more than 20 years since the initial observation of triplet trimethylenemethane (TMM).2 The overwhelming majority of such studies have involved non-Kekuli: molecules3 (delocalized biradicals). These are fully conjugated K systems, for which one cannot write a classical Kekuli structure with all the electrons paired into bonds. Examples include T M M , m-quinodimethane and derivatives, and the 1,8-naphthoq~inodimethanes.~ The non-KekulC nature and, to a considerable extent, the spin preferences of such structures are set by the topologies of the K system^.^ In contrast, direct spectroscopic studies of localized hydrocarbon biradicals are rare.6 Localized biradicals contain two well-defined radical substructures which are not in conjugation with one another in a classical K sense. Note that the radical units themselves can be classically delocalized, such as allyl or benzyl. Localized biradicals are fundamentally different from non-KekulC or delocalized biradicals in that there are no simple topological rules for predicting ground spin state, etc. Rather, a subtle and sometimes complicated interplay among factors such as ring strain, (a) NSF Predoctoral Fellow, 1982-1985. (b) JPL-CSMT Fellow, (c) Camille and Henry Dreyfus Teacher-Scholar, 1984-1989. Dowd, P. J . Am. Chem. SOC.1966, 88, 2587-2589. Dewar, M. J. S . The Molecular Orbital Theory of Organic Chemistry; McGraw-Hill: New York, 1969; pp 232-233. (4) Borden, W. T., Ed. Diradicals; Wiley: New York, 1982. (5) For recent discussions of such rules, see: (a) Ovchinnikov, A. A. Theor. Chim. Acta 1978, 47, 297-304. (b) Klein, D. J.; Nelin, C. J.; Alexander, S.; Matsen, F. A. J . Chem. Phys. 1982, 77, 3101-3108. (c) Borden, W. T.; Davidson, E. R. J . Am. Chem. SOC.1977, 99, 4587-4594. See, however: Dowd, P.; Chang, W.; Paik, Y . H. J . Am. Chem. SOC.1986,108,7416-7417; 1987,109, 5284-5285; Du, P.; Borden, W. T. J . Am. Chem. Sot. 1987,109, (1) 1987. (2) (3)

930-931. (6) Although they are generally not hydrocarbons, localized biradicals derived from Norrish type I and Norrish type I1 photoreactions are clearly relevant. An overview of early work involving primarily CIDNP (type I) and laser flash photolysis (type 11) studies, as well as the dehydrobenzenes, is provided in: Dervan, P. B.; Dougherty, D. A. In Diradicals; Borden, W. T., Ed.; Wiley: New York, 1982; pp 107-149. A list of more recent works follows, which is not intended to be exhaustive. Type I1 biradicals: Scaiano, J. C. Arc. Chem. Res. 1982, 15, 252-258. Caldwell, R. A,; Majima, T.; Pac, C. J . Am. Chem. SOC.1982, 104, 629-630. Scaiano, J. C.; Wagner, P. J. J . Am. Chem. SOC.1984, 106, 4626-4627. Johnston, L. J.; Scaiano, J. C.; Sheppard, J. W.; Bays, J. P. Chem. Phys. Lett. 1986,124,493-498. Akiyama, K.; Ikegami, Y.: Tero-Kuhta, S. J . Am. Chem. SOC.1987, 109, 2538-2539. Type I biradicals and the hydrocarbon biradicals derived by CO loss: Caldwell, R. A.; Sakuragi, H.; Majima, T. J . Am. Chem. SOC. 1984, 106, 2471-2473. Weir, D.; Scaiano, J. C. Chem. Phys. Left. 1985,118, 526-529. Zimmt, M. B.; Doubleday, C., Jr.; Gould, I. R.; Turro, N. J. J . A m . Chem. SOC.1985, 107,6724-6726. Zimmt, M. B.; Doubleday, C., Jr.; Turro, N. J. J . Am. Chem. SOC.1985,107.6726-6727; 1986,108,3618-3620. Johnston, L. J.; Scaiano, J. C. J . Am. Chem. Sac. 1986, 108, 2349-2353. Closs, G. L.; Forbes, M. D. E. J . Am. Chem. SOC.1987, 109, 6185-6187. See also: Mizuno, K.; Ichinose,N.; Otsuji, Y . ;Caldwell, R. A. J . A m . Chem. SOC.1985, 107, 5797-5798. Sugawara, T.; Bethell, D.; Iwamura, H. Tetrahedron Lett. 1984, 25, 2375-2378. Localized, triplet biradicals can be trapped by dioxygen. See, for example: Wilson, R. M.; Geiser, F. J . Am. Chem. Sac. 1978, 100, 2225-2226.

0002-7863/88/15 10-1356$01S O / O

steric effects, and second-order electronic effects (e.g., throughbond coupling) determines the electronic structure.' In 1984, we reported the E P R spectrum of the localized hydrocarbon triplet biradical 1,3-dimethyl-l,3-~yclobutanediyl (1Me).* Almost 10 years earlier, Closs had reported the E P R detection of triplet 1,3-~yclopentanediyl(2).9 Closs' classic study

0 2

was the first and, until our work, the only effort of its kind. At the time, the observation of 2 appeared to signal a turning point in the study of this important class of reactive intermediate^.^ For the first time, one would be able to directly probe the spectroscopy and reactivity of localized biradicals, and the many questions concerning the effects of substituents on singlet-triplet preferences and reactivity could be addressed directly. However, the cyclopentanediyl framework proved to be quite sensitive. Minor perturbations led to weak EPR signals or no signals a t aIL9 Thus, a systematic study of the role of substituents on localized biradical spectroscopy and reactivity was not possible. We now report that the cyclobutanediyl framework is much more robust. A variety of structures 1 have been prepared, all of which give intense, interpretable E P R spectra. We describe here the synthetic approaches to such molecules and the E P R spectra of the first key members of the series. Trends in the EPR zero-field splitting (zfs) parameter D have provided insights into the spin densities in these structures. In addition, interpretable hyperfine couplings (hfc) to several sets of hydrogens, but especially to those of the ring C H 2 groups, have provided further information on the molecular and electronic structures of these biradicals.

R,+R2

1.Me

Ri=Rz=CH3

I-CD3

R~=Rz=CDI

1.Et

Ri =Rz=CHzCH3

I-Pr

Rx=Rz=n-C3H7

1 . m

RI=CHS Rz=Ph

1.EV

Ri=CHzCH3 Rz-CHCH2

I-Ph

Ri=Rn=Ph

1.Vin

Ri=Rz=CHCHz

1.Vin-d~

RI=Rz=CDCDZ

(7) See, for example: (a) Doubleday, C., Jr.; McIver, J. W., Jr.; Page, M. J . Am. Chem. SOC.1982, 104, 6533-6542. (b) Goldberg, A. H.; Dougherty, D. A. J . Am. Chem. SOC.1983, 205, 284-290. (c) Salem, L.; Rowland, C. Angew. Chem., Int. Ed. Engl. 1972, 11, 92-11], (8) Jain, R.; Snyder, G. J.; Dougherty, D. A. J . Am. Chem. Soc. 1984, 106, 7294-7295. (9) Buchwalter, S. L.; Closs, G. L. J . Am. Chem. Sac. 1975, 97, 3857-3858. Buchwalter, S. L.; Closs, G. L. J . Am. Chem. SOC.1979, 101, 4688-4694.

0 1988 American Chemical Society

J . Am. Chem. SOC.,Vol. 110, No. 5, 1988 1357

Cyclobutanediyls

Wittig reactions. However, its synthesis proved to be surprisingly difficult. We first attempted the direct reduction of 14 to 15 using both diisobutylaluminum hydride and sodium bis(2-methoxyethoxy)aluminum hydride, but neither method was successful. Therefore, we adopted a two-step procedure in which the ester functionalities are first reduced to hydroxymethyl groups, producing diol 16. This can be achieved easily by using either lithium borohydride in T H F ” or sodium borohydride in methanol.I6 1 3 4 Oxidation of the diol was quite difficult, and many procedures, laboratories’0 showed that addition of N-methyltriazolinedione including chromium-based reagents,” Swern,’*lead t e t r a a ~ e t a t e , ’ ~ ( M T A D ) across the strained bond of a bicyclobutane is a conand N-phenyltriazolinedione20oxidations, failed. Though some venient method for the synthesis of such urazoles.” One key to of these methods left the starting material unchanged, others the synthesis of the diazenes, then, is to obtain the appropriately appeared to fail only in isolation of the product. Believing that functionalized bicyclobutanes for the M T A D addition. hydration of the dialdehyde might be the problem, we sought a A fairly general route to substituted bicyclobutanes is based completely water-free method. The pyruvate photolysis procedure on the reductive coupling of the readily available 1,3-dihaloemployed by Binkley2’ appeared attractive, since the aldehyde cyclobutanes. For example, the synthesis of the methyl phenyl product is obtained in solution without any workup. Pyrex-filtered urazole 5 began with the readily available 3-phenyl~yclobutanone~~ photolysis of the dipyruvate ester 17 in dilute benzene solution (eq 2). Treatment with methyllithium, conversion of the alcohol does give the dialdehyde 15, which indeed hydrates readily. Therefore, the product was used without isolation in Wittig rePh !’h actions to give diene urazoles 18, 19, and 20 (eq 5).

Synthesis

The immediate precursors to biradicals 1 are the substituted 2,3-diazabicyclo[2.1.1] hex-2-enes (3), which, in turn, are prepared from the appropriate urazoles 4 (eq 1). Earlier work in our

to the bromide 7, and benzylic bromination gave the dibromides 8 as a mixture of diastereomers. Closure of the dibromides to 1-methyl-3-phenylbicyclobutane(9) was accomplished by treatment with lithium amalgam. Thermal addition of M T A D in refluxing n-hexane then afforded 5 in good yield (eq 3). As with

8 11

R=CH, R=Ph

9 R=CH,

12

R=Ph

5 R=CH,

10

R=Ph

18 R = I I 19 R:CH3 20 R = P h

Deuteriated divinyl urazole 18-d6was prepared by following the above procedure using sodium borodeuteride in the reduction step and deuteriated phosphonium salt in the Wittig reaction. Proton N M R analysis revealed that the vinylic methylene groups were 68% deuteriated. The syntheses of the diethyl (21) and dipropyl (22) urazoles also utilized the diol 16 (eq 6). The bismesylate (23) of diol 16

our earlier reported thermal reactions,’h,b the addition of MTAD proceeds more cleanly in nonpolar solvents. The synthesis of the diphenyl urazole 10 is strictly analogous to that of 5 beginning with 1,3-dibromo-l,3-diphenylcyclobutane (11)’) as a 1:l mixture of diastereomers. The thermal addition of M T A D to bicyclobutanes is sensitive to substituents a t the bridgehead positions. For instance, 1,3dimethylbicyclobutane reacts rapidly with M T A D a t room temperature (as monitored by the disappearance of the red color of MTAD), but 1,3-dicarbomethoxybicyclobutane(13) fails to react even at high temperatures. We now report that the photochemicallocs’l addition of M T A D to 13 produces urazole 14 in good yield (eq 4). Given the ready availability of 13 in large quantity,I4 and the potential conversion of the ester functionality of 14 into a variety of substituents, we saw this as a general route to substituted diazenes and biradicals.

was treated with sodium iodide in refluxing 2-butanone to give diiodide 24. Lithium dimethylcopperZZthen converts 24 to diethyl urazole 21. Another route to 21 involves catalytic hydrogenation of divinyl urazole 18. The dipropyl urazole (22) was similarly prepared by hydrogenation of 19. Synthesis of hexadeuteriated dimethyl urazole 25-d6 (eq 7) involves mesylation of 16-d4followed by treatment with sodium borodeuteride in H M P A 2 3 to afford 25-d,.

-

16dJ

13

14

Dialdehyde 15 was selected as the key target in an approach to the synthesis of several hydrocarbon-substituted urazoles via (10)(a) Chang, M. H.; Dougherty, D. A. J . Org. Chem. 1981, 46, 4092-4093. (b) Chang, M.H.; Jain, R.; Dougherty, D. A. J. Am. Chem. SOC. 1984,106,4211-4217. (c) Snyder, G.J.; Dougherty, D. A. J . Am. Chem. SOC.1985,107, 1774-1775. (11)See also: Amey, R. L.; Smart, B. E. J . Org. Chem. 1981, 46, 4090-4092. (12)Falmagne, J.-B.; Escudero, J.; Taleb-Sahraoui, S.; Ghosez, L. Angew. Chem., Inr. Ed. Engl. 1981,20, 870-880. We have found that 3-phenylcyclobutanone can be easily prepared in multigram quantities by reductive dechlorination of 2,2-dichloro-3-phenylcyclobutanone(Krepski, L. R.; Hassner, A. J. Org. Chem. 1978.43,2879-2882) with zinc in refluxing acetic acid (Bak, D. A.; Brady, W. T. J. Org. Chem. 1979,44,107-110. Stewart, E.G.,

unpublished results). (13)Farnum, D. G.;Mostashari, A.; Hagedorn, A. A,, 111. J . Org. Chem. 1971,36,698-702. (14)Coffey, C. E. U.S. Patent 3657317,1972.

,

cn,

..

i>

25d6

The unsymmetrical ethyl vinyl urazole 26 was prepared by two methods. Catalytic hydrogenation of 18 using 1 equiv of hydrogen resulted in a l:2:1 mixture of 18, 26, and 21, respectively, which could be separated by preparative T L C and H P L C . Due to the difficulty of the separation, an alternative synthesis was developed which utilizes an epoxide protecting group (eq 8). The mono(15)Nystrom, R. F.;Chaikin, S. W.; Brown, W. G.J , A m . Chem. SOC. 1949,71, 3245-3246. (16)Brown, M.S.;Rapoport, H. J . Org. Chem. 1963,28, 3261-3263. (17)(a) Herscovici, J.; Egron, M.-J.;Antonakis, K. J . Chem. SOC.,Perkin Trans. 1 1982,1967-1973. (b) Corey, E.J.; Suggs, J. W. Tetrahedron Left. 1975,2647-2650.(c) Sharpless, K. B.; Akashi, K. J . Am. Chem. SOC.1975, 97, 5927-5928. (18)Mancuso, A. J.; Huang, S.-L.; Swern, D. J . Org. Chem. 1978,43, 2480-2482. (19) Partch, R. E. J . Org. Chem. 1965,30, 2498-2502. (20)Cookson, R. C.; Stevens, I. D. R.; Watts, C. T. J . Chem. SOC.,Chem. Commun. 1966,744. (21) Binkley, R. W. J . Org. Chem. 1976,41, 3030-3031. (22)Ireland, R. E.; Daub, J . P. J . Org. Chem. 1981,46, 479-485. (23)Boland, W.; Ney, P.; Jaenicke, L. Synthesis 1980,1015-1017.

Jain et al.

1358 J. A m . Chem. SOC.,Vol. 110, No. 5, 1988 epoxide (27) of 18 was catalytically hydrogenated to yield saturated epoxide 28, which was deoxygenated by treatment with potassium ~ e l e n o c y a n a t eto~ ~give 26.

28

' 26

Conversion of the diester urazole 14 to dibromo urazole 29 (eq 9) was accomplished in two steps: hydrolysis, followed by decarboxylative bromination. Since the urazole moiety is labile

30

29

toward nucleophiles, acid-catalyzed hydrolysis was required to convert 14 to dicarboxylic acid 30. Subjecting 30 to the conditions of the modified Hunsdiecker reaction25afforded dibromo urazole 29 in good yield. These studies, and others we have described,'& illustrate the insensitivity of the urazole moiety to a wide variety of synthetic procedures. Thus, we anticipate that an even wider array of substituted diazabicyclohexenes will be accessible, and we are pursuing such structures. Two methods were employed for the conversion of the urazoles to diazenes. The standard route involving a base hydrolysis (typically refluxing KOH/2-propanol) followed by oxidation via the copper complexes26was used for most of the stable diazenes (eq 10). Minor variations of this procedure also worked for the

4

3

thermally unstable (vide infra) diazenes 3-Vin and 3-EV. However, an alternative procedure developed in our laboratories1&was found to be more satisfactory for these diazenes and for others on which the standard method failed. This procedure involves partial hydrolysis of the urazoles and isolation of the product semicarbazides (31) (eq 11). Oxidation of the semicarbazides with nickel peroxide27 then affords the desired diazenes.

An obstacle to the success of the nickel peroxide method was the propensity of several of the semicarbazides to rearrange. These rearrangements were not studied extensively, but they appear to involve cleavage of a ring C-C bond to give five-membered-ring heterocycles. For instance, hydrolysis of the dibromo urazole 29 with potassium hydroxide in refluxing 2-propanol gave pyrazole 3 2 as a single product. Semicarbazides lacking a good leaving group (e.g., R = phenyl or vinyl) decomposed to give heterocycles such as 33. These side reactions were minimized by using pre-

cautions such as room temperature hydrolyses, exclusion of oxygen, and immediate oxidation of the semicarbazides. Rearrangement (24) Behan, J. M.; Johnstone, R. A. W.; Wright, M. J. J . Chem. Soc., Perkin Trans. I 1975, 1216-1211. (25) Della, E. W.; Patney, H. K. Synthesis 1976, 251-252. (26) Gassman, P. G.; Mansfield, K. T. Org. Synth. 1969, 49, 1-6. We have obtained better yields using cupric bromide instead of cupric chloride. (27) George, M. V.; Balachandran, K. S. Chem. Reu. 1975,75,491-519.

was especially fast in the hydrolysis of 20, precluding the isolation of the distyryl diazene. Diazene Reactivity. As for the parent (3-H),I0" diazabicyclohexenes with hydrocarbon substituents thermally decompose to produce the analogous bicyclobutane as the sole product. The stability of the diazenes is highly dependent upon the radicaldelocalizing ability of the bridgehead substituents.28 The parent and alkyl-substituted diazenes are stable up to 100 "C, while the ethyl vinyl diazene 3-EV decomposes with a half-life of ca. 6 h a t room temperature. The divinyl and diphenyl diazenes (3-Vin and 3-Ph) both decompose a t temperatures above -30 "C. The photochemistry of the various diazenes differs dramatically. The simplest cases are 3-Vin, 3-EV, and 3-Ph, all of which give only the corresponding bicyclobutanes upon either direct or sensitized photolysis, independent of temperature. The photochemistry of the parent diazene (3-H) has previously been reported,29 and the dimethyl diazene 3-Me shows completely analogous behavior. For these diazenes, direct photolysis gives mainly the corresponding bicyclobutane, but triplet-sensitized photolysis affords predominantly nitrogen-retained products, 1,2-diazabicyclo[3.1.0]hex-2-enes (34). Product ratios are also strongly temperature dependent, reflecting increasing yields of triplet product 34 a t lower temperature^.^^

We also prepared the divinyl azoxy compound, 35, in order to investigate its viability as a precursor to 1-Vin. An azoxy precursor has been used successfully in the synthesis of a reactive isoindene.N We followed the general synthetic procedure of Olsen and Snyder:" potassium hydroxide hydrolysis of urazole 18 in the presence of hydrogen peroxide. As expected from the behavior of the other azoxy compounds, 35 is significantly more stable than the diazene 3-Vin, being indefinitely stable at room temperature. Thus, ease of synthesis and handling make 35 attractive as a possible alternate precursor. Its photochemistry, however, proved to be far .too inefficient to allow accumulation of 1-Vin under cryogenic conditions.

EPR Spectroscopy General Considerations. The E P R spectroscopy of organic triplets is highly informative but somewhat different from the more common EPR of doublets (free radicals). For that reason we will provide a brief overview of the basic principle^.^^-^^ The expert can move to the next section. There are two important differences between doublet and triplet EPR, as they are conventionally applied. The first concerns the energy level pattern. At zero field, the two spin states (m, = of a doublet are degenerate. Application of an external magnetic field splits these levels, and an allowed (Am, = 1) transition between them can be observed. The spectrum consists of one line, which may be extensively split by hyperfine coupling. In contrast, the three spin states of a triplet (m, = 0, f l ) , are nondegenerate, even in the absence of an external magnetic field. This zero-field splitting (zfs) results from a dipolar coupling between the unpaired spins. In effect, the unpaired spins create an internal magnetic field which splits the magnetic sublevels. Application of an external magnetic field greatly magnifies this splitting and produces the usual resonance conditions. Most importantly, the dipolar (28) Engel, P. S. Chem. Reu. 1980,80, 99-150. (29) Chang, M. H.; Dougherty, D. A. J . Am. Chem. SOC.1982, 104, 2333-2334. (30) Dolbier, W. R., Jr.; Matsui, K.; Michl, J.; Horak, D. V. J . Am. Chem. SOC.1977, 99, 3816-3817. (31) Olsen, H.; Snyder, J. P. J . Am. Chem. SOC.1977, 99, 1524-1536. (32) Wertz, J. E.; Bolton, J. R. Electron Spin Resonance: Elemenrary Theory and Practical Applications; McGraw-Hill: New York, 1972; pp .. .. 223-251. (33) Berson, J. A. In The Chemistry of the Functional Groups, in press. (34) Wasserman, E.; Snyder, L. C.; Yager, W. A. J . Chem. Phys. 1964, 41, 1763-1712. ~~~~

~~~

J. Am. Chem. SOC.,Vol. 110, No. 5, 1988 1359

Cyclobutanediyls Table I. Zero-Field SDlittina Parameters for Cyclobutanediyls obsd" calcda*b biradical IDlhcl IElhcl IDlhcl IElhcl 0.1 12 0.005 1-Me 0.1 12 0.005 0.005 0.005 0.1 12 1-CD, 0.112 0.005 0.112 0.005 0.112 I-Et 0.005 0.005 0.112 0.112 I-Pr 0.003 0.08 1 0.003 1-MP 0.082 0.003 0.074 0.003 0.074 1-EV 0.002 0.061 0.002 I-Ph 0.060 0.001 I-Vin 0.050 0.001 0.050 0.050 0.001 0.001 0.050 1-Vin-d, "In cm". bCalculated D and E values are derived from the fits of Figures 2 and 3, respectively. coupling, and hence the zfs, depends strongly on the average distance, r, between the unpaired spins.. In particular, the interaction varies as l / ? . The relative energies of the three triplet levels at zero field are typically described by two zfs parameters, D and E . It is D which varies as l/?, while E is a probe of the symmetry of the biradical. The second difference between doublet and triplet EPR is that, typically, the former is obtained in fluid media, while the latter is obtained in frozen glasses or solids. Thus, doublet spectra are nicely isotropic, but triplet spectra are highly anisotropic. Since there are three levels, one would expect two allowed ( A m , = 1) transitions in a triplet. However, in a sample of randomly oriented, but nonreorienting triplets, each orientation produces its own pair of resonances. This leads to an essentially limitless number of lines in the absorption spectrum. Fortunately, though, one in 'effect selectively observes only those molecules which have one of their three principal magnetic axes nearly aligned with the external magnetic field (the canonical orientations). There are thus 3 X 2 = 6 transitions observed. In addition, a technically "forbidden" Ams = 2 transition is frequently seen a t half-field.35 As discussed below, this transition is intrinsically less anisotropic than the Ams = 1 transitions, and so only a single, broad line is observed. It is a simple matter to extract the zfs parameters D and E from such spectra,32 and thereby obtain an estimate of the average separation of the spins. Thus, triplet EPR provides an extremely informative probe of biradical structure. Zero-Field Splittings. Photolysis of frozen solutions of the diazenes 3 in 2-methyltetrahydrofuran ( M T H F ) and a variety of other solvents a t 4 K produces E P R spectra for the corresponding triplet cyclobutanediyls (1). Figure 1 shows some of these spectra, and Table I lists the observed zfs parameters. The anticipated six lines are clearly visible in the spectrum of 1-Me, along with the Ams = 2 transition. For the other structures, the value of E is relatively small. This leads to an apparent four-line spectrum.32 As expected, the biradicals with fully saturated substituents show identical zfs, with ID/hc( = 0.112 cm-'. This D value has been compared* with ID/hcl = 0.084 cm-I, which has been reported for The larger D value in the dialkylcyclobutanediyls is a consequence of the l / r 3 dependence of D, since the four-membered ring brings the radical centers closer together than the five-membered ring. When R , and R 2 are delocalizing substituents, the value of D decreases. On average, the unpaired spins are further apart in the delocalized cyclobutanediyls, and the 1/ r 3 dependence leads to a decrease in D. Qualitatively, the trend along the series is as expected. It may a t first seem surprising that 1-Ph shows a significantly larger D value than 1-Vin. Recall, however, that the Hiickel coefficient at the C H 2 in the nonbonding molecular orbital (NBMO) of benzyl (2/v'7) is larger than that in allyl ( l / d 2 ) . Thus, the spin density a t the ring carbons is higher in the diphenyl structure. Given the 1/? distance dependence, this effect dominates the D value. The D values in Table I can be correlated to theoretical values obtained by using a semiempirical method for zfs calculations (35) (a) Kottis, P.; Lefebvre, R. J . Chem. Phys. 1963, 39, 393-403. (b) Kottis, P.; Lefebvre, R. J . Chem. Phys. 1964, 41, 379-393.

h

CH+

Ph

r'

I\

I

v

1800 G 3200 G Figure 1. EPR spectra (first derivative) of several biradicals (1) in order of decreasing D value. The biradicals were generated by irradiation of the appropriate diazene precursors (3) in an MTHF matrix at 4 K. Lines in the region of 3200 G arise from free radicals and from double quantum

transition^.'^ described p r e v i ~ u s l y . ~The ~ distances between the radical centers in the ring were taken from ab initio optimized geometries of the and of 2 (2.37 A).38 The parent cyclobutanediyl (1-H, 2.10 A)7b,37 use of Huckel spin densities in the calculations allowed semiquantitative reproduction of the D values. However, it is wellknown that in delocalized monoradicals such as allyl and benzyl, spin polarization effects produce spin densities that deviate significantly from those predicted by Huckel theory.39 W e have (36) Rule, M.; Matlin, A. R.; Seeger, D. E.; Hilinski, E. F.; Dougherty, D. A.; Berson, J. A. Tetrahedron 1982, 38, 787-798. (37) Sponsler, M. B. Ph.D. Dissertation, California Institute of Technology, 1987. For zfs calculations, all substituents were coplanar with the cyclobutanediylring. For 1-Vin the C,, structure was used, although the C, form eives almost identical D and E values. (3g) Conrad, M. P.; Pitzer, R. M.; Schaefer, H. F., 111 J . Am. Chem. SOC. 1979, 101, 2245-2246.

Jain et ai.

1360 J . Am. Chem. SOC.,Vol. 110, No. 5, 1988 Observed

Calculated

ir' 1-MP

LEV ""

008

010

012

014

0'6

0 8

02C

CalCubted

Figure 2. Observed vs calculated ID/hcl values (cm-') for 1 (open boxes) The value for 2 (filled diamond) was not included in the least-squares analysis.

1-Ph

1.Vin

mtertept = 5 628e 4 H

W G 1.Vin4

Figure 4. Hyperfine structure in the Am, = 2 region of several biradicals 1.

0 0 0 0 4 .

0002

, 0004

.

I

0006

.

I

oooa

.

I

0010

.

, 0012

,

I OOIL

calculated

Figure 3. Observed vs calculated IE/hcl values (cm-') for 1 (open boxes). The value for 2 (filled diamond) was not included in the least-squares

analysis. approximated such spin polarization effects in our calculations by using "experimental" spin densities determined for allyl and benzyl radicals from hyperfine measurement^.^^ We assumed complete localization for alkyl-substituted radicals. With this approach, very good agreement between theory and experiment was obtained. A plot of experimental vs calculated D values (Figure 2) shows an excellent correlation, giving a least-squares line with a near-zero intercept. The calculated value for cyclopentanediyl (2) was not included in the least-squares fit, but as shown in Figure 2, it falls very close to the line. The calculated D values, which have been scaled by using the fit from Figure 2, are shown in Table I for comparison with the experimental values. A similar correlation has been obtained for the experimental and calculated E values (Figure 3, Table I). The accurate calculation of E values is more difficult, since E represents much smaller energy separations than D. Still, the correlation of calculated and observed E values across the cyclobutanediyl series is quite acceptable, although the value for 2 is significantly off the line. Hyperfine Coupling. Six of the cyclobutanediyls display splittings due to hfc in their half-field (Am, = 2) transitions (Figure 4). It was at first surprising to see these splittings, some of them quite well resolved, because of the anisotropies involved in powder spectra. Indeed, the observation of interpretable hyperfine in triplet EPR is relatively rare,10c.41except for single-crystal sam(39) Salem, L. The Molecular Orbital Theory of Conjugated Systems; W.

p l e ~ . We ~ ~ have found, however, that these splittings can be readily interpreted, providing values for the hfc constants (aH) of several protons in these structures. Of course, these coupling constants provide important structural information through standard relationship^'^^^^ such as the McConnell equation (eq 12) for a protons (those directly attached to radical centers) and UOH OBH

=A

= QP

+ pC cos2 0

(12) (13)

a related expression (eq 13) for p protons (those on carbons adjacent to radical centers). In these equations, p is the spin density at a radical center; Q,A , and C a r e empirical constants with typical values of 20-30, 0-5, and 40-45 G, respectively; and 0 is the angle between the axis of the radical p orbital on the a carbon and the (3 C-H bond. A first source of anisotropy to be considered is that which is intrinsic to the hfc tensor, and a and /3 couplings are different in this regard. It is generally recognized that the hfc tensor for p couplings is relatively i s o t r o p i ~ ,and ~ ~ ,so ~ ~splittings due to /3 couplings can be directly related to isotropic abH values. In contrast, the a hfc tensor is generally quite anisotropic.* However, S t e r n l i ~ h has t ~ ~shown that for planar organic radicals in randomly oriented, nonreorienting (powder) samples, the splittings observed from a hfc are approximately equal to the isotropic hfc constants (amH)and the lines are fairly symmetrical. Thus, anisotropies introduced by the hfc tensors should be relatively small in our spectra. Our goal, of course, is to relate the splittings we observe in our triplet spectra to spin densities, as is done for radical spectra. There is, however, an important difference between biradicals and simple radicals. A typical magnetic nucleus in a localized biradical will effectively interact with only one of the two spins of the triplet. This is always true for a protons and will be true for p protons

A. Benjamin: New York, 1966; Chapter 5.

(40) Snyder, L. C.; Amos, T. J . Chem. Phys. 1965, 42, 3670-3683. (41) Muller, J.-F.; Muller, D.; Dewey, H. J.; Michl, J. J . Am. Chem. SOC. 1978, 100, 1629-1630. Yamaguchi, T.; Irie, M.; Yoshida, H . Chem. Lett. 1973, 975-978. Grivit, Ph. Mol. Phys. 1970, 19, 389-398. Dowd, P.; Chang, W.; Paik, Y. H. J . Am. Chem. SOC. 1986, 108, 7416-7417. Jain, R.; McElwee-White, L.; Dougherty, D. A. J . Am. Chem. SOC.1988, /IO, 552-560. Wagner, P. J.; May, M. J. Chem. Phys. Lett. 1976, 39. 350-352.

(42) See, for example: Claesson, 0.;Lund, A,; Gillbro, 7.; Ichikawa, T.; Edlund, 0.;Yoshida, H. J . Chem. Phys. 1980, 72, 1463-1470. (43) Fischer, H . I n Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. 2, pp 435-491. (44) Morton, J R. Chem. Rev. 1964, 6 4 , 453-471. (45) Sternlicht, H . J . Chem. Phys. 1960, 33, 1128-1132.

J . A m . Chem. Soc., Vol. 110, No. 5, 1988 1361

Cyclobutanediyls

Gauv

Gau-i

Figure 5. Simulated absorbance spectra in the Ams = 2 region of I-Me (a) and 1-Vin (b) in the absence of hyperfine coupling. Table 11. Hyperfine Simulation Parameters biradical a ~ u " ~ b

l-CD, 1-EV

3 (2D), 1.5 (4D) vinyl: 2 ( l H ) , 7 (2H) ethyl: 5 (1 H), 21 ( 1 H) 1-Ph 22.5 3 (4 H; ortho); 3 ( 2 H; para); 0 . 5 (4 H; meta) I-Vin 19 2 ( 2 H), 7 (4 H) 1.5 (4 D) 1-Vin-d, 19 "In gauss. bCoupling constants rounded to the nearest 0 . 5 G 32 23

in the R groups of 1. When electron-exchange terms are much greater than the hyperfine interaction, nuclei of this type are expected to produce couplings which are half those seen in analogous free radical^.^^-^* For example, in allyl radical, the magnitudes of the a couplings are ca. 14 G for the terminal (CH,) hydrogens and ca. 4 G for the central hydrogen. On the basis of the above arguments, one would expect the analogous hydrogens in 1-Vin to produce 7- and 2-G splittings, respectively, and this is what is observed (see below). The exception to this rule is the set of @ hydrogens that are on the four-membered ring of 1. These can couple directly to both radical centers, and so the splittings that arise from them should be of the same magnitude as analogous splittings in simple radicals. As described below our simulations fully support this analysis. There is another source of anisotropy in these triplet spectra, namely, the intrinsic orientational anisotropy of the Ams = 2 t r a n ~ i t i o n . ~Just ' as there are, in effect, two observable Am, = 1 transitions for each of the three canonical orientations of a triplet, so there is one Am, = 2 transition for each canonical orientation. In addition, a fourth Am, = 2 transition, termed H,,,, occurs at lower field than the other three (H,, H,,and H,). The positions of these transitions can be calculated easily from the zfs parame t e r ~ .In~ addition, ~ the intensities of these transitions decrease through the series H,,,, H,,H y , H , (Figure 5 ) . Thus, the largest contributions to the line shape come from resonances at H,,,. H,, and Hy. From Figure 5, it is clear that for biradicals with relatively small D values (e.g., I-Vin and 1-Ph) the separation between H,,, and H, is on the order of the spectral line width (5-6 G). Thus, the spectra are very nearly isotropic, and simple splitting patterns can be observed. In contrast, biradicals with large D values ( e g , 1-CD3)have a large separation between H,, and H , (30 G), and as a result the line shape for the Am, = 2 transition should be broad and unsymmetrical, even in the absence of any hfc. If there is hfc, it is superimposed onto the patterns of Figure 5. Quantitative interpretation of the hyperfine splitting patterns of Figure 4 required spectral simulation. Our program, based on the algorithm of Kottis and Lefebvre," assumes isotropic hfc and equal transition probabilities for all orientations. The simulations explicitly include the orientational anisotropy of the Ams = 2 transition. The resulting calculated spectra are shown for the five cyclobutanediyls which exhibit interpretable hyperfine in Figure 4. The agreement between experiment and theory is quite satisfactory, given the approximations involved. In Table (46) McConnell, H. M. J . Chem. Phys. 1960, 33. 1 1 5-1 21. (47) Reitz, D. C.; Weissman, S. I. J . Chem. Phys. 1960, 33, 700-704. (48) Glarum, S . H.; Marshall, J. H. J . G e m . Phys. 1967,47, 1374-1378.

0.5

0.6

0.7

0.8

0.9

1.0

1.1

SPIN DENSITY

Figure 6 . Correlation of u B for ~ 1 with spin densities according to eq 13, using the coupling constants of Table 11. 1-EV is not included (see text).

I1 we list the coupling constants used in the simulations. Two of the biradicals, 1-Ph and 1-Vin-d,, display well-defined five-line patterns in their half-field transitions, which may be attributed to the dominant hyperfine interaction with the four p hydrogens of the cyclobutanediyl ring. But despite the apparent simplicity of the patterns, simulations show that the @-hyperfine interaction alone does not account for the observed spectrum. Accurate simulations required the inclusion of the smaller couplings to other nuclei. In addition, the simulation of l-Vin-d6 is a statistically weighted average over the d2 through d6 compounds present in the sample due to incomplete deuteriation in the synthesis. Due to the dominant @-hyperfineinteraction, 1-Ph and 1-Vin-d6 also exhibit hyperfine splitting patterns in the Am, = 1 transitions that are identical with those in the Ams = 2 region, though they are less well resolved. With the data from l-Vin-d6 in hand, the simulation for 1-Vin was straightforward. For both 1-Ph and 1-Vin the a couplings observed in the substituents are as expected-they are half the values seen for analogous protons in the benzyl and allyl radicals. In 1-CD,, a simple five-line pattern as for 1-Ph is not observed because of the anisotropy of the Am, = 2 transition. The simulation, though, treats this spectrum well, although the line broadening required to achieve a good fit was much greater for 1-CD3 than for other spectra. An important feature of these spectra is the fact that the ring C H , protons of a given biradical all have identical coupling constants. Given the dependence of on 6 this requires that the cyclobutanediyl ring is planar in all these structures. This is the first structural information of this kind for a localized biradical and is in accord with our theoretical prediction^.^' The observation of equivalent couplings would also be consistent with a pair of rapidly interconverting nonplanar forms, but this would seem to be unlikely at 3.8 K. Also, the intrinsic broadness of triplet EPR lines introduces some uncertainty, in that small differences in hfc constants would be absorbed into the line widths. Our simulations, though, indicate that differences as small as 1.0 G, implying differences in 6 of ca. 2O, could be detected in the spectra of 1-Ph. The uncertainty is greater in a more complex spectrum, such as that from 1-CD,. Thus, it is perhaps best to say that cyclobutanediyls are planar, or nearly so. Note that in the biradical precursors (azoalkanes 3) the cyclobutane moiety is very highly puckered (flap angle ca. 58'). Clearly, a considerable structural reorganization occurs within the matrix site after N, loss from 3. Apparently, the restraining forces of the matrix material are not overwhelming, an observation that is quite relevant to efforts to characterize the matrix decay kinetics of such biradicals.49 As indicated in eq 13, the values of aBHfor the ring protons provide another gauge of spin density at the cyclobutanediyl radical carbons. It is apparent from Table I1 that aBHdoes decrease with increasing delocalization, just as the zfs D value does. We have only three accurately determined values for aBH,but if we plot them according to eq 13, the result of Figure 6 is obtained. The intercept is 1.2 G, a quite reasonable value for A . The slope is

1362 J . Am. Chem. SOC.,Vol. 110, No. 5, 1988 30.9 G. If we set C = 42.5 G, the value of 0 is 31', which is in good agreement with our theoretically determined37value of 27'. Of course, three data points a r e hardly enough to provide an accurate determination of 8. However, the results summarized in Figure 6 again show that the spectral data we have obtained for the cyclobutanediyls agree both in absolute magnitude and in trends along the series with expectations based upon analogies to conventional radical chemistry. For example, the 32-G value of upH seen for l-CD3 is in good agreement with an expected value of 34 G derived from measurements on the cyclobutyl radical.50 These results indicate that hfc in triplet spectra could be a more widely applicable tool than had previously been anticipated. For example, splittings are ciearly observable in the Am, = 2 transition for Closs' biradical Z9Based on our results, reasonable estimates of uH values would be 32 G for the C 2 @ protons, 16 G (i.e., 32 + 2) for the C 4 and C5 p protons, and 12 G (half of 24 G, a common value for uaH) for C1 and C 3 a protons. Using these values, the essential features of the spectrum are well reproduced. The level of complexity in the hyperfine patterns of triplet spectra that can be interpreted is limited. For example, only a broad featureless Ams = 2 line is observed for 1-Me, and the pattern for 1 - M P (Figure 4) has not yielded to simulation. This could be a consequence of multiple conformations for the methyl groups. Remembering the angle dependence of eq 13, this could greatly complicate the spectra. Even if the methyls are rapidly rotating a t 4 K (perhaps via tunneling), the number of lines they would produce, superimposed on the quintet from the ring p protons, would create a poorly resolved spectrum. We have been able to simulate the spectrum for 1-EV, but the simulations required five large couplings, instead of the usual four due to the ring protons. This could indicate that one of the CH2 protons of the ethyl group has 0 = Oo, which, after application of the factor of one-half, would produce a coupling similar to the ring protons. This uncertainty, coupled with the fact that it is not completely clear what value of p should be used for the ring protons, is the reason that the value of upH for 1-EV was not included in the fit of Figure 6. If it is included, the line obtained does not change significantly.

Discussion W e have found that a wide range of triplet cyclobutanediyls

(1) can be prepared and directly observed by E P R spectroscopy. Most importantly, delocalizing substituents such as phenyl and vinyl are tolerated. In fact, they increase biradical stability.49 It would thus appear that the triplet cyclobutanediyl framework is more tolerant to substituent perturbations than that of cyclopentanediyl (2), although the substitutional studies on 2 were limited. W e propose two factors that could contribute to this difference. First, theory predicted some time ago7b(in fact, long before the observation of 1-Me) that the intrinsic preference for a triplet ground state is roughly twice as large for 1-H as for 2 (1.7 vs 0.9 kcal/mol). Although we have no direct data on the ground spin state, the fact that we can observe all the cyclobutanediyls at temperatures as low as 3.8 K does strongly support this prediction. A second factor is the diminished conformational flexibility of the four-membered ring vs the five-membered ring. Taken together, these two arguments suggest that perhaps in the cyclopentanediyl, relatively small substituent changes alter the conformation of the ring enough to produce a singlet ground state. Singlet-triplet gaps are well-known to be sensitive to the relative orientations of the orbitals i n ~ o l v e d and , ~ the existence of a triplet ground state appears to be a necessary criterion for success in these experiments. The E P R spectra of the cyclobutanediyls provide valuable structural information. The D value is a sensitive probe of spin distribution, and we have shown that the trends in the data can be quantitatively modeled by using a relatively simple scheme for zfs calculations. It is, however, necessary to use wave functions that include the spin polarization effects known to strongly influence the spin distributions in delocalized radicals and biradicals. In the present case, this could be done by using "experimental" spin densities determined for allyl and benzyl.

Jain et ul. Similarly, hfc to the ring CH, protons was observed to be quantitatively correlated to the spin densities a t the ring radical centers. Importantly, the hfc shows that all the cyclobutanediyls observed thus far are effectively planar. The present work establishes a foundation for the systematic study of the effects of substituents on the spectroscopy and reactivity of localized 1,3-biradicals. We are intensively investigating the matrix isolation reactivities of the hydrocarbon biradicals described herein and will report the results shortly.49 In addition, we are adapting the synthetic protocols developed to a wide array of new structures that will include polar and heavy-atom substituents. These should provide further insights into the fundamental nature of simple biradicals.

Experimental Section General. 'H NMR spectra were recorded on a Varian EM-390 spectrometer. Fourier transform NMR spectra ('H and "C) were recorded on a Varian XL-200, JEOL FX-90Q, or JEOL GX-400 spectrometer. An INEFTNONs' pulse sequence was employed to determine true carbon-hydrogen coupling constants. Ultraviolet spectra were recorded on a Hewlett-Packard 845 1A diode array spectrophotometer. Infrared spectra were recorded on a Shimadzu IR-435 or a Mattson Instruments Sirius 100 IT-IR spectrometer fitted with a Starlab minicomputer. Mass spectra were obtained by the Caltech Analytical Facility and the U.C. Riverside Analytical Facility. Analytical gas chromatography was performed on a Hewlett-Packard 5840A chromatograph equipped with a flame-ionization detector. Preparative gas chromatography was performed on a Varian Aerograph Model 920 chromatograph with a thermal conductivity detector. High-pressure liquid chromatog raphy was performed with a Perkin-Elmer Series 2 liquid chromatograph. Melting points are uncorrected. Tetrahydrofuran was distilled from benzophenone ketyl prior to use. All other solvents were reagent grade and used as purchased unless otherwise indicated. Column chromatography was performed by the method of Still'2 employing 230-400 mesh silica gel. 1-Methyl-3-phenylcyclohutanol( 6 ) . A solution of 3-phenylcyclobutanone'* (200 mg, 1.37 mmol) in 3 mL of dry ether was added dropwise to an ice-cold ether solution of methyllithium (1.3 mL, 2.0 mmol) over 20 min. After stirring at room temperature for 20 min, the reaction was quenched by addition of water (ca. 10 mL). The ether solution was dried over MgS04,concentrated, and purified by flash chromatography (7:3 petroleum ether-ethyl acetate) to give 195 mg (88%) of a diastereomeric mixture (83:17 syn:anti) of the alcohols as a colorless oil. Syn isomer: 'H NMR (CDCI,) 6 1.50 (s, 3 H), 2.00 (bs, 1 H), 2.22 (m, 2 H), 2.51 (m, 2 H), 3.06 (quintet, 1 H, J = 8.8 Hz), 7.27 (m, 5 H); I3C NMR (CDCIJ 6 27.33, 30.24, 45.52, 68.96, 125.70, 126.31, 128.01, 128.05. Anti isomer: ' H NMR (CDCI,) 6 1.36 (s, 3 H), 1.69 (bs, 1 H), 2.25 (m, 2 H), 2.50 (m, 2 H), 3.75 (quintet, 1 H, J = 8.5 Hz), 7.24 (in, 5 H). 1-Bromo-1-methyl-3-phenylcyclobutane (7). To a solution of 6 ( 1 12 mg, 0.691 mmol) in 6.5 mL of dry acetonitrile under argon were added triphenylphosphine (615 mg, 2.35 mmol) and carbon tetrabromide (780 mg, 2.35 mmol). After stirring at room temperature for 16 h, the mixture was filtered and concentrated to give a yellow oil. Careful flash chromatography (petroleum ether-benzene, 0-5%, gradient) gave 80 mg (45%) of 7 as a mixture of diastereomers (65:35 anti:syn). Anti isomer: ' H NMR (CDCI,) 6 1.90 (s, 3 H), 2.45 (m, 2 H), 3.0 (m, 2 H), 4.05 (quintet, 1 H, J = 8.5 Hz), 7.3 (m, 5 H). Syn isomer: 'H NMR (CDCI,) 2.10 (s, 3 H), 2.90 (m, 2 H), 3.0 (m, 2 H), 3.45 (quintet, 1 H , J = 8.5 Hz), 7.3 (m, 5 H). 1,3-LXbromo-l-methyl-3-phenylcyclobutane(8). A solution containing 7 (72 mg, 0.32 mmol), N-bromosuccinimide (57 mg, 0.32 mmol), and dibenzoyl peroxide (3 mg) in 3 mL of carbon tetrachloride was allowed to reflux under argon until the NBS had been consumed (1.5 h). The cooled solution was filtered and concentrated to afford 97 mg of a 1:l diastereomeric mixture of 8 as a viscous oil. Syn isomer: 'H NMR (CDCIj) 6 1.60 (s, 3 H), 3.5 (m, 2 H), 3.8 (m, 2 H), 7.4 (m, 5 H). Anti isomer: 'H NMR (CDCl,) 6 2.30 (s, 3 H), 3.5 (m, 2 H), 3.8 (m, 2 H), 7.4 (m, 5 H). (49)

Sponsler, M . B.; Jain, R.; Coms, F. D.; Dougherty, D. A,, manuscript

in preparation.

(50) This value was obtained by taking the observed upH for cyclobutyl (37 G;Fessenden, R. W.; Schuler, R. H. J . Chem. Phys. 1963,39,2147-2195) and scaling by 0.919, the correction for substitution by CH,. See ref 32, p 121. (51) Morris, G. A.; Freeman, R. J . Am. Chem. SOC.1979, 101, 760-762. Benn, R.; Giinther, H. Angew. Chem., Int. Ed. Engl. 1983, 22, 350-380. (52) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978,43, 2923-2925.

Cyclobutanediyls l-Methyl-lpbenylbicyci~l.l.O]butsne(9). To a solution of dibromide 8 (95 mg, 0.31 mmol) in 2 mL of tetrahydrofuran was added 1.5 g of freshly pulverized 2% lithium amalgam.s3 The heterogeneous mixture was allowed to stir at room temperature under argon for 12 h. After the dark gray slurry had been filtered through Celite, tetrahydrofuran (3 mL), CH2C12(10 mL), and 5% sodium bicarbonate were added to the filtrate and thoroughly mixed. The organic solution was dried over Na2S04and concentrated to afford 40 mg of a volatile oil, which was used immediately in the next step. 'H NMR (CDCI,) ii 1.20 (s, 2 H), 1.35 (s, 3 H), 2.05 (s, 2 H), 7.25 (m, 5 H); I3C NMR (CDCI,) 6 11.60 (q, J = 127 Hz), 16.02 (s), 20.08 (s), 34.60 (t, J = 161 Hz), 124.41 (d, J = 161 Hz), 125.27 (d, J = 158 Hz), 128.11 (d, J = 159 Hz), 138.33 (SI.

1,4-Dimethyl-7-phenyl-2,4,6tnazaticyclo[5.1 .1.@*61nonane-3,5-dione (5). To a solution of bicyclobutane 9 (45 mg, 0.31 mmol) in 30 mL of refluxing n-hexane under a stream of argon was added a solution of MTAD (47 mg, 0.42 mmol) in 5.5 mL of ether until the red color of MTAD persisted (ca.3.5 mL). After the hexane was removed, the crude solid was triturated with CH2CI2.The dichloromethane solution was then concentrated and purified by flash chromatography (4: 1 benzene-ethyl acetate) to afford 5 (30 mg, 37% from 7)as a white crystalline solid: mp 143.5-145.5 OC; 'H NMR (c6D6) 6 1.42 (m, 2 H), 1.57 (S, 3 H), 1.76 (m,2 H), 2.51 (s, 3 H), 7.15 (m, 5 H); I3C NMR (CDCI,) ii 16.10 (q, J = 128 Hz), 25.79 (q, J = 141 Hz), 47.87 (t, J = 145 Hz), 69.81 (s), 74.20 (s), 126.78 (d, J = 158 Hz), 128.12 (d, J = 159 Hz), 128.62 (d, J = 162 Hz), 133.18 (s), 159.20 (s), 159.74 (s); mass spectrum (EI), m / e 257, 242, 199, 185, 143 (loo), 129, 103, 91, 77; exact mass calcd for CI4Hl5N3O2 257.1 164, found 257.1167. Hydrolysis of 5. A suspension of urazole 5 (9.5 mg, 0.037 mmol)and freshly crushed KOH (19 mg, 0.3 mmol, 87%) in 2 mL of degassed 2-propanol was allowed to reflux for 10 min under argon. The cooled solution was then acidified by dropwise addition of degassed 3 N HCI until decarboxylation ceased (ca. 250 FL). The solution was then neutralized with degassed 1 N NH,. Degassed water (1 mL) was added to dissolve the salts, and then the solution was extracted with degassed CH2C12(2 X 2.5 mL). The organic solution was dried with Na2S0,, filtered, and concentrated to leave 8.5 mg of a light yellow oil. High-field 'H NMR analysis revealed that >90% of the product consisted of a 1:l mixture of semicarbazides 31-MPa and 31-MPb and