T H E J O U R N A L OF VOLUME52,NUMBER11
Organic Chemistry 0 Copyright 1987 by the American Chemical Society
MAY29, 1987
Irradiation of Tetraphenylborate Does Not Generate a Borene Anion John D. Wilkey and Gary B. Schuster* Department of Chemistry, Roger Adams Laboratory, University of Illinois, Urbana, Illinois 61801 Received February 12, 1987
In contrast to a previous claim, irradiation of sodium tetraphenylborate in acetonitrile or THF solution does not result in the formation of the diphenylborene anion. Stilbenes, formed from irradiation in the presence of diphenylacetylene and thought to have been evidence for formation of the diphenylborene anion, are shown to arise instead by an electron-transfer pathway.
Recent application of sensitive spectroscopictechniques
to the investigation of reactive intermediates has provided answers to questions that previously were difficult to resolve. Among the more fruitful areas of inquiry has been that of short-lived hypovalent intermediates such as carbenes, nitrenes, and silylenes.' These highly reactive species have been observed directly under actual reaction conditions and their chemical and physical properties have been characterized and classified. Obviously missing from this group are the hypovalent compounds of boron. It has been recognized for a long time that these compounds, variously called borene? boryne? or borylene: might have chemical and physical properties closely related to their more well-studied carbon, nitrogen, or silicon analogues.6 However, difficulties associated with uncertain preparation of borenes have caused considerable confusion. Indeed, there remains some doubt that an authentic example of this intermediate has ever been prepared in solution (vide infra). One way that the preparation of borenes has been attempted is the a-elimination of two groups bound to boron (typically halogens) with an active metal reducing agent. When this reaction was carried out in solutions containing presumed traps for the borene, in some cases, products were obtained that seemed to signal their transitory existence?*' However, this approach has been criticized since (1) Abramovitch, R. A. Reactive Intermediates; Plenum Press: New York, 1980; Vol. 1. Abramovitch, R. A. Reactive Intermediates; Plenum Press: New York, 1982; Vol. 2. Schuster, G. B. Adu. Phys. Org. Chem. 1986,23, 311. (2) van der Kerk, S. M.; Van Eekeren, A. L. M.; van der Kerk, G. J. M. J. Organornet. Chem. 1980, 190, C8. (3) Ramsey, B. G.; Anjo, D. J. Am. Chern. SOC. 1977, 99, 3182. (4) Eisch, J. J.; Becker, H. P. J. Organomet. Chern. 1979, 171, 141. (5) Nefedov, 0. M.; Manakov, M. N. Angew. Chem., Znt. Ed. Engl. 1966, 5, 1021. (6) van der Kerk, S. M.; Boersma, J.; van der Kerk, G. J. M. Tetrahedron Lett. 1976, 4765. (7) Joy, F.; Lappert, M. F.; Prokai, B. J. Organornet. Chern. 1966,5, 506.
0022-3263/87/1952-2117$01.50/0
it was shown that often the products identified as coming from the borene can arise by other route^.^ A second, potentially more general and useful (for our purposes) route for borene synthesis employed the photolysis of triarylboranes or tetraarylborates. In particular, Ramsey and Anjo claimed that irradiation of tri-lnaphthylborane gave 1-naphthylborene in 40% yield.3 However, our reinvestigation of this reaction showed that the evidence had been misinterpreted and that irradiation of this borane simply causes carbon-boron bond homolysis, which serves to initiate common free radical processes.8 A similar approach recently resulted in the successful preparation of triphenylsilylborene by irradiation of alkyl or aryl bis(triphenylsilyl)b~ranes.~ The photolysis of tetraarylborates [ (Ar,B)-] under a range of conditions was first studied by Williams and his co-workers.10 They found that irradiation of tetraphenylborate (1) in protic solvents containing dissolved oxygen led to biphenyl and diphenylborinic acid (which suffered further oxidation), eq 1. Labeling studies showed
that the two phenyl groups of the biphenyl originate from one tetraphenylborate and that the carbon-carbon bond linking the two phenyl rings in this product is formed from carbons that were formerly bound to boron. This, and other evidence, led to the postulation of a mechanism that (8) Calhoun, G. C.; Schuster, G. B. J. Org. Chem. 1984, 49, 1925. (9) Pachaly, B.; West, R. Angew. Chem., Znt. Ed. Engl. 1984,23,454. (10) (a) Williams, J. L. R.; Doty, J. C.; Grisdale, P. J.; Searle, R.; Regan, T. H.; Happ, G. P.; Maier, D. P. J.Am. Chem. SOC. 1967,89,5153. (b) Williams, J. L. R.; Doty, J. C.; Grisdale, P. J.; Regan, T. H.; Happ, G. P.; Maier, D. P. J. Am. Chern. SOC.1968, 90, 53. (c) Williams, J. L. R.; Grisdale, P. J.; Doty, J. C.; Glogowski, M. E.; Babb, B. E.; Maier, D. P. J. Organornet. Chern. 1968,14,53-62. (d) Grisdale, P. J.; Babb, B. E.;
Doty, J. C.; Regan, T. H.; Maier, D. P.; Williams, J. L. R. J. Organomet. Chem. 1968, 14,63. (e) Grisdale, P. J.; Williams, J. L. R.; Glogowski, M. E.; Babb, B. E. J. Org. Chern. 1971, 36, 544.
0 1987 American Chemical Society
2118
J. Org. Chem., Vol. 52, No. 11, 1987
Wilkey and Schuster
n
Scheme I
Ph
L Diphenylcarbene (as a singlet)
J
Diphenylbora t e (1) (as a singlet)
featured bridging boron intermediates and did not require the intervention of a hypovalent borene. More recently, Eisch and co-workers reported the photolysis of tetraphenylborate under an inert, oxygen-free atmosphere in ether solvents containing diphenylacetylene as a presumed trap for the borene." Protodeboronation of this reaction mixture with deuterioacetic acid gave biphenyl, hydrogen, and some dideuterated stilbene. The latter product was assumed to arise from reaction of a boracyclopropene intermediate formed from capture of a borene anion [diphenylborate(I)] by the acetylene, eq 2 and 3. Some support for this proposal comes from the
250
Results An unrecognized complication in the previous investigation of borate 1 is that it is not possible to irradiate the borate in the presence of diphenylacetylene without simultaneously irradiating the acetylene.'l The absorption spectra of these two compounds are shown in Figure 1. The extinction coefficient of tetraphenylborate at 254 nm (the reported irradiation wavelength) in acetonitrile is 4800 M-' cm-' and that of diphenylacetylene is 14000 M-l cm-l. We have found that the simultaneous photolysis of diphenylacetylene opens up an electron-transfer reaction channel that ultimately leads to the products that were attributed to reaction of the borene anion. Direct Irradiation of Tetraphenylborate. Photolysis of an oxygen-saturated acetonitrile solution of M+(Ph)4B(the results do not depend on the identity of M+, M = K, Na, R4N) a t 254 nm in a Rayonet reactor gives biphenyl in 98% yield and a light yellow solid precipitate. Analysis of the solution by llB N M R spectroscopy shows no product resonances. All of the boron-containing products are contained in the precipitate, which was shown to be diphenylborinate ([(Ph)2BO-]M+]by comparison with an authentic sample independently prepared. Evidently the borinate is protected from the further oxidation it suffers in protic solvents by its precipitation from acetonitrile. (11) Eisch, J. J.; Tamao, K.;Wilcsek, R. J. J.Am. Chem. SOC.1975, 97,895. (12) Walsh, T.D.;Powers, D.R. Tetrahedron Lett. 1970, 3855.
325
Figure 1. UV-vis absorption spectra of sodium tetraphenylborate (-, 1.9 X lo4 M) and diphenylacetylene (---, 5.5 X M) in
acetonitrile. 1.00
i-'
015
observation that irradiation of tetraphenylmethane, which is isoelectronic with tetraphenylborate, is reported to give diphenylcarbene, which is isoelectronic with the presumed borene intermediate, Scheme I.12 Also, addition of the borene to the triple bond of diphenylacetylene seems at first glance to be a reasonable extrapolation from the known chemistry of carbenes. However, our investigation requires an alternative explanation for these results.
275 300 Wavelength ( n m )
0
'
1.7 300
350
400
450
Wavelength
500 (N MI
4
550
Figure 2. UV-vis absorption s ectrum of a sodium tetraphenylborate solution (6.08 X 10- M in acetonitrile) following irradiation at 254 nm.
P
Photolysis of tetraphenylborate in acetonitrile or THF under an inert atmosphere leads to strikingly different results. No precipitate forms, but the solution turns red. The UV-vis spectrum of this solution, shown in Figure 2, reveals two new features: an intense UV band centered at 315 nm and a broad, much weaker visible band at 445 nm. When oxygen is added to this solution, both bands are quenched and biphenyl is isolated in quantitative yield. The same absorbing species is created when borate 1 is irradiated at 266 nm with the output of a Q-switched Nd:YAG laser. Under these conditions this product is formed within the 20-ns rise time of the laser pulse. This indicates its probable unimolecular origin. If a portion of the freshly irradiated, solution of borate 1 is transferred under nitrogen to an NMR tube, the llB NMR spectrum, shown in Figure 3, reveals a single boron-containing product resonance at -27.3 ppm (-27.6 when the irradiation and NMR spectral analysis are performed in THF). This intermediate reacts with oxygen to give a yellow precipitate of diphenylborinate identified above. Acting on the assumption that the detected intermediate could be the borene anion identified by Eisch, we examined its reaction with several trapping agents. The intermediate does not react with diphenylacetylene. Addition of a large excess of the acetylene to the red solution does not discharge the color and does not lead to any change in the llB resonance at -27.3 ppm. Nor does the intermediate react with other reagents known to com-
J. Org. Chem., Vol. 52, No. 11, 1987 2119
Irradiation of Tetraphenylborate 1
a
0
-5
-io
-is
-25
-20
-50
P P ~
b
I
~ . . , . . . . , . . . . , . . . . , . . . . , . . . . ( . . . . , . . . . , . 1 . . , . . . . , . . . . ,
6 0
5 5
S O
1 5
1 0
3 5
3 0
1 5
1 0
I 5
I O
Figure 3. (a) 80-MHz llB NMR spectrum of an acetonitrile solution of sodium tetraphenylborate (5.65 X M) following irradiation at 254 nm. The broad resonances are due to llB contained in the sample tube. (b) 300-MHz 'H NMR spectrum M) of a THF-$ solution of sodium tetraphenylborate (2.8 X following irradiation at 254 nm. Resonances labeled with an X are due to impurities present in the solvent prior to irradiation. For assignment of resonances, see structure 6.
bine rapidly with diphenylcarbene. However, addition of acetic acid leads to immediate conversion of the intermedaite with llB resonance at -27.3 ppm to phenylboronic acid [PhB(OH),, 28.3 ppm] and boric acid [19.1 ppm]. Quenching the intermediate with acetic acid also generates some biphenyl and a new product identified as 1phenyl-1,4-cyclohexadiene(a dihydrobiphenyl), eq 4. If
1-
Ph
the acetic acid is present in the reaction mixture during the irradiation, the intermediate is not detected by UV or by llB NMR spectroscopy and the dihydrobiphenyl is the major organic product. When acetic acid-0-d is substituted for the unlabeled acid in this experiment, both the biphenyl and the dihydrobiphenyl incorporate some deuterium. Analysis by lH NMR spectroscopy of the photolysis solution that exhibits the -27.3 ppm llB NMR resonance reveals five one-proton multiplets with chemical shifts in the aliphatic and olefinic regions. This group of proton signals shows the same sensitivity to oxygen and acetic acid as do the llB and UV signals. This spectrum, shown in Figure 3, and a series of decoupling experiments are described completely in the Experimental Section. These NMR data are consistent with assignment of a bridgedborate structure, like those considered previously by Williams and co-workers, to the product formed from photolysis of tetraphenylborate in acetonitrile or T H F under an inert atmosphere. The postulation of such an intermediate can explain the chemical and spectroscopic observations we have made about the direct irradiation of
tetraphenylborate. We will present a proposal for the specific structure of this bridged intermediate in the Discussion section. Quite significantly, analysis of the products formed from careful photolysis of tetraphenylborate in rigorously degassed acetonitrile or THF solution by lH NMR spectroscopy reveals that biphenyl is not among them. The biphenyl that is eventually isolated from this reaction is formed only after oxygen is admitted to the photolysis solution. Of course, direct formation of the borene anion in the photolysis requires the simultaneous cogeneration of biphenyl. We sought an explanation for the reported1' generation of hydrogen when acetic acid is added to photolyzed solutions of tetraphenylborate. We find that irradiation of the primary photoproduct in THF solution with 350-nm light (where tetraphenylborate does not adsorb) results in generation of two borohydrides [llB NMR 6 -9.0, (d, JBH = 76 Hz), -16.4 (t, JBH= 74 Hz] and biphenyl. Prolonged irradiation of tetraphenylborate at 254 nm eventually gives the same borohydrides, evidently by secondary irradiation of the primary product. These borohydrides, of course, will generate hydrogen when exposed to acetic acid. Finally, we investigated the photolysis of benzyl(tripheny1)borate (2) in an attempt to distinguish dissociative carbon-boron bond cleavage to form radicals from processes that do not involve bond homolysis in the product-determining step. This experiment derives from the assumption that bond cleavage from the excited state of borate 2 will give the benzyl radical rather than the much less stable phenyl radical. If the benzyl radical is formed, the expected product, by analogy with biphenyl formation from tetraphenylborate, is diphenylmethane. Irradiation of borate 2 in deoxygenated acetonitrile solution at 254 nm gives biphenyl in 82% yield and a small amount of toluene. Diphenylmethane cannot be detected by gas chromatographic analysis, eq 5. This finding supports postulation of a product-determining step for the direct photolysis of these borates that is insensitive to the radical stability of the migrating group. IPhCH2-B(Ph)3] 2
hv
+
PhCHl
15)
Electron-Transfer-Initiated Photoreactions of Tetraphenylborate. I t has been known for a long time that tetraarylborates are easily oxidized compounds. In 1959 Geske reported that electrochemical oxidation of tetraphenylborate gives biphenyl in good yield.13 Spin trapping experiments following electrochemical oxidation failed to detect any phenyl radical even though the n-butyl radical was easily detected from a similar oxidation of tetra-n-b~ty1borate.l~Labeling studies showed that the two phenyl groups of the biphenyl formed by electrochemical oxidation come from the same borate and that the bond linking the benzene rings is formed between carbons that had been bound to boron.15 The properties and characteristics of singlet excited 1,4-dicyanonaphthalene (DCN*') and its radical anion (DCN'-) make it an ideal probe for the involvement of photoinitiated electron-transfer reactions.16 The fluorescence of DCN is quenched with a rate constant of 1.8 X 10'O M-'s-l in acetonitrile (the diffusion limit) by tetraphenylborate. This is not a surprise since AG,, for this process is calculated to be -34 kcal/mol. When this (13) Geske, D. H. J. Phys. Chem. 1959,63,1062. (14) Bancroft, E.E.; Blount, H. N.: Janzen, E. G. J . Am. Chem. SOC.
1979,101, 3692. (15) Geske, D. H. J . Phys. Chem. 1962,66,1743. (16)Peacock, N.J.; Schuster, G. B. J. Am. Chem. SOC.1983,105,3632.
2120 J. Org. Chem., Vol. 52, No. 11, 1987
reaction is initiated by irradiation with a nitrogen laser (337 nm, 13 ns,7 mJ, only DCN absorbs the light), the transient absorption spectrum recorded ca. 200 ns after the laser pulse shows bands at 390 and 510 nm. These features are characteristic of DCN'-. There are no additional absorption bands in the observable regions of the spectrum that might be assigned to a boranyl radical [(Ph)4B']'7 or to some other product derived from oxidation of the tetraphenylborate. To insure that the boranyl radical spectrum is not being obscured by that of DCN'-, the experiment was repeated with 1-cyanopyrene as the electron acceptor. In this case, only the spectrum of the cyanopyrene radical anion at 495 nm is observed. The same results are obtained when THF is substituted for acetonitrile as the solvent for this experiment. Biphenyl is formed in nearly quantitative yield from the DCN-sensitized electron-transfer reaction of tetraphenylborate. Crossover experiments employing a 1:1 mixture of borate-h2, and borate-d,, show that each molecule of biphenyl comes from one molecule of the borate. Reaction of tetra-p-tolylborate under these conditions gives only 4,4'-dimethylbiphenyl. This shows that the bond between the rings of these biphenyls are formed from carbons that were bound to boron. Significantly, the appearance of biphenyl in the reactions of tetraphenylborate initiated by photolysis of DCN does not require the addition of oxygen to the reaction mixture. Under these conditions biphenyl is a primary product. Also, analysis of the reaction mixture by llB NMR spectroscopy reveals none of the intermediate absorbing at -27 ppm that is formed on direct irradiation. The evidence indicates that the photoinitiated electron-transfer reaction is proceeding by a mechanism similar to that for the electrochemical process. The key observation supporting the involvement of a borene anion in the earlier investigation of tetraphenylborate photolysis was the formation of stilbene following protodeboronation of reaction mixtures containing diphenylacetylene." We find that irradiation of an oxygen-free THF solution containing tetraphenylborate (0.1 M) and diphenylacetylene (0.03 M) at 350 nm (only the acetylene absorbs light!) gives, after protodeboronation, biphenyl (17% based on consumed borate) and cis-stilbene (10% based on consumed acetylene). Irradiation of a similar solution at 254 nm (both compounds absorb light) produces cis- and trans-stilbenes in a total yield of 20%. Boron-11 NMR spectroscopic investigation of such solutions following irradiation at 254 nm or 350 nm does not reveal any new features which could possibly be assigned to the key boracyclopropene intermediate required to support the claim of borene anion formation. The boron-containing product isolated from these reaction is diphenylborinate. The production of biphenyl in the absence of oxygen from photolyses in which only diphenylacetylene absorbs light is indicative of the operation of the electron-transfer mechanism. Calculation from the known redox potentials and the acetylene excited state energy shows that AG,, for this process is -24 kcal/mol and thus electron transfer is expected to be quite rapid. Discussion There are two major issues to be addressed. The first is a description of the reaction mechanisms for the direct and electron-transfer-initiated photoreaction of tetraphenylborate. The second is an analysis of the claim that (17) Horii, H.;Taniguchi, S. J . Chem. SOC.,Chem. Comrnun. 1986,915.
Wilkey and Schuster direct irradiation of this borate gives a borene anion. The direct irradiation of tetraphenylborate in inert (acetonitrile or THF) solvent leads to a single boron-containing primary product. Our attempts to isolate this compound from solution were not successful. Thus we are forced to rely on chemical and spectroscopic probes to assign its structure. However, there are several significant results that make the assignment of a structure of this compound possible. The boron-containing photoproduct is the precursor to biphenyl in the absence of acetic acid and to the dihydrobiphenyl when acetic acid is present. Also, in the primary direct irradiation, biphenyl is not formed until after oxygen is admitted to the solution. Since the eventual boron-containing product is the diphenylborinate, it is reasonable to conclude that this primary product has not lost any phenyl groups and must therefore be an isomer of tetraphenylborate. The oberved 'lB chemical shift of this product at -27.3 ppm indicates that the boron atom is still bound as a tetracoordinate, anionic complex. These considerations lead to a family of possible isomeric compounds for the boron-containing photoproduct that is typified by structures 3 and 4.
j ]
-
p@ph
4
1-
Reaction of oxygen with either 3 or 4 can lead eventually to biphenyl and the diphenylborinate. Similarly, it is clear that protodeboronation of either 3 or 4 will give a dihydrobiphenyl. Both sets of compounds will incorporate deuterium from acetic acid-0-d during the latter reaction. However, the 'H NMR spectrum of the primary product is consistent only with structures related to 3. There are three isomeric possibilities for compounds based on general structure 3. These structures, 5 through 7, are the 1-,2-, and 3-phenyl-substituted isomers, re-
spectively. The lH NMR spectrum of the photolysis product shows a ratio of three vinyl hydrogens to two alkyl hydrogens. This finding eliminates structure 5, and the splitting patterns for the vinyl hydrogens is consistent only with 6. Homonuclear decoupling experiments, described in the Experimental Section, corroborate this assignment. The intermediate formation of bridged intermediates related to 6 was suggested also by Williams and co-workers on the basis of isotope labeling studies.1° Our findings are fully consistent with their proposal. The mechanism we favor for the photolysis of tetraarylborates is outlined in Scheme 11. It differs from the earlier suggestion by postulating 1,3-bonding of two phenyl groups in a di-rmethane sense rather than carbon-boron bond homolysis to form a phenyl radical as the initial step. The experiment with borate 2 rules the latter route less likely.ls (18)It is possible that other isomers of 6 (particularly a cyclohexadienyl anion, or a boracycloheptatrieneanion) exist in equilibrium. This may account for the visible absorption of the photolysis s o l ~ t i o n . ' ~ However, the I'B and 'H Nh4R spectroscopic results indicate that, if such an equilibrium does exist, it greatly favors 6 since no other characteristic signals are observed.
J. Org. Chem., Vol. 52, No. 11, 1987
Irradiation of Tetraphenylborate
2121
Scheme IV
Scheme I1
r
L
6
Ph,EONa
".I-,
u
@+)
+ Ph,EONa
u
+ Ph,BONa
'QPh
D
0)
H
Scheme I11 r
-I-
b
r
8
There are some remaining uncertainties of detail about this proposed mechanism. However, a key point is that it does not include the previously suggested formation of the borene anion. There are no experimental observations that require or support such a postulate. The electron-transfer-initiated reactions of borate 1 follow a somewhat different route to the same final products. Under these conditions we are not able to identify any intermediate boron-containing compounds. In particular, the IlB NMR spectrum of the reaction mixture does not show any resonance at -27.3 ppm. The mechanism we suggest for this reaction is shown in Scheme 111. Some of these steps are well-established by the data, others are less certain. The initial interaction between the excited electron acceptor (EA*) and the borate is a single electron transfer. (19)Gourdenne, A.; Sigwah, P. Eur. Poly. J . 1967, 3, 481.
This generates the reduced electron acceptor (EA'-), detected spectroscopically, and the oxidized borate. , Migration of a phenyl group, perhaps as an in-cage radical pair as suggested by Williams and co-workers for the direct irradiation, to an a-carbon in the oxidized borate has close analogy in many other organoboron reactions. This process generates a substituted cyclohexadienyl radical that will have a lifetime sufficiently long to react with the reduced acceptor. The intermediate formed in this reaction is the source of the eventually isolated products. We note here also that the results are accommodated without the involvement of a borene anion. The critical observation that led to postulation of the borene anion in the first place was formation of stilbene when tetraphenylborate and diphenylacetylene were irradiated. Stilbene was presumed to be formed by protodeboronation of a metastable intermediate boracyclopropene. The IIB NMR spectroscopic results clearly rule out formation of any such intermediate. The recognition that photolysis of solutions of the borate-containing diphenylacetylene initiates an electron-transfer sequence provides another, quite reasonable route to stilbene, Scheme IV. The key step in the mechanism proposed in Scheme IV is the combination of the rearranged boranyl radical 8 with diphenylacetylene radical anion to give biphenyl and the boron-substituted vinyl anion 9. Reactions such as this are well-precedented from the work of Davies and coworkers who studied the closely related SH2reactions of organoboranes.20 Of course, 9 will be rapidly protonated by acetic acid and will eventually give stilbene. The exclusive formation of the cis isomer from this intermediate is not unreasonable and has already been suggested.21 The relevant point of course is that the experimental justification previously used to support formation of the diphenylborene anion is rendered invalid by these observations.
Conclusion We undertook this investigation with the hope of directly detecting the diphenylborene anion and comparing its reactivity with that of diphenylcarbene. Instead, we are forced to the conclusion that the previous claim for the preparation of this intermediate is without justification. There are no experimental observations at all that suggest borene anion involvement in the photochemical reactions of tetraphenylborate. We are continuing in our efforts to prepare a boron analogue of a carbene. (20) Davies, A. G. Pure Appl. Chem. 1974, 39, 497. (21) Pelter, A. Rearr. Grnd. Ercit. State 1980, 2, 95.
2122 J. Org. Chem., Vol. 52, No. 11, 1987 Experimental Section General. Diethyl ether and tetrahydrofuran were distilled from Na/benzophenone immediately prior to use. Acetonitrile was distilled from calcium hydride immediately prior to use. Glassware was oven-dried overnight and cooled under a stream of dry nitrogen. Proton magnetic resonance ('H NMR) spectra were recorded on a Varian Assoc. XL-200 or a General Electric QE-300 FT-NMR spectrometer. Boron-11 NMR spectra were recorded on the University of Illinois MSL 250 MHz FT-NMR spectrometer. UV-vis absorption spectra were recorded on a Perkin-Elmer Model 552 spectrophotometer. Fluorescencespectra were recorded on a Farrand Mark I spectrofluorometer. Product yields were determined on a Varian Model 3700 gas chromatograph equipped with a Hewlett-Packard Model 3390A integrator. Reverse-phase HPLC was performed on a Perkin-Elmer Series 2 liquid chromatograph with a LC-75 spectrophotometric detector using a 25-cm Alltech CN column from Alltech Associates and 30:70 acetonitrile/water as eluant. Tetramethylammonium Tetraphenylborate. The preparation of N(CH3),BPh4 was modeled after the procedure of Reynard and co-workers.22 Diphenylchloroborane (0.31 g, 1.5 mmol) in 7 mL of diethyl ether was added dropwise to a stirred solution of phenylmagnesium bromide (3.1 mmol) in ether and the resulting mixture was heated to reflux. Following the removal of the ether, water (10 mL) and Li2C03(0.12 g, 1.6 mmol) were added, and the reaction mixture was filtered to remove MgCOB. The aqueous solution was saturated with solid N(CH3),Br, and the product was isolated by filtration. Recrystallization from acetonitrile/water gave 0.35 g (60%) of N(CH3),BPh4: 'H NMR (CD3CN) 6 3.02 (s, 12 H), 6.84 (t, 4 H) 6.99 (t, 8 H), 7.27 (m, 8 H); liB NMR (CH,CN) 6 -6.75 (s). Anal. Calcd for C28H32BN: C, 85.49; H, 8.20; N, 3.56. Found: C, 85.31; H, 8.38; N, 3.55. Sodium Tetra-p -tolyborate. The procedure for the preparation of sodium tetra-p-tolyborate is similar to that of Williams and co-workers.lOaA solution of p-tolyllithium, prepared from p-bromotoluene (1.80 g, 10.5 mmol) and n-butyllithium (10.6 mmol) in 6 mL of diethyl ether, was added dropwise to a stirred solution of BF3.Eh0 (0.38 g, 2.6 mmol) in 5 mL of ether and the mixture heated at reflux for 1h. The reaction mixture was poured into water (25 mL) and washed with ligroin (2 X 45 mL). The aqueous layer was saturated with solid NaCl and the product isolated by filtration. Recrystallization from chloroform/dimethoxyethane gave 0.082 g (8%) of N&@-C6H&H3)4: 'H NMR (CD,CN) 6 2.18 (s, 1 2 H), 6.81 (d, J = 8 Hz, 8 H), 7.10 (m, 8 H); IIB NMR (CH3CN) 6 -7.43 (s);XCHaCNmsx 270 nm (log 3.5), 279 (3.4). Anal. Calcd for NaCzsHzsB: C, 84.43; H, 7.08. Found: C, 84.09; H, 7.28. Sodium Tetraphenylborate-dzP The preparation of NaBPh,-d,, was modeled after the procedure of Wittig and Raff.23 To bromobenzene-d, (1.9 g, 11.6 mmol) in 10 mL of diethyl ether was added dropwise n-butyllithium (12.1 mmol) in hexanes, and the solution was stirred for 10 min. An ethereal solution of BF3.Eh0 (0.4 g, 2.8 mmol) was added dropwise and the mixture refluxed for 1 h. The ether was removed, and the residue was dissolved in water (30 mL) and washed with ligroin (2 X 30 mL).
Wilkey and Schuster The aqueous layer was saturated with solid NaCl and the product isolated by filtration. Recrystallizationfrom acetone/toluene gave 0.34 g (34%) of NaBPh4-dzo:llB NMR (CH3CN)6 -6.91. Anal. Calcd for NaC%D&: C, 79.55; H, 5.56. Found: C, 79.32; H, 5.77. Direct Irradiation-Typical Procedure. A 1.1 x M solution of N(CH3)4BPh, in acetonitrile was placed in a 1.0-cm quartz cell equipped with a stir bar and Teflon stopcock. The solution was purged with nitrogen for 8 min and a balloon filled with nitrogen was fitted to the cell via a septum and syringe needle. The sample was irradiated in a Rayonet photochemical reactor with 254-nm lamps while being stirred magnetically. Portions of the solution were removed by syringe and analyzed by UV-vis spectroscopy and boron-11 NMR spectroscopy. Product yields were determined by GC. Relative conversion of starting material was determined by reverse-phase HPLC. Solutions purged with oxygen were not fitted with a balloon. 'H NMR Spectrum of the Primary Photoproduct of Tetraphenylborate (6). A solution of sodium tetraphenylborate (0.028 M) in THF-d8was degassed (freeze-pump-thaw, lo4 mm, three cycles) and sealed in an NMR sample tube. Irradiation at 254 nm produced 6: 'H NMR (THF-d8)6 1.19 (m, 1 H), 1.27 (d, 1 H), 4.90 (dd, 1 H), 5.34 (d, 1 H), 6.10 (dd, 1 H) (aromatic hydrogens not visible due to overlapping signals from starting material); I'B NMR 6 -27.6 (s). Decoupling of the simple firstorder spectrum supported the structural assignment. Irradiation at 6.1 ppm collapsed the alkyl hydrogens to an AB quartet and the 4.90 resonance to a doublet. Irradiation at 4.90 ppm collapsed the 6.10 resonance to a doublet and the 5.34 resonance to a singlet. Finally, irradiation of the alkyl hydrogens collapsed the 6.10 resonance to a doublet. The following coupling constants were obtained (see structure 6): J A B = 8.1 Hz; JBc = 5.7 Hz; JCD = 8.7 Hz; J D E = 6.6 Hz. Reaction of 6 with excess acetic acid produced 1-phenyl-1,4cyclohe~adiene:~~ 'H NMR (CDC13)6 2.90 (m, 2 H), 3.08 (m, 2 H), 5.82 (m, 2 H), 6.12 (m, 1 H), 7.30-7.40 (m, 6 H); XQ'clohexmemax 247 nm. Electron-TransferSensitization-Typical Procedure. An M) and NaBPh, (9.47 acetonitrile solution of DCN (5.2 X X M) was placed in a 1.0-cm Pyrex cell and fitted as above. The sample was irradiated in a Rayonet photochemical reactor equipped with 350-nm bulbs, DCN being the only species absorbing in this region. The solutidh was analyzed as above. Irradiation of 1 and Diphenylacetylene. A THF solution of 1 (0.102 M) and diphenylacetylerie (2.89 X M) in a 1-cm Pyrex cell was purged with nitrogen for 11min and fitted as above with a balloon of nitrogen. The solution was irradiated in a Rayonet photoreactor equipped with 350-nm lamps for 2 h while being stirred magnetically. Protodeboronation with excess acetic acid followed by gas chromatographic analysis revealed the formation of cis-stilbene in 10% yield on the basis of consumed acetylene. Irradiation of an identical solution (1, 0.100 M; diphenylacetylene, 2.86 X lo-' M) in a 1-cm quartz cell at 254 nm for 4 h followed by an identical workup procedure produced cisand trans-stilbene in 20% combined yield.
Acknowledgment. This work was supported by a grant from the National Science Foundation.
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(22) Reynard, K. E,; Sherman, R. E.; Smith, H. D., Jr.; Hanstedt, L.
F.Inorg. Synth. 1973, 14, 52.
(23) Wittig, G.; Raff, P. Justus Liebigs Ann. Chem. 1951, 573, 195.
(24) Grisdale, P. J.; Regan, T. H.; Doty, J. C.; Figueras, J.; Williams, J. L. R. J. Org. Chem. 1968, 33, 1116.
