Diphenylacetylene and the lickor superbase: o,o′-Dimetalation and reaction with electrophiles. A convenient synthesis of o,o′-disubstituted diphenylacetylenes

Full text of DOI:10.1021/jo001533t

J . Org. Chem. 2001, 66, 3229-3231
3229
bene, which undergoes further ring ortho-lithiation6 if
the reaction is carried out in hexane in the presence of
TMEDA. An elegant reinvestigation by Schleyer et al.7
provided the spectroscopic and crystallographic proof of
structure. In contrast to n-butyllithium, the LICKOR
Dip h en yla cetylen e a n d th e LICKOR
Su p er ba se: o,o′-Dim eta la tion a n d Rea ction
w ith Electr op h iles. A Con ven ien t Syn th esis
of o,o′-Disu bstitu ted Dip h en yla cetylen es†
(Lochmann-Schlosser) superbase
(n-butyllithium/
J anusz Kowalik* and Laren M. Tolbert
t-BuOK)8 does not affect the triple bond in lithium
phenylacetylide, producing ortho-metalated material in
high yields and regioselectivity.9 It has been shown that
the ortho-directing and activating effects of the lithium
acetylide group are analogous although somewhat weaker
in strength to those of OCH3, CH2NMe2, SLi, sulfonyl,
and silyl or amido groups.9c,10 The nature of the resulting
dianion is not known but is assumed to involve coordina-
tion of the potassium with the aromatic ring. The
structure of that species might be as complex as that of
the LICKOR superbase itself.11
School of Chemistry and Biochemistry, Georgia Institute of
Technology, Atlanta, Georgia 30332-0400, U.S.A.
janusz.kowalik@chemistry.gatech.edu
Received October 30, 2000
The renaissance of acetylene chemistry in recent years
can be attributed in great part to research in the area of
materials chemistry.1,2 Breakthroughs in palladium-
catalyzed cross-coupling reactions involving acetylenes
opened the doors to a plethora of exotic structures,
interesting for their theoretical ramifications on, for
example, the limits to aromaticity 3 and for their practical
synthetic utility as well. In this context, the chemistry
of diphenylacetylene and its derivatives continues to
attract significant interest, as they constitute intermedi-
ate or final components of many of the technologically
important structures. Nevertheless, only a handful of
convenient synthetic methods are available for the
preparation of substituted derivatives of diphenylacety-
lene. For example, one of the most versatile derivatives
of diphenylacetylene, o,o′-diiododiphenylacetylene (2b),
is produced by halogen exchange of o,o′-dibromodiphe-
nylacetylene,4 which is itself obtained through a tedious
multistep process.5 Our direct approach is to employ the
easily available diphenylacetylene, which upon double
ortho-metalation, followed by reaction with electrophilic
agents, should result in a symmetrical double substitu-
tion. To our knowledge, there is no report in the literature
about such a direct ortho-functionalization of dipheny-
lacetylene without affecting the acetylene unit. It is
known that diphenylacetylene reacts with n-butyllithium
in THF to form an addition product, 1-lithio-2-butylstil-
Addition of diphenylacetylene to
a
preformed12
LICKOR superbase (made of equimolar amounts of
potassium tert-butoxide and n-butyllithium) in THF/
hexane at -78 °C produces o,o′-dimetallodiphenyl-
acetylene 1, which reacts smoothly with various elec-
trophiles including methyl iodide, chlorotrimethyl-
silane, chlorodiphenylphosphine, methyl disulfide, and
chlorotri-n-butyltin, providing corresponding o,o′-di-
substituted derivatives 2 in reasonable isolated yields
(47-86%) (Scheme 1) (Table 1).
In the case of iodine as an electrophile it is found
that much better results can be obtained when the
original o,o′-dipotassiodiphenylacetylene (1) is converted
(6) (a) Mulvaney, J . E.; Garlund, Z. G.; Garlund, S. L. J . Am. Chem.
Soc. 1963, 85, 3897. (b) Mulvaney, J . E.; Garlund, Z. G.; Garlund, S.
L.; Newton, D. J . J . Am. Chem. Soc. 1966, 88, 476. (c) Mulvaney, J .
E.; Carr, L. J . J . Org. Chem. 1968, 33, 3286. (d) Mulvaney, J . E.;
Newton, D. J . J . Org. Chem. 1969, 34, 1936.
(7) Bauer, W.; Feigel, M.; Mu¨ller, G.; Schleyer, P. v. R. J . Am. Chem.
Soc. 1988, 110, 6033.
(8) (a) Lochmann, L.; Posp´ısˇil, J .; L´ım, D. Tetrahedron Lett. 1966,
257. (b) Schlosser, M. J . Organomet. Chem. 1967, 8, 9.
(9) (a) Hommes, H.; Verkruijsse, H. D.; Brandsma, L. J . Chem. Soc.,
Chem. Commun. 1981, 366. (b) Hommes, H.; Verkruijsse, H. D.;
Brandsma, L. Tetrahedron Lett. 1981, 2495. (c) Brandsma, L.; Hommes,
H.; Verkruijsse, H. D.; deJ ong, R. L. P. Recl. Trav. Chim. Pays-Bas
1985, 104, 226.
* Fax: (404) 894-7452.
(10) For a review, see: (a) Schlosser, M.; Faigl, F.; Franzini, L.;
Geneste, H.; Katsoulos, G.; Zhong, G. Pure Appl. Chem. 1994, 66, 1439.
(b) Chadwick, S. T.; Rennels, R. A.; Rutheford, J . L.; Collum, D. B. J .
Am. Chem. Soc. 2000, 122, 8640 and references therein. (c) For a review
on CIPE effects, see: Beak, P.; Meyers, A. I. Acc. Chem. Res. 1986,
19, 356. (d) Snieckus, V. Bull. Chem. Soc. Fr. 1988, 1, 67. (e) Block,
E.; Eswarakrishnan, V.; Gernon, M.; Ofori-Okai, G.; Saha, C.; Tang,
K.; Zubieta, J . J . Am. Chem. Soc. 1989, 111, 658. (f) Figuly, G. D.;
Loop, C. K.; Martin, J . C. J . Am. Chem. Soc. 1989, 111, 654. (g) Gais,
H.-J .; Vollhardt, J . Tetrahedron Lett. 1988, 1529. (h) Tamao, K.; Yao,
H.; Tsutsumi, Y.; Abe, H.; Hayashi, T.; Ito, Y. Tetrahedron Lett. 1990,
2925. (i) J ayasuriya, K.; Iyer, S. Int. J . Quantum Chem. 1988, 34, 199.
(11) For a recent review on reaction of organolithium compounds
with alkali metal alkoxides and reactions of “superbases”, see: (a)
Lochmann, L. Eur. J . Inorg. Chem. 2000, 1115. Schlosser, M. Pure
Appl. Chem. 1988, 60, 1627. (b) Kremer, T.; Harder, S.; J unge, M.;
Schleyer, P. v. R. Organometallics 1996, 15, 585. (c) den Besten, R.;
Lakin, M. T.; Veldman, N.; Spek, A. L.; Brandsma, L. J . Organomet.
Chem. 1996, 514, 191. (d) Harder, S.; Streitwieser, A. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 1066. (e) Lochmann, L.; J akubov, A.;
Brandsma, L. Collect. Czech. Chem. Commun. 1993, 58, 1445. (f) Bauer,
W.; Lochmann, L. J . Am. Chem. Soc. 1992, 114, 7482.
† Dedicated to Professor Paul von Rague´ Schleyer on the occasion
of his 70th birthday.
(1) Modern Acetylene Chemistry; Stang, P. J ., Diederich, F., Eds.;
VCH Publishers: New York, 1995.
(2) For an excellent review, see: (a) Haley, M. M. Synlett 1998, 7,
557. (b) Seminario, J . M.; Zacarias, A. G.; Tour, J . M. J . Am. Chem.
Soc. 2000, 122, 3015. Diederich, F.; Rubin, Y. Angew. Chem., Int. Ed.
Engl. 1992, 31, 1101. (c) Katayama, N.; Czarnecki, M. A.; Satoh, M.;
Watanabe, T.; Ozaki, Y. Appl. Spectrosc. 1997, 51, 487. (d) Taniike,
K.; Katayama, N.; Sato, T.; Ozaki, Y.; Czarnecki, M. A.; Satoh, M.;
Watanabe, T.; Yasuda, A. Mikrochim. Acta 1997, 14, 581. (e) Sarala,
S.; Roy, A.; Madhusudana, N. V.; Nguyen, H. T.; Destrade, C.; Cluzeau,
P. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1995, 261, 1. (f) Cluzeau,
P.; Nguyen, H. T.; Destrade, C.; Isaert, N.; Barois, P.; Babeau, A. Mol.
Cryst. Liq. Cryst. Sci. Technol., Sect. A 1995, 260, 69. (g) Godt, A.;
Fre´chet, J . M.; Beecher, J . E.; Willand, C. Macromol. Chem. Phys. 1995,
196, 133. (h) J uang, T. M.; Chen, Y. N.; Lung, S. H.; Lu, Y. H.; Hsu, C.
S. Liq. Cryst. 1993, 15, 529. (i) Morichere, D.; Chollet, P. A.; Fleming,
W.; (j) J urich, M.; Smith, B. A.; Swalen, J . D. J . Opt. Soc. Am. B 1993,
10, 1894. (k) Wu, S. T.; Hsu, C. S.; Chen, Y. N.; Wang, S. R.; Lung, S.
H. Opt. Eng. 1993, 32, 1792.
(3) Wan, W. B.; Kimball, D. B.; Haley, M. M. Tetrahedron Lett. 1998,
6795.
(4) (a) Dierks, R.; Vollhardt, K. P. C. J . Am. Chem. Soc. 1986, 108,
3150. (b) Whitlock, B. J .; Whitlock, H. W. J . Org. Chem. 1972, 37, 3559.
(5) (a) Letzinger, R. L.; Nazy, J . R.; J . Am. Chem. Soc. 1959, 81,
3013. (b) Diercks, R.; Vollhardt, K. P. C. Angew. Chem. 1986, 98, 268.
(12) It was found that changing the procedure to a potentially more
convenient one, in situ formation of the LICKOR superbase with
diphenylacetylene present in the reaction mixture followed by quench-
ing with chlorotrimethylsilane, produced a complex mixture of at least
nine compounds with the yield of the desired o,o′-disubstitution not
exceeding 30% (by gas chromatography, uncorrected).
10.1021/jo001533t CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/12/2001

3230 J . Org. Chem., Vol. 66, No. 9, 2001
Notes
Sch em e 1
Sch em e 2
ferent from the way the other first group metals coordi-
nate the anion.15a In the case of potassium, the crystal
structure reveals monomeric species with η6-coordination
of the metal ion and a single THF molecule in the
coordination sphere. In contrast, tolyl and diphenyl-
methyl anions coordinate potassium and rubidium ions
with the formation of the polymeric entities, with no
involvement of the solvent.15b,c In all of these complexes
chelating triamine (PMDTA) completes the coordination
sphere of the metal ion.
Activation of an ortho position by an adjacent acetylene
function is precedented.9 The Hammett σp value for the
acetylene moiety at 0.2316 classifies it as modestly
electron-withdrawing. The first metalation in the ortho
position may provide an additional activation of the ortho
position in the other phenyl ring by facilitating formation
of a bimetallic complex through, perhaps, the interven-
tion of the triple bond as a σ-donor. Such a synergistic
effect should actually facilitate the second metalation
step and explain the observed preference for o,o′-dimeta-
lation of diphenylacetylene. P. v. R. Schleyer has sug-
gested a similar explanation,17 “The metalation cluster
complexes with the triple bond and this ‘activates’ the
ortho CH bonds in the immediate vicinity.”
In summary, the reaction of diphenylacetylene with the
LICKOR superbase (n-BuLi/KOt-Bu) produces the cor-
responding o,o′-dimetalated species with high regiose-
lectivity. Subsequent substitution with halogen-, carbon-,
sulfur-, silicon-, tin-, and phosphorus-centered electro-
philes results in moderate-to-good yields of o,o′-disubsti-
tuted diphenylacetylenes.
Ta ble 1. Rea ction of o,o′-Dim eta llod ip h en yla cetylen e
w ith Electr op h iles
entry
electrophile
reaction product
yield (%)
1
2
3
4
5
6
TMSCl
I2
CH3I
CH3S-SCH3
Ph2PCl
(n-Bu)3SnCl
2a
2b
2c
2d
2e
2f
63
52a
86
59
47
62
a
via Grignard reagent (Method A).
to a much less basic Grignard reagent by adding 2 equiv
of magnesium bromide etherate, relative to diphenyl-
acetylene, before addition of iodine (Method A). Pre-
Exp er im en ta l Section
9b
Gen er a l Meth od s. 1H and 13C NMR spectra were recorded
on a Gemini 300 spectrometer at 300 MHz and 75 MHz,
respectively, using tetramethylsilane (TMS) as an internal
standard. The solvent was CDCl3 unless noted otherwise.
Chemical shifts (ppm) are reported relative to TMS for 1H and
13C NMR spectra and 85% H3PO4 for 31P NMR spectra (external
capillary). Mass spectra were measured on a VG Instruments
70 SE system. FTIR (KBr pellet) spectra were recorded on a
Nicolet 60SXR. Melting points were determined on Electrother-
mal melting point apparatus and are uncorrected. TLC analyses
were performed using Analtech silica gel GF TLC plates; Rf
values are reported for a n-hexane (95+ %)/acetone (5:1) solvent
system. THF and hexane were distilled under argon over sodium/
benzophenone and lithium aluminum hydride, respectively. All
other reagents were used as received. All experiments were
performed on a vacuum line in an argon atmosphere.
Gen er a l P r oced u r e. Bis(2-tr im eth ylsilylp h en yl)a cety-
len e (2a ). A solution of potassium tert-butoxide (1.20 g, 10.7
mmol) in THF/hexane (18/15 mL) was cooled to -78 °C, and
while stirred, a 2.5 M hexane solution of n-butyllithium (3.5 mL,
8.5 mmol) was added dropwise via syringe over 15 min, resulting
in a yellow solution. Stirring at -78 °C was continued for 20
min, followed by dropwise addition of diphenylacetylene (0.70
g, 3.93 mmol) dissolved in THF (3.5 mL) over 10 min. The
reaction was continued at -75 °C for 40 min, followed by 1.5 h
sumably, the initially formed 1, a very strong base, is
capable of eliminating hydrogen iodide from the mono-
or diiodinated diphenylacetylene, leading to a very reac-
tive benzyne derivatives and thus depleting the yield of
the desired o,o′-diiododiphenylacetylene. We have also
found that the most convenient preparation of this
compound includes a two-step process, where the readily
available o,o′-bis(trimethylsilyl)diphenylacetylene (2a ),
obtained in 63% yield, is allowed to react with iodine
chloride in methylene chloride to give the target structure
in 74% isolated yield, after crystallization (Method B)
(Scheme 2).
The observed preference for the o,o′-dimetalation reac-
tion can be explained by analogy with the double lithia-
tion of biphenyl13 or diphenyl ether.14 According to the
calculational studies of Schleyer et al., the o,o′-dilithiated
derivatives of both exist as symmetrically doubly bridged
structures,13a,b a result that was confirmed by an X-ray
crystal structure.13c,d However, in this case steric con-
straints prevent such bridging, and the role of potassium
needs to be acknowledged. As has been shown in the case
of trityl alkali metal structures, the location (and binding)
of potassium and rubidium cations is dramatically dif-
(15) (a) Hoffmann, D.; Bauer, W.; Schleyer, P. v. R.; Pieper, U.;
Stalke, D.; Organometallics 1993, 12, 1193. (b) Hoffmann, D.; Bauer,
W.; Hampel, F.; Hommes, N. J .; Schleyer, P. v. R.; Otto, P.; Pieper,
U.; Stalke, D.; Wright, D. S.; Snaith, R. J . Am. Chem. Soc. 1994, 116,
528. (c) Refs 34 and 40 in ref 15a.
(16) Landgrebe, J . A.; Rynbrandt, R. H. J . Org. Chem. 1966, 31,
2585.
(13) (a) Kos, A. J .; Schleyer, P. v. R. J . Am. Chem. Soc. 1980, 102,
7928. (b) Neugebauer, W.; Kos, A. J .; Schleyer, P. v. R. J . Organomet.
Chem. 1982, 228, 107. (c) Schubert, U.; Neugebauer, W.; Schleyer, P.
v. R. J . Chem. Soc., Chem. Commun. 1982, 1184. (d) Ref 7d in ref 14.
(14) Schleyer, P. v. R. Pure Appl. Chem. 1984, 56, 151.
(17) Schleyer, P. v. R. private communication.

Notes
J . Org. Chem., Vol. 66, No. 9, 2001 3231
at -25 °C. The initially red solution changed to very deep
burgundy. The temperature of the reaction mixture was lowered
to -43 °C, and chlorotrimethylsilane (1.5 mL) was added via
syringe over 2 min. Stirring was continued at -40 °C for 30 min,
and then the cooling bath was changed to ice and the reaction
mixture was allowed to reach room temperature (5 h). The color
of the reaction mixture changed to blue, blue-green, green-
yellow, and finally light yellow. Solvents were removed in vacuo,
ether (80 mL) was added, and the mixture was washed with
saturated aqueous citric acid solution and water. The ether
extracts were dried (MgSO4) and filtered, and the solvent was
removed in vacuo to give an oil (1.00 g), which solidified upon
standing. Crystallization from hexane yielded 0.80 g (63%) of
13C NMR δ 140.17, 132.06, 129.65, 128.37, 125.75, 123.49, 92.34,
20.72. EIMS 206(100), 205(74.98), 204(17.11), 203(17.88), 202-
(17.39), 191(40.04), 178(39.75), 165(17.68), 115(14.56).
Bis(2-m eth ylth iop h en yl)a cetylen e (2d ). Yield 59%, mp
122-124 °C, Rf 0.57. 1H NMR δ 7.54 (dd, J ) 7.6 Hz, 1.3 Hz,
2H); 7.30 (dt, J ) 7.5 Hz, 1.4 Hz, 2H); 7.18 (dd, J ) 7.9 Hz, 1.3
Hz, 2H); 7.11 (dt, J ) 7.5 Hz, 1.3 Hz, 2H). 13C NMR δ 141.59,
132.59, 128.84, 124.23, 121.38, 93.15, 15.24; EIMS 270(5.37),
255(50.86), 240(100), 222(10.54), 221(19.58), 176(2.08); IR(νmax
,
cm-1) 1583, 1471, 1433, 1278, 1244, 1074, 1035, 954, 752, 718,
680, 563, 519, 446. Anal. Calcd for C16H14S2: C, 71.07; H, 5.22;
S, 23.71. Found: C, 71.09; H, 5.17; S, 23.62.
Bis(2-d ip h en ylp h osp h in ylp h en yl)a cetylen e (2e). Yield
47%, mp 167-169 °C(dec); Rf 0.59. 1H NMR (acetone-d6) δ 7.41-
7.38 (m, 12H); 7.33-7.23 (m, 12H); 7.19-7.15 (m, 2H); 6.79-
6.75 (m, 2H); 13C NMR (acetone-d6) δ 140.11 (d, J CP ) 12.6 Hz),
136.56 (d, J CP ) 11.5 Hz), 134.06 (d, J CP ) 20.3 Hz), 132.38 (d,
J CP ) 20.6 Hz), 128.66, 128.45 (d, J CP ) 7.3 Hz), 128.11 (d, J CP
) 9.1 Hz), 127.85 (d, J CP ) 20.6 Hz), 90.09; 31P NMR δ -8.5.
CIMS 547(84.54), 471(100); EIMS 546(19.19), 469(100), 392-
(14.46), 357(2.01), 313(9.73), 283(19.62), 273(15.62), 252(7.01),
183(10.91); IR(νmax, cm-1) 3068, 3054, 3026, 3010, 3001, 1581,
1486, 1434, 1305, 1180, 1158, 1120, 1090, 1068, 1026, 998, 758,
745, 731, 694, 585, 544, 527, 492, 474, 411. Anal. Calcd for
C38H28P2: C, 83.50; H, 5.16. Found: C, 82.86; H, 5.31.
Bis(2-tr i-n -bu tylsta n n ylp h en yl)a cetylen e (2f). Yield 62%,
oil, purified by column chromatography on silica gel, elution with
hexanes, Rf 0.94. 1H NMR δ 7.53-7.47 (m, 4H), 7.33-7.24 (m,
4H), 1.68-1.42 (m, 12H), 1.29 (sextet, J ) 7.2 Hz, 12H), 1.21-
1.01 (m, 12H), 0.83 (t, J ) 7.2 Hz, 18H); 13C NMR δ 146.40,
136.55, 131.78, 131.20, 128.11, 127.46, 91.98, 29.03, 27.21, 13.44,
9.81; EIMS 699(6.1), 643(60.4), 585(5.0), 529(4.1), 473(1.6), 411-
1
colorless crystalline product, mp 72-74 °C, Rf 0.91. H NMR δ
7.55 + 7.54 (d + d, J ) 7.5 Hz, 2H); 7.40-7.29 (m, 2 H); 0.43 (s,
18 H). 13C NMR δ 142.54, 134.25, 132.06, 129.04, 128.84, 127.57,
93.72, -1.19. EIMS 322(49.49), 307(32.16), 291(8.52), 247(6.47),
233(57.42), 191(12.49), 129(10.02), 73(100), 59(30.83). IR (νmax
,
cm-1) 3069, 3050, 2951, 2896, 1581, 1466, 1243, 1128, 1076, 855,
838, 781, 751, 721, 691, 676, 621, 559, 451. Anal. Calcd for
C20H26Si2: C, 74.46; H, 8.12. Found: C, 74.17; H, 8.11.
Bis(2-iod op h en yl)a cetylen e (2b). Meth od A. The initially
formed 1 was converted to the corresponding Grignard reagent
by the addition of 2 equiv of magnesium bromide etherate,
relative to diphenylacetylene, followed by the reaction with
iodine and the workup as above. Yield 52%, mp 98-100 °C, Rf
1
0.77. H NMR δ 7.88 (dd, J ) 7.6 Hz, 0.4 Hz, 2H); 7.61 (dd, J )
7.6 Hz, 1.6 Hz, 2H); 7.33 (dt, J ) 7.6 Hz, 0.4 Hz, 2H); 7.02 (dt,
J ) 7.6 Hz, 1.6 Hz, 2H). 13C NMR δ 138.69, 132.89, 129.58,
129.42, 127.70, 100.45, 94.65. EIMS 430(100), 303(11.02), 215-
(3.39), 176(84.39), 150(14.87), 127(2.81), 126(4.64), 88(19.19). IR-
(νmax, cm-1) 1474, 1428, 1413, 1012, 750, 728, 649, 552, 434. Anal.
Calcd for C14H8I2: C, 39.10; H, 1.88; I, 59.02. Found: C, 38.37;
H, 1.83; I, 59.88.
(18.1), 353(26.5), 291(100), 235(41.2), 179(42.9); IR (neat, νmax
,
cm-1) 3052, 2956, 2930, 2871, 2863, 1484, 1434, 1418, 1376,
1340, 1292, 1284, 1248, 1181, 1150, 1109, 1072, 980, 879, 845,
758, 718, 890, 887, 850, 597, 558, 507; Anal. Calcd for C38H62
Sn2: C, 60.35; H, 8.26. Found: C, 60.44; H, 8.29.
-
Bis(2-iod op h en yl)a cetylen e (2b). Meth od B. A 1.0 M
solution of iodine chloride in methylene chloride (5.8 mL, 5.80
mmol) was added to a stirred solution of 2a (0.91 g, 2.82 mmol)
in methylene chloride (25 mL), and the reaction mixture was
stirred at room temperature overnight. The solvent was evapo-
rated in vacuo, hexane was added, and the crystalline product
was filtered to give 0.90 g of a pale yellow material. Upon cooling
of the mother liqueur, another 0.20 g of crystalline material was
recovered. Recrystallization from 95% ethanol yielded 0.90 g
(74%) material, identical with that obtained using direct iodi-
nation of o,o′-bis(bromomagnesio)diphenylacetylene.
Ack n ow led gm en t. The financial support from the
U.S. Department of Energy is gratefully acknowledged.
We thank Professors Lambert Brandsma and Paul von
Rague´ Schleyer for valuable discussions.
J O001533T
(18) Reported mp 28.7 °C (from ethanol, Coops, J .; Hojitink, G. J .;
Kramer, T. J . E.; Faber, A. C. Recl. Trav. Chim. Pays-Bas 1953, 72,
781). According to the GC analysis our sample contained ca. 10% of
the monomethylated derivative, not separable by column chromatog-
raphy or distillation.
Bis(2-m eth ylp h en yl)a cetylen e (2c). Yield 86%, oil.18 1H
NMR δ 7.52 (d, J ) 7.2 Hz, 2H); 7.27-7.15 (m, 6H); 2.53(s, 6H).