2786
J . Org. Chem. 1998, 63, 2786-2787
Communications
Ca r ba n ion : Th e F ir st Exa m p le To Gen er a te
Th er m od yn a m ica lly Un fa vor a ble
9-(Dicya n om eth yl)flu or en id e An ion
Xiao-Qing Zhu*,†,‡ and You-Cheng Liu†
National Laboratory of Applied Organic Chemistry
and Department of Chemistry, Lanzhou University,
Lanzhou 730000, China and Department of Chemistry,
Nankai University, Tianjin 300071, China
Received October 28, 1997
Carbanions as a class of very important intermediates in
many organic and bioorganic reactions have been extensively
studied and continue to be of interest. Generally, two
methods are used in the preparation of carbanions, i.e., (1)
addition of a nucleophile with a negative charge to unsatur-
ated neutral species (precursors) and (2) removal of a group
with a positive charge (e.g., proton) from neutral saturated
substrates (the other precursors).1 For 9-(dicyanometh-
yl)fluorenide carbanion (1) as a thermodynamically unfavor-
able anion, 9-fluorenylidenemalononitrile (3) and 9-fluore-
nylmalononitrile (4) would be its two typical precursors. In
fact, the former only gave 9-fluorenylmalononitrile carbanion
(2) rather than anion 1 by addition of a hydride (from LiAlH4
as an example),2 and the latter by removal of a proton also
gave sole anion 2,2 which were rationalized by the much
larger thermodynamic stability of 2 than that of 1 (pKa )
22 for fluorene and pKa ) 11 for dicyanomethane in
DMSO).3,4 In this paper, we wish to report the first example
of the generation of carbanion 1 by a hydride transfer from
1-benzyl-1,4-dihydronicotinamide (BNAH) to compound 3 in
dry CH3CN.
F igu r e 1. UV-vis spectrum of the reaction mixture.
Ta ble 1. P r od u cts a n d Yield sa (%) a t Differ en t Tim e
In ter va ls for th e Th er m olysis of Sa lt 5
time (h)
6
7
0
5
10
20
40
60
trace
8
16
24
36
41
0
trace
7
11
15
17
a
The yields are based on the amount of compound 3 used.
peak at m/z ) 164 (100) and a weak peak at m/z ) 165 (16)
owing to the fragments of fluorenylidene and fluorenyl,
respectively. In the 1H NMR spectrum of the reaction
mixture, no characteristic peaks of compound 4 were ob-
served, but the peaks at δ (CD3CN) 9.24 (1H, s), 8.92 (1H,
d), 8.75 (1H, d), 8.03 (1H, t), 7.36 (5H, s), 5.75 (2H, s) were
originated from the BNA+ cation.6 When the red solution
was treated with acetic acid, the red color disappeared
immediately and the solution subjected to chromatography
to give product 4 of yield 86% based on 3. Treatment of the
red solution with CH3CO2D gave 9-D-fluorenylmalononitrile
(yield 71% based on 3).7,8 These obtained results clearly
indicate the formation of carbanion 1 in the reaction of 2
with BNAH (see eq 1).9,10
3 (214 mg, 1 mmol) and BNAH (228 mg, 1 mmol) were
dissolved in dry acetonitrile to make a suitable oversaturated
solution. The solution (yellow) was rigorously deaerated and
stirred under argon in a closed system at room temperature,
and a deep red color gradually appeared. Sixteen hours
later, UV-vis spectroscopy showed the appearance of a new
absorption at λmax ) 540 nm (Figure 1), which is close to
that of a reported fluorenide carbanion (λmax ) 521 nm).5
Mass spectrometry of the reaction mixture exhibited a strong
To further confirm the carbanion 1 mentioned above, the
thermolytic reaction of salt (5) was studied. When the red
(6) Caughey, W. S.; Schellenberg, K. A. J . Org. Chem. 1966, 31, 1978.
(7) Zhu, X.-Q.; Yang, L.; Guo, Q.-X.; Wu, L.-M.; Liu, Y.-C. Chin. J . Magn.
Reson. 1996, 13(3), 248.
(8) In addition, there was a small amount of compound 4 obtained from
the products, which is attributed to hydrogen-deuterium exchange between
the deuterated product and the media during the workup (cf. Cram, D. J .;
Gosser, L. J . Am. Chem. Soc. 1964, 86, 5445).
(9) Although the thermodynamic stability of ion 2 is quite larger than
that of ion 1, the rearrangement of ion 1 into ion 2 by proton [1,2] transfer
was forbidden in the reaction based on the orbital correlation rule.
(10) The transfer of amide protons in BNAH and BNA+ to ion 1 was not
observed from the 1H NMR spectrum of the reaction mixture. The reason
could be the weaker acidity of the amide protons compared to the 9-proton
in compound 4 (pKIa ) 22 for nicotinamide in DMSO, cf. ref 3). In fact, ion
1 was not free in the reaction system, which must combined with BNA+ to
be more stable.
* To whom correspondence should be addressed at Nankai University.
† Lanzhou University.
‡ Nankai University.
(1) McManus, S. P. Organic Reaction Intermediates; Academic Press:
New York and London, 1973; pp 337-422.
(2) Zhu, X. Q. Ph.D. Thesis, Lanzhou University, 1996.
(3) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456-463.
(4) The hyperconjugative cyano group stabilization of ion 1 could be quite
substantial, making the stability difference much smaller than that
predicted by the difference between fluorene and dicyanomethane, but the
stability of ion 2 is still quite larger than that of ion 1, which can be
confirmed by the fact that treatment of compound 4 with NaH in dry CH3-
CN and followed by addition of CH3OD gave solely R-D-fluorenylmalono-
nitrile.
(5) Chan, L. L.; Smid, J . J . Am. Chem. Soc. 1968, 90, 4654.
S0022-3263(97)01969-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/02/1998
Zhu, Xiao-Qing
