10644
J. Am. Chem. Soc. 1999, 121, 10644-10645
of 1,8-dimethoxy-9-dimethoxymethylanthracene monocation (1)
as the first fully characterized hypervalent 10-C-5 compound.
Preparation of 1 is illustrated in Scheme 1. Commercially
available 1,8-dihydroxyanthraquinone (2) was converted to 5 via
methylation17 followed by reduction18 and trifluoromethane-
sulfonation. Carbon monoxide insertion19 to 5 in methanol
mediated by Pd(PPh3)4 gave 6 in 52% yield. 6 was treated with
trimethyloxonium tetrafluoroborate (Me3O+BF4-) under CH2Cl2
Synthesis and Isolation of Stable Hypervalent
Carbon Compound (10-C-5) Bearing a
1,8-Dimethoxyanthracene Ligand
Kin-ya Akiba,*,† Makoto Yamashita,†
Yohsuke Yamamoto,† and Shigeru Nagase‡
Department of Chemistry, Graduate School of Science
Hiroshima UniVersity, 1-3-1 Kagamiyama
Higashi-Hiroshima 739-8526, Japan
Department of Chemistry, Graduate School of Science
Tokyo Metropolitan UniVersity, 1-1 Minami-osawa
Hachioji, Tokyo 192-0397, Japan
reflux for 20 h, and after filtration of excess Me3O+BF4 and
-
removal of the solvent, 1 was obtained as a yellow-green solid.
1 is thermally stable but is sensitive to atmospheric moisture. 13
C
NMR chemical shift of the central carbon is found at δ 192.58
ppm, which is confirmed by independent preparation of 1 using
carbon monoxide (13C 99%) in the process to convert 5 to 6.
Crystals of 1 suitable for X-ray analysis were obtained by
careful recrystallization from dry CDCl3, and the X-ray structure
ReceiVed August 2, 1999
The bimolecular nucleophilic substitution (SN2) reaction at
saturated carbon such as hydrolysis of methyl halides is one of
the most important and the most popular reactions in organic
chemistry.1,2 The mechanism of the reaction invoking inversion
of configuration of the central carbon is one of the fundamental
ideas of organic reactions and is described commonly in textbooks
for undergraduate students.3 The structure of the transition state
(TS) of SN2 should be trigonal bipyramid (TBP) around the central
carbon.4 Hence the bonding about the carbon involves, at least
formally, expansion of the valence shell and is called hypervalent.5
Due to the fundamental importance of SN2, there have been a
variety of efforts to stabilize the TS and even to prepare model
compounds of TS; typical examples are by Hojo6 and Martin.7-10
They claimed that they observed symmetrical TBP structure for
the model compounds in solution. However, the X-ray structure
of dimethyl-1-fluorenylcarbenium hexachloroantimonate bearing
two methylthio groups at the 9-position6 revealed that the two
S-C+ distances were different and the compound should be
regarded as a sulfonium structure,6 and X-ray analysis of the
9-anthracenylmethyl dication bearing phenylthio groups at 1 and
8 positions has not been reported.7-10 Theoretical calculations on
SN2 are also numerous and they conclude that hypervalent 10-
C-5 species should be TBP and energy maximum.11 On the other
-
is shown in Figure 1.20 The counteranion is B2F7 unexpectedly
but it is well separated from the cationic part. The structure clearly
shows the symmetrical nature of the compound. The sum of the
angles (C9-C19-O3, C9-C19-O4, and O3-C19-O4) around
the central carbon is 360.0°, indicating that the carbon is planar
with sp2 hybridization. The angles around the oxygen atoms of
the methoxy groups are 119.2(9)° (C1-O1-C15) and 120.3(9)°
(C8-O2-C16), showing that both oxygen atoms have sp2
hybridization. Since the carbon atoms of the methoxy groups at
1,8-positions are in the plane of the anthracene, one of the lone
pairs of each oxygen atom should be directed toward the empty
p-orbital of the central carbocation at the 9 position. Therefore,
geometry around the central carbon atom is TBP, which is only
slightly distorted. The two O- -C distances are almost identical
(2.43(1) and 2.45(1) Å), which is significantly longer than that
of a covalent C-O bond (1.43 Å)21 but shorter than the sum of
the van der Waals radius (3.25 Å).21
To elucidate the property and the degree of interaction between
the central carbon atom and the two oxygen atoms in 1, several
compounds were synthesized and were structurally characterized
for comparison.22 The side views of crystal structures of 1,8,9-
tribromo- (7), 1,8-dimethoxy-9-trifluoromethanesulfonyloxy- (5),
and 1,8-dimethoxy-9-cyanoanthracene (8) are shown in Figure 2
together with 1. In the structure of tribromo derivative 7, the three
hand, recently reported exotic highly coordinate carbon species
+ 12,13
such as CH5
and CLi5,14 CLi6,15 and (Ph3PAu)5C+ 16 are
electron deficient carbocations and/or are stabilized by metal-
metal cage interactions. Therefore, these compounds cannot be
regarded as hypervalent 10-C-5 species for models of the SN2
transition state. Here we report the synthesis and crystal structure
(16) Scherbaum, F.; Grohmann, A.; Mu¨ller, G.; Schmidbaur, H. Angew.
Chem., Int. Ed. Engl. 1989, 28, 463.
(17) Quast, H.; Fuchsbauer, H.-L. Chem. Ber. 1986, 119, 1016.
(18) Prinz, H.; Wiegrebe, W.; Muller, K. J. Org. Chem. 1996, 61, 2853.
(19) Dolle, R. E.; Schmidt, S. J.; Kruse, L. I. J. Chem. Soc., Chem. Commun.
1987, 904.
† Hiroshima University.
‡ Tokyo Metropolitan University.
(20) Data were collected at 200 K on a MacScience DIP2030 imaging plate
equipped with graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å).
Unit cell parameters were determined by autoindexing several images in each
data set separately with the program DENZO. For each data set, rotation images
were collected in 6 ° increments with a total rotation of 180° about φ. Data
were processed by using SCALEPACK. The structure was solved using the
teXsan system and refined by full-matrix least-squares. Final R ) 0.105 (Rw
) 0.163) for 1356 observed reflections (290 parameters) with I > 3σ(I). Crystal
data for 1: monoclinic system, space group P21/c (no. 14), a ) 10.9810(2)
Å, b ) 14.7010(4) Å, c ) 13.4300(3) Å, â ) 107.81(2)°, V ) 2064.2(8) Å3,
Z ) 4, Fcalc ) 1.503 g cm-3. Details of the X-ray structure determination are
available; see Supporting Information. The programs DENZO and SCALEPACK
are available from Mac Science Co. Z. Otwinowski, University of Texas,
Southwestern Medical Center. The program teXsan is available from Rigaku
Co.
(1) For N-X-L designation: X, central atom; N, formal valence-shell
electrons about an X; L, the number of ligands. Perkins, C. W.; Martin, J. C.;
Arduengo, A. J.; Lau, W.; Alegria, A.; Kochi, J. K. J. Am. Chem. Soc. 1980,
102, 7753.
(2) Ingold, C. K. Structure and Mechanism in Organic Chemistry, 2nd ed.;
Cornell University Press: Ithaca, New York, 1969.
(3) Morrison, R. T.; Boyd, R. N. Organic Chemistry, 6th ed.; Prentice Hall
Inc.: New Jersey, 1992; Chapter 5.
(4) Chabinyc, M. L.; Craig, S. L.; Regan, C. K.; Brauman, J. I. Science
1998, 279, 1882.
(5) Akiba, K.-y. Chemistry of HyperValent Compounds; Wiley-VCH: New
York, 1999.
(6) Hojo, M.; Ichi, T.; Shibato, K. J. Org. Chem. 1985, 50, 1478.
(7) Forbus, T. R., Jr.; Martin, J. C. J. Am. Chem. Soc. 1979, 101, 5057.
(8) Forbus, T. R., Jr.; Martin, J. C. Heteroatom Chem. 1993, 4, 113.
(9) Forbus, T. R., Jr.; Martin, J. C. Heteroatom Chem. 1993, 4, 129.
(10) Forbus, T. R., Jr.; Kahl, J. L.; Faulkner, L. R.; Martin, J. C. Heteroatom
Chem. 1993, 4, 137.
(21) Dean, J. A. Lange’s Handbook of Chemistry, 11th ed.; McGraw-Hill:
New York, 1973; pp 3-8 and 3-9.
(22) Crystal data for 5: triclinic system, space group P1h (no. 2), a ) 8.5460-
(6) Å, b ) 9.4300(8) Å, c ) 11.1650(7) Å, R ) 73.016(4)°, â ) 82.571(5)
°, γ ) 81.014(4)°, V ) 846.6(1) Å3, Z ) 2, Fcalc ) 1.52 g cm-3. R ) 0.0510
(I > 3σ(I)), λ(Mo KR) ) 0.710 73 Å. Crystal data for 7: monoclinic system,
space group P21/c (no. 14), a ) 10.1530(5) Å, b ) 7.2480(2) Å, c ) 17.3580-
(9) Å, â ) 106.064(2)°, V ) 1227.48(9) Å3, Z ) 4, Fcalc ) 2.25 g cm-3. R )
0.0397 (I > 3σ(I)), λ(Mo KR) ) 0.710 73 Å. Crystal data for 8: tetragonal
system, space group I41/a (no. 88), a ) b ) 25.057(3) Å, c ) 8.3710(5) Å,V
) 5255.8(7) Å3, Z ) 16, Fcalc ) 1.33 g cm-3. R ) 0.0645 (I >3σ(I)), λ(Mo
KR) ) 0.710 73 Å.
(11) Deng, L.; Branchadell, V.; Ziegler, T. J. Am. Chem. Soc. 1994, 116,
10645.
(12) Olah, G. A.; White, A. M.; O’Brien, D. H. Chem. ReV. 1970, 70,
561.
(13) Olah, G. A.; Rasul, G. Acc. Chem. Res. 1997, 30, 245.
(14) Schleyer, P. v. R.; Wu¨rthwein, E.-U.; Kaufmann, E.; Clark, T.; Pople,
J. A. J. Am. Chem. Soc. 1983, 105, 5930.
(15) Kudo, H. Nature 1992, 355, 432.
10.1021/ja992719g CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/30/1999