An in-fluorosilaphane: The largest in-functional group is a uniquely encapsulated fluorine atom [11]

Full text of DOI:10.1021/ja981009s

J. Am. Chem. Soc. 1998, 120, 6421-6422
6421
Table 1. Computational Data for in-Cyclophanes and Their
out-Isomers
An in-Fluorosilaphane: The Largest in-Functional
Group Is a Uniquely Encapsulated Fluorine Atom
Steven Dell, Nancy J. Vogelaar, Douglas M. Ho, and
Robert A. Pascal, Jr.*
Department of Chemistry
Princeton UniVersity
∆Hf (AM1, kcal/mol)a,b
in-isomer out-isomer (in - out, kcal/mol)
Cyclophanes with Two-Atom Links (A)
Princeton, New Jersey 08544
differencec
central atoms
ReceiVed March 25, 1998
X ) P, Y ) lpd (3)
X ) Si, Y ) H (4)
X ) P, Y ) O (5)
X ) Si, Y ) F (6)
165.65
116.28
104.64
55.57
171.06
116.29
87.86
-5.41
-0.01
16.78
27.88
in/out stereoisomerism is a common phenomenon in medium-
and large-ring bicyclic organic structures. For the in-isomers of
such molecules, schematically represented by 1, there must be
sufficient space to accommodate the inwardly directed function-
alities X-Y. In a recent, comprehensive review of in/out
stereoisomerism,1 Alder and East noted that the vast majority of
inside functional groups are methines (X-Y ) C-H), amines
(N-lp), and ammonium ions (N+-H), but the only compounds
containing in-functionalities with Y larger than a hydrogen atom
or lone pair are the in,out-isomers of Whitlock’s enormous,
macrocyclic bis(phosphine oxides),2 where the linking arms (L)
between the phosphine oxide bridgeheads are 14-16 atoms long
and steric congestion is not an issue. We now report the synthesis
and crystallographic characterization of in-cyclophane 2, in which
an in-fluorosilane (X-Y ) Si-F) is pressed into the face of a
benzene ring.
27.69
Cyclophanes with Three-Atom Links (B)
X ) P, Y ) lpd (7)
X ) Si, Y ) H (8)
X ) P, Y ) O (9)
X ) Si, Y ) F (2)
X ) Si, Y ) Me (10)
140.56
86.07
55.91
3.55
146.88
93.40
61.41
4.10
-6.32
-7.33
-5.50
-0.55
8.20
91.32
83.12
a See ref 6 for computational details. b All structures possess C3
symmetry. c Negative values favor the in-isomer. d lp ) lone pair
electrons.
yield,9 but vigorous oxidation of 7 led only to the trisulfone 12;10
none of the phosphine oxide was formed (Scheme 1). X-ray
analysis confirmed the in-geometry of 12 (not shown). All
attempts to form 9 or a related phane by the direct cyclization of
tris[2-(chloromethyl)phenyl]phosphine oxide7 (13) with a variety
of tripodal nucleophiles under many different conditions were
unsuccessful.
Fluorosilanes are larger than phosphine oxides (typical bond
distances: C3SisF, 1.64 Å; C3PdO, 1.49 Å), but with the former
there is no chance of neighboring group participation by the
fluorine in the cyclization reactions (a possible complicating factor
with phosphine oxides). For the synthesis of the fluorosilaphane
2, tri(o-tolyl)silane11 (14) was fluorinated at silicon with AgF,
and then brominated to yield tris[2-(bromomethyl)phenyl]fluo-
rosilane (16). Condensation of 16 with 1,3,5-tris(mercaptometh-
yl)benzene gave the desired 2, but in an abysmal 0.4% yield.12
Even so, the isolation of 2 was achieved only by its direct
crystallization from an otherwise intractable chromatographic
fraction! The X-ray structure of 2 (Figure 1) unambiguously
establishes the inside location of the fluorine atom.13 Cyclophane
2 has approximate C3 symmetry, and the distance from the fluorine
to the mean plane of the basal aromatic ring is 2.826(7) Å.
The syntheses of in-phosphine 3 and in-silane 4 have been
reported previously,3,4 but attempts to place larger apical func-
tionality in that framework were unsuccessful. AM1 calcula-
tions5,6 (Table 1, series A) indicate that in-geometries are favored
for 3 and 4, but the X-ray structure of 4 shows significant
distortions of the basal ring due to pressure from the in-hydrogen,4
so it is not surprising that the in-phosphine oxide 5 and
in-fluorosilane 6 are calculated to be much more strained than
the corresponding out-isomers. However, the addition of one
methylene group to each of the linking bridges in these cyclo-
phanes (Table 1, series B) would seem to permit the inclusion of
a non-hydrogen in-atom.
1
Interestingly, the H NMR and mass spectra of 2 suggest that a
small amount (∼15%) of the in-hydroxo cyclophane is present,
(7) Letsinger, R. L.; Nazy, J. R.; Hussey, A. S. J. Org. Chem. 1958, 23,
1806-1807.
(8) Nakazaki, M.; Yamamoto, K.; Miura, Y. J. Org. Chem. 1978, 43, 1041-
We first attempted to prepare an in-oxide such as 9. Conden-
sation of tris[2-(chloromethyl)phenyl]phosphine7 (11) with 1,3,5-
tris(mercaptomethyl)benzene8 gave the in-phosphaphane 7 in 59%
1044.
1
(9) For 7: mp 288-291 °C; H NMR (CDCl3) δ 3.69 (s, 6 H), 3.75 (d, J
) 4 Hz, 6 H), 6.57 (ddd, J ) 8, 4, 1 Hz, 3 H), 7.07 (ddd, J ) 8, 8, 1 Hz, 3
H), 7.26 (s, 3 H), 7.28 (ddd, J ) 8, 8, 1 Hz, 3 H), 7.44 (ddd, J ) 8, 5, 1 Hz,
3 H); 13C NMR (CDCl3) δ 31.4 (d, JPC ) 32 Hz), 36.6, 127.5, 129.4, 129.6,
129.9 (d, JPC ) 4 Hz), 134.4, 134.6 (d, JPC ) 13 Hz), 139.2 (d, JPC ) 2 Hz),
142.8 (d, JPC ) 27 Hz); MS m/z 514 (M+, 86), 481 (M - SH, 4), 71 (100);
exact mass 514.1029, calcd for C30H27PS3 514.1015.
(1) Alder, R. W.; East, S. P. Chem. ReV. 1996, 96, 2097-2111.
(2) (a) Friedrichsen, B. P.; Whitlock, H. W. J. Am. Chem. Soc. 1989, 111,
9132-9134. (b) Friedrichsen, B. P.; Powell, D. R.; Whitlock, H. W. J. Am.
Chem. Soc. 1990, 112, 8931-8941.
(3) Pascal, R. A., Jr.; West, A. P., Jr.; Van Engen, D. J. Am. Chem. Soc.
1990, 112, 6406-6407.
(10) For 12: mp > 400 °C; 1H NMR (DMSO-d6) δ 4.69 (br s, 6 H), 4.82
(s, 6 H), 7.40 (m, 12 H), 7.77 (s, 3 H); MS m/z 610 (M+, 27), 546 (M - SO2,
12), 482 (M - 2SO2, 6), 418 (M - 3SO2, 100); exact mass 610.0705, calcd
for C30H27PS3O6 610.0709.
(4) L’Esperance, R. P.; West, A. P., Jr.; Van Engen, D.; Pascal, R. A., Jr.
J. Am. Chem. Soc. 1991, 113, 2672-2676.
(5) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am.
Chem. Soc. 1985, 107, 3902-3909.
(11) Benkeser, R. A.; Riel, Frank J. J. Am. Chem. Soc. 1951, 73, 3472-
3474.
(6) Semiempirical molecular orbital calculations were performed by using
the SPARTAN program package (version 5.0; Wavefunction, Inc., Irvine, CA),
and its built-in default thresholds for wave function and gradient convergence
were employed.
(12) For 2: 1H NMR (CD2Cl2) δ 3.71 (s, 6 H), 3.77 (s, 6 H), 7.14 (m, 6
H), 7.16 (s, 3 H), 7.36 (m, 6 H); 19F NMR (CD2Cl2) δ 5.3; MS m/z 530 (M+,
19), 439 (M - C7H7, 5), 408 (M - C7H6S, 10), 178 (100); exact mass
530.1018, calcd for C30H27FS3Si 530.1030.
S0002-7863(98)01009-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/11/1998

6422 J. Am. Chem. Soc., Vol. 120, No. 25, 1998
Communications to the Editor
Scheme 1
Figure 1. Molecular structure of in-cyclophane 2. Thermal ellipsoids
have been drawn at the 50% probability level.
but the X-ray structure shows no evidence of partial occupancy
by oxygen in the fluorine site.
The fluorosilane in 2 is quite unusual. First, the Si-F bond
distance of 1.591(3) Å is very short for a tri(alkyl/aryl)fluorosilane
(other fluorosilanes can have shorter bonds16). The only tetra-
coordinate triarylfluorosilane in the Cambridge Structural Data-
base17 is tris{2-[(dimethylamino)methyl]phenyl}fluorosilane,18
where the Si-F distance is 1.627 Å, and the mean Si-F distance
for all tetracoordinate tri(alkyl/aryl)fluorosilanes in the database
is 1.64 Å, with none less than 1.604 Å. Second, the 19F NMR
resonance for 2 (δ 5.3) is 155 ppm upfield from that of tri(o-
tolyl)fluorosilane (15, δ 160.6). Such enormous shielding must
arise from the proximity of the fluorine to both the silicon atom
and the center of the basal aromatic ring. Taken together, the
observed bond distance and NMR shielding suggest that the Si-F
bond has been shortened due to steric compression.19 In such
circumstances, one might also expect to observe compressional
frequency enhancement of the Si-F stretching band in the IR
spectrum of 2, a common phenomenon in cyclophanes with in-
C-H and in-Si-H bonds,4,20 but the Si-F stretch falls in a
complex region of the IR (800-1000 cm-1), making a definite
assignment of this band impossible. The reactivity of 2 has yet
to be explored in detail, but an attempt to invert the configuration
at silicon, by treatment with 0.1 M KF in refluxing methanol,
returned only unchanged 2.
It is clear from the 100-fold difference in the yields of the
syntheses of 7 and 2 that the presence of the in-atom strongly
inhibits macrocyclization. Not only is the in-atom a steric
impediment to the approach of the sulfur nucleophile, but it may
also bias the conformation of partly cyclized intermediates to give
unfavorable geometries for cyclization. For this reason we suspect
that the placement of still larger in-functionalities in small
cyclophanes will require an entirely different synthetic approach.
(13) Crystal data for 2: C30H27FS3Si; monoclinic, space group P21/c; a )
10.38(2) Å, b ) 17.27(4) Å, c ) 15.41(2) Å, â ) 108.97(11)°, V ) 2612(8)
Å3, Z ) 4, Dcalcd ) 1.350 g/cm3. Intensity data were collected with 2θ e 93°
by using Cu KR radiation (1.541 78 Å) at 298 K on a Rigaku R-AXIS IIC
image plate system. A total of 13 353 reflections were indexed, integrated,
and corrected for Lorentz and polarization effects (using the program
DENZO14), and the data were scaled and merged to give 2197 unique
reflections (Rint ) 0.030). The structure was solved by direct methods; non-
hydrogen atoms were refined anisotropically, with hydrogens riding [C-H
) 0.93 or 0.97 Å, U(H) ) 1.2U(C)] (SHELXTL15). The refinement converged
to R(F) ) 0.0276, wR(F2) ) 0.0724, and S ) 1.044 for 2110 reflections with
I > 2σ(I), and R(F) ) 0.0311, wR(F2) ) 0.0948, and S ) 1.032 for 2196
unique reflections, 317 parameters, and 0 restraints (one reflection was
suppressed). Full details are provided in the Supporting Information.
(14) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307-326.
(15) Sheldrick, G. M. SHELXTL, Version 5; Siemens Analytical X-ray
Instruments: Madison, WI, 1996.
Acknowledgment. This work was supported by NSF Grant CHE-
9707958, which is gratefully acknowledged.
Supporting Information Available: Experimental procedures for the
syntheses of cyclophanes 2, 7, and 12, and crystal structure report for
compound 2 (including full experimental details, tables of atomic
coordinates, bond distances, bond angles, and thermal parameters, and
selected figures). An X-ray crystallographic file, in CIF format, is
available through the Internet only. See any current masthead page for
ordering information and Web access instructions.
(16) The presence of additional fluorine atoms (or other heteroatoms)
significantly shortens the Si-F bond. For example, the average Si-F bond
distance for C2XSi-F, where X ) O or F, is 1.60 Å, and, among these, Si-F
distances of 1.57 Å are not uncommon.
(17) Allen, F. H.; Kennard, O.; Taylor, R. Acc. Chem. Res. 1983, 16, 146-
153.
(18) Breliere, C.; Carre, F.; Corriu, R. J. P.; Royo, G.; Wong Chi Man, M.
Organometallics 1994, 13, 307-314.
(19) However, the compression cannot be too great: the apical silicon is
only slightly flattened, with average C-Si-F and C-Si-C bond angles of
108.6° and 110.4°, respectively.
JA981009S
(20) (a) Pascal, R. A., Jr.; Grossman, R. B.; Van Engen, D. J. Am. Chem.
Soc. 1987, 109, 6878-6880. (b) Pascal, R. A., Jr.; Winans, C. G.; Van Engen,
D. J. Am. Chem. Soc. 1989, 111, 3007-3010.