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Hexakis(trimethylsilylethynyl)[3]radialene A Carbon-Rich Chromophore with Unusual Electronic Properties.

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To obtain the unprotected glycopeptide, the acid-labile protecting groups of 10 were cleaved with trifluoroacetic acid in the
presence of ethyl methyl sulfide and anisole as cation scavengers.[221The 0-acetyl groups were removed by a sodium
methoxide catalyzed transesterification in
The 0glycopeptide 11 was purified by gel permeation chromatography on Sephadex G-I 5 and characterized by RP-HPLC, highfield N M R spectroscopy, and FAB-MS.[231
These results show that allylic anchoring of the C-terminal
amino acid to the HYCRON anchor provides the basis for an
efficient solid-phase peptide synthesis. Without changing the
allylic anchor. both Fmoc and Boc groups can be used as temporary protecting groups during the synthesis. Furthermore, both
acid- and base-labile structural elements are maintained in the
palladium(0)-catalyzed cleavage reaction, for example tert-butyl
ester and trrt-butyl ether groups, the Fmoc group, and the 0glycosidic bond in the synthesized glycopeptides 6 and 10. With
this method, glycopeptides can be obtained in high yields and in
high purity directly after deblocking.
Received: December 7, 1994 [Z7529IE]
German version: Angew. Chem. 1995, 107, 901
-
[20] R . Knorr. A. Trzeciak. W. Bannwarth, D. Gillessen, Tefruhi,dronLeft. 1989.30,
1927.
[21] The total yield is based on the initial amino acid load. Obviously, it is impossible for the total yield to be higher than the detachment yield. This deviation,
however, is within the experimental error of the amino acid analysis. which is
the basis of all calculations of yields on the solid phase.
[22] E. Atherton, R. C. Sheppard, J. D. Wade, J. Chem. Soc. C'hem. Commun. 1983,
1060.
[23] R, (Spherisorb ODSII Cl8/5p, 250 x 4 mm, 1 mL min- ' ) = 17.2 min (A: 0.1
TFA/MeCN, B: 0.1% TFA/H,O; 0 to 2 min: 1 YOA In B. 2 to 24min: 1 % A
in B to 30% A in B), [.I? = -112.5"C ( c = 1 , H,O); ' H N M R (400 MHz,
D,O): S = 4.82 (H-1'). 4.75 (D'), 4.48 (T'), 4.33 (Tp),2.75 (Dp); I 3 C N M R
(100.6 MHz, D,O): S = 98.6(C-1'), 75.2(Tp), 50.8 (D"), 29.4(DP); MS(FAB):
m / z =1126.8 ( M + H').
Hexakis(trimethylsilylethynyl)[3]r adialene :
A Carbon-Rich Chromophore with Unusual
Electronic Properties""
Tim Lange, Volker Gramlich, Walter Amrein,
Franqois Diederich,* Maurice Gross, Corinne Boudon,
and Jean-Paul Gisselbrecht
Keywords: solid-phase synthesis glycopeptide peptide
[ I ] a) R B. Mernfield, J. A m Chem. So<. 1963. 85, 2149; b) G. B. Fields. R. L.
.
Res. 1990. 35, 161 ; c) P. Lloyd-Williams, F. AlberiNoble, I n / . J P P ~Prorein
cio, E. Giralt. ?i,truherlron 1993, 49. 11065.
[2] See, for example. G. Jung. A. G. Beck-Sickinger, Angew. Chem. 1992,104,357;
Angcii.. Chrri?.hit. E d EngI. 1992. 31, 375.
[3] See. for examole. W. H . Moos. G. D. Green. M. P. Pavia. Annu. RPD.Med.
Chiw. 1993. 28. 315.
H. Kunz. B. Dombo (ORPEGEN GmhH). DE-A 3720269.3, 1987; US-A
4929671. 1990.
a) H. Kunz. B. Dombo. Angew. Chem. 1988, I M , 732; Angew. Chem. In!. Ed.
EngI. 1988, 27, 71 1 ; h) H. Kunz. B. Dombo, W. Kosch in Pepfides 1988 (Eds.:
G. Jung, E. Bayer), de Gruyter, Berlin, 1989, 154; c) B. Blankemeyer-Menge.
R . Frank, Tmahedrun Lett. 1988. 29. 5871; d) F. Guibe. 0. Dangles. G. Balavoine. A . Loffet, ibid. 1989, 30, 2641.
a ) H Kunz. H. Waldmann. Angen. Chem. 1984.96,49;Angew. Chem. I n / . Ed.
Engl. 1984, 23, 71 ; b) M. Ciommer. H. Kunz, Synlert 1991, 593.
a ) H Paulsen. G. M e n . U. Weichert. Angeir. Chem 1988, f00.1425; Angeris.
Chem. In!. E d Engl. 1988. 27, 1365; b) B. Liining, T. Norherg. J. Tejbrant, J
Chem. Sor. Chem. Commun. 1987, 1267; c) I. Laczko. M. Hollosi, L. Urge,
K. E. Ugen. D. B. Weiner, H . H. Mautsch, J. Thurin, L. Otvos, Biuchemisfrj
1990. 2Y, 4282; d) mucin-type glycopeptides: M. Meldal, S. Mouritsen, K.
Bock, An?. Chrm. Soic Synip. Ser. 1993, 519, 19.
a ) E . Atherton. R. L. Cameron, R. C. Sbeppard. Tetrahedron 1988,44.843; b)
F. Albericio. N . Kneib-Cordonier. S. Biancalana, L. Geva, R. I. Masada. D .
Hudsen. G. Barany. J. Org. Chum. 1990. 55. 3730.
a) E. Atherton. D. L. J. Clive, R. C. Sheppard. J Am. Chem. Soc. 1975, 97.
6584: b) F. Albericio. G. Barany, I n f . J. Pept. Protein Res. 1985, 26, 92.
W. Koscb. J. Miirz, H. Kunz, React. Po/ym. 1994, 22, 181, and references
therein.
W. Kosch. Dissertation, Universitit Mainz. 1992.
a) W. Rapp. L. Zhang. R. Hibich. E. Bayer i n Pepfrdes (Eds.: G. Jung, E.
Bayer). de Gruyter. Berlin. 1988. 199; b) E. Bayer, Angew. Chem. 1991, f03,
117: Angiw. Chrm. Inr. Ed. Engf. 1991, 30, 113.
a ) M. Kowalski. J. Potz. L. Basiripour. T. Dorfman, W. C. Goh, E. Terwilliger,
A . Dayton. G. Rosen. W Haseltine. J. Sodroski. Science 1987, 237. 1351; b)
C B. Pert, J. M. Hill, M. R. Ruff, R. M. Berman, W. G. Robey. L. 0. Arthur,
F. W. Ruscetti. W. L. Farrar, Pror. Nut/. Acud. Sci. USA 1986, 83, 9254.
D. Sarantakis. J. Teichman, E. L. Lien, R. L. Fenichel, Biochem. Biophvs. Res.
C'omrmm. 1976. 73. 336.
W. Kdnig. R. Geiger, Chen?.Brr. 1970, 103, 788.
Detachment yield = 10q1-(b,,,/bU A,.)n/fbp../bg~l,.)u]; h = loading. n =
after detachment, v = before detachment.
This yield is based on the original load of the polymer with p-alanine. Basing
the yield on the initial amino acid load is not reasonable, because the amino
dcid analysis of hydroxydmino acids generally results in low values and thus in
high yields.
J. Hilkens. M. J. L. Ligtenberg, H . L. Vos, S. L. Litvinov, TIES 1992. 17,359.
U. Karsten. G. Papsdorf. A. Pauly. B. Vojtesek. R. Moll, E. B. Lane, H.
Ciausen. P. Stosiek, M. Kasper, Difltrenricifion (Berlin) 1993, 54, 55.
Angel! . ( % i w . In/,Ed. Engl. 1995. 34. No. 7
8 VCH
In our program targeting the construction of novel all-carbon
molecules and polymeric networks,['' we became interested in
the synthesis of hexaethynyl[3]radialene ( I a, C,,H,), which had
been independently identified both by Hopf and Maas''] and by
one of
as a suitable molecular precursor for a regular
two-dimensional all-carbon network. A variety of [3]radialenes
have been prepared, and their electronic and redox properties
are strongly dependent on the nature of the substituents at the
double bonds.['] Derivatives with alkyl substituents generally
are colorless and difficult to reduce, whereas those with electron-accepting groups, for example hexacyano[3]radialene (2)
and hexakis(methoxycarbony1)[3]radialene (3),13] are colored
and reducible to the corresponding radical anions and subsequently to the dianions. Radialene 2 is yellow and very unstable;
it is readily reduced in two consecutive reversible one-electron
steps to the stable dianion 22- (see Table 1) .[31 Radical anion 2'-
xYx
R
R
laR= H
l b R = MeSSi
Ic R = (CPr)3Si
2 X=CN
3 X = C02Me
4
[*I Prof. F. Diederich, Dr. W. Amrein, T. Lange
Laboratorium fur Organische Chemie, ETH Zentrum
Universititstrdsse 16. CH-8092 Zurich (Switzerland)
Telefax: Int. code + (1)632-1109
Dr. V. Gramlich
Institut fur Kristallographie und Petrographie. ETH-Zentrum
Sonneggstrasse 5. CH-8092 Zurich (Switzerland)
Prof. M. Gross. Dr. C. Boudon, Dr. J.-P. Gisselbrecht
Laboratoire dElectrochimie et de Chimie Physique du Corps Solide
U. R. A. a u C . N. R. S. no 405, Faculte de Chimie, Universite Louis Pasteur
1 et 4, rue Blaise Pascal, B. P. 296, F-67008 Strasbourg Cedex (France)
[**I This work was supported by the Schweizerische Nationalfonds zur Forderung
der wissenschaftlichen Forschung.
Verlugsgesell.d~rrfimhH, 0-69451 Wernheim, 1995
0570-0833/95/0707-0805$ 10.00+ .ZS;0
805
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acts as acceptor component in a variety of donor-acceptor
complexes with both organic and organometallic donors, which
had been prepared as potential organic ferromagnet~.[~I
In contrast, the yellow radialene 3 is a stable compound which can be
reduced to the dianion 32- via a short-lived radical anion 3'-.[31
MO calculations, I3C NMR spectra, and X-ray structural
data[4b1
suggest that resonance structure 4 featuring an aromatic
cyclopropenium cation and negative charges delocalized over
the electron-withdrawing groups contributes significantly to the
ground states of 22- and 32-.
Hexaethynyl[3]radialene (1 a) is isoelectronic and isosteric with
the hexacyano derivative 2, and it therefore was of interest to
compare the structural and electronic properties of the two compounds. Since previous studies had shown that (R,Si-C=C),C=
fragments were electron accepting[51and that dialkynylmethyl
anions (R,Si-C=C),CR'- had significant stability,[61we expected
1 to show the characteristic properties of an electron-accepting
radialene. Additional incentive for the synthesis of 1 was provided by the computational prediction of its structure in high-level
calculations by Schaefer and co-workers.~'1Here we describe the
synthesis of hexakis(trimethylsilylethynyl)[3]radialene (1 b) and
report its low-temperature X-ray crystal structure and its
unusual electronic and redox properties.
Initial attempts to prepare 1 b or the triisopropylsilyl-protected analogue 1 c from 5 a, b by the carbenoid cyclooligomerization method of Iyoda et al.[*l yielded the persilylethynylated
Atbutatrienes 6a, b but no trace of radialene (Scheme
tempts to add carbenes or carbenoids, including those formed
R
R
5a R = Me3Si
5b R = (i-Pr)3Si
B
A
1
I
- 90 "C
,
654.4
100
R
R
1. BuLi, Et20,
yielded [3]radialene 1 b as a dark-red solid which melts under
decomposition at 183OC. DDQ was chosen as the oxidizing
agent since it is sufficiently soluble in THF at the low temperatures required for the oxidation of 1 b2- to 1 b. The new radialene is stable under laboratory conditions and dissolves in all
common aprotic solvents. The low yield of 5 % must be ascribed
to side reactions, due in part to the instability of the intermediate dianion 1 b2- and to decomposition of 1 b during chromatographic purification. The iPr,Si derivative 1 c cannot be
prepared by the same route, presumably because of the severe
steric overcrowding of the six bulky iPr,Si groups in the radialene product. Attempts to deprotect 1 b under mildest conditions with borax in solvents ranging from MeOH to THF and
mixtures thereof converted 1 b to a new product as indicated by
thin-layer chromatography; however, these solutions so far
have been too unstable to allow spectral identification of this
product as l a .
Radialene 1 b was characterized by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. The spectrum obtained in the positive-ion mode with
2,5-dihydroxybenzoic acid (DHB) as matrix showed the molecular ion at m/z 654.4 as base peak in a highly resolved molecular
ion cluster with an isotopic pattern in perfect agreement with that
calculatl (Fig. I). Besides a complete absence of fragmentation,
[%I
R
R
6a R = Me3Si
6b R = (i-Pr)$i
\
J
I b R = Me3Si
l c R = (CPr)3Si
0
200
400
Scheme 1. Unsuccessful attempts to prepare [3]radialene 1b.
1. MeMgBr, THF
-
Me3Si
2. Me3Si-CHpBr,
CuBr, 50 "C, 3 h, 93 %
7
1. t-BuLi, THF, - 78 "C
CI, ,CI
2.
,-78"C
CI
CI
p\
-
3.OOQ, - 78 + 20 "C,
5%
Scheme 2. Synthesis of [3]radialene 1 b.
806
0 VCH Verlagsgescllschafi mbH, 0-69451
800
1000
1200
m/z
from 5a, b, to the central double bond in 6a, b also failed. The
preparation of 1 b eventually succeeded starting from 1,5bis(trimethylsilyl)-penta-1,4-diyne (7)['] following, with modifications, the route employed by Fukunaga in the synthesis of 2
and 3 (Scheme 2) .[3a1 Addition of tetrachlorocyclopropene at
- 78 "C to the carbanion obtained by deprotonation of 6 with
iBuLi, followed by oxidation with 2,3-dichloro-5,6-dicyano-pbenzoquinone (DDQ) of the presumably formed dianion 1 b2-
Me@-H
600
Weinheim. 1995
1400
-
Fig. 1. MALDI-TOF mass spectrum of l b . Insert: The experimentally observed
molecular ion cluster (A) is in excellent agreement with the calculated isotope pattern (8).
the reflectron spectrum also featured an unusually complete suppression of any matrix interference.["] In contrast, electron impact mass spectrometry (70 eV) only gave a molecular ion with a
relative intensity less than 0.05% besides a multitude of fragments
due to extensive decay processes of the Me,Si groups. This
underscores once more the importance of LD-TOF mass spectrometry for the characterization of carbon-rich materials.[' ']
In the IR spectrum (KBr) of 1 b, the C-C stretches are observed at 2132 cm-', and the strong absorption at 1512cm-' is
assigned to the double bonds. The 'HNMR spectrum
(500 MHz, CDC1,) contains one singlet at 6 = 0.23, and the 3C
NMR spectrum (125.7 MHz, CDCI,) displays the expected five
peaks at 6 = - 0.2, 92.5, 105.4, 106.0, and 126.7.
Radialene 1b is highly colored; solutions in hexane are, quite
unexpectedly, intensely purple-red and crystals are deep red.
The electronic absorption spectrum in hexane (Fig. 2) extends to
an end absorption around i= 620 nm with the strong longest
'
lb
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A n g w . Chem. I n f . Ed. Engl. 1995, 34, No. 7
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A [om]
-
Fig. 2. Electronic absorption spectrum of 1 b in hexane, T = 298 K. d
( ' = ?.37X 1 0 - ' M .
= 1 cm,
wavelength band at A,,,, = 567 nm ( E = 41 000). These optical
properties contrast those of yellow 2 and 3, and rather resemble
those of the highly colored tris(fluoren-9-ylidene)cyclopropane
(S)rSbland tris(9-anthron-10-y1idene)cyclopropane 9,11*1 n-acceptor radialenes (Table 1) with much larger n-electron chromophores." 31
0
reductions occurred at -0.40 and -0.95 V, respectively, in
T H F (Fig. 4) and at -0.52 and - 1.09 V in CH,CI, (vs. Ag/AgCI, see Table 1). The slopes of the waves obtained on the platinum RDE were close to the 58 mV expected for a one-electron
9
Unambiguous structural proof for 1 b was obtained by lowtemperature (143 K) X-ray analysis of red platelets grown by
slowly evaporating a solution of CH,CN and CH,Cl,
(Fig. 3).[14] The carbon chromophore is nearly planar with a
maximum deviation of 0.046 b; out of the best plane. The ideal
D,, symmetry of 1b is reduced in the crystal presumably because of crystal packing effects. Significant distortions are seen
owing to the steric compression between pairs of Me,Si-C-C
groups attached to neighboring exocyclic double bonds. The
angles at the acetylenic C atoms deviate from linearity; the
strongest angle reduction is observed for the bond angle C(44)C(45)-Si(46) (165.9(2)"). In addition, the three -C-C-C=
angles are reduced to an average value of 114.3". Crystal packing effects rather than the steric compression of the Me,Si
groups seem to determine the extent (between 0.04 and 0.44 A)
to which the six Si atoms are turned out of the best plane of the
carbon chromophore.
The C-C bond lengths in the three-membered ring adopt
values between 1.420(5) and 1.431(3) A, which is in the range of
the endocyclic bonds observed in the parent [3]radialene and its
hexamethyl derivative."
At values between 1.350(4) and
1.358(3) A, the exocyclic C - C bonds adopt values typical for
C = C bonds; Schaefer and co-workers had calculated by DZP
SCF a b initio methods a value of 1.338 b; for 1a.
The redox properties of 1 b were examined by stationary
voltammetry on a platinum rotating disk electrode (RDE) and
by cyclic voltammetry (CV) in either T H F o r CH,CI, containing Bu,NPF, as supporting electrolyte. The radialene was readily reduced in two reversible one-electron charge transfers; the
Anycir. U w m . I t i t . Ed. h y l . 1995, 34. N o . 7
C VCH
Fig. 3. Molecular structure of1 b in the crystal. Selected bond lengths [A] and bond
angles ["I: C(l)-C(2) 1.429(4), C(l)-C(3) 1.420(5), C(l)-C(4) 1.355(4), C(2)-C(3)
1.431(3), C(2)-C(S) 1.358(3), C(3)-C(6) 1.350(4). C(4)-C(41) 1.429(4), C(4)-C(44)
1.436(3), C(S)-C(Sl) 1.431(4), C(S)-C(S4) 1.430(4),C(6)-C(61) 1.439(4), C(6)-C(64)
1.428(4), all CmC bonds are between 1.204(4) and 1.211(5); C(l)-C(2)-C(3) 59.5(2),
C( 1)-C(2)-C(5) 150.1(3), C(l)-C(3)-C(2) 60.2(2), C( 1)-C(3)-C(6) 149.4(2), C(2)C ( ] ) - C ( 360.3(2),
)
C(2)-C(3)-C(6) 150.4(3), C(2)-C(l)-C(4)148.2(3).C(3)-C(l)-C(4)
151.S(2), C(3)-C(2)-C(S) 150.4(3), C(41)-C(4)-C(44) 113.3(2). C(Sl)-C(S)-C(S4)
114.4(2), C(61)-C(6)-C(64) 115.5(2), C(4)-C(44)-C(45) 169.6(3), C(44)-C(4S)-Si(46)
165.9(2), C(5)-C(54)-C(SS) 171.3(3), C(S4)-C(55)-Si(56) 165.7(2), C(b)-C(64)-C(65)
172.0(3), C(64)-C(65)-S1(66) 169.8(2), all other C - G C and C=C-Si angles hetween 171.3(3) and 177.8(2).
8
1;
I [PA]
0
-4
-8
J
-1.5
-1.0
- 0.5
0.0
0.5
1.0
E IVI
Fig. 4. Cyclic voltammogram of 1b in THF (0.1 M Bu,NPF,) recorded with a Pt
electrode at a sweep rate of 0.1 V s - ' ; Ag/AgCl reference electrode.
Nernstian charge transfer. In cyclic voltammetry the two reductions had the characteristics of a reversible one-electron charge
transfer for scan rates 20.1 Vs-', whereas for lower sweep
rates the second charge transfer became slightly "irreversible",
characteristic of a subsequent chemical step associated with the
second charge transfer under the experimental conditions.["j]
Thin-layer spectroelectrochemical measurements were carried out on an optically transparent thin-layer electrode. The
conversion of 1b to 1b'- in T H F showed well-defined isosbestic
points at 1. = 306, 345, 432, and 608 nm (Fig. 5 ) . The radical
Verlugsgesellschufi mbH, 0-69451 Weinheim, 1995
o57U-Os33195jO707-0807$ lO.U(J
+3 j O
807
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11
300
400
5M)
600
Experimental Procedure
700
To a solution of 7 (0.8 g, 3.84 mmol) in dry THF (25 mL) at -78 "C was added
dropwise under Ar 1 . 7 tBuLi
~
(2.2 mL. 3.84 mmol) in pentane. The mixture was
stirred for 1 h at -78 "C, then tetrachlorocyclopropene (114mg. 0.64mmol) in
TH F (1 mL) was added dropwise. and the red solution was stirred a further 10 min.
At - 78 "C, DDQ (0.22 g. 0.96 mmol) in THF (2 mL) was added. and the solution
was allowed to warm to 20°C. Pentane was added. and the organic phase was
~ (1 x ) and saturated aqueous NH,C1(2 x ) , dried (MgSO,),
washed with 0 . 5 HCI
and concentrated to dryness. The reaction mixture was dry-loaded on a plug of
SO,, the plug eluted with hexane, and the filtrate. after concentration, chromatographed (80 g SO,. hexane/CH,CI, 8: 1) to afford 1b (21 mg. 5%) as a red
solid; m.p. 183 "C (decomp). IR (KBr): Y [cm-'1 = 2958 s , 2896 m, 2132 m, 1512 s,
1405 w, 1248 s, 1089 w. 851 s. 157 s. 699 m, 662 w, 628 m; 'H N MR (500 MHz,
CDCI,): 6 = 0.23 (s); "C NMR(125.7 MHz. CDCI,): 6 = 126.7, 106.0, 105.4.92.5.
-0.2; UV/Vis (hexane): L,,, ( E ) = 567 (41 000). 516 (36100). 479 (sh, 18400). 333
(23200). 314 (24400). 298 (sh, 164001, 230 (22600). 221 (21 500); MALDI-TOFMS: ml; 654 ( M ' ) .
800
I [nm]
Received November 9, 1994 [274641E]
German version AngeM Chem 1995, 107, 898
Fig 5. Time-resolved UV/Vis spectra for the reduction of 1b in THF (0.1 M
Bu,NPF,) on an optically transparent thin-layer electrode: first reduction step at a
controlled potential of - 1.2 V vs. ferrocene.
Keywords: hydrocarbons
anion 1 b'- could be quantitatively oxidized back to the neutral
radialene only for short electrolysis times (less than 120 s); for
longer reduction times, only a 90 % recovery of 1 b was possible.
Further reduction of 1 b'- resulted in a decrease of the bands
at around 700-800 nm, and the generated species were no
longer oxidizable back to l b - nor to the initial neutral species
l b , probably as a result of chemical reactions of the strongly
basic dianion. Large-scale electrolysis in CH,CI, in a classical
three-electrode cell showed for the first reduction step a color
change from purple-red to violet and for the second reduction
step a color change from violet to light green.
Table 1. Redox potentials of electron-accepting [3]radialenes.
Radialene
Ef!,' (I)
Ef;,' (11)
Solvent
Reference
electrode
lb
-0.40
-0.52
+1.13
+0.16
-0.27
+0.02
-0.95
-1.09
+0.34
-0.07
-0.75
-0.28
THF
CH,CI,
CH,CN
CH,CN
DMF
CH,CN
Ag/AgCl [a1
Ag/AgCI [a1
SCE
SCE
Ag/AgCI
SCE
2
3
8
9
Ref.
-
I3 a1
I3 a1
[8 bl
WbI
[a] The redox potentials of 1b were measured against ferrocene as internal standard
as -0.96 and - 1.51 V (in THF) and -0.94 and - 1.51 V (in CH,CI,). Ferrocene
was oxidized in THF at + 0.56 V and in CH,CI, at + 0.42 V vs. Ag/AgCI.
With its six electron-withdrawing alkyne substituents at the
double bonds, radialene 1b exhibits redox properties similar to
those of other derivatives with electron-accepting substituents
and can be reduced stepwise and, under CV conditions, reversibly to the radical anion 1 b'- and subsequently to dianion
1 b2-. The cyclopropenium resonance structure 4 should contribute less to the stabilization of the ground state in 1 b2- than
in 2'- and 3,-, since CEC bonds stabilize negative charges in
a position less effectively than cyano and methoxycarbonyl
groups. The different stabilization of negative charges by the six
substituents in the radical anion and dianion explains why 1 b is
reduced at much more negative potentials than 2 and 3
(Table 1). No good explanation is currently available for the
unexpectedly strong bathochromic shift of the electronic absorption spectrum o f purple-red-colored 1 b compared to the
spectra of yellow-colored 2 and 3; further clarification of the
electronic properties of 1 b and its reduced forms awaits highlevel computational studies.
808
Q YCH Ver/agSgeSe//schuft mhH, 0-69451 Weinheim, 199s
. radialenes
. spectroelectrochemistry
[l] a) F. Diederich. Y Rubin, Angew. Chem. 1992, 104,1123-1 146: Angew. Chem.
l n t . Ed. Engl. 1992, 31. 1101-1123; b) F, Diederich, Nature [London) 1994.
369. 199-207.
121 H. Hopf. G. Maas. Angew. Chem. 1992.104.953-977; Angew. Chem. In[. Ed.
Engl. 1992.31.931 -954.
[ 3 ] a) T. Fukunaga, J. Am. Chem. Sor. 1976,98,610-611; b) T. Fukunaga, M. D.
Gordon, P. J. Krusic, h i d . 1976. 98. 611-613.
[4] a) J. S. Miller, A. J. Epstein, Angew. Chem. 1994.106,399-432; Angew. Chem.
hi.Ed. Engl. 1994, 33, 385-41 5; b) M. D. Ward, Organometatlics 1987, 6.
754-762; c) J. S. Miller, D. T. Glatzhofer. D. M. OHare, W. M. Reiff, A.
Chakraborty, A. J. Epstein. fnorg. Chem. 1989,28,2930-2939; d) T. Sugimoto,
Y. Misaki, Z.-I. Yoshida, J. Yamauchi, Mot. Cryst. Liy. Cryst. 1989, 176,259270; e) R. Breslow, ibid. 1985, f25, 261 -267; f ) T. J. LePage. R. Breslow, J.
Am. Chem. Soc. 1987, 109, 6412-6421.
[5] J.-D. van Loon, P. Seiler, F. Diederich. Angew. Chem. 1993. 105, 1235-1238;
Angew. Chem. f n t . Ed. Engl. 1993,32, 1187- 1190.
[6] a) H. Hauptmann, Tetrahedron Lett. 1974, 3593-3596; b) J. Anthony, C.
Boudon, F. Diederich, J.-P. Gisselbrecht, V. Gramlich, M. Gross, M. Hobi, P.
Seiler, Angew. Chem. 1994, f06,794-798; Angew. Chem. fnr. Ed. Engl. 1994,
33,763-166.
[7] B. Ma, H. M. Sulzbach, Y Xie, H. F. Schaefer 111, J. Am. Chem. Soc. 1994, 116,
3529-3538.
[S] a ) M. Iyoda, H. Otani, M. Oda. J. Am. Chem. SOC.1986, 108, 5371-5372;
b) M. lyoda, H. Otani, M. Oda, Angew. Chem. 1988, 100,1131- 1132; Angew.
Chem. f n f . Ed. Engl. 1988,27.1080-1081; c) M. Iyoda, H. Kurdta, M. Oda, C.
Okubo, K. Nishimoto, Angew. Chem. 1993,105,97-98; Angew. Chem. f n t . Ed.
Eng/. 1993, 32, 89-90.
PI Compound 7 was prepared according to general procedures for the synthesis
of 1.4-pentadiynes in : L. Brandsma, Prepararive Acetyknic Chemistry, 2nd ed.,
Elsevier, Amsterdam, 1988.
[lo] The complete suppression of matrix interference suggests that the ionization of
1 b in the DHB matrix must proceed by a pure charge transfer (CT) process at
a laser fluence well below the normal threshold of Droton transfer in MALDI.
This is consistent with recent results from Ledingham and co-workers. (C. T. J.
Scott, C. Kosmidis, W. J. Jia, K. W. D. Ledingham. R. P. Singhal. Rap. Commum Mass Spectrom. 1994, 8 , 829-832), who report extensive subthreshold
photoactivity of DHB. Alternatively, CT ionization of 1b can be viewed as
a matrix-assisted resonance two-photon ionization process, because the
radialene absorbs strongly at the laser wavelength of 337 nm (3.67 eV).
[ l l ] A. M. Boldi, F. Diederich, Angew. Chem. 1994,106,482-485; Angew. Chem.
h i . Ed. Engl. 1994, 33, 482-485.
[12] a) J. L. Benham, R. West, J. A. T. Norman, J. Am. Chem. SOC.1980,102.50475053; b) J. L. Benham, R. West, ibid. 1980, 102, 5054-5058.
=
,400 nm ( E 16300,
[13] Longest absorption wavelength maxima of 3 [3b]: i.,,
CH,CN), of 8 [Sb]: A,,, = 675 nm ( E 52500, CH,CI,), and of 9 [12a]:
A,, = 850 nm ( E 15100, CHCI,).
1141 Crystals of 1 b grown at room temperature crystallized in the triclinic space
group PT. On cooling to 143 K, a phase transition to the monoclinic space
group P2Jn took place with doubling of the cell volume and without destruction of the crystal. X-ray crystal data for 1 b (C36H54Si6,
M , = 655.3): Monoclinic space group P2,/0, pEplrd
=1.020 gcm--l, 2 = 4, a = 15 909(5),
h = 9.440(3), c = 28.410(11) A. p = 90.39(3)", V = 4267(3) A', MoKZI.radiation. 2 0 I 50", 5322 unique reflections, T = 143K. The structure was solved bv
direct methods (SHELXTL PLUS) and refined by full-matrix least squares
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;in;ilysis using unit weights (heavy atoms anisotropic. H atoms fixed. whereby
H positions are based on stereochemical considerations). Final R ( F ) = 0.0368.
I I R ( F ) = 0.058 [or 596 variables and 4864 observed reflections with F > 5u(Fj.
Furthcr detiiils of the crystal structure investigations may he obtained from the
Director of the Cambridge Crystallographic Data Centre. University Chemical
L,iboratory. 12 Union Road. GB-Cambridge CB2 1EZ (UK), on quoting the
full piirnal citation.
[IS] ; I ) I D. dun it^. A . Magnoli. H e i r . Chini. Acru 1966. 49. 1680-1681: b) E. A.
Dorko. 1. L Hencher. S. H. Bauer. Terroheclron 1968, 24. 2425-2434: c) H.
Dietrich. H Dierks, Airgew. Chon. 1968, 80, 487 488: Angrw. Cheni. Inr. Ed.
~ , ~1968.
~ i 7.465.
.
[ I 61 Although the solvents were dried. traces of moisture in the electrochemical cells
cannot be excluded. Therefore. the chemical step mentioned could be the protonation o f t h e dianion Ib 2 - .
MepSi(CI)H
-MepsiGI,
96%
Bis(pentafluoropheny1)borane: Synthesis,
Properties, and Hydroboration Chemistry
of a Highly Electrophilic Borane Reagent**
Daniel J. Parks, R u p e r t E. von H. Spence,
and Warren E. Piers*
Hydroboration is one of the most widely used reactions in
organic synthesis!'] A wide variety of borane reagents have been
reported such that a library of reagents exists tailored to many
different chemical situations. We were interested in the electrophilic and potentially very reactive borane HB(C,F,), for possible applications in generation of soluble Ziegler -Natta-type
olefin polymerization catalysts. These studies revealed this borane to be an exceptionally active hydroboration reagent. In this
communication we describe its synthesis, characterization, and
a preliminary survey of its activity towards alkenes and alkynes.
Bis(pentafluoropheny1)borane (1) was prepared from the
known chloroborane (C,F,),BCI[21 in the absence of Lewis
bases by reaction with hydride sources such as [Cp,Zr(CI)H],,
Bu,SnH. and Me,Si(CI)H (Scheme 1). Traditional metathetical
methods for transformations of this typeL3]were not advisable
because they necessitated the use of donor solvents which were
difficult to remove completely (if at all) owing to the high Lewis
acidityC4'of l .[51 The most convenient hydride transfer agent
proved to be Me,Si(CI)H since it also served as solvent for the
reaction and the by-product Me,SiCI, was easily removed; 1
was observed to precipitate over the course of one hour and was
isolated in high yield by filtration. The overall yield of 1 from
bromopentafluorobenzene was 52 YO.
Borane 1 is a white, microcrystalline, oxygen- and moisturesensitive solid which was found to be to stable (i.e. it retained
100 YOof its activity) for at least three months when stored under
an inert atmosphere at room temperature. Spectroscopic evidence indicated that the solid material is dimeric in structure.
The infrared spectrum of the solid showed a strong band at
1550 cm- characteristic[61of the vaSymin-phase mode for a B(p-H),-B unit, and no bands corresponding to stretching vibrations of terminal B-H bonds were evident.[71 Solution N M R
data were, however, consistent with the presence of monomeric
[*I
[**I
Prof. W. E. Piers. D. J. Parks. Dr. R. E. von H . Spence
Guelph-Waterloo Centre for Graduate Work in Chemistry
Guelph Campus. Department of Chemistry and Biochemistry
University of Guelph
Guelph. Ontario N I G 2W1 (Canada)
Telet:dx: Int. code + (519)766-1499
Financis1 support for this work was provided by the Novacor Research and
Technology Corporation ofCalgary, Alberta, and by the Natural Sciences and
Engineering Council of Canada (postgraduate scholarship for D. J. P.).
A n p i . . Chiwi. Inr Ed. ErigI. 1995, 34. N o . 7
ti>
1
Scheme 1. Synthesis of (C,F,),BH (1). In the last step the reagent Me2Si(CIjHalso
serves as the solvent.
(C,F,),BH. Most convincingly, the "B N M R spectrum revealed the presence of a minor species ( zI0 %) with a signal at
6 = 60.1 ppm in addition to a major species giving rise to a signal
at 6 = 18.0. The downfield resonance appears in the region associated with monomeric dialkylboranes,[*I while the signal due to
the major species is more typical of a dimeric borane.['] Thus,
although borane 1 is a dimer in the solid state, dissociation into
a monomeric borane is facile in aromatic solvents.
Bis(pentafluoropheny1)borane is a highly active hydroboration
reagent towards a range of simple alkenes and alkynes (Table 1).
Addition of the olefin or alkyne to a suspension of the borane
in benzene led to the rapid dissolution of the solid, and the
reaction was complete within two minutes.["] Even sterically
demanding olefins (Table 1, entry 1) were hydroborated very
rapidly, and the rates of hydroboration of methylcyclohexene
and methylcyclopentene were almost identical." '. '*I These observations are in contrast with those seen in reactions employing
the common hydroboration reagent 9-borabicyclo[3.3.1]nonane
(9-BBN) wlich, under identical conditions, required several
hours to go to completion with these substrates. The only substrates that d o not react rapidly with 1 are those having a
B(C,F,), functionality attached, in other words those formed
upon monohydroboration of alkynes (entries 6-8). Not only
are these alkenes more sterically encumbered, the electron-withdrawing effect of the B(C,F,), substituent likely deactivates the
double bond towards subsequent hydroboration. By comparison, hydroborations of alkynes with 9-BBN generally proceed
more rapidly for the second hydroboration, often precluding
isolation of the singly hydroborated product unless excess substrate is employed.['31
Entries 2-4 in Table 1 demonstrate that 1 hydroborates the
substrates by the commonly accepted cis addition mechanism.['"I
Regioselectivity was found to be excellent for substrates for which
multiple products were possible. For example, styrene was hydroborated to give the isomer shown with >98 YOselectivity (as
shown by 'H N M R spectroscopy and oxidation to 2-phenylethanol, vide infra). In some instances (entries 1-3) facile isomerization by means of a retrohydroboration - rehydroboration
sequence was observed over the course of a few hours at room
temperature. This behavior is typical for substrates with thexyl
substituents," 'I but for other substrates elevated temperatures
are usually required to induce these rearrangements.[' Fortu-
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trimethylsilylethynyl, properties, electronica, hexakis, radialene, chromophore, unusual, rich, carbon
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