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CD Spectroelectrochemistry a Method for the Characterization of Chiral Electron Transfer Reagents and Chiral Reaction Intermediates.

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contrast, from Scheme 1 we would expect elimination of
the silyl and the alkyl ligand to yield SiMe, and [(cO),cOH] [reaction (f)]. We find CH4, not SiMe, by 'H-NMR
H
I
(CO),Co-Me
I
-
CD Spectroelectrochemistry, a Method for the
Characterization of Chiral Electron Transfer
Reagents and Chiral Reaction Intermediates**
By Jorg Daub,* Josef Salbeck, and Irmgard Aurbach
SiMea + (C0)3Co-H
(0
SiMez
(6=0.18) and [(CO),Co-SiMe,] by FTIR. We cannot completely rule out the possibility that some SiMe, is also
formed, but formation of CH4 is certainly the predominant
reaction.
We have thus found evidence for all steps postulated in
Scheme 2. Formation of CH4, not SiMe,, in the reaction of
[(CO),Co-Me] with Me,SiH is clearly inconsistent with the
Chalk-Harrod mechanism, Scheme 1. We therefore propose the mechanism depicted in Scheme 2 for hydrosilation catalysis, at least with carbonylcobalt complexes. This
new mechanism involves as key step the insertion of an
olefin into a Co-Si bond.
Received: September 23, 1987 [Z 2439 IEI
German version: Angew. Chem. 100 (1988) 281
[ I ] J . F. Harrod, A. J. Chalk in 1. Wender, P. Pino (Ed.): Organic Syntheses
uia Metal Carbonyls. Vol. 2. Wiley, New York 1977, p. 673.
121 J. L. Speier, Adc. Organornet Chem. 17 (1979) 407.
[3] a ) A. J Chalk, J. F. Harrod, J . Am. Chem. Sac 8 7 (1965) 16; b) J. F.
Harrod, A. J. Chalk, ibid. 8 7 (1965) 1133.
141 a) G W. Parshall: Homogeneous Cafa/yis,Wiley, New York 1980, p.
29; b) N. M. Doherty. J. E. Bercaw, J. Am. Chem. Sac. 107 (1985) 2670;
c) D. C. Roe, J . Am. Chem SOC.I05 (1983) 7770.
151 a) A. J. Blakeney, J. A. Gladysz, lnorg. Chim. Acfa 53 (1980) L25; b) K.
C . Brinkman, A. J. Blakeney, W. Krone-Schmidt, J. A. Gladysz, Organometallic.s 3 (1984) 1325.
[61 M. A. Schroeder. M. S. Wrighton, J . Organomet. Chem. 128 (1977) 345.
[7] R. G. Austin, R. S. Paonessa, P. J. Giordano, M. S. Wrighton, Adu.
Chem. Ser. 168 (1978) 189.
[a] C. L. Reichel, M. S. Wrighton, lnorg. Chem. 19 (1980) 3858.
[9] a) A. Millan, M. J. Fernandez, P. Bentz, P. M. Maitlis, J . Mol. Card 26
(1984) 89: b) A. J. Cornish, M. F. Lappen, J . Orgonomet. Chem. 271
(1984) 153; c ) A. Onopchenko, E. T. Sabourin, D. L. Beach, J . Org.
Chem. 49 (1984) 3389; d) A. Onopchenko, E. T. Sabourin, D. L. Beach,
rhtd 48 (1983) 5101; e) Y. Seki, K. Takeshita, K. Kawamoto, S. Murai,
N. Sonoda, Angew. Chem. 92 (1980) 974; Angew. Chem. Inr Ed. Engl. 19
(1980) 928; f ) A. N. Nesmeyanov, R. K. Freidlina, E. C. Chukovskaya,
R. G. Petrova, A. B. Belyavsky, Tetrahedron 1 7 (1962) 61; g) B. Marciniec, J. Gulinski, J . Organornet. Chem. 253 (1983) 349.
[lo] C. L. Randolph, M. S. Wrighton, J . Am. Chem. Sac. 108 (1986) 3366.
[ I l l F. R. Anderson, M. S. Wrighton, J . A m . Chem. Sac. I06 (1984)995.
[I21 IR spectroscopic data (CO stretching region) for relevant complexes
[cm 'I: [(C0)4Co-SiEt,]: 2089 m, 2026, 1995 s ; [(CO),Co-SiEt,l: 1957 s,
1953 s: [(CO),(C2H4)Co-SiEtJ: 1968 sh, 1961 s; [(CO),CoC(0)CH2CH,]: 2105 m, 2045 m, 2023 s, 2002 s.
1131 When a solution of [(CO),Co-SiEt,] in ethylene containing [D&oluene
is irradiated, signals at 6 = 1.15 (m)for Et,Si and 3.00 (s) for coordinated
ethylene are observed. For the resonance of coordinated ethylene cf. Y.M. Wuu. J. G. Bentsen, C. G. Brinkley, M. S. Wrighton, Inorg. Chem. 26
(1987) 530
[141 Under I atm of CO at O T , 210 mg (0.3 mmol) of [(Ph,P),N][Co(CO),] in
2 m L of T H F was added to 200 mg (1.1 mmol) of [Et,O][BF,]. After stirring the solution for 15 min, the solvent was removed in vacuo. The residue was redissolved in I m L of methylcyclohexane. [(Ph,P)2N][BF,J
and excess [Et,Ol[BF,I was removed via filtration and the solution was
used without further purification. Only signals due to [(CO)4CoC ( 0 ) C H K H J were observed in the FTIR spectrum; cf. L. Marko, G.
Bor, G . Almasy, P. Szabo, Brennsf. Chem. 44 (1963) 184.
1151 At 0°C' 45 mg (0.3 mmol) of Me1 was added to a solution of 70 mg
(0.4 m m o l ) of Na[Co(CO),] in I m L [D,]THF. After 2 min the resulting
[(CO),C'o-Me] was allowed to react with 100 mg (1.3 mmol) of Me,SiH.
The solution was strrred for 20 min at 0°C and then warmed up. Immediately dfter the solution reached room temperature, a 'H-NMR spectrum was recorded: 6=0.18 for methane. Formation of [(CO),CoC(O)C'HJ accompanies formation of [(CO)aCo-Mel in the synthesis (see
[I411 and the thermolysis of the [(CO),Co-Me]/J(CO),Co-C(0)CH3]
mixture i n the presence of Me&H gives CH,CHO in addition to CH,
(cf. a k o R. W. Wegman, Orgonometallrcs 5 (1986) 707, which shows formation of C H K H O from reaction of [(CO)3(PPh,)Co-C(0)CH3] with
EtSiH or Ph,SiH).
Angew C'lrenr. Inr Ed. Enql. 27 11988) No. 2
The combination stereochernistry/electron transfer (ET)
has so far never been dealt with in monographs o n stereochemistry even though a close relationship exists between
both fields.['.'] Only a slight stereoselectivity has thus far
been observed in ET reactions carried out in homogeneous
solution. This holds true for diastereoselective reactions
and in particular for enantioselective reactions.I3I
We now show that chiral ET reagents can be investigated by the use of chiroptic methods (here measurement
of the circular dichroism) in spectroelectrochemical measurement~.'~'
For this purpose a thin-layer cell developed
for spectrochemical investigations, which consisted of an
optically transparent working electrode (OITLE = optically transparent thin-layer electrode, indium oxide doped
with tin oxide, coated on glass), a platinum disk as counter
electrode and an Ag/AgC1 reference ele~trode,'~]was
mounted in the beam of a CD and UV/VIS spectrometer.
By means of this instrumental coupling of potentiostatic
intermediate generation and electron spectroscopic (UV/
VIS) and chiroptic (CD) detection, both electrochemical as
well as structural properties of chiral intermediates can be
determined. Thus, a method is available which provides
access to important information about the properties of
chiral electron transfer compounds and about their use for
enantioselective redox reactions.I6' Herein we shall outline
the application of the method to the study of the axially
chiral bisanthraquinone 1.
0
( R ) - l - (R)-5
n
(S)-1 - ( S ) - 5
1, R = C 0 2 C H I ; 2, R = C H , ; 3, R = C H B r 2 ; 4, R = C H O ; 5 , R = C 0 2 H
The enantiomeric forms of I are readily accessible on a
preparative scale: starting from rac-2,2'-dimethyl-l , I '-bianthraquinone 2, the dicarboxylic acid rac-5 was prepared
via rac-3 and rac-4;17]this was then separated into the two
enantiomers (R)-(- ) - 5 and (S)-(+)-5.[*l Subsequent reaction with diazomethane furnished the optically active esters (R)-(-)- 1 and (S)-(+)-1 respectively, in good yields.'']
The optical purity of both enantiomers was determined
'H-NMR spectroscopically (250 MHz): in the presence of
( -)-9-( I -hydroxy-2,2,2-trifluoroethyl)anthraceneas chiral
auxiliary reagent the 'H-NMR spectra of the enantiomers
each show only the signals of one diastereomeric associate.
The absolute configurations of (R)-(-)-1 and (S)-(+)-1
were derived from the known configurations of (R)-(-)-2,
(S)-(+ ) - 2 and (R)-( -)-5, (S)-(+)-5.['01
J. Salbeck, DipLChem. 1. Aurbach
Institut fur Organische Chemie der Universitat
Universitatsstrasse 3 1, D-8400 Regensburg (FRG)
This work was supported by the Stiftung Volkswagenwerk a n d the
Deutsche Forschungsgemeinschaft We thank Prof. H . Emnner for placing the CD spectrometer (circular dlchrograph J-40A, Jasco) at our disposal and Frau D . Andert for instructions o n the use of the apparatus.
[*] Prof. Dr. J. Daub, Dipl:Chem.
["I
0 V C H Verlagsgesellscha~m b H . 0-6940 Weinheim. 1988
0570-0833/88/0202-0291 $ 02.50/0
291
For the electrochemical and spectroscopic measurements, 1 was dissolved in acetonitrile. Bu4NPF6 served as
supporting electrolyte; the substrate concentration was
5.4~
moI L-'.
IS[
10
a'
1;
The cyclovoltammograms of (S)- and ( R ) -1 show the reversible reaction cycle (a) with the following half-wave potentials: E'=1190mV; Ez= -1360mV,E3=E4= -1780mV
(Fig. 1). In the electronic spectra obtained by UV/VIS
-15
,
,
1
,
~
500
LOO
300
-J,
600
,
700
h inml61L nm
b)
A
30000-
-60
t 2oooo
-50
&
1
10000 -
-40
0
-30
300
A
I
-
600
500
LOO
InmI
+
700
Fig. 2. Spectroelectrochemistryof ( R ) - (-)- 1 and (S)-( )- 1. a) CD spectra of
the enantiomeric bisradical anions 1 2'oe'. b) Electronic spectrum of (S)-(+)-
-20
12'0e).
[@I
i-10
0
10
0,
20
Scheme 1.
30
I
0
I
-400
I
I
-800 -1200 --la30
~[rnVlvs.FOC
I
-Moo
t
as bisradical anion
absorptions of the radical anion 6 0 e appear at A,, = 390,
410, and 540 nm. The dianion 620 has its most intense band at /1,,,=480 nm and a weaker absorption at
645 nm.
Fig. I. Cyclovoltammogram of (.S)-(+)-l in CH3CN,c=5.4x IO-'rnol L - ' ,
scan 200 mV s - ' ; FOC = ferrocene.
5
spectroelectrochemical inspection of 1 the formation of
the monoanion 1 O e is recognizable only by one isosbestic
point at A,, = 360 nm. 1 O e is thereby formed at a stationary potential of - 1275 mV. The dianion 12'o'e)(absorptions at 403 and 614 nm, Fig. 2b) is formed at a stationary
potential of - 1600 mV. At a potential of - 2200 mV the
tetraanion 14e is then formed. Its absorption spectrum
shows an intense band at A,,,=490 nm with a shoulder
extending to 800 nm.
is a bisradical anion (Scheme I), as
The dianion 1
is clearly shown by comparison of its electronic spectrum
(Fig. 2b) with that of 9,10-anthraquinone 6 obtained by
UV/VIS spectroelectrochemistry (Fig. 3). The most intense
292
0 V C H Verlagsgesellschafl mbH. 0-6940 Weinheim. 1988
t L
lo-&€
3
2
1
0
300
LOO
500
Fig. 3. UV/VIS spectroelectrochemistry
- 2300 mV.
0570-0833/88/0202-0292 $ 02.50/0
-
600
hlnml
700
of 6 . Potential
800
range 0 to
Angew. Chem. Int. Ed. Engl. 27 (1988) No. 2
The C D spectra of electrochemically generated ions of
(R)-(- )- 1 and (S)-(+)-1 were recorded over the potential
range 0 to - 2200 mV. Figure 2a shows the C D spectra of
both enantiomeric forms of 12(oe',
which were generated
at a stationary potential of = - 1600 rnV.["] (S)-lz'oc)
shows a positive A&for the longest wavelength absorption;
in the case of (R)-12'oQ)the corresponding band has a negative AE value of equal intensity. The tetraanion (S)-(+)1 4 0 generated at a potential of -2200 mV has a positive
AE value for the intense band at d,,,=490 nm. A further
band with positive A& is observed at 630 nm.
The reaction cycle between bianthraquinone 1 and bisradical anion 1
is electrochemically reversible over
several measuring cycles. Racemization could not be observed. On the other hand, during the electrochemical formation of the tetraanion 140 with Bu4NPF6 as supporting
electrolyte, a chemical secondary reaction can already be
observed after the first
Received: August 17, 1987;
revised: October 12, 1987 [Z 2402 IE]
German version: Angew. Chem. 100 (1988) 278
[I] J . D. Morrison (Ed.): Asymmetric Synthesis. Vol. 1-5, Academic Press,
New York 1983-1985.
[2] a) J. Deisenhofer, 0. Epp, K. Miki, R. Huber, H. Michel, J. Mol. Biol.
180 (1984) 385; b) R. J. Cave, P. Siders, R. A. Marcus, 1. Chem. Phys. 90
(1986) 1436.
[3] For studies o n this theme see: a) B. Grossman, R. G. Wilkins, J . Am.
Chem. SOC.8 9 (1967) 4230; b) N. A. P. Kane-Maguire, R. M. Tollison,
D. E. Richardson, Inorg. Chert. 15 (1976) 499; c) M. Hatano, T. Nozawa, S . Ikeda, T. Yamamoto, Mukromol. Chem. 141 (1979) 11 ; d ) G. 8.
Porter, R. H. Sparks, J. Chem. Soc. Chem. Commun. 1979, 1094; e) D. A.
Geselowitz, H. Taube, J. A m . Chem. SOC.102 (1980) 4525; 0 Y. Kaizu,
T. Mori, H. Kobayashi, J. Phys. Chem. 8 9 (1985) 332; g) B. Pispisa, A.
Palleschi, M. Bateri, S. Nardini, J . Phys Chem. 89 (1985) 1767; h) P.
Osvath, A. G. Lappin, Inorg. Chem. 26 (1987) 195.
141 In the redox titrations of soluble ferredoxin, isolated from spinach
leaves, C D spectroscopy was employed as method of analytical detection: F. M. Hawkridge, B. Ke, Anal. Biochem. 78 (1977) 76.
[ S ] a) J. Bindl, P. Seitz, U. Seitz, E. Salbeck, J. Salbeck, J. Daub, Chem. Ber.
120 (1987) 1747; b) all potentials referred to ferrocene (FOC); c) for a
review of UV/VIS spectroelectrochemistry see: T. Kuwana, W. R.
Heineman, Acc. Chem. Res. 9 (1976) 241.
[6] J. Bindl, G. Pilidis, J . Daub, Angew. Chem. 96 (1984) 294; Angew. Chem.
Int. Ed. Engl. 23 (1984) 314; J. Daub, L. Jakob, J. Salbeck, Y. Okamoto,
Chimiu 39 (1985) 393.
[7] F. Bell. D. H. Waring, 1. Chem. SOC.1949, 1579.
[8] F. Bell, W. H. D. Morgan, J . Chem. SOC.1950, 1963; cf. also R. Kuhn, 0.
Albrecht, Justus Liebigs Ann. Chem. 464 (1928) 91.
[9] Physical data of rac-1, (R)-(-)-1 and (S)-( + ) - 1 . ruc-1: m.p. 259-260°C;
IR (KBr): 5=3080,2970, 1740, 1680, 1595, 1575, 1320, 1270, 1230, 1150,
1005, 955, 725crn-'; ( R ) - ( - ) - l : [a]$= - 132 (c=0.25, CH3CN}, m.p.
198°C; (S)-(+ ) - I : [a]$=
124 (c=0.37, CH,CN), m.p. 200°C.
[lo] S. Yamada, H. Akimoto, Tetrahedron Lett. 1968. 3967; H. Akimoto, S.
Yamada, Tetrahedron 27 (1971) 5999.
[ I I ] The C D spectra of the two enantiomers were fitted 10 the same zero
line.
1121 In order to observe reversibility u p to formation of the tetraanion,
Me,NBF, must be used as supporting electrolyte.
+
from, inter alia, the Mo-Mo distances of between 272 and
278 pm. In the case of titanium, however, far fewer cluster
compounds are known. Examples are [ C P ~ T ~ ~ ( ~with
~-S)~]
bridging sulfur atoms161and [ C ~ ~ T i ~ ( p ~with
- 0 )bridging
~]
oxygen atoms;['] in these compounds the titanium has an
oxidation number between +I11 and + I V and the Ti-Ti
distances are on average 3 17 and 289 pm, respectively. The
Ti-Ti distances in [(MeC5H4),Ti4S80Z]are of the same order of magnitude;[81this compound has been described as
a cluster, although according to Cotton's definition['l it
should not be regarded as a cluster, since the titanium has
oxidation number + IV and therefore no metal-metal
bonds should be present. We report here on a further Ti'"
compound with a structure typical for clusters.
In the reaction of titanium tetrachloride with bis(trimethylsilyl) sulfide [Eq. (a)] a black precipitate is formed
which mainly consists of TiSCL,. When the precipitate is
treated with a solution of tetraphenylphosphonium chloride or tetraethylammonium chloride in dichloromethane
the greater part turns green or orange in color respectively;
the tetrachlorothiotitanate ion 1 is formed.
TiCI,
+ S(SiMe,)2
-
TiSCI,
+ ZCISiMe,
J +><.la
[TiSCl4I2' 1
At the same time a small amount of the precipitate dissolves, resulting in a red solution. After addition of CCI,,
red (PPh,),[Ti,(p3-O)(p-S2)3C161
2a or (NEtd2[Ti3(p3O)(p-Sz)3C16] 2b, respectively, crystallizes from the solution. The oxygen in the anion 2 presumably comes from
the inert gas (anhydrous Nz containing small amounts of
0,).This assumption is supported by the finding that the
yield of 2 is considerably greater on repeating the experiment in an atmosphere of pure oxygen [Eq. (b)].
3 TiC1,
+ 6 S(SiMe3)2+ 312 0,
--t
[Ti30(S2),Cl4]+ 8 CISiMe,
+ 2 O(SiMe3)>
1+ 2CIQ
(b)
[Ti30(S2)3CI,]2Q2
(PPh4),[TiSCl4] l a and (NEt4)z[TiSC14]l b can be recrystallized from warm acetonitrile. Crystalline l a is green,
crystalline l b orange. l b crystallizes tetragonally with the
[TiSC14]2Qions on fourfold axes of
The ions
have a square-pyramidal structure with a short TiS bond
(21 1 pm) (Fig. 1). This can be interpreted in terms of a Ti-S
double bond, consistent with the I R spectrum
(v(TiS)= 530 cm-'1.
[TiSCl4jZQ
and [Ti30(S2)3C16]Z",
a Multinuclear
Complex with a Structure Typical for Clusters
By Ulrich Miiller* and Volker Krug
For molybdenum, a series of cluster compounds of the
composition [ M O , ( ~ ~ - S ) ( ~ - S , ) with
~ X ~molybdenum
]~~
in
the oxidation state IV (X = C1, CN, S etc.) are known.['-51
Such compounds contain metal-metal bonds, as is evident
+
[*I
Prof. Dr. U. Muller, DipLChem. V. Krug
Fachbereich Chemie der Universitat
Hans-Meerwein-Strasse, D-3550 Marburg (FRG)
Angew. Chem In1 Ed. Engl 27(1988) N o 2
I
C
Fig. I. Structure of the [TiSC14]2e ion in the crystal of Ib, with ellipsoids of
thermal vibration (50% probability level at 22OC). Bond lengths in prn, standard deviations: 0.2 pm.
When recrystallized from acetonitrile, 2a contains one
mol. equivalent of CH3CN. Its structure analysis"" reveals
0 VCH Verlagsgesellschuft mbH. 0-6940 Weinheim. 1988
0570-0833/88/0202-0293 $ 02 50/0
293
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