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Asymmetric Heck Reactions via Neutral Intermediates Enhanced Enantioselectivity with Halide Additives Gives Mechanistic Insights.

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assembled at 25 "C from homogeneous fluoride solutions of
Tergitol 15-S-9 and 15-S-12 (2.0 and 2.1 nm, respectively) are
smaller than those obtained from more concentrated, fluoridefree gels of the same surfactants (2.5 and 3.1 nm).1'71
The unprecedented temperature dependence of MSU-X pore
structures may be explained in terms of the amphiphilic character of the PEO-based structure directors. These surfactants promote framework assembly through hydrogen bond formation
between the hydrophilic (EO), segments and the silanol groups
of the neutral inorganic precursor. The hydrogen bonding interactions are expected to decrease as the assembly temperature
approaches the cloud point of the surfactant. At the cloud point
all amphiphilic character is lost, surfactant phase segregation
occurs, and no mesostructure is formed. Conformational
changes (e.g., coil to rod) in the (EO), segment with increasing
temperature up to the cloud point will cause both the hydrophilicity and the overall size of the EO head group to decrease. Consequently, the curvature of the micelle will decrease,
and the size of the hydrophobic core, which determines the
framework pore size, will increase with increasing temperature.
This model is supported further by the decrease in the wall
thickness and the increase in basal spacing with increasing assembly temperature (see Table 1).
As the assembly temperature of an MSU-X silica approaches
the cloud point of the surfactant, the intensity of the reflections
in the diffraction pattern weaken and they broaden, signifying
a loss of channel structure and a reduction in the fidelity of the
pore diameter. For example, NoIo assembly with Tergitol 15-S-7
at ambient temperature does not form a highly mesostructured
silica, because the cloud point of this surfactant is very low
(CPx37 "C). For Tergitol15-S-9 (CP=6O0C), there is progressive loss of channel structure as the synthesis temperature is
increased above room temperature. A similar behavior is observed for mesostructures prepared above 50 "C from Triton
X-100 ( C P z 6 5 "C). Mesostructures assembled from Tergitol
15-S-12 (CP=85"C), 15-S-15 (CP>IOO"C) or Igepal RC-760
(CP>IOO"C) exhibit XRD intensities and line widths that
change little with increasing synthesis temperature until the CP
is approached. Thus, these high-CP surfactants are best suited
for controlling the pore sizes of MSU-X mesostructures by judicious choice of synthesis temperature.
Experimental Section
The preparation of a MSU-1 silica with Tergitol 15-S-12 as structure director illustrates a typical pore-size-specific Nolo synthesis. To a sonicated 0.02 M solution of
the surfactant in water at room temperature was added sufficient tetraethyl orthosilicate (TEOS) to obtain a molar TEOS/Tergitol ratio of 8.0. Sonication was continued for a short period to obtain a milky suspension. The mixture was allowed to age
without agitation for 12 h to give a clear, colorless solution. At this stage no
mesostructure formation had occurred. A 0.24 M solution of sodium fluoride was
then added dropwise with stirring to the TEOSisurfactant solution to obtain a
molar NaF/TEOS ratio of0.025. The solution was placed in a shaking bath (shaking
\peed, 40 rpm) at 25 to 65 'C for 48 h. A white precipitate formed progressively.
After 48 h the precipitate was separated from the solution by centrifugation, dried
in air at 70°C and then at 200'C for 6 h with a hearing rate of 5"Cmin.'. Finally.
the product was calcined at 600' C for 6 h to remove the surfactant.
For comparison purposes a mesostructured silica was assembled from Tergitol
15-S-12 by the same solution procedure described above, except that NaF was
omitted from the reaction mixture and the reaction time at 35°C was increased to
125 h
XRD patterns were obtained with a Rigaku Rotaflex diffractometer equipped with
ii rotating anode and Cu,, radiation (i.= 0.15418 nm). N, isotherms were obtained
o n a Coulter Omnisorp 360CX Sorptometer operated under continuous adsorption
conditions.
Received: July 22, 1996
Revised version: November 15, 1996 [Z93641E]
German version: Angeu. Chem. 1997. 109, 533-536
Keywords: mesoporosity
tants
518
-
molecular sieves silicon
-
surfac-
Q VCH Verlugsgesellschufr mhH, D-69451 Weinheim. 1997
c.T. Kresge. M. E. Leonowicz, W. J.
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[l]
Asymmetric Heck Reactions via Neutral
Intermediates: Enhanced Enantioselectivity
with Halide Additives Gives
Mechanistic Insights**
Larry E. Overman* and Daniel J. Poon
Since the first reports in 1989,"l catalytic asymmetric Heck
reactions have emerged as powerful transformations for controlling absolute stereochemistry in the formation of C -C
bonds.[21IntramoIecular asymmetric Heck reactions have received particular attention and have served as a cornerstone of
asymmetric total syntheses of natural products.[31The vast majority of asymmetric Heck reactions reported to date are of
triflates or halides in the presence of silver or thallium salts and
enantiopure chiral bis(phosphane)pailadium
Counter to the prevailing dogma at the time,''] we revealed in
[*) Prof. L. E. Overman. D. J. Poon
[**I
Department of Chemistry
University of California, Irvine
516 Physical Sciences 1, Irvine, CA 92697-2025 (USA)
Fax: Int. code +(714)824-3866
e-mail: leoverma(4uci.edu
This work was supported by NIH NIGMS grant GM-30859 and NSF grant
CHE-9412266. NMR and mass spectra were determined at UCI on instruments acquired with the assistance of Shared Instrumentation Grants from
NSF and NIH. We wish to thank Professor M. A. Calter and Dr. T. K. Hollis
for their insightful discussion and S. Pitram for assistance in preparing 16.
0570-0833197/3605-0518S 15 00+ .25l0
Angen.. Chem. I n t . Ed. Engl. 1997, 36, No. 5
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1992 that a halide scavenger was not obligatory for realizing
high enantioselection in bis(ph0sphane)palladium-catalyzed
asymmetric Heck insertions of halide substrates.[61We demonstrated that the sense of enantioselection could be opposite in
Heck cyclizations of aryl iodides conducted in the presence or
absence of silver salts (1 -+ 2),[61and that some substrates give
higher enantioselectivities without silver ions.[3b1
0
\LNMe
I0 Yo Pd-(R)-BINAP,
1-2 equiv Ag3P04,
MeCONMe2, 80 "C,
(81%)
(S)-(+)-2 71 %W
1
MeCONMe2,80 "C.
(77%)
(R)-(-)-2 66 % ee
It is widely accepted that the insertion step in bis(phosphane)palladium-catalyzed asymmetric Heck reactions of triflates or halides in the presence of silver or thallium salts involves cationic Pd(Ir)
The
, , intermediates (Scheme 1, 3 -+ 4)."'
pathway of related Heck insertions of halide substrates conducted in the absence of halide scavengers is less clear. Three possibilities for the insertion step of this neutral pathway (that is, via
the neutral intermediate 5)are illustrated in Scheme 1 : insertion
from a four-coordinate intermediate generated by phosphane dissociation (5 -+ 6 48),[5.638]insertion from a fourcoordinate intermediate generated by halide dissociation
(5 + 7 -+ 8),['l or insertion from a five-coordinate intermediate
(5 + 9 + 8).16."-"l
In this communication we report a) that
halide additives can increase enantioselection in Heck cyclizations of triflates by diverting the insertion to the neutral path-
way, and b) that the neutral pathway does not proceed by phosphane dissociation.
We recently investigated asymmetric Heck cyclizations of
(Z)-butenanilide triflate 10 and iodide 11 with Pd-(R)-BINAP
(generated in situ from 5 % [Pd,(dba),].CHCI, and 1 2 % (R)BINAP) to form (R)-oxindole 12 under identical reaction conditions." 3 1 Enantiomeric purity was determined by converting 12
into oxindole alcohol 13, which was analyzed by HPLC.[141
As
we had reported earlier,[3b1enantioselection in the cyclization of
iodide 11 is higher in the absence of a silver salt (Table 1, entries 9, 13). Identical low enantioselection (43 O h ee) is observed
in the cyclization of triflate 10 or iodide 11 in the presence of
AgOTf (compare entries 1 and 13 in Table I), observations that
are consistent with a common stereochemistry-determining step
for both cyclizations. The beneficial effect of halide salts on
intramolecular Heck insertions of triflate 10 was striking. Cyclization of 10 in the presence of one equivalent of tetrabutylammonium iodide, bromide, or chloride affords 13 in 90-93 % ee,
within experimental uncertainty of the ee observed in the cyclization of iodide 11 (Table 1, entries 2-4 and 9 ) . Nearly identical results are obtained when the cyclization of 10 is conducted
in the presence of one equivalent of the hydrohalide salts of
1,2,2,6,6-pentamethylpiperidine(PMP) (Table 1. entries 6 - 8 ) .
That this enhancement is due to the halide and not a general salt
effect was signaled by the negligible influence of added tetrabutylammonium triflate (Table 1, entry 5 ) . Not surprisingly,
TBDMSO
OTBDMS
10% Pd-(R)-BINAP,
PMP (4 equiv),
MeCONMe2, 100 "C
Me
10 X = OTf
11X=l
12
6
,OH
HCh Hz0,THF
NaBH4
cationic pathway:
'
N
PMP = $Me
Me
07-13
-AArX +
Scheme 2. BINAP = 2,2'-bis(diphenylphosphino)-l.l'-binaphthyl. dba = ( E X ) di benzylideneacetone.
AsOTf
OTf
OTf
4
3
Table 1. Asymmetric cyclizations of 10 and Il.["
neutral pathway:
Entry
I
2
3
4
5
6
\X
P-P'
I
5
7
(;>pd<;
X'
8
T-)"
9
Scheme 1 . Cationic and neutral pathways for intramolecular Heck reactions
A n R m Chrm. lnr Ed Eiigl. 1997. 36. No. 5
C
8
9
10
11
12
13
Cmpd.
Additive
(equiv)
Oxindole (R)-13
Yield [%] [b]
(v [%] [c]
10
-
10
10
10
nBu,NI( 1)
nBu,NBr( 1 )
nBu,NCI( 1)
nBu,NOTf( 1)
PMP Hl(1)
PMP HBr(1)
PMP . HCI(I)
72 [d]
62
59
52
70 [dl
40 [el
62
60
76
62
45
75 [dl
10
10
10
10
11
11
11
I1
11
-
PMP . HI(1)
PMP HBr(1)
PMP HCI(1)
AgOTf(6)
- [fl
~
~
~~~~~
[a] Conditions are descrlbed in Scheme 2 and ref. [13]; [substrate] = 0 1 0 ~ All
.
reactions proceeded to completion unless noted otherwise. [b] O v e r ~ l lyield over
three steps. [c] By HPLC analysis; the number of experiments is shown In parentheses and the mean ee is reported. [dl Overall yield of oxindole aldehyde [el Reactant
10 was recovered in 34% yield. [Q Not determined.
VCH Verlagsgesellschajt m h H , 0-69451 Weinhein,, 1997
057O-OK33197~360j-0jf9$ 15.00 + .ZSiO
519
COMMUNlCATlONS
added halide salts had little effect on cyclizations of iodide 11
(Table 1, entries 10-12). The results summarized in Table 1
demonstrate that added halide can divert cyclization of triflate
substrates to the neutral pathway,['51 which in the case of a
(Z)-butenanifde occurs with much higher stereoinduction.
To examine the possibility of phosphane dissociation in the
neutral asymmetric Heck pathway, we studied the cyclization of
iodide 11 in the presence of the monophosphane (R)-BINAP
analogs 14-16. The ligands 14 and 15 were prepared as described by Hayashi et a1.,[161
while monophosphane 16, which is
a close model of monocoordinated BINAP, was synthesized
from (R)-ester 17.[17. Cyclizations of 11in the presence of 5 O h
In summary, these studies demonstrate for the first time that
a) enantioselection in asymmetric Heck insertions of triflates
can be enhanced dramatically by the addition of halide
and b) both the chiral bis(phosphane) and the halide can be
coordinated to palladium during the enantioselective step of
asymmetric Heck reactions that achieve high stereoinduction.
Further investigations will be required to establish the scope of
these observations, particularly with regard to variations of the
substrate and solvent. Nonetheless, the finding that under some
reaction conditions the enantioselective step of asymmetric
Heck reactions involves a five-coordinate intermediate broadens the vista for the design of asymmetric ligands for this and
related reactions.
Received: October 29. 1996 [Z97001E]
German version: Angew. Chmi. 1997. 109. 536-538
14 X=OTBDMS
15 X = OCPr
Keywords: asymmetric catalysis * asymmetric synthesis Heck
reactions palladium
16 X=CHPh2
-
17 X=CO,Me
(R)-BINAP X = PPh2
of [Pd,(dba),].CHCl, and 11 % of the monophosphanes 14, 15,
or 16 (4 equiv PMP, 0.1 M in MeCONMe,, 100 "C) provided
(S)-oxindole alcohol 13 with low ee (14 2 7 % ee, 15 23% ee,
16 19% ee). Two observations suggest that a (phosphane)palladium species is the active catalyst under these conditions:
1) Heck cyclizations of 11 in the presence of ligands 14- 16 take
place within minutes at room temperature; in marked contrast,
cyclizations of 11 under analogous conditions with [Pd,(dba),].CHCI, require 60-80 min at 55 "C, while cyclizations
with (R)-BINAP take 20-60 min at 100 "C to go to completion.
2) Nearly identical results are obtained when the ratio of
monophosphane 14 to Pd is increased through 1.6:1 (21 % ee)
and 2.1:l (21 % ee) to 2.9:l (24% ee). The observation that
cyclization of iodide 11 in the presence of Pd-(R)-16 proceeds
with low enantioselection to deliver the opposite oxindole enantiomer to that produced with Pd-(R)-BINAP provides strong
evidence that both phosphanes of BINAP are coordinated to Pd
during the enantioselective step, ruling out the sequence
5 + 6 8 of Scheme 1.
What then can we say about the mechanism of the neutral
pathway? Certainly, insertion from a pentacoordinate intermediate can not be ruled out. However, both the general theoretical
treatment by Thorn and Hoffrnann[''] and Samsel and Norton's kinetic investigation1191indicate that insertions from fivecoordinate intermediates have much higher barriers than those
from four-coordinate square-planar complexes. Thus, we favor
7 8 for the insertion step of the neutral pathway. However, to
be consistent with our observations, the influence of a chiral
bis(phosphane) ligand on the formation or insertion ofintermediate 7 would have to be distinctly different from its influence on
the nearly identical intermediate 3 of the cationic pathway.
Since Jutand and Mosleh have demonstrated through conductivity studies that tuuns-[ArPd(PPh,),OTfJ is fully dissociated in
N,N-dirnethylformamide,[' 5a1 the different nature of the counterion in 3 and 7 is likely not sufficient to rationalize the disparate stereoinduction seen in cationic and neutral asymmetric
Heck reactions.[6. We suggest an alternative explanation in
which the enantioselective step of the neutral pathway occurs
during formation of cationic four-coordinate intermediate 7 by
associative displacement of X (5 -+ 9 -+ 7) .I2 - 241 The key
feature is that this step would be influenced quite differently b y
a chiral bis(phosphane) ligand than an enantioselective step involving in-plane coordination of the alkene (for example, dissociative alkene coordination to generate 3 from a triflate precursor or conversion of 3 4 4).
-+
520
VCH ~ ~ t . l u g . ~ g e . ~ ~mbH.
l l . ~ cD-6Y451
h~~
Weinlreim, 1997
[I] a ) Y. Sato, M. Sodeoka. M. Shibasaki, J. Org. Chmi 1989, 54,4738; b) N. E.
Carpenter. D. J. Kucera. L. E. Overman, hid. 1989. 54, 5846
[2] For a complete listing of pertinent references. see references in T.Ohshiind, K.
Kagechika. M. Adachi. M. Sodeoka, M. Shibasaki, J. Am. Chem. SOC.1996,
ffH,7108.
131 K. Kagechika. M. Shibasaki, J. Org. Cliem. 1991.56.4093; b) A. Ashimori, T.
Matsuurd, L. E. Overman. D. J. Poon, ihid. 1993, 58, 6949; c) A. Kojima. T.
Takemoto, M. Sodeoka. M. Shibasaki, ibid. 1996,61.4876;d)T. Takemoto, M.
Sodeoka, H. Sasai, M. Shibasaki, J. Am. Chem. Sac. 1993, lf5, 8477; e) K.
Ohrai, K . Kondo, M. Sodeoka, M. Shibasaki. rbid.1994.116, 11737, f)ref. [2].
14) Notable exception: 0. Loiseleur. P. Meier, A. Phltz, Angew. Chem. 1996, 108,
218; Ange>v Chen?.i n f . Ed. Engl. 1996. 35.200.
[ S ] a) F. Ozawd, A. Kubo, T. Hayashi. J Am. Chem. Sor. 1991, 113, 1417: b) M
Shibasaki, Y. Sato. K.Kagechika. J Synrh. Org. Cliem. Jupuri 1992. SO, 826.
[6] A. Ashimori. L. E. Overman, J Org. Chem 1992, 57, 4571
[7] a) Recent brief survey: T. Jeffery in A h . if7 Mef.-Org Chem. 1996, 5, 249;
b) incisive recent studies: J. M. Brown, J. J. Perez-Torrente, N. W. Alcock. H.
J. Clase. Ot-gmon?eta//ics1995. f4, 207; J. M. Brown, K. K. M. Hii, Angeit.
C h r w 1996. 108. 679; Angel?. Clrem. 1111.Ed Engl. 1996, 35, 657.
[Sl W. Cabri, L. Cdndiani, Acc. CAem. Rrs. 1995, 28, 2 ; b) J. S. Brumbdugh, R R
Whittle. M. Parvez. A. Sen, O,;qunometo//i~.r,1990. 9, 1735
[9] a ) W. Cabri. I. Candiani, A. Bedeschi. S Penco. R. Santi, J: U r g . Chem 1992,
57.1481; h) M. Portnoy, Y. Ben-David, 1. Rousso. D. Mdstein, Orgunometa/lics. 1994, f3. 3465.
[lo] a) C. Amatore. A. Jutand. A. Suarez, J An?. Cliem. Sac. 1993. 115,9531; h) W.
Cabri. I. Candiani. A. Bedeschi, R Santi, J Org. Chem.1993,58,7421.
[I 11 The other five-coordinate intermediates that would intervene in the insertion
process are not shown in Scheme 1 [12]
[12] D. L. Thorn. R. Hoffmann. J Am. Chem. Soc. 1978, 100.2079.
[I 31 Representative experimental procedure for Heck cyclizdtions reported in Table
1: A base-washed. flame-dried lOmL Schlenk flask was charged with
[Pd,(dba),] CHCI, (8.1 mg, 0.008 mmol) and (R)-BINAP (11.2mg,
0.018 mmol) and purged with Ar Dry NNdirnethylacetamide (DMAC,
0.8 mL) was added. and the resulting purple-brown suspension was stirred for
2 h to give a bright orange solution. A solution of triflate 10 (75 mg,
0.16 mmol), PMP (120 pL, 0.66 mmol), nBu,NCI.H,O ( 5 3 mg, 0.18 mmol),
and dry DMAC (0.8 mL) was added, the suspension stirred until homogeneous
(about 15 min). and the resulting red-orange solution degassed (three freezepump-thaw cycles) and then heated at 100 C for 23 h. After cooling to 23 C,
the reaction mixture was partitioned between saturated aqueous NaHCO,
solution and EtOAc. and the combined organic layers were dried (NaJO,),
filtered. and concentrated. The resulting dark brown residue was partlally
purified by chromatography on silica gel (4:l hexanes!EtOAc) t o give a 20: 1
mixture of (€)- and (2)-eneoxysilanes 12 (contaminated with some dha) as a
yellow oil. This mixture was hydrolyzed at 2 3 ' C i n 1,'l T H F . ' ( ~ HCI),
N
and the
resulting crude product was purified on silica gel (4:l to 211 hexanes/EtOAc)
to give 21 mg (65%) of the corresponding aldehyde. This intermediate was
reduced withNaBH,and E t O H a t 2 3 Ctoafford17 mg(52%overdllforthree
steps) of (R)-13 [3b] as a colorless oil: 93% ee. Chiralcel AS column. 9:l
hexane%propanol. 1.0 r n l m i n - l ) .
[I41 A Diacel Chiralpak AS or OB-H: column was employed; precision in measuring ee is kZo/'.
[15] The formation of ~rrrns-[ArPdX(PPh,),] from the reaction of aryl triflates with
palladium catalysts in the presence of added chloride has been described and
shown to proceed without formation of a cationic triflate intermediate by
initial oxidative addition of ArOTf to [PdCI(PPh,),]-: a ) A. Jutand. A
Mosleh, Orgonon?e/aNic.r, 1995, 14. 1810; b) ref. [lea]
[I61 Y. Uozumi. A. Tanahashi, S. Y. Lee. T. Hayashi. ./ Utx. Chem. 1993. 58, 1945.
~570-0833IY7i36(~5-0520
8 lS.OO+ .25,'0
Angeir.. Ckem. I n f . Ed. E n d . 1997. 36. No. 5
COMMUNICATIONS
[I71 Y. Uozumi, N. Suzuki, A. Ogiwara. T. Hayashi, Tc/mhr&on 1994, SO, 4291.
[I81 By sequential reaction of 17with excess PhLi, BH, (to protect the PPh, group).
Ct',CO,H, and NaBH,.
[I91 E. G. Samsel. J. R. Norton. ./. An?. Chrni. Sor. 1984, 106, 5505.
[20] N,N-dimethylacetamide and N,N-dimethylformamide have nearly identical
solvent properties: R. Carlson. T. Lundstedt, C. Albano, Actir Chrni. Scand.
1985, 39, 79.
[21] Ligand substitution would involve multistep rearrangement of an initially
formed. square-pyramidal complex 9 to a square-pyramidal isomer having X
in apical position. the latter being the immediate precursor of 7;IZ2'for brevity
these intcrmediates are not shown in Scheme 1.
[22] Substitution reactions of 1he square-planar complexes are typically associative: R. J. Cross, A h , . Inorg. Chem. 1989, 34, 219.
[23] Palladium(n) halide complexes o f general formula [RPdC1(N-N')(q2-alkene)]
are well known and have been crystallographically characterized: V. G. Albano, C. Caatellari. M. E. Cucciolito, A. Panuzi, A. Vitnlgliano, Orgunonzelulhc,y, 1990. 9. 1269.
[24] This step could he either the formation of a pentacoordinate intermediate from
5 or the evolution of a pentacoordinate intermediate to 7. The first irreversible
enantiodifferentiating step would determine asymmetric induction: C. R. Landis, J. Halpern, J An7. Chem. Soc. 1987, IOY, 425.
[25] There arc scattered reports of various effects of halide additives on Heck
reactions o f triflates: a) modified regioselection: C M. Anderson. A. Hallberg, J. Org. Chem. 1988,53, 2112; b) modified yield: W. Cabri, I. Candiani. S.
DeBernardinis, F Francalanci, S. Penco, R. Santi, ihid. 1991, 56, 5796;
c) decreased enantioselection: C. Sonesson, M. Larhed, C Nyqvist, A. Hallberg, ihid. 1996, 61, 4756, and ref. [4]; d) Shibasaki [2] has reported that
addition of sodium halides to an asymmetric Heck cycli~ation:~~-allylpalladium carbanion trapping reaction resulted in moderate increases in enantioselectivity that were ascribed to complexation with the sodium enolate.
Structural Rigidity and Luminescence of Chiral
Lanthanide Tetraamide Complexes Based on
1,4,7,10-Tetraazacyclododecane**
Rachel S. Dickins, Judith A. K . Howard,
Christian W. Lehmann, Janet Moloney, David Parker,"
and Robert D . Peacock
N-Substituted octadentate ligands derived from 1,4,7,10tetraazacyclododecane form kinetically robust square-antiprismatic complexes in aqueous solution with metal ions that prefer
a coordination number of eight, such as calcium, the lanthanides,", 21 and lead.[3,41 When the N substituents contain
amide groups, the neutral ligand forms tripositive complexes
21 Such comcapable of binding further to one water
plexes (e.g. of La) have been considered as artificial nucleases
and may also function as NMR shift reagents for anions in
aqueous media.
We now report the structural characterization and chiroptical
properties of selected lanthanide complexes of the chiral tetraamide ligand 1 and compare their behavior to that of complexes
of the achiral analogue 2.15]Chiral europium and terbium com[*] Prof. D. Parker, R. S. Dickins, Prof. J. A. K. Howard, Dr. C. W. Lehmann,
J. Moloncy
Department o f Chemistry. University of Durham
South Road, Durham, D H I 3LE (UK)
Fax: Int. code +(191) 3861127
e-mail: david.parker(o;dur.ac.uk
Dr. R. D. Peacock
Departmcnt of Chemistry, University of Glasgow
Glasgow, GI2 8QQ (UK)
Fax: Int code +(141) 3304888
e-mail : r.peacock(&hem.gla.ac.uk
[**I We thank the Medical Research Council, the Engineering and Physical Sciences Research Council, and the University o f Durham for support of this
research. Circular dichroism spectra were provided by the National Chiroptical
Service based at King's College London.
Angew. Chcni. I n t . Ed. Engl. 1997. 36, No. 5
MeYph
MeHNOd
2
1
plexes are of particular interest since they may function as emissive chiral probes in biological media and are particularly
amenable to analysis by circularly polarized luminescence spectroscopy.[61In order to be useful as a probe that may interrogate
interactions with other chiral molecules or macromolecules, a
single enantiomer of the complex is desirable that is conformationally rigid on the time scale of the metal-based emission lifetime; that is, of the order of milliseconds.
Chiral anionic complexes incorporating propionate or benzylphosphinate pendant groups have been describcd previously
that are conformationally rigid on the NMR time scale.171.The
Eu and Tb carboxylate complexes exist as a diastereoisomeric
mixture in solution and are only weakly emissive (and lack a
suitable organic chromophore) . The chiral phosphinate complexes exist as a diastereoisomeric mixture in solution in which
one diastereoisomer predominates, and whilst these diastereoisomers have yet to be resolved they may be studied as chiral
probes following circularly polarized excitation.
Reaction of ( S ) - or (R)-N-2-chloroethanoyl-2-phenylethylamine with 1,4,7,10-tetraazacyclododecane(DMF, K,CO,,
60 "C) afforded the tetraamide ligands (S)-1or (R)-1 (m.p. 151
152"C, [a];' = + 241.7 (c = 0.12 in methanol) for (R) enantiomer) . Reaction of 1 with lanthanide trifluoromethanesulfonates (Ln = Pr, Eu, Tb, Dy, Yb) in CH,CN afforded
cationic complexes, which were crystallized from acetonitrile as
their triflate or trifluoroacetate complexes. The enantiomeric
terbium complexes showed equal and opposite rotations, for
example for [Tb.(R)-1I3', [a];' = +104.2 (c = 2.21 in methanol); [Tb.(S)-1I3', [XI;' = -104.2 (c = 0.14 in methanol).
The CD spectra of the enantiomeric terbium complexes are
mirror images of each othcr, but show no evidence for exciton
coupling betwcen the aryl chromophores above 200 nm. Crystal
structures"] of the enantiomeric [Eu. 113+complexes (the S
complex as its trifluoromethanesulfonate salt, the R complex as
a trifluoroacetate) reveal that the chirality of the remote amide
stereocenter dctermines the handedness of the arrangement of
the N substituents as well as the conformation of the twelvemembered tetraaza macrocycle (Figure 1). The complex contains one metal-bound water molecule, and the E u - 0 distances
(Eu-0 = 2.435(4) A) are very similar to those previously reported for nine-coordinate europium-aqua complexes.[2,91 The
mean E u - 0 and Eu-N distances are 2.39 and 2.67 A. respectively, while the isostructural dysprosium complex of (S)-1
(as its triflate salt) has slightly shorter bond lengths (mean
D y - 0 = 2.34 A, Dy-N = 2.64 A) in accord with the smaller
ionic radius of Dy3+.
The hydrogen atoms of the lanthanide-bound water molecule
engage in hydrogen-bonding interactions with the oxygen atoms
of the proximate counterions (Figure 2). This stabilizing interaction in the solid-state lattice may help to explain, at least in
part, the extraordinarily slow rate of water exchange for the
'C, VCH Verlagsgesellsch~~t
nibH. 0.69451 Weinheim,1997
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0570-0833/97:3605-O521$15.0O+ .25 0
521
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