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High-Pressure Synthesis of LiSi Three-Dimensional Network of Three-Bonded Si Ions.

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residue occurs with five hydrophobic phenylalanine (Phe)
residues per 30-mer. When the five Phe residues were placed
as far from the Asp residue as possible without changing the
filamentous [j-spiral molecular structure of the polypentapeptides with three pentamers per turn of helix,"3. l41 the
pK, was shifted to 6.7 from the usual value of 3.9.[91When
the five Phe residues were placed as close to the Asp residue
as possible but still consistent with the retention of the
molecular structure, a remarkable pK, of 10.0 was found
(D. W. Urry, D. C. Gowda, S. Peng, T.M. Parker, N. Jing,
R. D. Harris, unpublished results). This is a pK, shift of 6,
twice as great as that found for the excessively charge-dense
poly(methacry1ic acid). Thus it would appear that the capacity to shift pK, values is greater when the apolar-polar repulsion mechanism is employed.
Our understanding of electrostatic-induced pK, shifts
comes from Coulombs law, in which the increase in energy
(i.e., repulsive energy) between a pair of like charged species,
q1 and q z , is the product of the charges q i q 2 ,divided by the
distance, r, and the effective dielectric constant, c, between
the two charges: E = q,q,/w. Polymers have been explicitly
treated by Harris and Rice["] and by Katchalsky et al.,[161
and Honig et al. have extensively considered proteins.[" "1
There is as yet n o formal expression for the apolar-polar
repulsive free energy of hydration, but, as the name intends
to convey, it involves the interference that each apolar (hydrophobic) and polar (e.g., charged) moiety experiences due
to the presence of the other in a structurally constrained, and
thereby, hydration-limiting polymer such as a protein or
polypeptide. In short, when the distance between apolar and
polar moieties is sufficiently short, the intervening water
molecules cannot simultaneously provide the full free energy
of hydration for both the apolar and polar species.
When, in the hydrophobic domain where apolar hydration dominates, the carboxyl species, for example, cannot
achieve the hydration required of the carboxylate moiety, a
higher hydroxyl ion concentration is required before the
COOH proton can be removed to form COO-.[1o1In this
case the change in chemical potential, A p , of proton, as
Gibbs free energy per mole, is given by Ap =
- 2.3 RTApK,, where R =1.987 c a l K - ' m o l - '
and T i s in
degrees Kelvin. Under these circumstances, Ap becomes a
direct measure of the repulsive apolar-polar free energy of
hydration. On the other hand, when the charged species does
form, its pull for hydration is sufficient to destructure the
water of hydrophobic hydration, the driving force for hydrophobic folding and assembly is lost, and disassociation and
unfolding occur.
In fact, as the number of ionized species increases, the
temperature 7;, at which the hydrophobic folding and assembly transition occurs, increases.I", ' ' ] When the value of
7; becomes greater than the working temperature of the
polypeptide or protein, disassembly and unfolding sets in.
Also, as seen by differential scanning calorimetry, the endothermic heat of the transition, which is interpreted as the
heat required to destructure the more structured hydrophobic hydration to form less ordered bulk water, decreases as
the degree of ionization increases, consistent with the
charged species destructuring of the hydrophobic hydration.1'81A more extensive discussion of these hydrophobic
effects is available in references[". 19].
E.uperimental Procedures
The family of polypentdpeptides. poly~~(lPGVG),/,,(IPGDG)]
with,/;, = 1.O 0.06 for a total of ten polypentapeptides were Characterized by using classical
acid- base titrations to determine the pK, values. The specific syntheses, which
will be described elsewhere. were carried out analogously to those in [20] for
1442
'l,VCH Verlug~~ercll~rl~uft
mbH 0.69451 Wernlierm. IY93
Glu-containing peptides hut with care to avoid the a/P exchange of the Asp
residue, and were verified by means of amino acld analyses and I3C N M R
spectroscopy, which also provided accurate values for/, (see Table 1). The
acid-base titrations for the COOH/COO- couple. carried out at 3 7 ' C as
previously described I201 under argon, allowed 45 min per data point, thus
requiring approximately 24 h per titration for concentrations of Asp-containing pentamers ranging from 22 to 29 mM. In general, the samples remained in
a viscoelastic state until the degree of ionization was sufficient for dissolution.
Received: April 30, 1993 [Z 6050 IE]
German version: Angew. Chem. 1993. 105, 1523
K. Langetsmo. J. A. Fuchs. C. Woodward. Biochemisrrj 1991, 3U, 76037609.
K. Langetsmo, J. A. Fuchs. C. Woodward, K. A. Sharp, Biuchemi.str.v
1991, 30, 7609-7614.
M. Inoue, H. Ydmada. T. Yasukochi, R. Kuroki, T. Miki. T. Horiuchi. T.
Imoto, Biorhemistri. 1992, 31. 5545-5553.
I. Auzdt, JLR. Garel, Protein Sci. 1992. 1, 254-258.
M. Y Okamura, G. Feher, Annu. Rev. Biuchem. 1992, 61, 861 -896.
C. Tdnford, Adv. Prof& Chem. 1962, 17, 69-165.
K . Sharp. B. Honig, Annu. Rev. Biophp. Biophys. Chem. 1990, 1Y. 301 332.
D. W. Urry. D. C. Gowda, S . W Peng, T. M. Parker, R. D. Harris, J. Am.
Chem. Suc. 1992, 114. 8716-8717.
J. T. Edsdll. J. Wyman. Biophvsicul Chemistry, Academie. New York, NY,
1958, pp. 452-453.
D W. Urry. Angew. Chem. 1993, 105, 859-883; Angeic. Clwm. lnt. Ed.
EngI. 1993. 31, 819-841.
D. W. Urry, Prog. Biopli!s. Mol. B i d . 1992, 57. 23-57.
A. Katchalsky. J Poljm. Sci. 1951, 7, 393-412.
D W. Urry, J Prorein Chern. 1984. 3, 403-436.
D. W. Urry, in Muleculur ConJormurion und Biological Interactions (Eds.:
P. Bdbdrdm, S. Ramdseshan), Indian Academy of Sciences, Bangalore,
India. 1991, p. 555-583.
F. E Harris, S. A. Rlce, J. P/7vs. Chem. 1954, 58, 725-732.
A. Katchalsky. S. Lifson. I. Michaeli, M. Zwick in Size & Shupe of Contrurtile Polyners Conversion of Chemicul & Merhunical Energ.v (Ed.: A.
Wasserman), Pergdmon, New York, 1960, pp. 1-40.
M. E. McGrath. J. R. Vasquez, C. S. Craik. A. S. Yang, B. Honig, R. J
Fletterick, Biochemi.rtrj 1992, 31, 3059-3064.
D. W. Urry. C.-H. Luan, R. D. Harris. K. U. Prdsdd, P o l w Prep. Am.
Chem. Suc. D I V .PolFm. Chem. 1990.31, 188-189.
For hydrophobic hydration and hydrophobic interactions see also W.
Blokzijl, J. B. F. N. Engberts, Angew. Chern 1993, 105, No. 11: Angew.
Cl7em. In[. Ed. Engl. 1993. 32, No. 11.
D. W. Urry. S. Q. Peng, T. M. Parker, Biopoljmers 1992, 32, 373L379.
D. W. Urry, D. C. Gowda, T.M. Parker, C.-H. Luan. M. C. Reid, C. M.
Harris. A. Pattanaik, R. D. Harris, Biopolymers 1992. 32, 1243-1250.
'
High-pressure Synthesis of LiSi: ThreeDimensional Network of Three-Bonded Si- Ions**
By Jiirgen Evers,' Gilbert Oehlinger, and Gerhard Sexfl
Lithium germanide LiGe"] crystallizes under normal
pressure (NP) with MgGa"' structure and beIongs to the
Zintl phases.[31According to current textbooks,I41 several
binary Li-Si compounds such as Li,iSi,,[51 Li,,Si,,[61
Li,,Si,,['] and Li,,Si.,['l have been characterized, however,
"LiSi is not known".141
Our investigations also confirmed the results of previous
experimenters: simple fusion of equimolar amounts of Li
and Si at N P always results in a phase mixture, but never in
LiSi. LiSi should be stabilized with respect to a mixture
[*] Priv.-Doz. Dr. J. Evers. G . Oehlinger
Institut fur Anorganische Chemie der Universitlt
Meiserstrdsse 1 , D-80333 Munchen (FRG)
Telefax: Int. code + (89)5902-451
[**I
Dr. G. Sextl
Degussd AG
Rodenbdcher Chaussee 4, D-63457 Hanau (FRG)
This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemtschen Industrie.
(i57U-OR33193jf(~fO-1442
$ I 0 00+ 2510
Angen Chem In1 Ed Engl 1993. 32, N o 10
consisting of 1/12 Li,,Si, and 5/12 Si, however, if, under
sufficiently high pressure, it could be considerably more
densely packed than the mixture. For this reason, a highpressure (HP) synthesis was attempted in boron nitride and
molybdenum crucibles by using the Belt apparatus according to Equation (a). Typical reaction conditions were p =
4 GPa, T = 600 "C, t = 5 min: after this time the reaction
Li12Si,
+ 5 S i G 1 2 LiSi
(a)
mixture was quenched to room temperature and the pressure
relieved. It proved favorable to use an excess of Li (10
atom %) in the starting mixture. Due to the high sensitivity
of the reactions towards both air and moisture, all preparative work with open crucibles was performed in a glove box.
The Guinier diffractogram (CuKIIradiation) of the reaction product was indexed by the Dicvol method[91and found
to have a tetragonal unit cell and very good reliability indices
(F20= 36. M,, = 40).['01The sequence of the reflections and
the course of the intensities indicate that the structures
of LiSi and NP-LiGe"] are isotypic (MgGa structurel2])
(Rietveld method" 'I). A single-crystal X-ray
' 31
confirmed the resuits of the powder experiment.
According to the Zintl- Klemm concept,[31 silicon in
Li+Si-, like a phosphorus atom in elemental phosphorus,
possesses five valence electrons and has, correspondingly,
three nearest Si neighbors (241.7(6) (2 x ) and 250.2(6) pm).
The Si-Si-Si bond angles are 107.2(2), 110.7(2), and 117.9(2)".
The Si-Si bond length of 250.2 pm in LiSi is one of the longest
yet observed in binary metal silicides. Longer Si -Si distances
have only been found in organometallic compounds with
bulky rm-butyl groups, such as in hexa(tert-buty1)disilane
(Si-Si = 269.7 pm).r143'51 Typical Si-Si bond lengths in
LiI4Si6and Lil,Si, lie in the range 233-239 pm.[7.s1
Lithium silicide forms a three-dimensional network in
which each Si- ion is connected with three further Si- ions
(Fig. 1). A three-dimensional arrangement of Si- ions is also
network of Mns- atoms, whose cavities are filled by MndC
atoms. In the space group P4,32 (a = 631.5(2) pm), the
three-bonded Mn6- atoms (bond length 236.4(2) pm),[211
like the Si- ions in NP-SrSi, (a = 653.5(3) pm, bond length
239.2(1) pm),['7.221occupy the 8c site with very similar site
parameters.
In LiSi, the three-dimensional network contains fourfold
screw axes of silicon atoms aligned along the a axis; neighboring screw axes are of opposite chirality. The two shorter
Si-Si distances (241.7 pm) and the smaller Si-Si-Si bond
angles (107.2, 110.7") are found between atoms of one particular fourfold axis. Cross-linking of the fourfold screw axes
with one another leads to formation of the longer Si-Si
bond (250.2 pm) and the larger Si-Si-Si bond angle (1 17.9").
The puckered Si, rings formed in this manner are centered
on z = l/S, 3/8, S j S , and 7/8. The Li' cations are arranged as
tetrahedra and are located in the cavities formed by the Si,
rings. The tetrahedron centers are displaced by c/2 with respect to the Si, rings. Each Li+ ion has six LiC neighbors
within a distance of 305 pm; three of these are in the same
tetrahedron, two in the next nearest and one in the next
nearest but one. The long Li-Li distances effectively rule out
a discussion of tetrahedral Li, clusters with bonding Li-Li
interactions.
Each Li' cation has eight Si neighbors within a distance
of 326pm (and vice versa). Five of these come from the
nearest fourfold Si screw axis, two from the next nearest and
one from the next nearest but one. This implies a formal 8:8
coordination in the three-dimensional network structure of
LiSi. 1 :1 silicides of the heavier alkali metals (Na-Cs) have
lower coordination numbers. In these alkali metal silicides
the three-bonded Si- ions form isolated tetrahedra, as also
found for P atoms in white phosphorus. NaSi with the NaSi
structure[231formally has a 6:6 coordination, as have NPKSi, NP-RbSi, and NP-CsSi with the KGe structure.[241In
contrast, HP-KSi, HP-RbSi, and HP-CsSi[2s]with the NaPb
structure[261achieve 7: 7 coordination. NaSi could possibly
be converted into another modification under HP, but a new
phase has not yet been proven on quenching from pressures
of up to 4 GPa.
5 Li
+ 7 LiSieLi,,Si,
12 LiSieLi,,Si,
Fig. 1 . Lithium silicide. LiSi (MgGa structure): three-dimensional network of
three-bonded Si- ions. Right: view along [OOl]; left: view along [100].0= Li.
0 = SI.
found in SrSi2"6, "I and in cc-ThSi,.[". l 9 ] As in LiSi, the
cavities in these three-dimensional networks are also filled by
cations. A stabilization of this type in structures formed by
elemental phosphorus requires that these disproportionate
to form cations and anions. For this reason, the structures
formed by elemental phosphorus that have been observed
until now, all consist of isolated tetrahedra and layers of
three-bonded P atoms which pack together without the formation of larger cavities. The structure of ,Y-manganese.['*, '']
o n the other hand, may be considered as a three-dimensional
+ 5 Si
(b)
(C)
Although it is synthesized under high-pressure conditions,
LiSi is, somewhat surprisingly, not a metastable H P phase
under normal conditions. This is confirmed first by coulometric titrations and second by thermal analysis. Reaction
(b) (NP, 400 "C, LiCljKCl melt) can be reversed by reversing
the polarity, resulting in a negative free energy of format i ~ n . ~ The
~ ' ] thermogram (NP, Ta crucible, Ar atmosphere)
shows an endothermic signal between 470-490 "C
( A H = + 0.9T0.2 kJmol-') which may be attributed to
reaction (c). A strongly endothermic signal is observed at
590 T 10 "C, which corresponds to the melting point of the
eutectic in the Li-Si phase diagram.[281In attempting to
prepare LiSi by cooling a LiSi melt, it has to be taken into
consideration that NP-LiSi is thermodynamically unstable
above 470°C under NP. Thus, LiSi can only be prepared
under N P at temperatures below 470 "C. The formation of
LiSi from the phase mixture is extremely slow under these
conditions due to kinetic hindrance. In ignorance of this,
earlier experiments a t N P failed due to a lack of patience. At
H P (4 GPa, 600 "C, 5 min), the synthesis is achieved in a
shorter time, since LiSi is about 6 % more densely packed
than the mixture, and thus a rise in pressure leads to its
formation in accordance with Le Chatelier's principle ( - p ,
A V).
Received: May 4. 1993 [Z6058IE]
Cierman version: A n g w . Chem 1993, 105. 1532
[ l ] E. Menges. V. Hopf. H. Schdfer. A . WeiO. Z . Nrrtirr/or.sch. B 1969.24. 1351
121 K . Schubert. F. Gauui. K . Frank. 2.Mrtnllkd. 1963. 54. 422. MgGa-type
(1132. 1 4 , : ~ .u =1053. c = 553 pm. 16 Mg in 16f (0.070. 0.874. 0.129).
16 Ga in l 6 f (0.106, 0.948. 0.629)).
[ 3 ] H. Schiifer. B. Eisenmann. W. Miiller. A i i g c i i . C%eiii. 1973.85. 742: Angeit.
<./7em. / I ? / Ed. Engl. 1973. 12. 694.
[4] Hollenian-Wiberg. Lrlirhirch &r Anorgunischcvl Chemic. de Gruyter.
Berlin. 1985. p. 737.
[ S ] R. Nesper. H. G. von Schnering. J. Solid Srutr C h o n 1987, 70. 48.
[6] U Frank, W. Miiller. H. Schdfer. Z. Nururfor.w/i. B 1975. 30, 10.
[7] H. G . von Schnering. R. Nesper. K.-F. Tebbe. J. Curda, Z. Mctullkd. 1980.
71, 357.
181 R. Nesper. H. G . voii Schnering. J. Curda, CIwii. Brr. 1986, 1 / 9 . 3576.
[Y] D. Louer. R. Vargas, J. Appl Crytallugr. 1982. 15. 542.
1101 G. S. Smith. R. L . Synder. J. Appl. Cri,.stullugr 1979, 12. 60.
111) D. B. Whiles. R. A. Young!, J. Appl. Crytullugr. 1981. 14. 149. LiSi: 3500
measurement points. N-range: &43 R , = 0.071: R, = 0.057.
[12] LiSi crystal, cuboid with no well-defined surfaces (0.5 x 0.5 x0.25 mm3).
Nicolet diffractometer. filtered Cu,, radiation. 365 measured reflections
(/i:O
10. k : k 10. I - 0 5 ) . 170equivalent reflections. 153 reflections with
F > 3u(F), internal R value = 0.0803. SHELXTL program system, 11 parameters, R = 0.0645, R,, = 0.0516, R<,= R, = 0.0855. u = 935.3(1). (' =
574 X 1 ) pm; 16 Li i n 16f(0.082(2). 0.885(2). 0.058(4)). L , , = 0.031(7)
16 Si in 16f(0.1109(3). 0.9526(3), 0.5941(5)). Cs,= 0.0149(14) A'.
11 31 Further details of the crystal structure investigation may be obtained from
the Fachinform;rtionszentruin Karlsruhe. Gesellschaft fur wissenschaftlich-technische Information mbH, D-76344 Eggenstein-Leopoldshafen
( F R G ) on quoting the depository number CSD-57346. the names of the
authors. and the journal citation.
(141 N . Wiberg. H. Schuster. A. Simon. K. Peters, Ang'w. Clicm. 1986, 98. 100;
A n g c w Chem I n / . Ed. Engl. 1986, 25, 79.
1151 H. Bock. J. Meuret. K Ruppert, Angeit. Clitm. 1993. 105. 413; Angot.
Cl7m In!. Ed. Engl. 1993. 32. 414.
1161 K . Janzon, H. Schiifer, A. Weiss, Angew. Chon. 1965, 77. 258: Angeit.
Chein. / t i / . Ed. Ej7gI. 1965, 4. 245.
[17] J. Evcrs. G . Oehlinger, A . Weiss. Angmt. Ci7et77. 1978. 90. 562; Aiigrii..
Ch(w. I n ! . Ed. Eng/. 1978. 17. 53X.
[lX] G. Brauer. A. Mitius. Z.Anorg. A&. C/iem. 1942, 249. 325.
[19] J. Evers. .I. Solid Sirrtr C h n . 1978. 24. 199; ihid. 1979. 28, 369.
[20] G. D. Preston, Philux Mug. 1928, 5, 1207.
1211 C. B. Shoemaker. D P. Shoemaker. T. E. Hopkins, S. Yindepit. Acto
Cr,r.stal/igr. Sect. B 1978. 34. 3573.
[22] J. Evers. Habilitation Thesis, UniversitPt Miinchen. 1982.
[23] J. Witte. H. G. von Schnermg. Z. Anurg. A& C h m . 1964. 327. 260.
[24] E. Busmann. Z. Anurg. Allg. Climi. 1961, 313, 90.
1251 J. Evers. G. Oehllnger, G. Sextl, A. Weiss, Angcw. Chew. 1984, 96. 512:
Angm.. C h c ~ iJnt.
. Ed. Engl. 1984. 23, 528.
1261 R. E. Marsh, D. P. Shoemaker, Actu Crj..stu//ugr. 1953. 6, 197.
1271 H. G . von Schnering, R. Nesper, J. Curda (Max-Planck-Institut for
Festkorperforschung, Stuttgart). personal communication 1987.
[ZS] C. van der Marel. G. J. B. Vinke. W. van der Lugt. Sdid State Comniun.
1985. 54. 917.
.
+
compounds involve coupling of CD-ring and A-ring fragments by olefination procedures such as the Juliar4] and
W a d s w o r t h - E m r n o n ~ reactions.
~~~
This strategy is particularly attractive for the synthesis of semisynthetic analogs of
1 and 2 as drug candidates;['] combination of modified Aand CD-ring fragments prepared by chemical elaboration of
degradatively derived materials allows access to a wide range
of unnatural vitamin D, mimics.
o",,
Ho'"
+
A':
Vitamin D, Synthetic Studies: Enantiospecific
Synthesis of the CD Ring Fragment**
By Martin C. Clasby, Donald Craig,* and Andrew Marsh
The discovery that lsc,25-dihydroxy vitamin D, (l), the
hormonally active form of vitamin D, (2),[" induces cellular
differentiation and suppresses cell proliferation['] has
aroused renewed interest in this classical synthetic target.[31
Many of the published synthetic approaches to this class of
[*I
[**I
Dr. D. Craig, M. C. Clashy, A. Marsh
Department of Chemistry
Imperial College of Science. Technology and Medicine
GB-London SW72AY ( U K )
Telefax: Int. code + (71j5XY 3869
This research was supported by the British Science and Engineering Research Council (SERC) (Quota Studentships to M. C . C . and A. M.).
Ut
X
+
[ 4 + 21 ; reduction
X = H, PhSO,;
0
Y
-,.,,A-ring
Y = 0 : H, P(O)Ph,
HO+'
In this context, there has recently been an upsurge in research aimed at developing both new methods for A-ring
assembly and improved procedures for coupling these intermediates with CD-ring units.['] We have embarked on a program seeking on synthesize analogs of vitamin D, in which
the CD-ring fragment is assembled by total synthesis. Such
an approach is uniquely able to deliver analogs of the natural
system incorporating extremely subtle structural changes,
including isotopically labeled materials, which may serve as
probes of the origins of biological activity.
We selected the known[4b1bicycfic sulfones 3 as the synthetic target. We envisaged that 3 would be accessible by
stereoselective dihydrogenation of the product of the intramolecular Diels -Alder (IMDA) reaction['] of dienynylsulfone 4.[9. This approach contrasts with existing IMDAbased strategies,'"] in that the C8-C9 and C13-CI4 bonds
rather than theC11 -C12andC13-C14 bondsareformedin
the cyclization reaction. The stereocenters in IMDA substratF 4 would be installed by using a combination of the
c h i d pool['I' and the well-established asymmetric enolate
alkylation methodology developed by Evans et al.[' 31
Our synthesis of 3 started from (+)-(R)-citronellic acid,
which was prepared on about a 1 mol scale in 61 YO yield
from (+)-(R)-pulegone according to a published procedure.'14' Catalytic hydrogenation gave ( )-(R)-dihydrocitronellic acid, which was converted into the corresponding
acid chloride 5 by thionyl chloride. Treatment of the anion of
oxazolidone 6[15]with 5 gave the crystalline['61 N-acyloxazolidone 7 in good yield (Scheme 1). Allylation of the sodium enoiate["] of 7 at low temperature proceeded efficiently
with 2 2 0 : l diastereoselectivity to give alkene8 as an oil
after chromatographic separation of the diastereomers. Removal of the chiral auxiliary was attempted by several Iiter-
+
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