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Dynamic Kinetic Protonation of Racemic Allenylmetal Species for the Asymmetric Synthesis of Allenic Esters.

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[Ce(dtp),]: 1 (66 mg, 0.135 mmol) was allowed to react with [Ce(acac), . (H,O).]
(262mg) in 1,2,4-trichlorobenzene at reflux (12mL) for 3 h [17]. Column chromatography of the reaction mixture on alumina with hexanejCHC1, (2jl) as eluent
allowed the isolation of [Ce(dtp),] as the first fraction (4 mg, 5 % ) . 'H NMR
(270 MHz, [DJtoluene, - 4 0 ° C PhCD,H): 6 =10 19 (d, ' J =7.81 Hz, 4H ; o-endo-H in C,H,Me), 9.19 (s, 4 H ; meso), 8.93 (d, 'J = 4.40 Hz, 4H; 8-pyrrole), 8.85
(d, ,J = 4.40 Hz, 4 H ; p-pyrrole), 8.78 (d, ' J = 4.40 Hz, 4 H ; 8-pyrrole), 8.74 (d,
3J = 4.40 Hz, 4H ; 13-pyrrole), 8.20 (d, 'J =7.81 Hz, 4 H , m-endo-H in C,H,Me),
7.15 (d, ' J =7.81 Hz, 4 H ; m-exo-H in C,H,Me), 6.64 (d, 'J=7.81 Hz, 4 H ; oe.xoHinC6H,Me),2.80(s, 12H;CH,);UV/Vis(CHzC1,):i,,,(~)=
384(148000),
482 (11 500), 530.5 nm (7940); HR-MS (FAB) calcd for C,,H,,CeN,: 1116 3056
[M'], found: 1116.3085.
[Ce(motp),] and [Ce(motp)(tpp)]: 3 (50 mg, 0.065 mmol) was allowed to react with
[Cetacac), . (H,O).] (126 mg) in 1,2,4-trichlorobenzene ( 5 mL) at reflux for 14 h.
Flash column chromatography of the reaction mixture on silica gel with CHCI, as
eluent allowed the isolation of [Ce(motp),] as the second fraction ( 5 mg, 9.3%).
Likewise, [Ce(motp)(tpp)] was obtained from a mixture of 3 (65 mg, 0.083 mmol)
and 4 (25 mg, 0.042 mmol) and isolated as the third fraction by silica gel column
chromatography with CHCI, as eluent: [Ce(motp),]: 'H NMR (270 MHz,
[DJtoluene, 25'C, PhCD,H): 6 = 9.96 (br.s, 4H; o-endo-H in C,H,Me), 9.47 (s,
4H;o-endo-HinC6H,(OMe),),8.91
(d, , J = 4.64Hz,4H;B-pyrrole), 8.77(d,4H;
8-pyrrole), 8.76 (d, 4H ; 8-pyrrole), 8.63 (d, 3 J = 4.39Hz, 4H; P-pyrrole), 7.89
(br.s, 4 H ; m-endo-H in C,H,Me), 6.66 (br.s, 4H; o-exo-H in C,H,Me), 6.12 (s,
4 H; o-exo-H in C,H,(OMe),), 4.40 (s, 12H; endo-OCH,), 3.26 (s. 12H; exoOCH,), 2.67 (s, 12H; C,H,CH,), the signals due to m-exo-H in C,H,Me and p-H
in C,H,(OMe), overlapped with the solvent signal, UVjVis (benzene): i,,, ( E ) =
401.5 (257000), 482.5 (15800), 543.5 (12000), 635.5 nm (3240); HR-MS (FAB)
calcd for C,,,H,iCeN,O,. 1661.5232[M' + HI, found- 1661.5286
[Ce(motp)(tpp)]: 'H NMR (270 MHz, CDCl,, 2 5 ° C CHCI,): 6 = 9.66 (br.s, 4 H ;
o-endo-H in Ph), 9.44 (br.s, 2 H ; 0-endo-H in C,H,Me), 8.92 (s, 2 H ; o-endo-H in
C,H,(OMe),), 8.40 (d, 'J= 4.39 Hz, 4 H ; P-pyrrole in motp), 8.34 (s, 4 H , Bpyrrole in tpp), 8.29 (d, ' J = 4.39 Hz, 4H; P-pyrrole in motp). 8 22 (br.s, 4 H ;
8-pyrrole in tpp). 8.16 (br.s, 4 H ; m-endo-H in Ph), 7.92 (br,s. 2H; m-endo-H in
C,H,Me), 7.71 (t. 4H ; p- H in Ph). 7.08 (br.s, 2H; m-exo-H in C,H,Me), 6.87 (t,
4J = 2.20 Hz, 2H;p-H in C,H,(OMe),), 6.40 (br s, 4H; o-exo-H in Ph), 6.30 (br.s,
2 H; o-exo-H in C,H,Me), 4.43 (s, 6H; endo-OCH,), 3.52 (s, 6 H ; exo-OCH,), 2.72
(s. 6 H ; C,H,CH,), the signal due to m-exo-H in C,H,(OMe), overlapped with the
solvent signal; UVjVis (CH,CI,): ?.,,,ax = 398, 487, 542, 628.5 nm; HR-MS (FAB)
calcd for C,,H,,CeN,O,:
1513.4496 [ M i +HI, found: 1513.4581.
Optical resolution of [Zrfdtp),], [Zr(dip),]. and [Ce(motp),] was carried out on a
0.46125 cm HPLC column packed with silica gel coated with cellulose tris(3,5dimethylphenylcarbamate) as chiral stational phase. A solution (40 pL) of the racemate in CCI, was loaded on a JASCO Type TWINCLE equipped with a JASCO
Type 875-UV variable wavelength detector at a flow rate of 1.0 rnlmin-' at room
temperature. Retention times of the enantiomers (eluent): 6.7 and 8.8 min (hexanei
EtOH 98/2) for [Zr(dtp),], 5.6 and 8.6 min (hexane/EtOH 98/21 for [Zr(dip),], and
11.0 and 13.6 min (hexane/EtOH 92/8) for [Ce(motp),]
Racemization of the double-decker complexes was followed by the change in intensity of the CD bands. Protonic acid induced racemization was carried out in the
presence ofhydroquinone (4.5 x lo-' M) to prevent oxidation of thecomplexes [18],
and the CD spectra were taken after addition of Et,N.
Received. October 16, 1996
Revised version: January 2, 1997 [Z 9660 IE]
German version. Angew. Chem. 1997, 109, 882-884
Keywords: cerium . chirality
plexes . zirconium
.
porphyrinoids
.
sandwich com-
[I] a) 0. Bilsel, J. Rodriguez, D. Holten, J. Phys. Chem. 1990, 94, 3508-3512; b)
0. Bilsel, J. Rodriguez, D. Holten, G. S.Girolami, S. N. Milam, K. S. Suslick,
J. Am. Chem. Soc. 1990, 112,4075-4077; c) 0. Bilsel, J. H. Buchler, P. Hammerschmitt, J. Rodriguez, D. Holten, Chem. Phys. Lett. 1991, 182, 415-421
[2] J. W. Buchler, B. Scharbert, J. Am. Chem. Soc. 1988, 110, 4272-4276
[3] a) G. S. Girolami, P. A. Gorlin, K. S. Suslick, Inorg. Chem. 1994,33,626-627,
b) J. W Buchler, V. Eiermann, H. Hanssum, G. Heinz. H. Ruterjans. M.
Schwarzkopf, Chem. Ber. 1994,127,589-595; c) J. W. Buchler, G. Heinz, ibid.
1996, 129, 201 -205.
[4] a) J. W. Buchler, A. De Cian, J. Fischer, M. Kihn-Botulinski, H. Paulus, R.
Weiss, J. Am. Chem. SOC.1986, 108, 3652-3659; b) G. S. Girolami, S. N.
Milam, K. S. Suslick, ibid. 1988, 110. 2011 -2012; c) K. Kim, W. S. Lee, H.-J.
Kim, S:H. Cho, G. S. Girolami, P. A. Gorlin, K. S. Suslick. Inorg. Chem.
1991,30,2652-2656; d) D. Y Dawson, H. Brand, J. Arnold, J. Am. Chem. Soc.
1994, 116, 9797-9798.
151 a) R. Chong, P. S. Clezy, A. J. Liepa. A. W. Nichol, Ausr. J. Chem. 1969. 22,
229-238; b) P. S. Clezy, G. A. Smythe, ibid. 1969, 22,239-249; c) J. S. Lindsey, R. W. Wagner, J. Org. Chem. 1989,54, 828-836.
[61 J. S. Manka, D. S. Lawrence, Tetrahedron Lett. 1989, 30, 6989-6992.
858
,Q VCH Verlagsgesellschuft mhH, 0-69451 Weinhelm. 1997
[7] Analysis with TLC. R, for [Ce(motp),], [Ce(tpp),], and [Ce(motp)(tpp)] on a
silica gel plate with CHCI, as eluent: 0.2, 0.8, and 0.6, respectively.
[8] With assistance of NOESY spectroscopy, the signals at 6 = 9.36 and 9.32
(- 20 "C) were assigned to the /I-pyrrolic protons close to the meso-Ar and
meso-H substituents, respectively. Upon elevation of the temperature the signals shifted upfield with a cross-over at 0°C.
[9] The difference in resonance frequency (Av) between the two exchangeable
B-pyrrolic proton signals reached a plateau at -60°C. The exchange rate
constant (keJ at the coalescence temperature was calculated by the Gutowsky
equation: k,, = (x/l/Z)Av (S. Gutowsky, C. H. Holm, J Chem. Phys. 1956,25,
1228- 1234).
[lo] Calculated from k,,/4, exchange of the pyrrole units occurs at every quarter
turn of the porphyrin ligand.
[ l l ] Analysis with TLC. R, for [Zr(dtp),], [Zr(dip),]. and [Zr(dtp)(dip)]on a silica
gel plate with hexaneiCHC1, ( l i l ) as eluent: 0.39, 0.49, and 0.44, respectively.
[12] Calculated from u,,, = k,.,[Zr(dtp),][TsOH] eejlOO with ee in %.
[13] We suggest that a possible intermediate for the ligand rotation is a monoprotonated double-decker complex, which has been reported for some tervalent
metal bis(p0rphyrinate)s (La"', Sm"', Lu"'): a) J. W Buchler, M. Kihn-Botulinski, J. Loffler, B. Scharbert, NewJ. Chem. 1992,16,545-553; b)G. A. Spyroulias, A. G. Coutsolelos, C. P. Raptopoulou, A. Terzis, Inorg. Chem. 1995, 34,
2476-2479; c) G. A. Spyroulias, A. G. Coutsolelos, [hid.1996,35,1382- 1385.
[14] The 4N, mean plane separations in [Zr(oep),] and [Ce(oep),] are 2.53 and
2.75 A, respectively: J. W Buchler, A. De Cian, S Elschner, J. Fischer, P.
Hammerschmitt, R. Weiss, Chem. Ber. 1992, 125, 107-115 and ref. [4a].
[l5] J. Arnold, D. Y Dawson, C. G. Hoffman, J Am. Chem. Soc. 1993,115,27072713.
1161 J. W. Buchler, A. De Cian, J. Fischer, P. Hammerschmitt, R. Weiss, Chem. Ber.
1991, 124, 1051-1058.
[17] J. W. Buchler, A. De Cian, J. Fischer, P. Hammerschmitt, J. Loffler, B. Scharbert, R. Weiss, Chem. Ber. 1989, 122, 2219-2228.
[18] The zirconium double-decker complexes, as observed by UV/Vis and EPR,
were subject to oxidation in the presence of protonic acids. to form the correspondingcation radicals (g = 2.0026, u N = 0.5 GI. N o racemization took place
without TsOH in the presence of hydroqninone.
Dynamic Kinetic Protonation of
Racemic Allenylmetal Species for the
Asymmetric Synthesis of Allenic Esters
Koichi Mikami* and Akihiro Yoshida
van? Hoff predicted the existence of chiral allenyl species in
1875.['] Since then, much attention has been paid to the asymmetric synthesis of allenic acids and esters in particular because
they constitute an important class of natural products and synthetic intermediates.['] The first optically active allenes synthesized, (+)- and (-)-1,3-diphenyl-1,3-di(1-naphthyl)allene, were
obtained, though in quite a low enantiomer excess (ee), by
asymmetric dehydration of 1,3-diphenyl-l,3-di( 1-naphthy1)prop-2-en-1-01 with (+ )- and (-)-camphor-10-sulfonic acid,
respectively.[31Subsequent asymmetric syntheses of allenes were
mainly based on chirality transfer from a chiral center to the
allene axis and optical resolution. Optical resolution of allenic
acids was reported through crystallization of the salts of natural
alkaloids such as brucine, which has a yield of 50% at best.[41
Chirality transfer reactions of propargylic alcohol derivatives
by chlorination with thionyl
substitution reactions
of organocuprates,[6]and sigmatropic rearrangements[" *I leading to allene derivatives generally give a high degree of chirality
[*] Prof. Dr. K. Mikami, A. Yoshida
Department of Chemical Technology
Tokyo Institute of Technology
Meguro-ku, Tokyo 152 (Japan)
Fax: Int. code +(3)5734-2776
e-mail: kmikami@ o.cc.titech.ac.jp
0570-0833/9713608-0858$17.50+ .50/0
Angew Chem. h i . Ed. Engl. 1997, 36, No. 8
COMMUNICATIONS
[Pd(PPh&1(5 mol%)
Sml,, fBuOH
-q
(R)-BINOL-TI Cat.
CHzCl2
(95%)
H<
(+)-I
C0,Me
-L
.-C02Me
2
THF
(53%)
94% ee
C02Me
p=H
12)(:
J ~ B U U(Eto)2Poa
,
P = PO(0Et)Z
(94%)
Scheme 1. Pdo-catalyzed reduction of 1 with Sml, in the presence of 1.1 equiv. IBuOH.
transmission. We describe here the asymmetric synthesis of allenic esters by enantiomer selective protonation of racemic allenylmetal compounds[g1by deracemization without chirality
transfer or "destruction" of the allenyl enantiomer. This dynamic kinetic resolution["] by asymmetric protonation,'"] in
other words dynamic kinetic protonation, is a powerful strategy
for the asymmetric synthesis of allenic compounds from racemic
allenylmetal species.
o-Allenyl- and o-propargylpalladium(1r) complexes[121are
mainly used in reactions with nucleophiles such as alkylmagnesium and -zinc reagents, hydrides, carbon monoxide, and mal~ n a t e s . ~Recently,
'~]
Inanaga and we reported["] on the umpol~ n g ~of' ~allenyl-/propargylpalladium(II)
]
complexes to give
nucleophilic species by reduction with samarium@) iodide."
This strategy was used to effect the highly regioselective reaction
of secondary propargylic phosphates with proton sources or
ketones to provide allenes rather than acetylenes. Here we
report the results of the dynamic kinetic protonation and deracemization of racemic allenylsamarium(I1I) intermediates with
Pdo/SmI, and a chiral proton source.
For the asymmetric synthesis of a-allenic esters, the chirality
transfer reaction was first attempted with an optically active
propargylic phosphate in the presence of Pdo/Sm1, and an achiral alcohol. For this purpose the chiral propargylic phosphate 1
was prepared from a conjugated ~ n a l [ ' ~by] asymmetric carbony1-ene reaction catalyzed by a binaphthol-derived chiral
titanium complex[' *I followed by phosphorylation (Scheme l),
The Pdo-catalyzed reduction of 1 with SmI, in the presence of
1.1 equivalents of the achiral rut-butyl alcohol as a proton
source was attempted. The allenic product 2
was obtained without any contaminating
acetylenic product and surprisingly in
racemic form since rapid racemization occurred before protonation. This result suggests the possibility of the dynamic kinetic
protonation of allenylsamarium(Irr) species
using chiral proton sources even when
racemic phosphates serve as starting compounds (Scheme 2).
b- PB
Scheme 2. Dynamic kinetic protonation with Pdo. Sml,, and a chiral proton
source.
R = (cyclohex-1-en-1-yl)methyl
Scheme 3. Protonation of racernic 1 with a chiral proton source
Table 1. Pdo:SrnI,-mediated reduction and protonation of roc-1 to give (R)-2.
Yield [YO]
ee ["/.I
4
86
13
5
71
86
68
2 95
Entry
6
Chrral proton source
R
[a]
0
[a] Detemined by LIS N M R (LIS = lanthanide-induced shift) analysis using
was also obtained in
[Eu(hfc),] [lS]. [b] Methyl 5-(cyclohex-l-en-l-yI)pent-2-ynoate
7 % yield. [c] ( S ) - 2 was obtained.
Angrw. Chem. lnt. Ed. En$. 1997,36, No. 8
The dynamic kinetic protonation of the racemic propargylic
phosphate 1 was examined with various chiral proton sources
(Scheme 3, Table 1). The reactions with (R,R)-(
+ )-hydrobenzoin (entry 5, Table 1) and (R)-(
-)-pantola~tone['~](entry 6,
Table 1) provided allenic ester 2 regioselectively with high enantiomeric purity and reasonably high yield ( > 50 O h ) . The absolute configuration of 2 was deduced by application of the LoweBrewster rulef4]to be (R)because the compound is levorotatory
at the sodium D line. The result thus obtained is the first example of the asymmetric synthesis of allene derivatives by selective protonation of one enantiomer without destruction of the
other. The selective protonation by the (R,R)-diol to provide the
(R)-(
-)-allenic ester is in agreement with the proposed transition state model (Scheme 4), in which steric repulsion between
the alkyl groups in the allenylsamarium(II1) species and in the
chiral proton source
R'
plays a key role.
In summary, we
H.
H, i
O:<srn?
'
have disclosed the
O'
R>m
,?
,
A
asymmetric
of
allenic esters
synthesis
by dynamic kinetic protonation of racemic al-
8 VCH VerlagsgeselfschaffmbH, 0-69451 Wernheim.1997
H
4
R
H
$a"
E
E
Scheme 4 Transition state model.
OS70-0833/97/3608-0859 S 17.SO-k SO10
859
COMMUNICATIONS
lenylmetal species without the involvement of chirality transfer
or destruction of the allenyl enantiomer. This should become a
powerful strategy for the asymmetric synthesis of allenic compounds from racemic allenyl-/propargylmetal species.
Hexameric Aggregates in Crystalline
(Pentamethylcyclopentadienyl)gallium(I)
at 200 K**
Experimental Section
Dagmar Loos, Elke Baum, Achim Ecker,
Hansgeorg Schnockel,* and Anthony J. Downs
To a solution of the phosphate (0.5 mmol). [Pd(PPh,),] (29 mg. 0.025 mmol.
5 mol%), and the chiral alcohol (0.55 mmol) in anhydrous T HF (2.5 mL) was
added a 0.1 M solution of samarium(l1) iodide in T HF (10 mL, 1.0 mmol) at room
temperature under an argon atmosphere. After stirring for 10 min. the reaction
mixture was quenched with saturated NH,CI. Standard workup followed by silica
gel chromatography afforded the allenic ester 2.
Received: October 30, 1996 [Z9702IE]
German version: Angeu,. Chem. 1997, 109, 892-894
Keywords: allenes
samarium
*
asymmetric synthesis
.
palladium
*
J. H. van? Hoff, La Chimie duns IEspace, P. M. Bazendijk, Rotterdam, 1875,
p. 29.
a) H. F. Schuster, G. M. Coppola, Allenes in Organic Chemistrj, Wiley, New
York, 1984; b) S. R. Landor, The Chemistry of the Allenes, Academic Press,
New York, 1982.
a) P. Maitland, W. H. Mills, Nature 1935, f35,994: b) 1 Chem. SOC.1936,987.
Review: R. Rossi, P. Diversi, Synthesis 1973. 25.
First example of the asymmetric synthesis of an allene by chirality transfer.
R. J. D. Evans, S. R. Landor, R. Taylor-Smith, 1 Chem SOC.1963, 1506.
a) J. L. Luche, E. Barreiro, J. M. Dollat, P. Crabbt, Tetrahedron Lett. 1975,
4615; b) G . Tadema, R. H. Everhardus, H. Westmijze, P. Vermeer, ihid. 1978,
3935; c) W. H. Pirkle. C. W. Boeder, J Org. Chem. 1978, 43, 1950; d) L.-I.
Olsson, A. Claesson. Actu Chem. Scand. Ser. 5 1979, 33, 679; e) A. Haces,
E M. G. A. van Kruchten, W H. Okamura, Tetrahedron Lett. 1982.23, 2707;
f) A. Alexakis, 1. Marek, P. Mangeney, J. F. Normant, 3. Am. Chem. Soc. 1990,
112, 8042; g) 0 .W. Gooding, C. C. Beard, D. Y. Jackson, D. L. Wren. G F.
Cooper, J. Org. Chem. 1991,56,1083; h) A. Alexakis, Pure Appl. Chem. 1992.
64, 387.
Claisen rearrangement of orthoesters: M. A. Henderson, C. H. Heathcock, 1
Org. Chem. 1988.53.4736
[2,3]Wittig rearrangement: a) J. A. Marshall, E D. Robinson, A. Zapata, 3.
Org Chem. 1989.54, 5854; b) J. A. Marshall, X:j. Wang. ihid. 1991,56.4913.
Review of allenyl and propargyl organometallic compounds: H. Yamamoto
in Comprehensive Orgunic Synthesis, Vol. 2 (Eds.: B. M. Trost. I. Fleming).
Pergamon, Oxford, 1996, p. 81
Reviews: a) R. Noyori, M. Tokunaga, M. Kitamura, Bull. Chem. SOC.Jpn
1995, 68, 36: b) R. S. Ward, Tetrahedron: Asymmetry 1995, 6, 1475.
Reviews: a) L. Duhamel, P. Duhamel, J.-C. Launay, J.-C. Plaquevent, Bull.
Soc. Chim. Fr. 1984,II, 421 ;b) C. Fehr, Chimia 1991,45,253;c) H. Waldmann,
Nachr Chem. Tech. Lab. 1991.39.413; d) C. Fehr, Angew. Chem. 1996, 108,
2726; Angew. Chem. Int. Ed. Engl. 1996,35,2566; e) S . Hiinig in Houhen- Weyl,
Methods of Organic Chemistry, Vol. E2ld (Eds.: G Helmchen, R. W. Hoffmann, J. Mulzer, E. Schaumann), Thieme, Stuttgart, 1995. p. 3851.
C. J. Elsevier, H. Kleijn, J. Boersma, P. Vermeer, Organometallics 1986,5, 716.
Review: J. Tsuji, T. Mandai, Angesc. Chern. 1995, 107,2830, Angew Chem. Int.
Ed. Engl. 1995, 34, 2589.
complexes by diethylzinc(r1): Y.
Umpolung of allenyl-/propargylpaIladium(~~)
Tamaru, S. Goto, A. Tanaka, M. Shimizu, M. Kimura, Angew Chem. 1996,
108, 962; Angen-. Chem. In!. Ed. Engl. 1996, 35. 878.
K. Mikami, A. Yoshida, S. Matsumoto, F. Feng, Y Matsumoto, A. Sugino,
T. Hanamoto, J. Inanaga, Tetrahedron Lett. 1995, 36, 907.
Reviews: a) H. B. Kagan, J. L. Namy, Tetrahedron 1986,42,6573; b) J. Inanaga, J Synth. Org. Chenr. Jpn. 1989,47,200; c) H. B. Kagan, New J Chem. 1990.
f4.453; d) G. A. Molander in Comprehensive Organrc Synthesis, Vol. 1 (Eds.:
B. M. Trost. 1. Fleming), Pergamon, Oxford, 1996, p. S. 251 ; e) D. P Curran,
T. L. Fevig, C. P. Jasperse, M. J. Totleben, Synletr 1992. 943; f) N. E. Brandukova. Y s. Vygodskii. S V. Vinogradova, Russ. Chem. Rev. 1994, 63. 345;
g) T. Imamoto. Lanthanides in Organic Synthesis. Academic Press, 1994, p. 21 ;
h) G. A. Molander, Org. React. 1994,46,211; i) G. A. Molander. C. R. Harris,
Chem. Rev. 1996. 96, 307.
[17] K. Mikami, A. Yoshida, Y Matsumoto, Tetrahedron Lett. 1996. 37, 851 5.
[I81 a) K. Mikami, M. Terada, T. Nakai, J Am. Chem. Suc. 1990.112,3949; b) hid.
1989, ffl, 1940.
[191 (R)-(- )-pantolactone as a chiral proton source: U. Gerlach, S. Hiinig, Angen.
Chem. 1987, 99, 1323: Angerc Chem. Int. Ed. Engl. 1987, 26, 1283.
860
c) VCH Verlugsgesellschuff mhH, D-69451 Weinheim, lY97
Dedicated to Professor Hans Burger
on the occasion of his 60th birthday
Univalent derivatives of Group 13 metals with organic ligands have attracted considerable attention by virtue of their
synthetic potential and the teasing problems of aggregation and
structure that they pose.['] The first such gallium compounds[GaCp]['I (Cp = C,H,) and [Ga,{C(SiMe,),),]r31- were not
synthesized and authenticated until 1992. The pentamethylcyclopendadienyl ligand (Cp*), known for its stabilizing ability,
yields more or less robust derivatives [MCp*] (M = Al,14]Ga,'']
In,[,] or TI[']), which all vaporize as monomers with MC,,
skeletons of C,, symmetry (M = A1,I8]Ga,[91In,',] or Tl["]). In
the solid state the aluminum compound is a tetramer based on
a central Al, tetrahedron with comparatively short AI-A1
edges.[,] The indium compound is made up of "hexamers" incorporating near-octahedral In, units with long In-In edges,161
whereas the thallium compound forms polymeric zig-zag chains
~ ~ I we describe the cryswith even longer T1. . . T1 d i s t a n c e ~ .Here
tal structure of the missing link, namely [GaCp*] .
We succeeded in growing a single crystal, not from a solution,
but by cooling a molten sample of the pure, freshly condensed
material" in a rigorously preconditioned, Pyrex-glass capillary at about f 4 " C . Under these conditions, the oil affords
colorless crystals that belong to the trigonal space group R3
with 18 GaCp* units in the hexagonal "triple cell";['21 solid
[GaCp*] is thus isomorphous with [InCp*]I,[. The structure of
this gallium compound at 200 K consists of discrete hexameric
aggregates composed of a Ga, core enclosed by a shell of Cp*
ligands (Figure 1). The Ga, unit is not strictly octahedral but
compressed along a C, axis to give two distinct Ga, units; the
Ga . . . Ga distances within and between these units are 417.3(3)
and 407.3(2) pm, respectively (indicating a larger distortion
than in [In,Cp*,],[61 d(1n-.-In)= 396.3(1) and 394.2(1) pm).
The individual GaCp* units of the hexamer show $-coordination of the Cp* ligands; Ga-C distances range from 238.0(9) to
242.1(9) pm (average 240.0 pm), which gives a Ga-centroid distance of 208.1 pm. The corresponding dimensions for the
monomer, as determined by electron diffraction of the vapor
at 60 "C, are d(Ga-C) = 240.5(4) and d(Ga-centroid) =
208.1(5) ~ m . ' As
~ ] in the case of solid [ I ~ C P * ] ,the
[ ~ ~Ga-centroid vectors do not point towards the center of the hexameric
cluster but are tilted significantly towards the faces of the cluster
( S , symmetry). By contrast, the Al-centroid vectors of the tetramer [Al,Cp*,] point towards the center of the Al, tetrahedron.',]
The orientation of the Cp* ligands with respect to the M, core
of [M,Cp*,] (M = Ga or In) is consistent with a second-order
E.Baum. Dr. A. Ecker
Institut fur Anorganische Chemie der Universitiit
Engesserstrasse, Geb. 30.45, D-76128 Karlsruhe (Germany)
Fax: Int. code +(721)608-4854
e-mail: hg(u achpc9 chemie.uni-karlsruhe.de
Dr. D. Loos. Prof. A. J. Downs
Inorganic Chemistry Laboratory, University of Oxford (UK)
This work was supported in Germany by the Deutsche Forschungsgemeinschaft (postdoctoral grant to D. L.) and the Fonds der Chemischen Industrie,
and in the UK by the EPSRC. It was described in part at the XVIlth International Conference on organometallic Chemistry, Brisbane, July 1996.
[*) Prof Dr. H. Schnockel. Dr.
[**I
0570-0833/97/3608-0860S 17 SO+ 5010
Angen Chem. Int. Ed. Enat 1997. 36 Nn R
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