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Cyclopropenylidene Adducts of Divalent Germanium Tin and Lead.

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Me,SBHa
+ F-
Et3NBH,
+
OH-
MeSBH3-
+
+
Et,NBH,-
+ CH,F
+ H,C=CH,
(3)
+ H,O
(4)
For comparison, F- is completely unreactive in the gas-phase
with uncomplexed Me$, and OH --induced ethylene elimination from uncomplexed Et,N is actually endothermic by
14 kcalmol-118] and does not occur at room temperature. The
absence of Me,NBH; as a product from the reaction between
OH- and Me,NBH, indicates that Et,NBH; must be formed
by elimination rather than substitution. In neither of the above
reactions is the proton abstraction product observed.
In conclusion, complexation of methyl sulfides, methylamines, and methylphosphanes with BH, causes a dramatic
increase in the gas-phase acidity of the a-C-H bonds, amounting to 11-20 kcalmol-'. Deprotonation of borane Lewis base
complexes in the gas phase produces stable carbanions that do
not rearrange to the more stable "borate" forms. Borane coordination at a heteroatom is also found to enable new reactions
of the attached alkyl groups, such as a-substitution and p-elimination.
Cyclopropenylidene Adducts of Divalent
Germanium, Tin, and Lead**
Herbert Schumann,* Mario Glanz, Frank Girgsdies,
F. Ekkehardt Hahn, Matthias Tamm,* and
Alexander Grzegorzewski
Dedicated to Professor Wolfgang Beck
on the occasion of his 65th birthday
Adducts of nucleophilic carbenes"] (especially imidazol-2ylidenes[']) with main group Lewis acids have been studied intensively in recent years.[3]In the course of our work on ligands
containing cy~loheptatrienylium[~]
and cycl~propenylium[~~
rings we became interested in the coordination chemistry of
bis(dialkylamino)cyclopropenylidenes,which according to calculations[6.'1 have a stability comparable to imidazol-2-ylidenes. Whereas cyclopropenylidene transition metal complexes[*]
have been known for many years,[*]analogous derivatives with
main group elements have not yet been described.
The bis(dialky1amino)cyclopropenylidene (2) can be obtained
from cyclopropenylium salts like 1 by treatment with n-butyllithium. At low temperature 2 forms a stable lithium adduct that
Received: April 16, 1997 [Z10356IE]
German version: Angew. Chem. 1997, 109, 2330-2332
-
Keywords: acidity carbanions - gas-phase chemistry
molecule reactions Lewis acids
-
[l] a) D. A. Evans, D. M. Barnes, Tetrahedron Lett. 1997, 38, 57; b) Y. Hayashi,
J. J. Rohde, E. J. Corey, J. Am. Chem. Soc. 1996, If8,5502; c) D. A. Evans, J. A
Murry, M. C. Kozlowski, ibid. 1996, 118, 5814; d) K. Ishihara, H. Kurihara,
H. Yamamoto, ibid. 1996,118,3049; e) E. J. Corey, K. A. Cimprich, 2nd. 1994,
116, 3151.
[2] a) S. V. Kessar, P. Singh, Chem. Rev. 1997, 97, 721; b) Y. Nishigaichl, A.
Takuwa, Y. Naruta, K. Maruyama, Tetrahedron 1993,49,7395; c) L Deloux,
M. Srebnik, Chem. Rev.1993,93,763; d) H. B. Kagan, 0.Riant, ibid. 1992,92,
1007; e ) H. Yamamoto, K. Maruoka, K. Furuta in Setectivities in Lewis Acid
Promoted Reactrons (Ed.: D. Schinzer), Kluwer, Dordrecht, 1988, p. 281.
[3] P. Beak, D. B. Reitz, Chem. Rev. 1978, 78, 275.
[4] a) S. T. Graul, R. R. Squires, Mass Spectrom. Rev. 1988, 7, 263; b) P. J.
Marinelli, J. A. Paulino, L. S. Suuderlin, P. G. Wenthold, J. C. Poutsma, R. R.
Squires, Int. J Mass Spectrom. Ion Processes 1994, 130, 89.
[5] L. A. Curtis, K. Raghavachari, 1. A. Pople, J. Chem. Phys. 1993, 98, 1293.
[6] a) J. H. Stewart, R. H. Shapiro, C. H. DePuy, V. M. Bierbaum, J. Am. Chem.
Soc. 1977, 99, 7650; b) C. H. DePuy, V. M. Bierbaum, G. K. King, R. H.
Shapiro, ibid. 1978, 100, 2921.
[7] D. B. Workman, Ph D Thesis, Purdue University, 1990.
[8] S . G. Lias, J. E. Bartmess, J. F. Liebman, J. L. Holmes, R. D Levin, W. G.
Mallard, J Phys. Chem. Ref. Data 1988, f 7, Suppl. 1; updated: J. E. Bartmess,
NIST Std. Ref. Database 19B (Negative Ion energetics Database Ber. 3.00),
1993.
191 J. E. Bartmess, R. T. McIver, Jr. in Gas Phase Ion Chemistry, Vol. 2 (Ed.: M. T.
Bowers), Academic Press, London, 1979, p. 87.
[lo] AH,,,, = AGzcfd+ TAS.,,, For a concise description of the calculation of AS,,,,
by statistical mechanics, see: G. E. Davico, V M. Bierbaum, C. H. DePuy,
G. B. Ellison, R. R. Squires, J Am. Chem. Soc. 1995, If7,2590. Note that the
first term of Equation A2 in this paper contains a minor typographical error,
it should read 5/2R ln(T).
[ l l ] a) P. Speers, K. E. Laidig, A. Streitwieser, J Am. Chem. Soc. 1994,116. 9257,
b) K. B. Wiberg, H. Castejon, ibid. 1994, 116, 10489.
[12] K. M. Downard, J. C. Sheldon, J. H. Bowie, D. E. Lewis, R. N. Hayes, J. Am.
Chem. SOC.1989,111,8112.
[13] A. R. Muci, K R. Campos, D. A. Evans, J. Ant. Chem. Soc. 1995,117,9075.
[14] G. I. Mackay, R. S. Hemsworth, D. K. Bohme, Can. 1. Chem. 1976,54,1624.
[15] S Ingemann, N. M. M. Nibbering, J Chem. Soc. Perkin Trans. 2, 1985, 837.
Grabowski, et al. report a value of 384.2+ 3.2 kcalmol-I based on bracketing
experiments (J. J. Grabowski, P. D. Roy, R. Leone, ibid. 1988,1627). However,
B3LYPicc-PVTZ and CBS-4 calculations using an isodesmic reaction approach predict AH.,,,(PMe,) = 390 kcalmol-I, in support of the higher value.
2232
Q WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
iPr2N
ion-
CF,SOF
i
nBuLi
1. SnC12
2.Na[N(SiMe3)2]
2
4a: E = Ge
4b: E = Sn
4C: E = Pb
can be used directly for the synthesis of carbene complexe ~ . [ ~ . ' . Thus,
* ~ ] the reaction of 2 with the bisamides 3a and 3 c
affords the cyclopropenylidene complexes 4 a and 4 c, respectively. The analogous tin compound 4 b is synthesized by reaction of 2 with SnCl, followed by amidation of the intermediate
[*I Prof. Dr. H. Schumann, Dr. M. Glanz, DipLChem. F. Girgsdies
Institut fur Anorganische und Analytische Chemie der Technischen Universitat
Strasse des 17. Juni 135, D-10623 Berlin (Germany)
Fax: Int. code +(30)314-22168
e-mail: schumann@mailszrz.zrz.tu-berlin.de
Dr. M. Tamm, Prof. Dr. F. E. Hahn, Dip].-Chem. A. Grzegorzewski
Institut fur Anorganische und Analytische Chemie der Freien Universitat
Fabeckstrasse 34-36, D-14195 Berlin (Germany)
["I This work was supported by the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie.
0570-0833/97/3620-2232$17.5O+.SOjO
Angew. Chem. Int. Ed. Engl. 1997.36, No. 20
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carbene adduct. Compounds 4a-c are isolated as thermally
stable yellow crystals, which are soluble in aprotic solvents such
as tetrahydrofuran and diethyl ether and in aromatic and
aliphatic hydrocarbons. In the crystalline state the monomeric
complexes 4 have a trigonal-pyramidal structure (Figure 1, top)
with the E atom (E = Ge, Sn, Pb) on top and the two silylamineN atoms and the carbene-C atom forming the basal piane of the
pyramid (Figure 1, bottom). The plane defined by the threemembered carbene ring approximately includes the nitrogen
-\I\
/
c43
c22
E -N distances show the expected lengthening. Resonance stabilization of the carbene ligand is clearly indicated by the equivalence (30) of the C-C bonds (137-139 pm). The exocyclic
C-N distances (133- 134 pm) are significantly shorter than typical C-N single bonds.
The 'H NMR spectra of complexes 4 below - 60 "C exhibit
four multiplets for the methine protons and four doublets for
the methyl protons of the iso-propyl groups, which are all magnetically inequivalent due to hindered rotation around the
N-C(ring) and E-C bond axes. As the temperature increases
the signals broaden, and at room temperature only two very
broad resonances are observed for the CH, groups. As the exchange processes are concurrent, it is not possible to determine
the coalescence temperatures. The 13CNMR signals for the ring
carbon atoms in 4a-c are observed at 6~ 145-150 and fall in
the range typical for cyclopropenylium compounds.[5.71 In imidazol-2-ylidenes the carbene-C resonance is shifted significantly
to higher field upon
Here, an analogous
trend is purely speculative, as a free cyclopropenylidene 2 has
not yet been isolated and characterized by NMR spectroscopy.
Comparison of the '19Sn NMR resonance signals in 3 b
(6 = 766)"'l and in 4b (6 = 45) indicates strong shielding of
the tin nucleus by interaction with the carbene ligand.[3b.'1The
new compounds presented here illustrate that nucleophilic bis(dialky1amino)cyclopropenylidenescould be interesting alternatives to the intensively studied imidazol-2-ylidenes.
~
Experimental Section
(I(
Si12
c54
Figure 1. Top: ORTEP drawing [ l l ] o f 4 a (thermal ellipsoids at the 30% probability level); compounds 4h and 4 c have analogous structures Bottom: Schematic
ORTEP drawing [I 11 of 4 a (all methyl groups have been omitted for clarity).
Selected bond lengths [pm] and angles ["I [12]: Ge-C31 208.5(3), Ge-Nl 199.1(3),
Ge-N2 196.5(3); Nl-Ge-C31 98.67(12), N2-Ge-C31 98.65(12). Nl-Ge-N2
105.67(11). 4b: Sn-C31 230.3(9), Sn-Nl 221.2(7). Sn-N2 215.7(7); Nl-Sn-C31
95.1(3). N2-Sn-C31 94 ?(3j, Nl-Sn-N2 110.5(3). 4c: Pb-C31 242.3(8), Pb-N1
230.1(7), Pb-N2 230.8(7); Nl-Pb-C31 95.0(2), N2-Pb-C31 91.9(2), Nl-Pb-N2
110.2(2)
and secondary carbon atoms of the iso-propyl groups. The distances between the E atom and the carbene-C atom are appreciably longer than those in "true" germaethenes (180/183 pm)[91
and stannaethenes (202 pm)["] and fall in the range expected
for single bonds. This indicates a bond type best described by
the ylidic resonance structure A, which corresponds to that in
imidazol-2-ylidene ad duct^,[^^-^] rather than the metallaethene
structure B.
Comparison of the molecular structures of the adducts 4 with
the compound 3["'1reveals only slight changes in the N-E-N
angles upon coordination of the carbene ligand, whereas the
Anxeu. Chrm. h i Ed EngI. 1997, 36, No. 20
1 [18], 3a, and 3c [19] were prepared according to published procedures. NMR
spectra were recorded on Bruker AMX-200 and AMX-400 instruments. Microanalyses were performed on a Perkin-Elmer Series I1 CHNS/O 2400 analyzer.
4a: A solution of nBuLi (3.5 mL o f a 1 . 6 solution
~
in hexane. 5.6 mmol) was added
dropwise to a stirred suspension of 1 (2.35 g, 5.6 mmol) in THF (70 mL) at -78 "C.
After the mixture had been stirred for about 15 min, solid 3 a (2.20 g. 5.6 mmol) was
added in small portions to the clear solution. The reaction mixture was allowed to
warm up to 25 'C and was subsequently stirred for 12 h. After removal of the solvent
the residue was suspended in hexane (70 mL) and filtered. The solution was concentrated to about 35 mL, and yellow crystals of 4a were obtained at -78 "C. Yield
2.15g (61 %). M.p. (0.1 mbar) 125°C (decomp); 'HNMR (400MHz, C,D,,
25 "C): b = 1.23 (br. s, 120 Hz, 12H; CHCH,), 0.86 (br. s, 120 Hz, 12H; CHCH,),
0.47 (s, 36H; SiCH,); ' H N M R (400MHz, C,D,, -73°C): 6 =4.95 (m, 1 H ;
CHCH,), 3.92 (m. 1 H; CHCH,), 2.88 (m, 1 H ; CHCH,), 2.74 (m, 1H; CHCH,),
1.39 (d, 'J(H,H) = 5.7Hz, 6 H ; CHCH,), 1.09 (d, 3J(H.H) = 5.8 Hz, 6 H ;
CHCH,), 0.77 (d, '4H.H) = 6.1 Hz. 6 H ; CHCH,). 0.67 (d, 'J(H,H) = 6.0 Hz,
6 H ; CHCH,), 0.57 (s, 36H; SiCH,); I3C NMR (100.64 MHz, C,D,, 25°C):
b =145.45(CC,), 144.62(CC,),52.50(250 Hz; CHCH,), 49.26(250 Hz; CHCH,),
22.51 (120 Hz; CHCH,), 21.70(120 Hz CHCH,), 6.74 (SiCH,); elemental analysis
calcd (%) for C,,H,,GeN,Si,
(629.76) C 51.50, H 10.24, N 8.90; found: C 51.84,
H 10.01, N 8.47.
4b: A solution of nBuLi (3.9 mL of a 1 . 6 solution
~
in hexane. 6.22 mmol) was
added dropwise to a stirred suspension of 1 (2.61 g, 6.2 mmol) in T H F (70 mL) at
-78'C. After the mixture had been stirred for about 15 min, solid SnC1, (1.18 g,
6.2 mmol) was added in small portions to the clear solution. The reaction mixture
was allowed to warm up to 25'C and was subsequently stirred for 12 h.
Na[N(SiMe,),] (2.27 g, 12.3 mmol) was added and the mixture was stirred for another 12 h. Compound 4 b was isolated as described for4a. Yellow crystals of 4 b are
obtained from hexane at -78°C. Yield 2.30 g ( 5 5 % ) . m.p. (0.1 mbar) 137'C (decamp); 'HNMR (400 MHz, C,D,, 25°C): b = 1.26 (br. s, 75 Hz, 12H; CHCH,),
0.85 (br. s, 75 Hz, 12H; CHCH,), 0.49 (s, 36H, SICH,); 'H NMR (400 MHz,
C,D,, -61 'C): 6 = 4.87 (m, 1 H ; CHCH,), 3.91 (m, 1 H; CHCH,). 2.80 (m. 2 H ;
CHCH,). 1.38 (d, 'J(H,H) = 6.3 Hz. 6 H ; CHCH,), 1.09 (d, 'J(H,H) = 6.1 Hz.
6 H ; CHCH,), 0.78 (d, 'J(H,H) = 6.8 Hz, 6 H ; CHCH,). 0.68 (d,
'J(H,H) = 6.2 Hz, 6 H ; CHCH,), 0.57 (s. 36H; SiCH,); 13C NMR (50.32 MHz,
C,D,, 25°C): 6 =150.17 (CC,), 146.66 (CC,). 51.13 (120 Hz: CHCH,). 49.73
(120 Hz; CHCH,). 21.89 (100 Hz; CHCH,). 7.09 (SiCH,); I4N NMR (28.91 MHz,
C,D,, 25°C): 6 = - 316.7, -353.1; *9Si NMR (79.49 MHz, C,D,, 25°C):
6 = - 3.03; Ii9SnNMR (149.21 MHz. C,D,, 25°C): b = - 44.69;elemental anal(675.86): C 47 98, H 9.54, N 8.29; found: C 46.76.
ysis calcd (%) for C,,H,,N,Si,Sn
H 8.85, N 7.90.
4c: 4 c was prepared from 1 (1.16 g, 2.8 mmol), nBuLi (1.8 mL of a 1 . 6 solution
~
in hexane, 2.8 mmol) and 3 c (1.45 g, 2.8 mmol) by a procedure similar to that for
4a. Yield 1.66 g (79%) of yellow crystals. M.p. (0.1 mbar) 135 'C (decomp);
'H NMR(400 MHz, C,D,, 25°C): b = 1.20(br. s, 120 Hz, 12H; CHCH,), 0.81 (br.
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim,
1997
0570-083319713620.2233 $17.50+.50/0
2233
COMMUNICATIONS
s, 120Hz, 1 2 H ; CHCH,), 0.47 (s, 36H; SiCH,); 'HNMR (400MHz, C,D,,
-61 "C): 6 = 4.92 (m, 1 H; CHCH,), 3.95 (m, 1 H ; CHCH,), 2.84 (m, 1 H ;
CHCH,), 2.78 (m, 1 H ; CHCH,), 1.32 (br. s, 15 Hz, 6 H ; CHCH,), 1.16 (br. s,
15 Hz, 6H; CHCH,), 0.84 (br. s, 15 Hz, 6H; CHCH,), 0.75 (br. s, 15 Hz, 6 H ;
CHCH,), 0.60 (s, 36H; SiCH,); "C NMR (100.64 MHz, C,D,, 25°C): 6 = 148.53
(C,), 50.71 (400 Hz; CHCH,), 21.46 (175 Hz; CHCH,), 6.89 (SiCH,); elemental
(764.37): C 42.43, H 8.44, N 7.33; found:
analysis calcd (%) for C,,H,,N,Si,Pb
C 42.54, H 8.45, N 6.66.
Received: April 4, 1997 [Z 10311 IE]
German version: Angeu.. Chem. 1997,109,2328-2330
-
-
Keywords: carbene complexes germanium lead * tin
[I] M. Regitz, Angew. Chem. 1996, 108, 791-794; Angew. Chem. Int. Ed. Eng/.
1996,35, 725-728.
121 H:W. Wanzlick, H:J. Schonherr, Angew. Chem. 1968,80,154; Angew. Chem.
Int. Ed. Engl. 1968, 7, 141-142; A. J. Arduengo 111, R. L. Harlow, M. Kline,
J. Am. Chem. SOC.1991, 113,361- 363; A. J. Arduengo 111, H. V. R. Dlaz, R. L.
Harlow, M. Kline, ibid. 1992, 114, 5530-5534; N. Kuhn, T.Kratz, Synthesis
1993, 561-562; W. A. Herrmann, M. Elison, J. Fischer, C. Kocher, G. R. J.
Artus, Chem. Eur. J. 1996, 2, 772-780.
[3] a) A. J. Arduengo 111, H. V. R. Diaz, J. C. Calabrese, F.Davidson, Inorg. Chem.
1993, 32, 1541-1542; b) N. Kuhn, T. Kratz, D. Blaser, R. Boese, Chem. Ber.
1995,128,245-250; c) A. Schafer, M. Weidenbruch, W. Saak, S . Pohl, J. Chem.
Sot. Chem. Commun. 1995, 1157-1158; d) A. J. Arduengo 111, H. V. R. Diaz,
1992, 114,9724-9725; e) N.
J. C. Calabrese, F. Davidson, J. Am. Chem. SOC.
Kuhn, G. Henkel, T. Kratz, J. Kreutzberg, R. Boese, A. H. Maulitz, Chem. Ber.
1993, 126, 2041-2045; f ) X.-W. Li, J. Su, G. H. Robinson, Chem. Commun.
1996,2683-2684.
[4] M. Tamm, A. Grzegorzewski, 1. Brudgam, J. Organomet. Chem. 1996, 519,
217-220; M. Tamm, A. Grzegorzewski, T. Steiner, W. Werncke, T. Jentzsch,
Organometallics 1996, 15, 4984-4990, M. Tamm, W. Werncke, T. Jentzsch,
ibid. 1997, 16, 1418-1424.
[5] M. Tamm, A. Grzegorzewski, F. E. Hahn, J. Organomet. Chem. 1995, 501,
309 -3 13.
[6] M. Driess, H. Grutzmacher, Angew. Chem. 1996,108,900-929; Angeus. Chem.
Int. Ed. Engl. 1996, 35, 828-856, and references therein.
[7] 2. Yoshida, Pure Appl. Chem. 1982,54, 1059-1074.
[8] a) K. ofele, Angew,. Chem. 1968,80,1032-1033; Angen. Chem. Int. Ed. Engl.
1968, 7,950-951 ; b) R. Gompper, E. Bartmann, ibid. 1978, 90,490-491 and
1978, 17,456-457; c) R. Weiss, C. Priesner, ibid. 1978, 90,491-492 and 1978,
17,457-458; d) R. D. Wilson, Y Kamitori, H. Ogoshi, Z. Yoshida, J. A. Ibers,
J. Organomet. Chem. 1979, 173, 199-209; e) U. Kirchgassner, H. Piana, U.
Schubert, J Am. Chem. SOC.1991,113,2228-2232; f) J. Schubert, S . Mock, U.
Schubert, Chem. Ber. 1993, 126, 657-664; g) M. S . Morton, J. P. Selegue, A.
Carillo, Organometallics 1996, 15, 4664-4666.
[9] a) H. Meyer, G. Baum, W Massa, A. Berndt, Angew. Chem. 1987,99,790-791;
Angew. Chem. Int. Ed. Engl. 1987,26, 198-799; b) M. Lazraq, J. Escudie, C.
Couret, J. Satge, M. Drager, R. Dammel, ibid. 1988, 100, 885-887 and 1988,
27,828-830.
[lo] H. Meyer, G. Baum, W. Massa, S . Berger, A. Berndt, Angew Chem. 1987, 99,
559-560; Angew. Chem. Int. Ed. Engl. 1987,26, 546-547.
[ l l ] L. Zsolnai, H. Prizkow, ORTEP-Program for Personal Computer, Universitat Heidelberg, 1994.
[12] General X-ray structure data: Enraf-Nonius CAD 4 diffractometer, Mo,,
i
pm), graphite-monochromator, T = 163(2) K, correcradiation (=71.069
tion for Lorentz and polarization effects, structure solution with direct methods (SHELXS 86 [13]), refinement against F 2 (SHELXL 93 [14]) with anisotropic thermal parameters for all non-hydrogen atoms, hydrogen positions
with fixed isotropical thermal parameters (U,,, = 0.08 x lo4 pm') on calculated
positions, R , =
- l&[l/xl&l, wR2 = [xu,(F: - F32/xu(F:)2]1'2.
Crystallographic data (excluding structure factors) for the structures reported
in this paper have been deposited as supplementary publication no. CCDC100297 with the Cambridge Crystallographic Data Centre. Copies of the data
can be obtained free of charge on application to the following address: The
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ (fax. int. code
+ (1223)336-033, e-mail: deposit@chemchrys.cam.ac.uk). 4 a : crystal dimensions 0.48 x 0.36 x 0.24 mm, monoclinic, P2,/c, a = 1629.8(4), h = 1283.6(5),
c = 1796.2(8) pm, b = 101.70(3)", V = 3680(3) x 10, pm3, Z = 4, pcslcd
=
1 . 1 3 7 ~lo3 kgrn-,, p = 0.984mm-', F(OO0) =1368, 2.56"120S54.92',
O l h l l 5 , O s k 1 1 6 , -2311123, 5500 data collected, 5207 unique data,
(R,",= 0.03161, 5197 data with I>2u(I), 345 refined parameters, G O F
(F') = 1.053, R , = 0.0360, wR2 = 0.0874, max./min. residual electron density
0.497/-0.313 x
epm-'. 4b: crystal dimensions 0.45 xO.36 xO.24 mm,
monoclinic, Cc, a = 1140.6(2),b = 1948.3(8), c = 1802.5(6) pm. 0 = 104.08(2)",
V = 3885(2)x106pm3, Z = 4 , pcaisd
= 1 . 1 5 6 ~ l O ~ k g m -p=0.801
~,
mm-',
F(OO0) =1440, 4.18"<28S54.88", O S h S 1 4 , O 1 k 1 2 5 , -2311122, 4661
data collected, 4659 unique data (R,",= 0.0142), 4651 data with I>2a(I). 345
refined parameters, GOF(F2) =1.061, R , = 0.0529, w R , = 0.1238, max./min.
xllF,l
2234
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
residual electron density 1.349/-2.409 x
e p m - j ; 4c: crystal dimensions
0.48 x 0.24 x 0.18 mm, monoclinic, P2,/n, a = 1239.8(3), b = 1873.4(4),
c = 1693.4(5) prn, fl = 103.54(2)", V = 3824(2) x lo6 pm3, Z = 4, pealEd
=
1 . 3 2 8 lo3
~ kgm-,, p =4.558 rnm-', F(OO0) =1568, 4.02"128<54.90",
O I h 1 1 6 , O I k S 1 8 , -1611116, 7590 data collected, 7298 unique data
(R,",= 0.0763). 5476 data with I>2o(I), empirical absorption correction DIFABS [15] (min. 0.786, max. 1.423 $3' 1.011). 345 refined parameters, GOF
( F 2 )=1.170, R , = 0.0436, wR2 = 0.1097, max./min. residual electron density
3.420/- 2.672 x
e ~ m - ~ .
[13] G. M. Sheldrick, SHELX-86, Program for Crystal Structure Solution, Universitat Gottingen, 1986.
[14] G. M. Sheldrick, SHELX-93, Program for Crystal Structure Determination,
Universitat Gottingen, 1993.
1151 N. Walker, D. Stuart, Acra Crystallogr. Secr. A 1983, 39, 158-166.
[16] T. Fjeldberg, H Hope, M. F.Lappert, P. P. Power, A. J. Thorne, J: Chem. SOC.
Chem. Commun. 1983,639-641 ; R. W. Chorley, P. B. Hitchcock, M. F. Lappert, W:P. Leung, P. P. Power, M. M. Olmstead, Inorg. Chim. Acra 1992,
198-200,203 -209.
[17] H. Braunschweig, R W. Chorley, P. B. Hitchcock, M. F. Lappert, J. Chem.
SOC.Chem. Commun. 1992, 1311-1312.
I181 2. Yoshida, Y Tawara, J Am. Chem. SOC.1971, 93, 2573-2574.
1191 M. J. S . Gynane, D. H. Harris, M. F. Lappert, P. P. Power, P Riviere, M.
Riviere-Baudet, J. Chem. SOC.Dalton Trans. 1977, 2004-2009.
On the Mechanism of the McMurry Reaction**
Martin Stahl, Ulrich Pidun, and Gernot Frenking*
The use of low-valent titanium reagents for the reductive
coupling of carbonyl compounds is an important synthetic technique in organic chemistry. Although an impressively broad
range of alkenes is accessible by this method, for a long time
only a few mechanistic details were known and only in the last
few years have experiments shed new light on the mechanism.
The current status of research in this field has recently been
described in a review article.[']
Typically the reaction is conducted with TiCl, as the source of
titanium and a reducing agent such as LiAlH,, Zn/Cu, or Mg.
It was long believed that highly dispersed Tio particles are initially formed and that the carbonyl coupling proceeds heterogeneously on their surface via ketyl radical intermediates.[' -41
New experiments by Fiirstner et al. and BogdanoviC et al., however, indicate that when the coupling is performed with Zn/Cu
in dimethoxyethane (DME), a different reaction path is followed.[5.61 They propose a nucleophilic rather than a radical
mechanism, as depicted in Scheme 1I'[.
Because the postulated intermediates 1-4 have not been characterized experimentally and the energetics of the reaction
course suggested by Fiirstner and BogdanoviC are not known,
we have carried out quantum mechanical calculations of the
proposed reaction profile by means of density functional theory
(DFT)"] using formaldehyde as a model carbonyl compound.
All intermediates and transition states have been located on the
singlet potential energy surface and confirmed by frequency
calculations. The transition states have been further characterized by calculating the intrinsic reaction coordinates (IRC) .[I6]
[*I
Prof. Dr. G. Frenking, Dr. M. Stahl, Dr. U. Pidun
Fachbereich Chemie der Universitat
Hans-Meerwein-Strasse, D-35032 Marburg (Germany)
Fax: Int. code +(6421)282189
e-mail: frenking@psl515.chemie.uni-marburg.de
[**I Theoretical Studies of Organometallic Compounds, Part XXVII. This work
was supported by the Deutsche Forscbungsgemeinschaft (SFB 260, Graduiertenkolleg Metallorganische Chemie) and the Fonds der Chemischen Industrie.
Part XXVI: R. K. Szilagyi, G. Frenking, Organometallics, 1997, in press.
0570-0833/97/3620-2234 $17.50+ .50/0
Angew. Chem. Int. Ed. Engl. 1997,36, No. 20
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