Tetrakis(2,4,6-triisopropylphenyl)diplumbene: A molecule with a lead- lead double bond

Full text of DOI:10.1002/(sici)1521-3773(19990115)38:1/2<187::aid-anie187>3.0.co;2-2

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[4] a) E. J. Corey, C. P. Decicco, R. C. Newbold, Tetrahedron Lett. 1991,
32, 5287 ± 5290; b) K. Ishihara, M. Miyata, K. Hattori, T. Tada, H.
Yamamoto, J. Am. Chem. Soc. 1994, 116, 10520 ± 10524; c) H. Ishitani,
M. Ueno, S. Kobayashi, J. Am. Chem. Soc. 1997, 119, 7153 ± 7154; d) E.
Hagiwara, A. Fujii, M. Sodeoka, J. Am. Chem. Soc. 1998, 120, 2474 ±
2475.
[5] a) J. Sheehan, D. W. Chapman, R. W. Roth, J. Am. Chem. Soc. 1952,
74, 3822 ± 3825; b) C. R. Mc Arthur, P. M. Worster , A. U. Okon Synth.
Commun. 1983, 13, 311 ± 318.
[6] W. N. Speckamp, H. Hiemstra, Tetrahedron 1985, 41, 4367 ± 4416.
[7] The crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publica-
tion no. CCDC-101780. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (fax: (‡44)1223-336-033; e-mail: deposit@ccdc.cam.
ac.uk).
[8] H. Waldmann, Liebigs Ann. Chem. 1990, 671 ± 680, and references
therein.
[9] J. O. Osby, M. G. Martin, B. Ganem, Tetrahedron Lett. 1984, 25, 2093 ±
2096.
[10] W. J. Mc Gahren, J. H. Martin, G. O. Morton, R. T. Hargreaves, R. A.
Leese, F. M. Lovell, G. A. Ellestad, E. OꢁBrien, J. S. E. Holker, J. Am.
Chem. Soc. 1990, 102, 1671 ± 1684.
[11] D. R. Kronenthal, C. Y. Han, M. K. Taylor, J. Org. Chem. 1982, 47,
2765 ± 2768.
Experimental Section
A solution of N-Pht-tert-leucine chloride (1.2 mmol) in CH2Cl2 (3 mL) was
added at 08C to a solution of the imine (1 mmol) in CH2Cl2 (2 mL). After
the mixture was stirred for 30 min at room temperature it was again cooled
to 08C, and the silylketene acetal (1.5 mmol) added. The reaction mixture
was stirred and warmed to room temperature over 72 h, then CH2Cl2
(10 mL) was added. The solution was then extracted with a 10% NaHCO3
solution (10 mL) and a saturated NaCl solution (10 mL). The organic layer
was dried with Na2SO4 and the solvent evaporated in vacuo. The products
were isolated from the remaining residue by flash chromatography on silica
gel with hexane/ethyl acetate mixtures as eluents. For yields and
diastereomer ratios see Table 1.
7k: m.p. 1468C; [a]D22
ˆ
98.5 (c ˆ 0.5 in CHCl3); 1H nmr (500 MHz,
CDCl3): d ˆ 0.65 (s, 3H; CH3, anisidine), 1.11 (s, 3H; tBu), 1.16 (s, 3H;
CH3), 1.54 (s, 3H; CH3), 3.60 (s, 3H; OCH3), 3.67 (s, 3H; OCH3), 3.95 (s,
3H; OCH3), 4.34 (s, 1H; aH, tLeu), 6.08 (d, 3J ˆ 8 Hz, 1H; oH, aryl), 6.44
(s, 1H; bH), 6.50 (d, 3J ˆ 8 Hz, 2H; mH, aryl), 6.71 ± 6.75 (br s, 2H; 3- and
3
3
5-H, anisidine), 6.86 (d, J ˆ 8 Hz, 1H; oH, aryl), 7.08 (dd, J1 ˆ 3J2 ˆ 8 Hz,
1H; 4-H, anisidine), 7.51 (d, J ˆ 7 Hz, 1H; oH, Pht), 7.61 (ddd, J1 ˆ 3J2 ˆ
7 Hz, 4J ˆ 1 Hz, 1H, mH, Pht), 7.65 (ddd, 3J1 ˆ 3J2 ˆ 7 Hz, 4J ˆ 1 Hz, 1H;
3
3
3
mH, Pht), 7.74 (d, J ˆ 7 Hz, 1H; oH, Pht); 13C nmr (125.8 MHz, CDCl3):
d ˆ 17.57 ((CH3)2C), 21.34 ((CH3)2C), 24.40 (CH3, anisidine), 27.86 (3C,
tBu), 37.32 ((CH3)2C), 50.14 (tBu), 51.69 (OCH3), 54.69 (OCH3), 54.92
(OCH3), 58.41 (b-CH), 65.52 (aCH-tLeu), 109.50 (3C, anisidine), 112.27
(m-C, aryl), 122.36 (5-C, anisidine), 122.71 (o-C, Pht), 122.95 (o-C, Pht),
127.11 (1-C, anisidine), 128.29 (6-C, anisidine), 129.12 (o-C, aryl), 130.68 (1-
C, aryl), 132.29 (C, Pht), 133.53 (m-C, Pht), 133.85 (4-C, anisidine), 141.07
(C, Pht), 155.87 (p-C, aryl), 158.74 (4-C, anisidine), 166.27 (C(O), tLeu),
167.47 (2C, C(O), Pht), 177.21 (CO2CH3); HR-MS: calcd for C35H40N2O7
‡
[M ] 600.2836; found: 600.2823; elemental analysis calcd for C35H40N2O7: C
69.98, H 6.71, N 4.66; found: C 69.86, H 6.72, N 4.83.
Tetrakis(2,4,6-triisopropylphenyl)diplumbene:
A Molecule with a Lead ± Lead Double Bond**
Received: June 5, 1998
Supplemented version: September 30, 1998 [Z11953IE]
German version: Angew. Chem. 1999, 111, 166 ± 169
Martin Stürmann, Wolfgang Saak, Heinrich
Marsmann, and Manfred Weidenbruch*
Keywords: amino acids
´ asymmetric synthesis ´ chiral
auxiliaries ´ Mannich reaction
Dedicated to Professor Helmut Werner
on the occasion of his 65th birthday
[1] a) M. Arend, B. Westermann, N. Risch, Angew. Chem. 1998, 110,
1096 ± 1122; Angew. Chem. Int. Ed. 1998, 37, 1044 ± 1070; b) E. F.
Kleinmann in Comprehensive Organic Synthesis, Vol. 2 (Eds.: B. M.
Trost, I. Fleming, C. H. Heathcock), Pergamon, Oxford, 1991,
pp. 893 ± 952.
[2] a) S. Krauthäuser, L. A. Christianson, D. R. Powell, S. H. Gellman, J.
Am. Chem. Soc. 1997, 119, 11719 ± 11720; b) D. Seebach, M. Over-
hand, F. N. M. Kühnle, D. Martioni, L. Oberer, U. Hommel, H.
Widmer, Helv. Chim. Acta 1996, 79, 913 ± 941.
[3] a) D. Enders, D. Ward, J. Arden, G. Raabe, Angew. Chem. 1996, 108,
1059 ± 1062; Angew. Chem. Int. Ed. Engl. 1996, 35, 981 ± 984; b) M.
Arend, N. Risch, Angew. Chem. 1995, 107, 2861 ± 2864; Angew. Chem.
Int. Ed. Engl. 1995, 34, 2861; c) H. Frauenrath, T. Arenz, G. Raabe, M.
Zorn, Angew. Chem. 1993, 105, 74 ± 76; Angew. Chem. Int. Ed. Engl.
1993, 32, 83; d) D. A. Evans, F. Urpi, T. C. Somers, J. S. Clark, M. T.
Bilodeau, J. Am. Chem. Soc. 1990, 112, 8215 ± 8216; e) W. Oppolzer,
R. Moretti, S. Thomi, Tetrahedron Lett. 1989, 30, 5603 ± 5606; f) D.
Seebach, C. Betschart, M. Schiess, Helv. Chim Acta 1984, 67, 1593 ±
1597; g) N. Risch, A. Esser, Liebigs Ann. Chem. 1992, 233 ± 237; h) K.
Broadley, S. G. Davies, Tetrahedron Lett. 1984, 25, 1743 ± 1744; i) W.
Oppolzer, P. Schneider, Helv. Chim. Acta 1986, 69, 1817 ± 1820; j) C.
Gennari, I. Venturini, G. Gislon, G. Schimperna, Tetrahedron Lett.
1987, 28, 227 ± 230; k) H. Kunz, W. Pfrengle, Angew. Chem. 1989, 101,
1041 ± 1042; Angew. Chem. Int. Ed. Engl. 1989, 28, 1067; l) H. Kunz, D.
Schanzenbach, Angew. Chem. 1989, 101, 1042 ± 1043; Angew. Chem.
Int. Ed. Engl. 1989, 28, 1068; m) H. Waldmann, M. Braun, M. Dräger,
Angew. Chem. 1990, 102, 1445 ± 1447; Angew. Chem. Int. Ed. Engl.
1990, 29, 1468, n) H. Waldmann, M. Braun, J. Org. Chem. 1992, 57,
4444 ± 4451.
Disilenes, digermenes, and distannenesÐcompounds with
Si Si, Ge Ge, and Sn Sn double bondsÐare now well-
established molecules, and their properties have been sum-
marized in several review articles.[1, 2] They are usually
prepared by the primary generation of the carbene analogues
:
R2E (E ˆ Si, Ge, Sn) and subsequent dimerization. Diplum-
benes, molecules with a lead ± lead double bond, were
unknown until now. Furthermore, structurally characterized
dialkyl-[3, 4] and diarylplumbylenes (-plumbanediyls)[5, 6] were
reported for the first time in the past few years. The latter
particles with electron sextets exist both in solution and in the
crystal as monomers without noticeable Pb Pb interactions.
[*] Prof. Dr. M. Weidenbruch, Dipl.-Chem. M. Stürmann,
Dipl.-Chem. W. Saak
Fachbereich Chemie der Universität
Carl-von-Ossietzky-Strasse 9 ± 11, D-26111 Oldenburg (Germany)
Fax : (‡ 1)441-798-3352
Prof. Dr. H. Marsmann
Fachbereich Chemie der Universität (GH)
Warburger Strasse 100, D-33095 Paderborn (Germany)
[**] Compounds of Germanium, Tin, and Lead. Part 28. This work was
supported by the Deutsche Forschungsgemeinschaft and the Fonds
der Chemischen Industrie. Part 27: M. Weidenbruch, U. Grobecker,
W. Saak, E.-M. Peters, K. Peters, Organometallics 1998, 17, 5206.
Angew. Chem. Int. Ed. 1999, 38, No. 1/2
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187

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A first breakthrough in the field of lead ± lead contacts
between plumbylene molecules was achieved just recently
with the isolation of the heteroleptic plumbylene dimer 1,
which exhibits a Pb Pb separation of 353.7(1) pm and a trans
bent angle of the substituents to the Pb Pb vector of 40.88.[7]
Since intramolecular interactions in 1 between the fluorine
at 233 and 253 K, as well as the temperature dependence of
the 13C NMR signal for the ipso-carbon atoms, which exhibits
a nonlinear shift to higher field on cooling from 298 (d ˆ
262.4) to 243 K (d ˆ 236.2), are suggestive of a possible
equilibrium between 3 and the diplumbene 4.[14]
An X-ray crystallographic analysis of the red crystals
(Figure 1)[15] showed that the diplumbene 4 actually does
exist in the solid state. The Pb Pb bond length of
atoms of the ortho-CF3 groups and the lead atoms should
weaken rather than favor a possible lead ± lead bond,[7] we
prepared, by the same route, another heteroleptic plumbylene
that exists in the solid state as the plumbylene dimer 2. In
comparison to 1, 2 has a shorter lead ± lead separation of
337.0(1) pm and a bending angle of 46.58.[8] However, the
observed lead ± lead distances in 2 and especially in 1 are still
markedly longer than the value of 295 ± 300 pm calculated for
[7, 9, 10]
ˆ
the parent compound H2Pb PbH2.
As the choice of substituents seems to be of decisive
importance for the formation of a lead ± lead double bond, we
turned our attention to the 2,4,6-triisopropylphenyl group
(Tip), which was previously employed with success not only
for the synthesis of the first tetrasilabuta-1,3-diene,[11] but also
in the generation of the as yet only distannene that does not
dissociate into stannylene molecules in solution.[12] The first
attempt to prepare the plumbylene 3 was reported by Okazaki
et al. Although these authors were able to demonstrate the
existence of this particle in solution up to 408C by means of
trapping reactions, they were only able to isolate the
plumbanes Tip3PbX (X ˆ Br, I) as solids.[13]
Figure 1. Structure of 4 in the crystal (hydrogen atoms omitted, ellipsoids
represent the 50% probability level). Selected bond lengths [pm] and
angles [8]: Pb1 Pb2 305.15(3), Pb1 C1 228.8(5), Pb1 C16 229.3(5),
Pb2 C31 230.5(5), Pb2 C46 231.2(5); C1-Pb1-C16 97.8(2), C31-Pb2-C46
102.3(2), C1-Pb1-Pb2 100.5(1), C16-Pb1-Pb2 131.8(2), C31-Pb2-Pb1
99.6(1), C46-Pb2-Pb1 123.5(1).
305.15(3) pm is merely 5 to 10 pm longer than that calculated
ˆ
for H2Pb PbH2. The folding angles of 43.98 and 51.28 are
somewhat smaller than the theoretically calculated values of
around 558.[7, 9, 10] Such deviations, which have also been
observed in the distannenes,[2] are possibly due to the
replacement of hydrogen atoms in the parent compound by
the bulky aryl groups. This may also be the reason for the only
moderate torsion of 21.78 about the double bond.
Reaction of the Grignard compound TipMgBr with lead(ii)
chloride at low temperature furnished a violet solution from
which red crystals were isolated by rapid workup (Scheme 1).
The product is light- and very air-sensitive and is thermally
stable up to 758C. Spectral data recorded in the gas phase and
With the synthesis of compound 4, homonuclear double
bonds between all elements of Group 14 have been realized
for the first time. In contrast to C C double bonds, the bonds
between the heavier elementsÐespecially in the case of
leadÐarise through a double donor± acceptor interaction
between the respective doubly occupied 6s orbitals and the
empty 6p orbitals of two singlet plumbylene molecules 3
(Figure 2). The low tendency for plumbylenes to undergo
dimerization and the weakness of the resultant Pb Pb double
Scheme 1. Synthesis of 3 and dimerization of 3 to 4.
in solution supported the presence of the plumbylene 3, which
exists for a short time in solution under strict exclusion of light
up to 298 K. However, the absence of a signal in the 207Pb
NMR spectra, recorded in the range between d ˆ 0 and 20000
Figure 2. Orbital interactions in the formation of a diplumbene from two
singlet plumbylenes.
188
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Angew. Chem. Int. Ed. 1999, 38, No. 1/2

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[12] S. Masamune, L. R. Sita, J. Am. Chem. Soc. 1985, 107, 6390; A.
Schäfer, M. Weidenbruch, W. Saak, S. Pohl, H. Marsmann, Angew.
Chem. 1991, 103, 873 and 978; Angew. Chem. Int. Ed. Engl. 1991, 30,
834 and 962; M. Weidenbruch, A. Schäfer, H. Kilian, S. Pohl, W. Saak,
H. Marsmann, Chem. Ber. 1992, 125, 563.
[13] K. Shibata, N. Tokitoh, R. Okazaki, Tetrahedron Lett. 1993, 34, 1495;
N. Tokitoh, N. Kano, K. Shibata, R. Okazaki, Organometallics 1995,
14, 3121.
[14] Similar observations have been made for the 119Sn signals of the
bond may be due in part to the relativistic contraction of the
6s electron pairs, which are only available for bond formation
to a limited extent.[17] Furthermore, this mode of bond
formation clearly demonstrates that the lengths of homonu-
clear double bonds between the heavier elements of Group 14
may well be in the same range as or even larger than the
corresponding single bonds.
:
ˆ
equilibrium 2R2Sn > R2Sn SnR2 (R ˆ 2-tBu-4,5,6-Me3C6H): M. A.
DellaBonna, M. C. Cassani, J. M. Keates, G. A. Lawless, M. F.
Lappert, M. Stürmann, M. Weidenbruch, J. Chem. Soc. Dalton Trans.
1998, 1187.
Experimental Section
[15] a) Crystal structure analysis of 4: STOE-IPDS diffractometer, MoKa
radiation, graphite monochromator. The reflections were recorded at
173 K from a shock-cooled crystal with dimensions 1.10 Â 0.44 Â
0.27 mm3 under an inert oil to 2qmax ˆ 528. The structure was solved
by direct methods (SHELXS-97) and refined by full-matrix least-
squares techniques against F 2 (SHELXL-97).[16] C60H92Pb2, Mr ˆ
1227.72, monoclinic, space group C2/c, Z ˆ 8, a ˆ 4755.2(3), b ˆ
1375.24(6), c ˆ 1912.83(9) pm, b ˆ 101.155(6)8, Vˆ 12273(1)  106 pm3,
1calcd ˆ 1.329 gcm 3, m ˆ 5.511 mm 1; of 50232 measured reflections,
11197 were independent and 8498 observed for I > 2s(I). Hydrogen
atoms were placed in calculated positions and refined with isotropic
temperature factors; all other atoms were refined anisotropically. R1 ˆ
0.0290, wR2 ˆ 0.0729 (all data) for 553 parameters; max./min. residual
electron density 1.070/ 0.622 eŠ 3. b) Crystallographic data (exclud-
ing structure factors) for the structure reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC-102629. Copies of the data can
be obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB21EZ, UK (fax: (‡44)1223-336-033; e-mail: deposit
@ccdc.cam.ac.uk).
At 1108C solid lead(ii) chloride (5.6 g, 20 mmol) was added to a solution
of the Grignard reagent TipMgBr, prepared from 1-bromo-2,4,6-triisopro-
pylbenzene (11.44 g, 40.4 mmol) and magnesium (3.0 g, 123 mmol) in THF
(100 mL), and the resultant mixture was warmed with vigorous stirring to
room temperature within 20 min. The THF solvent was removed by
distillation under vacuum, and the residue extracted with n-hexane (2 Â
50 mL). After separation of the magnesium salts the violet solution was
concentrated to a volume of 40 mL and allowed to crystallize at 508C.
Red crystals (5.8 g, 47%) of 4 were obtained: m.p. 758C (decomp.).
Spectroscopic data of 3 in solution: 1H NMR (300 MHz, [D8]toluene,
283 K): d ˆ 1.14 (d, 24H, 3J(H,H) ˆ 6.7 Hz), 1.23 (d, 12H, 3J(H,H) ˆ
6.9 Hz, 2.72 (m, 2H), 2.82 (m, 4H), 7.68 (s, 4H); 13C NMR (75 MHz,
[D8]toluene, 283 K): d ˆ 24.22, 24.37, 35.56, 36.28, 128.57, 147.45, 157,45,
1
255.62 (ipso-C, J(13C,207Pb) ˆ 1100 Hz at 298 K); MS (CI, isobutane): m/z
‡
(%): 614 (100, M ); UV/Vis (n-hexane): lmax (e) ˆ 321, 385, 541 nm (960).
Elemental analysis of 4 (C60H92Pb2): found (calcd): C 58.57 (58.70), H 7.68
(7.55).
Received: August 26, 1998 [Z12339IE]
German version: Angew. Chem. 1999, 111, 145 ± 147
[16] G. M. Sheldrick, SHELXL-97, Program for crystal structure refine-
ment, Universität Göttingen, Germany, 1997.
[17] P. Pyykkö, Chem. Rev. 1988, 88, 563.
Keywords: carbene analogues
elements ´ multiple bonds
´
lead
´
main group
[1] Reviews of disilenes: R. West, Angew. Chem. 1987, 99, 1231; Angew.
Chem. Int. Ed. Engl. 1987, 26, 1202; G. Raabe, J. Michl in The
Chemistry of Organic Silicon Compounds, Part 2 (Eds.: S. Patai, Z.
Rappoport), Wiley, Chichester, 1989, p. 1015; T. Tsumuraya, S. A.
Batcheller, S. Masamune, Angew. Chem. 1991, 103, 916; Angew.
Chem. Int. Ed. Engl. 1991, 30, 902; M. Weidenbruch, Coord. Chem.
Rev. 1994, 130, 275; R. Okazaki, R. West in Multiply Bonded Main
Group Metals and Metalloids (Eds.: R. West, F. G. A. Stone),
Academic Press, San Diego, 1996, p. 232.
[2] Reviews on digermenes and distannenes: K. M. Baines, W. G. Stibbs
in Multiply Bonded Main Group Metals and Metalloids (Eds.: R. West,
F. G. A. Stone), Academic Press, San Diego 1996, p. 275; M. Driess, H.
Grützmacher, Angew. Chem. 1996, 108, 900; Angew. Chem. Int. Ed.
Engl. 1996, 35, 827.
[1,3], [3,3], and [3,5] Sigmatropic Rearrange-
ments of Esters Are Pseudopericyclic**
David M. Birney,* Xiaolian Xu, and Sihyun Ham
In 1969 the famous paper by Woodward and Hoffmann
describing the conservation of orbital symmetry was pub-
lished in this journal.[1] All of the well-known rules regarding
pericyclic reactions assume that there is a cyclic loop of
interacting p orbitals. However, it is possible to have a
pericyclic reaction in which this condition is not fulfilled, that
is, in which there is not cyclic p overlap. The unique
characteristics of these ªpseudopericyclicº reactions[2] have
been described in detail elsewhere;[3] they can be briefly
summarized as follows: 1) These reactions have planar
transition states, 2) they may have very low barriers if the
[3] Pb[CH(SiMe3)2]2: synthesis: P. J. Davidson, M. F. Lappert, J. Chem.
Soc. Chem. Commun. 1973, 317; P. J. Davidson, D. H. Harris, M. F.
Lappert, J. Chem. Soc. Dalton Trans. 1976, 2268; structure: K. W.
Klinkhammer, W. Schwarz, unpublished results. We thank Dr. Klink-
hammer for advanced communication.
[4] C. Eaborn, T. Ganicz, P. B. Hitchcock, J. D. Smith, S. E. Sözerli,
Organometallics 1997, 16, 5621.
[5] S. Brooker, J.-K. Buijink, F. T. Edelmann, Organometallics 1991, 10,
25.
[6] R. S. Simons, L. Pu, M. M. Olmstead, P. P. Power, Organometallics
1997, 16, 1920.
[7] K. W. Klinkhammer, T. F. Fässler, H. Grützmacher, Angew. Chem.
1998, 110, 114; Angew. Chem. Int. Ed. 1998, 37, 124.
[8] M. Stürmann, M. Weidenbruch, K. W. Klinkhammer, F. Lissner, H.
Marsmann, Organometallics 1998, 17, 4425.
[*] Dr. D. M. Birney, X. Xu, Dr. S. Ham
Department of Chemistry and Biochemistry
Texas Tech University
Lubbock, TX 79409-1061 (USA)
Fax: (‡1)806-742-1289
E-mail: vddmb@ttu.edu
[**] This work was supported by the Robert A. Welch Foundation.
[9] G. Trinquier, J. Am. Chem. Soc. 1990, 112, 2130.
Supporting information for this article is available on the WWW
under http://www.wiley-vch.de/home/angewandte/ or from the au-
thor.
[10] H. Jacobsen, T. Ziegler, J. Am. Chem. Soc. 1994, 116, 3667.
[11] M. Weidenbruch, S. Willms, W. Saak, G. Henkel, Angew. Chem. 1997,
109, 2612; Angew. Chem. Int. Ed. Engl. 1997, 36, 2503.
Angew. Chem. Int. Ed. 1999, 38, No. 1/2
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1433-7851/99/3801-0189 $ 17.50+.50/0
189