Umpolung of P-H bonds

Article abstract of DOI:10.1002/1521-3773(20000901)39:17<3084::AID-ANIE3084>3.0.CO;2-R

The π-electron delocalization in the diazaphospholenes 1 leads not only to a weakening of the P-H bond, but also to an umpolung of its reactivity. Thus, compounds 1 react with acids with elimination of H₂, and display an inverse regioselectivit

Full text of DOI:10.1002/1521-3773(20000901)39:17<3084::AID-ANIE3084>3.0.CO;2-R

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12 Union Road, Cambridge CB21EZ, UK (fax: (‡44)1223-336-033;
À
The reactivity of P H bonds in phosphane derivatives is
e-mail: deposit@ccdc.cam.ac.uk).
generally determined by the protonic character of the hydro-
gen atom (Scheme 1), even though in view of the similar
electronegativities (cAR(H) ˆ 2.2, cAR(P) ˆ 2.06) and resulting
low bond polarities it appears conceivable to achieve an
umpolung of the reactivity by means of suitable substituent
effects.[3] Taking into account our observation that p-delocal-
ization effects which can be described in terms of s*
Received: March 31, 2000 [Z14927]
[1] M. F. Lappert, P. P. Power, A. R. Sanger, R. C. Srivastava, Metal and
Metalloid Amides, Ellis Horwood, Chichester, 1980.
[2] R. G. Pearson, J. Am. Chem. Soc. 1963, 85, 3533 ± 3539.
[3] a) W. Wojnowski, B. Becker, J. Saûmannshausen, E. M. Peter, K.
Peters, H. G. von Schnering, Z. Anorg. Allg. Chem. 1994, 620, 1417±
1421; b) P. J. Bonasia, D. E. Gindelberger, J. Arnold, Inorg. Chem.
1993, 32, 5126 ± 5131.
[4] a) J. Beck, J. Strähle, Angew. Chem. 1986, 98, 106 ± 107; Angew. Chem.
Intl. Ed. Engl. 1986, 25, 95 ± 96; b) E. Hartmann, J. Strähle, Z.
Naturforsch. 1989, 446, 1 ± 4.
aromaticity[4] render the P Cl bonds in P-chloro-1,3,2-diaza-
phospholenes 1 more polar and favor dissociation under
formation of the cations 2,[5] it appeared of interest to
establish if the same effects can also be applied to generate
hydridic reactivity of the P H bonds in P-hydrido-1,3,2-
diazaphospholenes 3 (Scheme 2).
À
À
[5] H. Schmidbaur, Gold: Chemistry, Biochemistry and Technology, Wiley,
New York, 1999.
[6] a) K. Angermaier, H. Schmidbaur, Chem. Ber. 1995, 128, 817± 822;
b) A. Shiotani, H. Schmidbaur, J. Am. Chem. Soc. 1970, 92, 7003 ± 7004.
[7] a) H. Schmidbaur, Gold Bull. 1990, 23, 11 ± 21; b) P. Pyykkö, Chem.
Rev. 1997, 97, 597.
[8] a) P. B. Hitchcock, M. F. Lappert, L. J.-M. Pierssens, Chem. Commun.
1996, 1189 ± 1190; b) P. Miele, J. D. Foulon, N. Hovnanian, J. Durand, L.
Cot, Eur. J. Solid State Inorg. Chem. 1992, 29, 573 ± 583; c) for an aryl
bridged example, see: S. Gambarotta, C. Floriani, A. Chiesi-Villa, C.
Guastini, J. Chem. Soc. Chem. Commun. 1983, 1087± 1089.
R2
R2
R2
R1
R1
R1
N
N
N
LiAlH4
THF
P
N
R2
P
N
R2
P+ Cl−
H
Cl
N
R2
3a-d
1a-d
2a-d[Cl]
Scheme 2. R1 ˆ H, R2 ˆ tBu (1a ± 3a), 2,4,6-Me3C6H2 (Mes) (1c ± 3c);
R1 ˆ Cl, R2 ˆ tBu (1b ± 3b), Mes (1d ± 3d).
The target compounds 3a ± d are formed in clean reactions
upon treatment of 1a ± d with stoichiometric amounts of
LiBEt3H or LiAlH4 in THF and were isolated as light yellow,
air- and moisture-sensitive oils or solids after distillative work-
up (3a, b) or crystallization (3d). The constitution of all
products can be deduced unequivocally from their spectro-
scopic data. The 31P NMR signals (d31P ˆ 57.1 (3a), 71.6 (3b),
64.0 (3c), 75.8 (3d)) appear at slightly lower field than in the
À
Umpolung of P H Bonds**
Dietrich Gudat,* Asadollah Haghverdi, and
Martin Nieger
Dedicated to Professor Gerd Becker
on the occasion of his 60th birthday
[6]
1,3,2-diazaphospholidine 4 (d31P ˆ 57.9 ). The P,H coupling
Bonds between p-block elements E and hydrogen are
employed as versatile synthons in numerous synthetic trans-
formations.[1] Their reactivity follows a systematic course that
is characterized by a change from hydridic (E H bonds
involving elements of Group 13) to protonic (E H bonds
involving elements of Groups 15 ± 17) character of the hydro-
gen atom. Hydrogen compounds of Group 14 elements are a
borderline case: whereas the protonic character of the
hydrogen atom dominates for C H bonds, Si H bonds may
react both as the source of a hydride or of a proton
(Scheme 1).[1, 2]
constants display a remarkable substitution dependence:
whereas the couplings in the N-
tBu
SiMe3
tert-butyl
derivatives
3a, b
219
À
N
R
R
N
(1J(P,H) ˆ 181
(3a),
PH
Si PH
N
À
N
(3b) Hz) are clearly larger than
in 4 (1J(P,H) ˆ 156 Hz [6]), those
in the N-mesityl compounds
3c, d (1J(P,H) ˆ 139 (3c), 147
tBu
SiMe3
4
5
À
À
(3d) Hz) come close to the extremely low values of the
1
four-membered heterocycles 5 (R ˆ Me, Ph; J(P,H) ˆ 125 ±
127Hz [7]). Following common concepts, the decrease of
1J(P,H) can be related with decreasing p-character and
H+X−
H+X−
K+H−
K+H−
R3Si− K+ + H2
R2P− K+ + H2
R3Si
R2P
X
X
+ H2
+ H2
R3Si
R2P
H
À
lengthening of the P H bond and should thus indicate a
bond-weakening effect. This hypothesis is corroborated by the
H
À
red shift of the P H stretching vibration frequencies in 3a ± d
(nÄ(PH) ˆ 2120 ± 2202 cmÀ1) as compared to those of known
cyclic and acyclic diaminophosphanes(nÄ(PH) ˆ 2220 ±
2340 cmÀ1[6, 8]), and by the result of a single-crystal X-ray
diffraction study of 3d.[9]
Scheme 1. X ˆ halogen, OR.
[*] Priv.-Doz. Dr. D. Gudat, Dipl.-Chem. A. Haghverdi, Dr. M. Nieger
Anorganisch-Chemisches Institut der Universität
Gerhard-Domagk Strasse 1, 53121 Bonn (Germany)
Fax : (‡49)228-73-53-27
The structure of crystalline 3d is composed of isolated
molecules that display no significant intermolecular interac-
tions (Figure 1).[11] The five-membered ring features an
ªenvelopeº conformation with the phosphorus atom sticking
out of the plane formed by the remaining ring atoms, and the
attached hydrogen atom adopting a ªflagpoleº position. The
E-mail: dgudat@uni-bonn.de
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
Supporting information for this article is available on the WWW under
http://www.wiley-vch.de/home/angewandte/ or from the author.
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Angew. Chem. Int. Ed. 2000, 39, No. 17

COMMUNICATIONS
2d[BF4] + Ph3CH
2d[CF3SO3] + H2
CF3SO3H
Mes
Mes
Mes
Cl
Cl
Cl
N
N
N
BH3
PhCHO
P
P
P
BH3THF
-THF
OCH2Ph
H
H
N
N
N
Mes
Mes
Mes
3d
8
6
Figure 1. Molecular structure of 3d in the crystal (50% probability
thermal ellipsoids); important distances [Š] and angles [8]: P(1)-H(1)
1.51(4), P(1)-N(5) 1.709(3), P(1)-N(2) 1.722(3), N(2)-C(3) 1.407(5), C(3)-
C(4) 1.327(5), C(3)-Cl(3) 1.720(4), C(4)-N(5) 1.410(5); N(5)-P(1)-N(2)
89.92(16), N(5)-P(1)-H(1) 97.8(14), N(2)-P(1)-H(1) 99.3(14).
[CpW(CO)3H]
- H2, - CO
Mes
Cl
N
N
P W(CO)2Cp + 1c + ...
Mes
7
À
À
P H distance (1.51(4) Š) is notably longer than known P H
Scheme 3. Reactions of 3d.
bond lengths in phosphanes (1.288 Æ 0.09 Š[12]). The endocy-
À
À
clic bond lengths (P N 1.709(3), 1.722(3), C N 1.407(5),
1.410(5), C C 1.327(5) Š) range as in 1a, c and 2a
[5, 13]
studies to occur spontaneously and without detectable
intermediates even at À788C in CH2Cl2. Evidence for a
quaternization, which is typical for secondary or tertiary
phosphanes, was obtained in no case, even if the formation of
the stable BH3 adduct 6 from 3d and BH3 ´ THF suggests a
sufficient basicity for the phosphorus atom.
ˆ
between those of genuine single and double bonds, even if
the extent of bond length equalization decreases from 2a over
1a, c to 3d.
In accord with model calculations[14] these findings may be
interpreted by assuming that hyperconjugative interaction
Reaction of 3d with the transition metal hydride
[CpW(CO)3H] affords as main product (ca. 50%) the
phosphenium complex 7,[16] which was spectroscopically
identified, together with 1c (ca. 25%) and further as yet not
characterized by-products. Whereas the formation of 7 can be
À
between the six p electrons in the C2N2 unit and the s*(P H)
orbital results (as in the case of 1[5]) in partial delocalization of
p electrons in the ring. The lower extent of this ªs*
aromaticityº[4] in 3d as compared to that in 1a, c is consistent
À
with the lower acceptor ability of the s*(P H) as compared to
that of a s*(P Cl) orbital. Direct consequences of this p
delocalization are a weakening of the P H bond and a
concomitant increase of charge density at the hydrogen atom
(i.e. increasing hydridic character), which become immedi-
ately evident from comparison of computed P H distances,
n(PH) vibrational frequencies, and atomic charges of the
model compounds A ± C (Table 1).
À
explained by condensation of the hydridic P H and acidic
À
À
W H functionalities of 3d and [CpW(CO)3H], respectively,
À
which proceeds with cleavage of H2 and subsequent elimi-
À
À
nation of CO, 1c arises presumably from C Cl/P H meta-
thesis between two molecules of 3d.[18]
À
Beside the observation of H2 elimination in reactions with
compounds containing acidic protons, the umpolung of the
À
P H bonds in 3 becomes further manifest in an inverse
Table 1. Computed (at the MP2/6-31 ‡ g(d,p(P-H))-level)[14] P H distan-
regioselectivity during the addition to carbonyl compounds.
Thus, reaction of 3d with benzaldehyde proceeds not as
expected[19] with formation of a a-hydroxybenzyl phosphane,
but yields rather as the only product the benzyloxy derivative
8, which was isolated as a yellow oil and characterized by
spectroscopic techniques.[16] The regioselectivity of the addi-
tion can be deduced unequivocally from the appearance of the
signal of a benzylic CH2 moiety in the 13C{1H} DEPT
spectrum, and the absence of a characteristic n(OH) band in
the IR spectrum.
À
ces, n(P ± H) vibrational frequencies, and atomic charges q(H) (from
natural bond order (NBO) population analyses) in the model compounds
A ± C.
À
r(P H) [Š]
1.4471.426
22172361
À 0.14
1.406
2509
À 0.11
À1
À
n(P H) [cm
]
q(H)
À 0.08
Experimental Section
3a ± d: A solution of 1a ± d (10 mmol) in THF (50 mL) was added dropwise
at 48C to a suspension of LiAlH4 (2.5 mmol) in THF (10 mL). Stirring was
continued for 1 h, the solvent removed in vacuo, and the oily residue
dissolved in hexane (50 mL). The formed precipitate was filtered off over
Celite and the solvent removed in vacuo. The crude product was
characterized by NMR spectroscopy (3c), or purified by means of vacuum
distillation (3a, b) or recrystallization from pentane at À208C (3d). 3a:
b.p. 50 8C (1 mbar), yield 65%; 31P NMR (121.5 MHz, C6D6): d ˆ 57.1 (d,
1J(P,H) ˆ 181 Hz); 1H NMR (300 MHz, C6D6): d ˆ 1.14 (d, 4J(P,H) ˆ
0.9 Hz, 18H), 5.97(d, 3J(P,H) ˆ 4.0 Hz, 2H), 6.07(d, 1J(P,H) ˆ 181 Hz,
Chemical studies confirm that the enhancement of the
À
hydridic character of the P H bond, which has been inferred
from spectroscopic and structural criteria, leads in fact to an
‡
umpolung of the reactivity. Thus, 3d reacts with [Ph3C] [BF4]À
under hydride transfer to give a quantitative yield of
triphenylmethane and the phosphenium ion 2d (Scheme 3).[15]
The same cation is also formed upon treatment of 3d with
trifluoromethanesulfonic acid. The reaction proceeds with gas
evolution (H2) and was found by 31P NMR spectroscopic
‡
‡
1H); MS (16 eV): m/z (%): 200 (10) [M ], 199 (78) [M À H], 143 (28)
Angew. Chem. Int. Ed. 2000, 39, No. 17
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‡
‡
[M À C4H8], 87(100) [ M À 2C4H8]; IR (gas): nÄ ˆ 2176 cmÀ1 (P H). 3b:
b.p. 55 8C (1 mbar), yield 78%; 31P NMR (121.5 MHz, C6D6): d ˆ 71.6 (d,
1J(P,H) ˆ 219 Hz); 1H NMR (300 MHz, C6D6): d ˆ 0.99 (d, 4J(P,H) ˆ
0.7Hz, 9H), 1.33 (d, 4J(P,H) ˆ 1.7Hz, 9H), 6.16 (d, 3J(P,H) ˆ 3.3 Hz,
Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill,
B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon,
E. S. Replogle, J. A. Pople, Gaussian, Inc., Pittsburgh PA, 1998.
[15] All products were identified by comparison of their 1H and 31P NMR
data with those of authentic samples.
À
1H), 6.23 (d, 1J(P,H) ˆ 219 Hz, 1H); IR (Nujol): nÄ ˆ 2202 cmÀ1 (P H). 3c:
À
31P NMR (121.5 MHz, C6D6): d ˆ 64.0 (d, 1J(P,H) ˆ 139 Hz); 1H NMR
(300 MHz, C6D6): d ˆ 2.44 (s, 6H), 2.48 (s, 6H), 2.56 (s, 3H), 2.59 (s, 3H),
6.16 (d, 3J(P,H) ˆ 1.8 Hz, 2H), 7.04 (br, 4H), 7.13 (d,1J(P,H) ˆ 139 Hz, 1H);
3d: m.p. 87± 89 8C, yield 72%; 31P NMR (121.5 MHz, C6D6): d ˆ 75.8 (d,
1J(P,H) ˆ 147Hz); 1H NMR (300 MHz, C6D6): d ˆ 2.09 (s, 6H), 2.23 (s,
6H), 2.23 (s, 3H), 2.37(s, 3H), 5.87(d, 3J(P,H) ˆ 0.9 Hz, 1H), 6.67(s, 2H),
6.70 (s, 1H), 6.72 (s, 1H), 7.17 (d, 1J(P,H) ˆ 147Hz, 1H); MS (16 eV): m/z
[16] Characteristic spectroscopic data: 6: 31P NMR (121.5 MHz, C6D6):
d ˆ 85.9 (br d, 1J(PH) ˆ 353 Hz); 1H NMR (300 MHz, C6D6): d ˆ 1.29
(br, 3H; BH3), 2.03 (s, 9H), 2.13 (s, 3H), 2.38 (s, 3H), 2.43 (s, 3H), 5.32
(d, 3J(P,H) ˆ 10.3 Hz, 1H), 6.62 (s, 1H), 6.67(s, 1H), 6.70 (s, 2H), 8.01
(d, 1J(P,H) ˆ 353 Hz, 1H); 11B{1H} NMR (96.2 MHz, C6D6): d ˆ À34.3
(d, 1J(P,B) ˆ 52 Hz); 7: 31P NMR (121.5 MHz, C6D6): d ˆ 173 (d,
3J(P,H) ˆ 8 Hz, 1J(W,P) ˆ 765 Hz); 1H NMR (300 MHz, C6D6): d ˆ
2.05 (s, 3H), 2.06 (s, 3H), 2.27(s, 6H), 2.29 (s, 6H), 4.52 (s, 5H;
C5H5), 5.93 (d, 3J(P,H) ˆ 7.4 Hz, 1H), 6.78 (s, 2H), 6.79 (s, 2H); IR
(CH2Cl2): nÄ ˆ 1841, 1959 cmÀ1 (CO); 8: 31P NMR (121.5 MHz, C6D6):
d ˆ 121.3 (t, 3J(P,H) ˆ 9 Hz); 1H NMR (300 MHz, C6D6): 2.12 (s, 6H),
2.27(s, 3H), 2.34 (s, 3H), 2.38 (s, 3H), 2.45 (s, 3H), 4.50 (d, 3J(P,H) ˆ
(%): 358(43) [M ], 357(100) [M À H]; IR (gas): nÄ ˆ 2120 cmÀ1 (P H).
‡
‡
À
Received: April 17, 2000 [Z14998]
3
9.0 Hz, 2H; OCH2), 5.83 (d, J(P,H) ˆ 0.7Hz, 1H; 5-H), 6.7± 7.0 (m,
[1] Inorganic Reactions and Methods, Vol. 1 (Eds.: J. J. Zuckerman),
VCH, Deerfield Beach, 1986.
[2] S. Pawlenko in Methoden Org. Chem. (Houben-Weyl) 4th ed. 1952 ± ,
9H; m-H and Ph); 13C{1H} NMR (75.4 MHz, C6D6): d ˆ 66.9 (d,
‡
2J(PC) ˆ 25.6 Hz; OCH2); MS (16 eV, 1008C): m/z (%): 464(12) [M ],
‡
357(14) [M À OCH2Ph].
Vol. 13/5, 1980, p. 272, 350.
À
[17] The formation of 6 rather than a conceivable product 2d[BH4] agrees
with the observed addition reactions of phosphenium ions to BH4À: M.
Bürklin, E. Hanecker, H. Nöth, W. Storch, Angew. Chem. 1985, 97,
980; Angew. Chem. Int. Ed. Engl. 1985, 24, 999; G. Jochem, A.
Schmidpeter, H. Nöth, Z. Naturforsch. B 1996, 51, 267.
[3] For recent reports on the hydridic character of P H bonds in
Â
hypervalent Lewis base adducts of phosphanes, see: a) F. Carre, C.
Chuit, R. J. P. Corriu, A. Mehdi, C. Reye, J. Organomet. Chem. 1997,
529, 59; b) J.-P. Bezombes, F. Carre, C. Chuit, R. J. P. Corriu, A.
Mehdi, C. Reye, J. Organomet. Chem. 1997, 535, 81.
Â
Â
Â
[18] Evidence for the activity of 3d as a hydride transfer reagent was also
obtained in further cases; thus, reaction of CH2Cl2 with 3d was found
to proceed within several hours at 208C with quantitative formation of
the chlorination product 1d.
[19] G. Elsner, Methoden Org. Chem. (Houben-Weyl) 4th ed. 1952 ± , Vol.
13/E1, 1980, p. 122.
[4] a) A. Göller, H.Heydt, T. Clark, J. Org. Chem. 1996, 61, 5840; b) A.
Göller, T. Clark, J. Mol. Model 2000, 6, 133.
[5] D. Gudat, A. Haghverdi, M. Nieger, Chem. Eur. J. 2000, 6, 3414.
[6] R. B. King, P. M. Sundaram, J. Org. Chem. 1984, 49, 1784.
[7] E. Niecke, A. Nickloweit-Lüke, R. Rüger, Phosphorus Sulfur 1982, 12,
213.
[8] E. Niecke, W. Güth, Z. Naturforsch. B 1985, 40, 1049.
[9] Crystal structure determination of 3d: C20H24ClN2P, yellow crystals,
crystal size 0.05  0.10  0.30 mm; Mr ˆ 358.8; monoclinic, space
group P21/n (no. 14), a ˆ 8.4888(7), b ˆ 7.0438(7), c ˆ 30.940(3) Š,
b ˆ 93.629(5)8, Vˆ 1846.3(3) Š3, Z ˆ 4, m(MoKa) ˆ 0.297mm À1, Tˆ
123(2) K, F(000) ˆ 760. Of 7837 reflections which were collected on
a Nonius KappaCCD diffractometer using MoKa radiation up to
2qmax ˆ 508, 2857were independent and used in all further calcula-
Stereoselective Synthesis and Palladium-
Catalyzed Transformations of 2-Alkylidene-
5-vinyltetrahydrofurans**
tions. The structure was solved with direct methods (SHELXS-97[10a]
)
and refined anisotropically against F 2; the hydrogen atom at the
phosphorus center was refined free and the remaining ones using a
riding model (program: SHELXL-97[10b]). The final wR2(F2) was
0.136 and the conventional R value R(F) ˆ 0.065 for 227parameters.
Crystallographic data (excluding structure factors) for the structure
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no.
CCDC-142914. 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).
Peter Langer* and Edith Holtz
Domino and sequential reactions are of interest in modern
organic chemistry since they enable the rapid assembly of
complex products.[1] In the course of our studies on the
development of domino reactions of dianions and dianion
equivalents,[2] we have recently reported the first cyclizations
of dilithiated 1,3-dicarbonyl compounds with oxalic acid
dielectrophiles.[3] These reactions allow an efficient, regio-
and stereoselective synthesis of the pharmacologically rele-
vant substance class of g-alkylidenebutenolides. Although a
variety of simple condensation reactions of dianions with
monofunctional alkyl halides are known, only a few domino
dialkylation reactions of dianions with difunctional alkyl
[10] a) G. M. Sheldrick, SHELXS-97, Acta Crystallogr. Sect. A 1990, 46,
467; b) G. M. Sheldrick, SHELXL-97, Universität Göttingen, 1997.
[11] For a structurally characterized diaminophosphane with intermolec-
ular hydrogen bonds, see: M. M. Olmstead, P. P. Power, G. A. Sigel,
Inorg. Chem. 1988, 27, 2045.
[12] Average and standard deviation of the result of a query in the CCSD
À
database for P H distances in compounds HnY3ÀnP (Yˆ substituent
bound through a p-block element) with a three-coordinate phospho-
rus center.
[13] M. K. Denk, S. Gupta, A. J. Lough, Eur. J. Inorg. Chem. 1999, 41.
[14] All computations were performed at the MP2/6-31 ‡ g(d,p(P-H))-
level with the Gaussian package of programs: Gaussian 98 (Rev. A.7),
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Strat-
mann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N.
Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R.
Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J.Ochterski,
G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B.Foresman, J. Cioslowski, J. V.
Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi,
R. Gomperts, R. L.Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y.
[*] Dr. P. Langer, E. Holtz
Institut für Organische Chemie
Georg-August-Universität Göttingen
Tammannstrasse 2, 37077 Göttingen (Germany)
Fax : (‡49)551-399475
E-mail: planger@uni-goettingen.de
[**] This work was supported by the Fonds der Chemischen Industrie
(Liebig scholarship and funds for P.L.) and by the Deutsche
Forschungsgemeinschaft. P.L. thanks Prof. Dr. A. de Meijere for his
support.
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Angew. Chem. Int. Ed. 2000, 39, No. 17