1,3-diethynylallenes: New modules for three-dimensional acetylenic scaffolding

Article abstract of DOI:10.1002/1521-3773(20010618)40:12<2334::AID-ANIE2334>3.0.CO;2-7

Regioselective Pd⁰-catalized cross-coupling of bispropargylic precursors 1 with silyl-protected alkynes gave rise to the first 1,3-diethynylallenes 2 [Eq. (1), LG=leaving group]. In enantiometrically pure form, these novel carbon-rich modules c

Full text of DOI:10.1002/1521-3773(20010618)40:12<2334::AID-ANIE2334>3.0.CO;2-7

COMMUNICATIONS
ment with the presence of methylidene tantalum species. Determi-
nation of the exact structure of these surface complexes is underway.
[6] The reaction of 13C-monolabeled ethane on 1‡2 gave the partially
scrambled metathesis products of ethane along with unlabeled NpH
and partially labeled NpMe (35% 13C incorporated). This shows that
at least 70% of this compound arises from cross-metathesis. The
excess of unlabeled compound probably arises from rearrangement of
1‡2. Heating 1‡2 under Ar at 1508C for 15 h gives 0.25 equiv of NpH
and 0.01 equiv of NpMe.
R
R
R
R
R
R
1
2
(±)-3
5
6
ˆ
[7] While the addition of a C C bond to a M C bond has never been
observed before in organometallic chemistry, there are examples of
additions of C H bonds: L. R. Chamberlain, I. P. Rothwell, J. C.
Huffman, J. Am. Chem. Soc. 1986, 108, 1502; H. van der Heijden, B.
Hessen, Chem. Commun. 1995, 145.
4
Scheme 1. Diethynyl- and tetraethynyl[n]cumulenes (n ˆ 1 ± 3) as carbon-
rich modules for acetylenic scaffolding.
[8] a-H abstraction is commonly observed for tantalum alkyl complexes:
R. R. Schrock, J. D. Fellmann, J. Am. Chem. Soc. 1978, 100, 3359.
[9] This is an upper limit, since neopentane can also be formed by direct
display low-energy electronic absorption bands with remark-
ably high molar extinction coefficients,[5] and to photochromic
molecules for photochemical switching without competing
thermal isomerization pathways.[6] Expansion of the central
olefinic fragment in DEEs and TEEs leads to the di- and
tetraethynylated allenes (Æ)-3 and 4, and butatrienes 5 and 6
(Scheme 1). Whereas silylated derivatives of peralkynylated
butatriene 6 have been reported,[7] 1,3-diethynyl- and tetra-
ethynylallenes have remained elusive, despite intensive ef-
forts aimed at their preparation.[8] The major problems
encountered in previous attempts to synthesize these novel
building blocks for three-dimensional acetylenic scaffolding
were their high tendency for rearrangement and facile [2‡2]
cycloaddition, which occurred readily even at room temper-
ature or below.[9]
a-H abstraction of
a neighboring ligand without involving an
intermolecular C H bond. For the reference experiment under Ar,
see ref. [6].
[10] For examples of s-bond metathesis on electrophilic metal centers, see
P. L. Watson, J. Am. Chem. Soc. 1983, 105, 6491; C. M. Fendrick, T. J.
Marks, J. Am. Chem. Soc. 1984, 106, 2214; P. L. Watson, G. W.
Parshall, Acc. Chem. Res. 1985, 18, 51; C. M. Fendrick, T. J. Marks, J.
Am. Chem. Soc. 1986, 108, 425; M. E. Thompson, S. M. Baxter, A. R.
Bulls, B. J. Burger, M. C. Nolan, B. D. Santarsiero, W. P. Schaefer, J. E.
Bercaw, J. Am. Chem. Soc. 1987, 109, 203; I. P. Rothwell, Acc. Chem.
Res. 1988, 21, 153; R. H. Crabtree, Chem. Rev. 1995, 95, 987.
Herein we describe the synthesis of the first 1,3-diethynyl-
allenes (Æ)-7a ± d. As a result of the inherent 908 twist of the
1,3-Diethynylallenes: New Modules for Three-
Dimensional Acetylenic Scaffolding**
SiiPr3
R
Robert C. Livingston, Liam R. Cox, Volker Gramlich,
and François Diederich*
R = CH2OSitBuMe2
R = n-C6H13
R = p-CH2C4H6OMe
R = C6H5
(±)-7a:
(±)-7b:
(±)-7c:
(±)-7d:
R
Oxidative coupling[1] of multiply ethynylated building
blocks has facilitated the assembly of well-defined molecular
architecture that displays unusual electronic and optical
properties.[2] In particular, derivatives of (E)-1,2-diethynyl-
ethene (1, (E)-DEE, (E)-hex-3-ene-1,5-diyne) and tetraethy-
nylethene (2, TEE, 3,4-diethynylhex-3-ene-1,5-diyne) have
provided a unique class of p-conjugated precursors for the
modular construction of one- and two-dimensional carbon-
rich scaffolds (Scheme 1).[3] They have provided access to
monodisperse, linearly p-conjugated polytriacetylene (PTA)
oligomers that extend up to 18 nm in length,[4] to perethyny-
lated expanded radialenes that feature all-carbon cores (C30 to
C60), which undergo facile electrochemical reduction and
iPr3Si
allene moiety, alkyne deprotection and oxidative acetylenic
coupling of these compounds in enantiomerically pure form
promises access to a fascinating new class of three-dimen-
sional helical oligomers and polymers.[10] Optically active
helical polymers[11] are attracting increasing interest as
materials that exhibit circularly polarized electrolumines-
cence[12] or as dopants for cholesteric liquid-crystalline
phases.[13]
Palladium-catalyzed cross-coupling reactions have proven
valuable in the preparation of alkynylallenes from substrates
that bear propargylic leaving groups such as halides, epoxides,
acetates, and carbonates.[14] Given an appropriate regiochem-
ical bias, application of these conditions to bispropargylic
precursors should yield the corresponding 1,3-diethynylal-
lenes. Thus, target compound (Æ)-7a was obtained from
bispropargylic epoxide precursor (Æ)-8[15] (prepared in five
[*] Prof. Dr. F. Diederich, Dr. R. C. Livingston, Dr. L. R. Cox
Laboratorium für Organische Chemie, ETH-Zentrum
Universitätstrasse 16, 8092 Zürich (Switzerland)
Fax : (‡41)1-632-1109
E-mail: diederich@org.chem.ethz.ch
ꢀ
steps from but-2-yne-1,4-diol, Scheme 2) and iPr3SiC CH
Prof. Dr. V. Gramlich
Laboratorium für Kristallographie, ETH-Zentrum
Sonneggstrasse 5, 8092 Zürich (Switzerland)
under standard palladium-catalyzed cross-coupling conditions
([Pd(PPh3)4], CuI, HNiPr2, CH2Cl2). In situ protection of the
primary alcohol that results from epoxide ring opening as the
SitBuMe2 ether gave 1,3-diethynylallene (Æ)-7a (Table 1)
[**] We thank the ETH Research Council and the Fonds der Chemischen
Industrie for their support of this work.
2334
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4012-2334 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2001, 40, No. 12

COMMUNICATIONS
The synthesis of cross-conjugated 1,3-diethynyl-1,3-di-
phenylallene (Æ)-7d (Table 1) was hampered by the insta-
bility of the desired propargylic carbonate precursor, even in
solution at low temperature. Similarly, other acylated deriv-
atives could not be isolated in pure form. Ultimately, access to
the desired allene was achieved in a one-pot procedure
through the intermediate formation of the benzoate from
tertiary alcohol (Æ)-11, followed by direct application of the
cross-coupling conditions (Scheme 4). This allene proved
especially susceptible to thermal dimerization, which pro-
ceeded within minutes for the neat compound at room
temperature, and therefore it could only be handled and
characterized in solution.
OH
a–c
d,e
HO
OH
OSitBuMe2
iPr3Si
SiiPr3
O
Me2tBuSiO
iPr3Si
f
OSitBuMe2
OSitBuMe2
iPr3Si
(±)-8
(±)-7a
Scheme 2. Synthesis of 1,3-diethynylallene (Æ)-7a by means of epoxide
ring opening. Reagents and conditions: a) NaH, tBuMe2SiCl, 08C !RT,
ꢀ
65%; b) MnO2, Et2O, RT; c) nBuLi, iPr3SiC CH, THF, 788C !RT, 34 %
(over two steps); d) MnO2, Et2O, RT, 77%; e) MeLi ´ LiBr, CH2I2, THF,
788C !RT, 79%; f) [Pd(PPh3)4], CuI, HNiPr2, CH2Cl2, RT; then
tBuMe2SiCl, imidazole, DMAP, RT, 52%. DMAP ˆ 4-(dimethylamino)-
pyridine.
SiiPr3
OH
Ph
(±)-11
Ph
as a colorless solid in 52% yield from (Æ)-8. Gratifyingly, the
difference in steric bulk of the two alkyne substituents (SiiPr3
vs. CH2OSitBuMe2) in (Æ)-8 had induced the desired regio-
selectivity in the cross-coupling reaction.[16]
a–c
d
Ph
iPr3Si
Ph
Ph
iPr3Si
(±)-7d
Scheme 4. Synthesis of cross-conjugated allene (Æ)-7d via the bispropar-
gylic benzoate, prepared in situ. Reagents and conditions: a) nBuLi,
PhCHO, THF, 788C !RT; b) MnO2, Et2O, RT, 99% (over two steps);
The structural limitations imposed by the epoxide opening
were circumvented through the use of carbonate as the
propargylic leaving group. Under the same cross-coupling
conditions used above, bispropargylic precursor (Æ)-9 (pre-
ꢀ
c) nBuLi, iPr3SiC CH, THF, 788C !RT, 96 %; d) nBuLi, PhCOCl; then
[Pd(PPh3)4], CuI, HNiPr2, THF, reflux, 38%.
ꢀ
pared in four steps from iPr3SiC CH, Scheme 3) provided the
Allene (Æ)-7a is solid and stable at room temperature,
which allowed unambiguous structural determination by
X-ray crystallography.[18] The unit cell contains two molecules
each of the P and M enantiomers. The ORTEP representation
(Figure 1A) shows that although there is significant thermal
and orientational disorder in the silyl protecting groups, the
structure of the diethynylallene backbone is well defined, with
all bond lengths and angles within the expected ranges. The
view down the allene axis (Figure 1B) clearly shows the
O
a,b
c,d
iPr3Si
iPr3Si
SiiPr3
OCOOMe
R
e
R
R
iPr3Si
R
iPr3Si
ꢀ
orthogonal disposition of the C CSiiPr3 and CH2OSitBuMe2
R = n-C6H13
(±)-9:
(±)-10:
R = n-C H
6 13
(±)-7b:
(±)-7c:
substituents.
R = p-CH C H OMe
R = p-CH C H OMe
2
4 6
2 4 6
Scheme 3. Synthesis of 1,3-diethynylallenes (Æ)-7b and (Æ)-7c from
bispropargylic carbonates. Reagents and conditions: a) nBuLi,
CH3(CH2)5CHO, THF, 788C !RT, 79%; b) (COCl)2, Me2SO, Et3N,
CH2Cl2,
d) nBuLi, MeOCOCl, THF,
HNiPr2, CH2Cl2, RT, 94% ((Æ)-7b), 57% ((Æ)-7c). Carbonate (Æ)-10
was prepared analogously to (Æ)-9.
788C, 94%; c) nBuLi, 1-octyne, THF,
788C !RT, 66 %;
788C !RT, 71%; e) [Pd(PPh3)4], CuI,
dihexyl-substituted 1,3-diethynylallene (Æ)-7b (Table 1) in
94% yield, with complete conversion observed after only
30 min at room temperature. The preparation of the di(4-
methoxybenzyl)-substituted allene (Æ)-7c (Table 1) from (Æ)-
10 proceeded analogously, but the cross-coupling provided
the target in only 57% yield, possibly as a result of the acidity
of the benzylic protons. Unlike the more sterically encum-
bered derivative (Æ)-7a, compounds (Æ)-7b and (Æ)-7c were
susceptible to thermal dimerization, presumably through a
stepwise radical mechanism.[17] The half-life of neat (Æ)-7b
was in the order of one day at room temperature, and this
process was not entirely suppressed by storage at low temper-
ature, although dilute solutions showed minimal dimerization
over the course of several months.
Figure 1. A) ORTEP representation of (Æ)-7a with vibrational ellipsoids
at the 30% probability level. Bond lengths [Š] and angles [8] for the
diethynylallene backbone: C1-C2 1.297(6), C2-C3 1.433(7), C3-C4 1.205(6),
C2-C1-C2' 177.7(7), C1-C2-C3 121.7(5), C2-C3-C4 176.6(6), C3-C4-Si1
175.8(5), C1-C2-C14 123.1(4), C3-C2-C14 115.2(5). B) Stick representation
of (Æ)-7a showing the orthogonal orientation of the allene substituents.
Bisalkynylation has a profound effect on the allenic
13C NMR resonances. In tetraalkylallenes, the resonance for
the central allene carbon atom appears at around d ˆ 200,
whereas in tetraphenylallene the signal is shifted downfield to
Angew. Chem. Int. Ed. 2001, 40, No. 12
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4012-2335 $ 17.50+.50/0
2335

COMMUNICATIONS
d ˆ 208.[19] In the 1,3-diethynylallenes, this resonance appears
further downfield at d ˆ 215 ± 217 in (Æ)-7a ± c and at d ˆ
222.6 in (Æ)-7d. The terminal allenic resonance in (Æ)-7a ±
d is located between d ˆ 99 and 102, whereas the acetylenic
signals appear between d ˆ 92 and 97 (Table 1).
[1] P. Siemsen, R. C. Livingston, F. Diederich, Angew. Chem. 2000, 112,
2740 ± 2767; Angew. Chem. Int. Ed. 2000, 39, 2632 ± 2657.
[2] a) L. T. Scott, M. J. Cooney in Modern Acetylene Chemistry (Eds.: P. J.
Stang, F. Diederich), VCH, Weinheim, 1995, pp. 321 ± 351; b) J. S.
Moore, Acc. Chem. Res. 1997, 30, 402 ± 413; c) U. H. F. Bunz, Y. Rubin,
Y. Tobe, Chem. Soc. Rev. 1999, 28, 107 ± 119; d) W. J. Youngs, C. A.
Tessier, J. D. Bradshaw, Chem. Rev. 1999, 99, 3153 ± 3180; e) A.
de Meijere, S. I. Kozhushkov, Top. Curr. Chem. 1999, 201, 1 ± 42;
f) M. M. Haley, J. J. Park, S. C. Brand, Top. Curr. Chem. 1999, 201, 81 ±
130; g) U. H. F. Bunz, Top. Curr. Chem. 1999, 201, 131 ± 161; h) J. M.
Tour, Acc. Chem. Res. 2000, 33, 791 ± 804; i) H. Hopf, Classics in
Hydrocarbon Chemistry, Wiley-VCH, Weinheim, 2000.
Table 1. Selected physical and spectroscopic data of 1,3-diethynylallenes.
(Æ)-7a: Rf(SiO2) ˆ 0.45 (hexane/CH2Cl2 4:1); white solid; m.p. 56 ± 578C;
IR (film): nÄ ˆ 2944, 2891, 2147, 1944, 1464, 1388, 1361, 1333, 1254, 1111, 994,
878, 833, 779, 667 cm 1; 1H NMR (300 MHz, CDCl3): d ˆ 4.20 (s, 4H), 1.07
(s, 42H), 0.89 (s, 18H), 0.08 (s, 12H); 13C NMR (75 MHz, CDCl3): d ˆ
214.9, 98.7, 95.9, 95.1, 64.1, 25.7, 18.5, 18.1, 11.2, 5.4, 5.5; EI-MS: m/z:
[3] a) R. R. Tykwinski, F. Diederich, Liebigs Ann. 1997, 649 ± 661; b) F.
Diederich, L. Gobbi, Top. Curr. Chem. 1999, 201, 43 ± 79; c) F.
Diederich, Chem. Commun. 2001, 219 ± 227.
‡
688.4 [M ]; elemental analysis calcd for C39H76O2Si4 (689.38) (%): C 67.95,
[4] M. J. Edelmann, M. A. Estermann, V. Gramlich, F. Diederich, Helv.
Chim. Acta 2001, 84, 473 ± 480.
[5] M. B. Nielsen, M. Schreiber, Y. G. Baek, P. Seiler, S. Lecomte, C.
Boudon, R. R. Tykwinski, J.-P. Gisselbrecht, V. Gramlich, P. J.
Skinner, C. Bosshard, P. Günter, F. Diederich, Chem. Eur. J. 2001, 7,
issue 15.
[6] a) L. Gobbi, P. Seiler, F. Diederich, Angew. Chem. 1999, 111, 740 ± 743;
Angew. Chem. Int. Ed. 1999, 38, 674 ± 678; b) L. Gobbi, P. Seiler, F.
Diederich, V. Gramlich, Helv. Chim. Acta 2000, 83, 1711 ± 1723.
[7] J.-D. van Loon, P. Seiler, F. Diederich, Angew. Chem. 1993, 105, 1235 ±
1238; Angew. Chem. Int. Ed. Engl. 1993, 32, 1187 ± 1189.
[8] During an attempted preparation of tetraethynylmethane, evidence
for the formation of 1,1,3-triethynylallene in a mixture of products was
obtained: A. H. Alberts, H. Wynberg, J. Chem. Soc. Chem. Commun.
1988, 748 ± 749. For some syntheses of other acetylenic allenes, see
a) P. D. Landor in The Chemistry of the Allenes, Vol. 1 (Ed.: S. R.
Landor), Academic Press, New York, 1982, pp. 229 ± 233; b) H. F.
Schuster, G. M. Coppola in Allenes in Organic Synthesis, Wiley, New
York, 1984, pp. 114 ± 125.
H 11.11; found: C 67.85, H 11.13
(Æ)-7b: Rf(SiO2) ˆ 0.60 (hexane); colorless oil; IR (film): nÄ ˆ 2927, 2851,
2133, 1933, 1464, 1380, 1154, 1072, 990, 883, 672 cm 1; 1H NMR (200 MHz,
CDCl3): d ˆ 2.15 (t, J ˆ 7.1 Hz, 4H), 1.58 ± 1.43 (m, 4H), 1.40 ± 1.22 (m,
12H), 1.07 (s, 42H), 0.88 (t, J ˆ 6.6 Hz, 6H); 13C NMR (50 MHz, CDCl3):
d ˆ 216.2, 101.7, 92.6, 91.8, 34.1, 31.6, 28.2, 27.4, 22.4, 18.5, 14.0, 11.2; EI-
‡
MS: m/z: 568.6 [M ]; elemental analysis calcd for C37H68Si2 (569.12) (%): C
78.09, H 12.04; found: C 78.22, H 12.08
(Æ)-7c: Rf(SiO2) ˆ 0.47 (hexane/EtOAc 10:1); colorless oil; IR (film): nÄ ˆ
2943, 2851, 2133, 1939, 1611, 1512, 1464, 1303, 1248, 1174, 1039, 883, 815,
677 cm 1; 1H NMR (200 MHz, CDCl3): d ˆ 7.00 (d, J ˆ 8.7 Hz, 4H), 6.75 (d,
J ˆ 8.7 Hz, 4H), 3.78 (s, 6H), 3.34 (s, 2H), 3.33 (s, 2H), 1.03 (s, 42H);
13C NMR (75 MHz, CDCl3): d ˆ 216.9, 158.4, 130.2, 130.0, 113.6, 101.0, 93.9,
‡
93.2, 55.1, 39.8, 18.4, 11.1; EI-MS: m/z: 640.3 [M ]; HR-EI-MS calcd for
‡
C41H60O2Si2 [M ]: 640.4132; found 640.4129
(Æ)-7d: Rf(SiO2) ˆ 0.65 (hexane/CH2Cl2 10:1); yellow oil; IR (film): nÄ ˆ
2944, 2862, 2144, 1944, 1492, 1462, 1062, 990, 882, 759, 676 cm 1; 1H NMR
(300 MHz, CDCl3): d ˆ 7.65 ± 7.61 (m, 2H), 7.40 ± 7.27 (m, 8H), 1.15 (s,
42H); 13C NMR (75 MHz, CDCl3): d ˆ 222.6, 132.7, 128.7, 128.2, 126.8,
98.7, 97.6, 97.3, 18.5, 11.2. For the [2‡2] dimer of (Æ)-7d: EI-MS: m/z:
[9] a) J.-D. van Loon, P. Seiler, F. Diederich, Angew. Chem. 1993, 105,
1817 ± 1820; Angew. Chem. Int. Ed. Engl. 1993, 32, 1706 ± 1709; b) T.
Lange, J.-D. van Loon, R. R. Tykwinski, M. Schreiber, F. Diederich,
Synthesis 1996, 537 ± 550.
‡
1104.7 [M ]; elemental analysis calcd for C74H104Si4 (1106.00) (%): C 80.36,
H 9.48; found: C 80.17, H 9.34
[10] Preliminary AM1 geometry optimizations (MacSpartan Pro, Wave-
function, Inc., Irvine, CA 92612, 2000) suggest favorable helical
conformations for a dodecamer formed by oxidative coupling of the
parent 1,3-diethynylallene ((Æ)-3, R ˆ H).
These first representative examples show how the overall
stability of 1,3-diethynylallenes is affected by the electronic
and steric properties of the substituents. The substantial
stability of compound (Æ)-7a clearly demonstrates that bulky
side chains prevent the approach of two allene units and are
capable of mitigating the undesired thermal [2‡2] cyclo-
addition to an appreciable extent. Since removal of the
alkyne-protecting SiiPr3 groups from (Æ)-7b with Bu4NF
cleanly yields the corresponding bis-terminally deprotected
[11] For some recent examples, see: a) D. S. Schlitzer, B. M. Novak, J. Am.
Chem. Soc. 1998, 120, 2196 ± 2197; b) T. Takata, Y. Furusho, K.
Murakawa, T. Endo, H. Matsuoka, T. Hirasa, J. Matsuo, M. Sisido, J.
Am. Chem. Soc. 1998, 120, 4530 ± 4531; c) M. J. Mio, R. B. Prince, J. S.
Moore, C. Kubel, D. C. Martin, J. Am. Chem. Soc. 2000, 122, 6134 ±
6135.
[12] a) B. M. W. Langeveld-Voss, R. A. J. Janssen, M. P. T. Christiaans,
S. C. J. Meskers, H. P. J. M. Dekkers, E. W. Meijer, J. Am. Chem. Soc.
1996, 118, 4908 ± 4909; b) E. Peeters, M. P. T. Christiaans, R. A. J.
Janssen, H. F. M. Schoo, H. P. J. M. Dekkers, E. W. Meijer, J. Am.
Chem. Soc. 1997, 119, 9909 ± 9910.
diethynylallene (1H NMR spectrum (CDCl3): dCꢀ ˆ 2.94),
C-H
oxidative oligomerization of enantiomerically pure 1,3-diethy-
nylallenes is expected to lead to the proposed helical
oligomers with unusual chiroptical properties.
Â
[13] G. Solladie, R. G. Zimmermann, Angew. Chem. 1984, 96, 335 ± 349;
Angew. Chem. Int. Ed. Engl. 1984, 23, 348 ± 362.
[14] a) T. Jeffery-Luong, G. Linstrumelle, Tetrahedron Lett. 1980, 21,
5019 ± 5020; b) K. Ruitenberg, H. Kleijn, C. J. Elsevier, J. Meijer, P.
Vermeer, Tetrahedron Lett. 1981, 22, 1451 ± 1452; c) H. Kleijn, J.
Meijer, G. C. Overbeek, P. Vermeer, Recl. Trav. Chim. Pays-Bas 1982,
101, 97 ± 101; d) E. A. Oostveen, C. J. Elsevier, J. Meijer, P. Vermeer,
Recl. Trav. Chim. Pays-Bas 1982, 101, 382 ± 385; e) K. Ruitenberg, H.
Kleijn, H. Westmijze, J. Meijer, P. Vermeer, Recl. Trav. Chim. Pays-
Bas 1982, 101, 405 ± 409; f) C. J. Elsevier, P. M. Stehouwer, H.
Westmijze, P. Vermeer, J. Org. Chem. 1983, 48, 1103 ± 1105; g) C. J.
Elsevier, H. Kleijn, K. Ruitenberg, P. Vermeer, J. Chem. Soc. Chem.
Commun. 1983, 1529 ± 1530; h) W. de Graaf, A. Smits, J. Boersma, G.
van Koten, W. P. M. Hoekstra, Tetrahedron 1988, 44, 6699 ± 6704; i) T.
Mandai, T. Nakata, H. Murayama, H. Yamaoki, M. Ogawa, M.
Kawada, J. Tsuji, Tetrahedron Lett. 1990, 31, 7179 ± 7180; j) T. Mandai,
H. Murayama, T. Nakata, H. Yamaoki, M. Ogawa, M. Kawada, J.
Tsuji, J. Organomet. Chem. 1991, 417, 305 ± 311; k) S. Gueugnot, G.
Linstrumelle, Tetrahedron Lett. 1993, 34, 3853 ± 3856; l) P. H. Dixneuf,
Experimental Section
ꢀ
(Æ)-7b: iPr3SiC CH (34 mL, 27 mg, 0.15 mmol) and HNiPr2 (28 mL, 20 mg,
0.20 mmol) were added to (Æ)-8 (46 mg, 0.10 mmol) in CH2Cl2 (1 mL) at
room temperature. The solution was purged with Ar for several minutes,
and [Pd(PPh3)4] (5.8 mg, 5.0 mmol) and CuI (1.9 mg, 10 mmol) were added
sequentially. After an additional minute of degassing, the solution was
allowed to stir at room temperature for 30 min, at which point TLC showed
complete conversion. The mixture was diluted with hexanes (5 mL) and
filtered through celite. The solvent was removed under reduced pressure,
and the residue was purified by flash chromatography on SiO2 (hexane) to
give (Æ)-7b (54 mg, 94%) as a clear oil.
Received: February 19, 2001 [Z16640]
2336
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4012-2336 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2001, 40, No. 12

COMMUNICATIONS
T. Guyot, M. D. Ness, S. M. Roberts, Chem. Commun. 1997,
2083 ± 2084; m) S. Condon-Gueugnot, G. Linstrumelle, Tetrahedron
2000, 56, 1851 ± 1858.
idyldimethylsilyl (2-PyMe2Si) group as a removable directing
group (intramolecular ligand).[5±8] For example, highly effi-
cient carbopalladation of vinylsilanes can be achieved with
2-PyMe2Si group as a removable directing group.[6] In ongoing
efforts to exploit the utility of this removable directing group
in carbometalation, we investigated directed intermolecular
carbomagnesation across pyridyl-substituted vinylsilanes.
2-Pyridyldimethylvinylsilane (1) was treated with iPrMgCl
in Et2O, and the mixture was stirred for 3 h at room
temperature. The reaction gave the corresponding a-silyl
organomagnesium compound, and 2aa was isolated after
aqueous workup in 91% yield [Eq. (1)].[9] Much milder
[15] New compounds were fully characterized by 1H and 13C NMR
spectroscopy, FT-IR spectroscopy, EI-MS, and elemental analysis or
high-resolution EI-MS.
[16] Electronic factors cannot be ruled out, and their influence is currently
under investigation.
[17] a) H. Hopf in The Chemistry of the Allenes, Vol. 2 (Ed.: S. R. Landor),
Academic Press, New York, 1982, pp. 536 ± 546; b) M. Christl, S.
Groetsch, K. Günther, Angew. Chem. 2000, 112, 3395 ± 3397; Angew.
Chem. Int. Ed. 2000, 39, 3261 ± 3263.
[18] X-ray crystal data of (Æ)-7a (C39H76O2Si4, Mr ˆ 689.36): orthorhombic
3
space group Pbcn, Z ˆ 4, Dc ˆ 0.963 gcm
,
a ˆ 14.767(5), b ˆ
11.746(4), c ˆ 27.420(13) Š, Vˆ 4756(3) Š3, MoKa radiation, 2229
independent reflections, 2 < q < 208. Single crystals were grown at
48C from CH2Cl2/MeCN. The structure was calculated by direct
methods (SHELXS-86) and refined by full-matrix least-squares
methods (SHELXL-93). Final R(F) ˆ 0.0689, wR(F 2) ˆ 0.1888 for
199 variables and 2229 observed reflections with I > 2s(I). Crystallo-
graphic data (excluding structure factors) for the structures reported
in this paper were deposited with the Cambridge Crystallographic
Data Centre as supplementary publication no. CCDC-157566. 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).
R'MgCl
N
MgCl
N
R'
R
Et2O
RT, 3 h
Si
Me2
Si
Me2
R
N
1: R = H
3: R = nBu
H2O
H (D)
(1)
R'
or D2O
Si
Me2
R
[19] M. A. Hashem, P. Weyerstahl, Tetrahedron 1981, 37, 2473 ± 2476.
91%
93%
2aa: R = H
2ba: R = H
2bb: R = H
2ca: R = H
2da: R = H
R' = iPr
R' = nBu
R' = nBu
R' = Ph (PhMgBr)
R' = CH2CH=CH2
83% (>95% D)
73% (under reflux)
94%
91%
4ba: R = nBu R' = nBu
Directed Intermolecular Carbomagnesation
across Vinylsilanes: 2-PyMe2Si Group as a
Removable Directing Group**
reaction conditions and much shorter reaction times com-
pared to the previous reported reactions with vinylsilanes
clearly indicate the enhanced reactivity of 1.[2] We assume that
this carbomagnesation was facilitated by the complex-induced
proximity effect (CIPE).[10] The reaction presumably involves
a pre-equilibrium complex of 1 and RMgX, and this makes the
subsequent carbomagnesation step intramolecular in nature.
The importance of this pre-equilibrium complex was further
supported by the observation of dramatic solvent effects:
weakly coordinating solvents such as Et2O favor this reaction,
whereas strongly coordinating solvents such as THF disfavor
it. These results may be attributed to inhibition of the
formation of the pre-equilibrium complex by the coordinating
solvent.[11] In addition to this kinetic preference, we assume
that stabilization of the generated a-silyl organomagnesium
compound by intramolecular coordination of the pyridyl
group is also responsible for the efficiency of this carbomag-
nesation process.[8a]
Kenichiro Itami, Koichi Mitsudo, and Jun-ichi Yoshida*
The addition of organolithium reagents to vinylsilanes is
one of the most powerful methods for the generation of
synthetically useful a-silyl carbanions.[1] Although Grignard
reagents are viable alternatives for this addition reaction,[2]
serious limitations are associated with the carbomagnesation
methodology: 1) Activating groups on silicon (e.g., chloro,
alkoxy, and amino groups) are needed for the addition (no
reaction with trialkyl(vinyl)silanes); 2) substitutions at the
silicon atom are often observed as unavoidable side reactions
when these activating groups are used; and 3) primary alkyl
Grignard reagents are not applicable in the reaction.[3, 4] These
drawbacks profoundly diminish the synthetic usefulness of
this otherwise attractive methodology.
Recently, we initiated a program to develop highly efficient
hydro- and carbometalation reactions by utilizing the 2-pyr-
Next we investigated the addition of a primary alkyl
Grignard reagent, and found that nBuMgCl also adds to 1
at room temperature [Eq. (1)]. To our knowledge, this is the
first example of the efficient addition of a primary alkyl
Grignard reagent to a vinylsilane. In addition to secondary
and primary alkyl Grignard reagents, PhMgBr[12] and
[*] Dr. K. Itami, K. Mitsudo, Prof. J. Yoshida
Department of Synthetic Chemistry and Biological Chemistry
Graduate School of Engineering, Kyoto University
Yoshida, Kyoto 606-8501 (Japan)
Fax : (‡81)75-753-5911
E-mail: yoshida@sbchem.kyoto-u.ac.jp
ˆ
CH2 CHCH2MgCl also added across 1 [Eq. (1)]. The addi-
[**] This work was supported by a Grant-in-Aid for Scientific Research
from the Ministry of Education, Science, Sports, and Culture, Japan,
and in part by the Mitsubishi Foundation. K.M. thanks the Japan
Society for the Promotion of Science for Young Scientists.
ˆ
tion of CH2 CHCH2MgCl was complete within 1 h even at
08C (90%). Moreover, the addition to b-substituted vinyl-
silane 3, which represents a more difficult class of substrate,
also took place [Eq. (1)]. Quantitative incorporation of
deuterium in the position a to silicon occurred on quenching
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
Angew. Chem. Int. Ed. 2001, 40, No. 12
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4012-2337 $ 17.50+.50/0
2337