Probing the effect of organic and organometallic functionalization on [1,5]-silicon shifts in indenylsilanes

Article abstract of DOI:10.1021/om990650f

Indenylsilanes bearing organic and organometallic substituents have been prepared in order to probe the effect of substitution on the rate of [1,5]-silicon shifts in this class of compounds. In an attempt to prepare 1,1,3-tris(trimethylsilyl)indene (7), the hitherto unknown silicon-functionalized bis(trimethylsilyl)dibenzo[a,d]fulvalene (9) was unexpectedly generated; this species was characterized by use of both NMR spectroscopy and X-ray crystallography and was rationally prepared in 68% yield from 3,3′-bis(1-(trimethylsilyl))-indene (16). The molecular dynamics of 1,3-dimethyl-1-(trimethylsilyl)indene (18) and the crystallographically characterized chromium complex (η⁶1,3-dimethyl-1-exo-(trimethylsilyl)-indene)tricarbonylchromium (22) were examined by use of 1D-selective inversion and 2D-EXSY NMR techniques; surprisingly, the presence of chromium and methyl substituents has a negligible effect on the rate of [1,5]-silicon shifts (ΔG^? = 23-24 kcal mol^-1) versus the parent compound 1-(trimethylsilyl)indene (3) (ΔG^? ≈ 24 kcal mol^-1). In the case of 18, the intermediate isoindene 18-iso was intercepted with tetracyanoethylene as the crystallographically characterized [4 + 2] cycloadduct 5,6-benzo-2,2,3,3-tetracyano-1,4-dimethyl-7-(trimethylsilyl)bicyclo(2.2.1)hept- 5-ene (19).

Full text of DOI:10.1021/om990650f

590
Organometallics 2000, 19, 590-601
P r obin g th e Effect of Or ga n ic a n d Or ga n om eta llic
F u n ction a liza tion on [1,5]-Silicon Sh ifts in
In d en ylsila n es
Mark Stradiotto,*,† Paul Hazendonk, Alex D. Bain, Michael A. Brook,* and
Michael J . McGlinchey*,‡
Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1
Received August 12, 1999
Indenylsilanes bearing organic and organometallic substituents have been prepared in
order to probe the effect of substitution on the rate of [1,5]-silicon shifts in this class of
compounds. In an attempt to prepare 1,1,3-tris(trimethylsilyl)indene (7), the hitherto
unknown silicon-functionalized bis(trimethylsilyl)dibenzo[a,d]fulvalene (9) was unexpectedly
generated; this species was characterized by use of both NMR spectroscopy and X-ray
crystallography and was rationally prepared in 68% yield from 3,3′-bis(1-(trimethylsilyl))-
indene (16). The molecular dynamics of 1,3-dimethyl-1-(trimethylsilyl)indene (18) and the
crystallographically characterized chromium complex (η6-1,3-dimethyl-1-exo-(trimethylsilyl)-
indene)tricarbonylchromium (22) were examined by use of 1D-selective inversion and 2D-
EXSY NMR techniques; surprisingly, the presence of chromium and methyl substituents
has a negligible effect on the rate of [1,5]-silicon shifts (∆Gq ) 23-24 kcal mol-1) versus the
parent compound 1-(trimethylsilyl)indene (3) (∆Gq ≈ 24 kcal mol-1). In the case of 18, the
intermediate isoindene 18-iso was intercepted with tetracyanoethylene as the crystallo-
graphically characterized [4 + 2] cycloadduct 5,6-benzo-2,2,3,3-tetracyano-1,4-dimethyl-7-
(trimethylsilyl)bicyclo(2.2.1)hept-5-ene (19).
In tr od u ction
dynamic behavior of these stereochemically nonrigid
σ-indenyl compounds can be rationalized in terms of
symmetry-allowed, suprafacial [1,5]-sigmatropic shifts.4
In several cases, Diels-Alder trapping with reactive
dienophiles such as tetracyanoethylene (TCNE), yield-
ing adducts analogous to 2, has been advanced as
evidence1d,e in support of the existence of transient
isoindenes (1-iso).5 Indeed, in the course of studying the
molecular dynamics and reactivity of main-group and
transition-metal σ-indenyl complexes, we have crystal-
lographically characterized several such [4 + 2] cyclo-
adducts (Scheme 1).6
Kinetic data obtained for the interconversion of 1-S
and 1-R (via 1-iso) in a variety of simple 1-trialkylsilyl-
indenes, including 1-(trimethylsilyl)indene (3) suggest
that the nature of the alkyl substituents on silicon has
little effect on the barrier to [1,5]-silicon shifts (∆Gq ≈
24 kcal mol-1),1,6 a trend that has also been noted in
the related cyclopentadienyl series.7 However, Rakita
et al. noted that the introduction of methyl substituents
Although quasi-fluxional molecules of the type (1-
indenyl)nER4-n (E ) Si, Ge, Sn) have been known for
over 3 decades,1 the widespread use2 of these and
related main-group species as ligands in the preparation
of chiral ansa-metallocene precatalysts has brought
about renewed interest in this class of molecules.3 The
† Current address: Department of Chemistry, University of Cali-
fornia at Berkeley, Berkeley, CA 94720-1460.
‡ Phone: (905) 525-9140, ext. 27318. FAX: (905) 522-2509. E-mail:
mcglinc@mcmaster.ca.
(1) (a) Rakita, P. E.; Davison, A. Inorg. Chem. 1969, 8, 1164. (b)
Rakita, P. E.; Davison, A. Inorg. Chem. 1970, 9, 289. (c) Davison, A.;
Rakita, P. E. J . Organomet. Chem. 1970, 23, 407. (d) Larrabee, R. B.;
Dowden, B. F. Tetrahedron Lett. 1970, 915. (e) Ashe, A. J ., III.
Tetrahedron Lett. 1970, 24, 2105. (f) McLean, S.; Reed, G. W. B. Can.
J . Chem. 1970, 48, 3110. (g) Sergeyev, N. M.; Grishin, Yu. K.; Luzikov,
Yu. N.; Ustynyuk, Yu. A. J . Organomet. Chem. 1972, 38, C1. (h)
Luzikov, Yu. N.; Sergeyev, N. M.; Ustynyuk, Yu. A. J . Organomet.
Chem. 1974, 65, 303. (i) Sergeyev, N. M.; Avramenko, G. I.; Kisin, A.
V.; Korenevsky, V. A.; Ustynyuk, Yu. A. J . Organomet. Chem. 1971,
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Am. Chem. Soc. 1982, 104, 2109. (p) Atwood, J . L.; McMaster, A. D.;
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Int. Ed. 1999, 38, 428. (b) Halterman, R. L. Chem. Rev. 1992, 92, 965.
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Chem. 1997, 527, 297. (e) Nifant’ev, I. E.; Ivchenko, P. V. Organo-
metallics 1997, 16, 713.
(3) For a recent examination of the dynamic behavior of ansa-
bridged group IV metallocenes, see: Miyake, S.; Henling, L. M.;
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(5) A crystallographically characterized metallocene arising from the
intramolecular trapping of an isoindene by a metal center has been
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metallics 1997, 16, 3050.
(6) (a) Rigby, S. S.; Girard, L.; Bain, A. D.; McGlinchey, M. J .
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Hughes, D. W.; Brook, M. A.; Bain, A. D.; McGlinchey, M. J . Organo-
metallics 1996, 15, 5645. (c) Stradiotto, M.; Hughes, D. W.; Brook, M.
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10.1021/om990650f CCC: $19.00 © 2000 American Chemical Society
Publication on Web 01/26/2000

[1,5]-Silicon Shifts in Indenylsilanes
Organometallics, Vol. 19, No. 4, 2000 591
Sch em e 1. [1,5]-Sigm a tr op ic Sh ifts a n d Isoin d en e
Cycloa d d ition Ch em istr y of Meta lloin d en es
Ch a r t 1
In an attempt to prepare the target compound 7, the
standard synthetic approach involving the reaction of
a carbanion with an appropriate chlorosilane was
employed. Unfortunately, all attempts, either via three
consecutive additions of base, each followed by quench-
ing with chlorotrimethylsilane (Scheme 3), or directly
from a mixture of 5 and 6, did not produce the desired
product 7, the isoindene 8, or any other dimerization
product derived from these species. However, careful
chromatographic separation of the products in these
reactions consistently yielded the bright red solid prod-
at the C(4) and C(7) positions on the indene framework
in 4a (Chart 1) modestly reduces this barrier (21.8 kcal
mol-1), while the presence of a C(2) methyl group leads
to a slightly increased barrier to silicon migrations (26.5
kcal mol-1). On the basis of data obtained from extended
Hu¨ckel calculations, these workers postulated that the
reduced barrier in 4a arises as a result of electronic
stabilization of the [1,5]-silicon shift transition-state
structure.8 More recently, we have demonstrated that
such silatropic shifts are also electronically facilitated
by the incorporation of fused benzo rings in 4b (22 kcal
mol-1) and 4c (18 kcal mol-1), presumably attributable
to an increase in the aromatic character of the transi-
tion-state species.9 In continuation of these studies, we
report herein on our attempts to examine the effect on
the rate of [1,5]-silicon shifts of incorporating carbon,
silicon, and transition-metal substituents6d into the
indenylsilane architecture.
1
uct 9, in low yield. Although the simplicity of the H,
13C, and 29Si NMR spectral data suggested the presence
of one indene and one silicon environment in this
compound, mass spectrometric data for 9 indicated a
dimeric structure, comprised of two (SiMe3)C9H5 frag-
ments. This inference was further substantiated by the
nuclear Overhauser effect (NOE) enhancement of the
H(7) signal when the H(2) signal was irradiated, an
observation which cannot be rationalized for a com-
pound containing only one indenyl unit. The connectiv-
ity in 9, initially ascertained on the basis of these data,
was subsequently verified by a single-crystal X-ray
diffraction study; the structure appears as Figure 1.
Although the crystal structure is seriously disordered
(preventing a satisfactory anisotropic refinement), all
of the heavy atoms were found and the location of the
trimethylsilyl groups at the C(3) positions of bis-
(trimethylsilyl)dibenzo[a,d]fulvalene (9) is evident. The
X-ray crystal structure of the corresponding hydrocar-
bon trans-1,1′-bis(indenylidene) has been reported and
reveals an overall molecular structure which is quali-
tatively similar to that of the silylated complex 9.11
Resu lts a n d Discu ssion
In Sea r ch of th e Ster ica lly Loa d ed Isoin d en e
Tr is(tr im eth ylsilyl)isoin d en e (8). In an extension of
the work of Rakita et al., metalloindenes comprising
sterically demanding functionalities at the C(1) and C(3)
positions, rather than at the C(4) and C(7) sites, were
sought. Our initial approach in this regard was to create
a system in which the bulky C(1) and C(3) substituents
were identical with the migrating fragment, so as to
facilitate the generation of sterically loaded isoindenes,
while avoiding complications arising due to the genera-
tion of complex isomeric mixtures. Toward this end,
1,1,3-tris(trimethylsilyl)indene (7) was identified as an
attractive synthetic target, in which steric interactions
between the trimethylsilyl groups on C(1) could facili-
tate isomerization to the corresponding tris(trimethyl-
silyl)isoindene 8 (Scheme 2). The choice of 7 appeared
especially judicious in light of the fact that the logical
synthetic precursors, 1,1- and 1,3-bis(trimethylsilyl)-
indene (5 and 6), are known1c and because the exchang-
ing trimethylsilyl environments in 8, arising due to
[1,5]-silatropic shifts, could be readily monitored by use
of 2D-EXSY NMR.10
Although the formation of the dibenzofulvalene 9 is
surprising, precedent for the generation of dimeric
indene species from isoindenes does exist. Warrener and
co-workers report that, in addition to the well-known
cycloaddition chemistry exhibited by molecules which
contain the o-xylylene moiety, the dimerization of iso-
indenes via ene-type mechanisms can occur if a hydro-
gen atom is present at the C(2) position.12-14 It is also
noteworthy that dimeric compounds closely related to
9 have received recent attention in the literature.
Olmstead and co-workers15 have discussed the unex-
pected isolation of 10 (Chart 2) in their attempt to
(11) Capparelli, M. V.; Machado, R.; De Sanctis, Y.; Arce, A. J . Acta
Crystallogr. 1996, C52, 947.
(8) (a) Andrews, M. N.; Rakita, P. E.; Taylor, G. A. Tetrahedron Lett.
1973, 21, 1851. (b) Andrews, M. N.; Rakita, P. E.; Taylor, G. A. Inorg.
Chim. Acta 1975, 13, 191.
(9) (a) Rigby, S. S.; Gupta, H. K.; Werstiuk, N. H.; Bain, A. D.;
McGlinchey, M. J . Polyhedron 1995, 14, 2787. (b) Rigby, S. S.; Gupta,
H. K.; Werstiuk, N. H.; Bain, A. D.; McGlinchey, M. J . Inorg. Chim.
Acta 1996, 251, 355.
(12) Warrener, R. N.; Harrison, P. A.; Russell, R. A. J . Chem. Soc.,
Chem. Commun. 1982, 1134.
(13) Warrener, R. N.; Pitt, I. G.; Russell, R. A. J . Chem. Soc., Chem.
Commun. 1982, 1136.
(14) Warrener, R. N.; Pitt, I. G.; Russell, R. A. Aust. J . Chem. 1993,
46, 1845.
(15) Olmstead, M. M.; Hitchcock, S. R.; Nantz, M. H. Acta Crystal-
logr. 1996, C52, 1523.
(10) Perrin, C. L.; Dwyer, T. J . Chem. Rev. 1990, 90, 935.

592 Organometallics, Vol. 19, No. 4, 2000
Stradiotto et al.
Sch em e 2. P r op osed Syn th etic Rou te to 1,1,3-Tr is(tr im eth ylsilyl)in d en e (7) a n d Its Isom er iza tion to th e
Cor r esp on d in g Isoin d en e 8
Sch em e 3. Un exp ected Gen er a tion of 9 d u r in g th e
Attem p ted P r ep a r a tion of 7a
Ch a r t 2
generate an indenide ion, as depicted in Scheme 4.
Formation of the substituted 1,1′-biindene 12, followed
by attack by base, would lead to the generation of the
dianion 13. Subsequent oxidative coupling of the teth-
ered anions in 13 produces the dibenzofulvalene 9.
Although the origin of the oxidant is not certain, such
a process would be greatly facilitated by the eventual
formation of the highly conjugated, 18-π-electron system
in 9. Indeed, molecular oxygen itself has proven effec-
tive20 in the formation of related conjugated organic
structures.21
a
Dibenzofulvalenes are named according to the bonds that
are benzannulated.
Extended planar aromatic systems are of considerable
interest, both as ligands in the preparation of organo-
metallic complexes22 and as precursors to materials with
novel electronic, magnetic, and conductive properties.23
Given the potential utility of compound 9, its rational
preparation was undertaken, using the approach sum-
marized in Scheme 5. With 3,3′-biindene (14) and/or
1-(indanylidene)-1-indene (15) as starting materials,
treatment with base followed by quenching with chloro-
trimethylsilane yielded 16 as an inseparable mixture
of meso and d,l isomers. Subsequent double deprotona-
tion of 16 generated the previously invoked dianion 13,
which was efficiently oxidized, by use of iodine, to the
dibenzofulvalene 9.
To summarize, in an attempt to prepare the sterically
protected isoindene 8, the silicon-substituted dibenzo-
fulvalene 9 was serendipitously generated; this latter
compound was subsequently prepared in a rational
manner from the dimer of 1-(trimethylsilyl)indene, 16,
via oxidation of the corresponding dianion 13. Although
the generation of 9 represents a chemically interesting
result, the fact that the desired compound, 7, could not
be synthesized is perplexing. Given that the bis(tri-
methylsilyl)indenes 5 and 6 are readily prepared as a
mixture (∼1:1) of interconverting isomers, it is evident
that the Si-C(1) indenyl bonds in 5 are sufficiently long
F igu r e 1. Structure of 9, determined by X-ray crystal-
lography.
generate an ansa-titanocene, while Youngs et al.16,17
demonstrated that geometrically constrained bis(tri-
methylsilyl)dibenzo[a,f]fulvalenes, such as 11, can be
readily prepared via oxidative coupling using group 13
metal halides.
In the absence of conclusive experimental data,
decisive commentary on the mechanistic pathway lead-
ing to the formation of 9 cannot be provided. Neverthe-
less, given the paucity of evidence for either 7 or 8, and
in consideration of the highly basic experimental condi-
tions employed, it is possible to rationalize the genera-
tion of 9 on the basis of known synthetic methodolo-
gies.18,19 In the presence of excess base, abstraction of
(20) Escher, A.; Rutsch, W.; Neuenschwander, M. Helv. Chim. Acta
1986, 69, 1644.
+
either SiMe3 (in 5) or a proton (in 3 or 6) would
(21) As noted by an astute reviewer, the formation of compound 9
via oxidative coupling of indenyl fragments followed by reductive
elimination of hexamethyldisilane cannot be ruled out. We were unable
to conclusively identify Me3SiSiMe3 in the product mixture by use of
NMR spectroscopic techniques.
(22) McGovern, P. A.; Vollhardt, K. P. C. J . Chem. Soc., Chem.
Commun. 1996, 1593.
(16) Malaba, D.; Djebli, A.; Chen, L.; Zarate, E. A.; Tessier, C. A.;
Youngs, W. J . Organometallics 1993, 12, 1266.
(17) Malaba, D.; Tessier, C. A.; Youngs, W. J . Organometallics 1996,
15, 2918.
(18) Simmross, U.; Mu¨llen, K. Chem. Ber. 1993, 126, 969.
(19) Rutsch, W.; Escher, A.; Neuenschwander, M. Chimia 1983, 37,
160.
(23) Baumgarten, M.; Tyutyulkov, N. Chem. Eur. J . 1998, 4, 987.

[1,5]-Silicon Shifts in Indenylsilanes
Organometallics, Vol. 19, No. 4, 2000 593
Sch em e 4. Gen er a lized Mech a n istic Ra tion a le for
th e F or m a tion of 9
Sch em e 6. Molecu la r Rea r r a n gem en ts In volvin g
18 a n d Diels-Ald er Tr a p p in g of th e
Cor r esp on d in g Isoin d en e 18-iso w ith
Tetr a cya n oeth ylen e
were unable to develop a suitable synthesis of this
molecule. Data obtained from dynamic NMR studies
involving 18 would, however, be instructive, given that
the contracted metal-indene distance in this molecule,
relative to 17, would be expected to lead to an increased
ground state energy for the indene structure and
possibly to a reduced barrier to [1,5]-silicon shifts. In
contrast to the 1-indenylsilane 4a , which possesses C(4)
and C(7) methyl substituents, any reduction in the
barrier to silicon migrations in 18, relative to 3, is best
rationalized not in terms of electronic stabilization but,
rather, as arising from the alleviation of unfavorable
steric interactions between the indene methyl groups
and the methyl substituents on the silicon fragment.
The considerable appeal of 18 prompted a reinvestiga-
tion of this compound.
Sch em e 5. Ra tion a l Syn th esis of 9 via 16, th e
Dim er of 3
In an effort to prepare 1,3-dimethyl-1-(trimethylsilyl)-
indene (18), Davison and Rakita utilized a synthetic
protocol that involved quenching of (dimethylindenyl)-
lithium with chlorotrimethylsilane in hexane, followed
by workup with deuterium oxide (D2O). The formation
of the isolated product, 1-deuterio-1,3-dimethylindene,
can be rationalized both in terms of the quenching of
the unreacted lithium indenide and as arising from the
hydrolytic cleavage of the silicon-indene bond in the
desired product 18. Interestingly, these workers omitted
this workup step in the successful synthesis of the
indenylstannane 17.
On the basis of the successful preparation of 17, (1,3-
dimethylindenyl)lithium was quenched with chlorotri-
methylsilane in tetrahydrofuran, without aqueous work-
up. Following purification via flash chromatography on
silica, 1,3-dimethyl-1-(trimethylsilyl)indene (18) was
isolated as an analytically pure solid in 89% yield;
indeed, during the preparation of this report, Rausch
and co-workers reported the preparation of 18 under
anhydrous conditions.26
so as to accommodate two gem-trimethylsilyl groups.
Additional evidence in support of the viability of 7 comes
from the existence of the corresponding quasi-fluxional
tris(trimethylsilyl)cyclopentadiene series, which is readily
prepared via deprotonation of the cyclopentadienyl
analogue of 5, followed by quenching with chlorotri-
methylsilane.24,25 As such, unfavorable steric require-
ments in 7 can be ruled out as a significant factor
preventing the generation of this compound, and with-
out a persuasive argument based on electronic effects,
one is left to consider the experimental conditions
employed. On the basis of the tentative mechanism
depicted in Scheme 4, a critical step leading to the
formation of 9 may involve the attack of an indenyl
anion on a substituted indene, giving rise to 12. Perhaps
more dilute conditions would lead to the preferential
addition of a third trimethylsilyl group to the anion
derived from 5 and 6. Efforts to prepare 7 via alternative
synthetic routes are ongoing.
Syn th esis a n d Dyn a m ics of a Dim eth ylin d en yl-
sila n e. In 1970, Davison and Rakita noted that the
presence of methyl groups at the indenyl C(1) and C(3)
positions in 1,3-dimethyl-1-(trimethylstannyl)indene
(17) does little to lower the barrier to tin migrations,
relative to the parent compound 1-(trimethylstannyl)-
indene;1c a comparative study involving the correspond-
ing 1-indenylsilane 18 proved impossible, since they
Having successfully prepared the desired dimethyl-
indenylsilane 18, the dynamic behavior of this com-
pound (Scheme 6) was examined by use of 2D-EXSY and
1
single selective inversion NMR techniques.27 The H-
1H EXSY spectra obtained for 18 clearly indicate the
(26) Ready, T. E.; Chien, J . C. W.; Rausch, M. D. J . Organomet.
Chem. 1999, 583, 11.
(24) Ustynyuk, Yu. A.; Luzikov, Yu. N.; Mstislavsky, U. J .; Azizov,
A. A.; Pribytkova, J . M. J . Organomet. Chem. 1975, 96, 335.
(25) J utzi, P.; Sauer, J . J . Organomet. Chem. 1973, 50, 29.
(27) (a) Bain, A. D.; Cramer, J . A. J . Magn. Reson. 1993, A103, 217.
(b) Bain, A. D.; Cramer, J . A. J . Phys. Chem. 1993, 97, 2884. (c)
Muhandiram, D. R.; McClung, R. E. D. J . Magn. Reson. 1987, 71, 187.

594 Organometallics, Vol. 19, No. 4, 2000
Stradiotto et al.
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
P a r a m eter s for 19 a n d 22
19
22
empirical formula
mol wt
C
20H20N4Si
C17H20O3SiCr
352.42
344.49
description
temp, K
colorless prism
299(2)
yellow plate
299(2)
wavelength, Å
cryst syst
space group
a, Å
b, Å
c, Å
0.710 73 (Mo KR) 0.710 73 (Mo KR)
monoclinic
P21/ c
triclinic
P1h
9.601(1)
14.782(2)
13.977(2)
90.000
105.040(3)
90.000
9.691(11)
9.877(9)
10.395(12)
67.98(3)
81.98(2)
82.94(2)
910(2)
R, deg
â, deg
γ, deg
V, Å3
1915.7(5)
4
Z
2
calcd density, g/cm3
scan mode
F(000)
1.194
1.285
ω scans
728
ω scans
368
θ range for collecn, deg
2.04-22.50
-10 e h e 10
-15 e k e 15
-14 e l e 15
11384
2.12-22.50
-11 e h e 12
-12 e k e 12
-12 e l e 13
5572
index ranges
F igu r e 2. Crystallographically determined structure of 19
(shown with 30% thermal displacement ellipsoids).
no. of rflns collected
no. of indep rflns
no. of data/restraints/
params
2501
2501/0/226
2376
2353/0/200
operation of an exchange process which permutes the
C(1)-CH3 and C(3)-CH3 chemical environments; the
corresponding off-diagonal peaks linking the aromatic
hydrogen atom resonances are much more difficult to
discern, owing to the extensive overlap of these signals.
Moreover, as the mixing time is increased, the off-
diagonal peaks linking the C(1)-CH3 and the C(3)-CH3
sites in 18 grow in intensity and are consistently of the
same phase as the diagonal signals; this serves to
identify the origin of these off-diagonal signals as arising
from chemical exchange, not NOE interactions. Data
obtained from single selective inversion NMR experi-
ments allowed the barrier to [1,5]-silicon shifts in 18 to
be estimated (∆Gq ) 23 ( 1 kcal mol-1). This value does
not deviate significantly from those found for 1-(tri-
methylsilyl)indene (3; ∼24 kcal mol-1) and the C(4) and
C(7) methylated 1-indenylsilane 4a (∼22 kcal mol-1).
As such, it is evident that methyl substitution at the
C(1) and C(3) positions on indene does not contribute
in any significant way to a reduction in the barrier to
silicon migrations in 18. It is apparent that if sterically
promoted rearrangements of 1-indenylsilanes are to be
realized, the introduction of even bulkier molecular
fragments will be required in order to perturb the
indene ground-state structure.
Although the introduction of the indenyl methyl
substituents in 18 does not lead to an increased rate of
[1,5]-silicon shifts, the fact that these migrations con-
tinue to occur is important in and of itself. Indeed 18
serves as a model compound in the development of
systems which have the capacity to undergo intramo-
lecular Diels-Alder reactions. With the quasi-fluxional
nature of this compound ascertained, the propensity of
the corresponding isoindene 18-iso for [4 + 2] cyclo-
additions was examined by stirring a solution of 18 with
tetracyanoethylene (Scheme 6).
goodness of fit on F2
0.854
0.995
final R indices (I > 2σ(I)) R1 ) 0.0448
R1 ) 0.0672
wR2 ) 0.1507
R1 ) 0.1317
wR2 ) 0.1644
0.005
<0.001
0.901, 0.571
0.31
wR2 ) 0.0870
R indices (all data)
R1 ) 0.1104
wR2 ) 0.1016
<0.001
mean shift/error
max shift/error
transmissn (max, min)
largest diff Peak, e/Å3
largest diff Hole, e/Å3
<0.001
0.979, 0.690
0.152
-0.195
-0.29
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg.) for 19 a n d 22
19
22
Si(1)-C(15)
Si(1)-C(14)
Si(1)-C(16)
Si(1)-C(7)
C(1)-C(6)
C(1)-C(12)
C(1)-C(7)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(13)
C(4)-C(5)
C(4)-C(7)
C(5)-C(6)
C(5)-C(11)
C(6)-C(8)
C(8)-C(9)
C(9)-C(10)
C(10)-C(11)
1.850(3)
1.857(3)
1.859(3)
1.913(3)
1.511(4)
1.519(4)
1.547(4)
1.608(4)
1.591(4)
1.612(4)
1.505(4)
1.509(4)
1.554(4)
1.377(4)
1.387(4)
1.379(4)
1.393(4)
1.368(5)
1.388(5)
Cr(1)-C(3A)
Cr(1)-C(5)
Cr(1)-C(6)
Cr(1)-C(4)
Cr(1)-C(7)
Cr(1)-C(7A)
Si(1)-C(1)
Si(1)-C(8)
Si(1)-C(9)
Si(1)-C(10)
C(1)-C(2)
2.280(7)
2.172(8)
2.205(9)
2.217(6)
2.237(8)
2.293(6)
1.938(7)
1.847(8)
1.853(9)
1.831(7)
1.479(9)
1.496(8)
1.540(8)
1.34(1)
C(1)-C(7A)
C(1)-C(11)
C(2)-C(3)
C(3)-C(33)
C(3A)-C(7A)
C(3A)-C(4)
C(4)-C(5)
1.51(1)
1.413(8)
1.42(1)
1.40(1)
1.40(1)
C(5)-C(6)
C(6)-C(7)
1.38(1)
1.403(9)
C(7A)-C(7)
C(1)-C(7)-Si(1)
C(4)-C(7)-Si(1)
C(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(2)-C(1)-C(6)
C(3)-C(4)-C(5)
C(12)-C(1)-C(7)
C(13)-C(4)-C(7)
117.9(2)
119.6(2)
102.3(2)
102.7(2)
103.5(2)
103.8(2)
120.7(3)
120.9(3)
C(2)-C(1)-C(7A)
C(2)-C(1)-C(11)
C(2)-C(1)-Si(1)
C(7A)-C(1)-C(11)
C(7A)-C(1)-Si(1)
C(11)-C(1)-Si(1)
100.9(5)
116.1(7)
106.3(4)
115.4(6)
107.4(4)
109.8(5)
The expected Diels-Alder cycloaddition product 19
was isolated in 83% yield and fully characterized by use
of standard spectroscopic techniques and also by single-
crystal X-ray diffraction methods; the structure of 19
appears as Figure 2, while crystallographic data and
selected metrical parameters are collected in Tables 1
and 2, respectively. In the crystal 19 possesses an
effective σ-plane defined by Si(1) and C(7) and bisecting
the C(2)-C(3) bond, giving rise to identical bond lengths

[1,5]-Silicon Shifts in Indenylsilanes
Organometallics, Vol. 19, No. 4, 2000 595
Sch em e 7. Attem p ted P r ep a r a tion of th e
η6-Cr (CO)3-Com p lexed In d en ylsila n e 20
Sch em e 8. Molecu la r Rea r r a n gem en ts In volvin g
22 a n d th e Attem p ted Diels-Ald er Tr a p p in g of th e
Isoin d en e 22-iso w ith Tetr a cya n oeth ylen e
and angles (within experimental error) for all fragments
related by this mirror plane. Moreover, despite the
presence of the methyl substituents at the bridgehead
positions in 19, the geometric characteristics of this
molecule in the solid state parallel those found in other
crystallographically characterized metalloisoindene cy-
cloadducts of TCNE, including (a) anti-oriented tri-
methylsilyl and TCNE fragments, (b) elongated incipi-
ent Diels-Alder (C(1)-C(2) and C(3)-C(4)) and for-
merly olefinic (C(2)-C(3)) bonds, and (c) expanded
Si(1)-C(7)-C(1) and Si(1)-C(7)-C(4) angles (average
∼119°).6
supported by a report from Ceccon and co-workers, who
noted that the rearrangement of 1-phenylindene to
3-phenylindene is accelerated by the addition of (CH3-
CN)3Cr(CO)3 in tetrahydrofuran.31
Although the low-temperature metalation of 3 using
(CH3CN)3Cr(CO)3 represents a feasible route to 20,32
complexation of the dimethylindenylsilane 18 was se-
lected as an alternative means of generating a chromium-
containing 1-indenylsilane. Since the barrier to [1,5]-
methyl shifts is considerably higher than that associated
with hydrogen migrations, it was anticipated that the
use of 18 as a precursor would circumvent the isomer-
ization pathways which eventually lead to the formation
of sp2-bonded silicon complexes. Using the aforemen-
tioned synthetic approach, 18 and excess Cr(CO)6 were
allowed to react in a refluxing mixture of dibutyl ether
and tetrahydrofuran for 96 h. Gratifyingly, the single
product isolated from this reaction yielded spectroscopic
data consistent with the formation of a single complex
containing the required C(1)-bonded trimethylsilyl unit.
The exo geometry of the product, depicted in Scheme 8,
was ascertained by use of X-ray crystallographic tech-
niques; the structure of 22 appears as Figure 3, while
crystallographic data and selected metrical parameters
are collected in Tables 1 and 2.
Assessin g th e Im p a ct of π-Cr (CO)3 Com p lex-
a tion . Both Ku¨ndig28 and Butenscho¨n29 and have
demonstrated that the ring opening of benzocyclobutenes
to the corresponding o-xylylene is tolerant to the pres-
ence of a coordinated η6-tricarbonylchromium moiety.
In light of the similarity between this ring-opening
process and the isomerization of indenes to the corre-
sponding isoindenes, the effect of chromium complex-
ation on the rate of [1,5]-silicon shifts in 1-indenylsilanes
was examined. The tricarbonylchromium complex of
1-(trimethylsilyl)indene (20) was initially identified as
a candidate for such dynamic NMR studies, and its
synthesis was undertaken. Using the method described
by Fischer and Kriebitzsch for the preparation of
(indene)tricarbonylchromium,30 1-(trimethylsilyl)indene
(3) and excess Cr(CO)6 were heated to reflux in a
mixture of dibutyl ether and tetrahydrofuran for 72 h.
However, the indenylchromium product isolated from
this reaction, following chromatographic purification,
was not the desired complex 20 but rather the corre-
sponding 3-indenyl isomer 21, for which [1,5]-silicon
migrations are not possible; the 1H and 13C NMR signals
attributable to the methylene group and the single
vinylic methine unit were diagnostic of the structure
depicted in Scheme 7. The generation of 21 can be
rationalized in terms of two consecutive [1,5]-hydrogen
shifts that become possible at the high temperatures
employed in the synthesis. It is not known whether
these hydrogen migrations occur in the precursor 3, in
the complex 20, or via some bimolecular chromium-
mediated reaction pathway. The last possibility is
Congestion at the quaternary C(1) center in 22 results
in an elongated C(1)-silicon bond (C(1)-Si(1) ) 1.938(7)
Å) relative to tris(1-indenyl)silane33 (C-Si(1) ) 1.896(5),
1.889(4), and 1.895(5) Å), although statistically signifi-
cant lengthening of the C(1)-Me bond in 22 (C(1)-C(11)
) 1.540(8) Å), in comparison to the other carbon-methyl
distance in this compound (C(3)-C(33) ) 1.508(9) Å)
and those found in the cycloadduct 19 (C-CH3
)
1.519(4), 1.505(4) Å), is not observed. Moreover, while
the C5 and C6 rings in 19 deviate only modestly from
planarity (0.0217 and 0.0128 Å, respectively), the plane
defined by the allylic unit (C(1)sC(2)dC(3)) intersects
the C6 plane (C(3a), C(4), C(5), C(6), C(7), and C(7a)) at
an angle of approximately 3.2°, possibly due to unfavor-
able steric interactions between the C(1)-methyl group
and the chromium fragment. Such a phenomenon may
also be responsible for the asymmetric bonding that is
observed between the chromium group and the arene
(28) Ku¨ndig, E. P.; Bernardinelli, G.; Leresche, J .; Romanens, P.
Angew. Chem., Int. Ed. Engl. 1990, 29, 407.
(29) (a) Wey, H. G.; Butenscho¨n, H. Angew. Chem., Int. Ed. Engl.
1991, 30, 880. (b) Brands, M.; Wey, H. G.; Butenscho¨n, H. J . Chem.
Soc., Chem. Commun. 1991, 1541. (c) Brands, M.; Wey, H. G.; Goddard,
R.; Butenscho¨n, H. Inorg. Chim. Acta 1994, 220, 175. (d) Brands, M.;
Wey, H. G.; Kro¨mer, R.; Kru¨ger, C.; Butenscho¨n, H. Liebigs Ann. Chem.
1995, 253. (e) Butenscho¨n, H. Synlett 1999, 680.
(31) Berno, P.; Ceccon, A.; Gambaro, A.; Venzo, A. Tetrahedron Lett.
1988, 29, 3489.
(32) Rausch, M. D.; Moser, G. A.; Zaiko, E. F.; Lipman, A. L. J .
Organomet. Chem. 1970, 23, 185.
(33) Stradiotto, M.; Brook, M. A.; McGlinchey, M. J . J . Chem. Soc.,
Perkin Trans. 2, submitted for publication.
(30) Fischer, E. O.; Kriebitzsch, N. Z. Naturforsch. 1960, 15b, 465.

596 Organometallics, Vol. 19, No. 4, 2000
Stradiotto et al.
F igu r e 3. Crystallographically determined structure of 22
(shown with 30% thermal displacement ellipsoids).
1
F igu r e 4. Indene-CH3 region of the H-1H EXSY spec-
trum of 22, acquired in tetrachloroethane-d2 at 105 °C with
a mixing time of 1.0 s.
ring. When the C6 unit is traversed from the annulated
edge (Cr(1)-C(3a) ) 2.280(7) Å, Cr(1)-C(7a) ) 2.293(6)
Å), through the centroid of the ring (Cr(1)-C(4) )
2.217(6) Å, Cr(1)-C(7) ) 2.237(8) Å), to the rear edge
(Cr(1)-C(5) ) 2.172(8) Å, Cr(1)-C(6) ) 2.205(9) Å), the
distances between the chromium unit and the arene
ring gradually contract. While this differential bonding
pattern seems to be slightly more pronounced than in
related complexes reported by Kerber and Waldbaum34
and Ustynyuk and co-workers,35 such distortions in 22
are apparently less than those found in the crystal
structure of the η6-Cr(CO)3 complex of naphthalene.36
In the crystal, the tricarbonylchromium unit in 22
adopts an approximate staggered-anti orientation (Fig-
ure 3, inset); both staggered-anti and staggered-syn
geometries have been observed in such annulated poly-
cyclic chromium complexes.36,37
Having synthesized and fully characterized the re-
quired tricarbonylchromium complex 22, we probed the
resilience of the suprafacial [1,5]-silatropic shift process
in 1-indenylsilanes. On the basis of the aforementioned
findings by both Ku¨ndig and Butenscho¨n, it is reason-
able to suggest, a priori, that the presence of the
chromium fragment in 22 could possibly facilitate the
generation of isoindenes. However, it is also evident that
the conversion of the 1-indenylsilane 22 into the iso-
indene 22-iso gives rise to what can formally be
considered a 16-electron chromium center in this inter-
mediate species. If geometric constraints exist in the
isoindene that are not present in the o-xylylene (perhaps
arising as a result of the methylene unit in the former),
then possible long-range coordination to the C5 diene38
fragment may be prohibited in the isoindene, leading
to a destabilization of the 16-electron metalloisoindene
22-iso. Such a scenario could, in principle, lead to a
reduction in the rate of [1,5]-silicon shifts in 22, relative
to the uncomplexed compound 18.
The molecular dynamics of 22 were qualitatively
examined by use of 2D-EXSY NMR techniques; a
portion of a 1H-1H EXSY spectrum acquired from a
sample of this complex is presented as Figure 4. The
off-diagonal peaks linking the C(1)-CH3 and C(3)-CH3
sites verify that [1,5]-silicon migrations continue to
occur in 22. The low-frequency shift39 of the arene H
1
NMR resonances following complexation by a Cr(CO)3
fragment is particularly fortuitous in the present case,
as the heavily overlapped aromatic 1H NMR resonances
in 18 are converted into well-resolved multiplets in 22,
allowing for the observation of distinct 2D-EXSY off-
diagonal signals that correlate the exchanging arene
ring environments in 22.
Given that [1,5] silatropic shifts are operative in 22,
single selective inversion NMR experiments were con-
ducted in order to quantify the effect of chromium
complexation on the rate of [1,5]-silicon shifts. An
analysis of the temperature-dependent rate data ob-
tained during these dynamic NMR experiments re-
vealed the barrier to silicon shifts in 22 (∆Gq ) 24 ( 1
kcal mol-1) to be indistinguishable from the uncom-
plexed silane 18 (∆Gq ≈ 23 kcal mol-1). The fact that
the presence of the chromium fragment served neither
to increase nor to diminish the rate of [1,5]-silicon shifts
was unexpected, and as such, the behavior of the
Cr(CO)3 unit throughout the migratory process was
reexamined.
(34) Kerber, R. C.; Waldbaum, B. Organometallics 1995, 14, 4742.
(35) Nesmeyanov, A. N.; Ustynyuk, N. A.; Novikova, L. N.; Andri-
anov, V. G.; Struchkov, Y. T.; Ustynyuk, Y. A.; Oprunenko, Y. F.;
Luzikov, Y. N. J . Organomet. Chem. 1982, 226, 239.
(36) Kunz, V.; Nowacki, W. Helv. Chim. Acta 1967, 50, 1052.
(37) (a) Muetterties, E. L.; Bleeke, J . R.; Wucherer, E. J .; Albright,
T. A. Chem. Rev. 1982, 82, 499. (b) Wey, H. G.; Betz, P.; Topalovic, I.;
Butenscho¨n, H. J . Organomet. Chem. 1991, 411, 369. (c) Ku¨ndig, E.
P.; Grivet, C.; Wenger, E.; Bernardinelli, G. Helv. Chim. Acta 1991,
74, 2009. (d) Wey, H. G.; Betz, P.; Butenscho¨n, H. Chem. Ber. 1991,
124, 465.
During the course of this investigation, a report was
published by Ustynyuk and co-workers which is of
considerable relevance to the molecular dynamics of
22.40 These workers noted that, upon warming, (σ-
methyl)(η5-indenyl)tricarbonylchromium (24) is quan-
titatively converted into (η6-1-endo-methylindene)-
(39) Mingos, D. M. P. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press:
Toronto, 1982; Vol. 3, p 1.
(38) Yalpani, M.; Benn, R.; Goddard, R.; Wilke, G. J . Organomet.
Chem. 1982, 240, 49.

[1,5]-Silicon Shifts in Indenylsilanes
Organometallics, Vol. 19, No. 4, 2000 597
Sch em e 9. “Ricoch et” In ter -r in g Ha p totr op ic Rea r r a n gem en t of 24a
a
The numbering in the isoindene 26 is carried over from 25.
Ta ble 3. Com p u ta tion a lly a n d Exp er im en ta lly
Deter m in ed Bon d Len gth s (Å) for
In d en ylch r om iu m Com p lexes
Sch em e 10. Ch r om iu m -Rh od iu m Bim eta llic
In d en yl Com p lexes 28-30 (NBD ) Nor bor n a d ien e;
COD ) 1,5-Cycloocta d ien e) a n d a P ictor ia l
Rep r esen ta tion of th e Electr on Sh a r in g in Th ese
Com p ou n d s
26a
28b
29b
30c
Cr-C(3a)
Cr-C(7a)
Cr-C(4)
Cr-C(7)
Cr-C(5)
Cr-C(6)
2.440
2.417
2.247
2.230
2.209
2.206
2.375(4)
2.401(4)
2.239(4)
2.257(4)
2.181(5)
2.188(4)
2.448(3)
2.448(3)
2.258(3)
2.271(3)
2.208(4)
2.197(5)
2.434(7)
2.464(6)
2.255(7)
2.267(5)
2.194(6)
2.189(5)
a
b
DFT calculations.40 X-ray diffraction data.41 c X-ray diffrac-
tion data.42
tricarbonylchromium (25) and ultimately the thermody-
namically favored C(3)-methyl isomer 27 (Scheme 9).
Mechanistic aspects of these rearrangements were
examined by use of density functional theory (DFT)
calculations, which revealed that the conversion of 25
into 27 occurs via the isoindene intermediate 26. Once
the chromium-bonded methyl fragment in 24 is deliv-
ered to the indenyl C(1) position, the Cr(CO)3 group
migrates to the C6 ring rapidly and irreversibly; thus,
during the hydrogen migratory process which takes 25
via 26 into 27, the Cr(CO)3 fragment remains bonded
to the C6 ring of the indene skeleton. This hydrogen
migratory process is qualitatively similar to the conver-
sion of the metalated 1-indenylsilane 20 into the 3-in-
denylsilane 21 (Scheme 7). The energy-minimized struc-
ture provided by Ustynyuk and co-workers for 26
contains bond lengths (Table 3) that are indicative of
an η4-Cr(CO)3 fragment, especially when compared to
the metrical parameters associated with the crystallo-
graphically characterized η6-bonded indenylsilane 22;
short distances are observed between the chromium
center and C(4), C(5), C(6), and C(7) in 26, while the
bonds to C(3a) and C(7a) are considerably longer.
Experimental support for the viability of complexes
possessing such geometries has also been provided by
Ceccon and various co-workers, who have crystallo-
graphically characterized a series of bimetallic indenyl
complexes, including 28-30 (Scheme 10).41,42 In these
compounds, the chromium and rhodium moieties share
the same face of the indenyl ligand, which forces these
fragments toward the periphery of the C6 and C5 rings,
respectively. In fact, the solid-state structures of these
stable molecules exhibit approximate η4-η3 configura-
tions, despite the fact that the chromium and rhodium
fragments in these molecules preferentially coordinate
to indene via η6 and η5 bonding modes, respectively, in
Ch a r t 3
the absence of the second organometallic unit (31 and
32 in Scheme 10).
In light of the computational and crystallographic
results presented above, it appears that although a 16-
electron chromium intermediate (22-iso) can formally
be invoked during the interconversion of the 18-electron
species 22-S and 22-R, the energy cost associated with
this process is not sufficiently large as to lead to a
statistically significant increase in the barrier to [1,5]-
silicon shifts, relative to the unmetalated compound 18.
In effect, the chromium fragment in this system can be
classified as a spectator group with respect to the
operation of suprafacial [1,5]-silicon shifts. As a result,
the dynamic behavior of the η6-Cr(CO)3-containing η1-
indenyl complex of iridium 33 (Chart 3) need not be
rationalized in terms of symmetry-forbidden [1,3]-shifts,
as originally suggested by Ceccon et al.43 It is plausible
that this mechanistic pathway was proposed by these
workers so as to avoid invoking the intermediacy of a
high-energy η4-Cr(CO)3-complexed metalloisoindene and
because of the presumed static nature of (η5-C5H5)Fe-
(CO)2(η1-C9H7) (34).44 However, in light of the fact that
[1,5]-iron shifts in 34 have recently been detected,6c it
is possible to envision an analogous symmetry-allowed
[1,5]-iridium shift process involving 33.
The assertion that the Cr(CO)3 unit remains fixed to
the C6 ring during the interconversion of 22-S and 22-R
(40) Trifonova, O. I.; Ochertyanova, E. A.; Akhmedov, N. G.;
Roznyatovsky, V. A.; Laikov, D. N.; Ustynyuk, N. A.; Ustynyuk, Y. A.
Inorg. Chim. Acta 1998, 280, 328.
(41) Bonifaci, C.; Ceccon, A.; Gambaro, A.; Ganis, P.; Santi, S.; Valle,
G.; Venzo, A. Organometallics 1993, 12, 4211.
(42) Bonifaci, C.; Ceccon, A.; Gambaro, A.; Ganis, P.; Santi, S.; Valle,
G.; Venzo, A. J . Organomet. Chem. 1995, 492, 35.
(43) Cecchetto, P.; Ceccon, A.; Gambaro, A.; Santi, S.; Ganis, P.;
Gobetto, R.; Valle, G.; Venzo, A. Organometallics 1998, 17, 752.
(44) (a) Bennett, M. J .; Cotton, F. A.; Davison, A.; Faller, J . W.;
Lippard, S. J .; Morehouse, S. M. J . Am. Chem. Soc. 1966, 88, 4371.
(b) Cotton, F. A.; Musco, A.; Yagupsky, G. J . Am. Chem. Soc. 1967,
89, 6136.

598 Organometallics, Vol. 19, No. 4, 2000
Stradiotto et al.
suggests that the C5 diene unit in the purported
intermediate, 22-iso, should be available to participate
in cycloaddition reactions. In an attempt to intercept
this intermediate, pale yellow solutions of 22 and
tetracyanoethylene were combined, generating a dark
blue mixture typically observed during these trapping
reactions involving TCNE. However, in contrast to the
successful Diels-Alder trapping of 18-iso, the reaction
involving 22 yielded a dark solid, from which only a
small amount (approximately 10%, based on 22) of the
uncomplexed cycloadduct 19 could be extracted. The
deeply colored solution formed upon addition of TCNE
to a solution containing a metalloindene is consistent
with the formation of a charge-transfer (CT) complex;
arene45 and (arene)tricarbonylchromium,46,47 CT com-
plexes with TCNE are well-documented. Although the
significance of these CT complexes as precursors to the
isolated isoindene cycloadducts is unclear, it is possible
that, in the case of the chromium complexes 22 and 22-
iso, CT makes available reaction pathways which result
in decomposition. Ku¨ndig and co-workers have recently
reported that the addition of an olefinic compound
capable of η4-coordination, such as methyl acrylate, to
(benzene)tricarbonylchromium leads to rapid cleavage
of the arene-metal linkage under mild conditions.48
Since (arene)tricarbonylchromium compounds are known
to react with nitriles,49,50 it is feasible that TCNE may
also serve to promote demetalation of the indenyl ligand
in 22-iso, via addition to what can be described as a
coordinatively and electronically unsaturated chromium
center in this molecule. The isolation of 19 as a side
product in reactions involving 22 is in keeping with the
occurrence of a decomplexation event. Finally, it is also
possible to argue that the formation of 23 is disfavored
on steric grounds, since the generation of such a
cycloadduct possessing anti-disposed silyl and dieno-
philic fragments would require that TCNE add to the
isoindene 22-iso on the face occupied by the bulky
Cr(CO)3 unit. Although chromium complexes similar to
the desired adduct 23 have been prepared51,52 and
crystallographically characterized,53 the generation of
these species typically involves the introduction of a
chromium fragment to a preformed benzonorbornene.
indenyl skeleton and at the migrating silicon center.1,6
In the present study, the robustness of this silatropic
shift phenomenon in indenylsilanes was revealed by 2D-
EXSY experiments, which exposed the quasi-fluxional
character of both the methylated indenylsilane 18 and
the chromium complex 22. Moreover, kinetic data
obtained from 1D-selective inversion NMR experiments
revealed migratory barriers for 18 and 22 which do not
deviate significantly from the parent species, 1-(tri-
methylsilyl)indene (3). In the case of 18, Diels-Alder
trapping with tetracyanoethylene yielded experimental
evidence in support of the intermediacy of isoindenes
in these rearrangements; crystallographic data were
obtained for the cycloadduct 19.
During the pursuit of a sterically loaded indene, 7,
the conjugated dimeric compound 9 was unexpectedly
generated; while a mechanism has been proposed in an
attempt to rationalize the formation of this compound,
conclusive experimental evidence is still lacking. The
processes leading to 9 and the ensuing chemistry of this
compound are of considerable interest and will be the
focus of future study.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All reactions were carried out
under an atmosphere of dry nitrogen in oven-dried glassware,
using freshly distilled solvents that were purified according
to standard procedures. Unless otherwise stated, reagents
were obtained from Aldrich Chemical Co. and used as received;
hexacarbonylchromium was purchased from Strem Chemical
and deuterated NMR solvents were obtained from Cambridge
Isotope Laboratories and used as received. Mass spectrometric
data were obtained using a Finnigan 4500 spectrometer by
direct electron impact (DEI) or by chemical ionization (CI) with
NH3. Infrared spectra were obtained on a Bio-Rad FTS-40 FT-
IR spectrometer. Melting points were measured in open glass
capillaries using a Thomas-Hoover Unimelt capillary melting
point apparatus. Microanalyses were performed by Guelph
Chemical Laboratories (Guelph, Ontario, Canada). One-
dimensional NMR spectroscopic data were recorded using a
Bruker AC-200, Bruker AC-300, or Bruker Avance DRX-500
spectrometer. In all cases, NMR spectra were obtained from
samples dissolved in deuterated solvents, with NMR signals
referenced internally to tetramethylsilane. Two-dimensional
NMR experiments were conducted on the DRX-500 instru-
ment, equipped with an 11.74 T superconducting magnet, a
Bruker B-VT 2000 temperature controller, and a 5 mm
multinuclear inverse probe possessing three-axis gradient
capabilities. These consisted of 1H-1H COSY, 1H-13C HSQC,
1H-13C HMBC, 1H-1H EXSY, and 1H-29Si HMBC experi-
ments. Sample temperatures were set and maintained through-
out the course of these studies by use of the temperature
controller and externally calibrated by placing a copper-
constantan thermocouple, contained within an NMR tube, into
the probe. Using this procedure, absolute temperatures were
measured and maintained within (0.5 °C.
Con clu sion s
In comparison to complexes comprising group 13 or
group 15 elements, the role of substituents in determin-
ing the rate of [1,5]-metalloid shifts in group 14 cyclo-
pentadienyls is considerably less important;7b a similar
scenario has emerged for the indenylsilane series.
Indeed, the suprafacial [1,5]-silicon shift process in these
compounds has demonstrated remarkable tolerance to
organic and organometallic derivatization, both on the
1H-1H EXSY (Exch a n ge Sp ectr oscop y) Exp er im en ts.
Two-dimensional 1H-1H EXSY10 spectra were recorded in the
phase-sensitive mode, using the gradient implementation of
the NOESY (90°-t1-90°-tm-90°-ACQ) pulse sequence. In a
typical experiment, 512 FID’s were recorded in the f2 dimen-
sion, with each FID acquired in 16 scans. These FID data were
subsequently Fourier-transformed using Gaussian window
functions in both f1 and f2, with line broadening set to 3.0 Hz.
For both 18 and 22, tetrachloroethane-d2 was employed as the
solvent, and experiments were conducted at 105 °C, with
mixing times ranging from 0.3 to 1.5 s; the relaxation delay
and the initial value for the 2D evolution were set to 1.0 s
and 10 µs, respectively.
(45) Fatiadi, A. J . Synthesis 1986, 249.
(46) Sennikov, P. G.; Kuznetsov, V. A.; Egorochkin, A. N.; Sirotkin,
N. I.; Nazarova, R. G.; Razuvaev, G. A. J . Organomet. Chem. 1980,
190, 167.
(47) Fatiadi, A. J . Synthesis 1987, 959.
(48) Ku¨ndig, E. P.; Kondratenko, M.; Romanens, P. Angew. Chem.,
Int. Ed. 1998, 37, 3146.
(49) Guttenberger, J .-F.; Strohmeier, W. Chem. Ber. 1967, 100, 2807.
(50) Sneeden, R. P. A. Organochromium Compounds; Academic
Press: New York, 1975; p 193.
(51) Wege, D.; Wilkinson, S. P. Aust. J . Chem. 1973, 26, 1751.
(52) Bly, R. S.; Maier, T. L. J . Org. Chem. 1978, 43, 614.
(53) Merkert, J . W.; Geiger, W. E.; Paddon-Row, M. N.; Oliver, A.
M.; Rheingold, A. L. Organometallics 1992, 11, 4109.

[1,5]-Silicon Shifts in Indenylsilanes
Organometallics, Vol. 19, No. 4, 2000 599
Sin gle Selective In ver sion NMR Exp er im en ts. All
kinetic data were obtained in the slow-exchange regime by use
of 1H selective inversion NMR experiments,27 in which the low-
frequency methyl resonance was perturbed using a 90°-τ-
90° pulse sequence (τ ) 1/2∆ν), and then the return to
equilibrium of this and the other methyl signal involved in
the chemical exchange process was monitored as a function
of time, as in an inversion-recovery T1 experiment. These
experiments were conducted on a Bruker AC-300 spectrometer,
equipped with a 7.65 T superconducting magnet, a Bruker
B-VT 2000 temperature controller, and a 5 mm QNP probe;
spectral data were obtained between 95 and 110 °C. Nonlinear
least-squares fitting of the experimental data was performed
using a C programming language version of the SIFIT program
provided by McClung,27c which permitted the extraction of rate
data. Analysis of these temperature-dependent rates using
Eyring theory allowed for an estimate of the free energy of
activation associated with the dynamic process; in both cases
∆Gq is reported at 303 K.
X-r a y Cr ysta llogr a p h y. X-ray crystallographic data for 19
and 22 were collected from single-crystal samples, which were
mounted on a glass fiber. Single-crystal diffraction experiments
were carried out using a P4 Siemens diffractometer, equipped
with a Siemens SMART 1K charge-coupled device (CCD) area
detector (using the program SMART) and a rotating anode
using graphite-monochromated Mo KR radiation (λ ) 0.710 73
Å). The crystal-to-detector distance was 3.991 cm, and the data
collection was carried out in 512 × 512 pixel mode, utilizing 2
× 2 pixel binning. The initial unit cell parameters were
determined by a least-squares fit of the angular settings of
the strong reflections, collected by a 4.5° scan in 15 frames
over three different parts of reciprocal space (45 frames total).
One complete hemisphere of data was collected, to better than
0.8 Å resolution. Upon completion of the data collection, the
first 50 frames were re-collected in order to improve the decay
corrections analysis. Processing was carried out by use of the
program SAINT, which applied Lorentz and polarization
corrections to three-dimensionally integrated diffraction spots.
The program SADABS was utilized for the scaling of diffrac-
tion data, the application of a decay correction, and an
empirical absorption correction based on redundant reflections.
Structures were solved by using the direct methods procedure
in the Siemens SHELXTL program library, and refinement
was carried out by using full-matrix least-squares methods
with anisotropic thermal parameters for all non-hydrogen
atoms. Hydrogen atoms were added at calculated positions and
refined using a riding model with isotropic displacement
parameters equal to 1.2 times the equivalent isotropic dis-
placement parameter of the attached carbon.54
136.3, 128.6, 125.0, 123.8, 123.6, 123.3, 122.5, 121.5, 121.3
(vinyl and aromatic); mass spectra (DEI, m/z (%)) 260 (21,
[M]+), 245 (38, [M - CH3]+), 73 (100, [Si(CH3)3]+); mass spectra
12
(high resolution, DEI) calculated mass for
260.1417 amu, observed mass 260.1430 amu.
C
H24Si2 (M)+
15
Bis(tr im eth ylsilyl)d iben zo[a ,d ]fu lva len e (9). Meth od
a . In an effort to synthesize 1,1,3-tris(trimethylsilyl)indene (7),
n-butyllithium (43.1 mL of a 1.6 M hexane solution, 68 mmol)
was added dropwise over a 1 h period to a solution of indene
(4.0 g, 34 mmol) in freshly distilled diethyl ether (100 mL) at
-78 °C. After the addition of chlorotrimethylsilane (7.5 g, 68
mmol), the solution was stirred for 1 h and additional n-
butyllithium (21.5 mL, 34 mmol) and chlorotrimethylsilane
(3.8 g, 34 mmol) were added. The mixture was then warmed
to room temperature and stirred for an additional 18 h.
Extraction of the resulting opaque milky solution with water
(3 × 100 mL) gave rise to a clear, bright red organic phase,
which was dried over anhydrous MgSO4; alternatively, the
lithium salts can be removed directly from the crude reaction
mixture by filtration. After removal of the solvent, the organic
residue was subjected to flash chromatography on silica gel.
Elution with hexanes gave 1,1- and 1,3-bis(trimethylsilyl)-
indene (identified by comparison to an authentic sample; see
5 and 6) as a yellow oil (4.12 g, 15.9 mmol, 46%, based on
indene). Subsequent elution with hexanes/dichloromethane (5:
1) yielded 9 as a bright red powder, mp 129-130 °C (0.38 g,
1.0 mmol, approximately 6% isolated yield based on indene;
in analogous experiments, yields of 9 ranged between 5 and
10%).
Meth od b. To a mixture of 5 and 6 (4.51 g, 17.3 mmol) in
freshly distilled diethyl ether (120 mL) at -78 °C was added
n-butyllithium (10.8 mL of a 1.6 M hexane solution, 17.3 mmol)
dropwise over a 1 h period, followed by the addition of
chlorotrimethylsilane (1.88 g, 17.3 mmol). The mixture was
then warmed to room temperature and was stirred for an
additional 18 h. Following extraction with water (3 × 100 mL)
the organic phase was dried over anhydrous MgSO4, the
solvent was removed, and the organic residue was subjected
to flash chromatography on silica gel. Compound 9 was
isolated by elution with hexanes/dichloromethane (5:1) and
isolated in yields ranging from 5 to 10%. Data for 9: 1H NMR
(CDCl3, 500 MHz) δ 7.93 (d, 3J ) 7.4 Hz, 2H, H(4)), 7.54 (s,
3
2H, H(2)), 7.36 (d, J ) 7.3 Hz, 2H, H(7)), 7.24 (m, 2H, H(6)),
7.19 (m, 2H, H(5)), 0.36 (s, 18H, Si(CH3)3); 13C NMR (CDCl3,
125 MHz) δ 150.7 (C(1)), 147.2 (C(7a)), 141.1 (C(3)), 136.1
(C(2)), 138.0 (C(3a)), 128.3 (C(6)), 125.4 (C(5)), 125.2 (C(4)),
122.7 (C(7)), -1.1 (Si(CH3)3); 29Si NMR (CDCl3, 99.35 MHz) δ
-9.0; IR (CDCl3, cm-1) 3155.7 s, 1469.7 m, 1382.3 m; mass
spectra (DEI, m/z (%)) 372 (35, [M]+), 357 (17, [M - CH3]+),
1,1- a n d 1,3-Bis(tr im eth ylsilyl)in d en e (5 a n d 6). These
compounds, which exist as a set of interconverting isomers,
were prepared using previously reported methods.1c Data for
5: 1H NMR (CDCl3, 500 MHz) δ 7.12 (d, 3J ) 5.2 Hz, 1H, H(2)),
6.81 (d, 3J ) 5.2 Hz, 1H, H(3)), 0.04 (s, 18H, Si(CH3)3); 13C
NMR (CDCl3, 125 MHz) δ 47.1 (C(1)), -0.8 (CH3); 29Si NMR
(CDCl3, 99.35 MHz) δ 0.5. Data for 6: 1H NMR (CDCl3, 500
299 (14, [M - Si(CH3)3]+), 73 (100, [Si(CH3)3]+); mass spectra
12
(high resolution, DEI) calculated mass for
C
H28Si2 (M)+
24
372.1729 amu, observed mass 372.1736 amu. Anal. Calcd for
24H28Si2: C, 77.38; H, 7.58. Found: C, 77.42; H, 7.55. A
C
crystalline sample of this material, produced by slow growth
from a mixture of dichloromethane and acetone (1:1), was
mounted on a glass fiber and studied by single-crystal X-ray
diffraction. The following unit cell parameters were deter-
mined: a ) 7.080(1) Å, b ) 17.678(4) Å, c ) 8.955(2) Å, R )
90.000°, â ) 90.090(3)° and γ ) 90.000°. Subsequent refine-
ment in a monoclinic space group (P21/c) revealed the existence
of two independent half-molecules per asymmetric unit. How-
ever, in the latter stages of refinement, a disorder problem
prevented the structure from being refined to an acceptable
level (R1 > 35%). As a result, a discussion of the metrical
parameters associated with 9 is not presented. Some of the
difficulties in refining the structure (including the appearance
of nonpositive-definite atoms) may be a consequence of a
pseudo-orthorhombic twinning process; attempts to solve and
refine the structure of 9 based on an orthorhombic unit cell
were also unsuccessful. Such a disorder does appear plausible,
3
3
MHz) δ 6.97 (d, J ) 1.7 Hz, 1H, H(2)), 3.68 (d, J ) 5.2 Hz,
1H, H(1)), 0.44 (s, 9H, Si(CH3)3 at C(3)), 0.03 (s, 9H, Si(CH3)3
at C(1)); 13C NMR (CDCl3, 125 MHz) δ 49.2 (C(1)), -0.5
(Si(CH3)3 at C(1)), -2.3 (Si(CH3)3 at C(3)); 29Si NMR (CDCl3,
99.35 MHz) δ 1.6, -9.7. Data for 5 and 6: 1H NMR (CDCl3,
500 MHz) δ 7.67-7.24 (m, 8H, aromatic); 13C NMR (CDCl3,
125 MHz) δ 147.9, 146.9, 146.1, 145.8, 145.1, 141.8, 138.5,
(54) (a) Sheldrick, G. M. SMART, Release 4.05; Siemens Energy and
Automation, Inc., Madison, WI 53719, 1996. (b) Sheldrick, G. M. SAINT
1996, Release 4.05; Siemens Energy And Automation Inc., Madison,
WI 53719. (c) Sheldrick, G. M. SADABS (Siemens area detector
absorption corrections); Siemens Crystallographic Research Systems,
Madison, WI 53719, 1996. (d) Sheldrick, G. M. Siemens SHELXTL,
Version 5.03; Siemens Crystallographic Research Systems, Madison,
WI 53719, 1994.

600 Organometallics, Vol. 19, No. 4, 2000
Stradiotto et al.
in that a decrease in the â angle of only 0.1° would give rise
to an orthorhombic cell of higher crystallographic symmetry.
Si(CH3)3). 1H NMR (Cl2DCCDCl2, 500 MHz): δ 7.33-7.21 (m,
4H, H(4), H(5), H(6), and H(7)), 6.13 (s, 1H, H(2)), 2.16 (s, 3H,
C(3)-CH3), 1.41 (s, 3H, C(1)-CH3), -0.19 (s, 9H, Si(CH3)3).
13C NMR (CDCl3, 125 MHz): δ 151.0 (C(3a) or C(7a)), 144.6
(C(7a) or C(3a)), 137.7 (C(2)), 135.8 (C(3)), 124.7, 123.7, 121.7,
118.9 (C(4), C(5), C(6), and C(7)), 46.5 (C(1)), 16.0 (C(1)-CH3),
12.7 (C(3)-CH3), -3.56 (Si(CH3)3). 29Si NMR (CDCl3, 99.35
MHz): δ -0.1. MS (DEI, m/z (%)): 216 (61, [M]+), 201 (48, [M
- CH3]+), 143 (32, [M - Si(CH3)3]+), 73 (100, [Si(CH3)3]+). MS
(CI, NH3, m/z (%)): 217 (100, [M + H]+).
3,3′-Bis(1-tr im eth ylsilylin d en e) (16). A solution of 3,3′-
biindene (14)55 and/or 1-(indanylidene)-1-indene (15)56 (0.300
g, 1.3 mmol) in freshly distilled tetrahydrofuran (30 mL) was
cooled to -78 °C, and n-butyllithium (2.2 mL of a 1.3 M hexane
solution, 2.9 mmol) was added dropwise over a 1 h period. After
the mixture was stirred for an additional 3 h at ambient
temperature, chlorotrimethylsilane (0.45 g, 4 mmol) was added
at -78 °C, and the mixture was stirred while being slowly
warmed to ambient temperature over 18 h. The residue
obtained after removal of the solvent was extracted with
hexanes, and the insoluble lithium salts were separated by
filtration. The filtrate was then concentrated and the residue
subjected to flash chromatography on silica gel using hexanes,
yielding 16 (a meso and d,l mixture) as a viscous yellow oil
(0.200 g, 0.53 mmol, 41%). On the basis of the intensities of
the observed NMR signals, it is evident that one isomer (either
meso or d,l) predominates (∼1:1.3). Major isomer: 1H NMR
(CDCl3, 500 MHz) δ 7.62 (d, J ) 7.5 Hz, 2H, H(4)), 7.51 (d, J
) 7.5 Hz, 2H, H(7)), 7.30-7.22 (m, 4H, H(5) and H(6)), 6.85
(d, J ) 1.8 Hz, 2H, H(2)), 3.69 (d, J ) 1.8 Hz, 2H, H(1)), 0.02
(s, 18H, Si(CH3)3); 13C NMR (CDCl3, 125 MHz) δ 146.0 (C(7a)),
143.3 (C(3a)), 136.3 (C(3)), 132.7 (C(2)), 124.7 (C(6)), 123.9
(C(5)), 122.9 (C(7)), 120.5 (C(4)), 45.9 (C(1)), -2.4 (Si(CH3)3);
29Si NMR (CDCl3, 99.35 MHz) δ 4.7. Minor isomer: 1H NMR
(CDCl3, 500 MHz) δ 7.54 (d, J ) 7.1 Hz, 2H, H(4)), 7.49-7.26
(m, 6H, H(5), H(6), and H(7)), 6.80 (d, J ) 1.8 Hz, 2H, H(2)),
3.66 (d, J ) 1.8 Hz, 2H, H(1)), 0.03 (s, 18H, Si(CH3)3); 13C NMR
(CDCl3, 125 MHz) δ 146.1 (C(7a)), 143.6 (C(3a)), 136.8 (C(3)),
132.8 (C(2)), 124.7 (C(6)), 123.9 (C(5)), 122.8 (C(7)), 120.7 (C(4)),
45.8 (C(1)), -2.3 (Si(CH3)3); 29Si NMR (CDCl3, 99.35 MHz) δ
4.5. Mass spectra (DEI, m/z (%)): 374 (12, [M]+), 359 (16, [M
5,6-Ben zo-2,2,3,3-t et r a cya n o-1,4-d im et h yl-7-(t r im et h -
ylsilyl)bicyclo(2.2.1)h ep t-5-en e (19). The addition of tetra-
cyanoethylene (0.64 g, 5.0 mmol) to a solution of 18 (1.0 g, 4.6
mmol) in acetonitrile (50 mL) produced a dark blue solution;
the dark coloration subsided after 72 h. The residue obtained
after removal of the solvent was washed with hexane (1 × 25
mL) and diethyl ether (3 × 15 mL) and then dried to give 19
as a beige powder (1.32 g, 3.8 mmol, 83%); mp 214-215 °C
(with decomposition). Anal. Calcd for C20H20N4Si: C, 69.74;
H, 5.86; N 16.28. Found: C, 69.56; H, 5.91; N, 16.31. 1H NMR
(CD3CN, 500 MHz): δ 7.51-7.48 (m, 2H, H(8), H(11)), 7.47-
7.44 (m, 2H, H(9), H(10)), 2.01 (s, 1H, H(7)), 1.96 (s, 6H,
1
C-CH3), -0.23 (s, 9H, Si(CH3)3). H NMR (CDCl3, 500 MHz):
δ 7.50-7.44 (m, 2H, H(8), H(11)), 7.41-7.35 (m, 2H, H(9),
H(10)), 1.90 (s, 1H, H(7)), 1.96 (s, 6H, C-CH3), -0.20 (s, 9H,
Si(CH3)3). 13C NMR (CD3CN, 125 MHz): δ 143.2 (C(5), C(6)),
130.5 (C(9), C(10)), 123.6 (C(8), C(11)), 113.3 (pseudoequatorial
nitriles), 111.9 (pseudoaxial nitriles), 65.9 (C(1), C(4)), 57.6
(C(2), C(3), and C(7)), 15.5 (C-CH3), -0.93 (Si(CH3)3). 13C NMR
(CDCl3, 125 MHz): δ 141.6 (C(5), C(6)), 130.2 (C(9), C(10)),
122.5 (C(8), C(11)), 111.9 (pseudoequatorial nitriles), 110.3
(pseudoaxial nitriles), 65.3 (C(1), C(4)), 57.0 (C(2) and C(3) or
C(7)), 55.5 (C(7) or C(2) and C(3)), 15.2 (C-CH3), -0.87
(Si(CH3)3). 29Si NMR (CH2Cl2, 99.36 MHz): δ 2.4. Mass
spectrum (DEI, m/z (%)): 343 (6, [M - H]+), 329 (3, [M -
CH3]+), 270 (11, [M - Si(CH3)3]+), 216 (23, [M - C6N4]+), 128
(17, C6N4]+), 73 (100, [Si(CH3)3]+). MS (CI, NH3, m/z (%)): 362
(16, [M + NH4]+), 90 (42, [Si(CH3)3 + NH4]+). A sample suitable
for structural determination (0.38 × 0.25 × 0.20 mm3) by
single-crystal X-ray diffraction was obtained by recrystalliza-
tion from acetonitrile.
- Si(CH3)3]+), 73 (100, [Si(CH3)3]+). Mass spectra (high resolu-
12
tion, DEI): calculated mass for
observed mass 374.1884 amu.
C
H30Si2 (M)+ 374.1886 amu,
24
Oxid a tion of 16. On the basis of methods previously
described in the literature,18,19 a solution of 3,3′-bis(1-tri-
methylsilyl)indene (16; 0.082 g, 0.22 mmol) in freshly distilled
tetrahydrofuran (30 mL) was cooled to -78 °C, and n-
butyllithium (0.4 mL of a 1.3 M hexane solution, 0.5 mmol)
was added dropwise over a 1 h period. After the mixture was
stirred for an additional 3 h at ambient temperature, iodine
(0.110 g, 0.4 mmol) was added at -78 °C, and this mixture
was stirred while being slowly warmed to ambient tempera-
ture over 18 h. After removal of the tetrahydrofuran solvent,
the residue was subjected to flash chromatography on silica
gel using pentane, yielding 9 (0.055 g, 0.51 mmol, 68%).
(η6-3-(Tr im eth ylsilyl)in den e)tr icar bon ylch r om iu m (21).
To a solution of 1-(trimethylsilyl)indene (3; 1.5 g, 7.98 mmol)
in n-butyl ether (200 mL) was added freshly distilled tetrahy-
drofuran (20 mL) and hexacarbonylchromium (3.51 g, 15.95
mmol) under nitrogen. The mixture was then heated at reflux
for 72 h and the solvents removed under vacuum. The residue
obtained was subjected to flash chromatography on silica gel
using hexanes, yielding 21 as a bright yellow solid (1.71 g, 5.28
mmol, 66%); mp 94-95 °C. Anal. Calcd for C15H16SiCrO3: C,
55.55; H, 4.98. Found: C, 55.63; H, 4.83. IR (KBr, cm-1): νCO
1959.1, 1876.1. 1H NMR (CDCl3, 500 MHz): δ 6.66 (m, 1H,
H(2)), 5.29-5.13 (m, 2H, H(4), H(7)), 5.80-5.71 (m, 2H, H(5),
H(6)), 3.49 (m, 2H, CH2), 0.19 (s, 9H, Si(CH3)3). 13C NMR
(CDCl3, 125 MHz): δ 228.4 (CO), 146.5 (C(2)), 137.4 (C(3)),
91.0, 90.1, 89.5, 88.4 (C(4), C(5), C(6), and C(7)), 42.4 (C(1)),
-1.6 (Si(CH3)3). 29Si NMR (CH2Cl2, 99.35 MHz): δ -8.34. MS
(DEI, m/z (%)): 324 (22, [M]+), 268 (17, [M - 2(CO)]+), 240
1,3-Dim eth yl-1-(tr im eth ylsilyl)in d en e (18). The synthe-
sis of this compound has recently been reported in the
literature.26 1,3-Dimethylindene (3.3 g, 22.9 mmol) in freshly
distilled tetrahydrofuran (150 mL) was cooled to -78 °C, and
n-butyllithium (15 mL of a 1.6 M hexane solution, 24 mmol)
was added dropwise over 1 h, after which the mixture was
stirred and warmed to ambient temperature over 2 h. Chloro-
trimethylsilane (3.8 g, 35.2 mmol) was then added dropwise
at -78 °C over 2 h, and the mixture was warmed to room
temperature over 18 h followed by refluxing for an additional
6 h. The solvent was then removed and the residue extracted
using diethyl ether (3 × 75 mL). Subsequent purification of
the extracted material by flash chromatography on silica gel
using hexanes yielded 18 as a viscous yellow oil (4.4 g, 20.3
mmol, 89%). Anal. Calcd for C14H20Si: C, 77.73; H, 9.33.
Found: C, 77.68; H, 9.03. 1H NMR (CDCl3, 500 MHz): δ 7.44-
7.27 (m, 4H, H(4), H(5), H(6), and H(7)), 6.26 (s, 1H, H(2)),
2.28 (s, 3H, C(3)-CH3), 1.53 (s, 3H, C(1)-CH3), -0.06 (s, 9H,
(82, [M - 3(CO)]+), 73 (100, [Si(CH3)3]+). HRMS (DEI): calcd
12
for mass
amu.
C
H16SiCrO3 ([M]+) 324.0274 amu, found 324.0279
15
(η6-1,3-Dim et h yl-1-exo-(t r im et h ylsilyl)in d en e)t r ica r -
bon ylch r om iu m (22). To a solution of 18 (1.3 g, 6.0 mmol)
in n-butyl ether (100 mL) was added freshly distilled tetrahy-
drofuran (10 mL) and hexacarbonylchromium (2.5 g, 11.4). The
mixture was heated at reflux for 96 h, the solvent removed
under vacuum, and the residue subjected to flash chromatog-
raphy on silica gel. Elution using a mixture of hexanes and
dichloromethane (5:1) yielded 22 as a bright yellow solid (0.66
(55) Nicolet, P.; Sanchez, J .-Y.; Benaboura, A.; Abadie, M. J . M.
Synthesis 1987, 202.
(56) Majerus, G.; Yax, E.; Ourisson, G. Bull. Soc. Chim. Fr. 1967,
4147.
g, 1.9 mmol, 32%); mp 128-130 °C. Anal. Calcd for C17H20
-
SiCrO3: C, 57.94; H, 5.73. Found: C, 58.02; H, 5.61. IR (KBr,

[1,5]-Silicon Shifts in Indenylsilanes
Organometallics, Vol. 19, No. 4, 2000 601
cm-1): νCO 1956.6, 1879.3. 1H NMR (CDCl3, 500 MHz): δ 6.23
(s, 1H, H(2)), 5.78 (d, J ) 6.0 Hz, 1H, H(4) or H(7)), 5.48 (d, J
) 5.8 Hz, 1H, H(7) or H(4)), 5.42 (m, 1H, H(5) or H(6)), 5.01
(m, 1H, H(6) or H(5)), 2.06 (s, 3H, C(3)-CH3), 1.43 (s, 3H,
C(1)-CH3), -0.11 (s, 9H, Si(CH3)3). 1H NMR (Cl2DCCDCl2, 500
MHz): δ 6.24 (s, 1H, H(2)), 5.76 (d, J ) 6.0 Hz, 1H, H(4) or
H(7)), 5.51 (d, J ) 5.8 Hz, 1H, H(7) or H(4)), 5.38 (m, 1H, H(5)
or H(6)), 5.03 (m, 1H, H(6) or H(5)), 2.08 (s, 3H, C(3)-CH3),
1.47 (s, 3H, C(1)-CH3), -0.07 (s, 9H, Si(CH3)3). 13C NMR
(CDCl3, 125 MHz): δ 238.1 (CO), 143.2 (C(2)), 135.0 (C(3)),
122.6 (C(3a) or C(7a)), 117.6 (C(7a) or C(3a)), 93.8, 90.2, 86.5,
83.2 (C(4), C(5), C(6) and C(7)), 46.3 (C(1)), 18.7 (C(1)-CH3),
12.1 (C(3)-CH3), -3.68 Si(CH3)3). 29Si NMR (CDCl3, 99.36
MHz): δ 9.7. MS (DEI, m/z (%)): 352 (7, [M]+), 296 (11, [M -
2CO]+), 268 (19, [M - 3CO]+), 73 (100, [Si(CH3)3]+). A sample
suitable for structural determination (0.20 × 0.15 × 0.05 mm3)
by single-crystal X-ray diffraction was obtained by recrystal-
lization from dichloromethane.
Ackn owledgm en t. Financial support from the Natu-
ral Sciences and Engineering Research Council (NSERC)
of Canada, including NSERC postgraduate scholarships
(M.S. and P.H.), and also from the donors of the
Petroleum Research Fund, administered by the Ameri-
can Chemical Society, is gratefully acknowledged. M.S.
also wishes to thank McMaster University for a Centen-
nial Scholarship. Mass spectrometric data were obtained
courtesy of Dr. Kirk Green of the McMaster Regional
Center for Mass Spectrometry.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
positions, bond lengths and angles, anisotropic displacement
parameters, and hydrogen atom coordinates for 19 and 22.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM990650F