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substance, but generated in DMSO solution by treating
indene (3-H) with circa 1.05 equivalents of alkali tert-but-
oxide (KOtBu, NaOtBu, LiOtBu) or TMP-ZnX·LiX (X = Cl,
Br, TMP = 2,2,6,6-tetramethylpiperidyl).[9]
The potassium salt 3-K was combined with the quinone
methide 2b in DMSO solution. Aqueous acidic work up and
subsequent purification by column chromatography yielded
65% of 4 as a mixture of two diastereoisomers (Scheme 2).
Scheme 4. Reaction of 3-SiMe3 with 1a-Br in acetonitrile at ambient
temperature.
reaction progress showed the formation of silylated product 7
within 4 to 6 h, which was isolated in 46% yield. The
subsequent protodesilylation is much slower, and 49% of the
2-substituted indene 8 was isolated when the reaction mixture
was worked up after 5 days. Evidence for electrophilic attack
at 1- or 3-position of the indenyl moiety of 3-SiMe3 was not
found. Obviously, the formation of a benzyl cation stabilized
by a remote silyl group is preferred over the formation of a b-
silyl stabilized alkyl cation.
Scheme 2. Representative combinations of indenyl alkali and indenyl
zinc derivatives 3-Met with quinone methides and benzhydrylium salts.
[a] Determined by 1H NMR spectroscopy after chromatographic purifi-
cation.
Treatment of a DMSO solution of 3-H with a solution of
TMP-ZnCl·LiCl (ca. 1.1 equiv) and subsequent combination
with the benzhydrylium tetrafluoroborate 1l-BF4 gave crude
5 after aqueous work-up, which was recrystallized from
dichloromethane/ethanol (Scheme 2).
(1H-Inden-1-yl)trimethylstannane (3-SnMe3) was synthe-
sized in 52% yield in analogy to the reported procedure for
the corresponding tributylstannyl compound[10] by deproto-
nation of indene with n-butyllithium and subsequent addition
of trimethyltin chloride. (1H-Inden-1-yl)trimethylsilane (3-
SiMe3) was prepared analogously by adding trimethylsilyl
chloride to a solution of lithium indenide (3-Li) in THF.[11]
Combination of indenyltin 3-SnMe3 with 1e-BF4 in
dichloromethane at ambient temperature yielded 97% of 6
after filtration over Al2O3 and chromatographic purification
(Scheme 3).
The kinetic studies of the reactions of the metal deriva-
tives of indene with benzhydrylium ions 1 and quinone
methides 2 were performed in dichloromethane or DMSO
solution at 208C and monitored by UV/Vis spectroscopy at or
close to the absorption maxima of the electrophiles. By using
the indenyl derivatives 3-Met in large excess (> 10 equiv),
pseudo first-order kinetics were observed. The first-order rate
constants kobs were obtained by least-squares fitting of the
exponential function At = A0 exp(ꢁkobs t) + C to the time-
dependent absorbances At of the electrophile (Figure 1).
Second-order rate constants k2 (Table 2) were obtained from
linear correlations of kobs with the concentrations of the
organometallics as shown for the reaction of 3-K with 2c in
Figure 1, and for other reactions in the Supporting Informa-
tion.
As the reaction of 3-SiMe3 with the pregenerated
benzhydrylium salts, which were used for the kinetic inves-
tigations (see below), led to complex product mixtures, 3-
SiMe3 was combined with the covalent benzhydryl bromide
1a-Br in acetonitrile (Scheme 4). GC/MS monitoring of the
Figure 1. Plot of the absorbance A (at 486 nm) vs. time for the reaction
of 3-K (9.78ꢂ10ꢁ4 m) with 2c (2.03ꢂ10ꢁ5 m) in DMSO at 208C. Inset:
Plot of pseudo-first-order rate constants kobs versus nucleophile
*
*
Scheme 3. Reaction of 3-SnMe3 with the benzhydrylium tetrafluorobo-
rate 1e-BF4 in dichloromethane at ambient temperature.
concentrations. in absence of 18-crown-6, in presence of 1.2 equiv
of 18-crown-6 with respect to 3-K.
2
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
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