Relative rates of hydrosilylation of representative alkenes and alkynes by Cp*2YMe·THF and [Cp2sYMe]2

Article abstract of DOI:10.1021/jo9809027

Reactivity in the hydrosilylation of alkenes and alkynes catalyzed by the organoyttrium catalysts Cp*₂YMe·THF and [Cp₂^SYMe]₂ is generally determined by the steric environment of the substrate. Alkynes and conjugated alkenes show an increased reaction rate because of electronic effects; the magnitude of this increase is highly substrate dependent. The electron rich pyrrole system is particularly reactive, especially with the sterically open [Cp₂^TMSYMe]₂ precatalyst. For nonconjugated substrates, Cp*₂YMe·THF is generally a more selective catalyst than [Cp₂^TMSYMe]₂.

Full text of DOI:10.1021/jo9809027

J . Org. Chem. 1998, 63, 7009-7012
7009
Rela tive Ra tes of Hyd r osilyla tion of Rep r esen ta tive Alk en es a n d
TMS
Alk yn es by Cp *
2
YMe‚THF a n d [Cp
2 2
YMe]
Gary A. Molander* and Elizabeth E. Knight
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215
Received May 11, 1998
Reactivity in the hydrosilylation of alkenes and alkynes catalyzed by the organoyttrium catalysts
TMS
Cp*
2
YMe‚THF and [Cp
2 2
YMe] is generally determined by the steric environment of the substrate.
Alkynes and conjugated alkenes show an increased reaction rate because of electronic effects; the
magnitude of this increase is highly substrate dependent. The electron rich pyrrole system is
TMS
particularly reactive, especially with the sterically open [Cp
gated substrates, Cp*
2 2
YMe] precatalyst. For nonconju-
TMS
2
YMe‚THF is generally a more selective catalyst than [Cp
2 2
YMe] .
In tr od u ction
ously demonstrated effectiveness as selective hydrosily-
lation and cyclization/silylation catalysts. Relative rates
of reaction were determined by the competition method.
Organolanthanide and group 3 organometallic com-
pounds are emerging as potent catalysts in organic
synthesis. Initially used for the polymerization and
5
1
hydrogenation of simple alkenes, these complexes have
more recently catalyzed the cyclization/silylation of dienes,
enynes, and trienes to produce more complex molecules.1
As the substrates become more complex, however, with
multiple reaction sites available, the ability to predict the
chemoselectivity of the reaction before preparing the
starting material becomes a major concern.
i
As seen in a sample catalytic cycle of the cyclization/
silylation of a diene by Cp* YMe‚THF (Scheme 1), the
2
substitution pattern of the product obtained will depend
solely on the relative reactivity exhibited by the catalyst
in the initial insertion event. Numerous experiments
have qualitatively demonstrated that reactivity depends
on the steric environment of the alkene or alkyne.1 For
example, cyclization/silylation of substituted 1,5-dienes
d,2
and many 1,6-enynes by Cp* YMe‚THF provide a single
2
product from exclusive insertion at one select site of
unsaturation (eqs 1 and 2),2 but alkynes and conjugated
alkenes compete with less hindered alkenes to form
a,3
4
mixtures of products (eqs 3 and 4). Because this in-
formation was determined empirically, quantitative in-
formation for the relative rates of hydrosilylation of a
variety of substrates was sought. The catalysts investi-
gated in the studies described herein, Cp*
2
YMe‚THF and
TMS
[Cp
2 2
YMe] , were chosen on the basis of their previ-
(
1) (a) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51.
(
b) J eske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann,
H.; Marks, T. J . J . Am. Chem. Soc. 1985, 107, 8091. (c) Piers, W. E.;
Shapiro, P. J .; Bunel, E. E.; Bercaw, J . E. Synlett 1990, 74. (d) J eske,
G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T. J . J . Am.
Chem. Soc. 1985, 107, 8111. (e) Mitchell, J . P.; Hajela, S.; Brookhart,
S. K.; Hardcastle, K. I.; Henling, L. M.; Bercaw, J . E. J . Am. Chem.
Soc. 1996, 118, 1045. (f) Haar, C. M.; Stern, C. L.; Marks, T. J .
Organometallics 1996, 15, 1765. (g) Qian, C.; Xie, Z.; Huang, Y. J .
Organomet. Chem. 1987, 323, 285. (h) den Haan, K. H.; Wielstra, Y.;
Teuben, J . H. Organometallics 1987, 6, 2053. (i) Molander, G. A.
Chemtracts 1998, 11, 237 and references therein.
Resu lts a n d Discu ssion
To determine the relative rates of hydrosilylation, 1
equiv each of two substrates was mixed with 1.2 equiv
of silane, 0.5 equiv of dodecane as an internal standard,
and 0.6-2.9 mol % catalyst in cyclopentane. Competition
pairs were chosen such that the rate of the faster
(
2) (a) Molander, G. A.; Nichols, P. J . J . Am. Chem. Soc. 1995, 117,
4
4
415. (b) Molander, G. A.; Retsch, W. H. Organometallics 1995, 14,
570. (c) Molander, G. A.; J ulius, M. J . Org. Chem. 1992, 57, 6347. (d)
Molander, G. A.; Hoberg, J . O. J . Am. Chem. Soc. 1992, 114, 3123. (e)
Molander, G. A.; Hoberg, J . O. J . Org. Chem. 1992, 57, 3266.
(
3) Molander, G. A.; Retsch, W. H. J . Am. Chem. Soc. 1997, 119,
817.
4) (a) Nichols, P. J ., Ph.D. Thesis, University of Colorado, 1997.
b) Retsch, W. H., Ph.D. Thesis, University of Colorado, 1997.
8
(
(5) (a) Brown, H. C.; Chandrasekharan, J . J . Org. Chem. 1983, 48,
5080. (b) Ingold, C. K.; Shaw, F. R. J . Chem. Soc. 1927, 2918.
(
S0022-3263(98)00902-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/15/1998

7
010 J . Org. Chem., Vol. 63, No. 20, 1998
Molander and Knight
Sch em e 1
Ta ble 1. Rela tive Ra tes of Hyd r osilyla tion of Ter m in a l
Alk en es
a
Defined as rate ) 100. b Substrate is unreactive at the given
temperature.
substrate was not more than 10 times greater than that
of the slower; all rates were then normalized to a single
substrate (typically 1-decene) which was arbitrarily
defined as having a reaction rate ) 100. The tempera-
ture for a group of substrates was chosen to provide
reasonable reaction times (minutes or hours) so as to be
consistent with what would realistically be used in a
synthetic operation. It is important to note that because
the reaction temperature used was different for each
catalyst and/or type of substrate, one cannot necessarily
compare values from two different columns in any of the
following tables. While the reaction was in progress,
samples were taken at various intervals to measure the
amounts of substrates remaining against the internal
standard. A wide variation in the overall reaction times
and difficulty in sampling resulted in some deviation in
the times at which the measurements were taken.
However, as the Ingold-Shaw expression applies at any
a
The rate was more than 10 times faster than 1-decene. This
substrate could not be reacted in direct competition with vinyl-
naphthalene, as decomposition of vinylpyrrole occurs over the 1-3
days it takes for vinylnaphthalene competition reactions to reach
50% completion. b Rate defined as 100. c No reaction occurs at the
given temperature.
Ta ble 2. Com p a r ison of Rela tive Ra tes of
Hyd r osilyla tion to A Va lu esa
Cp*2YMe‚THF
(relative rate at
25 °C)
[CpTMS2YMe]2
(relative rate at
45 °C)
A value
(kcal/mol)
group
OSiMe3
Me
SiMe3
Ph
0.74
1.74
2.5
26
42
16
25
b
41
12
39
2.8
5
point during the reaction, calculated relative rates will
a
A values from reference 7. b Unreactive at the given temper-
be comparable for measurements taken between 50 and
ature.
1
00% of reaction completion (i.e., until 1 equiv total of
[CpTMS2YMe]2. Consequently, it is more difficult for
hindered substrates to approach the metal for insertion
into the metal hydride bond. This difference may also
the substrates has reacted).
The relative rates of hydrosilylation of terminal al-
TMS
kenes by Cp*
2
YMe‚THF and [Cp
2 2
YMe] are depicted
in Table 1. In general, both catalysts show a decrease
in the relative rate of hydrosilylation with an increase
in the steric hindrance of the substrate. Even the rate
of hydrosilylation of an allylsilane (entry 8, Table 1) is
related to its steric bulk, despite the stabilizing effect
silicon would have on any partial positive charge devel-
be why a trimethylsilyl-protected allyl alcohol is unre-
TMS
active with the more sterically open [Cp
2
2
YMe] at the
given temperature (Table 1, entry 6); the oxygen is
exposed enough to coordinate to yttrium and effectively
inhibit the catalytic cycle.8
Electronic effects appear to affect the reaction rate only
for vinylpyrrole and vinylnaphthalene (Table 1, entries
oping â to the silane.6 When comparing relative rates
7
of hydrosilylation to A values, substrates with OSiMe
3
1
and 2). These conjugated substrates are hydrosilylated
3
and SiMe groups are disproportionately slower than
faster than the sterically less crowded 1-decene. This is
in spite of the fact that hydrosilylations of styrene by
one would expect (Table 2). This indicates that the
yttrium catalysts are affected by steric bulk even some-
what removed from the double bond. This is particu-
larly evident in the protected allyl alcohols where
9
various organoyttrium catalysts and that of vinylpyrrole
TMS
10
compounds by [Cp
2
YMe]
2
do not proceed by the
typical “1,2 addition” where the yttrium is added to the
terminal carbon, but instead proceed via a sterically
unfavorable “2,1 addition” resulting in yttrium addition
to the more hindered carbon (Figure 1). As there is
Cp* YMe‚THF is generally a less reactive catalyst than
2
TMS
[
Cp
tivity difference can be attributed to the methyl groups
on the cyclopentadiene rings of Cp* YMe‚THF which
make its reactive site more crowded than that of
2 2
YMe] (entries 6 and 9-11, Table 1). This reac-
2
3
(8) For other examples of an OSiMe group coordinating to a metal
see (a) Chen, X.; Hortelano, E. R.; Eliel, E. L.; Frye, S. V. J . Am. Chem.
Soc. 1990, 112, 6130. (b) Reetz, M. T.; H u¨ llmann, M. J . Chem. Soc.,
Chem. Commun. 1986, 1600.
(9) Kretschmer, W. P.; Troyanov, S. I.; Meetsma, B. H.; Teuben, J .
H. Organometallics 1998, 17, 284, and references therein.
(10) Molander, G. A.; Schmitt, M. H. Unpublished results.
(
6) (a) Wierschke, S. G.; Chandrasekhar, J .; J orgensen, W. L. J . Am.
Chem. Soc. 1985, 107, 1496. (b) Lambert, J . B.; Wang, G. T.; Finzel,
R. B.; Teramura, D. H. J . Am. Chem. Soc. 1987, 109, 7838.
(7) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds;
Wiley & Sons: New York, 1994.

Hydrosilylation of Alkenes and Alkynes
J . Org. Chem., Vol. 63, No. 20, 1998 7011
Ta ble 4. Rela tive Ra tes of Hyd r osilyla tion of
1
,1-Disu bstitu ted Alk en es
F igu r e 1. Regioisomers derived from olefin insertion of
conjugated dienes into metallocene hydride complexes.
Ta ble 3. Rela tive Ra tes of Hyd r osilyla tion of Alk yn es
a
Defined as rate ) 100. b Unreactive at the temperature given.
evidence of organolanthanide compounds coordinating in
n
11
a η - fashion to benzyl and styryl ligands, it has been
suggested that 2,1-addition may be a result of initial
coordination of yttrium to the π bonds of the arene ring.12
This initial coordination stabilizes the electron deficient
metal center and lowers the energy of activation for olefin
insertion, and in doing so directs the regiochemistry of
the alkene insertion so that the yttrium remains near
the arene at the more hindered olefin site. The unusual
regiochemistry of reaction as well as the extraordinarily
high rate of hydrosilylation for vinylpyrrole and vinyl-
naphthalene is consistent with such a substrate-directed
reaction.13 This initial coordination is also supported by
a
Defined as rate ) 100.
Ta ble 5. Dir ect Com p a r ison of High ly Rea ctive
Su bstr a tes
a
b
Defined as rate ) 100. Substrate is unreactive at the given
the observation that electronic effects are particularly
temperature.
TMS
pronounced for [Cp
2 2
YMe] , which should be better able
to accommodate the arene as a ligand because it pos-
sesses more open space about the coordination sphere of
the metal.
Alkynes generally insert more rapidly into the cata-
lyst as seen by their faster relative rates of reaction
terminal alkenes and internal alkynes, steric effects
dominate the relative rates with the exception of the
pyrrole (Table 4, entry 2). The relative rate of hydrosi-
lylation of the pyrrole is closer to that of the sterically
less hindered methylenecyclohexane (Table 4, entry 1)
than to the sterically similar conjugated R-propylstyrene
(Table 3), but the overall reaction time is longer. The
alkynes undergo faster insertion because they are typi-
cally less sterically hindered and more electronically rich.
Once formed, however, the alkenylyttrium species is
sterically crowded, resulting in slow σ-bond methathesis
with the silane. Otherwise, alkynes follow the same
general pattern of reactivity as terminal alkenes in
(Table 4, entry 4). Surprisingly, the relative reaction rate
of R-propylstyrene seems to be determined almost solely
by its steric size, in contrast to vinylnaphthalene (entry
2
, Table 1) which showed a higher electronically influ-
enced reaction rate. This again points to the pronounced
steric effects of substituents at the allylic position and
beyond in various olefin substrates.
For a more direct comparison of the role played by
electronic effects, 1-decene, 2-decyne, and N-methyl-2-
that Cp*
2
YMe‚THF is a less reactive catalyst than
YMe] , and the more sterically hindered the
substrate, the slower the relative rate of hydrosilylation.
TMS
[Cp
2
2
TMS
It is unclear why [Cp
2 2
YMe] is unreactive with a
(1-methylvinyl)pyrrole were compared at the same reac-
TBDMS protected propargyl alcohol (Table 3, entry 5).
tion temperature (Table 5). Electronic effects are clearly
Perhaps ionization of this substrate by the Lewis acid
TMS
catalyst is responsible.4b
more important for the sterically open [Cp
for Cp* YMe‚THF as evidenced by the similar reaction
rate of the alkene, alkyne, and pyrrole. For Cp* YMe‚
2 2
YMe] than
2
The relative rates of hydrosilylation of 1,1-disub-
TMS
2
stituted alkenes were determined with [Cp
Table 4). The precatalyst Cp*
slowly or not at all with these substrates. As with
2 2
YMe]
THF, steric effects play a more important role as the
unhindered alkyne inserts relatively quickly, whereas the
electronically rich but sterically hindered pyrrole is
unreactive at room temperature.
(
2
YMe‚THF reacts only
2c
(11) (a) Mintz, E. A.; Moloy, K. G.; Marks, T. J .; Day, V. W. J . Am.
Chem. Soc. 1982, 104, 4692. (b) Evans, W. J .; Ulibarri, T. A.; Ziller, J .
W. J . Am. Chem. Soc. 1990, 112, 219.
Su m m a r y
(
12) Giardello, M. S.; Conticello, V. P.; Brard, L.; Gagn e´ , M. R.;
Marks, T. J . J . Am. Chem. Soc. 1994, 116, 10241.
13) Hoveyda, A. H.; Evans, D. A.; Fu, G. C. Chem. Rev. 1993, 93,
307.
For the catalysts Cp*
2
YMe‚THF and [CpTMS
2 2
YMe] , the
(
1
steric environment of the substrate determines the

7
012 J . Org. Chem., Vol. 63, No. 20, 1998
Molander and Knight
relative rates of hydrosilylation with the exception of
conjugated alkenes. Electronic effects cause an increased
reaction rate for conjugated alkenes, although the mag-
nitude of the increase is highly substrate dependent. The
electron rich pyrrole system is particularly reactive,
temperature. The flasks were opened to the air (thus quench-
ing the reaction) or taken to the glovebox to sample and then
analyzed as above.
4
-[(1,1-Dim eth yleth yl)dim eth ylsiloxy]-1-h exen e. 4-[(1,1-
Dimethylethyl)dimethylsiloxy]-1-hexene was prepared from
-hexen-4-ol and tert-butyldimethylsilyl chloride using stan-
1
especially with an “open” [CpTMS
nonconjugated substrates, Cp* YMe‚THF is generally a
YMe]
precatalyst. For
16
2
2
dard reaction conditions. Purification by flash chromatog-
raphy (silica gel, hexanes) and kugelrohr distillation (11
mmHg, 74-84 °C) resulted in the isolation of 4.76 g (88%) of
2
TMS
more selective catalyst than [Cp
2 2
YMe] . The results
4
-[(1,1-dimethylethyl)dimethylsiloxy]-1-hexene (98% pure by
reported herein not only demonstrate the reactivities that
can be achieved in insertion reactions with these metal-
locene catalysts on a quantitative basis, but also il-
lustrate in a dramatic fashion how reactivities and
selectivities can be changed by relatively simple manipu-
lations of the ligand system about the metal.
1
GC analysis): R
5
2
f
0.29; H NMR (300 MHz, CDCl
.75 (m, 1H), 5.04-4.98 (m, 2H), 3.60 (quint, J ) 5.74, 1H),
.18 (t, J ) 7.20, 2H), 1.47-1.37 (m, 2H), 0.87 (s, 9H), 0.85 (t,
3
) δ 5.84-
13
J ) 7.57, 3H), 0.03 (s, 6H); C NMR (75 MHz, CDCl
16.47, 73.18, 41.45, 29.46, 25.89 (3C), 18.16, 9.64, -4.44,
-4.55; IR (neat) 3077.9, 1640.7, 1434.0, 1360.6, 1254.2, 1058.0,
3
) δ 135.54,
1
-
1
+
3
26OSi (M - CH ) 199.1518,
9
10.4 cm ; HRMS calcd for C12
H
found 199.1516; LRMS (EI) m/z 173 (17), 157 (44), 129 (33),
Exp er im en ta l Section
1
15 (16), 99 (51), 73 (100), 59 (28), 41 (38), 29 (23).
15
The catalysts Cp*
2
YMe‚THF14 and [CpTMS
2
YMe]
2
were
3-(Tr im eth ylsilyl)-1-d ecen e. 3-(Trimethylsilyl)-1-decyne
was prepared from 1-decyne.18 Clean reduction of the alkyne
to 3-(trimethylsilyl)-1-decene was achieved by hydroboration
prepared by known methods. Substrates for Table 1, entries
, 3, 5, and 7, and Table 3, entries 2 and 3, were commercially
available. Substrates for Table 1, entry 1, and Table 4, entries
-7, were prepared via a Wittig reaction from commercially
2
1
9
by dicyclohexylborane followed by removal of the borane with
methanol.20 This compound has been previously reported in
1
1
6
21
available ketones. Substrates for Table 1, entries 4, 6, and
9
the literature.
-11, were alcohols protected by conventional methods.17
3-Meth yl-4-n on yn e. 3-Methyl-4-nonyne was prepared by
2
b
Preparations for all other substrates are described below.
NMR chemical shifts are reported in ppm relative to TMS
using the residual solvent resonance as an internal reference.
All chemicals were dried, degassed, and stored in a Vacuum
Atmospheres glovebox before use. Reactions were prepared
in the glovebox and performed with rigorous exclusion of air
and moisture. Reaction temperatures were maintained at (
alkylation of 3-methyl-1-pentyne with butyl iodide.
compound has been previously reported in the literature.
2-[(1,1-Dim eth yleth yl)d im eth ylsiloxy]-3-d ecyn e. 3-De-
This
2
2
4
b
cyn-2-ol was prepared from 1-octyne and acetaldehyde. The
alcohol was protected as the tert-butyldimethylsilyl ether by
standard methods.16 Purification by flash chromatography
(silica gel, 20:1 hexanes:ethyl acetate) followed by kugelrohr
distillation (0.3 mmHg, 79-89 °C) resulted in the isolation of
5.08 g (93%) of 2-[(1,1-dimethylethyl)dimethylsiloxy]-3-decyne
1
°C. Competition reactions were analyzed by GC, and relative
rates were calculated from the amounts of starting materials
remaining according to the Ingold-Shaw equation.5
R
f
0.55; 1H NMR (400 MHz,
(98% pure by GC analysis):
CDCl ) δ 4.48 (q, J ) 6.43, 1H), 2.15 (td, J ) 6.96, 1.87, 2H),
1.48-1.23 (m, 11H), 0.95-0.81 (m, 12 H), 0.10 (s, 3H), 0.09
(s, 3H); 13C NMR (125 MHz, CDCl
) δ 83.70, 82.81, 59.24,
31.36, 28.64, 28.52, 25.85 (3C), 25.78, 22.57, 18.68, 18.27,
Com p etition Rea ction s w ith Cp *
cal reaction, 2.00 mmol of each substrate, 2.5 mmol of PhSiH
.75 mL of cyclopentane, and 1.0 mmol of dodecane were
2
YMe‚THF . In a typi-
3
3
,
0
3
mixed, and a sample of the solution was taken to determine
time ) 0 concentrations. Meanwhile, a mixture of 0.011-0.058
mmol (0.6-2.9%) of the catalyst and 0.75 mL of cyclopentane
was prepared in a separate vial. After refrigerating both vials
at -23 °C for 5.0 min the catalyst mixture was added dropwise
to the substrate solution over 1-2 min. Samples were taken
immediately after addition of the catalyst was complete and
at various intervals throughout the course of the reaction. The
samples were quenched by addition to a vial containing a small
amount of Florisil. After dilution with MeOH and filtration
through a glass wool plug, the amounts of substrates remain-
ing were analyzed on a Hewlett-Packard 5890 GC with a 5%
phenylmethylsilicone column using the dodecane as an inter-
nal standard. Relative rates were determined by applying the
-
1
14.03, -4.56, -4.91; IR (neat) 2232.7, 1159.3, 1101.6 cm
;
+
HRMS calcd for C16H31OSi (M - H) 267.2144, found 267.2134;
LRMS (EI) m/z 268 (0.1), 253 (4), 211 (80), 167 (100).
2-(Tr im eth ylsilyl)-1-n on en e. tert-Butyllithium (13.0 mL,
1.7 M, 22.1 mmol) was slowly added to THF (20 mL) at -78
°C. (1-Bromovinyl)trimethylsilane (2.07 g, 11.6 mmol) was
slowly added, and the solution was stirred for 1.5 h. After
iodoheptane (2.26 g, 10.0 mmol) was slowly added, the reaction
was allowed to warm to room-temperature overnight. The
mixture was quenched with aqueous NaHCO
pentane, washed with water and brine, and dried over MgSO
3
, extracted with
4
.
Purification by flash chromatography (silica gel, hexanes)
followed by distillation (19 mmHg, 54 °C) resulted in the
isolation of 0.80 g (40%) of 2-(trimethylsilyl)-1-nonene (100%
pure by GC analysis). Spectral data matched that reported
in the literature.23
Ingold-Shaw equation: k
A
/k
B
) (ln[A]
0
t 0
- ln[A] )/(ln[B] - ln-
5
a
[B]
t
)
where t was such that the reaction was 50-100%
complete. Relative rates were normalized to 1-decene which
was defined with rate ) 100. Reaction pairs were chosen so
that their relative rates were within 10 times of each other.
Ack n ow led gm en t. We thank the National Insti-
tutes of Health (GM48580), NATO, and the National
Science Foundation (INT-942201) for their financial
support of this program. E.K. also thanks Mr. Eric
Dowdy for his generous donation of the Cp*2YMe‚THF
catalyst.
TMS
Com p etition Rea ction s w ith [Cp
2 2
YMe] . In a typical
reaction, 2.00 mmol of each substrate, 2.5 mmol of PhSiH
3
,
1
.0 mmol of dodecane, and 1.5 mL of cyclopentane were mixed,
and a sample was taken to determine time ) 0 concentrations.
After addition of 0.012-0.045 mmol (0.6-2.3%) of the yttrium
dimer the reaction was divided between two valved flasks, and
both were quickly transferred to an oil bath at the appropriate
J O9809027
(
14) (a) den Haan, K. H.; Teuben, J . H. J . Organomet. Chem. 1987,
22, 321. (b) den Haan, K. H.; Wielstra, Y.; Eshuis, J . J . W.; Teuben,
J . H. J . Organomet. Chem. 1987, 323, 181.
15) (a) Keitsch, M.; Schumann, H. Unpublished results. (b) For the
(18) Rajagopalan, S.; Zweifel, G. Synthesis 1984, 111.
(19) Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents; Academic
Press: London, 1988.
(20) Brown, H. C.; Molander, G. A. J . Org. Chem. 1986, 51, 4512.
(21) Tsuji, Y.; Kajita, S.; Isobe, S.; Funato, M. J . Org. Chem. 1993,
58, 3607.
(22) Suzuki, A.; Miyaura, N.; Abiko, S.; Itoh, M.; Brown, H. C.;
Sinclair, J . A.; Midland, M. M. J . Am. Chem. Soc. 1973, 95, 3080.
(23) Shiragami, H.; Kawamoto, T.; Imi, K.; Matsubara, S.; Ultimoto,
K.; Nozaki, H. Tetrahedron 1988, 44, 4009.
3
(
preparation of similar compounds, see Voskoboynikov, A. Z.; Parshina,
I. N.; Shestakova, A. K.; Butin, K. P.; Beletskaya, I. P.; Kuz′mina, L.
G.; Howard, J . A. K. Organometallics 1997, 16, 4041.
(
16) Fitjer, L.; Quabeck, U. Synth. Commun. 1985, 15, 855.
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