Siletanylmethyllithium: An ambiphilic organosilane

Article abstract of DOI:10.1039/b503110a

The adjacent centres of electrophilicity and nucleophilicity lead to interesting chemical reactivity in the title reagent. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b503110a

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COMMUNICATION
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Siletanylmethyllithium: an ambiphilic organosilane{
Mariya V. Kozytska and Gregory B. Dudley*
Received (in Bloomington, IN, USA) 2nd March 2005, Accepted 18th April 2005
First published as an Advance Article on the web 6th May 2005
DOI: 10.1039/b503110a
The adjacent centres of electrophilicity and nucleophilicity lead
to interesting chemical reactivity in the title reagent.
Siletanes (silacyclobutanes) are an emerging class of silane reagents
that attract attention by virtue of their ‘‘strain-release Lewis
1
acidity’’. Strained silacycles complex readily with nucleophiles to
Scheme 1 Nucleophilic addition to 3.
form hypervalent silicate complexes, which are less strained than
2
the neutral tetracoordinate silanes. This siletane electrophilicity
immune to self-annihilation, should have novel ambiphilic proper-
ties derived from its pseudo-ylide nature.
3
provides an advantage in aldol and allylation reactions.
4
Ring opening of siletanes has been used to trigger anionic
6
Our synthetic approach to 1 was based on addition to
11
chlorosiletane 3. We first examined silyl-based nucleophiles as
5
polymerization, transition metal-catalyzed coupling reactions,
7
and carbosilane oxidation.
models (Scheme 1). The results are fully consistent with past
With the exception of anionic polymerization, all of these
organosilane reactions parallel organoboron chemistry. Much of
the chemistry of these two metalloids (silicon and boron) is similar,
with boron reagents tending to be more reactive and silanes being
generally more stable. In many ways, siletanes combine attractive
features of organoboranes (mild reaction conditions, inherent
Lewis acidity) and conventional organosilanes (stability in multi-
step reaction sequences, ease of purification and handling).
The similarities notwithstanding, silicon and boron chemistries
diverge when it comes to metalloid-derived nucleophiles. It is
difficult to integrate the Lewis acidity of boranes with nucleophilic
experience: Grignard reagents react efficiently with 3, whereas
12
alkyllithiums afford little or none of the desired product.
Stannyl-based nucleophiles did not follow this trend, as attempts
1
3
to couple 5a with 3 failed to provide significant amounts of
1
4
stannane product analogous to 4. In contrast, 5b (generated from
15
6
by halogen–metal exchange) couples surprisingly well with 3,
providing multigram quantities of 1 after distillation [eqn. (1)]. This
is a dramatic illustration of subtle differences between silane and
stannane reagents.
ð1Þ
8
functionality. For example, the boron analog of the Peterson
9
olefination is limited to highly hindered organoborane reagents.
Siletane reagents, which incorporate features reminiscent of
boranes but with enhanced stability, may be particularly attractive
in this context.
The next phase of research focused on synthesis and applica-
n
tions of 2. It was not clear at the outset whether BuLi would react
faster with the siletane or stannane functionality of 1. Attack at
We therefore became interested in the synthesis and properties
of siletane-derived nucleophiles. Such compounds were expected to
display ambiphilic character and reactivity not seen in other
systems. We consider the organometallic reagents 1 and 2 to be
silicon would likely result in anionic polymerization, leading to
5
polymer-bound stannane. However, stannanes react rapidly with
n
16
BuLi to form ‘‘ate’’ complexes (e.g., A, Scheme 2), which in the
case of A would formally release siletanylmethyllithium 2.
It is the latter process—transmetallation—that predominates, as
1
0
‘
‘pseudo-ylides’’ (Fig. 1). Whereas a traditional ylide contains
formal positive and negative charges on adjacent atoms,
siletanylmethyl anions (e.g., 2) present adjacent nucleophilic and
electrophilic centres.
evidenced by trapping experiments with tribenzylsilyl chloride
n
(
Bn
at 278 uC for 30 min, followed by addition of Bn
in 52% yield based on 1 (entry 1) or 57–59% based on Bn
entries 2 and 3). This chemical reactivity is consistent with that
expected for siletanylmethyllithium intermediate 2. Increasing the
3
SiCl, Table 1). Treating 1 with an equimolar amount of BuLi
3
SiCl, provides 8
SiCl
We describe herein the synthesis and characterization of
stannane 1 and efforts to generate and trap 2. Reagent 2, if it is
3
(
n
amount of 1 (and BuLi) relative to Bn
3
SiCl is detrimental
(entry 4), probably due to alkyllithium-induced decomposition of
Fig. 1 Siletane ambiphiles.
{
Electronic supplementary information (ESI) available: procedures and
data for 1 and 8. See http://www.rsc.org/suppdata/cc/b5/b503110a/
gdudley@chem.fsu.edu
*
Scheme 2 Transmetallation to generate 2.
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Table 1 Formation and trapping of lithium 2
Scheme 3 Presumed olefination intermediate B.
n
Entry Equivalents 1/ BuLi Equivalents Bn
a
SiCl Yield (%)
3
The reaction described in entry 4 (with an excess of
siletanylmethyllithium 2) takes a different course: 9 was not
detected in the crude reaction mixture, but it forms gradually upon
treatment with acidic silica gel. This observation, surprising at first,
is consistent with our mechanistic hypothesis, which emphasizes
the importance of the siletane ring (Scheme 3).
1
2
3
4
a
1.0
1.2
1.5
2.0
1.2
1.0
1.0
1.0
52
57
59
26
Isolated yield of 8 following chromatographic purification.
5
the product (8). It thus appears that the carbanionic nature of 2
We interpret these results as follows: nucleophilic attack of 2 on
the carbonyl generates a lithium alkoxide, which, although inert to
(or A) shields the siletane ring against nucleophilic attack.
19
most vicinal silanes, coordinates with the electrophilic siletane.
Having concluded that siletane 2 is a viable chemical species, we
next examined its ambiphilic character and behaviour as a pseudo-
ylide species. Discussion of ylide chemistry calls to mind the Wittig
2
0
This intermediate (B) eventually decomposes into alkene 9. In
5
entry 4, the residual 2 likely promotes anionic polymerization of
1
7
silicate B. The resulting polymer-bound b-hydroxysilane (10) is no
longer activated by the silacycle strain-release Lewis acidity, and
therefore requires further treatment with acidic silica gel overnight
to promote elimination. In conclusion, this communication
describes the synthesis and ambiphilic reactivity of novel
organometallic reagents (1 and 2) derived from strained silacycles.
Stannane 1 is prepared conveniently and in high yield on a
olefination. The Peterson olefination with silylmethyl anions is a
well-known complementary method, but this is a two-step process
in which b-hydroxysilanes are isolated and then eliminated
9
regioselectively.
Silylmethyl anions are more reactive toward a range of ketones
than are phosphorus ylides, but they require an extra elimination
step, often involving acid or strong base, to complete the
olefination process. We expected: (1) that ambiphilic siletane 2
would maintain the high nucleophilicity associated with silylmethyl
anions, and (2) that the siletane electrophilicity would promote
elimination under mild conditions. Table 2 summarizes our
observations on the reaction of pseudo-ylide 2 (generated from 1
n
multigram scale. The reactivity of 1 with BuLi suggests that the
presence of the stannyl moiety suppresses polymerization of the
siletane ring. The ambiphilic properties of 2 present new avenues
for organosilicon chemistry that warrant further investigation.
Potential applications include the use of 2 for olefination or as a
synthetic equivalent of hydroxymethyllithium (in conjunction with
2
as described above) with benzophenone (Ph CO).
7
the siletane oxidation reported previously). Efforts aimed at
Addition of benzophenone to 2 affords 1,1-diphenylethylene (9),
which is obtained along with 7 as the sole identifiable products in
entries 1 and 3 (tributylmethyltin was produced in the experiment
described in entry 2). This one-step olefination stands in contrast
generating reagents of type 2 more efficiently by halogen–metal
2
1
exchange or by using the complex-induced proximity effect are
underway.
1
8
We thank the FSU Department of Chemistry and Biochemistry
for supporting this research.
to the traditional silane reactivity. Modest yields may reflect
inefficiencies in the transmetallation event (1 A 2). Nonetheless,
this reaction is interesting from a mechanistic perspective and for
the insight it provides into the ambiphilic nature of 2.
Mariya V. Kozytska and Gregory B. Dudley*
Department of Chemistry and Biochemistry, Florida State University,
Tallahassee, FL, USA. E-mail: gdudley@chem.fsu.edu;
Fax: +1-850-644-8281; Tel: +1-850-644-2333
Table 2 Reaction of 2 with benzophenone
Notes and references
1
2
S. E. Denmark and R. F. Sweis, Acc. Chem. Res., 2002, 35, 835.
R. Damrauer, A. J. Crowell and C. F. Craig, J. Am. Chem. Soc., 2003,
125, 10759.
3
4
(a) A. G. Myers, S. E. Kephart and H. Chen, J. Am. Chem. Soc., 1992,
114, 7922; (b) S. E. Denmark, B. D. Griedel, D. M. Coe and
M. E. Schnute, J. Am. Chem. Soc., 1994, 116, 7026.
(a) K. Matsumoto, K. Oshima and K. Utimoto, J. Org. Chem., 1994,
59, 7152; (b) K. Omoto, Y. Sawada and H. Fujimoto, J. Am. Chem.
Soc., 1996, 118, 1750.
n
Equivalents 1/ BuLi
a
Yield (%)
Entry
1
Equivalents Ph CO
2
1.0
1.0
1.0
1.2
1.0
1.0
1.5
1.0
33
36
b
2
c
3
4
a
40
d
0 A 42
5 For examples and leading references, see: (a) K. Matsumoto,
H. Shimazu, M. Deguchi and H. Yamaoka, J. Polym. Sci., Part A:
Polym. Chem., 1997, 35, 3207; (b) R. Knischka, H. Frey, U. Rapp and
F. J. Mayer-Posner, Macromol. Rapid Commun., 1998, 19, 455; (c)
R. K. Sheikh, K. Tharanikkarasu, I. Imae and Y. Kawakami,
Macromolecules, 2001, 34, 4384.
b
Isolated yield of 9 following chromatographic purification. In
entry 2, MeLi was used in place of BuLi for generating 2. Bu Sn
4
7) was isolated and quantified in this experiment. It was obtained in
d
4% yield. None of the expected product (9) was observed in the
crude reaction mixture by TLC or H NMR analysis. Alkene 9 was
obtained after stirring the crude product with silica gel in methylene
chloride overnight.
n
c
(
7
1
6
(a) S. E. Denmark and J. Y. Choi, J. Am. Chem. Soc., 1999, 121, 5821;
b) S. E. Denmark and Z. Wang, Synthesis, 2000, 999.
J. D. Sunderhaus, H. Lam and G. B. Dudley, Org. Lett., 2003, 5, 4571.
(
7
3
048 | Chem. Commun., 2005, 3047–3049
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8
9
(a) A. Pelter, Pure Appl. Chem., 1994, 66, 223; (b) T. Kawashima,
N. Yamashita and R. Okazaki, J. Am. Chem. Soc., 1995, 117, 6142.
D. J. Ager, Org. React., 1990, 38, 1.
17 B. E. Maryanoff and A. B. Reitz, Chem. Rev., 1989, 89, 863.
18 The two-step Peterson olefination of benzophenone using the
trimethylsilyl analog of 1 is known: D. E. Seitz and A. Zapata,
Tetrahedron Lett., 1980, 21, 3451.
19 For computational studies on siletane electrophilicity, see refs. 2
and 4b.
20 We are unable to conclude at this time if decomposition (B A 9) occurs
before or after aqueous quench. In situ spectroscopy studies that may
elucidate this detail will be reported in due course.
1
1
0 W. C. Kaska, Coord. Chem. Rev., 1983, 48, 1.
1 (a) Commercially available from Aldrich; (b) N. Auner and J. Grobe,
J. Organomet. Chem., 1980, 188, 25.
2 For coupling of 3 with an alkynyllithium (lithium acetylide), see ref. 6a.
3 D. Seyferth and E. G. Rochow, J. Org. Chem., 1955, 20, 250.
1
1
14 (a) E. Fillion and N. J. Taylor, J. Am. Chem. Soc., 2003, 125, 12700; (b)
T. Kauffmann and R. Kriegesmann, Chem. Ber., 1982, 115, 1810.
5 D. E. Seitz, J. J. Carroll, C. P. Cartaya M., S.-H. Lee and A. Zapata,
21 For relevant examples, see: (a) P. A. Brough, S. Fisher, B. Zhao,
R. C. Thomas and V. Snieckus, Tetrahedron Lett., 1996, 37, 2915; (b)
K. Itami, T. Kamei, K. Mitsudo, T. Nokami and J. Yoshida, J. Org.
Chem., 2001, 66, 3970.
1
Synth. Commun., 1983, 13, 129.
6 H. J. Reich and N. H. Phillips, J. Am. Chem. Soc., 1986, 108, 2102.
1
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