Potassium tert-butoxide-catalyzed dehydrogenative Si-O coupling: Reactivity pattern and mechanism of an underappreciated alcohol protection

Article abstract of DOI:10.1002/asia.200800426

A remarkable fen-butoxide-catalyzed coupling of alcohols and silanes is reported. Dihydrogen and not hydrochloric acid (generated in the prevalent, related coupling of alcohols and chlorosilanes) is formed as the sole by-product. A comprehensive survey of common silanes provides a reliable tool for the predictability of their reactivity under defined reaction conditions. The debated mechanism of this transformation is investigated monitoring the stereochemical course at the silicon atom by means of a silicon-stereogenic silane. On this basis, a transition state for the enantiospecific Si-O coupling step is suggested rationalizing the observed frontside attack and thus retention at the silicon atom.

Full text of DOI:10.1002/asia.200800426

FULL PAPERS
DOI: 10.1002/asia.200800426
ꢀ
Potassium tert-Butoxide-Catalyzed Dehydrogenative Si O Coupling:
Reactivity Pattern and Mechanism of an Underappreciated Alcohol
Protection
Andreas Weickgenannt and Martin Oestreich*[a]
Abstract: A remarkable tert-butoxide-
catalyzed coupling of alcohols and si-
lanes is reported. Dihydrogen and not
hydrochloric acid (generated in the
prevalent, related coupling of alcohols
and chlorosilanes) is formed as the sole
by-product. A comprehensive survey of
tool for the predictability of their reac-
tivity under defined reaction condi-
tions. The debated mechanism of this
transformation is investigated monitor-
ing the stereochemical course at the sil-
icon atom by means of a silicon-stereo-
genic silane. On this basis, a transition
ꢀ
state for the enantiospecific Si O cou-
Keywords: alcohols · nucleophilic
substitution · protective groups ·
reaction mechanisms · silylation
pling step is suggested rationalizing the
observed frontside attack and thus re-
tention at the silicon atom.
common silanes provides
a reliable
Introduction
erates dihydrogen as the sole by-product (Scheme 1).[5] Re-
markably, it is long known that this process is efficiently
mediated or catalyzed by several late transition metal com-
plexes[6] as well as Brønsted and Lewis acids/bases.[7]
Elegant complex-molecule synthesis is rated highly if it only
involves a minimal number or even no protective group ma-
nipulations.[1] Nevertheless, protective group chemistry will
certainly hold its place as a pivotal tool in organic synthesis.
The chemistry of protective groups itself as well as strategic
synthetic planning was greatly influenced and enhanced by
ꢀ
the Si O linkage for temporary hydroxy group protection.
Its extensive use stems from the convenient alteration of
both the steric and the electronic environment at the silicon
atom, which, in turn, oftentimes ensures orthogonality in the
protection and deprotection steps.[2]
ꢀ
Scheme 1. Dehydrogenative Si O coupling.
ꢀ
An impressive selection of Si O bond forming reactions
In connection with a novel kinetic resolution strategy, we
had recently developed diastereoselective, transition metal-
is available today, the prominent protocol being the treat-
ment of an alcohol with a moisture-sensitive chlorosilane in
the presence of a nucleophilic catalyst and a stoichiometric
amount of a base as a hydrochloric acid scavenger.[3] While
this setup, which eventually produces equimolar amounts of
a hydrochloride salt by-product, is commonly employed
without reconsidering alternatives, another powerful tech-
ꢀ
catalyzed Si O couplings of racemic mixtures of alcohols
and enantioenriched silicon-stereogenic silanes.[8,9] The cen-
[8]
[9]
ꢀ
ꢀ
tral step of these Cu H and Rh H catalyses is believed
to be a racemization-free s-bond metathesis of an inter-
mediate transition metal alkoxide and the Si H bond.
[8c]
ꢀ
The active catalyst is normally generated from the otherwise
inert transition metal tert-butoxide through alkoxide ex-
change. Sodium or potassium tert-butoxide and a transition
metal salt serve as viable precatalysts. It was in the course
of these investigations that we discovered the catalytic activ-
ity of potassium tert-butoxide alone, as indicated by the vig-
orous evolution of dihydrogen. We are aware of the fact
ꢀ
nique is only rarely used. The direct dehydrogenative Si O
coupling of alcohols A and readily available silanes B[4] lib-
[a] A. Weickgenannt, Prof. Dr. M. Oestreich
Organisch-Chemisches Institut
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Corrensstrasse 40, 48149 Mꢁnster (Germany)
Fax : (+49)251-83-36501
that such transition metal-free, alkoxide-catalyzed dehydro-
[5,7]
ꢀ
genative Si O couplings are not totally unprecedented,
E-mail: martin.oestreich@uni-muenster.de
but the present potassium tert-butoxide catalysis displays
406
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2009, 4, 406 – 410

ꢀ
Table 1. Reactivity pattern in the potassium tert-butoxide-catalyzed Si O coupling.
noteworthy generality and prac-
ticability. We therefore decided
to elaborate on this reaction
and wish to report a handy re-
activity survey of several
silane–solvent
combinations
and the corroboration of an un-
usual mechanism.
Entry
Silane 2[4]
Abbreviation
Yield of isolated product 3 [%]
THF
THF/DMF 4:1
DMF
ꢀ
1
2
3
4
5
6
7
Ph3Si H (2a)
TPS-H
99
99
76[a]
–
–
–
92
99
94
92[a]
97
–
98
97
94
99
ꢀ
MePh2Si H (2b)
MDPS-H
DMPS-H
TBDPS-H
TES-H
TBDMS-H
TIPS-H
ꢀ
Me2PhSi H (2c)
tBuPh2Si H (2d)
Results and Discussion
ꢀ
ꢀ
Reactivity Pattern
Et3Si H (2e)
95
tBuMe2Si H (2 f)
90[a]
68[b,c]
ꢀ
The pivotal experimental obser-
vation was made in a systematic
solvent screening. In the past,
we had always used toluene as
the solvent as well as a donor-
functionalized alcohol and steri-
ꢀ
iPr3Si H (2g)
–
–
[a] The reaction required double the amount of KOtBu (10 mol%) and extended reaction time (12 h). [b] The
reaction required double the amount of KOtBu (10 mol%), 18-crown-6 (10 mol%), and extended reaction
time (14 h). [c] Reaction in DMSO yielded 69% conversion at extended reaction time (14 h).
cally encumbered triorganosilanes.[8,9] Under those reaction
conditions, the transition metal was necessary to promote
attached to it. In order to render the less reactive silanes
more Lewis acidic, a Lewis base might be added to generate
a hypervalent and therefore more Lewis acidic silicon inter-
mediate (Lewis base activation of Lewis acids).[11] The ad-
mixture of DMF (or DMSO) and the reaction mixture
ꢀ
the Si O coupling, likely to be assisted by a directing effect
of the tethered donor. Conversely, performing the coupling
of a simple primary alcohol, for example, 1,[10] and an unhin-
dered silane 2 in THF immediately liberated dihydrogen ac-
companied by silicon ether formation (1!3, column 4,
Table 1). In detail, aryl-substituted silanes 2a–c reacted
cleanly in the presence of catalytic amounts of potassium
tert-butoxide (Table 1, entries 1–3) whereas trialkylsilanes
2e–g were completely inert (Table 1, entries 5–7); the appar-
ent activation by aryl groups at the silicon atom was overrid-
den by sterics, tert-butyldiphenylsilane (2d) showed no con-
version at all (Table 1, entry 4).
ꢀ
(column 5, Table 1) indeed facilitated the Si O coupling of
tert-butyldiphenylsilane (2d) as well as triethylsilane (2e)
(Table 1, entries 4 and 5) but hindered silanes 2 f and 2g re-
mained, again, untouched under these reaction conditions
(Table 1, entries 6 and 7). In DMF alone (column 6,
Table 1), the former was finally brought to reaction albeit
requiring double the amount of potassium tert-butoxide and
prolonged reaction time (Table 1, entry 6); tri-iso-propylsi-
lane (2g) was sterically too congested to participate in this
reaction under conventional conditions.[12] Either addition of
18-crown-6[5a] or a solvent switch to pure, slightly more
Lewis basic DMSO then facilitated turnover (Table 1,
entry 7).
ꢀ
The substantial decrease in reactivity in the order Ph3Si
ꢀ
ꢀ
ꢀ
ꢀ
HꢁMePh2Si H>Me2PhSi H@tBuPh2Si HꢁAlkyl3Si H
might be rationalized by the distinct Lewis acidities of these
silanes. As CACHTUNGTRENNUNG ACHUTNGTRENNGUN
(sp2) is more electronegative than C(sp3), the
Lewis acidity at the silicon atom will correlate with the
number of electron-withdrawing sp2-hybridized substituents
Verification of the General Procedure
We made several noteworthy observations: 1) While potassi-
um tert-butoxide (5.0 mol%) and 1 reacted cleanly with si-
lanes in the above solvent systems, almost no conversion
was detected neither with the potassium alkoxide of 1 alone
nor with a 1:1 mixture of the potassium alkoxide of 1 and
tert-butanol (5.0 mol%) and 1. The former outcome indi-
cates that a proton source (traces of water or the alcohol
itself) is needed for turnover[13] but reasons for the latter
outcome are less obvious. Solubility of the alkoxides in-
volved appears to be a crucial parameter in this reaction;
whereas potassium tert-butoxide (5.0 mol%) and 1 give a
clear solution, the potassium alkoxide of 1–t-butanol–1 cock-
tail remained turbid in THF, THF/DMF 4:1, and DMF.
2) The reaction rate increased by at least one order of mag-
nitude in the presence of 18-crown-6 likely owing to an in-
creased nucleophilicity of the alkoxide. 3) We note that this
reaction was also catalyzed by sodium tert-butoxide yet at
Abstract in German: Eine bemerkenswerte, tert-butanolat-
katalysierte Kupplung von Alkoholen und Silanen wird vor-
gestellt. Diwasserstoff und nicht Chlorwasserstoffsꢀure (aus
der weitverbreiteten, entsprechenden Kupplung von Alkoh-
len und Chlorsilanen) wird als einziges Abfallprodukt gebil-
det. Eine umfassende Untersuchung gebrꢀuchlicher Silane
stellt ein verlꢀssliches Instrument zur Vorhersagbarkeit ihrer
Reaktivitꢀt unter definierten Reaktionsbedingungen zur
Verfꢁgung. Der kontrovers diskutierte Mechanismus dieser
Umsetzung wird ꢁber den stereochemischen Verlauf am Sili-
ciumatom mit einem siliciumstereogenen Silan untersucht.
Auf dieser Grundlage wird ein ꢃbergangszustand fꢁr den
enantiospezifischen Kupplungsschritt vorgeschlagen, der den
beobachteten Vorderseitenangriff und damit die Retention
am Siliciumatom erklꢀrt.
Chem. Asian J. 2009, 4, 406 – 410
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
407

A. Weickgenannt and M. Oestreich
FULL PAPERS
ꢀ
ꢀ
Table 2. Brief verification of the potassium tert-butoxide-catalyzed Si O cou-
pling.
markedly lower rate. 4) In all reactions, the formation of Si
ꢀ ꢀ
OH and Si O Si by-products was almost suppressed by ad-
dition of molecular sieves although excessive amounts of it
inhibited the reaction. 5) Using 5.0 mol% of potassium tert-
butoxide turned out to be practicable, however, lower cata-
lyst loadings were possible. The general procedure was then
further verified for a few 18, 28, and even 38 alcohols
(Table 2). The high-yielding etherification of 1-adamantanol
in THF/DMF 4:1 or at a higher rate in THF/DMSO 4:1 is
remarkable (9!15, Table 2, entry 6) in that we had never
observed silylated tert-butanol.
Entry Alcohol
Product
Yield of isolat-
ed prod-
uct [%]
1
2
99
93
Reaction Mechanism
Over the last few years, we have been largely involved in
the chemistry of silicon-stereogenic silanes,[14] which serve as
stereochemical probes for a better understanding of reaction
mechanisms.[15] The mechanism of alkoxide-mediated dehy-
drogenative couplings was the subject of several investiga-
tions in the past,[7,13,16,17] and Wilzbach et al.[16] as well as
Sommer et al.[18] had already suggested reasonable mecha-
nisms, which rationalized the observed predominant stereo-
retention at the silicon atom.[19] On the other hand, Le
Bideau et al. recently suggested a mechanism for a hydrox-
ide-catalyzed coupling, which would result in racemiza-
tion.[5a] We therefore decided to also interrogate the stereo-
chemical course of our related potassium tert-butoxide-cata-
3
4
75
97
5
6
99
ꢀ
lyzed Si O coupling. As expected, acyclic tert-butylmethyl-
phenylsilane (99% ee) was too unreactive to undergo the
coupling using THF as solvent, which is why we employed
the strained silane (SiR)-16 (84–89% ee) recently introduced
by us.[8b,c] A dehydrogenative Si O coupling–reductive Si O
cleavage sequence would then reveal the enantiospecificity
of the coupling step (Table 3). When THF or THF/DMF 4:1
were used as solvents, the reaction proceeded with (almost)
immaculate stereoretention at the silicon atom [(SiR)-16!
98[a,b]
ꢀ
ꢀ
[a] The reaction was performed in THF/DMF 4:1 and extended reaction time
(14 h). [b] Quantitative conversion was seen in THF/DMSO 4:1 within 5 h.
(SiS)-17!
ACHTUNGTRENNUNG
(SiR)-16, entries 1 and 2, Table 3], whereas racemi-
zation was seen with DMF as solvent [(SiR)-16!rac-17!
rac-16, entry 3, Table 3]; the acyclic chiral silane also race-
mized in the latter solvent. The Lewis basicity as well as the
ET parameter (that is the polarity) of these solvents might
um tert-butoxide (5.0 mol%). It is particularly important to
note that (SiR)-16 is not racemized by pure DMF (vide
infra). An almost similar situation is seen for silicon ether
(SiS)-17 (entries 1–4, Table 5) but this time, potassium tert-
butoxide (1.0 equiv) brings about complete racemization
with concomitant decomposition, which, in turn, might ac-
count for the loss of stereochemical information observed in
be important [E
TACHTUNGTRENNUNG(THF)=37.4 vs ETACHTUNGTRENN(GUN DMF)=43.2 and
E CHTUNGTRENNUNG
(DMSO)=45.1].[20] DMF and DMSO might either in-
crease the Lewis acidity of tetravalent silicon compounds
through coordination and thus facilitate activation[11] or
simply stabilize charged species.
ꢀ
the Si O coupling through subsequent tert-butoxide-induced
racemization.
To further understand this curious solvent effect,[21]
namely the role of DMF,[22] in this catalysis, we conducted a
series of racemization experiments of both silane (SiR)-16
(Table 4) and silicon ether (SiS)-17 (Table 5). All reactions
were performed in DMF at room temperature; (SiR)-16 and
(SiS)-17 were configurationally stable in THF. Silane (SiR)-16
basically retained its stereochemical integrity in all cases
(entries 1–4, Table 4); marginal loss of enantioenrichment
was detected after prolonged treatment with potassium tert-
butoxide (1.0 equiv), which cannot explain completely race-
mized rac-17 in the presence of catalytic amounts of potassi-
As tert-butoxide is able to interact with (SiS)-17 (entry 2,
Table 5), the alkoxide of 1 (generated from KH and 1)
might do so even more. However, interaction of tetravalent
silicon with an alkoxide results in (partial) racemization ex-
clusively with sterically hindered systems (entry 2, Tables 4
and 5) while enantiospecific alkoxy–alkoxy exchange at sili-
con ethers is well-documented.[23,24] To understand this,
front- and backside attack at the silicon atom must be juxta-
posed: non-hindered alkoxides favor (concerted) frontside
SN2-type mechanisms;[23] conversely, hindered alkoxides
favor SN2-type backside attack because of sterical considera-
408
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ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2009, 4, 406 – 410

ꢀ
Potassium tert-Butoxide-Catalyzed Dehydrogenative Si O Coupling
Table 3. Solvent dependence of the stereochemical course at the silicon atom.[a]
terminated the reaction after
1 h instead of 14 h. After reduc-
Si
ꢀ
tive Si O cleavage, silane ( R)-
16 was isolated in 70% ee,
which is unambiguous evidence
for
a
racemization-free cou-
pling.
ꢀ
The alkoxide-catalyzed Si O
coupling is enantiospecific in all
three solvent systems. In THF,
the mechanism (1) agrees with
the seminal postulate by Wilz-
bach et al.[16] and (2) is similar
to the proposed cyclic transition
state by Sommer et al.;[19] a suf-
ficiently Lewis acidic silane B,
ꢀ
ꢀ
Entry
Si O coupling
Solvent
Si O cleavage
Yield of
16
Yield of
17
[%]
ee of
ACHTUNGTRENNUNG
(SiR)-
16
Stereochemical
course
ee of
16
[%]
system
strained
(SiR)-16
or
[%]
[%]
PhnMe3ꢀnSiH (n=3–1, 2a–c),
reacts directly with alcohol A
1
2
THF
THF/DMF
4:1
97
70
89
85
retention
retention
74
74
89
80
through transition state
forming
Scheme 2). The involvement of
both the potassium alkoxide of
A and tert-butanol or A itself in
D
C
(B!D!C,
3
DMF
82
84
racemization
72
0
[a] Enantiomeric excesses determined by HPLC analysis using a Daicel Chiralcel OJ-RH column (MeOH/
H2O=80:20 at 128C): 35.3 min for (SiR)-16 and 38.5 min for (SiS)-16.
Table 4. Racemization experiments of silane (SiR)-16 in DMF.
Entry Additive (1.0 equiv)
t [h] Enantiomeric excess [%][a]
before
after
1
2
3
–
24
24
24
84
84
89
80
60
89
potassium tert-butoxide
potassium alkoxide of 1
(generated from KH and 1)
alcohol 1
4
14
89
89
[a] Determined by HPLC analysis using
a Daicel Chiralcel OJ-RH
column (MeOH/H2O=80:20 at 128C): 35.3 min for (SiR)-16 and 38.5 min
for (SiS)-16.
Table 5. Racemization experiments of silicon ether (SiS)-17 in DMF.
Entry Additive (1.0 equiv)
t [h] Enantiomeric excess [%][a]
ꢀ
Scheme 2. Mechanistic picture of enantiospecific, dehydrogenative Si O
coupling with and without Lewis base-activation (LB=Lewis base).
before
after
1
2
3
–
14
14
24
89
89
89
89
0
89
potassium tert-butoxide
potassium alkoxide of 1
(generated from KH and 1)
alcohol 1
the concerted SN2-type displacement in D is experimentally
supported by a proton source (A or trace of water) as a key
requirement.[13] We think that, in contrast to the Sommer
mechanism,[19] the potassium cation is not an integral part of
this process since the reaction is still enantiospecific (!) in
the presence of 18-crown-6 (vide supra).[5a] In DMF, the sit-
uation is somewhat more complicated. The activation of
otherwise unreactive silanes 2d–f by this Lewis base (LB) is
a generally accepted mode of action (Table 1); nevertheless,
we do hesitate to propose a hypervalent silicon intermediate
as DMF and related donors cannot racemize any of the si-
lanes used in this study. We currently believe that a weak
Lewis acid–base interaction as depicted in B···LB activates
4
14
89
89
ꢀ
[a] Determined by racemization-free, reductive cleavage of the Si O
bond [(SiS)-17!
ACHTUNGTRENNUNG
(SiR)-16] followed by HPLC analysis using a Daicel Chir-
alcel OJ-RH column (MeOH/H2O=80:20 at 128C): 35.3 min for (SiR)-16
and 38.5 min for (SiS)-16.
tions,[25] thereby forming a trigonal bipyramidal intermediate
prone to pseudorotational processes.[26] In order to distin-
guish between racemization during or after the Si O cou-
ꢀ
pling step, we would have to prove post-coupling, tert-butox-
ide-catalyzed racemization [(SiS)-17!rac-17]. For this, we
ꢀ
ꢀ
repeated the Si O coupling in DMF (entry 3, Table 3) and
the silicon atom to enter the SN2-type Si O coupling
Chem. Asian J. 2009, 4, 406 – 410
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
409

A. Weickgenannt and M. Oestreich
FULL PAPERS
(B···LB!D···LB, Scheme 2).[25] Liberation of C might pro-
ceed through an associative (D···LB!C···LB!C, Scheme 2)
or a dissociative mechanism (D···LB!D!C, not shown).
[6] J. Y. Corey in Advances in Silicon Chemistry, Vol. 1 (Ed.: G.
Larson), JAI Press, Greenwich, 1991, pp. 327–387.
[7] For an authoritative review, see: E. Lukevics, M. Dzintara, J. Orga-
nomet. Chem. 1985, 295, 265–315.
[8] a) S. Rendler, G. Auer, M. Oestreich, Angew. Chem. 2005, 117,
7793–7797; Angew. Chem. Int. Ed. 2005, 44, 7620–7624; b) B. Kara-
tas, S. Rendler, R. Frçhlich, M. Oestreich, Org. Biomol. Chem. 2008,
6, 1435–1440; c) S. Rendler, O. Plefka, B. Karatas, G. Auer, R. Frçh-
lich, C. Mꢁck-Lichtenfeld, S. Grimme, M. Oestreich, Chem. Eur. J.
2008, 14, 11512–11528.
[9] H. F. T. Klare, M. Oestreich, Angew. Chem. 2007, 119, 9496–9499;
Angew. Chem. Int. Ed. 2007, 46, 9335–9338.
[10] Since separation of silanes and silicon ethers by flash chromatogra-
phy on silica gel was problematic with benzyl alcohol, we decided to
use cognate 1 as standard.
[11] a) S. E. Denmark, G. L. Beutner, Angew. Chem. 2008, 120, 1584–
1663; Angew. Chem. Int. Ed. 2008, 47, 1560–1638; b) S. Rendler, M.
Oestreich, Synthesis 2005, 1727–1747.
[12] Steric effects, namely iBu and tBu groups, were reported to hamper
silane hydrolysis: O. W. Steward, O. R. Pierce, J. Am. Chem. Soc.
1961, 83, 1916–1921 and references cited therein.
Conclusions
ꢀ
We believe that the alkoxide-catalyzed, dehydrogenative Si
O coupling using silanes deserves its place amongst the im-
portant techniques for alcohol protection. It is a mild and
reliable process with the predictability of orthogonality
(Table 1) required in silicon-based protective group chemis-
try. The mechanistic part of the present investigation shows
that the coupling itself is enantiospecific but base-induced
racemization of the silicon ethers formed is problematic in
very polar solvents.
[13] a) Mechanistic investigation including discussion of the role of water
(or related proton sources): F. P. Price, J. Am. Chem. Soc. 1947, 69,
2600–2604; b) Necessity of proton transfer in the transition state:
R. L. Schowen, R. Bacon, Tetrahedron Lett. 1970, 11, 4177–4180.
[14] M. Oestreich, Synlett 2007, 1629–1643.
[15] a) S. Rendler, M. Oestreich, C. P. Butts, G. C. Lloyd-Jones, J. Am.
Chem. Soc. 2007, 129, 502–503; b) S. Rendler, M. Oestreich, Angew.
Chem. 2008, 120, 6086–6089; Angew. Chem. Int. Ed. 2008, 47, 5997–
6000.
Experimental Section
A flame-dried Schlenk tube was charged under an argon atmosphere
with the indicated alcohol (Table 2, 0.525 mmol, 1.05 equiv), the silane
(Tables 1 and 2, 0.500 mmol), molecular sieves (30 mg, 3 ꢄ), and, accord-
ing to Table 1, the appropriate solvent mixture (0.5m). Then, potassium
tert-butoxide (2.8 mg, 0.025 mmol, 5.0 mol%) was added. After full con-
version (monitored by GLC) the reaction mixture was directly subjected
to flash column chromatography on silica gel using cyclohexane-t-butyl-
methylether solvent mixtures. The silicon ethers are isolated as colourless
oils or solids.
[16] Seminal mechanistic proposal: L. Kaplan, K. E. Wilzbach, J. Am.
Chem. Soc. 1955, 77, 1297–1302.
[17] Important previous mechanistic work: a) K. O’Donnell, R. Bacon,
K. L. Chellappa, R. L. Schowen, J. K. Lee, J. Am. Chem. Soc. 1972,
94, 2500–2505; b) C. R. Howie, J. K. Lee, R. L. Schowen, J. Am.
Chem. Soc. 1973, 95, 5286–5288; c) C. Eaborn, I. D. Jenkins, J. Or-
ganomet. Chem. 1974, 69, 185–192.
Acknowledgements
[18] L. H. Sommer, Stereochemistry, Mechanism and Silicon, McGraw-
Hill, New York, 1965, pp. 104–106.
[19] Cyclic transition state with KOtBu in tBuOH: L. H. Sommer, W. D.
Korte, C. L. Frye, J. Am. Chem. Soc. 1972, 94, 3463–3469.
[20] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry,
Wiley-VCH, Weinheim, 2003, pp. 418–424.
A.W. is indebted to the Fonds der Chemischen Industrie (predoctoral fel-
lowship, 2008–2010) and M.O. to the Aventis Foundation (Karl-Winnack-
er-Stipendium, 2006–2008). We thank Wacker Chemie AG (Burghausen,
Germany) for the donation of several organosilicon compounds.
[21] Influence of solvents: O. W. Steward, A. G. Lutkus, J. B. Geen-
[1] a) P. S. Baran, T. J. Maimone, J. M. Richter, Nature 2007, 446, 404–
408; b) R. W. Hoffmann, Synthesis 2006, 3531–3541.
shields, J. Organomet. Chem. 1978, 144, 147–154.
[22] Role of DMF in transition metal-catalyzed dehydrogenative Si O
couplings: J. J. Crusciel, Can. J. Chem. 2005, 83, 508–516.
[23] L. H. Sommer, Stereochemistry, Mechanism, and Silicon, McGraw-
Hill, New York, 1965, pp. 51–56.
ꢀ
[2] a) P. G. M. Wuts, T. W. Greene, Greeneꢀs Protective Groups in Or-
ganic Synthesis 4th ed., Wiley, New York, 2007, pp. 165–221; b) P. J.
´
Kociensky, Protecting Groups, 3rd ed., Thieme, Stuttgart, 2004,
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[24] a) Stereochemical course of alkoxy-alkoxy exchange: L. H. Sommer,
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ꢀ
reveals that, except tBuMe2Si X (X=H or Cl), the former are usu-
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ꢀ
[5] For recent examples of catalyzed dehydrogenative Si O couplings,
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Received: November 11, 2008
Published online: January 19, 2009
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