Switch of regioselectivity in palladium-catalyzed silaboration of terminal alkynes by ligand-dependent control of reductive elimination

Article abstract of DOI:10.1021/ja105096r

The regioselectivity in the addition of silylboronic esters to terminal alkynes can be switched by the choice of phosphorus ligands on the palladium catalysts. The silaboration proceeds with normal regisoselectivity in the presence of (η³-C

Full text of DOI:10.1021/ja105096r

Published on Web 08/11/2010
Switch of Regioselectivity in Palladium-Catalyzed Silaboration of Terminal
Alkynes by Ligand-Dependent Control of Reductive Elimination
Toshimichi Ohmura,* Kazuyuki Oshima, Hiroki Taniguchi, and Michinori Suginome*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Katsura, Kyoto 615-8510, Japan
Received June 11, 2010; E-mail: ohmura@sbchem.kyoto-u.ac.jp; suginome@sbchem.kyoto-u.ac.jp
Table 1. Ligand Effect on Pd-Catalyzed Silaboration of 1aa
Abstract: The regioselectivity in the addition of silylboronic esters
to terminal alkynes can be switched by the choice of phosphorus
ligands on the palladium catalysts. The silaboration proceeds with
normal regisoselectivity in the presence of (η3-C3H5)Pd(PPh3)Cl
(1.0 mol %) to give 1-boryl-2-silyl-1-alkenes in high yields. In sharp
contrast, selective formation of the inverse regioisomers, 2-boryl-
1-silyl-1-alkenes, takes place when the reaction is carried out with
a palladium catalyst bearing P(t-Bu)2(biphenyl-2-yl). A reaction
mechanism for the change of regioselectivity that involves
reversible insertion/ꢀ-boryl elimination steps is proposed.
entry
L
yield (%)b
3a:4ac
1
2
3
4
5
6
7
8
PPh3
P(n-Bu)3
PCy3
94
87
92
55
>99:1
>99:1
96:4
68:32
85:15
3:97
P(t-Bu)3
P(t-Bu)2(2-MeC6H4)
P(t-Bu)2(biphenyl-2-yl) 5
PPh2(biphenyl-2-yl)
PCy2(biphenyl-2-yl)
92
92, 90d
93
Transition-metal-catalyzed addition of the compounds H-E and
E-E′ (E, E′ ) B, Si, Sn, Ge, P, S, Se) across unsaturated carbon-carbon
bonds has received much attention as a straightforward, atom-
economical route to structurally defined, functionalized organic mol-
ecules.1 One of the important objects with these catalyses is to switch
the regio- and stereoselectivities of the additions by the nature of the
catalysts. Such systems are well-established in H-E additions, as
exemplified in the hydroboration of styrenes with HB(cat)2 as well as
in hydrosilylation,3 hydrothiolation,4 and hydrophosphination5 of
terminal alkynes. However, such regio- and stereochemical control in
the addition of bimetallic E-E′ compounds has rarely been achieved.
Silaboration of terminal alkynes proceeds under mild conditions
in the presence of palladium catalysts bearing an isocyanide or
phosphorus ligand.6 All of the reported catalysts resulted in
regioselective formation of (Z)-1-boryl-2-silyl-1-alkenes via cis
introduction of the boryl group to the terminal carbon and the silyl
group to the internal carbon. Our interests have been directed toward
efficient switching of the regio- and stereochemistry of silaboration.
In this regard, we established palladium-catalyzed trans silaboration
of terminal alkynes, which gives the E isomer with the same
regioselectivity.7 Herein, we describe a new palladium catalyst that
changes the regioselectivity in silaboration of terminal alkynes.
Silaboration of 1-octyne (1a) with ClMe2Si-B(pin) (2)8 was carried
out in toluene in the presence of (η3-C3H5)Pd(L)Cl (1.0 mol %, L )
tertiary phosphine)9 (Table 1). The product was then treated with
i-PrOH in the presence of pyridine to convert the Cl group on the
silicon atom to an i-PrO group. A palladium complex having PPh3
showed efficient catalyst activity for the addition at room temperature,
giving (Z)-1-boryl-2-silyloct-1-ene 3a in 94% yield with perfect
regioselectivity for the normal product (entry 1). The reaction was also
catalyzed by palladium complexes bearing trialkylphosphines (entries
2-4). Selective formation of 3a was observed with P(n-Bu)3 (entry
2), whereas a small amount of the (E)-2-boryl-1-silyloct-1-ene regioi-
somer 4a was formed in the reaction with PCy3 (entry 3). Formation
of 4a became more preferable with P(t-Bu)3 and P(t-Bu)2(2-MeC6H4)
(entries 4 and 5), indicating that electron-rich and sterically demanding
phosphines would change the regioselectivity of the reaction. We finally
established that a palladium catalyst bearing P(t-Bu)2(biphenyl-2-yl)
>99:1
97:3
96
a 1a (0.24 mmol), 2 (0.20 mmol), and (η3-C3H5)Pd(L)Cl (2.0 µmol)
were stirred in toluene (0.1 mL) at room temperature for 0.5-12 h. The
mixture was then reacted with pyridine (0.36 mmol) and i-PrOH (0.30
mmol) at room temperature for
1
h. b GC yield based on 2.
c Determined by GC analysis of the crude mixture. d Isolated yield in a
0.4 mmol scale reaction.
(5) achieved high “abnormal” regioselectivity for formation of 4a (3a:
4a ) 3:97; entry 6). Reactions with diphenyl- and dicyclohexyl
analogues of 5 did not form 4a (entries 7 and 8), indicating again the
requirement of electron-donating, bulky triorganophosphines.10
Various 1-alkynes 1 were subjected to silaboration using the Pd/5
catalyst (Table 2).11 Primary-alkyl-substituted 1b-h reacted smoothly
Table 2. Pd/5-Catalyzed Silaboration of Terminal Alkynesa
a 1 (0.48 mmol), 2 (0.40 mmol), and (η3-C3H5)Pd(L)Cl (L ) 5, 4.0
µmol). For details, see the Supporting Information. b Isolated total yield
of 3 and 4. c Determined by GC of the crude mixture. d Using 3.0 mol
% Pd at 50 °C for 24 h.
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10.1021/ja105096r  2010 American Chemical Society

C O M M U N I C A T I O N S
with 2 to give 4b-h in 80-99% yield with high regioselectivity
for the reverse addition (3:4 ) 2:98-3:97; entries 1-7). Functional
groups such as silyloxy (entries 3 and 4), AcO (entry 5), Cl (entry
6), and CN groups (entry 7) were tolerated under the conditions.
The regioselective silaboration was also applicable to sterically more
hindered 1i (entry 8). Although the silaborations of 1j and 1k
derived from secondary and tertiary alcohols were rather slow, the
regioselectivity was still acceptable (entries 9 and 10). In contrast,
a lower 3:4 ratio was observed in the reaction of phenylethyne (1l)
(entry 11). The electron-rich aromatic alkyne 1m reacted faster than
1l to give the adduct with a better 3:4 ratio (entry 12), whereas no
reaction took place with the electron-deficient alkyne 1n (entry 13).
The ligand-dependent change in regioselectivity was also ob-
served in the addition of Me2PhSi-B(pin) (6) to 1a (eq 1).
formation of 3o can be reasonably explained by the “normal” pathway
involving formation of A′. The failure of the cyclization (to form 9)
suggests that the reductive elimination step with the PPh3/Pd catalyst
is faster than the cyclization step. In contrast, reaction of 1o in the
presence of 5 as a ligand gave cyclization product 9 in good yield
with minor formation of 4o. The formation of 9 is inconsistent with
the formation of B in the “abnormal” path using 5, while the formation
of inverse addition product 4o is consistent with the mechanism. We
assume that in the silaboration using 5, formation of A′ is still
kinetically favored, but the subsequent reductive elimination is
significantly retarded by the effect of ligand 5. Alternatively, A′
undergoes cyclization with the intramolecular CdC bond to give 9 as
the major product. The formation of 4o can be explained by ꢀ-boryl
elimination from A′ back to O followed by formation of B.14,15 The
formation of 4 in the reaction shown in Table 2 is also explained by
the reversible insertion/ꢀ-boryl elimination (Scheme 1), by which
product formation finally takes place from B. Steric interactions between
the bulky ligand and the substituent on the double bond may destabilize
intermediate A, leading the equilibrium to the formation of B.
The possibility of ꢀ-boryl elimination from A was confirmed
by the reaction of (Z)-1-boryl-2-bromo-1-octene 10, which was
prepared separately, with Me2PhSiLi (Scheme 3). The reaction
afforded regioisomeric 7 and 8 in a 72:28 ratio in the presence of
the Pd/5 complex, while neither product was formed in the absence
of the palladium complex. These results indicate that 7 is obtained
through formation of complex 11 followed by silylation to form
A.16 Formation of 8 may be rationalized by the ꢀ-boryl elimination
from A, resulting in formation of O, which provides 8 via B.
We assume that the “normal” silaboration, which forms 3,
proceeds through formation of intermediate A by insertion of an
alkyne into the B-Pd bond of intermediate O, with B-C bond
formation occurring at the terminus of the alkyne (path a, Scheme
1).12 Another possibility, the formation of intermediate C via
insertion of the alkyne into the Si-Pd bond of O (path c), can be
neglected because of the much higher energy of the intermediate
as determined by theoretical studies.13 Likewise, any routes through
insertion into the Si-Pd bond of O (path d) can be neglected for
the same energetic reason. It is reasonable to assume that product
4 with inverse regiochemistry is obtained through formation of
intermediate B, which is derived by regioisomeric insertion of an
alkyne into the B-Pd bond of O (path b).
Scheme 3. Reaction of 10 with Me2PhSiLi Mediated by Pd/5
Scheme 1. Possible Mechanism
In conclusion, we have achieved reversal of regioselectivity in
the silaboration of terminal alkynes. Mechanistic details, which
involve unique ligand control of reductive elimination, are now
under investigation in this laboratory.
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research on Priority Areas (20037031, “Chemistry
of Concerto Catalysis”) from MEXT. K.O. acknowledges JSPS for
fellowship support.
Supporting Information Available: Experimental details and
characterization data for the products. This material is available free
of charge via the Internet at http://pubs.acs.org.
To gain insight into the mechanism, reactions of 1,6-enyne 1o were
carried out in the presence of either PPh3 or 5 (Scheme 2). In the
presence of PPh3, 1o selectively afforded uncyclized product 3o. The
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