Ni-catalyzed Si-B addition to 1,3-dienes: Disproportionation in lieu of silaboration

Article abstract of DOI:10.1021/ol060765x

Upon attempted silaboration of acyclic 1- and 1,4-substituted 1,3-dienes, a new disproportionation reaction was discovered, yielding 1:1 mixtures of allylsilanes and dienylboranes. It was demonstrated that, as a key step in this new catalytic process, hyd

Full text of DOI:10.1021/ol060765x

ORGANIC
LETTERS
2006
Vol. 8, No. 14
2929-2932
Ni-Catalyzed Si−B Addition to
1,3-Dienes: Disproportionation in Lieu
of Silaboration
Martin Gerdin and Christina Moberg*
Organic Chemistry, KTH School of Chemical Science and Engineering,
Stockholm SE 100 44, Sweden
kimo@kth.se
Received March 30, 2006
ABSTRACT
Upon attempted silaboration of acyclic 1- and 1,4-substituted 1,3-dienes, a new disproportionation reaction was discovered, yielding 1:1 mixtures
of allylsilanes and dienylboranes. It was demonstrated that, as a key step in this new catalytic process, hydrogen is being transferred from
one diene moiety to another.
Interelement1 compounds undergo additions to a wide range
of unsaturated substrates catalyzed by transition metals.2 In
one step, two functionalities are added to the same molecule,
giving a densely functionalized building block for further
transformations. The addition of interelement linkages to 1,3-
dienes has been accomplished using silicon-, germanium-,
tin-, and boron-containing homo- and heteroelement com-
pounds (Figure 1). Most commonly the addition proceeds
have most often been used as substrates, and few additions
to 1-substituted dienes3a,5 have been reported. Although
disilylations,6 silaborations,5 and diborations7 have been
achieved with cyclic 1,4-disubstituted 1,3-dienes, there are,
to the best of our knowledge, no reports of element-element
additions to acyclic 1,4-disubstituted dienes.
We have recently developed an asymmetric version of the
cis-1,4-silaboration of 1,3-cyclohexadiene using Pt(0) and
phosphoramidite ligands.8 In our efforts to widen the scope
of the reaction we decided to investigate the silaboration of
acyclic conjugated dienes. Previously butadiene, 2,3-dimeth-
yl-1,3-butadiene, isoprene, and 2-methyl-1,3-pentadiene have
been successfully silaborated, whereas attempted additions
of a silylborane to 2,4-hexadiene and 4-methyl-1,3-penta-
diene met with little success.9 We decided to start our
Figure 1. 1,4-Addition of an interelement compound to a 1,3-
diene. E,E′ ) Si, Ge, B or Sn.
(4) (a) Ishikawa, M.; Okazaki, S.; Naka, A.; Tachibana, A.; Kawauchi,
S.; Yamabe, T. Organometallics 1995, 14, 114-120. (b) Lee, J.; Lee, T.;
Lee, S. S.; Park, K.-M.; Kang, S. O.; Ko, J. Organometallics 2002, 21,
3922-3929.
in a 1,4 fashion,2a but 1,23 and 1,13b,4 additions have also
been observed. 2-Substituted and 2,3-disubstituted dienes
(5) Suginome, M.; Matsuda, T.; Yoshimoto, T.; Ito, Y. Org. Lett. 1999,
1, 1567-1569.
(1) Tamao, K.; Yamaguchi, S. J. Organomet. Chem. 2000, 611, 3-4.
(2) (a) Beletskaya, I.; Moberg, C. Chem. ReV. 2006, ASAP. (b) Suginome,
M.; Ito, Y. Chem. ReV. 2000, 100, 3221-3256. (c) Beletskaya, I.; Moberg,
C. Chem. ReV. 1999, 99, 3435-3461.
(3) (a) Ishiyama, T.; Yamamoto, M.; Miyaura, N. Chem. Commun. 1997,
689-690. (b) Tanaka, M.; Uchimaru, Y.; Lautenschlager, H.-J. Organo-
metallics 1991, 11, 16-18.
(6) Ishikawa, M.; Nisimura, Y.; Sakamoto, H.; Ono, T.; Ohshita, J.
Organometallics 1992, 11, 483-484.
(7) Clegg, W.; Johann, T. R. F.; Marder, T. B.; Norman, N. C.; Orpen,
A. G.; Peakman, T. M.; Quayle, M. J.; Rice, C. R.; Scott, A. S. J. Chem.
Soc., Dalton Trans. 1998, 1431-1438.
(8) Gerdin, M.; Moberg, C. AdV. Synth. Catal. 2005, 347, 749-753.
10.1021/ol060765x CCC: $33.50
© 2006 American Chemical Society
Published on Web 06/06/2006

investigation by the silaboration of (E,E)-5,7-dodecadiene
using 2 and different metal-ligand combinations.10 With Pt-
(0) and PPh3, PEt3, or P(OPh3) we observed little or no
conversion of the starting materials. Switching to Ni(0),
prepared by treatment of Ni(acac)2 with DIBALH, gave
immediate results. Although no reaction was observed using
PPh3 as the ligand, both PPh2Cy and PCy3 resulted in around
30% consumption of the starting silylborane whithin 24 h
at 80 °C. PEt3 turned out to give an almost complete
consumption of both starting materials, even though 2 equiv
of the diene were used. Instead of the expected 1:1 adduct,
two products, identified as 5-(dimethylphenylsilyl)-6-
dodecene (3) and 5-(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane)-5,7-dodecadiene (4), were formed in a 1:1 ratio (crude
1H NMR). Allylsilane 3, the product of a formal hydrosily-
lation of the diene,11 was isolated as the essentially pure Z
isomer, whereas dienylborane 4 was obtained as a 5:1
mixture of the E,E and Z,E isomers (Scheme 1). The same
a suitable substrate as it is 1,4-disubstituted, symmetrical,
and readily available.14 When subjecting 5 to the same
reaction conditions as 1, the outcome was very similar
(Scheme 2). The silylborane was almost completely con-
Scheme 2. Addition of Silylborane 2 to 2,4-Hexadienea
a Isolated yields, based on starting silylborane 2.
sumed, forming 6 and 7 in good yields and in almost equal
amounts (crude 1H NMR). The allylsilane 7 was formed with
high Z selectivity,15 whereas 6 was obtained as a 1.25:1
mixture of Z,E and E,E isomers. It is notable that the
unsubstituted double bond has pure E configuration, although
the starting 2,4-hexadiene consisted of a mixture of isomers.
We did not observe any products from normal silaboration.
At 60 °C the reaction was severely retarded and no
conversion of the starting silylborane was observed (1H
NMR).
Scheme 1. Addition of Silylborane 2 to
(E,E)-5,7-Dodecadienea
When the sterically demanding (E,E)-2,2,7,7-tetramethyl-
3,5-octadiene was subjected to the standard conditions, no
product formation or disapperance of starting materials was
observed, and the starting diene could be recovered by
chromatography. We next turned to the 1,3- and 1-substituted
dienes 9 and 14. Compound 9 has previously been silaborated
using ligand-free conditions, giving two 1,4-addition products
(11 and its regioisomer) in a 2:1 ratio.5,16 Using Ni(0)/PEt3
we obtained a mixture of at least four different products:
allylsilane 10 (>97:3 Z/E ratio), 1,4-silaboration product 11
(Z stereochemistry), 1,2-silaboration product 12 (E stereo-
chemistry), and dienylborane 13.
The effect of ligand structure on the distribution of
procucts obtained from 9 was studied. As can be seen from
Table 1 the ratio between the two silaboration products 11
and 12 is heavily influenced by the ligand. Using the small
and electron-rich PPhMe2, we obtained the two products in
a 1:1.3 ratio, whereas the bulkier and more electron-deficient
PPh3 and PPh2Cy only gave 11. The ratio between allylsilane
10 and silaboration products 11 and 12 was less affected by
the choice of ligand. It varied between 1.16 (entry 2) and
0.71 (entry 1). Compounds 11-13 constituted the major
products from the reaction; although they account for most
of the silicon added, the fate of a large part of the boron is
unknown.17 A 1:1 correspondence between allylsilane and
dienylborane could thus not be verified.
a Isolated yields, based on starting silylborane 2.
products were obtained when Ni(COD)2 was employed in
place of in situ formed Ni(0).
This type of disproportionation does not seem to have been
previously observed in element-element additions to 1,3-
dienes. However, hydrogen transfer was shown to accompany
the Rh-catalyzed addition of dialkyl disulfides to 1,2-dienes,
resulting in 1:1 mixtures of (E)-alkylthio-1,3-dienes and (E)-
2-alkylthio-2-alkenes.12 In addition, during dimerization-
double stannation of 1,3-dienes, hydrostannation was ob-
served when bulky distannanes were employed, but no
observation of stannyldienes was reported.13
These intriguing results prompted us to investigate the
generality of this disproportionation reaction. 2,4-Hexadiene
(5), which previously resisted silaboration,5 was selected as
(9) Silylborane and diene (2 equiv) were reacted in the presence of Ni-
(acac)2 (5 mol %) and DIBALH (10 mol %) in toluene at 80 °C. See ref 5.
(10) General Procedure. Silylborane 2, diene (2 equiv), ligand (10 mol
%), and M(acac)2 (5 mol %) were dissolved in toluene. DIBALH (10 mol
%) was then added at -35 °C to reduce the metal. The reaction mixture
was heated at 80 °C (Ni) or 110 °C (Pt) for 24 h.
(11) For reviews on hydrosilylation of 1,3-dienes, see: (a) Pietruzka, J.
In Science of Synthesis; Fleming, I., Ed.; Georg Thieme Verlag: Stuttgart,
2002; Vol. 4, pp 172-176. (b) ComprehensiVe Handbook on Hydrosily-
lation; Marciniec, B., Ed.; Pergamon: Oxford, 1992; pp 110-114. (c)
Ojima, I. In The Chemistry of Organic Silicon Compounds; Patai S.,
Rappaport, Z., Eds.; Wiley Interscience: New York 1989; Vol. 2, pp 1493-
1499.
(14) 2,4-Hexadiene, tech., 90%, mixture of isomers. Remaining 10% is
positional double bond isomers.
(15) As the starting 2,4-hexadiene was only 90% pure the selectivity of
the reaction is most probably higher than 93:7. We were not able to separate
7 from the isomeric impurities.
(16) Suginome, M.; Ito, Y. J. Organomet. Chem. 2003, 680, 43-50.
(17) Entry 2: compounds 10 + 11 + 12 ) 89%, while 11 + 12 + 13
) 53%.
(12) Arisawa, M.; Suwa, A.; Fujimoto, K.; Yamaguchi, M. AdV. Synth.
Catal. 2003, 345, 560-563.
(13) Tsuji, Y.; Kakekhi, T. J. Chem. Soc., Chem. Commun. 1992, 1000-
1001.
2930
Org. Lett., Vol. 8, No. 14, 2006

Table 1. Addition of Silylborane 2 to
(E)-2-Methyl-1,3-pentadiene
Table 2. Ni(0)/PEt3 Catalyzed Hydrosilylation of Dienes
yield (%)a
entry
ligand
10
11
12
13
a Isolated yield based on starting PhMe2SiH. Conditions: Ni(acac)2 (5
mol %), DIBALH (10 mol %), PEt3 (10 mol %), PhMe2SiH, diene (2 equiv),
toluene, 80 °C, 24 h.
1
2
3
4
5
PMe2Ph
PEt3
PPh3
PPh2Cy
P(OPh)3
38
48
41
37
0
23
32
46
49
0
31
9
0
0
0
11
12
32
19
0
element linkages, and final reductive elimination.19 In
silaborations,insertionintothemetal-boronbondispreferred.19a
The presently observed disproportionation can be explained
by â-elimination competing with reductive elimination. The
resulting Si-M-H species then reacts with another diene
molecule to yield the allylsilanes. Alternatively, initial sp2-
hydrogen activation, as suggested for additions of strained
Si-Si and Ge-Ge bonds to unsaturated substrates,20 fol-
lowed by reductive elimination to form the dienylborane and
hydrosilylation of a second diene would rationalize the
formation of the products observed. We favor the first
alternative, which avoids the formation of Ni(IV) intermedi-
ates.
a Based on starting silylborane 2 and determined by 1H NMR using
1-methoxynaphthalene as internal standard.
Subjecting 14 to the standard reaction conditions10 afforded
allylsilane 15 and dienylborane 16 as major products,
accompanied by products from silaboration. The allylsilane
15 was isolated in 76% yield with excellent Z stereoselec-
tivity, but we were not able to separate dienylborane 16 from
the silaboration products (Scheme 3). It was, however,
Scheme 3. Addition of Silylborane 2 to 1,3-Pentadienea
To shed light on the mechanism we decided to investigate
the disproportionation of 1 and 2, trying to identify the
hydrogen source. When toluene-d8 was used as solvent no
incorporation of deuterium in the products was observed.
To study whether hydrogen transfer from one diene moiety
to another occurred 5,8-dideuterido-1 was synthesized from
1-hexyne and DIBALD. When subjecting this substrate to
the standard reaction conditions, 5,8,8-trideuterio-3 and
8-deuterio-4 were formed, accompanied only by trace
amounts of 4 and dideuterio-3, attributable to isotopic
impurities present in the starting diene (Scheme 4). Thereby
it was verified that the diene acts as the hydrogen source in
the disproportionation reaction between 1 and 2.
a Isolated yield, based on starting silylborane 2. bBased on starting
silylborane 2 and determined by H NMR using 1-methoxynaph-
thalene as internal standard.
1
possible to determine the stereochemistry of 16 as E,E from
its coupling constants.
As the allylsilanes are produced in a formal hydrosilylation
reaction with the expected Z stereochemistry,11a,18 we decided
to study whether the same products were obtained by
hydrosilylation using dimethylphenylsilane instead of si-
lylborane 2, under otherwise identical reaction conditions.
As can be seen in Table 2 all dienes did indeed react
smoothly to furnish the expected products in good to
excellent yields with the same high stereoselectivity as in
the disproportionation reactions.
According to the commonly accepted mechanism, element-
element additions to 1,3-dienes, as well as to other unsatur-
ated compounds, proceed by oxidative addition of the
interelement compound to a metal(0) complex, followed by
insertion of the unsaturated bond into one of the metal-
The hydrogen transfer observed can thus be explained by
a mechanism where â-elimination from the Ni(II) complex
obtained by insertion of the diene competes with reductive
elimination. Addition of the simultaneously formed HNiSiMe2-
Ph (or DNiSiMe2Ph) to a second equivalent of diene then
affords the allylsilane with the expected Z configuration of
(19) For examples of mechanistic investigations, see: (a) Sagawa, T.;
Asano, Y.; Ozawa, F. Organometallics, 2002, 21, 5879-5886. (b) Ozawa,
F. J. Organomet. Chem. 2000, 611, 332-342. (c) Lesley, G.; Nguyen, P.;
Taylor, N. J.; Marder, T. B. Organometallics 1996, 15, 5137-5154. (d)
Iverson, C. N.; Smith, M. R., III. Organometallics 1996, 15, 5155-5165.
(e) Sagawa, T.; Sakamoto, Y.; Tanaka, R.; Katayama, H.; Ozawa, F.
Organometallics 2003, 22, 4433-4445.
(20) (a) Ishikawa, M.; Okazaki, S.; Naka, A.; Tachibana, A.; Kawauchi,
S.; Yamabe, T. Organometallics 1995, 14, 114-120. (b) Kang, Y.; Lee,
J.; Kong, Y. K.; Kang, S. O.; Ko, J. Organometallics 2000, 19, 1722-
1728. (c) Lee, J.; Lee, C.; Lee, S. S.; Kang, S. O.; Ko, J. Chem. Commun.
2001, 1730-1731.
(18) (a) Onozawa, S.; Sakakura, T.; Tanaka, M. Tetrahedron Lett. 1994,
35, 8177-8180. (b) Bareille, L.; Becht, S.; Cui, J. L.; Le Gendre, P.; Mo¨ıse,
C. Organometallics 2005, 24, 5802-5806.
Org. Lett., Vol. 8, No. 14, 2006
2931

Scheme 4. Deuterium Transfer
Scheme 5. Proposed Mechanism for the Ni-Catalyzed
Disproportionation of (E,E)-5,7-Dodecadiene with Silylborane 2
the olefinic bond.11a,18 The stereochemistry of the dienylbo-
rane, with pure E configuration of the nonfunctionalized
olefinic bond irrespective of the configuration of the starting
diene, provides strong support for the intermediacy of a
π-allyl nickel complex that can undergo syn-anti isomeriza-
tion.
In summary, a new disproportionation process was dis-
covered upon reaction of silylborane 2 with terminally
substituted dienes under Ni(0)/PEt3 catalysis. It seems that
the terminal substituents on the diene play an instrumental
role in the reaction; when 1,4-disubstituted dienes were used
only disproportionation and no silaboration was observed.
Dienes with only one terminal substituent gave a mixture of
the two types of products. In the reaction with (E,E)-5,7-
dodecadiene it was demonstrated that hydrogen is being
transferred from one diene moiety to another.
Supporting Information Available: Experimental pro-
cedures and characterizations including stereochemical as-
signments of all new compunds. This material is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by the
Swedish Research Council.
OL060765X
2932
Org. Lett., Vol. 8, No. 14, 2006