Alkene-pinacolborane hydroborations catalyzed by lanthanum tris[bis(trimethylsilyl)amide]

Article abstract of DOI:10.1055/s-2004-835645

Tris[bis(trimethylsilyl)amide] has been shown to be an effective catalyst for the hydroboration of representative alkenes and styrenes by pinacolborane.

Full text of DOI:10.1055/s-2004-835645

LETTER
2639
Alkene-Pinacolborane Hydroborations Catalyzed by Lanthanum
Tris[bis(trimethylsilyl)amide]
A
lkene-Pinacolbor
o
ane
H
ydrobo
s
rations hikazu Horino, Tom Livinghouse,* Magdalena Stan
Department of Chemistry, Montana State University, Bozeman, MT 59717, USA
E-mail: livinghouse@chemistry.montana.edu
Received 13 February 2004
This communication is dedicated to the memory of Heath Evans Freyer.
We initiated this study by examining the Group 3 amides
1a and Y[N(TMS)2]3 (1b) as prospective catalysts for the
hydroboration of 1-hexene (3a) with freshly distilled
catecholborane (4). Our early results were, if anything,
overly gratifying as both 1a and 1b (3.3 mol%) promoted
the hydroboration of 3a with 4 in 6 hours and 24 hours, re-
spectively at 25 °C. In this connection it is most relevant
that even N,N-dimethylacetamide has been shown to
significantly catalyze the hydroboration of simple alkenes
by 4.7 In view of this, we subsequently found that the sup-
posedly benign HN(TMS)2 (5, 10 mol%, C6D6) catalyzed
the hydroboration of 3a with 4 in 24 hours at 25 °C and
that direct exposure of 3a to 4 in the absence of any
‘catalyst’ led to the formation of 17% of the hydrobora-
tion product (1H NMR) after 2 days at 25 °C! These find-
ings serve as a caveat regarding the use of 4 for catalyst
evaluation as competing background hydroborations can
render the observed results inconclusive.
Abstract: Tris[bis(trimethylsilyl)amide] has been shown to be an
effective catalyst for the hydroboration of representative alkenes
and styrenes by pinacolborane.
Key words: hydroboration, lanthanides, catalysis
The hydroboration of alkenes is one of the most syntheti-
cally valuable methods for the preparation of myriad
organic molecules of commercial and medicinal impor-
tance.1 The classical direct variant of this reaction has
lately been augmented by Rh(I)-catalyzed hydroborations
involving catecholborane.2 Recently, Group 3 metallo-
cenes have been demonstrated to catalyze a mechanisti-
cally complementary process3a,b that has been extended to
the cyclization/boration of dienes.4 The high sensitivity of
Group 3 metallocene complexes to air coupled with the
comparative difficulty associated with their preparation
have hindered the widespread use of these compounds in
catalysis. We have previously disclosed that simple amido
derivatives of the Group 3 metals corresponding to the
formula Ln[N(TMS)2]3 1 (Ln = lanthanide) are competent
catalysts for intramolecular alkene hydroamination5 and
that La[N(TMS)2]3 (1a) is an efficient mediator of alkene
and diene hydrosilylations.6 In this communication we
show that commercially available 1a possesses activity as
a catalyst for the intermolecular hydroboration of alkenes
with pinacolborane (2).
Pinacolborane (2) is known to be a less reactive hydro-
borating agent than 48 and it as well as its derivatives are
also significantly less predisposed to undergo dispropor-
tionation. As had been expected, attempted hydroboration
of 3a with 2 (2 equivalents) either in the presence or
absence of HN(TMS)2 (5, 90 °C, C6D6, 24 h) led to no re-
action.9 By way of contrast, exposure of 3a to 2 (2 equiv-
alents) in the presence of 1a (3.0 mol%, C6D6, 90 °C, 19
h) gave rise to hexan-1-ol (6a) in 90% isolated yield
(Scheme 1).10 In consonance with our previous results
concerning the hydrosilylation of 3a with PhSiH3,
Y[N(TMS)2]3 (1b) proved vastly inferior to 1a as a cata-
lyst for hydroboration using 2.6 Alternative La3+ sources
were subsequently examined and found to be inactive as
catalysts for hydroboration. Accordingly, treatment of 3a
with 2 in the presence of 3 mol% of Cp3La or LaI311 (C6D6,
90 °C, 48 h) led to no reaction. The possibility that 1a was
simply catalyzing the disproportionation of 2 to BH3 was
addressed by the following control experiments. Hydro-
boration of 3a with 2 (2 equivalents) and 1a (3.0 mol%) in
the presence of Et3N (1 equiv) to serve as a trapping agent
for BH3 proceeded to 95% conversion over 50 hours at
90 °C.12 In addition, the attempted reaction of 2 with Et3N
(1 equivalent) either in the presence or absence of 1a (3.0
mol%, C6D6, 90 °C, 19 h) did not provide detectable
quantities (1H NMR) of BH3·NEt3.
O
O
B
1)
H
2
OH
n-H9C4
n-H9C4
La[N(TMS)2]3
90%
1a
3a
6a
(3.0 mol%)
H2O2, OH–
2)
Scheme 1
SYNLETT 2004, No. 14, pp 2639–2641
Advanced online publication: 10.11.2004
DOI: 10.1055/s-2004-835645; Art ID: S12004ST
© Georg Thieme Verlag Stuttgart · New York
2
2
.1
1
.2
0
0
4
Cyclohexene (3g), indene (3f) and several representative
styrenes (e.g., 3b–e) were then subjected to hydroboration

2640
Y. Horino et al.
LETTER
using 2 (2 equiv) and 1a (3.0 mol%). In every instance, In conclusion, we have demonstrated that commercially
hydroboration proceeded to >99% conversion (1H NMR) available15 amide La[N(TMS)2]3 (1a) is an effective
under the indicated conditions. In all cases, control exper- catalyst for representative hydroborations of alkenes with
iments were performed in which the alkene was treated pinacolborane (2). The utilization of lanthanum amides
with 2 (2 equiv) both alone and in the presence of possessing electronically and sterically varied ligand
HN(TMS)2 (5) and in no instance was hydroboration ob- environments, as well as other lanthanum chelates for
served. The results of this study are assembled in stereoselective hydrosilylation and catalyzed hydrobora-
Table 1.13
tion reactions will be reported shortly.
As is evident from these results, 1-hexene and styrene
substrates exhibit a pronounced bias toward anti-
Markovnikov hydroboration (Table 1, entries 1–6). Addi-
tionally, both electron deficient styrenes [e.g., 3d that pos-
sesses a strongly s-electron-withdrawing para substituent
(CF3)] and those that are electron rich (e.g., 3c) undergo
facile hydroboration. Increased alkene substitution
(Table 1, entries 5–7) results in suppression of the
reaction rate.
Acknowledgment
Generous financial support for this research was provided by the
National Institutes of Health and the National Science Foundation.
References
(1) Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents;
Academic Press: New York, 1988, and references cited
therein.
(2) (a) Evans, D. A.; Fu, G. C.; Hoveyda, A. H. J. Am. Chem.
Soc. 1992, 114, 6671; and references cited therein.
(b) Hayashi, T.; Matsumoto, Y.; Ito, Y. J. Am. Chem. Soc.
1989, 111, 3426. (c) Burgess, K.; Ohlmeyer, M. J. J. Org.
Chem. 1988, 53, 5179.
Although the identity of the actual catalyst in these reac-
tions has not yet been established, it is likely that reduc-
tion of the precatalyst 1a by 2 initiates the catalytic cycle
with concomitant generation of a lanthanum hydride of
the type 7. Anti-Markovnikov addition of 7 to 3 should
lead to the transient lanthanum alkyl 8 which could under-
go intermolecular s-bond metathesis with 2 to regenerate
7 with concomitant production of the boronic pinacolate 9
(Scheme 2).14
(3) (a) Harrison, K. N.; Marks, T. J. J. Am. Chem. Soc. 1992,
114, 9220. (b) Bijpost, E. A.; Duchateau, R.; Teubin, J. H. J.
Mol. Catal. A 1995, 95, 121.
(4) Molander, G. A.; Pfeiffer, D. Org. Lett. 2001, 3, 361.
Table 1 Catalyzed Hydroborations of Alkenes
Alkene
Reaction conditions
90 °C, 19 h
Producta
Isolated yield (%)
90% (>99:1)b
1
OH
n-H9C4
n-H9C4
3a
6a
2
3
4
5
90 °C, 20 h
90 °C, 22 h
90 °C, 25 h
90 °C, 6 d
85% (>99:1)b
86% (90:10)b
65% (>99:1)b
98% (>99:1)b
OH
H5C6
H5C6
3b
6b
OH
p-(MeO)H4C6
p-(MeO)H4C6
3c
6c
OH
p-(CF3)H4C6
p-(CF3)H4C6
3d
6d
OH
H5C6
H5C6
6e
3e
6
7
90 °C, 7 d
76% (5.75:1)b
OH
3f
6f
120 °C, 2 d
86%
OH
3g
6g
a Product obtained after oxidation.
b Regioisomeric ratio.
Synlett 2004, No. 14, 2639–2641 © Thieme Stuttgart · New York

LETTER
Alkene-Pinacolborane Hydroborations
2641
La[N(TMS)2]3
1a
H-La(L)2
7
H
La(L)2
O
O
R
B
H
8
2
R
3
O
O
B
H
2
O
B
O
R
H-La(L)2
9
7
Scheme 2
(5) (a) Kim, Y. K.; Livinghouse, T.; Bercaw, J. E. Tetrahedron
Lett. 2001, 42, 2933. (b) For the use of novel group 3
chelates to accelerate intramolecular aminoalkene
hydroaminations, see: Kim, Y. K.; Livinghouse, T. Angew.
Chem. Int. Ed. 2002, 41, 3645. (c) Kim, Y. K.; Livinghouse,
T.; Horino, Y. J. Am. Chem. Soc. 2003, 125, 9560.
(6) Horino, Y.; Livinghouse, T. Organometallics 2004, 23, 12.
(7) Garrett, C. E.; Fu, G. C. J. Org. Chem. 1996, 61, 3224.
(8) Srebnik has reported the use of (Ph3P)3RhCl as a catalyst for
the hydroboration of representative alkenes by 2: Pereira, S.;
Srebnik, M. Tetrahedron Lett. 1996, 37, 3283.
(13) A General procedure for alkene hydroborations is as
follows: In an argon-filled glove box, La[N(TMS)2]3 (1a,
18.6 mg, 0.03 mmol) and C6D6 (1.0 mL) were introduced
into a J. Young NMR tube equipped with teflon screw cap
and then pinacolborane (2, 256 mg, 2.0 mmol) and the
appropriate alkene (1.0 mmol) were added by micro syringe.
The homogeneous reaction mixture was maintained at 90 °C
or 120 °C, respectively, using an oil bath until the
hydroboration reaction was judged complete by the
disappearance of the appropriate olefinic resonances in the
1H NMR spectrum. The resulting solution was transferred to
a 25 mL round-bottomed flask and was cooled to 0 °C with
stirring whereupon 1:1 = THF:EtOH (6 mL), 2 N NaOH (3
mL) and 30% H2O2 (3 mL) were added. The reaction
mixture was allowed to warm to r.t. and was stirred for 9 h.
It was then extracted with Et2O (3 × 10 mL) and the
combined organic phases were dried over MgSO4. After
removal of the solvents in vacuo, the crude product was
purified by column chromatography on silica gel (5%
EtOAc–hexane for elution) to provide the corresponding
alcohol as a colorless oil.
(9) By way of contrast, Knochel has reported that crude 2 (2
equiv, prepared in situ from BH3·SMe2 and pinacol) is
capable of hydroborating alkenes (ClCH2CH2Cl, 50 °C, 48
h): Tucker, C. E.; Davidson, J.; Knochel, P. J. Org. Chem.
1992, 57, 3482.
(10) 1H NMR revealed >99% conversion of 3a to the
corresponding boronic pinacolate.
(11) Evans has reported that SmI3 is a catalyst for the
hydroboration of alkenes with catecholborane: Evans, D. A.;
Muci, A. R.; Stürmer, R. J. Org. Chem. 1993, 58, 5307.
(12) Temperatures of 125–200 °C are required for effecting the
hydroboration of alkenes using BH3·NEt3: Hawthorne, F. J.
Org. Chem. 1958, 23, 1788.
(14) An analogous catalytic cycle has been proposed in
connection with the hydroboration of alkenes by
catecholborane (4) catalyzed by permethyllanthanocene
complexes.3a
(15) Available from Gelest Inc., 612 William Leigh Drive,
Tullytown, PA 19007-6308, USA.
Synlett 2004, No. 14, 2639–2641 © Thieme Stuttgart · New York