A new facile approach to highly selective hydroboration of alkenes with sodium malonyloxyborohydride

Article abstract of DOI:10.1039/b100228g

The preparation and properties of sodium malonyloxyborohydride (SMB) are described. As a white solid, SMB is very sensitive to moisture in air. It is also highly sensitive to the structure of alkenes. Thus SMB, prepared in situ from the reaction of malonic acid with equimolar NaBH⁴ in dry THF, hydroborates alkenes showing that terminal aliphatic alkenes undergo facile hydroboration with 98% of the boron moiety placed on the terminal position and 2% at the 2 position. On the other hand, trisubstituted alkenes and cyclohexene show poor reactivity in the hydroboration as compared with terminal alkenes. These characteristics indicate that SMB is a highly regioselective monofunctional hydroborating reagent for terminal alkenes.

Full text of DOI:10.1039/b100228g

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A new facile approach to highly selective hydroboration of alkenes
with sodium malonyloxyborohydride
Shi-Wen Huang, Wei-Li Peng, Zi-Xing Shan and De-Jie Zhao
Department of Chemistry, W uhan University, W uhan, 430072, P.R. China.
E-mail: swhuang=public.wh.hb.cn; dejiezh=public.wh.hb.cn
Received (in Montpellier, France) 4th January 2001, Accepted 21st March 2001
First published as an Advance Article on the web 14th May 2001
The preparation and properties of sodium malonyloxyborohydride (SMB) are described. As a white solid, SMB is
very sensitive to moisture in air. It is also highly sensitive to the structure of alkenes. Thus SMB, prepared in situ
from the reaction of malonic acid with equimolar NaBH in dry THF, hydroborates alkenes showing that terminal
4
aliphatic alkenes undergo facile hydroboration with 98% of the boron moiety placed on the terminal position and
2% at the 2 position. On the other hand, trisubstituted alkenes and cyclohexene show poor reactivity in the
hydroboration as compared with terminal alkenes. These characteristics indicate that SMB is a highly
regioselective monofunctional hydroborating reagent for terminal alkenes.
yellow Ñame which indicates it is an organoboron compound.
The infrared spectrum of SMB did not show characteristic
Introduction
Hydroborations based on borane have become a highly
valuable tool and have extensive applications in organic syn-
thesis.1 To date, boraneÈtetrahydrofuran and boraneÈdimethyl
sulÐde (BMS) have been the two most widely used hydro-
borating reagents, however the development of new eco-
nomical, safe, easily used and highly selective hydroborating
reagents is still an important topic in hydroboration
research.2 Exotic borane hydroborating reagents are not
readily available. Sodium acetoxyborohydride (SAB,
absorption peaks at 2222, 2291 and 2383 cm~1 due to
NaBH ,8 whereas a clear absorption at 2470 cm~1, perhaps
4
due to the characteristic stretching vibration of the BÈH bond,
appeared. This absorption, however, was quite weak because
the BÈH bond in SMB is very sensitive to hydrolysis by traces
of moisture in air. On the other hand, the absence of absorp-
tion at 1740 cm~1 due to the carbonyl group of malonic acid
and a very strong absorption at 1688 cm~1 resulting from the
C2O stretching vibration of SMB was observed.
NaBH OAc),3 prepared in situ from acetic acid and sodium
3
borohydride, could provide an alternative for the hydro-
(1)
boration of alkenes as perhaps one of the most convenient
and efficient reagents.4 More systematic studies indicated that
the reaction of alkenes with SAB proceeds smoothly to di-
alkylboron species at 50 ¡C in THF; even in the presence of an
excess of alkenes the reaction does not proceed further.
However, it is difficult to halt the reaction at the mono-
alkylboron species stage; even in the case of a 1 : 1 molar ratio
at room temperature, the reaction is relatively slow and gives
an equivalent mixture of mono- and di-alkylboron species.5
On the other hand, the preparation of sodium mal-
onyloxyborohydride (SMB) as a weaker reducing agent was
reported.6 Monoalkylboron species have great synthetic
potential in carbonÈcarbon bond forming reactions to prevent
the loss of ““RÏÏ groups.7 We investigated the hydroboration
behavior of alkenes by using SMB as a monofunctional
hydroborating agent. In this paper, we report the preparation
and properties of SMB and its selective hydroboration of
alkenes in a one pot reaction.
The hydroboration of alkenes with SMB prepared in situ
has been studied in THF at various temperatures. We used
-heptene, 1-octene and 2-methyl-2-butene and cyclohexene as
representative terminal and internal alkenes respectively. For
example, the hydroboration of a terminal alkene with a 10%
1
excess of SMB, prepared in situ from the reaction of NaBH
with equimolar malonic acid in THF at room temperature for
4
1
h, proceeds in THF using an oil bath which was Ðrst heated
gradually to 60 ¡C and then was maintained at the same tem-
perature for 5 h. The alkylborate salt was hydrolyzed with 3
M sodium hydroxide and oxidized with 30% hydrogen per-
oxide in the usual manner9 to give the corresponding alcohol,
which was characterized by 1H NMR, IR and GC, as shown
in eqn. (2); the results are summarized in Table 1.
(2)
Results and discussion
The hydroborating reagent SMB was prepared from the inter-
action of sodium borohydride with equimolar malonic acid in
THF or diglyme (2,5,8-trioxanonane) at room temperature in
quantitative yield [eqn. (1)]. It is a white solid which does not
melt and decomposes at over 350 ¡C. It is insoluble in THF,
diglyme, chloroform and other non-protonic solvents and
highly sensitive to the moisture in air, decomposing imme-
diately to release hydrogen. It burns in air with a greenÈ
The data in Table 1 reveal that the hydroboration of ter-
minal alkenes with SMB prepared in situ is slow at room tem-
perature (25 ¡C). Fortunately at a moderate temperature
(60 ¡C) the hydroboration becomes sufficiently rapid to be
practical as a synthetic procedure. It is noteworthy that the
hydroboration of terminal alkenes with diborane or SAB pro-
ceeds in a regioselective manner to place 94 or 95% of the
boron atoms at the terminal position respectively.5,10 SMB,
DOI: 10.1039/b100228g
New J. Chem., 2001, 25, 869È871
869
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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Table 1 The hydroboration of alkenes with SMB
Regioselectivity of oxidation productsa
Alkene
T /¡C
t/h
1 position
2 position
Yieldb (%)
1
1
2
-Octene
-Heptene
-Methyl-2-butene
25
60c
25
ReÑux
60c
36
5
16
16
5
97.5
98
È
È
97
2.5
2
È
È
3
88
79.5d
30.3
10
Cyclohexene
-Heptene ] cyclohexenee
1
68.5d
a Determined by GC analysis using an E-30 column; 1 and 2 indicate the positions of carbon atoms of corresponding terminal alkenes. b The
overall yield of the mixture of alcohols determined by GC using an E-30 column. c The temperature of the oil bath. d The isolated yield of the
mixture of alcohols by distillation. e Using an equimolar mixture of 1-heptene and cyclohexene as starting materials for the hydroboration. The
reaction mixture was oxidized with alkaline hydrogen peroxide to give the products 1- and 2-heptanol; cyclohexanol was undetected by GC
analysis.
however, is more regioselective than diborane or SAB in the
hydroboration of terminal alkenes. For example, under
Experimental
optimum conditions, hydroboration in THF using a 10%
excess of SMB and the reaction Ñask immersed in an oil bath
at 60 ¡C for 5 h, terminal alkenes such as 1-heptene undergo
hydroboration to place 98% of boron on the terminal position
with 2% at the 2 position. On the other hand, trisubstituted
and cyclic internal alkenes such as 2-methyl-2-butene and
cyclohexene are hydroborated considerably more slowly than
terminal alkenes by SMB, which is probably due to steric hin-
drance involving the approach of the bulky SMB to the oleÐn-
ic center.
Materials and instrumentation
All alkenes were obtained from Aldrich and reagent grade
THF and diglyme were distilled from lithium aluminium
hydride and stored under nitrogen. Reagent grade malonic
acid and NaBH were used without further puriÐcation.
4
All glassware was dried at 140 ¡C for 10 h prior to use. All
reactions were carried out under a static pressure of nitrogen.
IR spectra were determined by a Nicolet-170SX FT IR
spectrometer, 1H NMR spectra on a Jeol FX-90Q spectro-
meter. The oxidation products were analyzed by GC using a
Consequently, the higher reactivity of the reagent SMB
toward terminal as compared with internal alkenes including
trisubstituted and cyclic oleÐns makes possible the selective
hydroboration of terminal alkenes in the presence of internal
alkenes. For example, the competitive hydroboration of 1-
heptene and cyclohexene with SMB was carried out using a
THF solution of 12.6 mmol of malonic acid added to a reac-
tion Ñask containing 12.6 mmol each of 1-heptene, cyclo-
hexene and NaBH4 in THF in an oil bath at 60 ¡C for 5 h,
followed by oxidation with alkaline hydrogen peroxide and
work-up following the general procedure. The products were
puriÐed by distillation and 1-heptanol as major product was
obtained in 68.5% yield with 97% isomeric purity by GC
analysis, and cyclohexanol was undetected. It is clear that
selective hydroboration of terminal alkene in a mixture of ter-
minal alkene and cyclohexene or of a terminal double bond in
a structure containing both terminal and cyclic double bonds
could be achieved.
1
02G gas chromatograph. Melting points were determined
with an X4 melting point apparatus and are uncorrected.
Preparation of SMB
A solution of malonic acid (1.04 g, 10 mmol) in THF (15 ml)
was added dropwise to a suspension of NaBH (0.38 g, 10
mmol) in THF (20 ml) with vigorous stirring at room tem-
perature; the hydrogen evolved was measured with a gas
buret. After stirring for one hour, the reaction mixture was
stirred at 60 ¡C until no hydrogen was released (about 3 h and
4
440 mL hydrogen were collected) to give a white precipitate
which can be used directly for the hydroboration. A small
amount of the precipitate thus obtained was Ðltered o† under
vacuum and washed with diglyme, then dried under vacuum
to give a sample of SMB for identiÐcation. IR (cm~1): 2470,
1688, 1591, 1360, 1077, 1009, 855, 696.
General procedure for the hydroboration of alkenes with SMB
In order to study the monohydroborating behavior of
SMB, we also examined the hydroboration of 1-heptene with
SMB in the molar ratio 2 : 1 in THF at 60 ¡C, followed by
The following procedure for the hydroboration of 1-heptene
with SMB is representative. To a suspension containing
dichlorocarbene insertion using 50% NaOHÈCHCl under
phase transfer conditions and oxidation with alkaline hydro-
NaBH (1.25 g, 33 mmol) and 1-heptene (2.95 g, 30 mmol) in
THF (30 mL), a solution of malonic acid (3.43 g, 33 mmol) in
3
4
gen peroxide. After the reaction mixture had been worked up
as described in the published procedure;9 the 1-heptanol was
isolated by distillation in 75.2% yield with 95.5% isomeric
purity by GC analysis and no insertion product, 8-
pentadecanone, was obtained. In our previous papers9 and
other groupsÏ work5,11 it was shown that the insertion of
dichlorocarbene into CÈB bonds of diorganyl borinates, fol-
lowed by reaction with alkaline hydrogen peroxide, gave the
corresponding ketones. The insertion of dichlorocarbene into
the CÈB bond of the alkylboronate did not occur. If SMB is a
difunctional hydroborating reagent, the insertion of dichloro-
carbene into the dialkylborinate, the intermediate of SMB
reacting with two mol of 1-heptene, should give 8-
pentadecanone. We now can conclude that SMB is a highly
regioselective monofunctional hydroborating reagent for ter-
minal alkenes to give monoalkylboron species in high yield
even using terminal alkenes and SMB in a 2 : 1 molar ratio.
THF (20 mL) was added dropwise over 45 min with efficient
stirring at room temperature. After completion of the addi-
tion, the reaction mixture was stirred at the same temperature
for 1 h. The reaction Ñask was immersed in an oil bath and
heated gradually to 60 ¡C within 1 h, then maintained at 60 ¡C
for 5 h with vigorous stirring. The reaction mixture was
cooled to room temperature and then treated carefully with 20
mL 3 M sodium hydroxide and 3.5 mL 30% hydrogen peroxi-
de. The contents were stirred Ðrst for 30 min at room tem-
perature, then for 2 h at 40È50 ¡C. After cooling to room
temperature, the reaction mixture was acidiÐed with 3 M
hydrochloric acid. The organic layer was separated and the
aqueous phase extracted with diethyl ether (3 ] 20 mL). The
combined organic phase was washed successively with satu-
rated brine and water, dried over anhydrous sodium sulfate
and analyzed by GC for the amounts of isomeric alcohol, 98%
1-heptanol and 2% 2-hepanol. The products, in this case 2.77
870
New J. Chem., 2001, 25, 869È871

View Article Online
g (79.5%), was obtained by distillation, bp 173È176 ¡C. IR:
Kanth, P. V. Dalvi and M. Zaidlewicz, J. Org. Chem., 1999, 64,
6
263; J. V. B. Kanth and H. C. Brown, Org. L ett., 1999, 1, 315; S.
3
347, 2957, 2926, 2856, 1415, 1395, 1057, 720 cm~1. 1H NMR
Narasimhan, S. Swarnalakshmi and R. Balakumar, Indian J.
Chem., Sect. B: Org. Chem. Incl. Med. Chem., 1998, 37, 1189.
For a recent review of the synthetic utility of SAB, see: G. W.
Gribble, Chem. Soc. Rev., 1998, 27, 395; G. W. Gribble, Chemtech,
(
CDCl ; TMS, d 0): d 3.50È3.68 (t, 2H, CH O), 1.70È2.10 (s,
3
2
1
H, OH), 1.10È1.70 (m, 10H, 5CH ), 0.70È1.05 (t, 3H, CH ).
3
2
3
1
996, 12, 26.
G. W. Gribble and C. F. Nutaitis, Org. Prep. Proc. Int., 1985, 17,
17; M. Periasamy, Curr. Sci., 1995, 68, 883.
C. Narayana and M. Periasamy, T etrahedron L ett., 1985, 26,
4
5
Acknowledgements
3
This work was supported by the National Natural Science
Foundation of China.
1
757; C. Narayana and M. Periasamy, T etrahedron L ett., 1985,
26, 6361.
6
7
B. C. Hui, V entron Alembic, 1980, 20, 1; Chem. Abstr., 94, 174242.
H. C. Brown, N. G. Bhat and V. S. Somayaji, Organometallics,
1
983, 2, 1311.
References
8
9
W. V. Hough, L. J. Edwards and A. D. McElory, J. Am. Chem.
Soc., 1958, 80, 1828.
1
H. C. Brown and P. V. Ramachandran, Pure Appl. Chem., 1994,
6, 201; H. C. Brown, Organic Syntheses via Boranes, Aldrich
F. Guo, S. W. Huang, Z. X. Shan and D. J. Zhao, J. Chem. Res.,
2000, 280; S. W. Huang, F. Guo, Z. X. Shan and D. J. Zhao,
Chin. J. Chem., 2001, 19, 202; D. J. Zhao, T. Y. Ke, C. M. Ke and
Z. X. Shan, Chin. Chem. L ett., 1993, 4, 471; T. Y. Ke, Z. X. Shan
and D. J. Zhao, J. W uhan Univ. (Nat. Sci. Ed.), 1995, 41, 702.
6
Chemical Co., Inc., Milwaukee, 1997; A. Pelter, K. Smith and
H. C. Brown, Borane Reagent, Academic Press, London, 1988.
M. Zaidlewicz, J. V. B. Kanth and H. C. Brown, J. Org. Chem.,
2
2
000, 65, 6697; H. C. Brown, J. V. B. Kanth, P. V. Dalvi and M.
10 H. C. Brown and G. Zweifel, J. Am. Chem. Soc., 1960, 82, 4708.
11 H. C. Brown and T. Imai, J. Org. Chem., 1984, 49, 892.
Zaidlewicz, J. Org. Chem., 2000, 65, 4655; H. C. Brown, J. V. B.
New J. Chem., 2001, 25, 869È871
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