Di(isopropylprenyl)borane: A new hydroboration reagent for the synthesis of alkyl and alkenyl boronic acids

Article abstract of DOI:10.1002/anie.200351312

The best of the old: The new hydroboration reagent 1 combines the selectivity of disiamylborane (2) with the reactivity towards carbonyls of allyl boranes 3. Conventional oxidative workup conditions are avoided, as facile hydrolysis in protic media provides the corresponding boronic acids with up to 99% anti-Markovnikov selectivities. A one-pot hydroboration/ Suzuki-Miyaura protocol adds to the synthetic value of 1.

Full text of DOI:10.1002/anie.200351312

Angewandte
Chemie
Hydroboration Reagents
Di(isopropylprenyl)borane: A New
Hydroboration Reagent for the Synthesis of Alkyl
and Alkenyl Boronic Acids
Alexey V. Kalinin, Stefan Scherer, and Victor Snieckus*
Dedicated to Professor H.C. Brown**
Scheme 2. Amalgamation of two classes of borane reagents in 1.
R=alkyl, alkenyl.
to mixtures of products, application of 9-BBN results in highly
selective reactions in certain cases. Of particular interest, 1:1
hydroboration of 2,5-dimethylhexa-2,4-diene (2, Scheme 3)
with 9-BBN leads to the anti-Markovnikov monohydrobora-
tion product in 90% yield.[14] Based on these studies, we
postulated that the reaction of borane with slightly more than
2 equivalents of 2 would mainly lead, by double anti-
Markovnikov hydroboration, to 1, which was expected to
exhibit hydroboration selectivity resembling that of disiamyl-
borane,[2,15] reactivity towards carbonyls similar to that of allyl
boranes,[3,16] and facile hydrolysis in protic media[17]
(Scheme 2). Consequently, we envisaged that 1 could serve
as a valuable hydroboration reagent, allowing conversion of
the product boranes into the corresponding boronic acids
under mild conditions similar to those used for the hydrolysis
of catechol-[18] and dihaloboranes,[19] without the need for the
standard strong oxidizing reagents.[2,20]
In pursuit of results to support this idea, we found that the
addition of dimethyl sulfate to a mixture of diene 2 and
NaBH4 in diglyme[21] at 0–58C was accompanied by intensive
gas evolution followed by the formation of a substantial
amount of a colorless precipitate upon ageing of the mixture
(3 h/08C). The precipitate was rapidly consumed in an exo-
thermic reaction with phenylacetylene, leading to the pre-
The discovery of the hydroboration reaction by Brown and
Subba Rao almost 50 years ago[1] launched the era of organo-
borane chemistry, which today constitutes a conventional
component of synthetic organic chemistry[2] sustained by the
availability of organoboranes either commercially or pre-
pared by reliable protocols.[2,3] The significance of organo-
boron compounds in synthesis surged when Suzuki and
Miyaura discovered the transition-metal-catalyzed cross-cou-
pling reaction,[4] which more recently has seen explosive
growth, particularly in industrial practice.[5] The current high
activity in this area is evidenced by the further development
of cross-coupling reactions of organoboron compounds for
[4–6]
and C–heteroatom bond formation,[7] C–H inser-
ꢀ
C C
tion,[8] allylic substitution,[9] Diels–Alder and dipolar cyclo-
addition,[10] and 1,2-/1,4-addition processes.[11] Consequently,
the quest for new reagents and procedures for the preparation
of organoboronic acids and their derivatives is a continuing
necessity.
Herein we report the development and synthetic utility of
di(isopropylprenyl)borane (1, iPP2BH), a new organoborane
reagent that is generated in situ and that provides rapid access
to alkyl and alkenyl boronic acids and their derivatives
conveniently, economically, and under mild conditions
(Scheme 1).
¼
sumed formation of borane 3a (Scheme 3, R = PhCH CH).
Detailed studies by Brown and co-workers[12] and Mikhai-
lov et al.[13] showed that although hydroboration of conju-
gated dienes with borane is a complex process that gives rise
Treatment of the reaction mixture with water and an aqueous
Scheme 1. General hydroboration process with 1.
[*] Dr. A. V. Kalinin
Department of Chemistry, University of Waterloo
Waterloo, Ontario, 2NL 3G1 (Canada)
Dr. S. Scherer
Clariant GmbH, Division LSE, BU Pharmaceuticals
65926 Frankfurt am Main (Germany)
Scheme 3. General protocol for the hydroboration of alkenes/alkynes
with 1. Reagents and conditions: a) NaBH4 (1 equiv), (MeO)2SO2
(1 equiv), diglyme, 0–58C, 3 h; b) BH3·THF (1m, 1 equiv), THF,
0–58C, 3 h. Method A: H2O quench, 30 min, room temperature; then
aqueous CH2O (1 equiv), 24h, room temperature; then
(HOCH2CH2)2NH (1.1 equiv). Method B: H2O quench, 30 min, room
temperature; then aqueous CH2O (1 equiv), 1 h, room temperature;
then HOCMe2CMe2OH (1.1 equiv), 24h, room temperature.
Method C: MeC(O)C(O)Me (1.1 equiv), 12 h, room temperature.
R=alkyl or alkenyl; diglyme=(MeOCH2CH2)2O.
Prof. Dr. V. Snieckus
Department of Chemistry, Queens University
Kingston, Ontario, K7L 3N6 (Canada)
Fax: (+1)613-533-6089
E-mail: snieckus@chem.queensu.ca
[**] This communication is in celebration of 91 years and with
admiration and appreciation of the “from little acorns to tall trees”
growth of his borane chemistry
Angew. Chem. Int. Ed. 2003, 42, 3399 – 3404
DOI: 10.1002/anie.200351312
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3399

Communications
Table 1: (Continued)
solution of formaldehyde followed by addition of diethanol-
amine led to the isolation of the corresponding adduct of
Entry
R
NaBH4
BH3·THF
¼
Selectivity Yield [%] Selectivity Yield [%]
styrylboronic acid 4a (R = PhCH CH) in 77% yield (meth-
od A, Scheme 3).[22]
4m
5m
6m
67
When pinacol was added instead of diethanolamine, the
corresponding borolane 5a was isolated in 83% yield and
> 99:1 anti-Markovnikov regioselectivity (GC/MS analysis of
the crude mixture; method B, Scheme 3) together with the
alcohol 7 (see Scheme 4).[23] Similar results were observed
when commercial BH3·THF complex in THF was used
(Table 1, entry 1), but in a homogeneous reaction.
For both procedures involving NaBH4 or BH3·THF, the
results may be rationalized by invoking the formation of 3,
>99
>99
>99
74
86
12
13
14
15
5n
6o
77
>99
–
72
–
98:2
73
55
Table 1: Synthesis of RB(OZ)2 by hydroboration of alkynes/alkenes with
1.[a]
4p
5p
>99
>99
68
47
16
17
4q
5r
–
–
>99
Entry
R
NaBH4
BH3·THF
Selectivity Yield [%] Selectivity Yield [%]
99:1
63
99:1
59
67
4a
5a
6a
77
1
2
3
99:1
99:1
99:1
83
72
4s
5s
4t
5t
18
19
98:2
91:9
97:3
96:4
71
4
4b
7499:1
66
70
8
4
4c
6c
5d
81
69
100
100
61
[a] Yields are for purified materials (recrystallized, distilled, or, for 6c and
6 m, after column chromatography) and are not optimized. Selectivities
were determined by GC and GC–MS analysis of the crude reaction
mixtures: an aliquot of the reaction mixture was worked up according to
method B by preparing the borolane derivative 5. [b] Yield is for the
mixture of a and b isomers; the yield of 98% pure (GC) 5d (a isomer) is
42%. [c] O-TMS-propargyl alcohol was used as the starting material.
62[b]
4
:16
96:484
6d
59
70
5e
83
17
–
–
5
6
which as expected for allyl boranes,[17] undergoes rapid
protonolysis via intermediate 8 to liberate alkene 9[24] and
yield the borinic acid 10 (Scheme 4). Reaction of 10 with
formaldehyde leads to 12 via intermediate 11. Hemiborate 12
is then esterified with diethanolamine (method A) or pinacol
(method B) to afford derivatives 4 and 5, respectively, and
alcohol 7 (Scheme 4).
As chemical proof for the formation of 3 and therefore, by
implication, of 1, repetition of the model hydroboration
experiment of phenylacetylene with biacetyl (1.1 equiv) as
the quenching reagent gave complex borolane 6a (R =
5 f
5g
98:2
61
98:2
55
7
8
5h
5i
96:467
97:3
99:1
59
73
55
98:2
93:7
72
41
¼
PhCH CH) in 72% yield as a single isomer, as judged from
9
5j
5k
4l
93:7
–
1
3
the H NMR spectrum (olefinic J = 18.4 Hz, C2 symmetry).
The formation of 6 (Scheme 5) may be rationalized by the
transition state 13, which is predisposed for the formation of
trans olefin 14. Intramolecular boron–carbonyl coordination
leads to the intermediate 15, in which repulsion between the
bulky side chains is minimized by adapting a pseudo-trans
borolane stereochemistry. Terminal collapse of 15 affords the
C2-symmetrical trans borolane 6 with trans stereochemistry in
both side chains.
10[c]
11
–
99:1
63
71
>99
68
>99
3400
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Scheme 4. Proposed hydrolysis and oxidation steps of 3 to products 4
or 5 and 7 (methods A and B).
Figure 1. X-ray crystal structure of 6m at the 50% probability level.
Hydrogen atoms have been omitted for clarity.
Figure 2. General trends in hydroboration with diisopropylprenylborane
(iPP2BH), dihaloborane complexes (Hal2BH·Me2S), catecholborane
(CthBH), and disiamylborane (Sia2BH). R=n-alkyl, Ph; Hal=Cl, Br;
X=O, Si.
Scheme 5. Proposed mechanism for the double oxidation of 3 by
biacetyl to form 6 (method C).
In view of the C2 symmetry of 6, the stereochemistry of
the borolane could not be confirmed by NMR spectroscopic
analysis. Fortunately, suitable crystals of 6m (see Table 1,
entry 12) were obtained and X-ray crystallographic analysis
established the trans stereochemistry of both the borolane
ring and of the alkenyl chains (Figure 1).[25] This result infers
its derivation from the parent 3 and strongly suggests that 1 is
the main product from the reaction of diene 2 with borane.
To explore the scope and limitations of the new boronic
acid synthesis, representative examples of alkenes and
alkynes were treated with 1 and the results are summarized
in Table 1. Several pertinent conclusions may be drawn:
a) hydroboration of terminal and symmetrical alkynes and
alkenes proceeds in a highly selective anti-Markovnikov
fashion (except Table 1, entry 5); b) both sources of borane
furnish hydroborated products with comparable regioselec-
tivity (except Table 1, entries 4 and 19); c) derivatives 4, 5,
and 6 (Scheme 3) are obtained in modest to good yields by
using conventional isolation techniques (distillation or crys-
tallization) and in upto 86% yield by using column
chromatography (Table 1, entry 12); d) sterically hindered
alkenes are less reactive under the tested conditions as
reflected in the modest yields of the final products (Table 1,
entries 16 and 19). Although direct hydroboration of a,b-
unsaturated esters (acrylates and cinnamates) was unsuccess-
ful, these compounds were readily accessed from their
protected precursors (Table 1, entries 6 and 17).[26]
and Hal2BH·Me2S,[19] the commonly used reagents to produce
the corresponding boronic acids under hydrolytic, nonoxida-
tive conditions (Figure 2). The similar selectivities achieved
with iPP2BH (1) and Sia2BH[2,15] may be attributed to their
comparable steric demand (see Scheme 2).
As a logical extension of the new hydroboration proce-
dure, the development of a sequential, “one-pot” hydro-
boration/Suzuki–Miyaura cross-coupling protocol was real-
ized (Scheme 6). In all experiments, borane 3a, generated
in situ from NaBH4, was coupled with 4-bromoanisole under
identical conditions. However, the hydrolysis–oxidation pro-
tocol (methods A1, B, C, and D) prior to the cross-coupling
stepwas varied for 3a.
Thus application of methods A1 and B to 3a led to the
intermediate styrylboronic acid (16) and pinacolate 5a,
respectively, which upon coupling with 4-bromoanisole,
afforded styrene 18 in virtually identical yields (60% and
62%). Borolane 6a was found to be a less effective coupling
partner, yielding 18 in 53% yield (method C). Interestingly,
cross-coupling of the borinic acid 17, a proposed intermediate
in the hydrolysis of 3a, also gave 18 but in a slightly lower
yield (46%, method D).
In conclusion, a new and general reagent for the hydro-
boration of alkenes and alkynes, iPP2BH (1), conveniently
prepared in situ by reaction of borane with 2,5-dimethylhexa-
2,4-diene (2), has been uncovered. Advantageous workup
protocols allow rapid access to alkyl and alkenyl boronic acids
and their derivatives under mild conditions. Extension to a
successful “one-pot” hydroboration/Suzuki–Miyaura cross-
The generally high hydroboration regioselectivities of
iPP2BH compare favorably with those observed for CthBH[18]
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analysis calcd for C12H16BNO2: C 66.40, H 7.43, N 6.45; found: C
66.36, H 7.35, N 6.45.
General Procedure (6m): Under an argon atmosphere, a flame-
dried flask was charged with a solution of 2 (8.10 mL, 56.92 mmol) in
THF (8 mL). The solution was cooled in an ice bath, and BH3·THF
solution (1m, 26 mL) was added while maintaining the reaction
temperature below 58C. The resulting solution was aged, with stirring,
for 3 h at 08C, and then treated with a solution of 9-vinylcarbazole
(5.00 g, 25.87 mmol) in THF (15 mL) while maintaining the temper-
ature below 58C. The reaction mixture was allowed to warm to room
temperature, stirred for 2 h, cooled in an ice bath, and slowly
quenched with 2,3-butanedione (2.72 mL, 31.04 mmol). The reaction
was stirred for 1 h, warmed to room temperature, and stirred for an
additional 12 h. Concentration under vacuum and purification by
flash column chromatography (silica gel, eluent: EtOAc/hexanes)
gave 6m (11.38 g, 22.16 mmol, 86%) as a colorless glass, which slowly
crystallized on standing. M.p. 79–818C (MeCN); IR (KBr): n˜ = 2959,
1630, 1598, 1485, 1464, 1453, 1380, 1347, 1325, 1237, 1081 cmꢀ1
;
1H NMR (400 MHz, CDCl3): d = 8.12 (d, 2H, J = 7.7 Hz), 7.51–7.45
(m, 4H), 7.25 (ddd, 2H, J = 7.9, 6.0, 2.2 Hz), 5.63 (dd, 2H, J = 15.8,
1.1 Hz), 5.36 (dd, 2H, J = 15.8, 6.6 Hz), 4.59–4.46 (m, 2H), 2.35–2.25
(m, 2H), 1.57–1.51 (m, 2H), 1.51 (s, 6H), 1.13 (s, 6H), 1.11 (s, 6H),
1.02 (d, 6H, J = 6.8 Hz), 1.01 ppm (d, 6H, J = 6.8 Hz); 13C NMR
(100 MHz, CDCl3): d = 139.9, 134.3, 134.2, 125.5, 122.9, 120.3, 118.6,
108.7, 92.4, 45.7, 38.8, 31.5, 25.8, 25.5, 22.6, 22.5, 18.7, 11.5 ppm (br);
MS (EI): m/z (%): 514 (M++1, 8), 513 (M+, 20), 293 (17), 292 (62), 291
(25), 181 (12), 180 (44), 112 (16), 111 (100); HRMS (EI): calcd for
C34H48BNO2: 513.3778; found: 513.3784.
Scheme 6. One-pot hydroboration/Suzuki–Miyaura cross-coupling pro-
tocol. Method A1: similar to method A, but without addition of
(HOCH2CH2)2NH. Method D: H2O quench, 1 h, room temperature.
Yields are for recrystallized material and are not optimized.
Received: March 3, 2003 [Z51312]
coupling protocol and perusal of the current state of organo-
boron chemistry[2–4] hold promise for further application of
these findings in organic synthesis.
Keywords: boranes · cross-coupling · homogeneous catalysis ·
.
hydroboration
Experimental Section
General procedure (4a): Under an argon atmosphere, a flame-dried
flask was charged with NaBH4 (757 mg, 20 mmol), 2 (7.1 mL,
50 mmol), and anhydrous diglyme (20 mL), and the resulting mixture
was cooled in an ice bath. (MeO)2SO2 (1.9 mL, 20 mmol) was added
to the stirred mixture over ~ 1 h while maintaining the temperature
below 58C. The addition was accompanied by an intensive gas
evolution and homogeneity of the reaction mixture. The mixture was
aged, with stirring, for 3 h at 08C, resulting in the gradual formation of
a new thick suspension, to which phenylacetylene (2.2 mL, 20 mmol)
was added slowly (~ 20 min) while maintaining the temperature below
58C. The resulting mixture was stirred for 1 h at 08C, slowly quenched
with H2O (3 mL) (additional gas evolution), stirred for 30 min at
room temperature, and treated with a solution of formaldehyde
(1.5 mL, 20 mmol, aq 37 wt% solution) in a single addition. The
resulting exothermic reaction was compensated by application of an
ambient-temperature bath. The reaction mixture was stirred for 24 h
at room temperature and diluted with EtOAc (40 mL). After
separation of layers, the organic phase was dried (Na2SO4), trans-
ferred into a flask containing diethanolamine (2.31 g, 22 mmol), and
the combined mixture was evaporated under vacuum (25 Torr, then
0.5 Torr, heating bath at 50 then 808C). The resulting solid residue
was recrystallized from MeCN to give 4a (3.34 g, 15.38 mmol, 77%
yield) as colorless needles. M.p. 197–1998C (MeCN); IR (KBr): n˜ =
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3000 (br), 1622, 1598, 1573, 1494, 1469, 1454, 1277, 1242, 1206 cmꢀ1
;
1H NMR (400 MHz, [D6]DMSO): d = 7.36 (d, 2H, J = 7.8 Hz), 7.27 (t,
2H, J = 7.5 Hz), 7.15 (t, 1H, J = 7.4 Hz), 6.84 (br s, 1H), 6.66 (d, 1H,
J = 18.1 Hz), 6.24 (d, 1H, J = 18.1 Hz), 3.81–3.73 (m, 2H), 3.71–3.64
(m, 2H), 3.08–2.99 (m, 2H), 2.26–2.70 ppm (m, 2H); 13C NMR
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125.7, 62.5, 50.4 ppm; MS (EI): m/z (%): 217 (M+, 1), 187 (9), 186
(100), 185 (8), 130 (7), 129 (9), 114 (87), 103 (25), 77 (15); elemental
3402
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[24] Experiments to isolate
ence [17]).
9 were not conducted (see refer-
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3404
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www.angewandte.org
Angew. Chem. Int. Ed. 2003, 42, 3399 – 3404