Combining high pressure and catalysis: Pinacol- or catecholborane hydroboration of functionalized olefins

Article abstract of DOI:10.1039/b004275g

The hydroboration of three families of functionalized olefins (1-bromo- and 1,3-dibromopropenes, allylamines, 2,3-dihydrofuran) by pinacolborane and catecholborane has been studied under various experimental conditions. For 1-bromo- and 1,3-dibromopropenes, pinacolborane (PBH) is a poor reagent that requires the use of high pressure in ethereal solvents and provides only by-products, resulting from the undesired β-bromoboronate regioisomer. By contrast, catecholborane (CBH) affords mainly the expected α-bromoboronate at atmospheric pressure. With dibenzylallylamine, PBH yields a mixture of the two possible regioisomers (β- and γ-amino-boronates), whatever the pressure, while CBH affords selectively the expected γ-isomer in good yields under atmospheric thermal conditions. For 2,3-dihydrofuran, only PBH gives an efficient access, in THF, to a mixture of the corresponding α- and β-regioisomers when combining the effects of 0.5% Wilkinson's catalyst and high pressure.

Full text of DOI:10.1039/b004275g

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Combining high pressure and catalysis: pinacol- or catecholborane
hydroboration of functionalized olefins†
Sylvie Colin,a Lucile Vaysse-Ludot,b Jean-Pierre Lecouvéb and Jacques Maddaluno*a
1
a Laboratoire des Fonctions Azotées et Oxygénées Complexes de l’IRCOF, Université de Rouen,
76821 Mont St Aignan, France. Fax: 33 (0)235522971; E-mail: jmaddalu@crihan.fr
b ORIL Industrie, 13 rue Auguste Desgenetais, 76210 Bolbec, France
Received (in Cambridge, UK) 30th May 2000, Accepted 25th September 2000
First published as an Advance Article on the web 7th November 2000
The hydroboration of three families of functionalized olefins (1-bromo- and 1,3-dibromopropenes, allylamines,
2,3-dihydrofuran) by pinacolborane and catecholborane has been studied under various experimental conditions.
For 1-bromo- and 1,3-dibromopropenes, pinacolborane (PBH) is a poor reagent that requires the use of high
pressure in ethereal solvents and provides only by-products, resulting from the undesired β-bromoboronate
regioisomer. By contrast, catecholborane (CBH) affords mainly the expected α-bromoboronate at atmospheric
pressure. With dibenzylallylamine, PBH yields a mixture of the two possible regioisomers (β- and γ-amino-
boronates), whatever the pressure, while CBH affords selectively the expected γ-isomer in good yields under
atmospheric thermal conditions. For 2,3-dihydrofuran, only PBH gives an efficient access, in THF, to a mixture
of the corresponding α- and β-regioisomers when combining the effects of 0.5% Wilkinson’s catalyst and high
pressure.
Introduction
Organoboron compounds play a pivotal role in modern organic
chemistry because of their functional versatility1 and their
implications in asymmetric synthesis.2 Currently boronic esters
and especially α-aminoboronates, are also of interest in medi-
cinal chemistry since they can behave as extremely efficient
enzyme inhibitors.3 Their interaction with serine proteases
has been particularly well studied. The importance of the tetra-
hedral ate-complex formed by the boron atom upon attack of
the serine hydroxy group seems to be responsible for the
inhibitory activity, thanks to its topological analogy with the
transition state occurring during hydrolysis of the peptide
amide function.4 One of the major therapeutic challenges in
this area is definitely the application of these compounds to
the selective inhibition of thrombin, in an effort to prevent
the transformation of fibrinogen into soluble fibrin, further
converted into insoluble fibrin under the action of coagulation
factor XIIIa. This fundamental event in the formation of fibrin
clots alters the very last step of the coagulation cascade of
Fig. 1
which control is essential in the treatment of serious affections
such as pulmonary embolism or cardiovascular diseases (e.g,
provides. However, Knochel et al. underline it needs to be used
unstable angina, deep vein thrombosis). Thus, several molecules
such as S183265 and DuP7146 (Fig. 1) have already been
evaluated in the corresponding clinical tests.
One of the key-steps in the synthesis of these compounds is
the chemo- and enantioselective access to the α-aminoboronic
moiety. Approaches based on the hydroboration of functional-
ized olefins by dialkoxyboranes are of special interest. These
reagents are indeed efficient and afford products bearing a
boron atom directly at the appropriate oxidation state.
in excess (2 eq.) and also note that its RhCl(PPh3)3 (Wilkinson)
catalysed version can be troublesome;8 and (ii) the influence of
high pressures (>10 kbar) and/or catalysis9 on the efficiency
and selectivity of these same reactions. Although the effect of
transition metal catalysis on the hydroboration reaction has
indeed been the object of extensive work compiled recently in a
very good review,10 the influence of high pressure on this type
of reaction has been scarcely studied.11
In this wide context, we have decided to evaluate and
compare (i) the efficiency and selectivity of the hydroboration
of variously functionalized olefins by catechol7 (CBH) and
pinacol8 (PBH) boranes. The latter is indeed known for its
selectivity and the increased stability of the boronic esters it
Results and discussion
Three different types of olefins have been considered, viz.
two bromopropenes, three allylamines and 2,3-dihydrofuran.
Various synthetic routes to transform the corresponding
α-functionalized boronates into α-aminoboronic esters can be
proposed and are summarised in Fig. 2. The PBH used in
this work has been either prepared in CH2Cl2–dimethyl sulfide
† The IUPAC name for pinacolborane is 4,4,5,5-tetramethyl-1,3,2-
dioxaborolane and for catecholborane is 1,3,2-benzodioxaborole.
DOI: 10.1039/b004275g
J. Chem. Soc., Perkin Trans. 1, 2000, 4505–4511
This journal is © The Royal Society of Chemistry 2000
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Table 1 Wilkinson catalyzed hydroboration of 1-bromopropene 1 by pinacol (PBH) and catechol (CBH) boranes
Borane
(eq.)
Cat.
(%)
Product
(% yield)
Entry
1 isomer
Pressure
T/ЊC
Solvent
Time/h
1a
2
E ϩ Z
Z
Z
E ϩ Z
E
Z
PBH (1)
PBH (1)
PBH (2)
CBH (1.5)
CBH (2)
CBH (2)
1
1 bar
12.5 kbar
12.5 kbar
1 bar
1 bar
1 bar
25
20
20
20
20
20
CH2Cl2
THF
Et2O
C6H6
—
3
24
24
8
6
3
5 (50)
6 (80)
5 (82)
7 (82)
7 (74)c
7 (75)c
0.5
0.5
0.1
0.5
0.5
3
4b
5
6
—
a Results taken from ref. 12. b Results taken from ref. 14b. c As product 7 was difficult to purify, conversions are given instead of yields.
ism would disfavour this route under high pressure. The 82%
yield we observe seems to indicate that it is not the case (entry 3).
When switching from ether to THF, 4-bromobutoxyborane 6
is selectively obtained in the same conditions (entry 2). The
occurrence of this four-carbon atom chain product can be
explained by a THF ring fission, probably under the influence
of the Lewis-acid bromoboronate 4. It is indeed unlikely
that PBH itself triggers this reaction since we will see below
that hydroboration of 2,3-dihydrofuran by PBH in THF is
chemically efficient.
It is worth noting that by resorting to CBH instead of PBH,
Elgendy et al.14 have observed the opposite regioselectivity in
the hydroboration of 1, and have recovered the corresponding
α-bromoboronate 7 in high yield (entry 4). We therefore tried to
use CBH in separate reactions with the (E)- and the (Z)-isomers
of 1-bromopropene (Scheme 2 and Table 1, entry 5 and 6).
Fig. 2
(DMS) from BH3–DMS and pinacol following the described
procedure8 or purchased neat or in solution in THF. The CBH
has only been purchased neat and added as such to the reaction
mixture.
Let us first discuss the case of vinylic bromides such as
1-bromopropene 1 and 1,3-dibromopropene 2. Srebnik and
Pereira have already studied the reaction of PBH with vinyl
bromide and a mixture of (E)- and (Z)-1.12 These authors
focused on the Wilkinson catalysed version of the reaction,
since its thermal activation requires elevated temperatures or a
large excess of PBH. Their results are presented in Table 1
(entry 1) with those we have obtained under high pressure,
taken as an alternative to prolonged heating. In Scheme 1 are
Scheme 2
Despite slight experimental differences, such as the absence of
solvent in our case or the amount of catalyst, our data are in
fine agreement with those of Elgendy. For total conversion,15
the propylboronate 8, more than likely derived from the “cen-
tral” boration of the olefin as described above, is the only side-
product formed during this reaction. The efficiency associated
with this transformation at atmospheric pressure and possible
hazards in the use of CBH under high pressure (vide infra)
discouraged us from considering a hyperbaric version of this
reaction.
We next considered the case of 1,3-dibromopropene 2, a
difunctionalized substrate much more suited to our synthetic
plan, and that is commercially available as a mixture of
(E)- and (Z)-isomers (E:Z ≈ 60:40). Both the thermal and
catalytic hydroboration of this olefin by catecholborane have
also been studied by Elgendy and co-workers14 who report
the recovery of the expected α,γ-dibromoboronate 10 in good
yields. The results we have obtained are summarised in Scheme
3 and Table 2. Our first set of experiments was conducted with
PBH in the absence of solvent and at atmospheric pressure.
With 2 eq. of PBH, the reaction is slow under thermal as well as
catalytic conditions, a low 40% yield in 9 being obtained after
5 h at 90 ЊC. This result could not be improved, even after 17 h,
by the addition of 0.5% Wilkinson catalyst (entry 1). In ether, it
takes the combination of 0.5% Wilkinson’s catalyst and a 13
kbar pressure to increase the yield to 75% (entry 2). In THF,
and still under high pressure, the solvent ring-fission side-
reaction described above takes place, leading mainly to borate 6
(entries 3 and 4). Therefore, for the two bromoolefins studied
Scheme 1
described the results of hyperbaric hydroboration of pure (Z)-1
in the presence of Wilkinson’s catalyst. In ether (entry 2), only
pinacol-derived 1-propylboronate 5 is obtained, as observed by
Srebnik under atmospheric pressure. Following previous obser-
vations,13 these authors proposed that a rapid β-syn-elimination
of bromoboronate 4 from primary hydroboration adduct 3 can
explain the temporary formation of propene, a very good sub-
strate for further hydroboration by a second equivalent of
PBH, thus leading to 5.12 One could expect that the positive
activation volume associated with such an elimination mechan-
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Table 2 Wilkinson catalyzed hydroboration of (E/Z)-1,3-dibromopropene 2 by pinacol (PBH) and catechol (CBG) boranes
Product
Entry
Borane (eq.)
Cat. (%)
Pressure
T/ЊC
Solvent
Time/h
(% yield)a
1
2
3
4
5
6
PBH (2)
PBH (2)
PBH (2)
PBH (2)
CBH (2)
CBH (2)
0.5
0.5
—
0.5
—
1 bar
13 kbar
13 kbar
13 kbar
1 bar
90
20
20
20
90
20
—
17
72
96
72
2
9 (40)
Et2O
THF
THF
—
9 (75)
9 (17) ϩ 6 (50)
9 (26) ϩ 6 (74)
10 (59)b
0.5
1 bar
—
24
10 (70)b
a As compounds 10–12 were difficult to purify, conversions are given instead of yields. b Accompanied by various amounts of by-products 11 and 12
(see text).
Scheme 3
here, PBH does not yield, under our experimental conditions,
the expected α-bromoboronates but only by-products derived
from the regioselective boration of the central carbon atom
instead of the terminal one as obtained with catecholborane.
The CBH hydroboration of 1,3-dibromopropene has been
performed only at atmospheric pressure (Scheme 3 and Table 2,
entries 5 and 6). Our results are in overall agreement with
those by Elgendy, but significant amounts of the “wrong”
regioisomer 11 (8 for entry 5, 15% for entry 6) and of its
β-elimination–hydroboration derivative 12 (33% for entry 5,
15% for entry 6) have been identified beside the expected
dibromoboronate 10.
In conclusion, catecholborane affords, in the cases of the two
bromoolefins considered here and whatever the experimental
conditions employed, a much higher regioselectivity in favour
of the expected α-bromoboronates than pinacolborane.
Scheme 4
We next considered the hydroborations of allylamines. The
reaction between primary, secondary and tertiary allylamines
and boranes such as BH3, 9-BBN or CBH has been previously
studied in literature.16 In fact, the complexation between the
boron and nitrogen atoms is known to lead first to a primary
amine–borane complex; upon warming, this later triggers the
hydroboration of the double bond. Baboulène and co-workers
have shown16c,e,f that the regioselectivity of this step depends on
the nature of the amino group and on that of the borane
employed. The interaction between the nitrogen and the boron
can also take place after the hydroboration has occurred,12b
eventually leading to cyclic structures called 1,2-azaborolidine.
We began this part of the study with allylamine itself that
had been first reacted with PBH. In the CH2Cl2–Me2S mixture
(resulting from the synthesis of PBH itself)8 no reaction was
observed, even after 7 days at room temperature under 11 kbar.
With CBH, the reaction was performed in the absence of any
solvent and led mainly to the expected terminal hydroboration
product 13, together with several contaminants that could not
be identified (Scheme 4). In addition, this reaction turned out to
be extremely exothermic and was once even explosive. We thus
decided not to further explore this system.
partial hydroboration, but also to rather extensive desilylation.
We then turned to a more robust protecting group for the amino
group and envisaged N,N-(dibenzyl)allylamine 14 as a well-
suited candidate. Reacting 14 with 2 eq. PBH under thermal
conditions in a mixture of solvents induces a total conversion
(Scheme 5) after 16 h but as a 50:50 mixture of the two regio-
Scheme 5
isomers 15a and 16a (Table 3, entry 1). Adding 1% Wilkinson’s
catalyst to the mixture left the starting materials unaltered after
1 h at room temperature and we had to warm the medium for
1 h to 100 ЊC to obtain results comparable to those in entry 1. Use
of 5% catalyst improves the kinetics such that the completion is
Protecting the amino group in the 1-allyl-2,2,5,5-tetramethyl-
1,2,5-azadisilolidine form and exposing it to CBH led to a
J. Chem. Soc., Perkin Trans. 1, 2000, 4505–4511
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Table 3 Wilkinson catalyzed hydroboration of N,N-dibenzylallylamine 14 by pinacol (PBH) and catechol (CBH) boranes
Entry
Borane (eq.)
Cat. (%)
Pressure
T/ЊC
Solvent
Xylenea
Time/h
Product (% conv.)
1
2
3
4
5
6
7
8
PBH (2.0)
PBH (2.0)
PBH (2.0)
PBH (2.0)
CBH (1.1)
CBH (1.1)
CBH (1.1)
CBH (2.0)
—
5
—
—
1
—
1
—
1 bar
1 bar
9 kbar
11 kbar
1 bar
1 bar
1 bar
110
25
25
25
25
95
100
95
16
16
168
48
16
8
15a (50) ϩ 16a (50)
15a (60) ϩ 16a (40)
14
15a (50) ϩ 16a (50)
14
14 (50) ϩ 16b (50)
15b (45) ϩ 16b (55)
16b (100)
a
—
THFa
a
—
THF
—
—
8
4
1 bar
—
a The medium contains also CH2Cl2 and dimethyl sulfide since PBH is prepared in this mixture.
Table 4 Wilkinson catalyzed hydroboration of 3,4-dihydrofuran 17 by pinacolborane (PBH)
Entry
PBH (eq.)
Cat. (%)
Pressure
T/ЊC
Solvent
Time/h
Product (Sel. %)
Yield (%)a
1
2
3
4
5
6
7
1
1
2
1
1
1
2
0.5
—
1 bar
25
25
25
25
25
25
25
THF
THF
THF
THF
THF
Et2O
Et2O
8
48
72
48
48
24
48
18 (41) ϩ 19 (24)
45
46
84
80
38
70
75
12.5 kbar
12.5 kbar
12.5 kbar
12.5 kbar
12.5 kbar
12.5 kbar
19 (25)b
0.5
5
18 (61) ϩ 19 (39)
18 (44) ϩ 19 (56)
18 (11) ϩ 19 (22)b
19 (36)b
0.5c
—
0.5
20 (50) ϩ 21 (50)
a Calculated on the basis of all products recovered. b Accompanied by various amounts of by-products 20 and/or 21 (see text). c [RhCl(COD)2] used
instead of Wilkinson’s catalyst.
reached after 16 h at room temperature (entry 2). Replacing the
high temperature and the catalyst by high pressure affords dis-
appointing results since no reaction is observed in THF, while a
50:50 mixture of the regioisomers is obtained in CH2Cl2–DMS
(entries 3 and 4). We therefore switched to CBH. Entry 5 indi-
cates that this reagent is ineffective toward dibenzylallylamine
14 when in solution in THF or toluene, even upon warming or
in the presence of Wilkinson’s catalyst. By contrast entry 6
shows a partial conversion using neat reagents in 8 h at 95 ЊC.
Adding 1% catalyst speeds up the reaction such that a total
conversion is observed after the same time at the same temper-
ature, but an almost 50:50 mixture of regioisomers 15b and 16b
is recovered (entry 7). Using two equivalents of CBH finally
offers a total conversion in 4 h and provides selectively the
expected isomer 16b in the absence of catalyst (entry 8).
To conclude with the hydroboration of dibenzylallylamine,
it appears that the expected terminal boration product can be
obtained in excellent yields in the absence of any catalyst and
at atmospheric pressure, provided two equivalents of neat
Scheme 6
catecholborane are used and the reaction mixture with the
amine (neat) is warmed to 95 ЊC for 4 h.
Finally, we considered the case of 2,3-dihydrofuran (DHF).
Its hydroboration has been the object of at least three import-
ant studies in the literature. Zweifel and Plamondon17 first
showed that the action of BH3 and other boranes leads to a
clean, efficient and regioselective hydroboration of the double
bond, providing, after oxidation, 3-hydroxytetrahydrofuran in
good yields. These results were confirmed later by Brown and
co-workers who improved the yields and conditions18 and
proposed an asymmetric version of this reaction.19 The regio-
selectivity is in fine agreement with other results on enol ethers
hydroborations,20 although it does not fit the synthetic pathway
presented in Fig. 2 that requires the opposite regioisomer.
The reaction of DHF with pinacolborane has been per-
formed in THF or ether (Scheme 6). At atmospheric pressure
and room temperature, pinacolborane is totally unreactive
after 8 h; the addition of 0.5% Wilkinson’s catalyst to the
medium increases the yield to 45%, the main product being the
unexpected (but desired) alkoxyborane 18 resulting from
α-boration (entry 1, Table 4), together with 24% 19 and 35% 21.
It therefore appeared that the catalyst could advantageously
reverse, at least partially, the regioselectivity of this reaction.
Resorting to high pressure did not improve either the efficiency
or the selectivity (entry 2). A mixture of the direct hydro-
boration product 19 was obtained together with but-3-
enyloxyborane 20 (50%) and its hydroboration product 21
(25%). The combination of high pressure and Wilkinson’s
catalyst improves both the reactivity and the selectivity, as can
be seen from entry 3 where both PBH and catalyst quantities
have been optimised. Such a low amount as 0.5% of the
rhodium complex indeed raises the yield significantly and gives
access to a mixture of the two regioisomers 18 and 19, in
absence of any rearrangement product, while higher catalyst
loadings decrease the selectivity (entry 4). In one case
Wilkinson’s catalyst was replaced by the rhodium() chloride–
cyclooctadienyl complex (entry 5) in an attempt to take advan-
tage of a ligand effect on the selectivity, as reported in many
circumstances.10,21 The results are disappointing considering the
presence of side-products (22% 20 ϩ 45% 21) and a mediocre
38% overall yield.22 A strong solvent effect can be noticed in the
two last entries of Table 4, since working in ether rather than
in THF yields a mixture of the undesired regioisomer 19 and
ring-fission derivatives 20 (36%) and 21 (28%) in the absence
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of catalyst (entry 6), while 0.5% Wilkinson’s catalyst gives
exclusively the two latter side-products (in a 50:50 ratio) as
noted in entry 7.
stake. Extensions to asymmetric versions23 of these results will
also be disclosed in due course.24
The recovering of but-3-enylborate 20 has been rationalised
by Zweifel and Plamondon17 on one hand, and Brown et al.18
on the other. These authors have indeed shown that the
cyclic ethers resulting from the hydroboration of DHF and
dihydropyran (DHP) undergo ring opening induced by inter- or
intramolecular borane complexation on the oxygen followed by
a β-elimination. A more-or-less concerted six-membered transi-
tion state, such as that depicted in Scheme 7, could be imagined
Experimental
General
Solvents were purified by conventional methods prior to use.
Reagents were purchased from common commercial suppliers.
High-pressure hydroboration reactions were performed in a
Unipress piston-cylinder apparatus for pressures up to 14 kbar.
TLC was performed on Kieselgel 60F-254–0.25 mm silica gel
plates and column chromatography over silica gel SI 60 (230–
400 mesh). Gas chromatography was performed on a Hewlett
Packard 5890 apparatus equipped with a 30 m, 0.25 mm ID,
0.25 µm J & W column. Gas chromatography coupled to a mass
spectrometer were realised on a ATI Unicam Automass spec-
trometer equipped with the same column. Elemental micro-
analyses were carried out on a Carlo Erba EA 1110 analyser.
NMR spectra were recorded in CDCl3 (unless precised) on a
Bruker AC-200 (200 MHz) or Advance DMX 400 (400 MHz);
chemical shifts (δ) are expressed in ppm relative to TMS for 1H
and 13C, BF3–Et2O for 11B; constants coupling (J) are given in
Hz; coupling multiplicities are reported using conventional
abbreviations. The infrared spectra were recorded on a Perkin-
Elmer 16PC FT-IR instrument. The hydroboration reactions
were carried out in an atmosphere of argon. The high-pressure
hydroboration flasks were usually 2 or 4 mL reactors.
Scheme 7
Hydroboration of 1-bromoprop-1-ene by pinacolborane
for the intramolecular pathway. The resulting double bond can,
in turn, be hydroborated by an excess of reagent (Scheme 7),
giving access, in our case, to compound 21.
The extension of this work to catecholborane is not reported
here since we have been unable to characterise the product(s)
obtained in the reaction mixture. The CDCl3 and C6D6 NMR
spectra seemed to correspond to the formation of oligomeric
mixtures of the borated adduct(s) in solution.
To conclude this part of the study, it is clear that the combin-
ation of catalysis and high pressure induces a partial inversion
of the “classical” regioselectivity of the hydroboration of DHF.
Up to 60% of the desired α-regioisomer can be obtained using
PBH in THF under these conditions.
1-Bromoprop-1-ene (0.34 g, 2.8 mmol) and RhCl(PPh3)3 (13.0
mg, 0.014 mmol) were dissolved in ether or THF (4.6 mL) and
pure pinacolborane (410 µL, 2.8 mmol) was added slowly at
20 ЊC. The reaction mixture was then placed under 12.5 kbar
for 24 hours. After release of the pressure, the excess ether or
THF was evaporated under vacuum. The product was then
purified by flash chromatography on silica gel with heptane–
AcOEt (80:20) as eluent, to yield 82% 2-propyl-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (colourless oil), when the
reaction was performed in ether, and 80% 2-(4-bromobutoxy)-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (colourless oil) when
in THF.
2-(4-Bromobutoxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
6. δH (200 MHz) 3.84 (t, 2H, J = 6.7 Hz), 3.40 (t, 2H, J = 6.7
Hz), 1.92 (quintet, 2H, J = 6.7 Hz), 1.70 (quintet, 2H, J = 6.7
Hz); δC (50 MHz) 83.1, 63.7, 33.5, 29.8, 28.9, 24.5; m/z (EI,
70 eV) (rel. int.): 279 (M, 15%), 199 (100), 137 (48).
Conclusion
Our two objectives in this study have led to the following
conclusions.
(i) Regarding selectivities, both catecholborane and pinacol-
borane can effect regioselective hydroboration of functional-
ized olefins. Interestingly, these two reagents tend to provide the
opposite regioisomers. The hydroborations by CBH have been
found to be efficient under thermal or catalytic conditions,
while the reactions with PBH seem to require high pressure and
catalysis together. However, strong solvent effects are to be
expected.
2-Propyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane 5. δH (200
MHz) 1.40 (sextet, 2H, J = 6.7 Hz), 1.24 (s, 12H), 0.90 (t, 3H,
J = 6.7 Hz), 0.75 (t, 2H, J = 6.7 Hz); δC (50 MHz) 83.0, 24.7,
17.4, 17.1; m/z (CI, CH4) (rel. int.): 171 (M ϩ 1, 100%), 155
(39), 129 (20), 101 (88), 85 (46), 71 (18), 57 (15).
Hydroboration of 1,3-dibromopropene
(ii) Regarding the relative interest of catalysis and high pressure
for hydroboration reactions, both modes of activation can be
efficient (separately) for hydroborations with dialkoxyboranes,
as already known for other boranes. However, the conjunction
of these two techniques can bring significant improvements to
the reactivity and selectivity of these reactions, as recently
underlined by Reiser for palladium-catalysed reactions.9f
Further studies of the utility of this combination of acti-
vation modes are certainly worth pursuing and should include
essential factors that have not been taken into consideration
here, such as variation of the catalyst (many transition metal
complexes have been successfully used for hydroborations)10
and of the nature of the hydroborating reagent. We believe the
data presented here show that significant synthetic gains are at
1,3-Dibromopropene (0.15 g, 0.75 mmol) and RhCl(PPh3)3
(3.47 mg, 0.004 mmol) were dissolved in ether (1.6 mL) and
pure pinacolborane (110 µL, 0.75 mmol) was added slowly at
20 ЊC. The reaction mixture was then placed under 12.5 kbar
for 3 days. After release of the pressure, excess ether was evap-
orated under vacuum. The product was then purified by flash
chromatography on silica gel with heptane–AcOEt (80:20) as
eluent, to yield 70% 2-(3-bromopropyl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane 9, as a colourless oil. δH (200 MHz) 3.40
(t, 2H, J = 6.7 Hz), 1.94 (quintet, 2H, J = 6.7 Hz), 1.24 (s, 12H),
0.90 (t, 2H, J = 6.7 Hz); δC (50 MHz) 83.1, 36.2, 27.4, 24.7; m/z
(EI, 70 eV) (rel. int.): 249 (M, 70%), 169 (100), 125 (35), 101
(75), 57 (45).
J. Chem. Soc., Perkin Trans. 1, 2000, 4505–4511
4509

View Article Online
Hydroboration of (dibenzyl)allylamine by pinacolborane
2-(But-3-enyloxy)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
20. δH (200 MHz) 5.90–5.65 (m, 1H), 5.15–4.92 (m, 2H), 3.85 (t,
2H, J = 6.8 Hz), 2.28 (m, 2H), 1.19 (s, 12H); δC (50 MHz) 134.6,
116.8, 82.9, 64.0, 35.9, 24.5; m/z (CI, CH4) (rel. int.): 199
(M ϩ 1, 5%), 183 (18), 101 (100).
A solution of pinacolborane in CH2Cl2 (10.4 mmol; 2 eq.),
prepared according to the literature,8 was added to
(dibenzyl)allylamine (5.2 mmol, 1 eq.) in the presence of
Wilkinson’s catalyst (1% mol, 0.052 mmol). The reaction mix-
ture was warmed to 100 ЊC for one hour. The NMR spectrum
of the crude mixture corresponded to 40% 2-(1-dibenz-
ylaminopropan-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
15a and 60% 2-(3-dibenzylaminopropyl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane 16a.
2-[4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)butoxy]-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane 21. δH (200 MHz) 3.78
(t, 2H, J = 6.8 Hz), 1.50 (m, 4H), 0.80 (m, 2H), 1.19 (s, 12H);
δC (50 MHz) 82.9, 64.7, 33.8, 24.7, 19.8; m/z (CI, CH4) (rel. int.):
327 (M ϩ 1, 2%), 227 (47), 183 (18), 101 (100).
2-(1-Dibenzylaminopropan-2-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane 15a. δH (200 MHz, C6D6) 7.53–7.08 (m, 10H, H),
4.20 (s, 4H), 2.60 (m, 2H), 2.05 (m, 1H), 1.18 (s, 12H), 0.75 (m,
3H); δC (50 MHz, C6D6) 140.1, 132.9, 128.9, 128.7, 128.2, 126.6,
82.4, 58.2, 53.1, 24.5, 11.5; m/z (EI, 70 eV) (rel. int.): 365
(M, 10%), 274 (10), 211 (100), 181 (40), 91 (100).
General procedure for hydroboration of 1-haloalk-1-enes by
catecholborane
Catecholborane (5.0 mmol, 2 eq.) was added dropwise to 1-
haloalk-1-ene (2.5 mmol, 1 eq.) in the presence of Wilkinson’s
catalyst (0.5% mol, 0.0125 mmol). The reaction mixture was
stirred at room temperature and monitored by 1H NMR,
following the disappearance of the olefinic protons, until the
reaction was complete. The desired product was purified
after transesterification with commercial (1S,2S,3R,5S)-(ϩ)-
pinane-2,3-diol according to the literature procedure.14a
2-(3-Dibenzylaminopropyl)-4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane 16a. δH (200 MHz, C6D6) 7.53–7.08 (m, 10H), 3.57
(s, 4H), 2.48 (t, 2H, J = 7.1 Hz), 1.79 (qt, 2H, J = 7.1 Hz),
1.16 (s, 12H), 1.01 (t, 2H, J = 7.1 Hz); δC (50 MHz, C6D6)
140.1, 132.9, 128.9, 128.6, 128.2, 126.6, 82.5, 58.2, 55.5, 24.5,
21.5; m/z (EI, 70 eV) (rel. int.): 365 (M, 3%), 350 (5), 210 (100),
91 (89).
2-(1-Bromopropyl)-1,3,2-benzodioxaborole 7. δH (200 MHz)
7.40–6.95 (m, 4H), 3.75 (t, 1H, J = 6.9 Hz), 2.16 (m, 2H), 1.14
(t, 3H, J = 6.9 Hz); m/z (EI, 70 eV) (rel. int.): 241 (M, 5%), 214
(22), 199 (35), 173 (30), 158 (24), 145 (71), 118 (30), 55 (100).
(1S,2S,3R,5S)-(ϩ)-Pinanediol derivative (overall yield = 60%):
δH (200 MHz) 4.41–4.20 (m, 2H), 3.38–3.24 (m, 2H), 2.44–2.30
(m, 2H), 2.30–2.13 (m, 2H), 2.04 (t, 2H, J = 4.7 Hz), 2.04–1.85
(m, 6H), 1.82–1.80 (m, 2H), 1.80–1.60 (m, 2H), 1.38 (s, 6H),
1.27 (s, 6H), 1.23–1.15 (m, 2H), 1.03 (t, 6H, J = 4.7 Hz), 0.84 (s,
6H); δC (50 MHz) 87.0, 86.9, 78.6, 78.5, 51.3, 51.2, 39.4, 38.2,
35.5, 28.6, 27.6, 27.0, 26.4, 23.9, 13.4; m/z (CI, CH4) (rel. int.):
302 (M ϩ 1, 5%), 217 (38), 189 (64), 134 (42), 83 (38), 55 (100).
Hydroboration of 2,3-dihydrofuran by pinacolborane
In THF. 2,3-Dihydrofuran (0.30 g, 4.3 mmol) and RhCl-
(PPh3)3 (19.8 mg, 0.0215 mmol) were dissolved in THF (3.4
mL) and pure pinacolborane (0.55 g, 4.3 mmol) was added
slowly at 20 ЊC. The reaction mixture was then placed under
12.5 kbar for 2 days. After release of the pressure, the excess
THF was evaporated under vacuum. The two products were
then purified by flash chromatography on silica gel with
heptane–AcOEt (50:50) as eluent, but their 50:50 mix-
ture could not be fully separated. An authentic sample
of pinacol-1-tetrahydrofuranyl-2-boronate 19 was prepared by
hydroboration of DHF by BH3-DMS, according to Brown’s
procedure,18 followed by an overnight treatment by pinacol in
THF at room temperature. This product was purified by flash
column chromatography in the same conditions as above
(75% yield). Its spectra were identical to that of the less polar
product in the previous mixture.
2-Propyl-1,3,2-benzodioxaborole 8. δH (200 MHz) 7.40–6.95
(m, 4H), 1.60 (m, 2H), 1.05 (t, 3H, J = 6.8 Hz), 0.95 (t, 2H,
J = 6.9 Hz); m/z (EI, 70 eV) (rel. int.): 162 (M, 19%), 134 (5),
120 (100), 92 (8), 63 (17). (1S,2S,3R,5S)-(ϩ)-Pinanediol deriv-
ative (overall yield = 20%): δH (200 MHz) 4.41–4.20 (m, 1H),
2.44–2.30 (m, 1H), 2.30–2.13 (m, 1H), 2.04–1.85 (m, 1H), 1.82–
1.80 (m, 1H), 1.80–1.60 (m, 1H), 1.45 (m, 2H), 1.38 (s, 3H), 1.27
(s, 3H), 1.23–1.15 (m, 1H), 0.95 (t, 3H, J = 6.7 Hz), 0.84 (s, 3H),
0.80 (t, 2H, J = 6.7 Hz); δC (50 MHz) 85.2, 78.3, 51.4, 39.2, 38.2,
35.2, 28.3, 27.0, 26.4, 23.9, 17.5, 16.9; m/z (CI, CH4) (rel. int.):
222 (M, 22%), 207 (72), 153 (95), 126 (58), 55 (100).
2-(Tetrahydrofuran-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane 18. δH (200 MHz) 3.83 (m, 2H), 3.37 (dd, 1H, J = 6.7,
2.2 Hz), 2.05 (m, 2H), 1.50 (m, 2H), 1.20 (s, 12H); δC (50 MHz)
82.4, 64.6, 33.9, 24.6, 19.9; m/z (CI, CH4) (rel. int.): 199 (M ϩ 1,
15%), 141 (48), 101 (53), 71 (100); C10H19BO3 requires C, 60.64;
H, 9.67; O, 24.23. Found: C, 60.43; H, 9.88.
2-(1,3-Dibromopropyl)-1,3,2-benzodioxaborole 10. δH (200
MHz) 7.36–6.96 (m, 4H), 4.05 (t, 1H, J = 6.4 Hz), 3.68 (t, 2H,
J = 6.4 Hz), 2.61 (q, 2H, J = 6.4 Hz); δC (50 MHz) 147.7, 122.9,
112.8, 36.3, 31.6; m/z (EI, 70 eV) (rel. int.): 320 (M, 5%), 252 (8),
200 (100), 159 (22), 136 (78), 120 (56), 92 (31), 77 (15), 54 (38).
(1S,2S,3R,5S)-(ϩ)-Pinanediol derivative (overall yield = 60%):
δH (200 MHz) 4.38–4.21 (m, 2H), 3.78–3.73 (m, 4H), 3.45 (t,
2H, J = 6.9 Hz), 2.40–2.32 (m, 4H), 2.32–2.25 (m, 4H), 2.25–
2.15 (m, 2H), 2.15–1.95 (m, 2H, J = 6.9 Hz), 1.95 (m, 2H), 1.95–
1.75 (m, 2H), 1.39 (s, 6H), 1.27 (s, 6H), 1.25–1.00 (m, 2H), 0.85
(s, 6H); δC (50 MHz) 87.0, 86.9, 78.6, 78.5, 51.3, 51.2, 39.4, 38.2,
36.3, 35.2, 31.6, 28.4, 27.0, 26.5, 24.0; δB 30.7; m/z (EI, 70 eV)
(rel. int.): 380 (M, 4%), 365 (5), 284 (8), 262 (12), 189 (22), 135
(82), 93 (100).
2-(Tetrahydrofuran-3-yl)-4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane 19. δH (200 MHz) 3.92 (t, 1H, J = 8.2 Hz), 3.75 (dt, 1H,
J = 8.2, 4.1 Hz), 3.65 (dd, 1H, J = 8.2, 4.1 Hz), 3.55 (t, 1H,
J = 8.2 Hz), 1.97 (m, 1H), 1.76 (m, 1H), 1.53 (m, 1H), 1.19 (s,
12H); δC (50 MHz) 83.3, 70.2, 68.4, 28.6, 24.6; m/z (CI, CH4)
(rel. int.): 199 (M ϩ 1, 70%), 183 (10), 145 (12), 101 (38), 85
(58), 71 (45), 55 (100); C10H19BO3 requires C, 60.64; H, 9.67; O,
24.23. Found: C, 60.52; H, 10.02.
In ether. 2,3-Dihydrofuran (0.40 g, 5.7 mmol) and RhCl-
(PPh3)3 (26.4 mg, 0.0285 mmol, 0.5% mol) were dissolved in
ether (3.4 mL) and pure pinacolborane (0.73 g, 5.7 mmol) was
added slowly at 20 ЊC. The reaction mixture was then placed
under 12.5 kbar for 2 days. After release of the pressure, excess
ether was evaporated under vacuum. The two products were
then purified by flash chromatography on silica gel with
heptane–AcOEt (50:50) as eluent, but their 50:50 mixture
could not be fully separated.
2-(1,3-Dibromopropan-2-yl)-1,3,2-benzodioxaborole 11. δH (200
MHz) 3.90 (m, 4H), 1.92 (m, 1H).
2-(3-Bromopropyl)-1,3,2-benzodioxaborole 12. δH (200 MHz)
7.21–6.95 (m, 4H), 3.51 (t, 2H, J = 6.7 Hz), 2.19 (qt, 2H, J = 6.7
Hz), 1.48 (t, 2H, J = 6.7 Hz); δC (50 MHz) 147.6, 122.6, 112.8,
4510
J. Chem. Soc., Perkin Trans. 1, 2000, 4505–4511

View Article Online
Acc. Chem. Res., 1988, 21, 287; (c) D. S. Matteson, Stereodirected
Synthesis with Organoboranes, Springer Verlag, Berlin, 1995;
(d) D. S. Matteson, CHEMTECH, July 1999, 6; (e) A. Chen, L. Ren
and C. M. Crudden, J. Org. Chem., 1999, 64, 9704.
3 J. Hiratake and J. Oda, Biosci. Biotechnol. Biochem., 1997, 61, 211.
4 (a) C. A. Kettner and A. B. Shenvi, J. Biol. Chem., 1984, 259, 15106;
(b) W. W. Bachovchin, W. Y. L. Wong, S. Farr-Jones, A. B. Shenvi
and C. A. Kettner, Biochemistry, 1988, 27, 7689.
5 (a) G. De Nanteuil, A. Rupin, P. Mennecier and T. J. Verbeuren,
Thromb. Haemostasis, 1995, 73, 1309; (b) G. De Nanteuil, A. Rupin,
T. J. Verbeuren, S. Simonet and M. O. Vallez, Thromb. Haemostasis,
1995, 73, 1310; (c) G. De Nanteuil and T. J. Verbeuren, Expert Opin.
Invest. Drugs, 1999, 8, 173.
6 (a) C. Kettner, L. Mersinger and R. Knabb, J. Biol. Chem.,
1990, 265, 18289; (b) M. A. Hussain, R. Knabb, B. J. Aungst and
C. Kettner, Peptides, 1991, 12, 1153; (c) J. Wityak, R. A. Earl,
M. M. Abelman, Y. B. Bethel, B. N. Fisher, G. S. Kauffman,
C. A. Kettner, P. Ma, J. L. McMillan, L. J. Mersinger, J. Pesti,
M. E. Pierce, F. W. Rankin, R. J. Chorvat and P. N. Confalone,
J. Org. Chem., 1995, 60, 3717.
7 C. F. Lane and G. W. Kabalka, Tetrahedron, 1976, 32, 981.
8 P. Knochel, C. E. Tucker and J. Davidson, J. Org. Chem., 1992, 57,
3482.
9 For examples involving catalysis and high pressure together, see, for
instance: (a) L. F. Tietze, C. Ott, K. Gerke and M. Buback, Angew.
Chem., Int. Ed. Engl., 1993, 32, 1485; (b) J. Knol, A. Meetsma and
B. L. Feringa, Tetrahedron: Asymmetry, 1995, 6, 1069; (c) S. Hillers
and O. Reiser, Chem. Commun., 1996, 2197; (d) G. Jenner,
New J. Chem., 1997, 21, 1085; (e) G. Jenner, Tetrahedron, 1997, 53,
2669; ( f ) O. Reiser, Top. Catal., 1998, 5, 105.
30.2, 27.3; m/z (EI, 70 eV) (rel. int.): 240 (M Ϫ 1, 22%), 200
(100), 84 (86).
General procedure for hydroboration of allylamines by
catecholborane
Catecholborane (10.5 mmol, 2 or 3 eq.) was added very slowly
to the allylamine (3.5 mmol, 1 eq.). The reaction mixture was
heated at 100–110 ЊC under argon and monitored by the dis-
appearance of the olefinic protons in the proton NMR. The
desired product was purified after transesterification with
commercial (1S,2S,3R,5S)-(ϩ)-pinanediol.14a CAUTION: the
reaction of allylamine with catecholborane is strongly exo-
thermic and can become explosive!
2-(3-Aminopropyl)-1,3,2-benzodioxaborole 13. δH (200 MHz)
7.24–6.85 (m, 4H), 3.65 (t, 2H, J = 7.0 Hz), 2.09 (qt, 2H, J = 7.0
Hz), 1.39 (t, 2H, J = 7.0 Hz); δB (128 MHz) 22.1; IR 3154, 1472,
1236, 1074, 912, 738 cmϪ1; m/z (CI, CH4) (rel. int.): 178 (M ϩ 1,
100%), 111 (21), 76 (50). (1S,2S,3R,5S)-Pinanediol aminoprop-
ylboronate derivative (overall yield = 50%): δH (200 MHz)
4.45–4.05 (m, 1H), 3.65 (m, 2H), 2.41–2.32 (m, 1H), 2.32–2.25
(m, 2H), 2.25–2.20 (m, 1H), 2.10 (m, 1H), 2.00–1.91 (m, 1H),
1.91–1.88 (m, 1H), 1.40 (s, 3H), 1.30 (s, 3H), 1.25–1.04 (m, 1H),
1.03 (m, 3H), 0.80 (s, 3H); δB (128 MHz) 22.3; IR 3214, 1486,
1234, 1064, 908, 738, 650 cmϪ1
.
10 I. Beletskaya and A. Pelter, Tetrahedron, 1997, 53, 4957.
11 See for instance: J. E. Rice and Y. Okamoto, J. Org. Chem., 1982, 47,
4189.
2-(1,1-Dibenzylaminopropan-2-yl)-1,3,2-benzodioxaborole
15b. δH (200 MHz) 7.46–6.90 (m, 14H), 4.25 (s, 4H), 3.00 (m,
2H), 2.10 (m, 1H), 0.75 (m, 3H).
12 S. Pereira and M. Srebnik, Tetrahedron Lett., 1996, 37, 3283.
13 H. C. Brown and R. L. Sharp, J. Am. Chem. Soc., 1968, 90, 2915.
14 (a) S. Elgendy, G. Claeson, V. V. Kakkar, D. Green, G. Patel,
C. A. Goodwin, J. A. Baban, M. F. Scully and J. Deadman,
Tetrahedron, 1994, 50, 3803; (b) S. Elgendy, G. Patel, V. V. Kakkar,
G. Claeson, D. Green, E. Skordalakes, J. A. Baban and J. Deadman,
Tetrahedron Lett., 1994, 35, 2435; (c) S. Elgendy, G. Patel, D. Green,
J. A. Baban, V. V. Kakkar and G. Claeson, Eur. Pat., 599633, 1994.
15 In the case of the CBH adducts, conversions (evaluated from the
crude NMR mixture) are given instead of yields in the Tables since
the catecholboronates are rapidly hydrolyzed during work-up. In
several cases, a transesterification step with pinanediol has been
performed directly after quenching the experiment, to provide the
more stable boronates that could be isolated.
16 Leading references: (a) H. C. Brown and M. K. Unni, J. Am. Chem.
Soc., 1968, 90, 2902; (b) Z. Polivska, V. Kubelka, N. Holubova
and M. Ferles, Collect. Czech. Chem. Commun., 1970, 35, 1131;
(c) M. Baboulène, J. L. Torregrosa, V. Speziale and A. Lattes,
Bull. Soc. Chim. Fr., 1980, 565; (d) J. L. Torregrosa, M. Baboulène,
V. Speziale and A. Lattes, Comptes Rendus Acad. Sci. Paris, 1983,
891; (e) A. Lattes, Z. Benmaarouf-Khallaayoun, M. Baboulène and
V. Speziale, J. Organomet. Chem., 1985, 289, 309; ( f ) S. Igueld,
M. Baboulène, A. Dicko and M. Montury, Synthesis, 1989, 200.
17 G. Zweifel and J. Plamondon, J. Org. Chem., 1970, 35, 898.
18 H. C. Brown, J. V. N. Vara Prasad and S. H. Zee, J. Org. Chem.,
1985, 50, 1582.
2-(3,3-Dibenzylaminopropyl)-1,3,2-benzodioxaborole 16b. Bp
(0.7 mmHg) = 250 ЊC; δH (200 MHz, CDCl3) 7.40–6.90 (m,
14H), 3.80 (s, 4H), 2.68 (t, 2H, J = 7.0 Hz), 1.73 (qt, 2H, J = 7.0
Hz), 1.04 (t, 2H, J = 7.0 Hz); δH (200 MHz, C6D6) 7.42–6.90 (m,
14H), 3.66 (s, 4H), 2.53 (t, 2H, J = 7.0 Hz), 1.72 (qt, 2H, J = 7.0
Hz), 1.03 (t, 2H, J = 7.0 Hz); δC (50 MHz, CDCl3) 150.5, 131.0,
129.4, 127.5, 122.9, 118.8, 112.3, 56.5, 51.8, 25.4; δB (128 MHz,
CDCl3) 14.3; IR 3050, 1493, 1240, 1065, 913 (br), 738 cmϪ1; m/z
(EI, 70 eV) (rel. int.): 357 (M, 6%), 210 (99), 181 (23), 151 (22),
91 (100). (1S,2S,3R,5S)-(ϩ)-Pinanediol derivative (overall
yield = 87%): δH (400 MHz, CDCl3) 7.38–7.19 (m, 10H), 4.19
(dd, 1H, J = 8.7, 1.9 Hz), 3.58 (s, 4H), 2.43 (t, 2H, J = 7.4 Hz),
2.27–2.07 (m, 2H), 1.99 (t, 1H, J = 5.5 Hz), 1.91–1.81 (m, 2H),
1.79–1.75 (m, 1H), 1.72–1.61 (m, 1H), 1.38 (s, 3H), 1.26 (s, 3H),
1.00 (m, 1H), 1.03 (t, 2H), 0.84 (s, 3H); δC (50 MHz, CDCl3)
144.1, 129.0, 128.1, 126.9, 85.4, 57.9, 55.1, 51.1, 39.4, 38.2, 35.2,
28.5, 27.0, 26.5, 23.9, 21.1; δB (128 MHz, CDCl3) 12.3; IR 2930,
1486, 1368, 1234, 1068, 908, 734 cmϪ1; m/z (EI, 70 eV) (rel. int.):
417 (M, 5%), 210 (98), 91 (100).
19 (a) H. C. Brown and J. V. N. Vara Prasad, J. Am. Chem. Soc., 1986,
108, 2049; (b) H. C. Brown and J. V. N. Vara Prasad, Heterocycles,
1987, 25, 641; (c) H. C. Brown, A. K. Gupta, M. V. Rangaishenvi
and J. V. N. Vara Prasad, Heterocycles, 1989, 28, 283.
20 See, for instance: (a) B. M. Mikhailov and T. A. Shchegoleva,
Izv., Akad. Nauk SSSR, Ser. Khim., 1959, 546; (b) D. J. Pasto and
C. C. Cumbo, J. Am. Chem. Soc., 1964, 86, 4343; (c) H. C. Brown
and R. L. Sharp, J. Am. Chem. Soc., 1968, 90, 2915.
Acknowledgements
S. C. thanks ORIL Industrie for a PhD grant. The high pressure
equipment used in this work was purchased thanks to the joint
support of the Conseil Régional de Haute-Normandie and the
DRRT to J. M.
21 Recent results: P. V. Ramachandran, M. P. Jennings and H. C.
Brown, Org. Lett., 1999, 1, 1399.
22 The presence of 45% of 21 explains such a low yield since it takes
two equivalents of borane to convert 17 into 21.
23 For recent results in the field, see, for instance: M. McCarthy,
M. W. Hooper and P. J. Guiry, Chem. Commun., 2000, 1333, and
references cited.
References
1 General references: (a) H. C. Brown, Hydroboration, W. A.
Benjamin Inc., New York, 1962; (b) H. C. Brown, Boranes in Organic
Chemistry, Cornell University Press, New York, 1972; (c) H. C.
Brown, Organic Syntheses via Boranes, Wiley Interscience
Publishers, 1975.
2 See for instance: (a) H. C. Brown, P. K. Jadhav and M. C. Desai,
Tetrahedron, 1984, 40, 1325; (b) H. C. Brown and B. Singaram,
24 S. Colin, L. Vaysse-Ludot, J. P. Lecouvé and J. Maddaluno,
unpublished results.
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4511