An efficient radical procedure for the halogenation and chalcogenation of B-alkylcatecholboranes

Article abstract of DOI:10.1002/adsc.200700531

An efficient formal anti-Markovnikov addition of HX (X = Cl, Br, I, SR and SeR) to olefins under mild reaction conditionsisdes cribed. The procedure isbas ed on the hydroboration of alkeneswith catecholborane. The conversion of the intermediate B-alkylcatecholboranesto the corresponding halides, sulfides and selenides is based on a common process, i.e., generation of a radical from the alkylborane followed by abstraction of a heteroatom from an aromatic sulfonyl reagent. The efficiency of these radical reactionsis remarkable. The mildness of the reaction conditions is well illustrated by the preparation of iodoalkanes. Despite the notorious reactivity of iodoalkanes under radical reaction conditions, no product degradation wasobs erved.

Full text of DOI:10.1002/adsc.200700531

FULL PAPERS
DOI: 10.1002/adsc.200700531
An Efficient Radical Procedure for the Halogenation and
Chalcogenation of B-Alkylcatecholboranes
Arnaud-Pierre Schaffner,a Florian Montermini,a Davide Pozzi,a
Vincent Darmency,a Eoin Martin Scanlan,a and Philippe Renauda,*
a
University of Berne, Department of Chemistry and Biochemistry, Freiestrasse 3, 3012 Berne, Switzerland
Fax : (+41)-31-631-3426; e-mail: philippe.renaud@ioc.unibe.ch
Received: November 9, 2007; Revised: February 21, 2008; Published online: April 11, 2008
Dedicated to Professor Andreas Pfaltz on the occasion of his 60th birthday
Supporting information for thisarticle isavailable on the WWW under http://asc.wiley-vch.de/home/.
Abstract: An efficient formal anti-Markovnikov ad- matic sulfonyl reagent. The efficiency of these radi-
dition of HX (X=Cl, Br, I, SR and SeR) to olefins cal reactionsisremarkable. The mildnesof the reac-
under mild reaction conditionsisdescribed. The pro-
cedure isbased on the hydroboration of alkeneswith
tion conditionsiswell illutsrated by the preparation
of iodoalkanes. Despite the notorious reactivity of
catecholborane. The conversion of the intermediate iodoalkanesunder radical reaction condition,s no
B-alkylcatecholboranesto the corresponding halide,s
product degradation wasobserved.
sulfides and selenides is based on a common process,
i.e., generation of a radical from the alkylborane fol- Keywords: boranes; halogenation; radicals; sele-
lowed by abstraction of a heteroatom from an aro- nides; sulfides; sulfonyl halides
Introduction
allylic sulfones.[11,15,16] In 1972 Daviesand Roberts
demonstrated that benzenesulfonyl bromide is a suita-
ble reagent for the bromination of tri-n-butylbor-
Organoboranesare very uesful reagentsfor organic
synthesis.[1] They are easily prepared by hydrobora- ane.[17] They assumed that this reaction is a radical
tion of alkenesand are very efficiently converted into
chain process in which a benzenesulfonyl radical dis-
placesan alkyl radical from tri- n-butylborane. Thisin-
[2]
alcoholsby oxidative treatment.
Thisreaction es-
quence represents one of the most efficient ways of teresting observation did not lead to any synthetic ap-
achieving the anti-Markovnikov addition of water to plication. Here, we report on the free radical-mediat-
alkenes. Extension of this chemistry to the formation ed halogenation, sulfanylation and selanylation of al-
[3]
[4]
[5]
À
À
À
of C Cl, C Br, and C I bondsby reaction of
the organoboraneswith halogenating agentshasbeen
reported. However, the high reactivity of the halogen-
ating agent and/or the use of alkaline condition make
these reactions only moderately attractive for synthet-
ic applications. The formation of carbon-sulfur bonds
has also been examined but is scarcely used for prepa-
rative purpose,[6] with the exception of the radical sul-
furization process mediated by Barton esters.[7,8] The
kylcatecholboranes using sulfonyl halides, sulfides and
selenides as key reagents according to Scheme 1.[18,19]
À
formation of a C Se bond from organoboranesislim-
ited to selenocyanates.[9] Recently, we discovered that
B-alkylcatecholboranesare extremely uesful radical
precursors that can participate in efficient carbon-
carbon[7,10–12] and carbon-oxygen[12,13] bond forma-
tion.[14] Chain reactionsinvolving areneuslfonyl radi-
calsand B-alkylcatecholboranesare particularly effi-
cient asdemontsrated by the allylation reaction with
Scheme 1.
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Arnaud-Pierre Schaffner et al.
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Results and Discussion
of the pinocampheyl halidesare detected by GC-MS
analysis. A similar stereochemical outcome was ob-
tained for the radical hydroallylation of a-pinene.[15]
Secondly, a radical probe experiment wasrun with
Halogenation
Different B-alkylcatecholboranes, prepared by hydro- the cyclopropylcarbinyl radical derived from (+)-2-
boration of olefins 1–3 under N,N-dimethylacetamide carene 13. Thisradical isexpected to rearrange with a
catalysis,[20] were allowed to react with arenesulfonyl rate constant higher than 109 sÀ1.[22] The rate constants
halidesaccording to Eq. (1) under either di- tert-butyl for the chlorination of primary and secondary alkyl
hyponitrite[21] initiation at 408C (method A) or di-tert- radicalsby PhCH 2SO2Cl have been measured by
butyl peroxide at 1408C using microwave heating Chatgilialoglu to be k=1.2–1.3”106 MÀ1 sÀ1 at
(method B) [Eq. (1)]. Results are summarized in 258C.[23] These values are also expected to be valid
for the reactionswith PhSO Cl.[24] This assumption is
2
confirmed by the recent report that PhSO2Cl reacts
with the cyclohexyl radical at a rate constant k=1.2”
106 MÀ1 sÀ1 at 188C.[25] These rates fit well with the ob-
servation that the reaction of (À)-2-carene 13 affords
exclusively the ring-opened product 14 in 56% yield
Table 1. Primary (entries1–6) and secondary (en-
tries7–9) chlorides, bromides and iodides were pre-
pared in good to excellent yields. For chlorides and
bromides, both reaction conditions could be used
without significant change in yields. Microwaves heat-
ing allowsusto reduce the reaction time down to
15 min. For the preparation of iodides, the reactions [Eq. (2)]. A non-radical rearrangement could not ex-
are run at 408C according to method A in order to plain the formation of thisproduct, asit wasetsab-
avoid thermal decomposition of the products. Under lished that no fragmentation occurs when the transfor-
these conditions, sensitive alkyl iodides are stable and mation proceeds via a polar mechanism or resulted
can be obtained in good yields.
from a concerted process.[26] In order to get a third in-
The radical nature of the halogenation process is sight into the reaction mechanism, the reaction was
supported by several observations. First, the stereose- run in the absence of a radical initiator. However, we
lectivity of the hydrohalogenation of a-pinene 3 noticed that hydroboration of (+)-b-pinene 1 fol-
(Table 1, entries7–9) is characteristic for a radical
lowed by reaction with benzenesulfonyl chloride at
process. An ionic process is expected to proceed with 408C proceedseven in the absence of di- tert-butyl hy-
retention of configuration and to afford exclusively ponitrite and chloride 4 is isolated in similar yields.
2À
isopinocampheyl halides. However, traces (about 3%) Tracesof oxygen and/or homolytic cleavage the SO
Table 1. Hydrohalogenation of alkenesaccording to Eq. (1).
Entry
Alkene
Trap
Product
Method:[a] Yield[b]
1
2
3
PhSO2Cl
PhSO2Br
p-TolSO2I
4
5
6
A: 94%[c]
A: 85%[c]
A: 88%[c]
1
2
4
5
6
PhSO2Cl
PhSO2Br
p-TolSO2I
7
8
9
B: 90%
B: 90%
A: 77%
7
8
9
PhSO2Cl
PhSO2Br
p-TolSO2I
10
11
12
A: 83%[d]
A: 90%[d]
A: 87%[d]
3
[a]
Reactionswere carried out on the 2.0 mmol scale.
Yield of isolated product.
9:1 mixture of diastereomers (major isomer shown).
Ratio of diastereomers dr ꢀ 97:3.
[b]
[c]
[d]
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An Efficient Radical Procedure for the Halogenation and Chalcogenation
FULL PAPERS
Cl bond are presumably initiating this efficient radical methodsgive clean and rapid reactionswith a variety
chain process.
of substrates (Table 2). For instance, primary (en-
tries1,2) and secondary (entries3–9) B-alkylcatechol-
boranes afford the expected sulfides and selenides
21–26 in good to excellent yields. Reaction at a ben-
zylic position is also possible as demonstrated by the
Chalcogenation
Extension of the reaction to the formation of carbon- reaction with stilbene 17 affording 27 (Table 2,
sulfur and carbon-selenium bonds using commercially entry 10). Dienes, such as b-citronelene 18 (entry 11)
available S-phenyl benzenesulfonothioate and Se- and (R)-limonene 19 (entry 12) give the monosulfur-
phenyl benzenesulfonoselenoate was investigated ized products 28 and 29 in good yields. In these two
next.[19] Reactionswere run under the conditionsde-
particular cases, the use of Wilkinsonꢁs catalyst for
veloped for the halogenation process using initiation the hydroboration step leads to higher selectivities
with either di-tert-butyl hyponitrite at 408C [Eq. (3), and yields.[28]
Reactionswith ( +)-2-carene 13 (entry 13) and 2-
phenylmethylenecyclopropane 20 (entry 14) afford ex-
clusively the products 30 and 31 resulting from the
opening of the cyclopropane ring. These two reactions
demonstrate again the radical nature of the process.
Interestingly, the radical derived from (R)-limonene
18 (entry 11) and b-citronelene 19 (entry 12) are also
method A] or di-tert-butyl peroxide under microwave expected to rearrange via 5-exo-trig and 6-exo-trig
irradiation at 1408C [Eq. (3), method B].[27] Both processes, respectively. Since the cyclization rate con-
Table 2. Benzenesulfanylation and benzeneselanylation of alkenes according to Eq. (3).
Entry
Alkene
X
Product
Method:[a] Yield[b]
1
2
3
4
5
6
7
8
2
2
15
15
15
3
S
S
S
S
Se
S
S
21
21
22
22
23
24
24
25
A: 64%
B: 80%
A: 73%
B: 98%
A: 90%
A: 90%[c]
B: 92%[c]
A: 93%[c]
3
3
Se
9
16
17
S
S
26
27
A: 59%
A: 77%
10
11
12
13
18
19
S
S
28
29
A: 75%[d,e]
A: 92%[d]
13
20
S
S
30
31
A: 61%
A: 63%
14
[a]
Reactionswere carried out on 2.0 mmol scale.
Yield of isolated product.
[b]
[c]
[d]
[e]
Ratio of diastereomers dr ꢀ 97:3.
Hydroboration performed with (Ph3P)3RhCl asa catalyst.
1:1 mixture of diastereomers.
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Arnaud-Pierre Schaffner et al.
FULL PAPERS
stants are kꢁ5”105 sÀ1 at 808C[29] and the concentra-
tion of PhSO2SPh is1.2M, the absence of cyclization
products suggests a rate constant kꢀ5”106 MÀ1 sÀ1 for
1–3 h), the solution turned black. The crude product was pu-
rified by flash chromatography.
the reaction of
a
primary alkyl radical with
General Procedure B (microwave heating)
PhSO2SPh. Thisrate contsant isabout one order of
magnitude faster than the rate constant for the reac-
tion of a primary alkyl radicalswith diphenyl disulfide
(k=2”105 MÀ1 sÀ1 at 808C[30]).
Catecholborane (0.319 mL, 3 mmol) wasadded dropwies at
08C to a solution of olefin (1.5 mmol) and N,N-dimethyl-
acetamide (0.014 mL, 0.15 mmol) in CH2Cl2 (2.0 mL) under
nitrogen. The reaction mixture washeated under reflux for
5 h. MeOH (0.200 mL, 4.8 mmol) wasadded at 0 8C and the
solution was stirred for 15 min at room temperature. The
solvent was evaporated with a stream of nitrogen and the
reaction mixture wastranfserred to a microwave reaction
vessel containing either dichlorobenzene or DMF (4.0 mL),
PhSO2X (1.8 mmol) and di-tert-butyl peroxide (25 mg,
0.15 mmol). The reaction vessel was closed and heated with
microwave (600 W) at 1408C for 15 min. The black reaction
mixture wasdepoisted onto a chromatography column and
eluted with pentane to remove dichlorobenzene and with
pentane/Et2O (95:5) to elute the product. When N,N-
The sulfurization process is not limited to the intro-
duction of an SPh group. Reagentsfor the introduc-
tion of S-alkyl residues can also be used. For instance,
the organoborane derived from a-pinene 3 wastreat-
ed with easily available S-methyl and S-benzyl ben-
A
sulfides 32 and 33 were obtained in good yields.
dimethylformamide was used as solvent, the crude mixture
U
was washed with water before flash chromatography.
General Procedure C (rhodium-catalyzed
hydroboration)
Conclusions
Catecholborane (0.468 mL, 4.4 mmol) wasadded dropwies
at 08C to a solution of alkene (2.0 mmol) and (Ph3P)3RhCl
(1 mol%, 18 mg) in CH2Cl2 (2.0 mL) under nitrogen. The re-
Halogenation and chalcogenation of B-alkylcatechol-
boraneswith ArSO X reagentsare mild and efficient
action mixture washeated to reflux for 5 h.
t-BuOH
2
procedures. The reactions can be carried out under
conventional and microwave heating. Yieldsare com-
parable for both methodsbut reaction time isgreatly
reduced by using microwave conditions. Due to the
exceptional reactivity of B-alkylcatecholboranesto-
wardsuslfonyl radical,s excellent yieldsare obtained
with only 1.2 equivalentsof the radical trap. The radi-
cal nature of the transformation is supported by ex-
perimentswith radical probesand by tsereochemical
considerations. The halogenation and chalcogenation
processes presented here compares favorably in term
of yieldsand mildnesof the reaction conditionswith
all known literature procedures.
(0.24 mL, 2.5 mmol) wasadded at 0 8C and the solution was
stirred for 15 min at room temperature. PhSO2X (2.4 mmol)
wasthen added and the solution waswarmed at reflux and
di-tert-butyl hyponitrite (10 mg, 0.06 mmol) wasadded every
1 h. The reaction wasmonitored by GC-MS. At the end of
the reaction time (typically 1–3 h), the solution turned
black. The crude product waspurified by flash chromatogra-
phy.
General Procedure D (iodination)
Catecholborane (0.468 mL, 4.4 mmol) wasadded dropwies
at 08C to a solution of alkene (2.0 mmol) and N,N-dimethy-
lacetamide (0.020 mL, 0.2 mmol) in CH2Cl2 (2.0 mL) under
nitrogen. The reaction mixture washeated to reflux for 5 h.
t-BuOH (0.24 mL, 2.5 mmol) wasadded at 0 8C and the so-
lution wasstirred for 15 min at room temperature. After ad-
dition of freshly prepared para-toluenesulfonyl iodide[32]
(846 mg, 30 mmol) at 08C. Air (60 mL, 0.5 mmol O2) wasin-
troduced over 90 min through a needle placed just above
the reaction surface. The reaction was monitored by GC-
MS. The solution turned to dark yellow and black in 30 to
90 min. The organic phase was washed with a 10% aqueous
Na2S2O3 solution. The resulting organic layer was dried over
MgSO4. After evaporation of the solvent, the crude product
waspurified by flash chromatography.
Experimental Section
General Procedure A (conventional heating)
Catecholborane (0.639 mL, 6 mmol) wasadded dropwies at
08C to a solution of olefin (2.0 mmol) and N,N-dimethyl-
acetamide (0.020 mL, 0.2 mmol) in CH2Cl2 (2.0 mL) under
nitrogen. The reaction mixture washeated under reflux for
5 h. MeOH (0.200 mL, 4.8 mmol) wasadded at 0 8C and the
solution was stirred for 15 min at room temperature.
PhSO2X (2.4 mmol) wasthen added and the oslution was
warmed at reflux and di-tert-butyl hyponitrite (10 mg,
0.06 mmol) wasadded every 1 h. The reaction wasmoni-
tored by GC-MS. At the end of the reaction time (typically
SupportingInformation
The detailed characterization of all compoundscan be
found in the Supporting Information.
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An Efficient Radical Procedure for the Halogenation and Chalcogenation
FULL PAPERS
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Acknowledgements
This work was supported by the Swiss National Science
Foundation (project 20–103627). We thank BASF Corpora-
tion for the generous gift of catecholborane.
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