Direct transformation of terminal alkynes to branched allylic sulfones

Article abstract of DOI:10.1021/ja509383r

A new strategy for the transformation of terminal alkynes to branched allylic sulfones was developed. Using a Rh(I)/DPEphos/benzoic acid catalyst system, terminal alkynes react with sulfonyl hydrazides to produce branched allylic sulfones with good to exc

Full text of DOI:10.1021/ja509383r

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Communication
Direct Transformation of Terminal Alkynes to Branched Allylic Sulfones
Kun Xu, Vahid Khakyzadeh, Timm Bury, and Bernhard Breit
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja509383r • Publication Date (Web): 03 Nov 2014
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Direct Transformation of Terminal Alkynes to Branched Allylic
Sulfones
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Kun Xu, Vahid Khakyzadeh, Timm Bury and Bernhard Breit*
Institut für Organische Chemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 21, 79104 Freiburg i. Brsg.,
Germany
Supporting Information Placeholder
on7 may facilitate the attack to rhodium-allyl intermedi-
ABSTRACT: A new strategy for the transformation of
ates;8 2) Sulfonyl hydrazides are easily accessible;9 3) Sul-
terminal alkynes to branched allylic sulfones was devel-
fones, particularly allylic sulfones, are useful building
oped. Using a Rh(I)/DPEphos/benzoic acid catalyst sys-
blocks in organic synthesis10 and pharmaceuticals.11 Sul-
tem, terminal alkynes react with sulfonyl hydrazides to
fone derivatives bearing a α–chiral center are an im-
produce branched allylic sulfones with good to excellent
portant class of compounds in biological research. For
yields and selectivities in general.
example, Dorzolamide, Tazobactam and Dalfopristin are
prescription drugs used for anti-glaucoma agent, antibi-
otic treatments and anti-infections respectively.11e-g Herein,
we report a rhodium catalyzed hydrosulfination of termi-
Transformation of simple and readily accessible starting
nal alkynes with sulfonyl hydrazides as an efficient meth-
materials to branched allylic derivatives is an important
od to the synthesis of branched allylic sulfones.7b,12
topic in organic synthesis because of the versatility of the
allylic moiety for further elaboration, and the stereogenic
Scheme 1. Transformation of Terminal Alkynes to
Branched Allylic Sulfones
center for asymmetric synthesis.1 In the last decades, sig-
nificant progress toward this goal has been achieved, in
particular with allylic substitution1 and allylic C-H oxida-
tion2 chemistry. However, these methods require pre-
installation of a leaving group or stoichiometric amounts
of an oxidant, respectively. More recently, our group has
discovered rhodium catalyzed coupling of allenes with
pronucleophiles3 towards the synthesis of branched allylic
products.4 Alternatively, the easily accessible terminal
alkynes5 can couple with carboxylic acids in the presence
of a suitable rhodium catalyst to form the corresponding
branched allylic esters.6a-b Unfortunately, replacement of
carboxylic acids with other pronucleophiles to provide
new branched allylic derivatives only led to no reaction or
traces of product after intensive efforts, possibly due to
the difficulty of the alkyne to allene isomerization or the
rhodium-allyl formation.6c
To test our hypothesis, the reaction of p-
toluenesulfonyl hydrazide (1.0 equiv.) and 1-octyne (1.5
equiv.) with [Rh(COD)Cl]2 (2.5 mol%) and (Oxydi-2,1-
phenylene)bis(diphenylphosphine) (DPEphos, 10 mol%)
in 1,2-dichloroethane (DCE, 0.4 M) was heated at 80 oC for
18 hours with 1.0 equivalent benzoic acid. To our delight,
the reaction gave a promising 58% nmr yield of the
branched allylic sulfone (B), albeit trace amount of vinyl
sulfones (V) were formed (B/V = 92/8, Table 1, entry 1).
Notably, the reaction without benzoic acid gave only 10%
of the desired product (Scheme 1). Encouraged by this
result, we then checked different parameters of the reac-
tion conditions. Bidentate phosphine ligands with differ-
ent bite angles were investigated, yet DPEphos was found
to be the most efficient in terms of reactivity and selectivi-
Mechanistic investigations on the coupling of carbox-
ylic aicds with terminal alkynes indicated that the reac-
tion involves rhodium/carboxylic acid catalyzed isomeri-
zation of alkyne to an allene, followed by formation of a
Rh-allyl species, which is the turnover determining inter-
mediate of the catalytic cycle.6c We wondered whether
such an in situ formed σ or π rhodium-allyl species could
be attacked by an external nucleophile to produce the
corresponding branched allylic derivative (Scheme 1). To
this end, sulfonyl hydrazides were chosen as the bench-
mark pronucleophile based on the following reasons: 1)
the high nucleophilicity of in situ generated sulfonyl ani-
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ty (Table 1, entry 2-5). The amount of benzoic acid is cru-
sulfonyl hydrazides can also be transferred to the valuable
3
4
5
6
7
8
9
cial for the reaction, as lower benzoic acid loadings led to
reduced yields and lower selectivities (Table 1, entry 6-7).
The branched allylic product (1a) was isolated with 92%
yield when higher alkyne loading (2.5 equiv.) was used
(Table 1, entry 8). Other carboxylic acids with different
pKa value were proved to be less efficient than benzoic
acid (Table 1, entry 9-10). Control experiments indicated
that both the rhodium precursor and the ligand were
necessary for the reaction to proceed.13
branched allylic sulfones (2j-k).
Table 2. Scope of Rh-Catalyzed Hydrosulfination
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Entry
1
R1
n-C5H11
R2
p-Tol
Yield/% a
92 (1a)
82 (1b)
63 (1c)c
61 (1d)c
90 (1e)
83 (1f)
70 (1g)c
78 (1h)
88 (1i)
74 (1j)
B/Vb
93/7
94/6
79/21
82/18
93/7
93/7
82/18
82/18
89/11
79/21
95/5
87/13
93/7
92/8
95/5
98/2
95/5
92/8
94/6
95/5
90/10
91/9
Table 1. Optimization of Rh-Catalyzed Hydrosulfina-
tion
2
n-C4H9
p-Tol
3
(CH3)2CH
Cyclopentyl
Ph(CH2)2
PhCH2
p-Tol
4
p-Tol
5
p-Tol
6
p-Tol
7
Ph
p-Tol
8
NC(CH2)2
CH3O2C(CH2)2
PhthN(CH2)2
TBSO(CH2)2
HO(CH2)8
n-C5H11
p-Tol
Yield/% a
B/Vb
92/8
93/7
9
p-Tol
Entry
Ligand
DPEphos 1.5
x
R’
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CH3
y
10
11
p-Tol
1
2
100
100
100
100
100
20
58
12
p-Tol
70 (1k)
79 (1l)
dppb
dppp
dppf
1.5
1.5
1.5
12
13
14
15
16
17
18
19
20
21
22
23
p-Tol
3
23
97/3
Ph
86 (2a)
80 (2b)
90 (2c)
86 (2d)
59 (2e)
78 (2f)
75 (2g)
62 (2h)
78 (2i)
71 (2j)
4
5
47
92/8
63/37
80/20
91/9
n-C5H11
1-Naphthyl
2-Naphthyl
2-Me-Ph
3-Me-Ph
4-F-Ph
4-Cl-Ph
4-Br-Ph
4-MeO-Ph
PhCH2
CH3(CH2)2
rac-binap 1.5
DPEphos 1.5
DPEphos 1.5
DPEphos 2.5
DPEphos 2.5
35
n-C5H11
6
7
47
n-C5H11
50
64
(92)c
35
n-C5H11
8
9
10
50
93/7
n-C5H11
50
58/42
89/11
DPEphos 2.5 pCF3Ph 50
54
n-C5H11
a 1H NMR yield of the branched product in the crude reaction
mixture using 1,3,5-trimethoxybenzene as internal standard. b
ratio of B/V was determined by 1H NMR of the crude reaction
n-C5H11
n-C5H11
n-C5H11
c
mixture. Isolated yield of the branched product. binap =
n-C5H11
74 (2k)c
86/14
2,2’-bis(diphenylphosphino)-1,1’-binaphthyl, cod
=
1,5-
a
b
cyclooctadiene, dppb = 1,4-bis(diphenylphosphino)butane,
dppf = 1,1’-bis(diphenylphosphino)ferrocene, dppp = l,3-
bis(diphenylphosphino)propane.
Isolated yield of branched products. Ratio of B/V was
1
c
determined by H NMR of the crude reaction mixture.
[RhCODCl]2 (5.0 mol%) and DPEphos (20 mol%) were used.
Phth = phthaloyl, TBS = tert-butylsilyl.
With the optimized conditions in hand, we then evalu-
ated the scope of the reaction. Various terminal alkynes
were coupled with p-toluenesulfonyl hydrazide to give the
corresponding branched allylic sulfones with good to
excellent yields and selectivities in most cases (table 2, 1a-
l). Several functional groups (cyano, ester, protected am-
ide and even free hydroxyl) were well tolerated (1h-l),
both aliphatic and aromatic substituted terminal alkynes
were compatible (1a-g), although sterically hindered ter-
minal alkynes gave lower yields and selectivities (1c-d). A
series of sulfonyl hydrazides reacted with 1-octyne to give
the corresponding branched allylic sulfones (table 2, 2a-k).
Diversely substituted aromatic sulfonyl hydrazides were
suitable substrates (2a-i). Benzyl and alkyl substituted
To probe the reaction mechanism, control experiments
of TsNHNH2 (N1) and 4-methylbenzenesulfinic acid (N2)
with branched benzoic ester (E1), allene (E2) and alkyne
(E3) were performed (Table 3). Benzoic acid had little
impact on the reactivity of ester (E1) with TsNHNH2 (Ta-
ble 3, Entry 1-2). This suggests that the ester (E1) can un-
dergo oxidative addition with rhodium (I) to form a rho-
dium-allyl species followed by nucleophilic attack under
both neutral and acidic conditions. The reaction of allene
(E2) with TsNHNH2 was accelerated in the presence of
50% benzoic acid (Table 3, Entry 3-4), which indicates
that benzoic acid is essential for the formation of rhodi-
um-allyl species.
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The reactions of 4-methylbenzenesulfinic acid (N2)
therefore the overall acidity of the reaction is dominated
by the acidity of benzoic acid.
with ester (E1) gave high yields of the desired product
(Table 3, Entry 5-6). However, the reactions of N2 with
allene (E2) and alkyne (E3), only led to traces or no prod-
uct formation (Table 3, Entry 7-10). We suspect that the
low reactivities of allene and alkyne with N2 arise from
the relatively higher acidity of N2 (pka ≈ 2.1) comparing to
benzoic aicd (pka = 4.2). The presence of large amounts of
acidic N2 would suppress the isomerization of alkyne, as
well as transformation of allene to rhodium-allyl species.
Scheme 2. Proposed reaction pathways
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Table 3. Control experiments
To explore the potential of this methodology in asym-
metric synthesis, a preliminary chiral ligand screening was
undertaken. One result with the chiral ligand (S)-iPr-
MeOBIPHEP, the branched allylic sulfone (1a) was ob-
tained with 68% isolated yield, excellent regioselectivity
(B/V > 99/1) and a promising 41% ee (Scheme 3).
Entry E1 - E3 n (equiv) N1, N2 z (mol%) Yield/%a
1
2
E1
E1
E2
E2
E1
E1
E2
E2
E3
E3
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2.5
2.5
N1
N1
N1
N1
N2
N2
N2
N2
N2
N2
0
50
0
59
61
3
25
Synthesis of 1a in a 5.0 mmol scale under the scope
conditions led to the isolation of 1.14 g (86%) of the de-
sired product without detrimental effect of the regioselec-
tivity, which proved the practicality of this methodology.13
4
5
50
0
70
80
6
7
50
0
77
Scheme 3. Asymmetric hydrosulfination
traceb
traceb
n.rb
n.rb
8
9
10
50
0
50
a isolated yield. b determined by 1HNMR of the crude reaction
mixture; n.r: no reaction.
According to the control experiments and previous
mechanistic investigation on rhodium-catalyzed coupling
of carboxylic acids with terminal alkynes,6c we propose
that the reaction of sulfonyl hydrazides with terminal
alkynes proceeds via the following pathways (Scheme 2)14:
1) Terminal alkyne (A1) is transferred to a σ or π rhodium-
allyl species (A2) via alkyne to allene isomerization fol-
lowed by hydrometallation in the presence of rhodi-
um/DPEphos/benzoic acid; 2) The rhodium-allyl species
(A2) can be attacked by in situ formed sulfonyl anion to
generate the desired branched allylic sulfone. Alternative-
ly, ligand exchange of A2 to B3 followed by reductive
elimination is also possible. The rhodium-allyl species (A2)
undergoes reductive elimination to produce the branched
allylic benzoate and [Rh(DPEphos)Cl], which is reversible.
As a side reaction, the vinyl sulfones are formed via reduc-
tive elimination of the rhodium-vinyl species, which are
generated by hydrometallation of rhodium hydride spe-
cies to terminal alkyne.
To conclude, we have developed the first rhodi-
um/benzoic acid catalyzed hydrosulfination of terminal
alkynes with sulfonyl hydrazides to afford the valuable
branched allylic sulfones in good to excellent yields and
selectivities. The success of hydrosulfination is a proof-of-
concept of transforming terminal alkynes to branched
allylic derivatives via rhodium/benzoic acid catalysis.
Application of this methodology to other nucleophiles,
their asymmetric variants, as well as mechanistic investi-
gations will be reported in due course.
ASSOCIATED CONTENT
Supporting Information. Experimental procedures and
1
analytic data for synthesized compounds, including H and
13C NMR spectra as well as HPLC data sheets for chiral com-
pounds. This material is available free of charge via the In-
ternet at http://pubs.acs.org.
AUTHOR INFORMATION
We speculate that a relatively weak acidity (pka around
4.0) of the reaction mixture is crucial. The in situ formed
sulfinic acid is consumed promptly via reaction with A2,
Corresponding Author
bernhard.breit@chemie.uni-freiburg.de
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(9)
Sulfonyl hydrazides can be obtianed in one step by re-
action of commercially available sulfonyl chlorides with hydra-
zine.
(10)
gamon: Oxford, 1993; (b) Patai, S.; Rapoport, Z.; Stirling, C. The
Chemistry Functional Groups: Sulfones and Sulfoxides; Wiley:
New york, 1988; c) Organosulfur Chemistry in Asymmetric Syn-
thesis; Toru, T., Bolm, C., Eds.; Wiley-VCH: Weinheim, 2008. d)
El-Awa, A.; NoShi, M. N.; Mollat du Jourdin, X.; Fuchs, P. L.
Chem. Rev. 2009, 109, 2315-2349. e) Alba, A. R.; Companyó, X.;
Rios, R. Chem. Soc. Rev. 2010, 39, 2018-2033.
ACKNOWLEDGMENTS
This work was supported by the DFG, the International Re-
search Training Group “Catalysts and Catalytic Reactions for
Organic Synthesis” (IRTG 1038), the Fonds der Chemischen
Industrie and the Krupp Foundation. We thank Umicore,
BASF and Wacker for generous gifts of chemicals.
(a) Simpkins, N. S. Sulfones in Organic Synthesis; Per-
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yl)benzenesulfonohydrazide under optimized condition (both
without and with 50% benzoic acid) only led to decomposition,
which indicates that nitrogen attack of the sulfonyl hydrazide to
the in situ formed rhodium-allyl benzoate species A2 followed by
releasing of nitrogen and hydrogen is unlikely.
See supporting information.
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