Ring-Opening of N-tert-Butanesulfinylethynylaziridines with Lithium Tris(dimethylphenylsilyl)zincate: Stereoselective Access to 4-Amino-1-allenylsilanes

Article abstract of DOI:10.1002/adsc.201500347

The ring-opening of N-tert-butanesulfinylethynylaziridines with lithium tris(dimethylphenylsilyl)zincate is reported. The reaction is demonstrated to be both stereoselective and stereospecific and to proceed through an anti-S_N2′ process. Further deprotection of the nitrogen atom under mild conditions allows access to 4-amino-1-(dimethylphenylsilyl)allenes with high yields and levels of stereoselectivity.

Full text of DOI:10.1002/adsc.201500347

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DOI: 10.1002/adsc.201500347
Ring-Opening of N-tert-Butanesulfinylethynylaziridines with
Lithium Tris(dimethylphenylsilyl)zincate: Stereoselective Access
to 4-Amino-1-allenylsilanes
Valentin N. Bochatay,a Youssouf Sanogo,a Fabrice Chemla,a,* Franck Ferreira,a,*
Olivier Jackowski,a and Alejandro PØrez-Lunaa,*
a
Sorbonne UniversitØs, UniversitØ Pierre et Marie Curie, UMR CNRS 8232, Institut Parisien de Chimie MolØculaire
(IPCM), Case 229, 4 place Jussieu, F-75252 Paris Cedex 05, France
Fax: (+33)-1-4427-7567; phone: (+33)-1-4427-5571; e-mail: fabrice.chemla@upmc.fr or franck.ferreira@upmc.fr or
alejandro.perez_luna@upmc.fr
Received: April 7, 2015; Revised: June 16, 2015; Published online: September 11, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500347.
Abstract: The ring-opening of N-tert-butanesulfinyl-
ethynylaziridines with lithium tris(dimethylphenylsi-
lyl)zincate is reported. The reaction is demonstrated
to be both stereoselective and stereospecific and to
proceed through an anti-SN2’ process. Further de-
protection of the nitrogen atom under mild condi-
tions allows access to 4-amino-1-(dimethylphenylsi-
lyl)allenes with high yields and levels of stereoselec-
tivity.
yields through an anti-SN2’ process (Scheme 1).[10,13]
Nevertheless, this easy-to-implement method has
never been used in synthesis probably because of the
harsh reaction conditions required for the removal of
the arylsulfonyl group. On the other hand, the Ns
group, easily cleavable under mild conditions,[14] has
been reported not to be compatible with these reac-
tions.[13b]
In this context, we reasoned that developing
a straightforward and stereoselective synthesis of 4-
amino-1-allenylsilanes 2 would be synthetically useful.
The tert-butanesulfinyl group is indeed known to be
easily removed under mild acidic conditions compati-
ble with most functional groups,[15] allowing possible
post-functionalizations. We thus aimed to obtain com-
pounds 2 by the reaction of N-tert-butanesulfinyl-
Keywords: allenes; asymmetric synthesis; aziri-
dines; diastereoselectivity; zincates
Since 4-aminoallenes I (Figure 1) constitute valuable ethynylaziridines 1 with silylcopper or silylzinc re-
building blocks for organic synthesis,[1] their synthesis agents (Scheme 1).
has attracted much attention in the past decade.[2–8]
The study was initiated with the stereoselective syn-
Surprisingly, despite an obvious synthetic potential as thesis of racemic N-tert-butanesulfinylethynylaziri-
propargylic anion equivalents in SE2’ substitution re- dines 1 obtained as pure trans and cis isomers in 3
actions,[9] only a few reports deal with the synthesis of steps following a methodology developed in our
4-amino-1-allenylsilanes II (Figure 1).[3a,8a,b,10]
group (Scheme 2).[16]
The SN2’ ring-opening of ethynylaziridines provides
We first investigated the reaction of aziridine 1a
a straightforward access to 4-aminoallenes.[11] In par- with cyano cuprate M1.[17] At À808C in THF, we were
ticular, alkylcopper reagents have been reported to
readily react with N-Mts, N-Mtr and N-Ts[12] ethynyl-
aziridines to give the corresponding 4-aminoallenes
with high levels of stereoselectivity and excellent
Figure 1. Structure of 4-aminoallenes I and 4-amino-1-alle-
nylsilanes II.
Scheme 1. Planned access to 4-amino-1-allenylsilanes 2.
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Table 1. Screening of the reaction conditions.
Scheme 2. Synthesis of racemic aziridines 1.
pleased to observe the formation of the desired
1
adduct 2a as a single isomer as seen by H NMR anal-
ysis.
Although encouraging, this result was however not
fully satisfactory because of the unexpected formation
of vinylsilane 3a. The side-product 3a was assumed to
be formed through the nucleophilic attack of the
copper species onto in situ generated 2a. Despite
many efforts, the formation of 3a could not be avoid-
ed. Our best result was obtained by adding a slight
sub-stoichiometric amount of M1 (0.9 equiv.) to
a THF solution of 1a at À808C. Under such condi-
tions, a full conversion was observed within 1 h and
a 2a:3a ratio of 80:20 was attained (Table 1, entry 1).
The same trend was observed with 28 alkyl and aro-
matic aziridines 1b and 1c for which the correspond-
ing side-products 3b and 3c were detected in the
crude reaction mixture by 1H NMR analysis in
2b:3b=72:28 and 2c:3c=84:16 ratios (Table 1, en-
tries 2 and 3).
Entry 1
M
equiv. T
[8C]
2:3[a]
dr[b]
2, yield
[%][c]
1
2
3
4
5
6
7
8
1a M1 0.9
1b M1 0.9
1c M1 0.9
1a M2 2.2
1a M2 2.2
1a M2 1.1
1a M3 2.2
1a M4 2.2
À80 80:20 >98:2
À80 72:28 >98:2
À80 84:16 >98:2
À80 >98:2 >98:2
À30[f] >98:2 >98:2
À30[f] >98:2 >98:2
2a, 63[d]
2b, 66[e]
2c, 63[d]
2a, 66
Being unable to avoid the formation of disilylated
side-product 3,[18] we thus planned to employ zincate
M2[19] instead of cuprate M1. To our great delight,
when carried out at À808C in THF, the reaction with
aziridine 1a was complete within 2 h and afforded 2a
in 66% isolated yield as a single isomer (Table 1,
entry 4). The yield could be improved to 80% by
adding 1a to M2 in THF at À608C and then running
the reaction at À308C for 1.5 h (Table 1, entry 5),
whereas using a stoichiometric amount of zincate spe-
cies M2 resulted in an incomplete conversion and
a significantly lower 47% isolated yield (Table 1,
entry 6). It is worthy of note that, in all these cases,
no formation of the side-product 3a was observed as
2a, 80
2a, 47[g]
À30[f] >98:2 (75:25)[h] 2a, (57)[i]
À30[f] >98:2 (96:4)[h] 2a, (78)[i]
[a]
[b]
[c]
1
Ratio determined by H NMR analysis of the crude reac-
tion mixture.
The dr of major product 2 was determined by H NMR
analysis of the crude reaction mixture.
Unless otherwise stated, isolated yield of diastereomeri-
1
cally pure major product 2.
Minor product 3 could not be isolated.
[d]
[e]
[f]
Isolated yield of 24% for minor product 3b.
Aziridine 1a was added to the organometallic species M
in THF at À608C and the resulting solution warmed to
À308C.
1
seen by H NMR analysis. As in this reaction two
PhMe2Si groups were lost, we next envisaged to use
mixed lithio zincates M3 and M4. Unfortunately, at
À308C degradation was observed leading to modest
yields, albeit a high dr of 96:4 was attained with M4
(Table 1, entry 8). In addition, in our hands, these re-
sults were not reproducible both in terms of stereose-
lectivity and yield. This was assumed to result from
the difficulty in controlling the stoichiometry of these
Conversion of 80% by 1H NMR analysis.
The stereoselectivity was not reproducible.
Unidentified side-products were observed by 1H NMR
analysis. The yield of crude 2a was not reproducible.
[g]
[h]
[i]
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Ring-Opening of N-tert-Butanesulfinylethynylaziridines
Table 2. Synthesis of 4-amino-1-allenylsilanes 2.[a]
Table 3. Synthesis of 4-amino-1-allenylsilane salts 4.[a]
[a]
Isolated yields. Unless otherwise stated, all salts 4 were
isolated as diastereomerically pure products (>98:2 dr).
Many side-products were observed.
Salt 4e could not be satisfactorily purified.
[b]
[c]
[a]
Isolated yields of 2 after chromatography over silica gel.
[d]
The reaction was carried out 5 h at 08C. The dr was de-
Unless otherwise stated, all 4-amino-1-allenylsilanes 2
were isolated in a diastereomerically pure form (>98:2
dr).
1
termined by H NMR analysis of the crude reaction mix-
ture. The two diastereomers could not be separated by
chromatography over silica gel.
[b]
1
The dr was determined by H NMR analysis of the crude
reaction mixture. The two diastereoisomers could not be
separated by flash chromatography over silica gel.
treatment with dry HCl in MeOH. In most cases, hy-
drochloride salts 4 were obtained in good yields and
high dr (Table 3). In the cases of compounds 2e and
mixed zincates, presumably because the accurate ti- 2h degradation was however observed to some extent.
tration of the PhMe2SiLi solution is impossible.[20]
Furthermore, we have demonstrated that the N-
The reactivity of M2 was next investigated with acetylation of salt 4a under basic conditions provided
other aziridines (Table 2). When subjected to the N-acetyl derivative 5 in 90% isolated yield over 2
above optimized conditions, all disubstituted aziri- steps (Scheme 3). The relative configuration of com-
dines tested, bearing 28 alkyl, alkenyl, phenyl or even pounds 2 was unambiguously assigned by the single
functionalized substituents, afforded the correspond- crystal X-ray analysis of the 3,5-dinitrobenzoyl deriva-
ing 4-amino-1-allenylsilanes 2a–g in good yields (59– tive 6[21] obtained in 60% yield through the reaction
85%) and high dr (ꢀ96:4). Interestingly, the reaction of salt 4c with 3,5-(NO2)2C6H3COCl under the same
could also be efficiently applied to the synthesis of 2h conditions.
possessing a tetrasubstituted stereogenic carbon atom.
We next explored the reactivity of M2 with cis azir-
At this stage we thus had a general method for the idine 1a’. The latter was proven to be less reactive
stereoselective synthesis of 4-amino-1-allenylsilanes 2. than its trans isomer 1a. Only partial conversion of
In order to demonstrate the synthetic potential of this ~30% was indeed reached within 1.5 h at À308C in
method, we then considered the removal of the N- THF leading to less than 10% isolated yield of 2a’.
tert-butanesulfinyl chiral auxiliary. As well document- However, increasing the reaction temperature to 08C
ed,[15] this group, whose reactivity is comparable with afforded 2a’ in 70% yield as a single isomer within
that of the Boc group, could be easily removed by 1.5 h (Scheme 4). Further removal of the N-tert-buta-
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Scheme 3. Determination of the relative configuration of
compounds 2.
Scheme 5. Mechanistic investigation.
To gain further insight into the mechanism, aziri-
dine 1a was reacted with (n-Bu)3ZnLi in situ generat-
ed by mixing n-BuLi (3 equiv.) with ZnBr2 (1 equiv.)
at À208C for 10 min.
Whereas no reaction occurred at À308C in THF,
deuterated aziridine D-1a was quantitatively obtained
when the reaction was performed for 2 h at 08C and
then quenched with CD3OD (Scheme 5). This result
indicates that (n-Bu)3ZnLi is able to afford acetylenic
zincate Zn-1a (through deprotonation and subsequent
transmetallation), but that the transfer of an n-Bu
group via an SNi mechanism does not take place in
contrast with the reactivity observed by Oku et al.
with propargylic electrophiles.[19,22] On the other hand,
two supplementary experiments were performed with
M2. Under our optimized conditions, upon quenching
with CD3OD, 4-amino-1-allenylsilane 2a was obtained
in 75% yield from 1a, with no incorporation of deute-
Scheme 4. Synthesis of 2a’ and 4a’.
nesulfinyl moiety produced hydrochloride salt 4a’ in rium atom. Additionally, from deuterium-labelled
1
74% yield. The H NMR spectrum of 4a’ was found aziridine D-1c no significant loss of deuterium incor-
to be different from that of 4a. This allowed us to poration was noted upon quenching with MeOH.
deduce the relative configuration of 4a’ since 4a and These results suggest that a mechanism reminiscent of
4a’ only differ in the configuration of the stereogenic that occuring for ethynylepoxides[23] that would entail
carbon atom a to the allenyl moiety.
deprotonation and subsequent anti-SNi transfer of
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Ring-Opening of N-tert-Butanesulfinylethynylaziridines
Experimental Section
Representative Procedure for the Ring-Opening of
Aziridines 1 with M2
Under argon, Li (557 mg, 80.0 mmol) was washed with an-
hydrous THF (”3). TMSCl (5.12 mL, 40.0 mmol) and THF
(20 mL) were added. After 15 min of sonication, Li was
washed with anhydrous THF (”4) and then taken up in an-
hydrous THF (40 mL). After an additional 5 min of sonica-
tion under argon, PhMe2SiCl (3.30 mL, 20.0 mmol) was
added and the resulting mixture was sonicated for 2 h at
08C to give a dark-red THF solution of PhMe2SiLi (~
0.4M). The solution could be stored at À188C for 3 weeks.
Under argon, to the above solution of PhMe2SiLi (~0.4M
in THF, 28.50 mL, ~11.40 mmol) was added dropwise a solu-
tion of ZnBr2 (1.00M in THF, 3.80 mL, 3.80 mmol) at
À208C, giving, after 10 min, a dark-green solution of M2.
This solution was cooled to À608C and aziridine 1a
(369.3 mg, 1.73 mmol) in THF (17 mL) was added. After
warming to À308C, the reaction mixture was stirred for
1.5 h and quenched with MeOH. The layers were separated
and the aqueous one extracted with Et2O (”3). The com-
bined organic layers were washed with H2O then brine,
dried over anhydrous MgSO4, filtered, and concentrated
under vacuum. The crude product was purified by flash
chromatography over silica gel (EtOAc/cyclohexane 20:80)
to afford analytically pure 2a as a yellow oil; yield: 484.1 mg
(80%).
Scheme 6. Mechanistic rationale.
Acknowledgements
The authors wish to thank the MESR for a grant to V. N. B. ,
Geoffrey Gontard from IPCM for single-crystal X-ray analy-
sis, and Omar Khaled from IPCM for HR-MS analysis.
PhMe2Si is unlikely. Such a process would afford the
deuterated 4-amino-1-allenylsilane D-2 (Scheme 6,
top). Conversely, a direct anti-SN2’ pathway fully ac-
counts for the results (Scheme 6, bottom).
All these results point out that the stereoselectivity
of the ring-opening only depends on the configuration
of the progargylic stereogenic carbon atom on the
aziridine. In particular, they indicate that the chiral
tert-butanesulfinyl group on the nitrogen atom has no
impact on the stereochemical outcome these reac-
tions.
In conclusion, we have disclosed a rapid and highly
stereoselective access to 4-amino-1-allenylsilanes by
the reaction of lithium tris(dimethylphenylsilyl) zin-
cate, (PhMe2Si)3ZnLi·3LiCl·2LiBr M2, with N-tert-bu-
tanesulfinylethynylaziridines. The reaction was shown
to be stereospecific and to proceed through an anti-
SN2’ pathway. The use of these compounds in SE2’
substitution reactions is currently being investigated
in our group.
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