New synthesis of pyrrolidines via reaction of γ-halocarbanions with imines

Article abstract of DOI:10.1002/hlca.200590132

γ-Chlorocarbanions of proper nucleophilicity, generated from 3-chloropropyl pentachlorophenyl sulfone (= pentachloro[(3-chloropropyl) sulfonyl]benzene; 1; Ar = C₆Cl₅), add to electron-deficient formal imines 3a-I to produce anionic adducts that enter intramolecular substitution leading to substituted pyrrolidines. This new and simple synthesis of pyrrolidines mimics a 1,3-dipolar cycloaddition, although it proceeds in two distinct steps.

Full text of DOI:10.1002/hlca.200590132

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Helvetica Chimica Acta Vol. 88 (2005)
New Synthesis of Pyrrolidines via Reaction of g-Halocarbanions with
Imines
by Mieczys¯aw MaŒkosza* and Marek Judka
Institute of Organic Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, PL-01 224 Warsaw
(e-mail: icho-s@icho.edu.pl)
Dedicated to Professor Rolf Huisgen on the occasion of his 85th birthday
g-Chlorocarbanions of proper nucleophilicity, generated from 3-chloropropyl pentachlorophenyl sulfone
(ˆ pentachloro[(3-chloropropyl)sulfonyl]benzene; 1; Arˆ C6Cl5), add to electron-deficient formal imines 3a
l
to produce anionic adducts that enter intramolecular substitution leading to substituted pyrrolidines. This new
and simple synthesis of pyrrolidines mimics a 1,3-dipolar cycloaddition, although it proceeds in two distinct
steps.
Substituted pyrrolidine rings are present in numerous alkaloids, pharmaceuticals,
plant protecting agents [1], ligands of transition metal catalysts [2], etc., thus, general
methods for the synthesis of this ring system are of great interest and demands. There
are many routes leading to substituted pyrrolidine rings, based on cationic cyclizations,
mainly via iminium ions [3], cyclization proceeding via free radicals [4], dipolar
cycloadditions [5], reactions proceeding via carbanions [6], etc.
Of particular value and interest is the construction of pyrrolidine and other five-
membered heterocyclic rings via 1,3-dipolar cycloaddition proceeding according to the
concept created and pioneered by Huisgen and co-workers [7]. Huisgen also
formulated basic structural elements of 1,3-dipoles and general mechanistic features
of this process [8]. Addition of readily generated azomethine ylides (ˆ iminium ylides)
to alkenes is a general route to pyrrolidines [5].
In this paper, we report a simple and efficient protocol of the synthesis of the
pyrrolidine ring via reactions of g-halocarbanions with formal imines, that to some
extent mimic 1,3-dipolar cycloaddition.
Known reactions of g-halocarbanions are limited to intramolecular substitution of
the halogen atom leading to three-membered rings. Synthesis of cyclopropanes via
alkylation of −methylenic× carbanions with 1,2-dihaloalkanes [9] and via addition of a-
halocarbanions to Michael acceptors [10] proceeds via intermediate formation of g-
halocarbanions. They are also intermediates in the Ramberg B‰cklund [11] and
Favorski reactions [12]. Due to the high rate of the intramolecular substitution
promoted by the proximity effect, intermolecular reactions of g-halocarbanions are
observed only when structural features decelerate [13] or hinder the intramolecular
reaction [14].
On the other hand, g-halocarbanions containing an electron-deficient C-atom
connected to the halogen atom and the nucleophilic carbanion center in a 1,3 relation-
¹ 2005 Verlag Helvetica Chimica Acta AG, Z¸rich

Helvetica Chimica Acta Vol. 88 (2005)
1677
ship can be considered as analogues of 1,3-dipoles, and upon addition to polar double
bonds, followed by 1,5-intramolecular substitution, they should form five-membered
rings (Scheme 1). This reaction shall proceed via two distinct time-separated steps.
Scheme 1
Indeed, we have shown recently that g-halocarbanions generated via deprotonation
of 3-chloropropyl phenyl sulfone (ˆ [(3-chloropropyl)sulfonyl]benzene), 4-chlorobu-
tanenitrile, and t-butyl 4-chlorobutanoate, in spite of very fast intramolecular
substitution leading to the corresponding cyclopropanes, can be trapped by active
external electrophiles such as aldehydes and ketones. The produced aldol-type anions
enter intramolecular substitution to give substituted tetrahydrofurans [15] (Scheme 2).
Scheme 2
Our attempts to extend reactions of these g-halocarbanions to other electrophilic
partners containing electron-deficient double bonds, i.e., Michael acceptors and imines
that should lead to cyclopentanes and pyrrolidines, was initially unsuccessful. Treat-
ment of mixtures of some of these electrophilic partners and the carbanion precursors,
shown in Scheme 2, with the base gave only cyclopropanes, whereas electrophiles
decomposed or polymerized. Obviously, the intramolecular substitution in the g-
halocarbanions was faster than the addition to these moderately active electrophiles.
This problem for the reaction with Michael acceptors was solved by tuning the acidity of
the g-halocarbanion precursor. We found that carbanions of pentachlorophenyl 3-
chloropropyl sulfone (ˆ pentachloro[(3-chloropropyl)sulfonyl]benzene; 1) are less
nucleophilic than the corresponding phenyl sulfone, hence, their lifetimes are
sufficiently long for addition to a variety of Michael acceptors. Subsequent intra-
molecular substitution in the anionic adducts results in the formation of substituted
cyclopentanes [16].
In spite of the extended lifetimes of carbanions of 1, the base-promoted reaction of
1 with a simple formal imine, i.e., N-benzylideneaniline, gave only (arylsulfonyl)cy-
clopropane 2, which is the product of the intramolecular substitution (Scheme 3). On
the other hand, in the reaction of 1 with the more-electrophilic N-tosyl-substituted
formal imine 3a, an N-benzylidenesulfonamide, the expected [(3-pentachlorophenyl)-

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Helvetica Chimica Acta Vol. 88 (2005)
sulfonyl]-2-phenyl-1-tosyl pyrrolidine (4a) was obtained. (Arylsulfonyl)cyclopropane 2
was formed as a minor side product.
Scheme 3
a) For a l, see the Table.
N-Tosyl-substituted formal imines derived from other aromatic aldehydes, cinna-
maldehyde, and pivalaldehyde reacted similarly giving the expected substituted 1-
tosylpyrrolidines usually in good yield (Table). In all experiments, minor amounts of
the cyclopropane 2 were formed.
Table. Synthesis of Pyrrolidines by the Reaction of Carbanions of 1 with Electron-Deficient Imines (see
Scheme 3)
Imine
R
Z
Pyrrolidines
Yield [%]
Yield of 2 [%]
3a
3b
3c
3d
3e
3f
3g
3h
3i
Ph
Tol SO2
Tol SO2
Tol SO2
Tol SO2
Tol SO2
Tol SO2
Tol SO2
EtOCO
(EtO)2PO
(EtO)2PO
PhSO
4-ClÀC6H4ÀSO
4a
4b
4c
4d
4e
4f
4g
4h
4i
75
73
54
72
60
63
52
45
68
75
4
5
9
10
5
16
8
12
4-MeÀC6H4
4-ClÀC6H4
3-CFÀ3 ÀC6H4
PhCHˆCH
2-furyl
t-Bu
a
Ph
Ph
)
)
)
)
a
a
3j
3k
3l
4-MeÀC6H4
4j
4k
4l
a
Ph
a
Ph
42
)
a) Not determined.
Arenemethanimines RÀCHˆNH activated by substitution at the N-atom with
some other electron-accepting groups, such as diethoxyphosphoryl and ethoxycarbonyl
(see 3h j), are also sufficiently active electrophiles to trap the carbanion of 1 and
subsequently produce substituted 1-(ethoxycarbonyl)- and 1-(diethoxyphosphoryl)-
pyrrolidines. Of particular interest in this reaction should be N-benzylidenebenzene-
sulfinamide (3k) because phenylsulfinyl- and alkylsulfinyl-substituted compounds can
be readily obtained in enantiomerically pure form and are known to be efficient chiral
auxiliaries [17]. The benzenesulfinamide 3k was, however, an insufficiently active
electrophile, and in the reaction with the carbanion of 1, the expected pyrrolidine 4k

Helvetica Chimica Acta Vol. 88 (2005)
1679
was formed only in minute amounts (see Table), the major process being cyclopropane
formation. This problem was solved simply by use of the 4-chlorobenzenesulfinamide 3l
that shows a stronger electron-accepting character. Thus the base-promoted reaction of
1 with 3l gave the expected pyrrolidine 4l in acceptable yield (see Table).
All pyrrolidines obtained according to Scheme 3 were formed as single diaster-
eoisomers in which the substituent R and the (pentachlorophenyl)sulfonyl group were
in trans relationship. It should be mentioned that the yields of the pyrrolidines given in
the Table are not optimized, and the reactions were carried out under standard
conditions. From these results, we can conclude that the reaction of g-halocarbanions of
properly tuned nucleophilicity with electron-deficient imines is an efficient and simple
new synthesis of functionalized pyrrolidines. Taking into account the facile desulfo-
nylation as well as the formation of sulfonyl-substituted carbanions, these products are
attractive for further reactions. For example, in the reaction of pyrrolidine 4b with ethyl
iodoacetate, the product of alkylation is formed in 80% yield, offering rather wide
synthetic possibilities. In this connection, an interesting synthesis of pyrrolidines
should be mentioned, which proceeds via reactions of g-iodoenolates
(ICH2CH2CHˆC(R)ÀOÀ) generated in situ by treatment of acylcyclopropanes with
MgI2 or Et2AII [18]. Although g-iodoenolates cyclize rapidly, this is a reversible
process, whereas addition to formal imines followed by cyclization to pyrrolidines
proceeds irreversibly.
This work was supported by KBN grant No. 4 T09A 056 25.
Experimental Part
General. Pentachlorophenyl 3-chloropropyl sulfone was obtained as described earlier [16]. N-Substituted
imines 3a l were synthesized according to literature procedures, i.e., 3a g [19], 3h [20], 3i,j [21], 3k,l [22].
Lithium diisopropylamide (LDA) was purchased from Aldrich. All reactions were performed in oven-dried
glassware under dry Ar. THF was freshly distilled from K/benzophenone. Column chromatography and prep.
TLC: Merck silica gel 60 and silica gel 60 PF254 resp. M.p.: uncorrected. 1H- and 13C-NMR Spectra: Varian
Gemini-200-MHz and Varian Mercury-400-MHz spectrometer; d in ppm, J in Hz. HR-MS: AMD-604
instrument; in m/z.
Reaction of [Pentachloro[(3-chloropropyl)sulfanoyl]benzene (1) with Substituted Imines, Synthesis of 4a
4: General Procedure. To a soln. of 1 (117 mg, 0.3 mmol) and N-tosyl-substituted arenemethanimine, (0.4 mmol)
in THF (5 ml) at À 308, 1.8m LDA (0.27 ml, 0.5 mmol) was added. The mixture was stirred for 5 min, warmed to
r.t., quenched with aq. NH4Cl soln., and extracted and the product purified by CC or prep. TLC (hexane/
AcOEt).
Procedure for the Synthesis of 4h: To a soln. of 1 (117 mg, 0.3 mmol) in THF (5 ml) at À 708 1.8m LDA
(0.27 ml, 0.5 mmol) was added. After 2 s, ethyl benzylidenecarbamate (3h; 70.8 mg, 0.4 mmol) in THF (0.5 ml)
was added, then the procedure as above was followed.
3-[(Pentachlorophenyl)sulfonyl]-2-phenyl-1-tosylpyrrolidine (4a): M.p. 1648 (CHCl3). 1H-NMR (400 MHz,
CDCl3): 2.25 2.41 (m, 2 H); 2.43 (s, 3 H); 3.76 (dt, 3J(H,H) ˆ 2.20, 6.78, 1 H); 4.30 4.35 (m, 2 H); 5.02 (d,
3J(H,H) ˆ 5.50, 1 H); 7.10 7.62 (m, 9 H). 13C-NMR (100 MHz, CDCl3): 21.58; 24.57; 48.44; 63.03; 70.62; 126.38;
127.75; 127.84; 128.02; 128.19; 128.50; 128.66; 129.65; 129.73; 133.81; 134.06; 134.34; 135.06; 139.21; 139.38;
‡
143.94. HR-EI-MS 610.91177 (C23H18Cl5NO4S2 ; calc. 610.91199).
2-(4-Methylphenyl)-3-[(pentachlorophenyl)sulfonyl]-1-tosylpyrrolidine (4b): M.p. 1738 (EtOH). 1H-NMR
(400 MHz, CDCl3): 2.27 (s, 3 H); 2.28 2.41 (m, 2 H); 2.43 (s, 3 H); 3.72 3.76 (m, 2 H); 3.32 4.39 (m, 1 H);
4.86 (d, 3J(H,H) ˆ 6.32, 1 H); 6.95 (s, 4 H); 7.26 7.59 (m, 4 H). 13C-NMR (100 MHz, CDCl3): 21.02; 21.55;
24.23; 48.30; 63.12; 70.13; 126.37; 126.42; 127.69; 128.99; 129.59; 129.67; 133.68; 134.07; 134.28; 134.96; 135.69;
138.33; 139.26; 143.87. Anal. calc. for C24H20Cl5NO4S2 (627.81): C 45.92, H 3.21, N 2.23, S 10.21; found: C 45.91, H
3.43, N 2.38, S 10.23.

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Helvetica Chimica Acta Vol. 88 (2005)
2-(4-Chlorophenyl)-3-[(pentachlorophenyl)sulfonyl]-1-tosylpyrrolidine (4c): Oil. 1H-NMR (400 MHz,
3
CDCl3): 2.23 2.36 (m, 2 H); 2.44 (s, 3 H); 3.68 3.79 (m, 2 H); 4.21 4.26 (m, 1 H); 5.02 (d, J(H,H) ˆ 5.22,
1 H); 7.10 7.72 (m, 8 H). 13C-NMR (100 MHz, CDCl3): 21.56; 24.73; 48.38; 62.15; 70.55; 133.59; 133.66; 133.74;
134.16; 134.22; 134.30; 134.88; 135.12; 136.73; 137.99; 139.59; 141.28; 143.56; 144.18. HR-ESI-MS: 667.8657
‡
‡
(C23H17Cl5NNaO4S2 [M ‡ Na] ; calc. 667.8622).
3-[(Pentachlorophenyl)sulfonyl]-2-[3-(trifluoromethyl)phenyl]-1-tosylpyrrolidine (4d): M.p. 1038 (EtOH).
1H-NMR (400 MHz, CDCl3): 2.29 2.41 (m, 2 H); 2.43 (s, 3 H); 3.77 3.82 (m, 2 H); 4.25 (dt, J(H,H) ˆ 5.37,
3
7.02, 1 H); 5.15 (d, 3J(H,H) ˆ 5.22, 1 H); 7.27 7.81 (m, 8 H). 13C-NMR (100 MHz, CDCl3): 21.55; 24.82; 48.62;
62.28; 70.76; 122.91;126.36; 127.00; 127.70; 129.21; 129.61; 129.72; 129.76; 130.19; 133.78; 134.18; 135.24; 139.73;
‡
‡
140.60; 144.35. HR-ESI-MS: 679.9054 (C24H18Cl5FNO4S2 , [M ‡ H] ; calc. 679.9067).
3-[(Pentachlorophenyl)sulfonyl]-2-(2-phenylethenyl)-1-tosylpyrrolidine (4e): M.p. 1808 (dec.; EtOH).
1H-NMR (400 MHz, CDCl3): 2.25 (m, 2 H); 2.42 (s, 3 H); 3.57 3.67 (m, 2 H); 4.20 4.25 (m, 1 H); 4.48
4.52 (m, 1 H); 5.84 (dd, 3J(H,H) ˆ 8.26, 15.59, 1 H); 6.20 (d, 3J(H,H) ˆ 15.77, 1 H); 7.09 7.71 (m, 9 H).
13C-NMR (100 MHz, CDCl3): 21.55; 24.26; 47.79; 62.13; 67.87; 125.67; 125.99; 126.37; 127.85; 128.48; 128.60;
‡
129.75; 133.44; 133.81; 134.31; 134.79; 135.18; 139.68; 144.08. HR-ESI-MS, 659.9167 (C25H20Cl5NNaO4S2 [M ‡
‡
Na] ; found 659.9169)
2-Furan-2-yl-3-[(pentachlorophenyl)sulfonyl]-1-tosylpyrrolidine (4f): M.p. 1668 (EtOH). 1H-NMR
(200 MHz, CDCl3): 2.29 2.41 (m, 2 H); 2.42 (s, 3 H); 3.56 3.65 (m, 1 H); 3.37 3.81 (m, 1 H); 4.62 (dt,
3J(H,H) ˆ 6.33, 7.70, 1 H); 5.15 (d, 3J(H,H) ˆ 6.19, 1 H); 6.13 (dd, 3J(H,H) ˆ 1.78, 3.30, 1 H); 6.14 6.16 (m,
1 H); 7.09 7.12 (m, 1 H); 7.28 (d, 3J(H,H) ˆ 7.96, 2 H); 7.58 (d, 3J(H,H) ˆ 8.24, 2 H). 13C-NMR (100 MHz,
CDCl3): 21.55; 24.69; 47.71; 56.58; 66.93; 109.55; 110.36; 127.48; 129.59; 129.66; 133.87; 134.52; 135.00; 139.34;
142.77; 143.87; 149.99. Anal. calc. for C21H16Cl5NO5S2 (603.75): C 41.78, H 2.67, N 2.32, S 10.62; found: C 41.96, H
2.50, N 2.33, S 10.75.
2-(tert-Butyl)-3-[(pentachlorophenyl)sulfonyl]-1-tosylpyrrolidine (4g): M.p. 2178 (EtOH). 1H-NMR
(200 MHz, CDCl3): 1.06 (s, 9 H); 1.70 1.78 (m, 1 H); 1.81 1.91 (m, 1 H); 2.45 (s, 3 H); 3.26 (ddd,
3J(H,H) ˆ 6.33, 10.45, 12.38, 1 H); 3.70 3.76 (m, 1 H); 4.25 (dt, 3J(H,H) ˆ 4.13, 8.66, 1 H); 4.56 (d, 3J(H,H) ˆ
3.98, 1 H); 7.33 (d, 3J(H,H) ˆ 7.98, 2 H); 7.85 (d, 3J(H,H) ˆ 8.25, 2 H). 13C-NMR (100 MHz, CDCl3): 21.54;
27.29; 28.10; 36.89; 48.25; 65.18; 67.94; 128.66; 129.54; 133.77; 135.09; 135.52; 135.73; 138.20; 143.65. Anal. calc.
for C21H22Cl5NO4S2 (593.79): C 42.48, H 3.73, N 2.36, S 10.80; found: C 42.48, H 3.72, N 2.36, S 10.72.
Ethyl 3-[(Pentachlorophenyl)sulfonyl]-2-phenylpyrrolidine-1-carboxylate (4h): M.p. 2048 (CHCl3).
1H-NMR (400 MHz, CDCl3): 1.15 (t, 3J(H,H) ˆ 7.17, 3 H); 2.60 2.67 (m, 1 H); 2.97 3.11 (m, 1 H); 4.14 (q,
3J(H,H) ˆ 7.02, 2 H); 4.54 (ddd, 3J(H,H) ˆ 6.56, 8.54, 10.52, 1 H); 4.64 4.73 (m, 2 H); 4.97 (dt, 3J(H,H) ˆ 1.67,
8.69, 1 H); 7.25 7.38 (m, 5 H). 13C-NMR (100 MHz, CDCl3): 14.44; 26.79; 62.70; 64.45; 73.85; 128.75; 129.45;
‡
131.92; 132.27; 133.18; 133.61; 134.74; 136.42; 137.22; 154.34. HR-ESI-MS: 551.9112 (C19H16Cl5NNaO4S [M ‡
‡
Na] ; calc. 551.9135).
Diethyl {3-[(Pentachlorophenyl)sulfonyl]-2-phenylpyrrolidin-1-yl}phosphonate (4i): Oil. 1H-NMR
(400 MHz, CDCl3): 1.01 (dt, 3J(H,H) ˆ 0.91, 7.02, 3 H); 1.31 (dt, 3J(H,H) ˆ 0.76, 7.02, 3 H); 2.38 2.45 (m,
1 H); 2.52 2.61 (m, 1 H); 3.59 3.69 (m, 2 H); 3.82 3.94 (m, 2 H); 3.98 4.05 (m, 2 H); 4.36 4.41 (m, 1 H);
5.11 (t, 3J(H,H) ˆ 5.19, 1 H); 7.11 7.24 (m, 5 H). 13C-NMR (100 MHz, CDCl3): 15.68; 15.76; 16.12; 16.18; 25.89;
25.96; 47.69; 47.74; 62.25; 62.31; 62.33; 62.39; 62.52; 62.58; 71.25; 71.35; 126.19; 127.87; 128.40; 133.88; 134.81;
‡
‡
135.05; 139.20; 140.97. HR-ESI-MS: 593.9381 (C20H22Cl5NO5PS , M ; calc. 593.9393).
Diethyl {2-(4-Methylphenyl)-3-[(pentachlorophenyl)sulfonyl]pyrrolidin-1-yl}phosphonate (4j): M.p. 1268
(EtOH). 1H-NMR (400 MHz, CDCl3): 1.00 (dt, 3J(H,H) ˆ 0.83, 7.01, 3 H); 1.30 (dt, 3J(H,H) ˆ 0.69, 7.01, 3 H);
2.27 (s, 3 H); 2.38 2.46 (m, 1 H); 2.53 2.61 (m, 1 H); 3.57 3.66 (m, 2 H); 3.80 3.93 (m, 2 H); 3.95 4.02 (m,
2 H); 4.41 (dt, 3J(H,H) ˆ 5.77, 7.43, 1 H); 4.97 (t, 3J(H,H) ˆ 5.50, 1 H); 6.95 7.00 (m, 4 H). 13C-NMR (100 MHz,
CDCl3): 15.67; 15.75; 16.11; 16.18; 20.97; 25.71; 25.78; 47.60; 47.65; 62.22; 62.28; 62.41; 62.47; 70.89; 70.98;
126.25; 128.91; 133.80; 134.76; 134.97; 137.57; 137.88; 139.10. Anal calc. for C21H23Cl5NO5PS (609.72): C 41.37, H
3.80, N 2.30, S 5.26; found: C 41.48, H 3.92, N 2.30, S 5.47.
1-(4-Chlorophenylsulfinyl)-3-[(pentachlorophenyl)sulfonyl]-2-phenylpyrrolidine (4l): M.p. 1658 (dec.;
1
3
EtOH). H-NMR (200 MHz, CDCl3): 2.65 2.98 (m, 2 H); 3.59 (ddd, J(H,H) ˆ 5.38, 7.91, 10.91, 1 H); 3.79
3.91 (m, 1 H); 4.54 4.66 (m, 1 H); 5.84 (d, 3J(H,H) ˆ 7.91, 1 H); 6.94 7.57 (m, 9 H). 13C-NMR (50 MHz,
CDCl3): 25.61; 47.62; 62.27; 69.42; 120.46; 127.15; 127.25; 127.49; 128.11; 128.16; 128.67; 129.02; 136.98; 137.12;
‡
‡
140.92. HR-ESI-MS: 637.8532 (C22H15Cl6NNaO3S2 , [M ‡ Na] ; calc. 637.8517).

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Received March 1, 2005