ortho-(substituted silyl)phenylsydnones via a novel sydnone to phenyl ring, lithiation-induced silicon migration

Article abstract of DOI:10.1081/SCC-120021032

3-(2-Bromophenyl)sydnone (5) reacts with LDA then substituted chlorosilanes to form the corresponding 4-silylsydnones 6, which rearrange to the novel ortho-silylphenylsydnones 8 on treatment with n-butyllithium.

Full text of DOI:10.1081/SCC-120021032

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ortho-(Substituted Silyl)phenylsydnones via a Novel
Sydnone to Phenyl Ring, Lithiation-Induced Silicon
Migration
Kenneth Turnbull a & Douglas M. Krein a
a Chemistry Department , Wright State University , Dayton, Ohio, USA
Published online: 17 Aug 2006.
To cite this article: Kenneth Turnbull & Douglas M. Krein (2003) ortho-(Substituted Silyl)phenylsydnones via a Novel
Sydnone to Phenyl Ring, Lithiation-Induced Silicon Migration, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 33:12, 2061-2067, DOI: 10.1081/SCC-120021032
To link to this article: http://dx.doi.org/10.1081/SCC-120021032
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 12, pp. 2061–2067, 2003
ortho-(Substituted Silyl)phenylsydnones via a
Novel Sydnone to Phenyl Ring,
Lithiation-Induced Silicon Migration
*
Kenneth Turnbull and Douglas M. Krein
Chemistry Department, Wright State University,
Dayton, Ohio, USA
ABSTRACT
3-(2-Bromophenyl)sydnone (5) reacts with LDA then substituted
chlorosilanes to form the corresponding 4-silylsydnones 6, which
rearrange to the novel ortho-silylphenylsydnones 8 on treatment
with n-butyllithium.
Key Words: Sydnones; Silyl rearrangement.
*Correspondence: Kenneth Turnbull, Associate Professor, Chemistry
Department, Wright State University, 3640 Colonel Glenn Highway, Dayton,
Ohio 45435, USA; Fax: (937) 775-2717; E-mail: kenneth.turnbull@wright.edu.
2061
DOI: 10.1081/SCC-120021032
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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2062
Turnbull and Krein
Scheme 1.
Recently, we have demonstrated that the sydnone ring is an effective
director of ortho-lithiation.[1] Therein, treatment of 3-phenylsydnone (1)
with 2.2 equiv. of n-BuLi in the presence of N,N,N0,N0-tetramethylethy-
lenediamine (TMEDA) provided the putative dilithio intermediate 2
which could be trapped with reactive electrophiles to yield the disubsti-
tuted sydnones 3[1] or with Weinreb’s amides to afford the o-acyl species 4
regiospecifically (Sch. 1).[2] The latter result was of particular interest
since much of our previous work had utilized ortho-substituted arylsyd-
nones as precursors to differently substituted sydnones,[3] to fused ring
sydnones[4] or to various heterocycles.[5] However, the utility of this
approach as a general route to ortho-substituted arylsydnones is limited
since only weak electrophiles show this selectivity. Accordingly, there was
a need for a lithiation protocol which would provide the desired o-sub-
stituted species without competitive reaction at the sydnone 4-position.
We postulated that this goal could be achieved by a sequence involving
protection of the sydnone 4-position of 3-(2-bromophenyl)sydnone (5) as
a silyl moiety (to yield 6), metal-halogen exchange followed by reaction
with an electrophile, and removal of the protective group with fluoride
ion (to yield 7) (Sch. 2).
Preparation of the 4-silyl species 6 was achieved in 79–85% yield
using LDA then the appropriate chlorosilane. However, all attempts to
effect halogen-metal exchange, followed by reaction with an electrophile,
led to product mixtures, the major component of which was the corre-
sponding o-silylated compound 8. Indeed, excellentyields (80–89%) of
these novel sydnones were forthcoming on treatment of 6 with n-butyl-
lithium in the absence of an electrophile, even at temperatures as low as
À100ꢀC (Table 1). Apparently, the rearrangement of 6 to 8 is very facile

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ortho-(Substituted Silyl)phenylsydnones
2063
Scheme 2.
Table 1. Preparation of the 4-silyl species 6 and conversion to their o-silyl
congeners 8.
R, R0, R00
Compound (in 6 & 8)
Yield (%) M.p. (ꢀC) Yield (%) M.p. (ꢀC)
of 6a
of 6
of 8a
of 8
a
b
c
Me, Me, Me
i
85
81
83
79
124–126
143–145
116–118
108–110
87
89
85
80
43–44
78–80
91–93
i
iPr, Pr, Pr
t
Me, Me, Bu
t
Ph, Ph, Bu
d
102–104
aAll compounds were fully characterized by IR, 1H NMR, 13C NMR, and
combustion analysis.
and is unaffected by steric encumbrance around the silicon atom. The
driving force is presumably the greater thermodynamic stabilty of the
sydnone anion. Thus, upon exposure to n-butyllithium, the bromosilyl-
sydnone 6 undoubtedly undergoes metal-halogen exchange to form the
corresponding unstable ortho-anion which attacks the 4-silyl moiety,
leaving the more stable sydnone anion. Protonation on work-up affords
the appropriate ortho-silylsydnone 8.
The identities of the products were confirmed by satisfactory com-
bustion analyses, the presence of the sydnone C¼O stretching vibration
at ꢁ1750 cmÀ1 and the singular sydnone ring C-H stretching vibration at
ꢁ3150 cmÀ1 in their infrared spectra, as well as the expected chemical
shifts and splitting patterns in their proton and carbon NMR spectra.

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2064
Turnbull and Krein
Overall, we have developed a useful preparation of ortho-silylaryl-
sydnones 8 which, due to the probable lability of the ortho-silyl moiety,
likely would be inaccessible by conventional preparations involving
manipulations with the appropriate ortho-substituted aniline derivatives.
Accordingly, we intend to explore further the scope and limitations of the
present discovery as a route to novel ortho-substituted arylsydnones.
EXPERIMENTAL
General Procedure for the Preparation of
3-(2-Bromophenyl)-4-(substituted silyl)sydnones 6
To a stirred solution of 3-(2-bromophenyl)sydnone (5) [1.00 g,
4.15 mmol] in dry THF (100 mL) at À78ꢀC was added lithium diiso-
propylamide ꢂ monotetrahydrofuran [3.04 mL, 4.56 mmol, 1.5 M in cyclo-
hexane] dropwise under an atmosphere of dry nitrogen. After 0.5 h, the
appropriate silyl chloride (4.98 mmol) was added and, after an additional
1 h, the mixture was allowed to warm to 0ꢀC whereupon itwas quenched
with saturated brine (100 mL), then extracted with CH2Cl2 (3 ꢃ 100 mL).
The combined organic layers were dried (MgSO4) and evaporated in
vacuo to yield an impure oil which was purified by column chromatog-
raphy (SiO2, CH2Cl2:hexane (4:1) as eluant), followed by recrystallization
(CH2Cl2:hexane) to afford colorless crystals of the title compound.
3-(2-Bromophenyl)-4-(trimethylsilyl)sydnone (6a). Using trimethylsilyl
chloride (0.63 mL) in the general procedure. Found: C, 42.54; H, 4.20; N,
8.98. Calcd. for C11H13BrN2O2Si: C, 42.18; H, 4.18; N, 8.94; ꢀmax (KBr):
3093, 2961, 2899, 1729, 1418, 1252, 1217, 848, 765 cmÀ1 1H NMR
;
(200 MHz; CDCl3): ꢁ 0.11 (s, 9H), 7.58 (m, 3H), 7.81 (m, 1H);
13C NMR (CDCl3): À2.08 (Si-(CH3)3), 106.29 (syd. C-4), 120.07,
128.00, 128.47, 133.33, 134.02, 135.46, 173.14 (syd. C¼O) ppm.
3-(2-Bromophenyl)-4-(triisopropylsilyl)sydnone (6b). Using triisopro-
pylsilyl chloride (1.06 mL) in the general procedure. Found: C, 51.46;
H, 6.36; N, 6.99. Calcd. for C17H25BrN2O2Si: C, 51.38; H, 6.34; N,
7.05; ꢀmax (KBr): 3072, 2950, 2866, 1729, 1469, 1401, 1207, 1027, 884,
763, 645 cmÀ1; 1H NMR (200 MHz; CDCl3): ꢁ 1.01 (d, 18H), 1.15 (septet,
3H), 7.55 (d, 3H), 7.78 (t, 1H); 13C NMR (CDCl3): 11.27 (Si-CH(CH3)2),
18.50 (Si-CH(CH3)2), 104.64 (syd. C-4), 120.84, 128.06, 128.14, 133.30,
134.21, 135.87, 173.85 (syd. C¼O) ppm.
3-(2-Bromophenyl)-4-(tert-butyldimethylsilyl)sydnone (6c). Using tert-
butyldimethylsilyl chloride (0.750 g) in the general procedure. Found:
C, 47.31; H, 5.44; N, 7.89. Calcd. for C14H19BrN2O2Si: C, 47.33; H, 5.39;

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ortho-(Substituted Silyl)phenylsydnones
2065
N, 7.88; ꢀmax (KBr): 3095, 2931, 2859, 1739, 1478, 1409, 1253, 1217, 1033,
846, 824, 770 cmÀ1; 1H NMR (200 MHz; CDCl3): ꢁ À0.35 (s, 3H), 0.23 (s,
3H), 0.93 (s, 9H), 7.55 (m, 3H), 7.77 (m, 1H); 13C NMR (CDCl3): À6.69,
À7.13 (Si-[C(CH3)3](CH3)2), 18.37 (Si-[C(CH3)3](CH3)2), 26.70 (Si-
[C(CH3)3](CH3)2), 105.31 (syd. C-4), 120.70, 128.15, 128.27, 133.26,
133.95, 135.68, 173.50 (syd. C¼O) ppm.
3-(2-Bromophenyl)-4-(tert-butyldiphenylsilyl)sydnone (6d). Using tert-
butyldiphenylsilyl chloride (1.29 mL) in the general procedure. Found: C,
60.30; H, 4.99; N, 5.82. Calcd. for C24H23BrN2O2Si: C, 60.12; H, 4.84; N,
5.84; ꢀmax (KBr): 3049, 2930, 2856, 1730, 1428, 1401, 1219, 1110, 858,
1
742, 704, 607, 506 cmÀ1; H NMR (200 MHz; CDCl3): ꢁ À0.01 (s, 6H),
0.11 (s, 3H), 7.37 (m, 10H), 7.73 (m, 4H); 13C NMR (CDCl3): 19.00 (Si-
[C(CH3)3](C6H5)2), 26.51, 28.22 (Si-[C(CH3)3](C6H5)2), 102.66 (syd. C-4),
119.89, 127.59, 127.84, 128.20, 129.49, 129.76, 130.58, 131.79, 132.42,
133.59, 134.73, 135.22, 135.67, 135.91, 173.56 (syd. C¼O) ppm.
General Procedure for the Preparation of
3-(2-(Substituted silyl)phenyl)sydnones 8
To a stirred solution of the appropriate 3-(2-bromophenyl)-4-(sub-
stituted silyl)sydnone 6 [5.00 mmol] in dry THF (150 mL) at À78ꢀC was
added n-butyllithium [3.67 mL, 5.5 mmol, 1.5 M in cyclohexane] dropwise
under dry nitrogen. After 2 h, the mixture was allowed to warm to 0ꢀC
whereupon it was quenched with saturated brine (100 mL) then extracted
with CH2Cl2 (3 ꢃ 100 mL). The combined organic layers were dried
(MgSO4) and evaporated in vacuo to yield an impure oil which was
purified via column chromatography (SiO2, CH2Cl2: EtOAc (10:1) as
eluant) followed by recrystallization (CH2Cl2: hexane) to afford colorless
crystals of the title compound.
3-(2-Trimethylsilylphenyl)sydnone (8a). Using 3-(2-bromophenyl)-4-
trimethylsilylsydnone (6a) [1.566 g] in the general procedure. Found: C,
56.61; H, 6.16; N, 11.83. Calcd. for C11H14N2O2Si: C, 56.38; H, 6.02; N,
11.95; ꢀmax (KBr): 3147, 3071, 2960, 2905, 1744, 1423, 1367, 1257, 941,
1
842, 767 cmÀ1; H NMR (200 MHz; CDCl3): ꢁ 0.60 (s, 9H), 6.94 (s, 1H),
7.80 (d, 1H), 7.99 (t, 2H), 8.12 (d, 1H); 13C NMR (CDCl3): À0.81 (Si-
(CH3)3), 97.18 (sydnone C-4), 124.62, 130.41, 131.43, 136.40, 136.59,
139.57, 168.73 (C¼O) ppm.
3-(2-Triisopropylsilylphenyl)sydnone (8b). Using 3-(2-bromophenyl)-
4-triisopropylsilylsydnone (6b) [1.986 g] in the general procedure. Found:
C, 64.19; H, 8.24; N, 8.81. Calcd. for C17H26N2O2Si: C, 64.11; H, 8.23; N,
8.80; ꢀmax (KBr): 3121, 2950, 2868, 1753, 1463, 1424, 1363, 1252, 883,

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2066
Turnbull and Krein
773, 674 cmÀ1; 1H NMR (200 MHz; CDCl3): ꢁ 0.95 (d, 18H), 1.11 (septet,
3H), 6.52 (s, 1H), 7.38 (d, 1H), 7.61 (t, 2H), 7.74 (d, 1H); 13C NMR
(CDCl3): 12.09 (Si-[CH(CH3)2]3), 19.02 (Si-[CH(CH3)2]3), 97.81 (sydnone
C-4), 126.27, 130.10, 130.80, 132.79, 138.12, 140.30, 168.59 (C¼O) ppm.
3-(2-tert-Butyldimethylsilyl)sydnone (8c). Using 3-(2-bromophenyl)-
4-tert-butyldimethylsilylsydnone (6c) [1.777 g] in the general procedure.
Found: C, 60.62; H, 7.45; N, 10.28. Calcd. for C14H20N2O2Si: C, 60.84;
H, 7.29; N, 10.13; ꢀmax (KBr): 3133, 2953, 2859, 1737, 1467, 1423, 1364,
1260, 1103, 939, 837, 773 cmÀ1 1H NMR (200 MHz; CDCl3): ꢁ 0.09
;
(s, 6H), 0.87 (s, 9H), 6.47 (s, 1H), 7.33 (d, 1H), 7.57 (t, 2H), 7.67 (d,
1H); 13C NMR (CDCl3): À5.33 (Si-[C(CH3)3](CH3)2), 17.48 (Si-
[C(CH3)3](CH3)2), 26.88 (Si-[C(CH3)3](CH3)2), 98.11 (sydnone C-4),
125.66, 130.18, 130.70, 134.59, 137.61, 139.69, 168.56 (C¼O) ppm.
3-(2-tert-butyldiphenylsilyl)sydnone (8d). Using 3-(2-bromophenyl)-4-
tert-butyldiphenylsilylsydnone (6d) [2.397 g] in the general procedure.
Found: C, 72.15; H, 6.13; N, 6.79. Calcd. for C24H24N2O2Si: C, 71.97;
H, 6.04; N, 6.99; ꢀmax (KBr): 3150, 3064, 2961, 2859, 1761, 1467, 1428,
1368, 1255, 1106, 938, 837, 703 cmÀ1; 1H NMR (200 MHz; CDCl3): ꢁ 1.14
(s, 9H), 5.86 (s, 1H), 7.51–7.30 (m, 11H), 7.72 (m, 2H), 8.18 (d, 1H);
13C NMR (CDCl3): 19.12 (Si-[C(CH3)3] (C6H5)2, 28.79 (Si-
[C(CH3)3](C6H5)2), 98.09 (sydnone C-4), 126.58, 127.81, 129.83, 130.77,
131.55, 132.33, 135.37, 135.62, 138.16, 140.86, 167.92 (C¼O) ppm.
REFERENCES
1. Turnbull, K.; Krein, D.M. The sydnone ring as an ortho-director of
lithiation. Dilithiation of 3-phenylsydnone and trapping by electro-
philes. Tetrahedron Lett. 1997, 38, 1165.
2. Turnbull, K.; Sun, C.; Krein, D.M. The sydnone ring as an ortho-
director of lithiation. 2. Dilithiation of 3-phenylsydnone and
regiospecific o-aryl acylation using N-methoxy-N-methylamides.
Tetrahedron Lett. 1998, 39, 1509.
3. (a) Turnbull, K. J. Bromination of sydnones. I. Reaction with 3-
arylsydnones containing electron-donors on the aryl ring.
Heterocycl. Chem. 1985, 22, 965; (b) Hodson, S.J.; Turnbull, K. J.
Bromination of sydnones. II. Bromination of 3-(2-aminophenyl)syd-
none and related compounds. Heterocycl. Chem. 1985, 22, 1223; (c)
Turnbull, K. Debromination of 3-aryl-4-bromosydnones with
sodium borohydride. Synthesis 1986, 334; (d) Turnbull, K.;
Blackburn, T.L.; Esterline, D.T. Bromination of sydnones. III.
Bromination of 3-(2-substituted-phenyl)sydnones and subsequent

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ortho-(Substituted Silyl)phenylsydnones
2067
side chain modification. J. Heterocycl. Chem. 1990, 27, 1259;
(e) McChord, K.L.; Tullis, S.A.; Turnbull, K. Debromination of 4-
bromo-3-arylsydnones with sodium sulfite. Synth. Commun. 1989,
19, 2249.
4. (a) Preston, P.N.; Turnbull, K. Approaches to the synthesis of com-
pounds containing fused mesoionic rings. J. Chem. Soc., Perkin
Trans. 1 1977, 1229; (b) Burson, W.C., III; Jones, D.R.; Turnbull,
K.; Preston, P.N. Intramolecular aza-wittig routes to sydnoquinox-
alines. Synthesis 1991, 745; (c) Saljoughian, M.; Turnbull, K. Acid
induced reactions of a sydnone ketoxime. J. Heterocycl. Chem. 1988,
25, 1817; (d) Turnbull, K.; Krein, D.M. Formation of 4-hydroxy-4-
substituted sydno[3,4-a](4H)indoles via dilithiation of 3-(2-bromo-
phenyl)sydnone. Synthesis 1996, 1183.
5. (a) Gelvin, C.R.; Turnbull, K. Sydnones as masked hydrazines for
heterocycle formation: reactions of 3-(2-substituted phenyl)sydnones
with HCl. Helv. Chim. Acta 1992, 75, 1931; (b) Marx, J.A.; Turnbull,
K. Formation of 1-bromocarbonylindazoles via cleavage of 4-bromo
ortho-substituted arylsydnones with HBr. Tetrahedron Lett. 1993,
239.
Received in the USA October 3, 2002

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