Addition of NOCl to cyclic vinylsilanes: An unexpected reversal of regiochemistry

Article abstract of DOI:10.1016/S0040-4039(01)00190-3

NOCl adds to cyclic vinylsilanes in a syn manner with NO⁺ bonding to the β-carbon and Cl^- to the α-carbon, which is a reversal of the regiochemistry expected from the β-silicon effect. The adducts dimerize to a single diastereomer containing enantiomeric pairs and/or give secondary products on further reaction.

Full text of DOI:10.1016/S0040-4039(01)00190-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2565–2568
Addition of NOCl to cyclic vinylsilanes: an unexpected reversal
of regiochemistry
M. Narendra Mallya,a Gopalpur Nagendrappa,a,* J. Shashidhara Prasad,b M. A. Sridhar,b
N. K. Lokanathb and N. S. Beguma
aDepartment of Chemistry, Bangalore University (Central College Campus), 560 001 Bangalore, India
bDepartment of Physics, University of Mysore, Manasagangotri, 570 006 Mysore, India
Received 19 July 2000; revised 16 January 2001; accepted 1 February 2001
Abstract—NOCl adds to cyclic vinylsilanes in a syn manner with NO+ bonding to the b-carbon and Cl− to the a-carbon, which
is a reversal of the regiochemistry expected from the b-silicon effect. The adducts dimerize to a single diastereomer containing
enantiomeric pairs and/or give secondary products on further reaction. © 2001 Elsevier Science Ltd. All rights reserved.
Electrophilic addition and substitution reactions of
vinylsilanes are valuable in organic synthesis. An
important aspect of the chemistry involved in these
reactions is the high degree of predictability of their
stereo- and regiochemical outcome arising from the
b-silicon effect.1
secondary products 13–17 were formed in some mea-
sure (Scheme 1). The results are summarized in Table 1.
A higher proportion of the secondary products were
isolated when solvents were used. Chloroketones 18–22
were also observed if NOCl was generated under
aqueous conditions, e.g. AmONO+conc. HCl+AcOH.
Vinylsilane 6 gave compound 12 alone while 2 gave
only secondary products under all conditions.
Electrophilic addition of NOCl to an olefin (Eq. (1)) is
a well studied reaction and is known to follow
Markovnikov’s mode of regiochemistry.2,3 Considerable
theoretical and experimental work has been recently
directed to the study of the structure of NOCl and
related molecules.4 Our work on NOCl addition to
cyclic vinylsilanes has yielded interesting results in the
context of the directing influence of silicon on stereo-
and regiochemistry, which are reported here.
The structures of 7 and 12 were established by single
crystal X-ray analysis (Figs. 1 and 2). The NO and Cl
groups are cis in both 7 and 12 with Cl on the a-carbon
and NO on the b-carbon, and in 12 they are in the exo
position (see ORTEP diagrams). It is interesting to note
that both 7 and 12 crystallized out as a single
diastereomeric dimer consisting of a pair of enan-
tiomers (1R,2S-2R,1S pair in the case of 7 and 2R,3S-
3R,2S pair in the case of 12) with a center of symmetry
passing through the NꢀN bond. The other two possible
diastereomers (1R,2S-2S,1R and 1S,2R-2R,1S in the
case of 7, and 2S,3R-3R,2S and 2R,3S-3S,2R in the
case of 12) are not formed in either case. Thus, there is
a type of molecular recognition in this dimer formation.
The other adducts, 9–11, did not give crystals suitable
The cyclic vinylsilanes 1–6 were employed for the
present study. Different reaction conditions and various
sources of NOCl were tried. The best yields of the
chloronitroso adducts, 7 and 9–12 isolated as dimers,
were obtained when NOCl was generated in situ by the
reaction of isoamylnitrite (AmONO) with chloro-
trimethylsilane (TMSCl) without solvent.5,6 In addition,
(1)
Keywords: NOCl; vinylsilanes; nitrososilanes; silylated diazene dioxides.
* Corresponding author. E-mail: nagendrappa@mailcity.com
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00190-3

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M. Narendra Mallya et al. / Tetrahedron Letters 42 (2001) 2565–2568
Scheme 1.
Table 1. NOCl addition to cyclic vinylsilanes 1–6
A striking feature of the addition is the regioselectivity,
which is the reverse of that expected from the polarity
of NOCl and the b-silicon effect. NOCl has been shown
by extensive theoretical and experimental work to be
reasonably ionic with NO bearing the positive charge
and chlorine the negative charge.4 Electrophilic addi-
tion and substitution reactions of vinylsilanes have been
found to be such that the electrophile mostly attacks
the a-carbon, due to b-cation stabilization by silicon,
provided the geometric requirement is fulfilled. How-
ever, electronic effects caused by other substituents or
steric factors can reverse this trend.1 Addition of ICl,
IOMe, IN3 and other reagents8–10 are found to conform
to this regioselectivity model in which positive and
negative segments of the reagents add to a- and b-car-
bons, respectively. We have observed the same pattern
Vinylsilane
Productsa (% yield)
1
2
7 (52)
8 (–)
13 (40)
14 (85)
15 (30)
16 (25)
17 (75)
–
3
9 (65)
10 (75)
11 (8)
12 (96)
4
5b
6
a The product distribution given here is only for the procedure
reported in Ref. 6.
b The conversion was about ꢀ40%. The yields of the products are
based on recovered 5.
for X-ray analysis and the stereochemistry could not be
proven in these cases. However, similarities in their
NMR spectra7 indicated that the regiochemistry is
likely to be the same with Cl and NO groups in a- and
b-positions, respectively.
+
in the nitration of 1–6 where NO2 attacks the a-car-
bon.11 Therefore, the NO+ group adding to the b-car-
bon and Cl− to the a-carbon in the present study is
rather unusual.
,
Figure 1. Crystal data for 7: C8H16ClNOSi, M=205.78, monoclinic, a=9.0610(8), b=11.2500(12), c=10.826(3) A, h=90,
3
i=95.344(13), k=90°, U=1098.8(3) A , T=293 K, space group P21/n, Z=4, v=0.416 mm−1, 1353 reflections measured, 1343
,
independent reflections (Rint= 2.7001), final R value [I>2|(I)]=0.0388, R (all data)=0.0546, wR(F2)=0.1321; selected bond
,
,
,
lengths: CꢁN=1.490(3) A, NꢁN=1.326(4) A, NꢁO=1.259(3) A.

M. Narendra Mallya et al. / Tetrahedron Letters 42 (2001) 2565–2568
2567
,
Figure 2. Crystal data for 12: C10H18ClNOSi, M=231.79, monoclinic, a=6.681(8), b=14.689(2), c=12.596(2) A, h=90,
3
i=104.09, k=90, U=1199.0(3) A , T=293 K, space group P21/c, Z=4, v=0.389 mm−1, 2289 reflections measured, 2097
,
independent reflections (Rint=0.0459), final R value [I>2|(I)]=0.0304, R (all data)=0.0339, wR(F2)=0.0736; selected bond
,
,
,
lengths: CꢁN=1.478(2) A, NꢁN=1.312(3) A, NꢁO=1.267(2) A.
It has not been easy to rationalize the regiochemical
outcome. One explanation that we consider to have
some merit is the following. If NOCl approaches the
double bond with NO approaching the a-carbon, the
positively charged oxygen would be spatially close to
the electropositive silicon 24, which possibly causes a
repulsive effect between the two atoms, either because
of orbital interactions or of electronegativity effects or
both. This can offset the delicately balanced stabilizing
effect of silicon on the incipient b-cation. However, in
23 in which NO is turned away from Si, this repulsion
is absent or diminished. Another plausible
explanation12 could be a weak Lewis acid effect of
silicon, i.e. formation of an ON···Cl···Si complex, in
which ‘ion-pairing’ of the SiCl complex and the cyclic
9–11 give the secondary products when treated with
NOCl. We have been able to isolate chlorooxime 26
from 10 under aqueous conditions. The adduct 12 also
produced the oxime 27 but only when heated to 50°C in
dioxane containing levulinic acid and HCl.
Acknowledgements
This study was partly financed by UGC-DRS program
and DST, New Delhi. Some equipment used in this
study was donated by the Alexander von Humboldt
Foundation, Germany. M.N.M. thanks UGC, New
Delhi, for a Senior Research Fellowship.
nitrosonium ion 25 may: (a) stabilize the intermediate;
(b) direct the chloride ion from the same side as the
NO, as well as to the a-position. Although NOCl
addition to olefins is found to be trans in most cases, cis
addition is not unknown. It is reported that norbornene
and some of its derivatives give only exo,cis adducts.13
In these cases the authors have proposed a four-cen-
tered cyclic transition state and the same mechanism
may be involved in our case.
References
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a consequence of isomerization of the primary adducts
to the respective a-chlorooximes which are known to
produce dichloronitroso compounds when treated with
NOCl or hydrolyze to chloroketones if water is
present.14,15 We have confirmed that the dimers 7 and
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M. Narendra Mallya et al. / Tetrahedron Letters 42 (2001) 2565–2568
5. For the generation of NOCl using TMSCl and organic or
C10H20ClNOSi: C, 51.48; H, 8.65; N, 6.01. Found: C,
51.43; H, 8.77; N, 6.17. Compound 10: mp 85–87°C. H
1
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6. Brief experimental procedure: A well-stirred mixture of
cyclic vinylsilane (10 mmol) and iso-amylnitrite (12.5
mmol) at −15°C was treated dropwise with TMSCl (12.5
mmol). After the disappearance of the vinylsilane (moni-
tored by GC), the solid dimeric primary adduct formed
was filtered rapidly, washed with cold ethanol (5 mL) and
dried. The filtrate was worked up in the usual manner
and the secondary products were separated by silica gel
chromatography (dichloronitroso compounds were eluted
with pure petroleum ether, bp 40–60°C and a-chloro-
ketones formed in aqueous conditions were eluted with
1% ethyl acetate in petroleum ether). All new compounds
were characterized by satisfactory elemental analysis and
spectral data. The known products were identified by
comparing their properties with those reported in the
literature and in many cases with authentic compounds
prepared from known procedures.
7. Selected characterization data (NMR spectra were taken
in CDCl3). Compound 7: mp 110–112°C. 1H NMR
(dimer): l 5.81 (t, J=8.0 Hz, 1H), 2.7 (m, 1H), 2.18 (m,
2H), 1.6–1.9 (m, 3H), 0.1 (s, 9H). 13C NMR: l 69.68,
66.94, 38.76, 27.73, 21.17, −3.17. Anal. calcd for
C8H16ClNOSi: C, 46.70; H, 7.84; N, 6.81. Found: C,
46.71; H, 8.12; N, 6.83. Compound 9: mp 90–92°C. 1H
NMR (monomer–dimer mixture in solution): l 5.87 (dd,
J=1.8 Hz, 2.1 Hz, dimer), 4.33 (dd, J=2.4 Hz, 1.8 Hz,
monomer) (together 1H), 2.13–2.64 (m, 3H), 1.57–1.96
(m, 7H), 0.19 (s), 0.16 (s) (together 9H). Anal. calcd for
NMR (monomer–dimer mixture in solution): l 6.01 (m,
dimer), 4.6 (m, monomer) (together 1H), 2.1–2.7 (m, 4H),
1.5–1.9 (m, 8H), 0.25 (s), 0.15 (s) (together 9H). Anal.
calcd for C11H22ClNOSi: C, 53.31; H, 8.95; N, 5.65.
Found: C, 53.32; H, 9.23; N, 5.92. Compound 11 could
not be fully characterized because of its poor yield and
difficulty in its purification. Compound 12: mp 150–
1
152°C. H NMR (dimer): l 4.93 (s, 1H), 2.73 (broad s,
1H), 2.5 (m, 2H), 1.2–1.7 (m, 5H), 0.3 (s, 9H). 13C NMR:
l 74.88, 68.12, 50.43, 41.33, 38.08, 26.37, 25.59, −0.67.
Anal. calcd for C10H18ClNOSi: C, 51.95; H, 7.79; N,
6.06. Found: C, 51.97; H, 7.90; N, 6.16.
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12. The authors thank the referee for this suggestion.
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