Electronic and steric effects of silyl groups in silicon-directed Norrish type cleavages

Article abstract of DOI:10.1016/S0022-328X(03)00666-1

Sterically congested cis-2-(t-butyl)-3-(organosilyl)cyclohexanones were irradiated with UV light to give a mixture of Norrish type I and II products, as well as the corresponding trans-2-(t-butyl)-3- (organosilyl)cyclohexanones. In comparison with the trans isomers, the quantum yields and rate constants of the photolytic reactions were greater for the cis isomers.

Full text of DOI:10.1016/S0022-328X(03)00666-1

Journal of Organometallic Chemistry 686 (2003) 198Á201
/
www.elsevier.com/locate/jorganchem
Electronic and steric effects of silyl groups in silicon-directed Norrish
type cleavages
Jih Ru Hwu a,b,*, Buh-Luen Chen b, Chien-Fu Lin a, Brown L. Murr b
a Organosilicon and Synthesis Laboratory, Department of Chemistry, National Tsing Hua University, 101, Section 2, Kuang Fu Road, Hsinchu 30043,
Taiwan, ROC
b Department of Chemistry, The Johns Hopkins University, Baltimore, MD 21218, USA
Received 3 April 2003; accepted 7 July 2003
Abstract
Sterically congested cis-2-(t-butyl)-3-(organosilyl)cyclohexanones were irradiated with UV light to give a mixture of Norrish type
I and II products, as well as the corresponding trans-2-(t-butyl)-3-(organosilyl)cyclohexanones. In comparison with the trans
isomers, the quantum yields and rate constants of the photolytic reactions were greater for the cis isomers.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Silicon; Norrish type cleavage; Electronic effect; Photolysis; sÁ
/
p Hyperconjugation
Irradiation of a,b-disubstituted cycloalkanones with
UV light may lead to products through the Norrish type
I and type II cleavages. The cis isomers may undergo
epimerization to give the corresponding trans isomers
[1]; this process has been used in organic synthesis [2].
On the other hand, a b-Me3Si group in cycloalkanones
can direct the Norrish type I cleavage to give an
By modifying Hudrlik’s procedure, we treated 2-t-
butyl-2-cyclohexen-1-one (1) [15] with LiSiMe3 [16,17] at
ꢀ78 to 0 8C to give a mixture of cis- and trans-
/
cyclohexanones 2a and 3a in a ratio of 32:1, as
determined by GC (see Scheme 2). After purification
by medium pressure liquid chromatography, pure 2a
was isolated in 81% yield. For the preparation of 2bÁ2d
/
from 1, we had to add cuprous cyanide to the reaction
flask containing LiSiMe2Ph, LiSiMePh2, or LiSiPh3.
After purification, we obtained pure 2b in 75% yield, 2c
in 51% yield, and 2d in 35% yield. On the other hand, we
heated a 1.0 N HCl methanolic aqueous solution of cis
excellent regioselectivity for a C1Ã
/
C2 bond cleavage, as
well as to increase the quantum yield and the reaction
rate [3]. This reaction involves biradical intermediates,
in which the b-Me3Si group can stabilize the carbor-
adical (see Scheme 1) [4,5]. Two factors may contribute
compounds 2aÁ
isomers 3aÁd were isolated in 95, 93, 85 and 65% yields,
respectively. Thus the trans isomers 3aÁd were thermo-
dynamically more stable than the corresponding cis
isomers 2aÁd.
To realize the conformation of 2aÁ
/d at reflux. The corresponding trans
to the stabilizing effect: the ‘sÁ
/
p hyperconjugation’ [6Á
/
/
11] and ‘pÁd homoconjugation’ [12,13]. Considering the
/
/
potential applicability of this recently developed silicon-
directed reaction [14], we investigated the geometric
/
factor associated with the alignment between the SiÃ
/C3
/d with the lowest
and the C2ÃC1 bonds in the SiÃC3ÃC2ÃC1ÄO moiety.
/
/
/
/
/
total energies, we performed the MM2 [18] molecular
modeling studies. Our results indicate that their most
stable conformations held a chair form. For example,
the conformation with the lowest energy for cis isomer
2d had an axial t-butyl group and an equatorial silyl
substituent (see 2dSi-equat in Scheme 3). Furthermore, we
obtained the lowest molecular energy of the cis- and the
Herein we report our findings that the electronic and the
steric effects resulting from a silyl group exhibited strong
impact on Norrish type cleavages during photolysis of
b-silyl cyclohexanones.
* Corresponding author. Tel.: ꢁ
1594.
E-mail address: jrhwu@mx.nthu.edu.tw (J.R. Hwu).
/
886-35-72-5813; fax: ꢁ886-35-72-
/
trans-cyclohexanones 2aÁ
/
d and 3aÁd by using the MM2,
/
MNDO [19], and AM1 [19,20] programs. All of the results
0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00666-1

J.R. Hwu et al. / Journal of Organometallic Chemistry 686 (2003) 198Á
/201
199
Scheme 1. Photolysis of cis-2-t-butyl-3-silylcyclohexan-1-ones (2aÁ
/
d) to give a mixture of products from the isomerization, Norrish type I and type
II processes.
indicate that the trans isomers 3aÁ
/
d were more stable
the trans compounds 3aÁ
positions.
To study the influence of a silyl group on the Norrish
type reactions, we irradiated 2aÁd in anhydrous metha-
nol containing NaHCO3 in Pyrex by UV light with l ꢀ
/
d, however, resided at axial
than the cis isomers 2aÁd. The energy differences ranged
/
from 7.06 to 8.71 kcal molꢀ1 by the MM2 method (see
/
Table 1). Both of the a-t-butyl and the b-silyl groups in
/
˚
3000 A for 9.0Á12 h. The isolated products and yields
/
are listed in Table 2. The ratios of the products through
Type I process to those through Type II process ranged
from 2.4 to 3.8 [21]. In a series of control experiments,
we monitored the photolytic reactions involving 2a by
detecting the products with GC. After 60 min, the
reaction mixture contained isomer 3a in 6% and a trace
amount of photo-cleaved products. Irradiation for 180
min gave 15% of 3a, 4% of Type I products, and 2% of a
Type II product. After 12 h, we obtained 35% of 3a, 32%
of Type I products (i.e. 4aꢁ5a), and 13% of a Type II
/
product (i.e. 6a) [22]. Our findings on isomerization of
the cis isomers 2 to the trans isomers 3 are in consistent
with those involving non-silylated compounds reported
by Barltrop and Coyle [23] as well as by Yang and Chen
[24]. Moreover, we found that none of the trans isomers
3aÁ/d isomerized to give the corresponding cis isomers
2aÁ/d.
Furthermore, we measured the quantum yields and
reaction rate constants for disappearance of the cis
isomers 2aÁ
than those of the corresponding trans isomers 3aÁ
respectively. For the Fcis/Ftrans, the ratios were found
/
d (see Table 3). They were always greater
/d,
Scheme 2. Synthesis of cis-and trans-2-t-butyl-3-silylcyclohexan-1-
ones.

200
J.R. Hwu et al. / Journal of Organometallic Chemistry 686 (2003) 198Á201
/
Scheme 3. Bond allignment in the photolysis of 2d and 3d.
We consider two factors that may contribute to
efficiency of the Norrish type cleavages of cis isomers
Table 1
The lowest molecular energy obtained by the MM2 calculations with
the CVFF for cis- and trans-2-(t-butyl)-3-silylcyclohexanones
2: (1) the repulsion of the two bulky groups (i.e. Ã
and Ãt-Bu) at the adjacent positions with cis config-
uration; and (2) the better alignment between the SiÃC3
and the C2ÃC1O bonds (see Scheme 3). An ideal bond
alignment in the structure of 2dSi-equat allows the ‘sÁ
/SiR3
/
a
2-t-Butyl-3-silyl-cyclohexa-
none
Elowest
(kcal molꢀ1
DE
/
)
(kcal molꢀ1
)
/
2a
3a
2b
3b
2c
3c
2d
3d
37.66
30.10
75.39
7.56
7.06
7.26
8.71
/
p
hyperconjugation’ to exert great influence on efficiency
of a-bond cleavage in the Norrish type I reaction. The
steric effect resulting from the three phenyl groups
attached to Si in 2d leads it to the thermodynamically
more stable conformational isomer 2dSi-equat. Because
68.33
115.16
107.90
155.46
146.75
a
DEꢂ
/
Ecis -2ꢀEtrans -3.
/
Table 3
Comparison of the quantum yields (F) and reaction rate constants
(tꢀ1) in the photolysis of cis- and trans-2-(t-butyl)-3-silylcyclohex-
between 1.20 and 22.6; for the tcꢀis1/t
, the ratios were
ꢀ1
trans
anones (i.e. 2aÁ
/
d and 3aÁ
/
d)
found between 1.22 and 20.0. The biggest ratios are
associated with the pair of 2d/3d, which possess the
triphenylsilyl group.
a
b
ꢀ1
trans
c
Cyclohexaone
F
Fcis /Ftrans
t
ꢀ1ꢃ
/
108 (sꢀ1
)
tcꢀis1/t
2a
3a
2b
3b
2c
3c
2d
3d
0.414 2.74
0.151
0.22
0.18
1.25
0.32
1.31
0.71
5.00
0.25
1.22
0.556 1.20
0.464
3.91
1.85
20.0
Table 2
Photolysis of cis-2-t-butyl-3-silylcyclohexan-1-ones 2aÁ
/d with UV
0.678 2.63
0.258
˚
light (l ꢀ
/
3000 A) for 9.0Á
Products (isolated yield)
Substrate Isomerization Through Type I
/
12 h
0.928 22.6
0.041
Through Type II
a
Quantum yield of disappearance of 2 or 3. 3-(Trimethylsilyl)cy-
clohexanone was used as an actinometry.
2a
2b
2c
2d
3a (35%)
3b (28%)
3c (42%)
3d (40%)
4a (4%)ꢀ
4b (5%)ꢀ
4c (6%)ꢀ
4d (0%)ꢀ
/
5a (28%)
5b (46%)
5c (30%)
6a (13%)
6b (14%)
6c (15%)
6d (12%)
b
/
Quantum yield ratio of cis isomers 2 (Fcis ) to trans isomers 3
/
(Ftrans ).
c
Rate constant ratio of cis isomers 2 (tcꢀis1) to trans isomers 3
/
5d (45%)
ꢀ1
trans
(t
).

J.R. Hwu et al. / Journal of Organometallic Chemistry 686 (2003) 198Á
/201
201
[9] E.W. Colvin, Silicon in Organic Synthesis (Chapter 3), Butter-
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/
p
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In conclusion, photolysis of various hindered cis-a-t-
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through the Norrish type I and II cleavages as well as
isomerization. The quantum yields and reaction rate
constants of these reactions are bigger than those from
the corresponding trans isomers 3aÁd. These results
/
indicate that the ‘sÁp hyperconjugation’ plays an
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¨
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this solution was irradiated with UV light that was filtered
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2.35 (m, 8H); 13C-NMR (CDCl3, 50 MHz) d ꢀ
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2.68
(SiCH3), 20.58, 24.50, 29.55, 31.96, 32.33, 34.50, 43.12, 51.27
(OCH3), 173.82 (CO); IR (CHCl3) 1730 (s, CÄO), 1249 (m, SiÃ
CH3), cmꢀ1; exact mass Calc. for C14H30O2Si:
/
0.06 (s, 9H, Si(CH3)3), 0.94 (s, 9H,
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