New generation of organosilyl radicals by photochemically induced homolytic cleavage of silicon-boron bonds

Article abstract of DOI:10.1021/jo000547w

Unlike bis(diethylamino)organosilylboranes, bis(diisopropylamino)organosilylboranes, which have UV absorption at longer wavelength than 300 nm, undergo photolysis to afford pairs of an organosilyl radical and a bis(diisopropylamino)boryl radical by homolytic scission of the silicon-boron bonds. Generation of organosilyl radical and organoboryl radical was confirmed by trapping experiments using TEMPO (2,2,6,6-tetramethylpiperidine N-oxyl). The organosilyl radical thus generated induces not only silylation of mono-olefins and silylative cyclization of dienes but also polymerization to afford polymers bearing organosilyl termini. On the other hand, the bis(diisopropylamino)boryl radical generated is not incorporated into the olefin adducts.

Full text of DOI:10.1021/jo000547w

J . Org. Chem. 2000, 65, 5707-5711
5707
New Gen er a tion of Or ga n osilyl Ra d ica ls by P h otoch em ica lly
In d u ced Hom olytic Clea va ge of Silicon -Bor on Bon d s
Akira Matsumoto† and Yoshihiko Ito*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Kyoto 606-8501, J apan
yoshi@sbchem.kyoto-u.ac.jp
Received April 11, 2000
Unlike bis(diethylamino)organosilylboranes, bis(diisopropylamino)organosilylboranes, which have
UV absorption at longer wavelength than 300 nm, undergo photolysis to afford pairs of an organosilyl
radical and a bis(diisopropylamino)boryl radical by homolytic scission of the silicon-boron bonds.
Generation of organosilyl radical and organoboryl radical was confirmed by trapping experiments
using TEMPO (2,2,6,6-tetramethylpiperidine N-oxyl). The organosilyl radical thus generated induces
not only silylation of mono-olefins and silylative cyclization of dienes but also polymerization to
afford polymers bearing organosilyl termini. On the other hand, the bis(diisopropylamino)boryl
radical generated is not incorporated into the olefin adducts.
In tr od u ction
dimethylphenylsilyl radical from organosilylboranes, which
can promote stereoselective silicon-carbon bond forma-
tion.
Combination of an organotin hydride and a conven-
tional radical initiator has been widely utilized in organic
synthesis.1 However, the use of organotin derivatives is
not desired from the environmental viewpoint. In addi-
tion, it is often difficult to remove any organotin impuri-
ties from the product.2 Although some attempts using a
catalytic amount of organotin hydride have been re-
ported,3 development of the alternatives may be recom-
mended in synthetic organic chemistry.
The silicon-boron σ-bonds in organosilylboranes had
been considered to be thermally inert.8 However, the
silicon-boron bond activation catalyzed by transition
metal complexes has been recently utilized in organic
synthesis, e.g., some silaborations of carbon-carbon
multiple bonds catalyzed by Pd, Pt, and Ni complexes
create both of silicon-carbon bonds and boron-carbon
bonds stereoselectively and regioselectively.9
On the other hand, usefulness of organosilyl radicals
in organic synthesis has also been reported,4 suggesting
that organotin radicals may be replaced with organosilyl
radicals. Unlike organotin radicals, however, generation
of organosilyl radical intermediates has not been practi-
cally convenient. Organosilyl radicals are often generated
by treatment of organosilyl hydride with a stoichiometric
amount of peroxide. Since a hydrogen atom of an orga-
nosilyl hydride is, however, resistant to abstraction by a
carbon-centered radical, organosilyl hydrides unlike or-
ganotin hydrides are not usually involved in a radical
chain process.5 Polarity-reversal catalyst is reported to
promote the efficient radical chain cycle.6 On the other
hand, it has been recently found that some organosilyl
hydrides such as tris(trimethylsilyl)silane (TTMSS) pro-
vide the corresponding tris(trimethylsilyl)silyl radical
intermediates via hydrogen atom abstraction without the
catalyst, which are utilized in organic synthesis.7 Herein,
we describe new and useful photochemical generation of
However, photochemistry of organosilylboranes has not
been investigated well. To our knowledge, a photochemi-
cal generation of triphenylsilylboranediyl (Ph3SiB:) from
bis- or tris(organosilyl)boranes has been noted.10
Resu lts a n d Discu ssion
UV spectra of a series of bis(dialkylamino)organosi-
lylboranes revealed that UV absorption of 1d was very
similar to that of 1f, suggesting no significant electronic
contribution of dialkylamino groups to UV absorption.11
Of note is, however, that sterically bulky substituents
on either silicon or boron atoms remarkably affected UV
absorption of bis(dialkylamino)organosilylboranes. The
silylborane 1c bearing a triphenylsilyl group absorbed
UV light up to around 320 nm (Figure 1). The charac-
(7) Chatgilialoglu, C. Acc. Chem. Res. 1992, 25, 188-194.
(8) (a) Seyferth, D.; Ko¨gler, H. P. J . Inorg. Nucl. Chem. 1960, 15,
99. (b) Cowley, A. H.; Sisler, H. H.; Ryschkewitsch, G. E. J . Am. Chem.
Soc. 1960, 82, 501-502. (c) No¨th, H.; Ho¨llerer, G. Chem. Ber. 1966,
99, 2197-2205. (d) Biffar, W.; No¨th, H.; Schwertho¨ffer, R. Liebigs Ann.
Chem. 1981, 2067-2080. (e) Buynak, J . D.; Geng, B. Organometallics
1995, 14, 3112-3115.
(9) (a) Suginome, M.; Nakamura, H.; Ito, Y. Chem. Commun. 1996,
2777-2778. (b) Onozawa, S.; Hatanaka, Y.; Tanaka, M. Chem. Com-
mun. 1997, 1229-1230. (c) Suginome, M.; Nakamura, H.; Ito, Y.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2516-2518. (d) Suginome, M.;
Nakamura, H.; Matsuda, T.; Ito, Y. J . Am. Chem. Soc. 1998, 120,
4248-4249. (e) Suginome, M.; Matsuda, T.; Nakamura, H.; Ito, Y.
Tetraheadron 1999, 55, 8787-8800.
(10) Pachaly, B.; West, R. Angew. Chem., Int. Ed. Engl. 1984, 23,
454-455.
(11) It has been reported that aminoboranes exhibit UV absorption
ascribed to π-bonding between boron and nitrogen: (a) Lappert, M.
F.; Pedley, J . B.; Riley, P. N. K.; Tweedale, A. Chem. Commun. 1966,
21, 788-789. (b) Fuss, W. Z. Naturforsch., Teil B 1974, 29, 514-523.
* To whom correspondence should be addressed. Phone: 81 75 753
5654. Fax: 81 75 753 5668.
† Additives, Ciba Specialty Chemicals K. K., J apan.
(1) (a) Neumann, W. P. Synthesis 1987, 665-683. (b) Curran, D. P.
Synthesis 1988, 417-439, and 489-513.
(2) Crich, D.; Sun, S. J . Org. Chem. 1996, 61, 7200-7201.
(3) (a) Stork, G.; Sher, P. M. J . Am. Chem. Soc. 1986, 108, 303-
304. (b) Hays, D. S.; Fu, G. C. J . Org. Chem. 1996, 61, 4-5. (c) Hays,
D. S.; Scholl, M.; Fu, G. C. J . Org. Chem. 1996, 61, 6751-6752. (d)
Lopez, R. M.; Hays, D. S.; Fu, G. C. J . Am. Chem. Soc. 1997, 119, 6949-
6950.
(4) Chatgilialoglu, C. Chem. Rev. 1995, 95, 1229-1251.
(5) El-Durini, N. M. K.; J ackson, R. A. J . Organomet. Chem. 1982,
232, 117-121.
(6) Roberts, B. P. Chem. Soc. Rev. 1999, 28, 25-35.
10.1021/jo000547w CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/12/2000

5708 J . Org. Chem., Vol. 65, No. 18, 2000
Matsumoto and Ito
Ta ble 1. 11B Ch em ica l Sh ifts of Or ga n osilylbor a n es 1
chemical
chemical
compound
shift (ppm)a
compound
shift (ppm)a
1a
1b
1c
39.5
41.2
35.5
1d
1e
36.5b
34.1
a
Measured in C6D6 with BF3‚Et2O as external reference.
b
Reported in ref 8e.
Ta ble 2. P h otoch em ica l Rea ction of 1b w ith Olefin s
olefin (equiv)
1-octene (4.0)
solvent
product
yield (%)
56
n-hexane
cyclohexene (4.0) n-hexane
56
46
MMA (2.0)
BnMA (2.0)
benzene
benzene
F igu r e 1. UV spectra of 1a -d in acetonitrile and 1b in
n-hexane at 5.0 × 10-5 M.
45
tively for 1.5 h at ambient temperature to afford a
complex product mixture, in which bis(diisopropylamino)-
borane14 and R1 Si-C6D5 were detected by 1H, 13C NMR,
3
and GC-MS. However, although 1c underwent sluggish
photolysis, no reaction took place under the same condi-
tion with 1d -f, which have very weak, if any, UV
absorption at >280 nm. It is noted that photolysis of 1b
in the presence of TEMPO in benzene-d6 gave PhMe2-
Si-TEMPO adduct 4 and (iPr2N)2B-TEMPO adduct 5
in a quantitative yield, indicating generation of pairs of
an organosilyl radical and an organoboryl radical (eq 1).
These reactions do not occur without irradiation. More-
over, organosilylboranes 1 are thermally stable and do
not decompose even at 180 °C.
F igu r e 2. UV spectra of 1e-f, 2, and 3 in acetonitrile at 5.0
× 10-5 M.
teristic tailing of the UV absorption was not observed for
a pinacol derivative 1e (Figure 2).
The high reactivity of organosilyl radicals toward
carbon-carbon multiple bonds is well-known.4 Indeed,
irradiation of a mixture of 1b and olefins led to addition
of organosilyl radicals across the carbon-carbon double
bonds to give silylated products (eq 2 and Table 2). UV
irradiation of 1b with 1-octene and cyclohexene in
The similar substituent effect at the boron was also
observed in the UV spectra of 1a and 1b having diiso-
propylamino groups, which exhibited higher intensities
than those of 1c and 1d , respectively. The UV absorption
of the silylboranes was not influenced by the polarity of
the solvent (Figure 1), suggesting that the ground state
does not involve a charge-transfer character.
It may be remarked that the characteristic UV absorp-
tion was not observed for the two reference compounds
ClB(NiPr2)2 (2)11 and Ph3SiMe (3) (Figure 2). The origin
of the absorption is not clear at present,12 but the effect
of the substituents at boron was also seen in 11B NMR.
The 11B chemical shifts of 1a ,b appeared in downfield
compared to those of 1c-e (Table 1).13
(12) Some organosilylboranes have been reported to have UV
absorption around 400 nm: Pillot, J .-P.; Birot, M.; Bonnefon, E.;
Dunogues, J .; Rayez, J .-C.; Rayez, M.-T.; Liotard, D.; Desvergne, J .-P.
Chem. Commun. 1997, 1535-1536.
(13) 11B NMR chemical shifts of some bis(dialkylamino)organo-
silylboranes reported so far are 33-36 ppm; see refs 8c, 8d, and 8e.
(14) Structural identity of bis(diisopropylamino)borane was con-
firmed by comparison (1H, 13C NMR) with an authentic sample
prepared from 2 and LiAlH4: see Chavant, P. Y.; Vaultier, M. J .
Organomet. Chem. 1993, 455, 37-46.
(15) Steric congestion of bis(diisopropylamino)boryl groups has been
discussed. See ref 14.
(16) Oxidation of â-boryl ester; see (a) Rasset, C.; Vaultier, M.
Tetrahedron 1994, 50, 3397-3406. (b) Matteson, D. S.; Michnick, T.
J . Organometallics 1990, 9, 3171-3177.
On irradiation with a high-pressure mercury lamp, 1a
and 1b underwent photolysis in benzene-d6 quantita-

New Generation of Organosilyl Radicals
J . Org. Chem., Vol. 65, No. 18, 2000 5709
n-hexane afforded dimethyl(n-octyl)phenylsilane (6) and
dimethyl(cyc-hexyl)phenylsilane (7), respectively. No re-
radiation of a mixture of 1b and 1d in benzene-d6
resulted in the consumption of 1b, leaving 1d unchanged.
This finding may exclude a mechanistic possibility of
sensitization of 1b followed by radical chain reaction.
The generation of trivalent boron-centered radical in
the present work deserves to be noted. Organoboryl
radicals (R2B•) have been so far detected by some
spectroscopic studies19 and proposed as transient inter-
mediates.20 Trapping of organoboryl radical with TEMPO
described in the present paper provides the first and
unambiguous evidence for a nonligated boryl radical
intermediate.21
Next, we attempted to demonstrate the usefulness of
the photochemical reaction of 1b in organic synthesis.
Hydrosilylation of olefins shown in eq 2 was carried out
in the presence of excess amounts of olefins. Use of a
stoichiometric amount of olefins in the hydrosilylation
resulted in low yields. However, the chemical yield was
improved by choice of ether solvent, which served as
hydrogen donor and terminated the radical reaction
effectively. Regioselective hydrosilylation of olefins took
place by irradiating a 1:1 mixture of 1b and 1-octene in
diethyl ether (eq 3). Similarly, methyl crotonate also
underwent the hydrosilylation by UV irradiation to afford
10 in a good yield.
gioisomeric adduct was obtained in the reaction of
1-octene. Photochemical reaction with R,â-unsaturated
esters such as methyl methacrylate (MMA) and benzyl
methacrylate (BnMA) in benzene also produced the
corresponding â-silyl carboesters 8 and 9 along with some
methacrylate oligomers. Hydrogen atoms, which termi-
nate the radical reactions, may have mainly come from
the diisopropylamino group. Irradiation of 1b (1.0 equiv)
with MMA (2.0 equiv) in THF-d8, 2H donor solvent,
2
yielded 8, in which 20% of H was incorporated at the
carbon R to the ester moiety. These reactions may involve
radical intermediates since the presence of TEMPO in
the reaction mixture completely inhibited the formation
of silylated products, affording TEMPO-adducts 4 and 5
in good yields.
In contrast to the reactive organosilyl radical, the
organoboryl radical, which may have been simulta-
neously generated, does not apparently participate in the
reaction with olefins. The organoboryl radical did not add
to olefins, probably due to the steric hindrance.15 In the
case of the reaction with BnMA, oxidation of the reaction
mixture using hydrogen peroxide in the presence of
sodium hydroxide afforded 9 in 44% yield without giving
the corresponding â-hydroxy carboester, which might
have arisen from a possible formation of the â-boryl
carboester.16 Monitoring the photochemical reaction with
1
MMA in benzene-d6 by H NMR revealed that neither of
boryl enolates nor silyl enolates were formed throughout
the photochemical reaction. Only bis(diisopropylamino)-
borane, which could be produced via hydrogen abstrac-
tion by the bis(diisopropylamino)boryl radical,14 was
identified as one of the products derived from the boryl
group.17 No reaction proceeded without irradiation in all
cases.
Trialkylboranes are known as a radical initiator in the
presence of a trace of oxygen (O2).18 However, oxygen
cannot initiate any reaction of 1a ,b. Under dry air
atmosphere, 1b in benzene-d6 was not decomposed at all.
Photochemical reactions of 1a ,b are unlikely to proceed
with the radical chain mechanism. Heating a mixture of
1b (1.0 equiv) and benzoyl peroxide (0.2 equiv) in
benzene-d6 at 95 °C for 2 h in a sealed tube led to partial
decomposition of 1b, giving a mixture of 1b and small
amounts of unidentified products. However, the photoly-
sis products such as PhMe2Si-C6D5 and bis(diisopropyl-
amino)borane were not observed in the mixture. We
assume that the BPO-induced decomposition of 1b may
be caused by hydrogen abstraction from the diisopropyl-
amino groups by the benzoyloxy radical. Moreover, ir-
Photochemical reaction of 1b with dienes induced
organosilylative cyclization. Irradiation of 1,6-dienes (11)
in the presense of 1b afforded 5-exo cyclized products (12)
exclusively in a stereoselective manner without using
conventional radical initiators (eq 4). Various functional
groups such as ether, ester, silyl ether, and amide are
tolerated in this reaction. Not only cyclopentane deriva-
tives but also heterocycles such as tetrahydrofurans and
pyrrolidines can be constructed by using organosilyl-
borane 1b. In contrast, radical-induced cyclization using
TTMSS in the presence of Et3B as a radical initiator
yields an undesired product mixture including bicyclic
compounds.22
(17) Attempts to isolate bis(diisopropylamino)borane from the reac-
tion mixture have all failed because it was highly moisture sensitive.
Usually, a complex mixture of boryl compounds was produced in the
photochemical reaction, probably due to the hydrogen abstraction from
the isopropyl groups.
Organosilyl radicals are also highly reactive for halo-
gen abstraction. The organosilylborane 1b promotes a
reductive cyclization of 1-bromo-5-hexene (13) to give
methylcyclopentane (14). Halogen abstraction by the silyl
radical and subsequent cyclization was followed by
hydrogen abstraction to give the product. This finding is
(18) Brown, H. C.; Midland, M. M. Angew. Chem., Int. Ed. Engl.
1972, 11, 692-700.
(19) (a) Herzberg, G.; J ohns, J . W. C. Proc. R. Soc. London, Ser. A
1967, 298, 142-159. (b) Rehorek, D.; Herzschuh, R.; Hennig, H. Inorg.
Chim. Acta 1980, 44, L75-L76.
(20) Hancock, K. G.; Uriarte, A. K. J . Am. Chem. Soc. 1970, 92,
6374-6376, and references therein.

5710 J . Org. Chem., Vol. 65, No. 18, 2000
Matsumoto and Ito
Ta ble 3. P h otoch em ica lly In d u ced Ra d ica l
P olym er iza tion w ith 1b
purified by distillation in the presence of appropriate drying
agents under nitrogen. All photochemical reactions were
carried out with a high-pressure mercury lamp (100 W) and
Pyrex glassware. Methyl methacrylate, methyl acrylate, benzyl
methacrylate, and vinyl acetate were distilled prior to use.
Other commercially available reagents were used without
further purification.
monomer (15)
yield (%)
Mn
Mw/Mn
15a
15b
15c
23
94
51
3.11 × 103
3.08 × 104
9.36 × 103
1.67
2.41
5.17
Syn t h esis of Or ga n osilylb or a n es (1). To a mixture of
lithium (0.56 g, 80 mmol) and THF (20 mL) was added PhMe2-
SiCl (3.36 mL, 20 mmol) dropwise. After the mixture was
stirred at room temperature for 6 h, the resulting supernatant
indicating that organosilylborane 1b may function as an
alternative of organotin hydride-radical initiator (eq 5).
23
solution of PhMe2SiLi was added dropwise to ClB(NiPr2)2
(4.75 g, 19.3 mmol) in n-hexane (20 mL), and then the mixture
was stirred at room temperature overnight. After the produced
precipitates were filtered off, the filtrate was evaporated and
distilled to afford 1b (5.13 g, 77% yield). The same procedure
was applied for the synthesis of Ph3SiB(NiPr2)2 (1a ).
The organosilylborane 1b can promote a radical po-
lymerization. Irradiation of 1b with large excess of
monomers (15) in benzene afforded polymers (eq 6 and
Table 3). Not only methyl methacrylate (15a ) and acry-
late (15b) but also vinyl acetate (15c) was polymerized
with 1b. 1H NMR analysis showed that the polymers
contained dimethylphenylsilyl groups at the polymer
termini. Addition of TEMPO inhibited the polymerization
completely, indicating that the polymerization took place
in a radical fashion. It is needless to say that no polymer
was obtained without irradiation.
Tr ip h en ylsilylbis(d iisop r op yla m in o)bor a n e (1a ): bp
1
190 °C at 0.1 mmHg; H NMR (C6D6) δ 1.07 (d, J ) 7.0 Hz,
24H), 3.94 (hept, J ) 7.0 Hz, 4H), 7.15-7.29 (m, 9H), 7.80-
7.92 (m, 6H); 13C NMR (C6D6) δ 25.4, 50.2, 127.9, 128.3, 136.8,
140.2; 11B NMR (C6D6) δ 39.5. Anal. Calcd for C30H43BN2Si:
H, 9.21; C, 76.57; N, 5.95. Found: H, 9.40; C, 76.30; N, 5.88.
Dim eth ylph en ylsilylbis(diisopr opylam in o)bor an e (1b):
1
bp 140 °C at 0.2 mmHg; H NMR (C6D6) δ 0.51 (s, 6H), 1.14
(d, J ) 6.8 Hz, 24H), 3.85 (hept, J ) 6.8 Hz, 4H), 7.15-7.32
(m, 3H), 7.62-7.70 (m, 2H); 13C NMR (C6D6) δ 1.0, 25.4, 49.2,
127.9, 128.3, 134.4, 144.2; 11B NMR (C6D6) δ 41.2; HRMS Calcd
for C20H39BN2Si (M+1) 347.3054, found 347.3059.
Silylboranes 1c-f were prepared by the reported proce-
dures.24,25 Physical and spectral data for new compounds are
given below.
Tr ip h en ylsilylbis(d ieth yla m in o)bor a n e (1c): bp 170 °C
at 0.1 mmHg; 1H NMR (C6D6) δ 0.84 (t, J ) 6.9 Hz, 12H), 2.93
(q, J ) 6.9 Hz, 8H), 7.16-7.25 (m, 9H), 7.82-7.87 (m, 6H);
13C NMR (C6D6) δ 15.3, 43.5, 128.6, 136.6, 140.1; 11B NMR
(C6D6) δ 35.5.
Tr ip h en ylsilylp in a colbor a n e (1e): 1H NMR (C6D6) δ 1.01
(s, 12H), 7.15-7.24 (m, 9H), 7.82-7.88 (m, 6H); 13C NMR
(C6D6) δ 24.8, 83.9, 128.3, 129.4, 135.7, 136.6; 11B NMR (C6D6)
δ 34.1; HRMS Calcd for C24H27BO2Si (M - C6H5) 309.1482;
found 309.1494.
Organosilylborane 1b is easily prepared via two steps
(eq 7). Treatment of excess diisopropylamine with BCl3‚
SMe2 yielded 2 without formation of (iPr2N)3B.23 Orga-
nosilylborane 1b was obtained by a reaction of dimeth-
ylphenylsilyllithium and 2. Organosilylborane 1b is
stable enough for storage for months under dry atmo-
sphere. Synthetic availability of organosilylborane 1b
may enhance the utilities of the present photochemical
reactions in organic synthesis.
P h otolysis of Or ga n osilylbor a n e 1b w ith TEMP O. In
5.0 mL of C6D6 were dissolved 1b (0.308 mmol) and TEMPO
(0.925 mmol). The solution was irradiated for 1.5 h at ambient
temperature in a water bath. Structures of products 4 and 5
1
were identified by H NMR. Structual data of 4 was identical
in all respect with an authentic sample prepared by the
reported procedure.26 After evaporation of the reaction mix-
ture, the resulting residue was subjected to a column chro-
matography on silica gel to give 5 in 51% yield.
1-Dim et h ylp h en ylsilyloxy-2,2,6,6-t et r a m et h ylp ip er i-
d in e (4): bp 100-105 °C at 0.5 mmHg; 1H NMR (C6D6) δ 0.51
(s, 6H), 1.08 (s, 6H), 1.16 (s, 6H), 1.00-1.22 (m, 2H), 1.26-
1.47 (m, 4H), 7.17-7.27 (m, 3H), 7.64-7.69 (m, 2H); 13C NMR
(C6D6) δ 0.2, 17.2, 19.4, 33.9, 40.0, 59.8, 129.3, 133.7, 140.5;
HRMS calcd for C17H29ONSi 291.2017, found 291.2011.
1-Bis(d iisop r op yla m in o)b or yloxy-2,2,6,6-t et r a m et h -
ylp ip er id in e (5): 1H NMR (C6D6) δ 1.23-1.33 (m, 36H), 1.39-
1.54 (m, 6H), 3.39 (hept, J ) 6.8 Hz, 2H), 4.62 (hept, J ) 7.1
Hz, 2H); 13C NMR (C6D6) δ 17.5, 21.9, 24.6, 25.2, 33.4, 40.3,
46.1, 47.0, 59.5; HRMS calcd for C21H46BN3O 367.3734, found
367.3728.
Con clu sion
Steric congestion in organosilylborane is crucial for the
bathochromic UV absorption, which causes homolytic
scission of Si-B bonds on UV irradiation. Utility of
organosilylboranes is also demonstrated. Organosilylbo-
rane 1b serves as a good photolatent dimethylphenylsilyl
radical in organic synthesis. This reagent can be an
alternative for organotin hydrides-conventional radical
initiators. In addition, 1b is useful as a radical initiator
for polymerization.
(21) Ligated boryl radicals; see (a) Dang, H. S.; Diart, V.; Roberts,
B. P.; Tocher, D. A. J . Chem. Soc. Perkin Trans. 2 1994, 1039-45. (b)
Lucarini, M.; Pedulli, G. F.; Valgimigli, L. J . Org. Chem. 1996, 61,
4309-4313, and references therein.
(22) Miura, K.; Oshima, K.; Utimoto, K. Bull. Chem. Soc. J pn. 1993,
66, 2348-2355.
(23) Synthesis of 2 has been reported in ref 14.
(24) Synthesis of organosilylbis(dialkylamino)boranes; see ref 8e.
(25) Synthesis of organosilylpinacolboranes; see Suginome, M.;
Matsuda, T.; Ito, Y. Organometallics, in press.
(26) Armbrecht, M.; Maringgele, W.; Meller, A.; Noltemeyer, M.;
Sheldrick, G. M. Z. Naturforsch., Teil B 1985, 40, 1113-1122.
(27) Fleming, I.; Lee, D. J . Chem. Soc., Perkin. Trans. 1 1998, 2701-
2710.
Exp er im en ta l Section
Gen er a l. All reactions were carried out under dry nitrogen,
unless otherwise mentioned. Reagents and solvents were
handled with standard syringe techniques. Solvents were

New Generation of Organosilyl Radicals
J . Org. Chem., Vol. 65, No. 18, 2000 5711
P h otolysis of 1b w ith Ben zyl Meth a cr yla te. A solution
of 1b (0.293 mmol) and benzyl methacrylate (0.586 mmol) in
benzene (30 mL) was irradiated for 1.5 h at ambient temper-
ature in
a water bath. After the reaction mixture was
evaporated, the residue was subjected to a column chroma-
tography on silica gel to give 9 in 45% yield.
Ben zyl 3-(dim eth ylph en ylsilyl)-2-m eth ylpr opion ate (9):
1H NMR (CDCl3) δ 0.30 (s, 3H), 0.31 (s, 3H), 0.95 (dd, J ) 7.2,
14.7 Hz, 1H), 1.18 (d, J ) 7.2 Hz, 3H), 1.34 (dd, J ) 7.5, 14.7
Hz, 1H), 2.55-2.67 (m, 1H), 4.97 (d, J ) 12.6 Hz, 1H), 5.04 (d,
J ) 12.6 Hz, 1H), 7.30-7.41 (m, 8H), 7.48-7.55 (m, 2H); 13C
NMR (CDCl3) δ -2.6, 20.5, 20.6, 35.6, 66.0, 127.8, 128.1, 128.5,
129.0, 133.6, 136.2, 138.7, 177.3. Anal. Calcd for C19H24O2Si:
H, 7.74; C, 73.03. Found: H, 8.00; C, 73.31.
Photolysis of 1b with MMA was carried out in the same
manner. The spectral data of the product 8 has been reported
in the literature.27 For reactions with 1-octene and cyclohexene,
a mixture of 1b and olefins in n-hexane was irradiated for 3 h
at ambient temperature in a water bath.
6.3 Hz, 3H), 1.13 (dd, J ) 14.9, 3.5 Hz, 1H), 1.62-1.83 (m,
2H), 3.19 (dd, J ) 8.5, 8.5 Hz, 1H), 3.25 (dd, J ) 8.5, 8.5 Hz,
1H), 3.87 (dd, J ) 8.4, 6.9 Hz, 1H), 3.93 (dd, J ) 8.4, 6.9 Hz,
1H), 7.32-7.38 (m, 3H), 7.46-7.53 (m, 2H); 13C NMR (CDCl3)
δ -2.7, -2.5, 15.2, 18.0, 43.2, 74.6, 75.2, 127.9, 129.1, 133.5,
139.0. Anal. Calcd for C14H22OSi: H, 9.46; C, 71.73. Found:
H, 9.52; C, 71.47. The stereochemistry was determined by NOE
in CD3OD.
Radical cyclization of 11b-e was carried out with the same
procedure. All 12b-e were isolated as a mixture of two
stereoisomers, the ratio of which was determined by capillary
GC. However, isolation as a single isomer with HPLC failed.
12b: 1H NMR (CDCl3) δ 0.30 (s, 6H), 0.62-0.95 (m, 5H),
1.21 (t, J ) 7.1 Hz, 6H), 1.81-2.18 (m, 4H), 2.25-2.40 (m, 2H),
4.07-4.21 (m, 4H), 7.28-7.38 (m, 3H), 7.45-7.56 (m, 2H);
HRMS calcd for C21H32O4Si 376.2070; found 376.2074. Anal.
Calcd for C21H32O4Si: H, 8.57; C, 66.98. Found: H, 8.58; C,
67.27.
Dim eth y(n -octyl)p h en ylsila n e (6): 1H NMR (CDCl3) δ
0.26 (s, 6H), 0.74 (t, J ) 7.8 Hz, 2H), 0.88 (t, J ) 6.8 Hz, 3H),
1.14-1.40 (m, 12H), 7.33-7.39 (m, 3H), 7.48-7.56 (m, 2H);
13C NMR (CDCl3) δ -3.1, 14.0, 15.6, 22.6, 23.8, 29.2, 31.9, 33.5,
127.7, 128.7, 133.6, 139.9. Anal. Calcd for C16H28Si: H, 11.36;
C, 77.34. Found: H, 11.32; C, 77.13.
1
Cycloh exyld im eth ylp h en ylsila n e (7): H NMR (CDCl3)
δ 0.24 (s, 6H), 0.72-0.88 (m, 1H), 0.97-1.33 (m, 5H), 1.56-
1.81 (m, 5H), 7.31-7.41 (m, 3H), 7.44-7.56 (m, 2H); 13C NMR
(CDCl3) δ -5.2, 25.8, 26.9, 27.4, 28.0, 127.6, 128.7, 133.9, 138.7.
Anal. Calcd for C14H22Si: H, 10.15; C, 76.99. Found: H, 10.38;
C, 76.71.
1
12c: H NMR (CDCl3) δ 0.00 (s, 12H), 0.28 (s, 6H), 0.87 (s,
18H), 0.55-1.18 (m, 7H), 1.40-2.11 (m, 4H), 3.28-3.43 (m,
4H), 7.29-7.37 (m, 3H), 7.46-7.54 (m, 2H); HRMS Calcd for
C29H56O2Si3 520.3588; found 520.3577. Anal. Calcd for C29H56O2-
Si3: H, 10.83; C, 66.85. Found: H, 11.07; C, 66.56.
Hyd r osilyla tion of 1-octen e w ith 1b in Dieth yl Eth er .
A solution of 1b (0.386 mmol) and 1-octene (0.322 mmol) in
39 mL of dry diethyl ether was irradiated under nitrogen
atmosphere for 1.5 h at ambient temperature with a high-
pressure mercury lamp. After evaporation of the reaction
mixture, the residue was treated with 5 mL of n-hexane and
filtered to remove the resultant precipitates with Celite. The
filtrate was concentrated under reduced pressure and then
subjected to a column chromatography on silica gel to give
dimethyl(n-octyl)phenylsilane (6) in 52%. Hydrosilylation of
methyl crotonate was carried out in the same manner.
Met h yl 3-(d im et h ylp h en ylsilyl)b u t yr a t e (10a , m a jor
isom er ): 1H NMR (CDCl3) δ 0.28 (s, 6H), 0.97 (d, J ) 7.5 Hz,
3H), 1.37-1.51 (m, 1H), 2.06 (dd, J ) 11.1, 15.3 Hz, 1H), 2.39
(dd, J ) 4.2, 15.3 Hz, 1H), 3.61 (s, 3H), 7.33-7.39 (m, 3H),
7.46-7.53 (m, 2H); 13C NMR (CDCl3) δ -5.5, -5.1, 14.4, 16.4,
36.6, 51.3, 127.8, 129.2, 133.9, 137.3, 174.5. Anal. Calcd for
1
12d : H NMR (CDCl3) δ 0.276 (s, 6H, minor isomer), 0.279
(s, 6H, major isomer), 0.52-0.94 (m, 5H), 1.00-1.98 (m, 8H),
7.30-7.38 (m, 3H), 7.47-7.56 (m, 2H); HRMS Calcd for C15H24
-
Si 232.1647; found 232.1637. Anal. Calcd for C15H24Si: H,
10.41; C, 77.51. Found: H, 10.26; C, 77.68.
1
12e: H NMR (CDCl3) δ 0.29-0.33 (m, 6H), 0.52-1.21 (m,
5H), 1.50-2.17 (m, 5H), 2.68-3.82 (m, 4H), 7.30-7.40 (m, 3H),
7.42-7.57 (m, 2H); HRMS Calcd for C16H25NOSi 275.1705;
found 275.1701. Anal. Calcd for C16H25NOSi: H, 9.15; C, 69.76;
N, 5.08. Found: H, 9.15; C, 69.83; N, 5.03.
Ra d ica l Cycliza tion of 1-Br om o-5-h exen e (13) w ith 1b.
A solution of 1b (0.354 mmol), 1-bromo-5-hexene (13) (0.295
mmol), and n-octane (26.0 mg) in 35 mL of diethyl ether was
irradiated under nitrogen atmosphere for 1.5 h with a high-
pressure mercury lamp. During the irradiation, the reaction
flask was kept at ambient temperature by a cooling water
bath. The yield of methylcyclopentane (14) was determined
by capillary GC with n-octane as an internal reference.
P h otoch em ica lly In d u ced P olym er iza tion w ith 1b. A
solution of 1b (0.10 mmol) in 1.0 mL of monomer (15) and 2.0
mL of benzene was irradiated under nitrogen atmosphere for
1.5 h with a high-pressure mercury lamp. During irradiation,
the apparatus are kept at ambient temperature by a cooling
water bath. After the reaction mixture was evaporated under
reduced pressure, the viscous residue was poured into 50 mL
of methanol (for PMMA and PMA) or n-hexane (for PVAc).
Precipitates produced were collected by filtration and dried.
The molecular weight of the polymer was determined by GPC
analysis with a polystyrene standard. 1H NMR indicated that
the polymerization products thus prepared contained the
dimethylphenylsilyl groups at the polymer ends. The dimeth-
ylphenylsilyl terminal of the polymer (1H NMR in CDCl3):
P oly(m eth yl m eth a cr yla te): δ 0.28 (bs, 6H), 7.26-7.50
(m, 5H).
C
13H20O2Si: H, 8.53; C, 66.05. Found: H, 8.77; C, 66.25.
Meth yl 2-(d im eth ylp h en ylsilyl)bu tyr a te (10b, m in or
isom er ): 1H NMR (CDCl3) δ 0.34 (s, 3H), 0.36 (s, 3H), 0.88 (t,
J ) 6.9 Hz, 3H), 1.42 (ddt, J ) 20.6, 6.9, 3.0 Hz, 1H), 1.79
(ddt, J ) 20.6, 11.6, 6.9 Hz, 1H), 2.12 (dd, J ) 11.6, 3.0 Hz,
1H), 3.55 (s, 3H), 7.32-7.41 (m, 3H), 7.45-7.54 (m, 2H); 13C
NMR (CDCl3) δ -4.9, -4.0, 15.0, 20.4, 39.5, 50.9, 127.9, 129.5,
133.9, 136.4, 175.7. Anal. Calcd for C13H20O2Si: H, 8.53; C,
66.05. Found: H, 8.69; C, 66.25.
Ra d ica l Cycliza tion of Dia llyl Eth er (11a ) w ith 1b. A
diene cyclization reaction was conducted in the same procedure
with the hydrosilylation. A column chromatography on silica
gel afforded the product 12a as a mixture of two stereoisomers.
The ratio of the isomers was determined by capillary GC. Each
isomer was isolated by HPLC.
cis-12a (m a jor , p ola r isom er ): 1H NMR (CDCl3) δ 0.30
(s, 6H), 0.71 (dd, J ) 14.4, 9.6 Hz, 1H), 0.88 (d, J ) 6.9 Hz,
3H), 0.89 (dd, J ) 14.4, 5.4 Hz, 1H), 2.07-2.19 (m, 1H), 2.20-
2.33 (m, 1H), 3.25 (dd, J ) 8.1, 8.1 Hz, 1H), 3.45 (dd, J ) 8.0,
3.6 Hz, 1H), 3.78 (dd, J ) 8.1, 8.1 Hz, 1H), 3.85 (dd, J ) 8.0,
6.0 Hz, 1H), 7.33-7.39 (m, 3H), 7.48-7.54 (m, 2H); 13C NMR
(CDCl3) δ -2.50, -2.47, 13.2, 13.3, 37.3, 38.3, 73.2, 75.0, 127.8,
129.0, 133.4, 139.0. Anal. Calcd for C14H22OSi: H, 9.46; C,
71.73. Found: H, 9.58; C, 71.84. The stereochemistry was
determined by NOE in C6D6.
P oly(m eth yl a cr yla te): δ 0.25 (bs, 6H), 7.26-7.50 (m, 5H).
P oly(vin yl a ceta te): δ 0.28-0.42 (m, 6H), 7.28-7.62 (m,
5H).
Ack n ow led gm en t. A.M. is sincerely grateful to Ciba
Specialty Chemicals K. K. for general support to this
research.
1
tr a n s-12a (m in or , less p ola r isom er ): H NMR (CDCl3)
δ 0.28 (s, 6H), 0.63 (dd, J ) 14.9, 10.3 Hz, 1H), 0.95 (d, J )
J O000547W