Nickel-catalyzed denitrogenative annulation reactions of 1,2,3-Benzotriazin-4(3 H)-ones with 1,3-dienes and alkenes

Article abstract of DOI:10.1021/jo1008756

(Figure presented) 1,2,3-Benzotriazin-4(3H)-ones react with 1,3-dienes in the presence of a nickel(0)/phosphine complex to give a variety of 3,4-dihydroisoquinolin-1(2H)-ones. Oxidative insertion of nickel(0) into the triazinone moiety prompts extrusion of dinitrogen to give a five-membered ring azanickelacyclic intermediate. Subsequent insertion of 1,3-dienes into the nickel-carbon bond followed by allylic amidation affords 3,4-dihydroisoquinolin- 1(2H)-ones. Alkenes also undergo insertion into the five-membered ring azanickelacyclic intermediate, and subsequent reductive elimination gives 3-substituted 3,4-dihydroisoquinolin-1(2H)-ones.

Full text of DOI:10.1021/jo1008756

pubs.acs.org/joc
sence of a rhodium catalyst to give indolizine derivatives
Nickel-Catalyzed Denitrogenative Annulation
Reactions of 1,2,3-Benzotriazin-4(3H)-ones with
1,3-Dienes and Alkenes
with extrusion of N2.3 Phthalimide,4a phthalic anhydride,4b
and isatoic anhydride4c were also utilized in the nickel-cata-
lyzed annulation reaction with alkynes to form isoquinolin-
1(2H)-ones, isochromen-1-ones, and quinolin-4(1H)-ones,
respectively, with extrusion of CO or CO2.5 We have recently
shown that nickel-catalyzed denitrogenative annulation reactions
of 1,2,3-benzotriazin-4(3H)-ones with alkynes6a and allenes6b
provide new synthetic approaches to isoquinolin-1(2H)-ones
and 4-methylene-3,4-dihydroisoquinolin-1(2H)-ones. Thus,
1,2,3-benzotriazin-4(3H)-ones can be exploited as a precursory
platform for the synthesis of isoquinolin-1(2H)-one derivatives,7
whicharefoundinawidevariety ofplant alkaloids and bioactive
compounds. We next examined the possibility of their reactions
with other unsaturated molecules to expand the reaction scope.
In this paper are described the results of the nickel-catalyzed
annulation reactions of 1,2,3-benzotriazin-4(3H)-ones with 1,3-
dienes and alkenes.
Tomoya Miura, Masao Morimoto, Motoshi Yamauchi, and
Masahiro Murakami*
Department of Synthetic Chemistry and Biological Chemistry,
Kyoto University, Katsura, Kyoto 615-8510, Japan
murakami@sbchem.kyoto-u.ac.jp
Received May 5, 2010
The model substrate, N-tolyl-1,2,3-benzotriazin-4(3H)-
one (1a), was readily prepared from methyl anthranilate
in two steps (eq 1);8 methyl anthranilate was diazotized by
NaNO2 and then coupled with 4-toluidine to give methyl
2-[3-(4-tolyl)triaz-2-enyl]benzoate. Subsequent heating in
refluxing ethanol prompted six-membered ring closure to
afford 1a as a white solid (78% yield over two steps).
1,2,3-Benzotriazin-4(3H)-ones react with 1,3-dienes in the
presence of a nickel(0)/phosphine complex to give a variety
of 3,4-dihydroisoquinolin-1(2H)-ones. Oxidative insertion
of nickel(0) into the triazinone moiety prompts extrusion
of dinitrogen to give a five-membered ring azanickelacyclic
intermediate. Subsequent insertion of 1,3-dienes into the
nickel-carbon bond followed by allylic amidation affords
3,4-dihydroisoquinolin-1(2H)-ones. Alkenes also undergo
insertion into the five-membered ring azanickelacyclic
intermediate, and subsequent reductive elimination gives
3-substituted 3,4-dihydroisoquinolin-1(2H)-ones.
In addition, we developed an alternative simple route to 1a
from commercially available NH-1,2,3-benzotriazin-4(3H)-
one through direct N-arylation catalyzed by copper (eq 2).9
When NH-1,2,3-benzotriazin-4(3H)-one was treated with 4-io-
dotoluene (1.5 equiv) in the presence of CuI (10 mol %) and
2-isobutyrylcyclohexanone (20 mol %) in DMSO at 80 °C, an
N-arylation reaction took place and 1a was obtained in 95%
yield. The isolated 1a was stable at room temperature and could
be kept for months without any decomposition.10
Transition-metal-catalyzed annulation reactions provide
an efficient synthetic route to heterocyclic compounds.1
Heterometalacyclic complexes are often involved as the key
intermediates, which can induce the incorporation of un-
saturated molecules into the heterocyclic skeleton. It has
been demonstrated that relatively stable heterocyclic com-
pounds act as precursors of heterometalacyclic complexes.
Oxidative addition to a low-valent transition metal and
extrusion of gaseous molecules such as N2, CO, and CO2
lead to the formation of new heterocyclic systems. For
example, pyridotriazoles2 reacted with alkynes in the pre-
(1) For reviews, see: (a) Nakamura, I.; Yamamoto, Y. Chem. Rev. 2004,
104, 2127. (b) Zeni, G.; Larock, R. C. Chem. Rev. 2004, 104, 2285.
€
(c) D’Souza, D. M.; Muller, T. J. J. Chem. Soc. Rev. 2007, 36, 1095.
We initiated our study by conducting a reaction of 1a with
2,3-dimethylbuta-1,3-diene (2a, 2 equiv) in THF at 60 °C in
(2) (a) Chuprakov, S.; Hwang, F. W.; Gevorgyan, V. Angew. Chem., Int.
Ed. 2007, 46, 4757. (b) Chuprakov, S.; Gevorgyan, V. Org. Lett. 2007, 9,
4463.
(3) For related examples, see: (a) Horneff, T.; Chuprakov, S.; Chernyak,
N.; Gevorgyan, V.; Fokin, V. V. J. Am. Chem. Soc. 2008, 130, 14972.
(b) Miura, T.; Yamauchi, M.; Murakami, M. Chem. Commun. 2009, 1470.
(c) Nakamura, I.; Nemoto, T.; Shiraiwa, N.; Terada, M. Org. Lett. 2009, 11,
1055. (d) Chuprakov, S.; Kwok, S. W.; Zhang, L.; Lercher, L.; Fokin, V. V.
J. Am. Chem. Soc. 2009, 131, 18034. (e) Grimster, N.; Zhang, L.; Fokin, V. V.
J. Am. Chem. Soc. 2010, 132, 2510.
(4) (a) Kajita, Y.; Matsubara, S.; Kurahashi, T. J. Am. Chem. Soc. 2008,
130, 6058. (b) Kajita, Y.; Kurahashi, T.; Matsubara, S. J. Am. Chem. Soc.
2008, 130, 17226. (c) Yoshino, Y.; Kurahashi, T.; Matsubara, S. J. Am.
Chem. Soc. 2009, 131, 7494.
(5) For related examples, see: (a) O’Brien, E. M.; Bercot, E. A.; Rovis, T.
J. Am. Chem. Soc. 2003, 125, 10498. (b) Wang, C.; Tunge, J. A. J. Am. Chem.
Soc. 2008, 130, 8118.
DOI: 10.1021/jo1008756
r
Published on Web 07/06/2010
J. Org. Chem. 2010, 75, 5359–5362 5359
2010 American Chemical Society

Miura et al.
JOCNote
SCHEME 1. Ni(0)-Catalyzed Reaction of 1a with 2a Using
PMe3 as the Ligand
TABLE 1. Ni(0)-Catalyzed Reaction of 1a with 2a: Screening of
Phosphine Ligandsa
entry
ligand (mol %)
yield of 3aa (%)
yield of 4aa (%)
1
2
3
4
5
6
7
PMe2Ph (20)
PMePh2 (20)
PPh3 (20)
P(n-Bu)3 (20)
P(t-Bu)3 (20)
Dppb (10)
21
<5
0
<5
0
52
9
<5
<5
0
12
<5
54
87
Dppf (10)
aConditions: 1a (0.2 mmol), 2a (0.4 mmol), Ni(cod)2 (20 μmol,
10 mol %), ligand (10 or 20 mol %) in THF (1 mL) at 60 °C for 12 h.
Dppb =1,4-Bis(diphenylphosphino)butane. Dppf =1,10- Bis(diphenyl-
phosphino)ferrocene.
TABLE 2. Ni(0)-Catalyzed Reaction of 1,2,3-Benzotriazin-4(3H)-ones
1b-g with 2,3-Dimethylbuta-1,3-diene (2a)a
the presence of a nickel(0) catalyst generated in situ from
Ni(cod)2 (10 mol %) and PMe3 (20 mol %) (Scheme 1). The
substrate 1a was consumed in 12 h, and after chromato-
graphic isolation, the noncyclized product 3aa and the deni-
trogenative annulation product 4aa were obtained in 51%
and 37% yields, respectively. We assume the following reac-
tion mechanism, which is analogous to what we previously
reported for the annulation reaction with allenes.6b Oxidative
addition of a C(O)N-N bond to nickel(0) prompts extru-
sion of N2 to generate five-membered ring azanickelacyclic
intermediate A. Subsequent insertion of a carbon-carbon
double bond of 2a into the nickel-carbon bond gives seven-
membered ring azanickelacyclic intermediate B, which is in
equilibrium with zwitterionic π-allylnickel species C. The
intermediate B undergoes β-hydride elimination and reduc-
tive elimination to afford 3aa along with nickel(0). On the
other hand, the intermediate C undergoes intramolecular
recombination to afford 4aa along with nickel(0).
entry
1
R1
R2
R3
solvent T (°C)
4
yield (%)
1
2
3
4
5
6
1b Ph
H
H
H
H
H
THF
toluene
THF
60 4ba
80 4ca
60 4da
87
85
88
24
74
86
1c 4-MeOC6H4
1d 4-CF3C6H4
1e Bn
1f Ph
1g Ph
H
H
H
H
toluene 110 4ea
60 4fa
80 4ga
CO2Me THF
toluene
MeO MeO
aConditions: 1 (0.2 mmol), 2 (0.4 mmol), Ni(cod)2 (20 μmol, 10 mol %),
Dppf (20 μmol, 10 mol %) in solvent (1 mL).
4aa in 40% yield. These contrasting results demonstrated the
effect of the Dppf ligand accelerating incorporation of 2a.
Next, several phosphine ligands were examined using 1a and
2a as the substrates (Table 1). Although the use of PMe2Ph
increased the yield of 4aa to 52% (entry 1), other monopho-
sphine ligands such as PMePh2, PPh3, P(n-Bu)3, and P(t-Bu)3
gave considerably inferior results in terms of reactivity (entries
2-5). The yield and product-selectivity were both improved
when bisphosphine ligands such as Dppb and Dppf were
employed (entries 6 and 7). In particular, the use of Dppf gave
4aa in 87% isolated yield along with a trace amount of 3aa.
As we previously reported,6b the five-membered ring
Under the optimized reaction conditions using Dppf as
the ligand, various benzotriazinones 1b-g were subjected
to the catalytic reaction with 2a (Table 2). Substrates 1b-d
possessing an aryl group on the nitrogen atom reacted
smoothly to afford the corresponding products 4ba-da in
high yield (entries 1-3). However, the reaction of benzyl-
substituted substrate 1e was slower even at the higher tem-
perature (110 °C) and the product 4ea was obtained in only
24% yield (entry 4). Benzotriazinones 1f and 1g having
electron-withdrawing and -donating substituents on the ben-
zene ring were both competent substrates for the annulation
reaction (entries 5 and 6).
azanickelacycle A Dppbenz was easily isolated by the treat-
3
ment of 1a with equimolar amounts of Ni(cod)2 and 1,2-bis-
(diphenylphosphino)benzene (Dppbenz). Although a reac-
tion of A Dppbenz with 2a (3 equiv) was attempted in toluene
3
at 110 °C, only a trace amount of 4aa was obtained (eq 3).
When Dppf was added to a mixture of A and 2a (3 equiv) as
an additional ligand and the reaction mixture was heated in
toluene at 110 °C, the annulation reaction proceeded to give
Next, various 1,3-dienes 2b-f were subjected to the annu-
lation reaction of 1a (Table 3). Symmetrical 1,3-dienes such
as buta-1,3-diene (2b) and 1,2-dimethylenecyclohexane (2c)
reacted well to give the corresponding products 4ab and
4ac in 81% and 92% yields, respectively (entries 1 and 2).
Unsymmetrical 1,3-dienes were also used, and the regio-
selectivity of the annulation reaction was examined. The
reaction with isoprene (2d) gave a mixture of regioisomers
4ad and 5ad in a 86:14 ratio. The two regioisomers could be
(6) (a) Miura, T.; Yamauchi, M.; Murakami, M. Org. Lett. 2008, 10,
3085. (b) Yamauchi, M.; Morimoto, M.; Miura, T.; Murakami, M. J. Am.
Chem. Soc. 2010, 132, 54.
(7) For a review of 1(2H)-isoquinolone synthesis, see: Glushkov, V. A.;
Shklyaev, Y. V. Chem. Heterocycl. Compd. 2001, 37, 663.
(8) Clark, A. S.; Deans, B.; Stevens, M. F. G.; Tisdale, M. J.; Wheelhouse,
R. T.; Denny, B. J.; Hartley, J. A. J. Med. Chem. 1995, 38, 1493.
(9) (a) Sugahara, M.; Ukita, T. Chem. Pharm. Bull. 1997, 45, 719.
(b) Shafir, A.; Buchwald, S. L. J. Am. Chem. Soc. 2006, 128, 8742.
(10) Barker, A. J.; Paterson, T. M.; Smalley, R. K.; Suschitzky, H.
J. Chem. Soc., Perkin Trans. 1 1979, 2203.
5360 J. Org. Chem. Vol. 75, No. 15, 2010

Miura et al.
JOCNote
TABLE 3. Ni(0)-Catalyzed Reaction of N-Tolyl-1,2,3-benzotriazin-
4(3H)-one (1a) with 1,3-Dienes 2b-fa
FIGURE 1. Other 1,3-dienes 2g-j.
SCHEME 2. Ni(0)-Catalyzed Asymmetric Reaction of 1a with
1,3-Dienes
SCHEME 3. Ni(0)-Catalyzed Reaction of 1a with 6a Using
PMe3 as the Ligand
aConditions: 1 (0.2 mmol), 2 (0.4 mmol), Ni(cod)2 (20 μmol, 10 mol %),
Dppf (20 μmol, 10 mol %) in THF (2 mL) at 60 °C for 6-12 h unless other-
acceptable yield (77%), albeit with low enantioelectivity
(20% ee). Although the use of (S)-Segphos was also exam-
ined in the reaction of 1a with 2d and 2e, the enantioselec-
tivities were 16% ee for 4ad and 52% ee for 4ae. These results
inferred that the reaction step of 1,3-diene insertion that
determines the enantioselectivity is different from that for
allene insertion.6b
We next turned our attention to the use of alkenes as
coupling partners instead of 1,3-dienes. When oct-1-ene was
subjected to the annulation reaction of 1a using PMe3 as the
ligand (110 °C in toluene), no reaction was observed. On the
other hand, the reaction with methyl acrylate (6a) proceeded
cleanly to give 3,4-dihydroisoquinolin-1(2H)-one 7aa in
90% yield as a single regioisomer. The methoxycarbonyl
group was bound to the C(3) position next to the nitrogen
atom. This regiochemistry resulted from the electronic de-
mand expected for the insertion process of the electron-
deficient carbon-carbon double bond of methyl acrylate
(6a) into the nucleophilic nickel-carbon bond. Therefore,
we assume the mechanism of the annulation reaction as
shown in Scheme 3. Insertion of 6a into the five-membered
ring azanickelacycle A gives seven-membered ring azanick-
elacycle D, and the following reductive elimination releases
7aa and nickel(0).
b
c
wise noted. The 4/5 ratios given in parentheses. Using toluene (2 mL)
at 80 °C. dE/Z= >95:5. ecis/trans = 12:88. fCombined yield of isomers.
gUsing Ni(cod)2 (40 μmol, 20 mol %) and Dppf (40 μmol, 20 mol %).
separated by silica gel chromatography, and the major isomer
4ad was obtained in 66% yield (entry 3). Myrcene (2e) exhibited
a similar regioselectivity (4ae/5ae = 83:17), and after chroma-
tographic isolation, the major isomer 4ae was obtained in
53% yield (entry 4). When (E)-penta-1,3-diene (2f) was em-
ployed, the product 4af was formed with good regioselectivity
(4af/5af = 90:10, entry 5).11 Thus, insertion of unsymmetrical
1,3-dienes occurs preferentially at the more substituted
carbon-carbon double bond.12
Other 1,3-dienes 2g-j shown in Figure 1 were less reactive
and failed to undergo the annulation reaction of 1a.
The asymmetric version was attempted using 1a and 2a as
the substrates (Scheme 2). Among various chiral ligands
examined,13 only (S)-Segphos gave the product 4aa in an
(11) The product 4af was obtained as a mixture of two diastereomers (cis/
trans = 12:88). This result infers a mechanism involving a π-σ-π isomer-
ization of π-allylnickel intermediate.
(12) (a) Ogoshi, S.; Tonomori, K.; Oka, M.; Kurosawa, H. J. Am. Chem.
Soc. 2006, 128, 7077. (b) Kimura, M.; Ezoe, A.; Mori, M.; Iwata, K.;
Tamaru, Y. J. Am. Chem. Soc. 2006, 128, 8559.
(13) Abbreviations: (S,S)-i-Pr-Foxap = (S,S)-[2-(40-isopropyloxazolin-20-
yl)ferrocenyl]diphenylphosphine. (R)-Quinap = (R)-1-(2-diphenylphosphino-
1-naphtyl)isoquinoline. (R)-Binap = (R)-2,20-bis(diphenylphosphino)-1,10-bi-
naphthyl. (R)-Tol-Binap = 2,20-bis(di-p-tolylphosphino)-1,10-binaphthyl.
(S)-Segphos=5,50-bis(diphenylphosphino)-4,40-bi-1,3-benzodioxole.
Other phosphine ligands were screened in the reaction of
1a with 6a, and P(n-Bu)3 gave the best result (Table 4, entries
1-4). Furthermore, the amount of P(n-Bu)3 could be re-
duced to 20 mol % (entry 5).
J. Org. Chem. Vol. 75, No. 15, 2010 5361

Miura et al.
JOCNote
TABLE 4. Ni(0)-Catalyzed Reaction of 1a with 6a: Screening of
TABLE 6. Ni(0)-Catalyzed Reaction of N-Tolyl-1,2,3-benzotriazin-
4(3H)-one (1a) with Alkene 6b-ga
Phosphine Ligandsa
entry
ligand (mol %)
yield of 7aa (%)
1
2
3
4
5
PPh3 (40)
96
99
<5
88
P(n-Bu)3 (40)
P(t-Bu)3 (40)
Dppf (20)
P(n-Bu)3 (20)
91
entry
6 (equiv)
R6
7
yield (%)
aConditions: 1a (0.1 mmol), 6a (0.15 mmol), Ni(cod)2 (10 μmol, 10 mol
%), ligand (20 or 40 mol %) in toluene (1 mL) at 110 °C for 13-14 h.
1
2
3
4
5
6
6b (1.5)
6c (3.0)
6d (1.5)
6e (3.0)
6f (1.5)
6g (1.5)
CN
CONMe2
COEt
2-pyridyl
4-pyridyl
4-CF3C6H4
7ab
7ac
7ad
7ae
7af
7ag
92
83
39b,c
83c
73d
11b
TABLE 5. Ni(0)-Catalyzed Reaction of 1,2,3-Benzotriazin-4(3H)-ones
1b-j with Methyl Acrylate (6a)a
aThe reaction conditions are the same as those in Table 5 unless
otherwise noted. bUsing mesitylene (2 mL) at 160 °C. cUsing PMe3
(80 μmol, 40 mol %). dUsing toluene (4 mL).
In summary, we have demonstrated that a five-membered
ring azanickelacycle generated from 1,2,3-benzotriazin-4(3H)-
ones with extrusion of N2 successfully incorporates 1,3-dienes
and alkenes, disclosing the potential as a versatile synthetic
intermediate. This catalytic system provides a convenient
and regioseletive method for the synthesis of substituted 3,4-
dihydroisoquinolin-1(2H)-ones from readily available pre-
cursors. Noteworthy is that dinitrogen is the only byproduct
of the present annulation reaction.
entry
1
R1
R2
R3
7
yield (%)
1
2
3
4
5
6
7
8
9
1b
1c
1d
1h
1i
1e
1j
1f
1g
Ph
H
H
H
H
H
H
H
H
MeO
H
H
H
H
H
H
H
7ba
7ca
7da
7ha
7ia
7ea
7ja
7fa
7ga
99
96
98
81
77
99
97
96
98
4-MeOC6H4
4-CF3C6H4
4-ClC6H4
2-MeOC6H4
Bn
Me
Ph
Ph
CO2Me
MeO
aConditions: 1(0.2 mmol), 6a (0.3 mmol), Ni(cod)2 (20 μmol, 10 mol %),
P(n-Bu)3 (40 μmol, 20 mol %) in toluene (2 mL) at 110 °C for 12 h.
Experimental Section
Representative Procedure for the Nickel-Catalyzed Annulation
Reaction of 1,2,3-Benzotriazin-4(3H)-ones with 1,3-Dienes
(Table 1, entry 7). To an oven-dried flask was added 1a (44.6
mg, 0.2 mmol), Ni(cod)2 (5.6 mg, 20 μmol), Dppf (11.0 mg,
20 μmol), THF (1 mL), and 2a (45 μL, 0.4 mmol). After being
heated at 60 °C for 12 h, the reaction mixture was cooled to room
temperature and stirred for 30 min in open air. The resulting
mixture was passed through a pad of Florisil and eluted with
ethyl acetate. The filtrate was concentrated under reduced
pressure. The residue was purified by preparative thin layer
chromatography (hexane/ethyl acetate 5:1) to give 4aa (48.2 mg,
Other examples of the reaction of benzotriazinones 1b-j
with methyl acrylate (6a) under the optimized reaction con-
ditions using P(n-Bu)3 as the ligand are listed in Table 5.
The reaction of aryl-substituted substrates 1b-d and 1h-i
with 6a afforded the corresponding products 7ba-da and
7ha-ia in yields ranging from 77% to 99% (entries 1-5).
Interestingly, unlike the results obtained with 1,3-diene 2a,
benzyl- and methyl-substituted substrates 1e and 1j could
readily participate in the reaction with 6a (entries 6 and 7).
Electronically perturbed benzotriazinones 1f and 1g were
also converted to the corresponding products 7fa and 7ga in
96% and 98% yields, respectively (entries 8 and 9).14
The results of the nickel-catalyzed annulation reaction of
1a with various electron-deficient alkenes 6b-g are summar-
ized in Table 6. Acrylonitrile (6b) and N,N-dimethylacryla-
mide (6c) were incorporated well to give the corresponding
products 7ab and 7ac in good yield (entries 1 and 2), whereas
ethyl vinyl ketone (6d) gave the product 7ad in only 39%
yield (entry 3). In addition to 6a-c, vinylpyridines 6e and 6f
successfully participated in the annulation reaction (entries 4
and 5). However, the reaction of the styrene derivative 6g
having a trifluoromethyl group was sluggish even under
more forcing conditions (entry 6).
0.174 mmol, 87% yield). IR (KBr): 2980, 1644, 1512, 1374 cm-1
.
1H NMR: δ = 1.31 (s, 3H), 1.71 (s, 3H), 2.37 (s, 3H), 3.18 (d, J =
15.9 Hz, 1H), 3.28 (d, J = 15.6 Hz, 1H), 4.93 (s, 1H), 5.05 (s, 1H),
7.12 (d, J = 7.5 Hz, 1H), 7.16-7.20 (m, 4H), 7.29-7.37 (m, 1H),
7.39-7.46 (m, 1H), 8.05-8.10 (m, 1H). 13C NMR: δ = 19.8,
21.1, 26.5, 41.2, 64.6, 114.8, 126.7, 126.9, 128.2, 128.8, 129.2,
129.6, 131.8, 136.1, 136.8, 137.1, 146.0, 165.5. HRMS (EIþ):
calcd for C20H21NO, Mþ 291.1623, found m/z 291.1626.
Acknowledgment. This work was supported in part by
MEXT (Scientific Research on Priority Areas “Chemistry of
Concerto Catalysis” No. 20037032), the Mitsubishi Chemi-
cal Corporation Fund, the Sumitomo Foundation, and the
Astellas Award in Synthetic Organic Chemistry, Japan.
Supporting Information Available: Experimental details,
C
structural data for all new compounds, and copies of 1H and 13
(14) We also attempted the asymmetric version using 1a and 6a as the
substrates. Although various chiral ligands were examined, both the yield
and the enantioselectivity of 7aa were low.
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
5362 J. Org. Chem. Vol. 75, No. 15, 2010