Photoarylation of alkenes and heteroaromatics by Dibromo-BINOLs in aqueous solution

Article abstract of DOI:10.1021/jo100349h

The photochemistry of 6,6′-dibromo-BINOL (BINOL = 2,2′-dihydroxy-1,1′-binaphthyl) under mild basic conditions and its methyl and triisopropylsilyl ethers has been investigated in neat and aqueous acetonitrile through product distribution analysis and laser flash photolysis. Arylation and alkylation have been successfully achieved in the presence of allyltrimethylsilane, ethyl vinyl ether, pyrrole, pyridine, thiophene, benzene, and indole. Such a photoreactivity offers a metal and protecting group free synthetic protocol toward mono- and disubstituted 6-aryl/alkyl BINOLs, since the BINOL chirality is preserved in the photoactivation process.

Full text of DOI:10.1021/jo100349h

pubs.acs.org/joc
SCHEME 1. Photogeneration of 4-Oxocyclohexa-2,5-dienyl-
idene Carbene (3) and 4-Hydroxyphenyl Cation (4) from
Photoarylation of Alkenes and Heteroaromatics by
Dibromo-BINOLs in Aqueous Solution
Chlorophenol 1 and Chloroanisole 2
Daniela Verga, Filippo Doria, Luca Pretali, and
Mauro Freccero*
ꢀ
Dipartimento di Chimica Organica, Universita di Pavia,
V.le Taramelli 10, 27100 Pavia, Italy
mauro.freccero@unipv.it
Received February 24, 2010
pollutants in water.4 Recently, the reactivity has been ex-
tended to organic solvents, suggesting new synthetic applica-
tions for
a
metal free arylation/alkylation process.5
Nevertheless, synthetic protocols based on the mild photo-
activation in aqueous solvents are still scarce.6 The photo-
reactivity of the 4-chlorophenol (1) and its methyl ether (2) as
prototypes of aryloxy halides has been rationalized as a
heterolytic process generating both 4-oxocyclohexa-2,5-die-
nylidene carbene (3) and 4-hydroxyphenyl cation (4). The
carbene 3 has been proposed to arise from the depro-
tonation of the 4 (Scheme 1). The latter has successfully
been trapped by several π-nucleophiles. Despite the
great effort on mechanistic and synthetic aspects related
to the photoreactivity of 1 and 2, only very recently the
photoreactivity of both the 6-bromonaphthols 5a-d7 and
the 1,6-dibromonaphthols 6a-d (Scheme 2),8 has been
investigated in the presence of aromatics, heteroaromatic,
and alkenes.
The reactive triplet carbenes 5C and 6C, photogenerated
from the bromonaphthols 5a and 6a (Scheme 2), have been
shown to be the key intermediates involved in the reactivity,
by LFP and trapping experiments.7,8 These carbene inter-
mediates have been efficiently trapped by O2 generating the
detectable carbonyl oxides 5CO7 and 6CO,8 which exhibit a
broad absorbance centered at 580 and 680 nm, respectively.
From a synthetic point of view, 6,60-dibromo-BINOL (8)
is much more interesting than the 6-bromonaphthol, since it
is widely used as precursor in the synthesis of both 6,60-
disubstituted chiral ligands,9 and several helicates with dif-
ferent cavity sizes and chemical properties.10 The above
aspects, conjugated with the interest of our group in the
The photochemistry of 6,60-dibromo-BINOL (BINOL =
2,20-dihydroxy-1,10-binaphthyl) under mild basic condi-
tions and its methyl and triisopropylsilyl ethers has been
investigated in neat and aqueous acetonitrile through
product distribution analysis and laser flash photolysis.
Arylation and alkylation have been successfully achieved
in the presence of allyltrimethylsilane, ethyl vinyl ether,
pyrrole, pyridine, thiophene, benzene, and indole. Such a
photoreactivity offers a metal and protecting group
free synthetic protocol toward mono- and disubstituted
6-aryl/alkyl BINOLs, since the BINOL chirality is pre-
served in the photoactivation process.
The activation of carbon-halogen bonds in aryl halides,
by photoexcitation, has been studied for decades by classic
product distribution analysis1 and fast kinetic techniques,
such as laser flash photolysis (LFP), detecting the generated
transient species by means of UV/vis.2 The mechanistic
investigations propelled the exploitation of such a reactivity
for synthetic applications3 and photodegradation of organic
(4) (a) Sykora, J.; Pado, M.; Tatarko, M.; Izakovic, M. J. Photochem.
Photobiol. A 1997, 110, 167–175. (b) Skurlatov, Y. I.; Ernestova, L. S.;
Vichutinskaya, E. V.; Samsonov, D. P.; Semenova, I. V.; Rodko, I. Y.;
Shvidky, V. O.; Pervunina, R. I.; Kemp, T. J. J. Photochem. Photobiol. A
1997, 107, 207–213.
(5) (a) Protti, S.; Fagnoni, M.; Albini, A. Org. Biomol. Chem. 2005, 3,
2868–2871. (b) Fagnoni, M.; Albini, A. Acc. Chem. Res. 2005, 38, 713-721.
(c) Protti, S.; Fagnoni, M.; Albini, A. Angew. Chem., Int. Ed. 2005, 44, 5675–5678.
(6) Freccero, M.; Fasani, E.; Mella, M.; Menet, I.; Monti, S.; Albini, A.
Chem.;Eur. J. 2008, 14, 653–663.
(1) (a) Schutt, L.; Bunce, N. J. In CRC Handbook of Organic Photochem-
istry and Photobiology, 2nd ed.; Horspool,W. M., Lenci, F., Eds.; CRC Press:
Boca Raton, FL, 2004; Chapters 38 and 39 and references cited therein.
(b) Grimshaw, J.; De Silva, A. P. Chem. Soc. Rev. 1981, 10, 181–203. (c) Protti,
S.; Fagnoni, M.; Mella, M.; Albini, A. J. Org. Chem. 2004, 69, 3465–3473.
(2) (a) Bonnichon, F.; Richard, C.; Grabner, G. Chem. Commun. 2001,
73–74. (b) Bonnichon, F.; Grabner, G.; Richard, C.; Lavedrine, B. New J.
Chem. 2003, 27, 591–596. (c) Grabner, G.; Richard, C.; Kohler, G. J. Am.
Chem. Soc. 1994, 116, 11470–11480. (d) Bonnichon, F.; Grabner, G.; Guyot,
G.; Richard, C. J. Chem. Soc., Perkin Trans. 2 1999, 1203–1210. (e) Arnold,
B. R.; Scaiano, J. C.; Bucher, G. F.; Sander, W. J. Org. Chem. 1992, 57, 6469–
6474.
(7) Pretali, L; Doria, F.; Verga, D.; Profumo, A.; Freccero, M. J. Org.
Chem. 2009, 74, 1034–1041.
(8) Verga, D.; Doria, F.; Mella, M.; Freccero, M. J. Org. Chem. 2009, 74,
5311–5319.
(3) (a) Lazzaroni, S.; Protti, S.; Fagnoni, M.; Albini, A. Org. Lett. 2009,
11, 349–352. (b) Fagnoni, M.; Albini, A. Acc. Chem. Res. 2005, 38, 713–721.
(c) Dichiarante, V.; Fagnoni, M. Synlett 2008, 787–800.
(9) Chen, Y.; Yekta, S.; Yudin, A. K. Chem. Rev. 2003, 103, 3155–3211.
(10) Bunzen, J.; Bruhn, T.; Bringmann, G.; Lutzen, A. J. Am. Chem. Soc.
2009, 131, 3621–3630.
DOI: 10.1021/jo100349h
r
Published on Web 04/02/2010
J. Org. Chem. 2010, 75, 3477–3480 3477
2010 American Chemical Society

Verga et al.
JOCNote
SCHEME 2
SCHEME 3. Photoreactivity of 6,60-Dibromo-BINOL (8) in the
Presence of Arenes and Alkenes
photoreactivity of BINOL-derivatives,11-13 prompted us to
explore the photochemistry of 6,60-dibromo-BINOL (8) and
the methyl and silyl ethers 9 and 10 (Scheme 2) in the presence
of alkenes and heterocycles, to assess whether the reactivity
of the resulting transient carbene may be exploited for the
synthesis of functionalized BINOLs.
The photoreactivity of 6,60-dibromo-BINOL (8) was first
explored in 1 ꢀ 10-3 M solution by irradiation with four
lamps (15 W) centered at 310 nm. The reaction was investi-
gated in cyclohexane, 2-propanol (IPA), acetonitrile (ACN),
and aqueous acetonitrile 1:1 NaPi buffer (phosphate) at pH
8. In cyclohexane a very sluggish mixture was recovered both
in the absence and in the presence of oxygen with the
formation of uncharacterized oligomer products. The main
isolated product was BINOL (7), in a very low yield (10%), in
aqueous ACN without oxygen. No photoreactivity was
detected in neat ACN. 7 became the only product through
a clean and quantitative reductive dehalogenation process
in IPA.
We next investigated the photoreactivity of 8 in the pre-
sence of both an equimolar amount of triethylamine (TEA)
and several arenes and alkenes in large excess (5 ꢀ 10-2 M) in
ACN solution. The choice of using TEA had been suggested
by our previous studies on 6-bromonaphthols (5a-d),6,7
where we observed an important improvement of the photo-
product yields, using a diluted solution of the amine. In
addition, the presence of TEA induces a red shift in the
UV-vis spectra of 8, allowing us also to explore the reactiv-
ity at 360 nm. The irradiation at 360 nm in the presence of
pyrrole, thiophene, 1H-indole, ethyl vinyl ether (EVE),
allyltrimethylsilane, pyridine, and benzene in ACN solution
with the addition of TEA gave reaction mixtures containing
three types of adducts: (i) 6,60-disubstituted BINOLs (11a,
14a, 15a), (ii) 6-monosubstituted BINOLs (11b-15b, 17b)
(Scheme 3), and (iii) BINOL (7). The irradiation of 8 in the
presence of morpholine afforded only 7, resulting from a
TABLE 1. Product Distribution from the Irradiation of 8 in Acetoni-
trile, neat Benzene, and Pyridinea
HNub
consumption (%)
products (% yield)c
pyrrole
thiophene
1H-indole
ethyl vinyl ether
allyltrimethylsilane
pyridine
100
80
80
60
100
25d
98e
70f
30g
57h
100
100i
7 (5), 11a (38), 11b (55)
7 (32), 12b (35)
7 (23), 13b (49)
7 (25), 14a (6), 14b (18)
7 (18), 15a (22), 15b (59)
16c (25)
16c (95)
7 (70)
7 (5), 17b (24)
7 (15), 17b (42)
7 (95)
benzene
morpholine
IPA
7 (95)
aIrradiation time 1 h, at 360 nm, T=30°C. [8] =1ꢀ 10-3 M. b[HNu] =
5 ꢀ 10-2 M. cThe product yields have been determined by HPLC.
dIn anhydrous pyridine without NEt3. eIn anhydrous pyridine with-
f
out NEt3, after 12 h of irradiation. In neat acetonitrile with pyridine
i
and NEt3. gIn neat benzene. hIn neat acetonitrile with 1 M benzene. In
neat IPA.
clean reductive dehalogenation process. The methyl ether 9
and the triisopropylsilyl (TIPS) ether 10 were both stable
upon irradiation at 310 and 360 nm after 5 h.
The product yields and reactant consumption irradiating 8
in neat benzene, pyridine, and acetonitrile in the presence of
π- and n-nucleophiles are listed in Table 1. Morpholine and
TEA, similarly to IPA, act as H-donor rather than nucleo-
philes, affording almost a quantitative conversion to 7,
through a clean reductive dehalogenation process. π-Nucle-
philes undergo an arylation/alkylation process, producing a
mixture of mono- and bis-arylated/alkylated BINOLs, with
the former product being always the major one. The irradia-
tion in ACN in the presence of anhydrous pyridine and TEA,
used as base, yields 7. The photoreaction becomes less
efficient by using pyridine without TEA, since it requires
longer irradiation time to generate the monoarylated-mono-
bromo BINOL 16c, but overall the photoreaction becomes
a cleaner process, free from the reductive dehalogenation.
(11) Colloredo-Mels, S.; Doria, F.; Verga, D.; Freccero, M. J. Org. Chem.
2006, 71, 3889–3895.
(12) Richter, S. N.; Maggi, S.; Colloredo Mels, S.; Palumbo, M.; Freccero,
M. J. Am. Chem. Soc. 2004, 126, 13973–13979.
(13) Doria, F.; Richter, S. N.; Nadai, M.; Colloredo-Mels, S.; Mella, M.;
Palumbo, M.; Freccero, M. J. Med. Chem. 2007, 50, 6570–6579.
3478 J. Org. Chem. Vol. 75, No. 10, 2010

Verga et al.
JOCNote
TABLE 2. Product Distribution from the Irradiation of 8 in MeCN:
NaPi Buffer pH 8 (1:1)a
HNub
consumption (%)
products (% yield)c
pyrrole
thiophene
1H-indole
ethyl vinyl ether
allyltrimethylsilane
98
80
75
50
99
7 (2), 11a (62), 11b (33)
7 (8), 12b (69)
7 (4), 13b (70)
7 (21), 14a (10), 14b (19)
7 (9), 15a (47), 15b (42)
aIrradiation time 1.5 h, at 360 nm, T = 30 °C. [8] = 1 ꢀ 10-3 M.
c
b[HNu] = 5 ꢀ 10-2 M. The product yields have been determined by
HPLC.
In fact, when pyridine is used instead of TEA, no H atom
donor is present in solution and no dehalogenation mecha-
nism is observed (Table 1). The product distribution appears
to be similar to that arising from the photoreactivity of
6-bromo-2-naphthols (5a-d)7 and 1,6-dibromo-2-naphthol
derivatives (6a-d).8
FIGURE 1. Transient absorption spectra of 8 in acetonitrile:
H2O = 1:1 with Et3N (5 ꢀ 10-3 M) and O2. The spectra were taken
immediately after the laser pulse (red line), and 0.04, 0.10, and
0.15 μs after the laser pulse, respectively (black lines). The inset
shows the decay traces monitored at 470 and 560 nm.
In these photoreactions the presence of a base is always
required in order to trap the generated acidity (HBr) and
improve the conversion. Unfortunately the presence of TEA
makes more efficient the competitive photodehalogenation
process due to its H-donor character. Therefore, in order to
minimize the reductive dehalogenation process and to im-
prove the efficiency of the arylation/alkylation pathways, we
decided to run the irradiation in buffered aqueous acetoni-
trile (H2O:MeCN = 1:1; NaPi buffer, pH 8). Product yields
and conversions, listed in Table 2, suggest that the selectivity
of the arylation/alkylation vs the reductive dehalogenation
process is improved under mild basic aqueous solution.
To clarify the mechanism, we run the reaction at low
conversion of the reactant 8 (<20%), monitoring the photo-
arylation process by time-dependent product distribution
analysis in ACN in the presence of TEA. The irradiation of 8
(in a merry-go-round photoreactor, with 4 lamps at 15 W,
360 nm; ca. 25 °C, and Ar purged) in the presence of
pyrrole after 10 min afforded a crude containing the bis-
substituted adduct 11a (4%), the monosubstituted derivative
11b (10%), and the monosubstituted-monobromo 11c (18%)
together with unreacted starting material (65%). The photo-
product 11c reaches its maximum after 10 min of irradiation.
Longer irradiation time causes a decrease of 11c and a
parallel rising of both 11a and 11b. The irradiation of 8
in the presence of thiophene after 10 min afforded a crude
containing the monosubstituted derivative 12b (15%)
and 12c (14%) together with unreacted starting material
(69%).
To evaluate the potential application of this photoaryla-
tion/alkylation reaction for synthetic purposes, we had to
asses the retention of the BINOL configuration in the
photoarylation process. Therefore, we decided to investigate
the photochemistry of the purified chiral (R)-8. We irra-
diated (R)-8 at 360 nm both in the presence of pyrrole and
TEA in ACN and in a 1:1 mixture ACN/water (buffered, pH
8), affording the chiral adduct (R)-11b (55%, 33% yields,
respectively), with complete retention of the binol moiety
configuration. The configuration assignment of (R)-11b has
been achieved by chiral HPLC chromatography, comparing
the crude arising from the irradiation of (R)-8 to the crude of
the same reaction run on racemic 8 (Supporting Information,
Figure S1).
SCHEME 4
Nd:YAG laser), in neat acetonitrile and aqueous acetonitrile
(pH 8, buffered conditions), yielded an intense transient
absorbance centered at λmax = 460 nm (Figure 1), which
was efficiently quenched by O2.
The above transient has been attributed to the triplet
excited state of 8 (38*) taking into account the spectroscopic
similarity of the triplet state of 5a (35a*, λmax 460 nm).7
Although very similar transients have been recorded also for
the methyl and TIPS ethers 9 and 10, respectively, their
lifetimes were not affected by TEA addition. The decay
profile of such a transient absorbance follows mainly a
second order kinetic, in the absence of O2. The decay
3
trace of 8* became a single exponential upon addition of
TEA, in the concentration range 3 ꢀ 10-4 to 5 ꢀ 10-3
M
[k2 = 3((0.5) ꢀ 108 M-1 s-1, which was slightly faster than
the quenching of 35a*, k2 = 9((1.0) ꢀ 107 M-1 s-1],7 where
in the presence of O2 a second transient with a broad
absorbance centered at 560 nm was detected (Figure 1).
The latter is almost superimposable to the absorbance
recently assigned to the carbonyl oxide 5-CO (λ 580 nm,
Scheme 2). Due to the striking similarities of the transient
species detected and the similar reactivity of 8 in comparison
to 5a, we believe that the transient with absorbance centered
at 560 nm has to be assigned to the carbonyl oxide 8CO,
resulting from the trapping of the triplet carbene by mole-
cular oxygen (Scheme 4). Therefore, the synthesis of the 6,
60-disubstituted (11a, 14a, 15a) and 6-monosubstituted
BINOLs (11b-15b, 17b) achieved by irradiation can be
rationalized as a sequential photogeneration of the two
BINOL-carbenes 8C and 8C0 (Scheme 4).
The photoactivation processes of 8 has also been studied
by LFP. Flashing a solution of 8, at 266 nm (4 mJ/pulse,
J. Org. Chem. Vol. 75, No. 10, 2010 3479

Verga et al.
JOCNote
In summary, we have described the arylation of alkenes
and arenes by 6,60-dibromo-BINOL using a mild (360 nm)
photochemical protocol in acetonitrile and in aqueous
buffered solution (pH 8). Mono- and bis-functionalized
BINOLs at 6,60 positions were achieved with good convers-
ions. The reaction yields were improved under aqueous
condition, since the competing reductive dehalogenation
process became much less efficient. The bis-arylated/alkylated
adducts were the main photoproducts in the presence of
electron rich π-nucleophiles, such as pyrrole and trimethyl-
allylsilane. Since it has been shown that the configuration of
the BINOL moiety is preserved in the photoactivation
process, the described photoreactivity represents a metal
and protecting group free synthetic protocol, toward
mono- and disubstituted 6-aryl/alkyl BINOLs, via a photo-
generated electrophilic triplet carbene intermediate.
7.30-7.50 (m, 5H), 7.85-8.05 (m, 4H), 8.55 (br s, 1H). 13C
NMR (CDCl3) δ 106.2, 110.2, 110.6, 111.0, 117.7, 118.2, 119.0,
121.8, 124.0, 124.1, 124.6, 124.8, 127.4, 128.3, 128.6, 129.3,
129.7, 131.0, 131.4, 131.79, 131.84, 133.2, 152.3, 152.6. Anal.
Calcd for C24H17NO2: C, 82.03; H, 4.88; N, 3.99; O, 9.11.
Found: C, 81.91; H, 4.90; N, 3.93.
General Procedure for the Irradiation of 8 in the Presence of
Alkenes in Aqueous ACN. An argon-purged solution of 8
(130 mg, 0.29 mmol), freshly distilled allyltrimethylsilane
(1.71 g, 15 mmol), and NaPi Buffer pH 8 (2 ꢀ 10-3 M) in
300 mL of aqueous ACN (1:1) was irradiated for 1.5 h by using
argon-purged solutions in Pyrex tubes (20 mL) and a multilamp
reactor fitted with four 15 W lamps, with maximum emission
centered at 360 nm. After ACN evaporation, the remaining
aqueous suspension was extracted 5 times with CH2Cl2. Chro-
matographic separation (cyclohexane:ethyl acetate = 7:3), fol-
lowing CH2Cl2 removal by vacuum concentration gave 50 mg of
15a (47%, yield) and 40 mg of 15b (42%, yield).
6,60-Diallyl-[1,10]-binaphthalenyl-2,20-diol (15a): pale yellow
1
Experimental Section
solid; Mp >185 °C dec; H NMR (CDCl3) δ 3.50 (d, J = 6.5
Hz, 4H), 5.00 (s, 2H), 5.05-5.20 (m, 4H), 5.95-6.15 (m, 2H),
7.15 (dd, J = 5.0, 1.5 Hz, 2H), 7.20 (d, J = 5.0 Hz, 2H), 7.35 (d,
J = 8.5 Hz, 2H), 7.70 (s, 2H), 7.90 (d, J = 10.0 Hz, 2H); 13C
NMR (CDCl3) δ 39.9, 110.7, 116.0, 117.7, 124.2, 127.1, 128.8,
129.5, 130.8, 131.8, 135.6, 137.1, 152.2. Anal. Calcd for
C26H22O2: C, 85.22; H, 6.05; O, 8.73. Found: C, 85.46; H, 6.12.
6-Allyl-[1,10]-binaphthyl-2,20-diol (15b): pale yellow solid; mp
130-132 °C; 1H NMR (CDCl3) δ 3.50 (d, J = 6.5 Hz, 2H), 5.00
(s, 1H), 5.05 (s, 1H), 5.10-5.20 (m, 2H), 5.90-6.20 (m, 1H),
7.05-7.25 (m, 2H), 7.30-7.45 (m, 4H), 7.70 (s, 2H), 7.85-8.05
(m, 3H); 13C NMR (CDCl3) δ 39.9, 110.6, 110.8, 116.0, 117.6,
117.7, 123.9, 124.1, 124.2, 127.1, 127.4, 128.4, 128.9, 129.3,
129.5, 130.9, 131.3, 131.8, 133.3, 135.7, 137.1, 152.2, 152.6.
Anal. Calcd for C23H18O2: C, 84.64; H, 5.56; O, 9.80. Found:
C, 84.64; H, 5.60.
8-10 have been synthesized via standard synthetic proce-
dures already published.14-16
General Procedure for the Irradiation of 8 in the Presence of
Heteroaromatics in ACN. An argon-purged solution of 8
(133 mg, 0.3 mmol), freshly distilled pyrrole (1.01 g, 15 mmol),
and Et3N (121 mg, 0.6 mmol) in 300 mL of ACN was irradiated
for 1 h by using argon-purged solutions in Pyrex tubes (20 mL)
and a multilamp reactor fitted with four 15 W lamps, with
maximum emission centered at 360 nm. Chromatographic
separation (cyclohexane:ethyl acetate = 7:3) following solvent
removal by vacuum concentration gave 47 mg of 11a (38%,
yield) and 61 mg of 11b (55%, yield).
6,60-Bis(1H-pyrrol-2-yl)-[1,10]-binaphthalenyl-2,20-diol (11a):
1
pale green solid; mp >180 °C dec; H NMR (CDCl3) δ 5.50
(br s, 1H), 6.30-6.35 (m, 2H), 6.55-6.60 (m, 2H), 6.85-6.90 (m,
2H), 7.15-7.25 (m, 2H), 7.35-7.55 (m, 4H), 7.85-8.05 (m, 4H),
8.55 (br s, 1H). 13C NMR (CDCl3) δ 106.3, 110.2, 118.2, 119.0,
121.8, 124.0, 124.7, 124.8, 127.5, 128.6, 129.7, 131.1, 131.7,
152.3. Anal. Calcd for C28H20N2O2: C, 80.75; H, 4.84; N,
6.73; O, 7.68. Found: C, 80.59; H, 4.92; N, 6.70.
Acknowledgment. Financial support from the Italian Min-
istry of University and Research (MIUR, FIRB-Ideas Project
RBID082ATK_003) and the Italian Association for Cancer
Research (Associazione Italiana per la Ricerca sul Cancro, or
AIRC Grant No. 5826) is gratefully acknowledged.
6-(1H-Pyrrol-2-yl)-[1,10]-binaphthalenyl-2,20-diol (11b): pale
green solid; mp >210 °C dec; 1H NMR (CDCl3) δ 5.05 (s,
1H), 6.30-6.35 (m, 1H), 6.85-6.94 (m, 1H), 7.10-7.25 (m, 2H),
Supporting Information Available: General Experimental
Methods and characterization data for 11c, 12b,c, 13b, 14a,b,
16c, and 17b and 1H NMR and 13C NMR spectra for the
adducts 11a-c 12b,c 13b, 14a,b 15a,b 16c, and 17b. This material
is available free of charge via the Internet at http://pubs.acs.org.
(14) Yao Sogah, G. D.; Cram, D. J. J. Am. Chem. Soc. 1979, 101, 3035–
3042.
(15) Groves, J. T.; Viski, P. J. Org. Chem. 1990, 55, 3628–3664.
(16) Sellner, H.; Faber, C.; Rheiner, P. B.; Seebach, D. Chem.;Eur. J.
2000, 6, 3692–3705.
3480 J. Org. Chem. Vol. 75, No. 10, 2010