C1-Symmetric BINOLs
COMMUNICATION
À
expected to afford large quantities of the homocoupled
products BINOL and 5. When the identical oxidative cou-
pling reaction showed in Scheme 1 was performed with
CuCl2, no reaction occurred, and when using the
ligand occurs and a mono-NHC Cu complex is the active
species. Interestingly, when the mono-NHC complex 10 was
tested under the oxidative coupling conditions (Scheme 4),
[CuCl(OH)ACHTUNGTRENNUNG(tmeda)] complex (tmeda=N,N,N’,N’-tetrame-
thylethyldiamine), only small amounts (8–10%) of BINOL
and the homocoupled 5 were isolated. Because catalyst 3
shows a preference for a mixed coupling product, it suggests
that the oxidative coupling may be occurring through a radi-
cal-addition coupling mechanism and not the common radi-
cal–radical mechanism traditionally observed with copper
catalysis.
In an effort to try to further elucidate the exact structure
of the catalyst in the mixed coupling, we were attracted to
recent work by Nolan and co-workers, who prepared a
ACHTUNGTRENNUNG
series of [CuI(NHC)2]BF4 complexes.[15] These complexes
Scheme 4. Mono-NHC–Cu complexes do not affect oxidative coupling
under the optimized reaction conditions.
have a strong similarity to the structure of 3, observed by
mass spectrometry. Consequently, we prepared the Cu com-
plex 9 and tested its reactivity in oxidative couplings
(Scheme 3). In the homocoupling of 4a, the Cu catalyst 9
was unreactive despite the improved solubility of the latter
in MeOH (the reaction with 9 is homogenous, whereas reac-
tion using 3 is heterogenous). Upon the addition of AgNO3
as an additive, complex 9 now afforded 5 in 42% yield.
When one equivalent of AgNO3 was added, catalyst 9 af-
forded an increase in the yield of 5 (from 42 to 66%). When
a mixed coupling of 4a (1 equiv) and 6a (1 equiv) was stud-
ied with catalyst 9, the results were very similar to that ob-
served for catalyst 3. Catalyst 9 also showed a preference
for the formation of the mixed coupling product 7a in simi-
lar yields as to what was observed with 3 (48 and 55% yield,
respectively).
no coupling was observed in either the presence or absence
of silver additives. The fact that 3 and 9 provide similar reac-
tivity patterns and selectivities suggests a structural similari-
ty between the two catalysts. In addition, the failure of
simple Cu salts (CuCl2, [CuCl(OH)ACTHNUTRGNEUNG(tmeda)]) or 10, which
only possesses a single NHC ligand, to perform the oxida-
tive coupling also supports a bis-NHC-type complex for cat-
alyst 3 as the active species.
The ability of catalyst 3 to promote mixed couplings of 2-
naphthols is intriguing. Indeed, efficient mixed couplings of
this type are rare,[8,9] and would be a complementary syn-
thetic tool for the synthesis of highly functionalized bi-
naphthyls with C1 symmetry. Consequently, mixed oxidative
couplings were then investigated by using 4a or 8a and an
excess of an electron-rich 2-naphthol (Table 2). By using a
slight excess of the 2-naphthol coupling partner, the mono-
ester product 7a was isolated in 80% yield. The sterically
encumbered ester 8a derived from (1R, 2S, 5R)-menthol did
not undergo homocoupling, perhaps owing to the bulkiness
of the ester substituent. However, 8a did undergo mixed
coupling with 6a and the product 8b was isolated in a 50%
yield.
Nolan and co-workers have studied complex 9 in hydrosi-
lylation and have proposed that dissociation of an NHC
Larger phenanthrol groups could also be used in the
mixed oxidative couplings. Coupling of 4a with 3-phenan-
throl afforded the product 7b in 70% isolated yield
(Table 2, entry 2), however, the analogous coupling with 9-
phenanthrol was problematic. It appears that 9-phenanthrol
rapidly oxidizes under the reaction conditions and only a
20% yield of the product 7c could be isolated. Electronic
effects are easily recognized when examining the mixed cou-
pling of ester 4a with either the electron-poor 6-bromo-2-
naphthol or electron-rich 7-methoxy-2-naphthol. The latter
undergoes very efficient coupling to afford the mixed
BINOL 7e in 90% yield, whereas the former undergoes
sluggish coupling to afford the product 7d in only 65% iso-
lated yield. Both the 6-methyl- and the 6-phenyl-2-naphthol
undergo mixed coupling with 4a to afford their correspond-
ing mixed coupling products 7 f and 7g in 81 and 95% yield,
Scheme 3. Comparing catalyst 3 and the structurally similar 9.
Chem. Eur. J. 2009, 15, 9655 – 9659
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9657