Chemistry Reference
In-Depth Information
9
10
·
·
R
R
R
R
3
BAC
:
R
=
H
BAC-Ph
2
:
R
=
Ph
BAC-b: R = H
BAC-Ph
2
-b: R = Ph
(BAC)
3
(a)
(b)
(c)
2
N
2
O
(BAC-Ph
2
)
2
BAC-Ph
2
(N
2
)
BAC-Ph
2
(O)
(d)
(e)
(f)
Figure 3.64
(a) Chemical structures of bis(9-anthryl)carbenes,
BAC
(lifetime 0
s) and
BAC-Ph
2
(half-
life 19 minutes). (b) Diradical type resonance structures,
BAC-b
and
BAC-Ph
2
-b
, with allene-bridged
bis(triphenylmethyl) diradical structures. (c) Chemical structures of carbene trimer, (
BAC)
3
,and(d)ofca -
benicdimer,
(BAC-Ph
2
)
2
.(e)Chemicalstructuresof
BAC-Ph
2
(N
2
)
astheprecursorof
BAC-Ph
2
and(f)ofketone
BAC-Ph
2
(O)
generated by irradiationof
BAC-Ph
2
(N
2
)
in the presence of oxygen. The phenyl groups effectively
preventthespinsfrom''leakingout''andthussuppressthereactionat10-positions.
.
5
µ
and two bromine groups in unsymmetrical and symmetrical fashions were designed and generated
94b
(Figures 3.63b,c).
Spin delocalization from the carbenic center in triplet carbenes was investigated in bis(9-anthryl)carbene
BAC
by Wasserman using EPR spectroscopy, elucidating a possible structure with a collinear divalent
carbon (allenic carbon) and perpendicularly arranged anthryl groups (Figures 3.64a,b).
98
These electronic
effects are expected to provide a thermodynamic stability in addition to the kinetic stability through
shielding the carbenic center with four
peri
-protons of the anthryl groups. In spite of these favorable stere-
oelectronic effects,
BAC
possesses low stability
99
with a lifetime of 0
s.
100
This is because
BAC
tends to
form a trimer
(BAC)
3
101
by a contribution of the diradical resonance structure
BAC-b
(Figures 3.64b,c). In
2001, Tomioka reported a bis(9-anthryl)carbene derivative with phenyl groups,
BAC-Ph
2
, showing a half-
life of 19 minutes in a solution state at room temperature by suppression of the trimerization reaction.
100
A
resonance structure
BAC-Ph
2
-b
can be regarded as a bis(triphenylmethyl radical) connected by an allenic
bond (Figure 3.64b). The contributions of the carbene and the diradical structures for
BAC-Ph
2
are esti-
mated by chemical reactivity with help of EPR spectroscopy.
BAC-Ph
2
forms a carbene dimer
(BAC-Ph
2
)
2
by double bond formation at the carbenic center (Figure 3.64d). Furthermore, irradiation of
BAC-Ph
2
(N
2
)
,
a precursor of
BAC-Ph
2
, in the presence of oxygen gives a ketone
BAC-Ph
2
(O)
(Figures 3.64e,f). Con-
sequently, these experimental results suggest that the electronic contribution of the carbenic character is
more important than that of the diradical one, although the spins are still extensively delocalized.
Tomioka's successful stabilization method for the triplet carbene enabled the design and synthesis of
stable high spin polycarbenes in ambient conditions (Figure 3.65).
102,103
Tris
BAC
, tricarbene system,
was designed based on bis(9-anthryl)carbene, and was generated in 2-methyltetrahydrofuran at 77 K with
.
5
µ
Search WWH ::
Custom Search