Chemistry Reference
In-Depth Information
O
2
O
O
O
H
3
C
CH
2
H
3
C
CH
3
H
3
C
CH
3
N
NN
N
N
N
N
N
NN
N
+
2
H
Ph
Ph
Ph
51
52
53
x2
O
Ph
H
3
C
N
NN
N
N
N
N
N
CH
3
Ph
O
54
Scheme 7.20
O
O
H
3
C
CH
3
H
3
C
O
2
N
NN
N
N
NN
N
+
R
Ph
R
Ph
51
55
Scheme 7.21
This reaction lies at the heart of the lack of long-term stability of many 1,5-dimethyl-6-oxoverdazyls.
Interestingly, the recently synthesized 1,5-diisopropyl 6-oxoverdazyl analogues (Scheme 7.8) appear to be
indefinitely stable with respect to disproportionation even though they still possess an
-CH proton.
46
Georges has harnessed this disproportionation reaction of 1,5-dimethyl-6-oxoverdazyls for synthetic
purposes. Alkenes serve as effective trapping agents for the azomethine imine intermediates
53
, leading to
bicyclic structures
55
viaa3
α
2 cycloaddition; running the reactions in the presence of oxygen significantly
improves the yields of
55
by converting the leucoverdazyl byproduct
52
back to a radical, whereupon the
radical can react with more alkene (Scheme 7.21).
112
+
7.2.3.4 Radical coupling reactions of verdazyls
The reactions of verdazyls with Grignards, organolithiums, and so on (Scheme 7.17) includes a coupling
reaction between a verdazyl radical and an alkyl radical, the latter of which is generated by electron transfer
from the initial alkyl anion. Alkylated verdazyls can be made by treatment of the corresponding leuco
compound with an alkyl halide and base,
19,42
but verdazyls also react cleanly with carbon-centered radicals
to give tetrazine compounds of general structure
49
(cf. Scheme 7.17). For example, heating verdazyls
in the presence of AIBN (2,2-azobisisobutyronitrile) generates the 2-cyanopropyl-substituted tetrazines
56
- which can be reverted back to radicals in high yield upon heating to 150
◦
C (Scheme 7.22).
42,83,113
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