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Competitive cyclisation of singlet and triplet nitrenes. Part II. Cyclisation of 2-nitrenophenyl- thiophens, -benzothiazoles and -benzimidazoles.
TetrahedronLatter8
HO. 12, pp 929
COMPETITIVE
- 932,
1976.
Pergtmon
w338.
Printed in Great Britain.
CXCLISATION OF SINGLET AND TRIPLET NITRENES.
CYCLISATION OF I-NITRBNOPHENYL-
THIOPHENS, -BENZOTHIAZOLES
By Ian M. McRobbre, Otto Meth-Cohn*,
PART II
AND -BENZIMIDAZOLES.
and Hans Suschrtzky
Department of Chennstry and Applred Chemrstry,
The Ramage LaboratorIes,
Unrversrty of Salford, Salford M5 4WT, Lancashrre. (Rmceived in UK 12 Juupry 1976; The
competrtron between srnglet and trrplet pathways In nltrene cyclrsatlons
demonstrated
in a preceedlng paper'.
on alternatrve
Surprisingly, an arylazzde, 2
.
by Its proneness
The electrophrllc
only one report has yet appeared of the Interactron 1 e. the cycllsatlon of 2-azrdothlobensophenone
the tetrahydro-2-phenylrndazole,
solution
that the longer wavelength
1
In
products)suggests
to the trlplet azrde
by the trrplet acetophenone
(6) no trace of (10) or any other srnglet derived product that strongly favoured the srnglet pathway
(ll), analogues of the benzothrazoles
(Scheme 2)
(l), have the potential
srnglet (attack at -N=) and triplet (attack at -CHR2) Intramolecular
phenone of the aside (11,
photolysls
gave solely trrplet-derrved
(1)) via photosensltlsatlon
could be detected even under condltlons
with the bensothrazoles,
Thrs gave unexpectedly
light excites the conIugated azlde drrectly leadrng to some slnglet
In the case of the thlophen
The benzrmrdazoles
(2) from the
lendlng some support to the o-qurnonold nature of
Thus minor process is In competrtron with the excrtatron
(and hence trlplet nrtrene
rrradratlon
The
srnce no product In
wrth Raney nrckel
(2), even U-I the triplet-sensrtlsed
(In whrch the pyrazoles
wrth
by a concerted non-nrtrene
The structure of the maJor product
rncrdentally
The formatIon of thus product
(1)
of sulphur nucleophiles
for nitrogen nucleophrles
thermal reactIons followed also from desulphurrsatlon
nltrene
1
We have exarmned two systems (1 and 6) where such InteractIon 1s possrble
either case derrved from attack at sulphur
acetophenone
character of a singlet nrtrene 1s
to attack by for example nitrogen nucleophlles
srnglet nrtrene clearly has strong preference
(2)
has been
We now wish to report further examples and also to comment
srnglet sites of reactron.
well demonstrated
mechanrsm
accepted for publmatron 16 fibruary 1976)
pathways
for both
(Scheme 3).
the srnglet reaction 1s hrghly effrclent and photolvsis
As
rn aceto-
R=H) agarn showed 'leakage' yleldlng some slnglet product by direct
Only with the Isopropyl derrvative
only under total 'trlplet-control'
(11,
R=Me) was CH-rnsertlon
by use of photosensrtrsatlon
929
observed and then
rn acetophenone.
Ho. 12
955
Scheme 1
Condltlons
a Some
1
Products
(X)
(21
(3)
(4)
-N02/lXP/cmene
54
4
0
13.5
N3fAlWl?e~~a
79
1
0
79 0
N3/hv/CH2C12
53
7
8
35
N3fhvfPhCOCH3=
19
4
34
05
(nltrene source/reagent/solvent)
blcumyl
also
Isolated,
b
smglet
stablllser,
c
tnplet
Scheme 2
MeQ -+Mem (7)
(8)
NH1
S/T
sensltlser
I
931
No. 12
Scheme 2 continued
Products
Condltlons (reagent/solvent) A/decalln hv/CH2C12 a h"/CH2Cl/
+ pyrene
b
hv/acetophenone
a Slnglet
(%)
(7)
(8)
(9)
35
40
0
0
trace
62
0
0
17
0
46
46
b stabrllser,
trlplet quencher.
Scheme 3
Y
slngl
Y-N=N-y
Y-N H, (14)
Condltlons R
H
Source
N02
Products Solvent
(12)
(13)
(14)
(15)
TEP
Cumene
83
0
trace
0
96
0
4
0
15
0
26
8
66
0
14
14
0
59
15
0
H
N3
A
H
N3
hv
PhCOMea
Me
N3 N3
(X)
Reagent
Cumene
Me
(15)
b
A
C6H5Br
hv
PhCOMea
a Triplet sensltlser,
b
trlplet promotor - heavy atom effect
932
AO. 12
The powerful demonstrated deactivates
'substltuent control' of the nltrene reactlon
by the thermolysxs
outlmed
ln Scheme 4.
(see ref
1) 1s further
The p-dmethylammo-group
totally
the slnglet state and hence no attack at N 1s observed while trlplet CH-InsertIon Thus, the ease of trlplet lnsertlon Into CH bonds follows
occurs even Into a methyl group
the expected order (CH > CH3) and It seems that all the lnsertlons In this paper are of higher energy than those we have observed before' photosensltlsatlon
m
acetophenone
produced
energetic
to Insert Into 'actxve' methyl groups or
tertiary centres but otherwIse gives rise to azo-compound
trlplet nltrenes, being vlbratmnally
attack of unactlvated
CH bonds
generated by
1s seen throughout our studies to be a low energy species.
This 'lazy triplet' 1s only sufflcrently favourable
Indeed, the trlplet mtrene
Thermally
formatmn
more excited, have greater potential
These characterlstlcs
for
are under active mvestlgat1on
References
1.
I.M. McRobble, 0
2.
J
Meth-Cohn
and H, Suschltzky, preceedlng paper, Part I
Ashby and H. Suschltzky, Tetrahedron
Letters, 1971, 1315.
We thank SRC for a grant to I M.McR.
Scheme 4
+
(16)
Y-NH, (19)
(18) (36X)
+
(15%)
Y-N=N-Y (20)
(trace)
HO. 12, pp 929
COMPETITIVE
- 932,
1976.
Pergtmon
w338.
Printed in Great Britain.
CXCLISATION OF SINGLET AND TRIPLET NITRENES.
CYCLISATION OF I-NITRBNOPHENYL-
THIOPHENS, -BENZOTHIAZOLES
By Ian M. McRobbre, Otto Meth-Cohn*,
PART II
AND -BENZIMIDAZOLES.
and Hans Suschrtzky
Department of Chennstry and Applred Chemrstry,
The Ramage LaboratorIes,
Unrversrty of Salford, Salford M5 4WT, Lancashrre. (Rmceived in UK 12 Juupry 1976; The
competrtron between srnglet and trrplet pathways In nltrene cyclrsatlons
demonstrated
in a preceedlng paper'.
on alternatrve
Surprisingly, an arylazzde, 2
.
by Its proneness
The electrophrllc
only one report has yet appeared of the Interactron 1 e. the cycllsatlon of 2-azrdothlobensophenone
the tetrahydro-2-phenylrndazole,
solution
that the longer wavelength
1
In
products)suggests
to the trlplet azrde
by the trrplet acetophenone
(6) no trace of (10) or any other srnglet derived product that strongly favoured the srnglet pathway
(ll), analogues of the benzothrazoles
(Scheme 2)
(l), have the potential
srnglet (attack at -N=) and triplet (attack at -CHR2) Intramolecular
phenone of the aside (11,
photolysls
gave solely trrplet-derrved
(1)) via photosensltlsatlon
could be detected even under condltlons
with the bensothrazoles,
Thrs gave unexpectedly
light excites the conIugated azlde drrectly leadrng to some slnglet
In the case of the thlophen
The benzrmrdazoles
(2) from the
lendlng some support to the o-qurnonold nature of
Thus minor process is In competrtron with the excrtatron
(and hence trlplet nrtrene
rrradratlon
The
srnce no product In
wrth Raney nrckel
(2), even U-I the triplet-sensrtlsed
(In whrch the pyrazoles
wrth
by a concerted non-nrtrene
The structure of the maJor product
rncrdentally
The formatIon of thus product
(1)
of sulphur nucleophiles
for nitrogen nucleophrles
thermal reactIons followed also from desulphurrsatlon
nltrene
1
We have exarmned two systems (1 and 6) where such InteractIon 1s possrble
either case derrved from attack at sulphur
acetophenone
character of a singlet nrtrene 1s
to attack by for example nitrogen nucleophlles
srnglet nrtrene clearly has strong preference
(2)
has been
We now wish to report further examples and also to comment
srnglet sites of reactron.
well demonstrated
mechanrsm
accepted for publmatron 16 fibruary 1976)
pathways
for both
(Scheme 3).
the srnglet reaction 1s hrghly effrclent and photolvsis
As
rn aceto-
R=H) agarn showed 'leakage' yleldlng some slnglet product by direct
Only with the Isopropyl derrvative
only under total 'trlplet-control'
(11,
R=Me) was CH-rnsertlon
by use of photosensrtrsatlon
929
observed and then
rn acetophenone.
Ho. 12
955
Scheme 1
Condltlons
a Some
1
Products
(X)
(21
(3)
(4)
-N02/lXP/cmene
54
4
0
13.5
N3fAlWl?e~~a
79
1
0
79 0
N3/hv/CH2C12
53
7
8
35
N3fhvfPhCOCH3=
19
4
34
05
(nltrene source/reagent/solvent)
blcumyl
also
Isolated,
b
smglet
stablllser,
c
tnplet
Scheme 2
MeQ -+Mem (7)
(8)
NH1
S/T
sensltlser
I
931
No. 12
Scheme 2 continued
Products
Condltlons (reagent/solvent) A/decalln hv/CH2C12 a h"/CH2Cl/
+ pyrene
b
hv/acetophenone
a Slnglet
(%)
(7)
(8)
(9)
35
40
0
0
trace
62
0
0
17
0
46
46
b stabrllser,
trlplet quencher.
Scheme 3
Y
slngl
Y-N=N-y
Y-N H, (14)
Condltlons R
H
Source
N02
Products Solvent
(12)
(13)
(14)
(15)
TEP
Cumene
83
0
trace
0
96
0
4
0
15
0
26
8
66
0
14
14
0
59
15
0
H
N3
A
H
N3
hv
PhCOMea
Me
N3 N3
(X)
Reagent
Cumene
Me
(15)
b
A
C6H5Br
hv
PhCOMea
a Triplet sensltlser,
b
trlplet promotor - heavy atom effect
932
AO. 12
The powerful demonstrated deactivates
'substltuent control' of the nltrene reactlon
by the thermolysxs
outlmed
ln Scheme 4.
(see ref
1) 1s further
The p-dmethylammo-group
totally
the slnglet state and hence no attack at N 1s observed while trlplet CH-InsertIon Thus, the ease of trlplet lnsertlon Into CH bonds follows
occurs even Into a methyl group
the expected order (CH > CH3) and It seems that all the lnsertlons In this paper are of higher energy than those we have observed before' photosensltlsatlon
m
acetophenone
produced
energetic
to Insert Into 'actxve' methyl groups or
tertiary centres but otherwIse gives rise to azo-compound
trlplet nltrenes, being vlbratmnally
attack of unactlvated
CH bonds
generated by
1s seen throughout our studies to be a low energy species.
This 'lazy triplet' 1s only sufflcrently favourable
Indeed, the trlplet mtrene
Thermally
formatmn
more excited, have greater potential
These characterlstlcs
for
are under active mvestlgat1on
References
1.
I.M. McRobble, 0
2.
J
Meth-Cohn
and H, Suschltzky, preceedlng paper, Part I
Ashby and H. Suschltzky, Tetrahedron
Letters, 1971, 1315.
We thank SRC for a grant to I M.McR.
Scheme 4
+
(16)
Y-NH, (19)
(18) (36X)
+
(15%)
Y-N=N-Y (20)
(trace)