Miscellaneous Methods

Chapter 15

Miscellaneous Methods

This chapter deals with synthetic methods for aliphatic diazo compounds that are quite useful in specific cases but do not offer general applicability. 15.1

Ring Cleavage of Heterocycles

Besides isomerizations of diazirines to diazoalkanes (see Section 1), mainly the ring opening of triazoles (see Sections 13.6.1 and 13.6.2), 3H-pyrazoles (Section 13.6.2), and 2,3-diazabicyclo[3.1.0]hex-2-enes (Section 13.4.4) are important synthetically. In the following, some mechanistically unrelated ring-opening reactions leading to diazoalkanes are discussed. Starting compounds for the synthesis of diazophenylacetonitrile (3) by cheletropic cycloreversion are the aziridines 1. "Hetero-olefination" with oxophenylacetonitrile leads to the hydrazones 2, which decompose upon heating in benzene to give rise to the a-diazonitrile 3 (7). N-N=P(C H ) 6

5

>-»=C

3

HcC 5^6 H

5^6

1

I: · C

2

6 5 H

3

When 5-acetylimino-3-benzylsydnone is hydrogenated, the benzyl group is cleaved off as toluene. The zwitterion 4 then possibly isomerizes to the oxadiazole 5, which is converted to the diazoacetamide 6 (2). However, the 534

15.1

535

Ring Cleavage of Heterocycles

reduction of 6 cannot be prevented entirely, thus giving rise to N-acetylacetamide as a by-product. H C -CH , 5

6

2

H2/CH3OH/

N„.

P d - catalyst

CH

%'^N-C

3

N-C 0

Ν

CH

3

NN >-NH-C,

N = CH 2

ring opening

CH

\

/

3

C-NH-C // \\ 0 0

0

partial

hydrogenation

HC 3

CH C-NH-C // \\ 0 0

3

The ring cleavage of 5-aryl-substituted dihydrofurandione in its reaction with α-diazocarbonyl compounds in refluxing benzene is not entirely unex­ pected: 2-diazo-l,3,5-tricarbonyl compounds are formed (3). A remarkable thiadiazole ring-opening reaction was observed when the vinylogous acid chloride 7 was treated with sodium azide. The S reaction leading to the azide 8 is followed by [1,5] ring cleavage to the α-diazothiomalonic acid derivative 9, which then undergoes ring closure to yield the diazomethylthiatriazole 10 (73%) (4). N

,C0 C H 2

2

P0 C H

5

2

2

Ν α Ν , acetone/water 3

N

5

[1.51-ring opening

3

.CO2C2H5 [1,51-cyclization

// - 3 C

S

N

}^~"\\

H C 0 C-Cj^ ^ N 10 5

2

2

s

2

9

Photolysis of the diazaheterocyclic compound 11 leads to cleavage of the pyrazoline ring and to simultaneous isomerization of the tricyclic carbon framework to 7-(2-diazo-2-phenylethyl)cycloheptatriene (12), which then, as a consequence of continued irradiation, loses N to give rise to a carbene (5). 2

15

536

Miscellaneous Methods

A combination of photochemical and thermal reactions leads to the formation of 1 -diazo-1,4-diphenyl-2-buten-4-one (15): 3,6-diphenylpyridazine N-oxide (13) isomerizes to the hypothetical oxaziridine 14, and then to 15 (diazo absorption at 2070 c m ) by a thermal reaction. Part of the diazoalkane is photolyzed to the vinylcarbene (16), which cyclizes to 2,5diphenylfuran (18); the other part undergoes thermal isomerization to the 3H-pyrazole 17, followed by aromatization (17 19) (6). - 1

11

12 HC, 5

6

thermal isomerization

H C 5

13

14

-N

15

hv

2

HC 5

6

6

16

17

[1,5]-ring closure

18

aromatization

19

C H 6

5

Also of interest in this context is the alkaline hydrolysis of iV-methoxypyridazine salts (20), leading to α,β-unsaturated diazomethanes (21), which are converted to the pyrazoles 22 upon heating (7). 4-Diazo-2-butenal (21a),

15.2

Condensation Reactions

537

4-diazo-4-phenyl-2-butenal (21b), 5-diazo-3-hexen-2-one (21c), and 5-diazo5-phenyl-3-penten-2-one (21d) have been synthesized by this route (7). Η

R

1

^-R2

C^R2

KOH/H 0,0 to 5°C 2

N

%^R2

Χ

2

Θft Φ ,

0CH

R ^ N

= < H R1

3

20

22

21(70-907.)

a:R' = R =H; 2

b: R = C H , R = Η 1

2

6

5

c:R = R =CH ; 1

d : R = C H , R = CH

2

1

3

2

6

5

3

The cyclic azine 23 isomerizes thermally to the intermediate 6-diazo-l,6diphenyl-l-hexene (24), which after N loss and formation of the carbene 25 finally produces a mixture of diastereoisomeric 1,6-diphenylbicyelo[3.1.0]hexanes 26 and 27 (70%, 2:3 ratio) (8). 2

CH 6

C H

5

AJ75 C 0

C H 6

γ^

6

5

2

-N

t H 6

5

23

6

5

C H

5

2

5

6

24

25

C H 6

[2+0

C H

intramoleculor

C H

5

6

Ό

•

26

15.2

5

Ό 27

Condensation Reactions

The formation of small yields of diazomethane in the reaction of N,Ndichloromethylamine and hydroxylamine has been known for a long time (9). It seems plausible to assume intermediates like methyldiazohydroxide or methyldiazotate. An analogous reaction with nitrosobenzene is also known (10), but there is no indication in the literature for the general applicability of this reaction type. H C-NCl 3

2

+

H N-0H-HCl 2

+

CH OH/ether, 20°C 3

3CH 0Na 3

H C=N 2

(-18 7.)

2

+

3NaCl

+

3 CH3OH

+

H 0 2

15

538

Miscellaneous Methods

The unsuccessful attempt to synthesize diisocyanogen (the isomer of dicyanogen) from chloroform and hydrazine produced diazomethane in­ stead, in 25% yield (11). Since hydrazine is methylated by diazomethane to some extent, this reaction was never employed for the general synthesis of diazoalkanes. However, the diazomethane yield can be doubled by carrying out the reaction in a two-phase system with phase-transfer catalysis by a crown ether (12). a. C H OH under N or 2

CHO3

+

H N-NH 2

+

2

5

2

b. ether, 18-crown-6-ether

3 KOH

H C=N 2

+

2

3KCI

+

3H 0 2

The conversion of dimesitylketoneimine (28) to diazodimesitylmethane (30) also seems to be rather unique (13). When 28 is nitrosated with dinitrogen tetroxide, the nitrosoimine 29 is formed and is reduced by LiAlH to 30 (13). 4

N2O4/CCU/ H C-COONa

M

\ /

Mes

L1AIH4 / ether - 30°C

\

3

C=NH

>

Mes

M

—

C=U-HO

Mes 28

e

s

\

•

/

C=N

2

Mes 29

30(53%)

HC 3

Mes = - v 3 ~

C

H

3

HC 3

Formally related to the Wittig olefin synthesis from carbonyl compounds is the preparation of diazomethane (22-25%) from methylenetriphenylphosphorane and dinitrogen monoxide (14). The mechanism of this reaction has not yet been studied, however. Since diazomethane is CH-acidic (see its metallation reactions, Section 14.2.1) and the ylide is a base, a reaction takes place to give rise to the salt 31, which may be hydrolyzed to diazomethane C H 6

C H

5

6

I

H C - P = CH 5

6

C H 6

+

2

N0 2

ether 0°C

H C -P=0 5

2

? HsCe-P-CHa 6 Η δ

C H 6

6

θ Ηϋ=Ν

Κ 0 Η / Η 2

*°

HC S

H

5

U

°

e / a )

6

- H C -P-CH /5

31

5

2

I® °

+

6

5

HC= N

5

I

— -

6

3

0

C H 6

6

H C=N 2

2

5.3

Cleavage Reactions

539

and methyltriphenylphosphonium hydroxide. The latter ultimately decomposes to benzene and methyldiphenylphosphine oxide. Acid hydrolysis of 31, on the other hand, produces isodiazomethane (see Section 1), which proves the constitution of 31 (14). 15.3

Cleavage Reactions

Purest base-free diazomethane is prepared by thermal decomposition of triphenylphosphazinomethane (32) in dioxane or toluene (15,16) and distillation of the product. However, this is not actually a preparative method but rather a method of purification for diazomethane, since 32 is formed by the reaction of triphenylphosphine and diazomethane (81%). Methylenetriphenylphosphorane, a possible by-product, has never been detected (15,16). H C 5

ether

6 s

P-

C H 6

5

+

N =CH 2

2

« A,dioxane or t o l u e n e

I

*

H C - P = N - N = CH 5

6

2

| ^6^5

32

2,2,6,6-Tetramethyl-4-triphenylphosphazino-1 -thiacyclohexane (34) decomposes to yield the diazoalkane 35 and triphenylphosphine (17). In this case, the phosphazino group is not formed by the addition of a diazo compound to the phosphine, but rather from the hydrazone 33, which is brominated and then reacted with triphenylphosphine. Photochemical, thermal, and proton-catalyzed decompositions of symmetric azine oxides 36 give rise to diazophenylmethane and diazodiphenylmethane (18). It is not possible to isolate the diazo compounds in pure form because of their subsequent reactions under the applied conditions. Upon photolysis and thermolysis, the azine 37 is formed, whereas acidic decomposition in alcohols leads to ethers of the type 38 (18). Another rather singular case is the reaction of the azine 39 with benzenesulfinate, leading to 92% bis(benzenesulfonyl)diazomethane (40) (79). It is assumed that nucleophilic substitution of all bromine atoms in 39 leads to tetra(benzenesulfonyl)azine (41), which is then cleaved (41 40). Hydrolytic work-up gives rise to 42, which supports the proposed mechanism (79).

540

15

H C 5

6 N

C H

Φ

6

Miscellaneous Methods hv,A or

5

H C 5

H®

C=N-N=C

6v

C=N C= 0

36

hv or Δ

H5C6

H^/R'-OH

H C

C6H5

5

Η

6 v

C R

X

R

37 R=H

or

Br

C H 6

0-R'

38 (R'=C2H C H9, < ^ ^ -

5

5>

4

Br

\

/

C=N-N = C

Br

N

r'

N

Br

7

39

- U NaBr

H C -S0 Na® e

5

6

H C -S0 5

6

C= N

- (H C -S0 ) Cl©Na® 5

2 v

2

6

2

3

S

2

o/

40( ·/ ) 92

H C -S0 5

6

2

S0 -C H

n

2

/

HsCe-SOz

6

β

5

C= N-N = C 7

N

S0 -C H 2

6

5

41 H 0

H C -S0

2 v

H C -S0

2

5

6

C= N-NH

2

5

6

2

42(90%)

Substituted diazoethanes like 1 -diazo- l-(4-nitrophenyl)ethane (46) are formed by solvolyses of azo compounds of the general structure 43 (20). A plausible mechanism involves solvolysis of 43 (dioxane-water) to resonancestabilized cations with partially charged carbon and nitrogen atoms (44), which convert to bridged chloronium ions (45) and finally decompose to diazoalkanes (46) (20).

5.3

541

Cleavage Reactions

CI

CI

C l - ^ ^ - C - Ν = ΝCH

C - ^ ^ - N 0 CH

3

2

3

43

dioxane/ water, 7 : 3 , 25°C

CI C l - ^ ^ - C - N=N— CH

C - ^ ^ - N 0 CH

3

2

3

I

CI*

CI C

I

C ~~

= Ν- ΝCH

C

N

0

2

C H ~~

3

3

44

II CI® CI - Q - C -

N

CH

N -

=

x

C - Q ^ - N 0

CH

3

3

45 I decomposition N0

CI

ci^M

t

Cl

ce CH

3

w

+

N =C 2

CH 46

2 HCl

H 0 2

CH

3

3

2

2

CI*

15

542

15.4

Miscellaneous Methods

Fragmentation of Heterocyclic Compounds

Upon flash pyrolysis, 3-substituted l,3,4-oxadiazolin-5-ones are frag­ mented to C 0 and nitrile-imine dipoles, which isomerize by sigmatropic substituent shift to diazo compounds. However, these are not isolable because of the rather drastic reaction conditions, and they lead to carbenes by loss of nitrogen (21-22). The heterocycle 47, for example, leads in a very selective reaction to the eneyne 51 (22), with 4-diazo-4-phenyl-l,2-butadiene (49) as the intermediate. Other intermediates in this reaction are the dipole 48 and the methylenecyclopropene 50. 2

PH -CECH 2

N-N

H Ce 5

i

^ ° 47

_

u

700°C/0005Torr ^

°

^

~

[3,3)-siqmotropic shift,

N

2

^ ^ ^ = 0 = 0 ^ 2

48

Γ ;

via carbene

H C 5

h

2

49

^

2 •

/-X

2

2

hS

"° C

HrfV-d^

·__g U

6

HsC6-C=C-C(

^

Η 51 (94%)

6

50

A photochemical fragmentation was observed for irans-3,5-diphenyl-A pyrazoline upon monochromatic irradiation, leading to diazophenylmethane and styrene (23). The thermal fragmentation of the thiatriazole 52 should be mentioned, even if there is already one diazo group in the starting material. With cleavage of the ring, ethyl cyanodiazoacetate (53) is formed (4). 1

ur'VY-u " 6

Η 5

°

N=N

6

5

'~ °

hV

2

X

'

H

5C -CH=N 6

2

*

H C -CH=CH 5

6

2

H

N-N N

2

52

N

2

5 3 (100%)

Oxidation of α-dicarbonyl monosemicarbazones by lead(IV)acetate, lead­ ing to the respective α-diazocarbonyl compounds, may be also interpreted as the cleavage of a heterocyclic system (54 55) (24). This oxidation reaction leads to the formation of 2,5-dihydro-l,3,4-oxadiazoles 56, which may either fragment directly to isocyanic acid and the α-diazocarbonyl compounds 55 or be converted to the oxadiazolinones 57, which lose C 0 to form 55 (24). 2

543

References

Pb(0C0CH k,CH Cl ,-5°C 3

-C0 .-NH 2

X

^R ^N-NH-CONH

?

7

3

2

2

54 Pb(0C0CH ) 3

3

^ ° \ - 2 H C-l P

4

b

,

0

3

V

-R N=N z

57

56

X

-R

yield

2

CH 6

5

OC H

C H

5

CH

6

50%

2

5

3

29%

35%

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

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