Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups. Part 2 / 0683 745

15. Photochemistry of amines and amino compounds				703
	Et	Et
CN		N
	Et2 NH			(37)
	hν
CH2 F
	Et2 NH			(38)
	hν
			CH2 NEt2
F			Me
	Et2 NH		+	(39)
	hν
			F
Me	H	NEt2	NEt2

The intramolecular photochemistry of 9-[2-(N-substituted aminomethyl)-1-naph- thyl]phenanthrenes has also been studied101 (equation 40). The pyrroline derivatives are obtained by the addition of the N H to the C-9 carbon atom of phenanthrene ring. Reasonable yields for the highly regioselective products are obtained by irradiation in benzene solution.

R1
CH2 N	hν	CH2
R2		R1
	∆

(40)

(102)R1 = C6 H5 , R2 = H

(103)R1 = CH3 , R2 = H

(104)R1 = C6 H11, R2 = H

(105)R1 = t−C4 H9 , R2 = H

704	Tong-Ing Ho and Yuan L. Chow

IV. PHOTOCHEMISTRY INVOLVING PRIMARY AMINES AND AMMONIA

The intermolecular fluorescence quenching of the singlet trans-stilbene and primary aliphatic amines have been studied8. The free energy for electron transfer from ground state ethylamine to singlet stilbene was calculated to be endothermic (0.8 eV). Thus, neither electron transfer fluorescence quenching nor exciplex formation are observed for the ethylamine and stilbene system. The low observed rate constant for fluorescence quenching of styrene by butylamine is also due to the high oxidation potential of the primary amine99. Direct irradiation of stilbene8 or styrene99 with a primary amine does not result in any addition products, while irradiation of primary styrylamine 106 in acetonitrile solution results in both isomerization and intramolecular addition to yield a mixture of a and b adducts (equation 41). The ratio of b/a is about 1.4.

	hν	N	+
					(41)
Ph NH2		H	Ph	N
		Ph		H
(106)		(a)		(b)

Intramolecular fluorescence quenching of phenylalkylamines 107 have been studied earlier102 but no exciplex was observed. An exciplex is believed to be an intermediate in the intramolecular styrylamine 106 photoreaction99.

(CH2 )nNH2

n = 1, 2, 3

(107)

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanobenzene (DCNB)103 106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNBŁ by t-S (or vice versa) yields the t-S cation radical (t-S)C. . Nucleophilic addition of ammonia or the primary amine to (t-S)Cž followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electrontransfer sensitized addition of nucleophiles to terminal arylolefins107,108.

							+			NHR
				+			NH2 R
		hν			RNH2
		DCNB
Ph	Ph		Ph	Ph		Ph	Ph		Ph	Ph

(42)

Irradiation of an equimolar mixture of t-S and DCNB in 9:1 acetonitrile water solution containing ammonia results in the formation of 1,2-diphenylethylamine in 46% yield106.

15. Photochemistry of amines and amino compounds									705
				hν, CH3 CN/H2 O
					t-S+
	t-S
	DCNB			−				RNH2
				DCNB
	Ph							Ph
							+
Ph	NHR			Ph				NH2 R

SCHEME 7

The regiochemistry for the substituted t-S with ammonia and DCNB was also studied (equation 43). When there is an alkoxy substituent on the para position, the reaction yields selectively 1-amino-2-aryl-1-phenylamine (equation 44).

			NH2
CH CHPh +		hν	CH2 CHPh
		NH3
		DCNB
Xn			Xn
(108)			(a)
			(43)
			NH2
		+	CHCH2 Ph
			Xn
			(b)
R1
R2	CH CH		R4 + NH3
R3	(109)	hν
			DCNB
R2			(44)
	1
	R
R3
	CH2	CH	R4
		NH2

706	Tong-Ing Ho and Yuan L. Chow

The photoamination of t-S having a methyl or chloro group in the para position or a methoxy group in the meta or ortho positions gave non-selective adducts. Irradiation of the primary aminoethyl and aminopropylstilbenes in acetonitrile water solution in the presence of DCNB results in the formation of isoquinoline and benzazepine products in good preparative yields24,28 (equations 45 and 46). The preparation of benzazepine represents an improvement in overall yield when compared to previously reported nonphotochemical synthetic routes.

	Ph		Ph
	NH2	hν, DCNB	NH	(45)
		CH3 CN/H2 O
	(110)		(111)
	Ph		Ph
		hν , DCNB	NH
	NH2			(46)
		CH3 CN/H2 O
	(112)		(113)

Photosensitized amination of several aromatic compounds including phenanthrene, anthracene, naphthalene with ammonia or primary amines has also been investigated106 (equations 47 49).

NHR

hν, m −DCNB

+ RNH2 (or NH3 )

MeCN−H2 O

(47)

NR2

hν, DCNB

+ R2 NH (R = H or Me) MeCN−H2 O

(48)

NH2

NH2

	+ NH3	hν, DCNB	+
		MeCN−H2 O

(49)

15. Photochemistry of amines and amino compounds

707

Recently, a photosensitized synthesis of phenanthrene heterocycles from 1- and 9-(aminoalkyl)phenanthrenes has been achieved109. Irradiation of 1-(2-aminoethyl)phen- anthrene and m-DCNB in 9:1 acetonitrile water solution for 2 hours gives 70% of the aporphine 114 and 15% of the dehydroaporphine 115 with 87% conversion of starting material (equation 50).

	NH2
	NH		NH
	hν	+
	CH3 CN/H2 O		(50)
	DCNB

(114) 70%

(115) 15%

Irradiation of the 9-(2-aminoethyl)phenanthrene (equation 51) under the same conditions but for 70 hours produces phenanthro[1,10-d,e]piperidine 116 and 9- methylphenanthrene 117.

		NH
NH2			Me
	hν, DCNB	+	(51)
	MeCN−H2 O

			(116) 30%	(117) 10%
Irradiation		of the 1-(3-aminopropyl)phenanthrene (equation 52) for 3 hours results
in	the formation		of the hexahydrophenanthro[10,	1-b,c]azepine 118 (68%) and
119	(12%)	with	81% conversion. Irradiation of	9-(3-aminopropyl)phenanthrene

(equation 53) for 4 hours results in the formation of 9,10-dihydrophenanthrene[9,10- b]piperidine 120 (13%), phenanthro[9,10-b]piperidine 121 (22%) and 1-aza[5,6;7,8- dibenzo]spiro[4,5]decane 122 (4%).

NH2
hν, DCNB	NH	NH
	+
MeCN−H2 O		(52)

(118)

(119)

708	Tong-Ing Ho and Yuan L. Chow

NH2

hν, DCNB

MeCN/H2 O

NH

(120)

(53)

NH

+	+
	NH
(121)	(122)

The triplet reaction of 2-nitrodibenzo[b,e][1,4]dioxin with primary amines (n- propylamine and benzylamine) was studied110 in polar and apolar solvents. In polar solvents, the irradiation results in the formation of two isomeric compounds, (alkylamino)hydroxynitrodiphenyl ether and N-(alkylamino)-2-nitrophenoxazine (equation 54). In apolar solvents, only the nitrophenoxazine is obtained. In polar solvents, the exciplex formed between the 2-nitrodibenzo[b,e][1,4]dioxin triplet state and amines dissociates to the solvated radical ions, from which the diphenyl ether arises. 1- Nitrodibenzo[b,e][1,4]dioxin is stable even on prolonged irradiation.

O	NO2	OH NHR
	hν	O	NO2
	RNH2
O	(R = n−C3 H7; PhCh2 )
(123)		(124)
OH	NHR
		NH	NO2
+	O	+
	NO2	O
(125)		(126)

(54)

15. Photochemistry of amines and amino compounds

709

The photochemical reaction between substituted benzenes and t-butylamine gave a

mixture of addition and substitution products53 (equations 55					57).
Cl
						Cl
	hν	+			+
	t-BuNH2
		Cl
		H		NHBu-t	H	NHBu-t
		NHBu-t
(127)		(128)	(129)			(130)
						(55)
						Cl
		Cl
		+			+
		H		NHBu-t	H	NHBu-t
			(131)			(132)
Cl

	hν	(56)
	t-BuNH2

NHBu-t

CH2 F

	hν	+	(57)
	t-BuNH2
		CH NHBu-t	CH2 NHBu-t

In the case of toluene and t-butylamine (equation 58), a novel acyclic adduct is obtained.

Me

Me

hν

+

t-BuNH2

Me

(58)

H

NHBu-t

H

NHBu-t

Me

+ Me

C

N

CH

CH

CH

CH

CMe2

Me

710	Tong-Ing Ho and Yuan L. Chow

The solid state photochemistry of the salts of carboxylic acids with optically active primary amines has been studied111. Enantiomeric excesses ranging from 14 80% can be achieved (equation 59).

COOY

COOY COOEt

EtOOC

hν

COOEt

COOY

(59)

+

H	+		H +		H	+
Y = H ,	N		N			N
			H		H
H		,		,
HOOC			MeOOC		HOCH2
V. PHOTOCHEMISTRY OF IMINE AND IMINIUM SALTS
The photochemistry of	imine	chromophores		shows similarity		to the ethylenic

or carbonyl group. For example, the imine double bond undergoes syn anti photoisomerization112, photocyclization113, cycloaddition114 and intramolecular hydrogen abstraction115 (equation 60). Earlier reviews on the photochemistry of imine compounds are available116. More recent reviews on the photochemistry of imine and iminium salts are also available117,118.

Ph	Ph
NR	NHR
hν	(60)
CH3	CH2
R = Ac

A. Photoinduced Electron Transfer Chemistry of Iminium Salts

The energies calculated for the electron transfer from electron-rich olefins to excited conjugated iminium salts are energetically favourable118. Thus, electron-rich olefins are excellent fluorescence quenchers of 2-phenyl-1-pyrrolinium perchlorate 133119. The

15. Photochemistry of amines and amino compounds

711

photochemistry between excited 133 and cyclohexene in methanol to obtain the addition product 134 and the methanol adduct 135 is illustrated in equation 61.

+	ClO4 −	+	hν		Ph
	Ph
N			MeOH	N
H				H
(133)				(134)	31%

(61)

Ph OMe

+

N

H

(135) 18%

The mechanism for the photoreaction between 133 and cyclohexene can be summarized as in Scheme 8. The initiating electron transfer fluorescence quenching of 133 by cyclohexene resulted in the formation of an ˛-amino radical radical cation pair 136. Proton transfer from the 2-position of the cyclohexene radical cation to the nitrogen atom of the ˛-amino radical leads to another radical cation radical pair 137. Recombination of 137 at the radical site affords the adduct 134, while nucleophilic attack at the cation radical of 136 leads to another radical pair 138 which is the precursor for the adduct 135.

Electron-poor olefins with higher oxidation potentials may decrease the rate of electron transfer and other processes competing for deactivation of the iminium salt excited states may increase. Alternate reaction pathways involving olefin-arene 2 C 2 cycloaddition may take place in the photochemistry of 133 with electron-poor olefins (equation 62)120,121.

							N H
+	Ph	+ CH2		CRZ	hν		R
N	−	(139) R = H, Z = CN
Cl O4							Z
H		(140) R = H, Z = CO2 Me
(133)							H
		(141) R = Me, Z = CO2 Me
							(62)
					+ N H		(142) R = H, Z = CN (44%)
				R			(143) R = H, Z = CO2 Me (52%)

(144) R = Me, Z = CO2 Me (54%)

Z

712

+	Ph +
N	−
ClO

4

H

(133)

Tong-Ing Ho and Yuan L. Chow

			+
	hν	N	Ph
	SET
			H
		H
			(136)

−H+ Nuc

OMe

+

N Ph

Ph

N

H H

H

(137)

(138)

	Ph	Ph	OMe
	+
	N	N
H	H	H
	(134)	(135)
		SCHEME 8

Intramolecular electron transfer initiated cyclization reaction of N-allyliminium salt systems may also generate 3-pyrrolidinyl ethers or alcohols in monocyclic and bridged or fused bicyclic systems (e.g. equations 63 65)122,123.

+ H N

CHPh

Me R

R = H or Me

	MeO	N	H
hν
MeOH	Me		(63)

R	Ph
(25− 28%)