Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups / 0423 477

10. Hydrogen bonding and complex formation	443
The product of tribenzylamine with bromine was reported154	to be

(C6H5CH2)3NBrC Br , but it is actually a mixture of tribenzylammonium and N,N- dibenzylbenzylideneaminium tribromides 50 and 51, respectively155.

+	+
(C6H5CH2)3 NH Br3	(C6H5CH2)2 NDCHC6H5 Br3
(50)	(51)

4. Intramolecular electron donor acceptor complexes

The interactions between the nitro and the amino groups in p-nitroanilines have been described in classical terms as intramolecular charge-transfer from an electron-donating substituent to an electron-accepting substituent. Usually this kind of internal interaction is named ‘through-conjugation’. This is described in structure 52 with strong separation of the charges in a quinonoid resonance structure which possesses a high degree of charge transfer character156.

H	+	H	H	+	H
	N			N	O	−
					N
					O
	N			X
−O	+	O−
	(52)			(53)

On the other hand, all the substituents in para position to the amino group may be shown to interact with the amino group by classical mesomeric and inductive electronic processes. This is the case of 4-substituted-2-nitroanilines: their absorption (UV/VIS) spectra157 are correlated with substituent constant pC in a Hammett plot.

Contributions by canonical structures like 53 are considered important. The absorption band of substituted anilines (with electron-withdrawing groups) contains an intramolecular charge-transfer transition which is strongly affected by the hydrogen bonding with protic solvents. The geometry of the amino nitrogen depends on the electron affinity of the electron-withdrawing substituent and on its position in relation to the amino group72.

In several examples including nitroanilines, the effect of twisting the chromophore from planarity decreases the absorption intensities. The reasons for the bathochromic effect as the angles of twist in the 4-aniline series increase is subject to discussion. When considering this (as well as in all attempts to obtain definitions of polarity of solvents by quantitative parameters) it is important to exclude or minimize the presence of hydrogen bonding overlapping158 other interactions.

The classical idea of through-conjugation is revised: the importance of structures 52 and 53 was criticized in particular by Hiberty and Ohanessian159. There is a tendency to explain physical and chemical properties of derivatives of p-nitroaniline without considering quinonoid structures like 52 to be the most important ones.

Crystal data (by X-ray diffraction analysis) and calculation of net -electron population for N,N-dialkyl-p-nitroaniline and for 3,5- and 2,6-dimethyl-4-nitroaniline indicate that

444	Luciano Forlani

the classical through-conjugation with full charge transfer from the amino to the nitro group may be considered to make a relatively small contribution to the structure of p-nitroanilines160.

It is possible to evaluate the strength of several acceptor parts (including some nitroaromatic derivatives) in intramolecular charge transfer complexes in substrates such as 54 and 55 which also contain electron-donor fragments (X D PhNH , PhO , Ph ) from the oxidation potential of the complex and of the donor species161.

Crystallographic structural analysis162 of compounds 56 bearing donor and acceptor moieties indicates the intramolecular origin for the charge-transfer (donor acceptor) complex. The acetylene bridges, which must participate in the migration of the charge, are not themselves structurally changed. System 56 possesses a single bond/triple bond alternation pattern with none of the quinonoid structure usually expected and the charges are highly localized. The strong interaction between the donor and the acceptor groups is evident, but, even when the triple bond bridges have to participate in the migration of the charge, they are not structurally altered.

	NO2		NO2
	NO2
O2 N	X		X
	O2 N		C
	NO2	O	O
	(54)	(55)

NH2 C6 H4 (C C)nC6 H4 NO2 (n = 0−3)

(56)

Thermal cis trans isomerization of 4-(diethylamino)-40-nitroazobenzene163 (equation 17) is sensitive to changes in the solvent, both protic and aprotic. The mechanism of the isomerization in equation 17 has been postulated to proceed via a rotation around the N N bond, facilitated by an intramolecular charge transfer interaction 57. Increased solvent polarity increases the rate of the isomerization (equation 17) by stabilizing 57. The isomerization is faster in hydrogen bonding solvents than in aprotic solvents of similar polarity. This fact is explained163 by a hydrogen bonding interaction between the solvent and the nitro group of the dye: this interaction stabilizes the transition state of the isomerization in equation 17.

		O	O 3 O
		1 N		N...a
				.
		O		. N
		+
		NEt2		N
−O2 N
	N	N		.H
			.
	N	O 2	..
			O. b
	(57)		(58)

10. Hydrogen bonding and complex formation

445

O2 N

	N	N	N	(17)
		N

O2 N

NEt2

NEt2

Crystallographic properties of the 2-N-(2,4,6-trinitrophenyl)pyridineamine (58) and of other related heterocycles164 indicate interand intra-molecular interactions between the amino group and nitro groups. In 58 the two nitro groups N(1) and N(2) are coplanar with the phenyl ring, while the nitro group N(3) is twisted by about 40° from the phenyl ring plane. The relative geometry of the nitro groups, in particular of N(3), and of the 2- aminoheterocyclic moiety indicates that the classical conjugation theory can hardly explain the UV/VIS spectroscopic data.

The red shift observed on substituting hydrogen by a methyl group on the 2-amino group (also for nitroanilines) may be explained by the presence of an internal charge transfer which does not require coplanarity of the involved groups. An internal hydrogen bond (b) is observed between the amino and the ortho-nitro groups.

Conjugation between the phenyl ring and the nitro group should be greater when the nitro group is coplanar with the phenyl ring. Consequently, the bond between the N of the nitro group and the phenyl carbon should be shorter than when only the inductive effect is operating. In 58 the length of all the N C(ring) bonds is the same, indicating that the electron-withdrawing power of the nitro group derives from an inductive rather than from a mesomeric effect. This conclusion agrees well with the quantitative evaluation of mesomeric effects by R values of a Hammett-like correlation, which for all the electronwithdrawing groups are near to a zero value165.

When the pyridine ring is replaced by a thiazole ring and a pyrimidine ring respectively (both bearing a trinitroaniline group in position 2), the intramolecular distance in 58

(a) changes to 2.821 A˚ and 2.692 and 2.765. On the other hand, the distance in the homocyclic series (2,4,6-trinitrodiphenylamine), a is 3.235 A˚ . This suggests the presence of an attractive interaction between the aza and nitro nitrogens and a consequent greater torsion angle of the nitro group compared to the benzene ring. This kind of interaction is an indication of the nature of molecular complexes between nitro derivatives and amines (see Section VI) which may be considered as responsible for catalytic behaviours in aromatic nucleophilic substitution reactions in apolar solvents.

Recent attempts to unify the polarity scales of solvents (for non-specific interactions) are of great interest in rationalizing the medium effects166. Generally, the spectroscopic properties of appropriate substances are used to check the solvating ability of solvents. 4-Nitroaniline is a useful indicator for estimating solvent polarity because it is an electron acceptor molecule which presents incomplete complexation with weak donor solvents167.

The problem of conformational dynamics on semi-rigid systems168 in which donor and acceptor moieties are separated by saturated hydrocarbon spacers169 is investigated by the

446	Luciano Forlani

time-tested fluorescence spectroscopy170. Intramolecular charge transfer is used to investigate ultrafast dynamics involving motions of molecules or solvent organization171,172. Systems like 59 (where A is an electron acceptor group and D is an electron donor group) show an intramolecular charge transfer interaction. In moderately polar solvents (ethers) the only fluorescence observed is from a dipolar species, while in strongly associating solvents (alkanes) a folded dipolar species with a contact charge transfer interaction is produced173.

D N

A

(59)

A

D N

(60)

Intramolecular charge transfer in p-anthracene-(CH2)3-p-N,N-dimethylaniline (61) has been observed174 in non-polar solvents. Measurements of fluorescence-decay (by the picosecond laser method) allow some conclusions about charge-transfer dynamics in solution: internal rotation is required to reach a favourable geometry for the formation of intramolecular charge-transfer between the donor (aniline) and the acceptor (anthracene).

NH

(CH2 )3

N(CH3 )2

(61)

(62)

Spontaneous ionization from the charge-transfer state of 2-anilinonaphthalene (62) in water/methanol mixtures175 shows (using picosecond spectroscopy) that the hydration of the electron limits the rate in the overall kinetics. For 8-(phenylamino)-1- naphthalenesulphonate, a water cluster (of 4 members) is the charge acceptor in the same way as observed for proton hydration175.

10. Hydrogen bonding and complex formation

447

III. NITROSO GROUP

Nitroso derivatives (with the nitroso group bound to a carbon atom) can exist in three molecular forms176,177: the monomer 63 and the dimers 64 and 65, Z and E, respectively.

Aliphatic C-nitroso compounds are mainly dimers178. Aromatic nitroso derivatives, in solution, may be monomers or dimers, depending on the concentration and temperature, and on the substituent on the aromatic ring. Nitrosobenzene itself is in the E dimer form in the solid state179.

	O	O	O	R
O	N N		N	N
R N	R	R	R	O
(63)	(64)		(65)

1,4- and 1,3-Dinitrosobenzenes are long-chain polymers180, probably in a cyclic form, based upon the E N2O2 group 66. Depolymerization to monomer occurs on heating and on vaporization of 66.

O

N

N

O	n

(66)

In principle, the addition of nucleophiles to the nitroso group produces adducts by the equilibrium equation 18 which may evolve to more stable reaction products181. The usual explanation is that the nitroso group is isoelectronic with the carbonyl group. The hydrated form of nitrosobenzene, C6H5N(OH)2, is considered unstable by electrochemical investigations182,183 and its life-time is very short184.

Nu +

+ Nu

N O

N	(18)
	O−

Reactions of nitrosobenzene with aliphatic amines185 yield (phenylazo)alkanes and azoxybenzene as the main products. The adduct 67 is assumed to be an intermediate in obtaining both products, as seen in Scheme 3.

The addition of hydroxyde ion to nitrosobenzene produces azoxybenzene186. Three techniques (electronic absorption spectroscopy, linear sweep voltammetry and d.c. polarography) have been used to study the equilibrium between nitrosobenzene and hydroxyde ions. The probable reaction pathway to obtain azoxybenzene is indicated by Scheme 4. The importance of the nitroso group in the reduction of nitro derivatives by alkoxide ions, when the electron-transfer mechanism is operating, has been explained187.

A kinetic study of reactions between 4-substituted nitrosobenzene and methoxide ions (in methanol), to yield 4-substituted azoxybenzenes in the presence of oxygen, indicated

448	Luciano Forlani
C6 H5NO + RR′CHNH2		C6 H5N				NCRR′
		OH				HH

(67)

C6 H5NH(OH) + NH CRR′

C6 H5N NCHRR′ + H2 O

C6 H5NO

C6 H5N N(O)C6 H5 + H2 O

R= H, R′ = Me, Et, n-Pr, i-Pr, n-Bu, t-Bu, Benzyl

R= R′ = Me, Et

SCHEME 3

						OΗ					O−
					−OH		−OH				+ H2 O
C6 H5		N		O		C6 H5 N		C6 H5 N
						O−					O−
								+ 5H+		+ C6 H5(NOH)O −
											(fast)
								C6 H5N(O)			NC6 H5 + 3H2 O

SCHEME 4

(mainly by a substituent electronic effect yielding a Hammett plot) the formation of RC6H4NOHÐ as the rate-determining step of reaction 19188.

slow
RC6H4NO C CH3O ! RC6H4NOHž C CH2O ž	19

Usually, protic acids decompose N-nitrosoamines189, but recently several salts of N- nitroso compounds with acids have been prepared. Thus, nitrosoamines produce stable salts (1:1) with perchloric and trifluoromethanesulphonic acids which can be isolated, recrystallized and characterized as pure salts.

Nitrosoamines and hexafluorophosphonic acid afford 2:1 salts. Scheme 5 shows some examples of such salts. The unusual stability of these salts may be attributed to the use of non-nucleophilic counter ions and solvents. When a nucleophilic counter ion is present, the N-nitrosoamine decomposes to amine and NOX (X D Cl , Br , SCN , I ). In denitrosation reactions in acid solutions, N-nitroso compounds are also found190 to yield hydrogen bonded complexes with a general acid HX as reported in 68 of Scheme 6. 68 may yield two different protonated species, since in nitrosoamines both oxygen and nitrogen are possible centres of protonation. Probably the more basic site is the oxygen atom191.

Reactions between nitroso compound and salts of nitro compounds afford nitrones, as illustrated by Scheme 7 involving the intermediate 69 which is the first step in the addition of the nitro carbanion to the NDO double bond192. In this case the leaving group is on the nucleophile.

10. Hydrogen bonding and complex formation

449

R

+

OH

N X

−

N

R

O

Η+ X −

R1

X− = ClO−4 , CF3 SO −3

N

N

+

R1

+

R

O H

O

R

N

N

N

N

X −

R1

R1

R= Me, R1 = t-Bu;

X− = PF6 −

R = R1 = Me, Et;

R

N

= piperidine, pyrrolidine, cyclohexylamine, morpholine

R1

SCHEME 5

+

O

H

−

−

N

N

X−

+

O

+

O

+

HX

. HX

N

N

N

N

H

O

N

N

+

X−

(68)

SCHEME 6

R1

O

R2

O−

R1

O−

C−

+

N

R1

C N

C N +

− NO2

−

O2 N

R2

R3

R3

R2

R3

NO2

(69)

R1 = H,

H

H

H

H

H

H

R2

= Me

Me

C(O)NMe

2

C(O)NHMe

CO Me

C(O)NMe

2

C(O)NHMe

2

R3

= Ph

Ph

Ph

Ph

t-Bu

t-Bu

t-Bu

SCHEME 7

450	Luciano Forlani

Addition of carbanions (which may be electrochemically generated), derived from active methylene compounds (such as fluorene or indene193), to nitrosobenzene produces the intermediate181 70, which is dehydrated to the azomethine 71 or may be oxidized to the nitrone derivative 72, as illustrated by Scheme 8.

O−

PhNO + −CHR1R2 PhN

CHR1R2

(70)

PhN CR1R2 + OH − PhN CR1R2 + 2e + H+

O

(71)(72)

CH2 R1R2 =

SCHEME 8

Nitrosobenzenes react with the carbonyl group of aldehydes to yield hydroxamic acids 73, according to reaction 20. Recently, the reactions between some X-substituted nitrosobenzenes (X D H, p-Me, p-Cl, m-Cl, p-Br) and formaldehyde were reported194 in order to investigate the mechanism of the hydroxamic acid formation. The mechanism reported in Scheme 9 involves a first equilibrium yielding the zwitterionic intermediate 74 which rearranges (by acid catalysis) into hydroxamic acid 75. The presence of a general acid catalysis, the substituent effect ( values of the Hammett equation equal 1.74),

O

O−

OH

O

+

+ H

+

+

+ ArN

HC

HC

NAr

HC

NAr

H

H

O

H

O

(74)

− H+

OH

OH

+

HC

NAr

HC

NAr

O

O−

(75)

SCHEME 9

10. Hydrogen bonding and complex formation

451

deuterium/hydrogen isotope effects and the presence of a general base catalysis are the main points which support the mechanism depicted by Scheme 9.

O						O				OH
RC	+	ArNO
											(20)
H
				R		C					NAr
									(73)

IV. NITRO GROUP

A. Hydrogen Bonding

1. Intermolecular hydrogen bonding

Recently195, the hydrogen bond basicity scale (pKHB as logarithm of the formation constant of 4-fluorophenol/base complexes in carbon tetrachloride, equilibrium 21) has been measured for several nitro derivatives (nitromethane, nitrobenzene, N-nitrocamphorimine, 2-nitropropane, 4-nitro-o-xylene, 4-nitroanisole, N,N-diethyl-4- nitroaniline, 1-dimethylamino-2-nitroethylene, 1-piperidino-2-nitroethylene):

B C p-FC6H4OH	KHB	p-FC6H4O HÐ Ð ÐB	21

The electronic effect of the substituents on nitro-aromatics is rationalized by the YukawaTsuno equation.

Complex 1:1 is considered the only complex present, but the hydrogen bond may be either two (76) or three center (77). Nitroenamines are more prone to complex with 4-fluorophenol than the nitroanilines and they form the strongest hydrogen bond presently known for nitro-bases. In particular, 1-piperidino-2-nitroethylene (78) and 1- dimethylamino-2-nitroethylene (79) (both in E form) present a hydrogen bond basicity comparable to that of tributylamine.

1	−		1	−
	2 O ..... H	O		2 O.
				.
N			N		...			O
							.H		N	H
						.
				..
	O			.
				O					C	C
1	−		1	−
									H	NO2
	2			2
	(76)					(77)			(78)
									O
	Me			O2 N						NO2
Me N			H
	C	C
	H		NO2						NO2
		(79)							(80)

452	Luciano Forlani

X-ray structural analysis of the charge-transfer complex between 2,4,7-trinitrofluore- none (80) and 2,6-dimethylnaphthalene shows the presence of C HÐ Ð ÐO hydrogen bonding196 involving both nitro and carbonyl groups of 80. These hydrogen bonds importantly influence the molecular arrangement in a nearly coplanar ribbon-like structure also for other aromatic and aliphatic nitro derivatives.

Crystal structures of donor acceptor complexes (1:1) between 4-nitrobenzoic acid and 4-N,N-(dimethylamino)benzoic acid197, 4-N,N-(dimethylamino)cinnamic acid and other related complexes containing nitro and dimethylamino groups are used to investigate198 (by X-ray diffraction analysis) feeble C HÐ Ð ÐO hydrogen bonds between nitro and dimethylamino groups, as depicted in 81 which is a frequent pattern in these cases. These feeble interactions may be of help in explaining the additional stabilization (to overcome entropic barriers) of organic and bioorganic molecular aggregates. Probably, they are effective for supramolecular architecture.

Another instance of C HÐ Ð ÐO hydrogen bond yielding a supramolecular assembly (at least in the solid state) is from complexes 82 and 83 between 1,3,5-trinitrobenzene and dibenzylideneacetone or 2,5-dibenzylidenecyclopentanone, respectively, which were

investigated by X-ray diffraction199. C HÐ Ð ÐO hydrogen bonds (such as those of 82 and 83) are three to five times weaker than N HÐ Ð ÐO (and O HÐ Ð ÐO) bonds200.

	O	H	CH2
	N		N
	O	H	CH2
		(81)
O	O		O	O
	N			N

O O O O N N N N

O H O O H O

H O H H O H

H H H H

(82)

(83)

2. Intramolecular hydrogen bonding

The molecular geometry of 2-nitrophenol201 and of 2-nitroresorcinol202 has been determined by a joint investigation of gas-phase electron diffraction and ab initio molecular orbital calculations. The molecule is planar and there is a strong, resonance-assisted intramolecular hydrogen bond between the nitro group oxygen and the hydroxy hydrogen as shown in 84 and in other resonance structures.