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Reactive Intermediate Chemistry

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SPECTROSCOPIC AND KINETIC STUDIES OF NITRENIUM IONS

637

N

ν = 1568, 1440, 1392 cm

Figure 13.65. Infrared bands for diphenylnitrenium ion.

species. The 1392-cm 1 band shifted by 16 cm-1 to lower frequencies when the central nitrogen was enriched with 15N. On this basis it was assigned to an asymmetric CNC vibration, analogous to the 1320-cm 1 band observed for the parent amine. The remaining bands were unperturbed by 15N substitution and thus assigned to C C stretches (Fig. 13.65).

Several 4-substituted phenylnitrenium ions 134/135 were also examined by TRIR and the results were compared with IR spectra generated from DFT calculations (Fig. 13.66).163 Each of these arytntrenium ions shows prominent bands in the region 1580–1628 cm 1. These signals were assigned to symmetric aromatic C C stretches. The frequencies of these bands correlate with the expected degree of charge delocalization. For example, the 4-methoxy derivative was expected to show the most charge delocalization and exhibit the highest degree of quinoidal character. This leads to an increase in bond alternation in the aromatic ring (i.e., resonance form 135 is favored) and thus the highest frequency for the C C. In contrast, the 4-chloro substituent renders the ring less susceptible to delocalization, and exhibits a C C frequency closer to that of an unperturbed aromatic ring (1604 cm 1). The 4-phenyl deriviative shows two bands: a more quinoidal band at 1612 cm 1, attributed to the more perturbed proximal ring, and a more aromatic band at 1584 cm 1 attributed to the less perturbed distal ring.

The DFT calculations (BPW91/cc-pVDZ) on the same singlet nitrenium ions give excellent agreement with these measurements. For calculated and experimental C C stretches, deviation between experiment and calculation is generally <5 cm 1. On the other hand, similar calculations carried out for the triplet states showed significantly greater deviation from the measured values.

N

 

 

Me

 

Me

 

 

 

 

N

 

 

 

 

 

 

r2

r1

R

R

134

135

R

C=C

r1

r2

Cl

1604

1.376

1.458

Me

1616

1.375

1.459

OMe

1628

1.367

1.463

Ph

1612

1.373

1.458

 

 

 

 

Figure 13.66. ˚ 1

Bond lengths (A) and aromatic C C stretching frequencies (cm ) for arylnitrenium ions.

638

NITRENIUM IONS

 

 

 

 

 

 

 

 

 

 

 

 

 

N

 

 

H

 

 

 

 

 

 

 

 

 

N

 

 

N

 

 

 

 

H

 

 

 

 

 

 

 

 

 

 

 

 

Ph

 

 

 

 

 

 

 

 

 

136

137

 

66

 

1637

1625

 

1604

 

1607

1607

 

1525

 

1567

1496

 

1478

 

1486

1422

 

1455

Figure 13.67. Selected Raman bands (in cm 1) for arylnitrenium ions.

Phillips and co-workers188 introduced the technique of TRRR to these studies. They reported detailed TRRR studies and DFT calculations on 2-fluorenynitrenium ion (136), the 4-biphenylylnitrenium ion (137), and the N,N-diphenylnitrenium ion (66, Fig. 13.67). The former two nitrenium ions were generated through azide photolysis in MeCN–H2O mixtures, and the latter nitrenium ion was generated via the N-aminopyridinium ion route. Some of the major Raman bands measured for these species appear in Figure 13.67. As with the TRIR experiments, excellent agreement is obtained between measured and DFT-calculated values.

5.4. Direct Detection of Intermediates in Nitrenium Ion Reactions

The LFP studies of the reaction of the N-methyl-N-4-biphenylylnitrenium ion with a series of arenes showed that no detectable intermediate formed in these reactions.162 The rate constants of these reactions correlated neither with the oxidation potentials of the traps (as would be expected were the initial step electron transfer) nor with the basicity of these traps (a proxy for their susceptibility toward direct formation of the sigma complex). Instead, a good correlation of these rate constants was found with the ability of the traps to form p complexes with picric acid (Fig. 13.68). On this basis, it was concluded the initial step in these reactions was the rapid formation of a p complex (140) between the nitrenium ion (138) and the arene (139). This was followed by s-complex formation and tautomerization to give adducts, or a relatively slow homolytic dissociation to give (ultimately) the parent amine.

In a subsequent study using diphenylnitrenium ion, several intermediates were detected. With 1,3,5-trimethoxybenzene or 1,3-dimethoxybenzene, the decay of the nitrenium ion occurred concurrently with the appearance of sigma adducts (141, Fig. 13.69).168 These were characterized on the basis of their absorption maxima and their behavior toward pyridine bases. On the other hand, when readily oxidized arenes, such as N,N-dimethylaniline were employed, the characteristic ion radicals were detected (Fig. 13.70).189

SPECTROSCOPIC AND KINETIC STUDIES OF NITRENIUM IONS

639

 

Ph

 

 

 

R1

 

 

 

 

 

R6

R2

Log k (trap)

 

 

 

 

 

R3

 

 

 

R5

 

 

 

 

7.5

 

 

 

 

NMe

R4

 

 

 

 

 

6.5

 

 

 

 

138

139

 

 

 

 

 

5.5

 

 

 

 

 

 

4.5

 

 

 

 

 

 

3.5

 

 

 

 

 

 

−0.2

0

0.2

0.4

0.6

0.8

NMe

 

 

log K (picric acid)

 

Ph

 

 

 

R6

R1

 

 

 

 

 

 

 

 

 

 

R5

R2

 

 

 

 

 

R4

R3

 

 

 

 

 

 

140

Figure 13.68. Arene trapping correlates with p complexation.

 

 

 

 

MeO

 

OMe

 

H

 

 

 

 

NPh2

Ph2N+

 

 

 

 

 

 

 

MeO

 

 

 

 

 

 

OMe

 

 

 

 

 

 

OMe

 

 

 

 

 

 

 

 

λmax = 330 nm

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

OMe

 

 

 

 

 

 

 

 

 

 

141

 

Figure 13.69. Sigma complex detected with diphenylnitrenium ion.

 

 

 

 

 

 

NMe2

 

 

 

 

 

 

 

NMe2

Ph2N +

 

 

 

 

 

 

 

Ph2N

 

 

 

 

 

 

 

 

 

 

 

 

 

 

λmax = 660 nm

λmax = 460 nm

Figure 13.70. Radicals detected from electron-transfer reaction.

These results lead to a general mechanism for the reaction nitrenium ions with aromatic compounds (Fig. 13.71). Initial encounter leads to a p-complex (141). The latter is converted into isomeric s complexes (142–144) which, in turn, either tautomerize to give stable adducts (145–147) or else dissociate to give radicals. The relative rates of these processes depend on the reactivities of the nitrenium ion and the arene. With less reactive nitrenium ions the p-complex is relatively long lived. With more reactive nitrenium ions the p complex forms in a low steady-state

640 NITRENIUM IONS

NR'

 

 

 

 

 

 

 

 

 

R

R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ktrap

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

141

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R''

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R''

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NR'

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

π-complex

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NR'

R''

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NR'

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R''

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

142

 

 

 

 

 

143

 

 

 

 

para-σ-complex

 

ortho-σ-complex

 

R''=H

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

−H+

 

 

 

~H+

 

 

 

 

 

 

 

 

 

 

 

 

 

−H+

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

~H+

 

 

 

 

 

 

 

 

 

 

R

NHR'

R

146

NHR'

145

NR' R

R''

R

H

R''N

R'

144

N-σ-complex

−H+

R

NR'

R''

147

Figure 13.71. Mechanism for arylnitrenium ion reactions with arenes.

concentration and proceeds to give radicals in cases where the arenes are readily oxidized, or to s complexes when they are not.

6. THE ROLE OF ARYLNITRENIUM IONS IN DNA DAMAGING REACTIONS

Arylnitrenium ions are considered to be key intermediates in carcinogenic DNA damage by metabolically activated amines. The enzymatic activation of amines and the propogation of DNA–amine adducts into mutations and cellular transformations is beyond the specific scope of this chapter. Presented here is an outline of the key mechanistic issues relevant to the interaction of DNA components with various arylnitrenium ions.

THE ROLE OF ARYLNITRENIUM IONS IN DNA DAMAGING REACTIONS

641

O

NN NH

 

H

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N

N NH2

 

 

 

 

 

 

 

R

 

 

 

 

 

 

O

 

 

 

 

O

 

 

H

 

 

 

 

 

 

 

 

 

 

 

 

N

NH

 

 

N

NH

 

 

 

 

 

N

 

 

 

 

 

 

 

 

 

 

 

 

 

N

N NH2

N

N N

 

 

 

 

 

 

R

 

R

H

 

 

 

 

 

 

 

 

 

NH2

 

 

 

 

 

 

 

 

 

 

 

 

148

 

 

 

 

 

 

 

149

 

Figure 13.72. Reaction of naphthylnitrenium ion with guanosine.

Boche and co-worker190 carried out detailed studies of the decomposition reactions of various N-hydroxylamine esters in the presence of DNA and deoxyguanosine, and characterized the adducts that resulted from these reactions. Thus, the nitrenium ion derived from O-acetyl-N-(2-aminofluorene) added to DNA to give an adduct joining the C8 position of the base with the nitrenium ion nitrogen. Similar experiments carried out with the precursor of 2-naphthylnitrenium ion gave N2ortho 148 and C8–N adducts 149 as shown in Figure 13.72.96

With regard to DNA damage, the most extensively studied arylnitrenium ions are the 2-fluorenenylnitrenium ion and the 4-biphenylylnitrenium ion. The toxicity of these species is explained by the fact that they are relatively long lived in water (>1 ms) yet react with DNA components at nearly the diffusion limit. Novak and Kennedy191 carried out extensive competitive trapping studies using N-acetyl-4- biphenylylnitrenium ion with various nucleosides. The pyrimidines, cytidine, uracidine, and thymidine did not show any reaction with this nitrenium ion (i.e., reaction with water was faster). On the other hand, gaunosine and its derivatives trapped this nitrenium ion with rate constants in excess of 109 M 1 s 1. Other purines, adenine, and inosine showed about a tenth of the reactivity of guanosine.

McClelland used LFP methods to measure rate constants for the reactions of

various substituted 4-biphenylylnitrenium ions with deoxyguanosine and found that they fell mostly in the narrow range of 1.0–2.2 109 M 1 s 1.129,131,183 An

exception was the highly stabilized 40-methoxy derivative that was trapped at 3:6 107 M 1 s 1. The 2-fluorenyl derivatives also showed somewhat attenuated reactivity, with rate constants with deoxyguanosine of 2–9 108 M 1 s 1.131

With free bases or nucleosides in solution, the main adduct arises from a net coupling of the C8 on gaunosine with the nitrenium ion nitrogen. This result is interesting for two reasons. First, most reactive nucleophiles tend to add to the ring carbons of the arylnitrenium ion rather than the nitrogen. In some cases, highly reactive arenes have been observed to give a mixture of regioisomers, including N adducts;

642 NITRENIUM IONS

only with guanine is the N adduct predominant. Second, whereas guanine bases in DNA react with a variety of electrophiles, the primary sites for addition seem to be N7 and O6. Adducts at C8 are apparently very rare. The reason for this divergent behavior with guanine has not been adequately explained.

A particularly important result from the point of view of chemical toxicology was the detection of an intermediate in the reaction of N-acetyl-2-fluorenylnitre- nium ion (150) with guanosine (Fig. 13.73).133 Concurrent with the decay of the nitrenium ion, a new absorbance band with lmax ¼ 330 nm appeared. On the basis of kinetic isotope effects, and the pH dependence of its decay, this intermediate was assigned to the s adduct generated by joining the nitrenium ion’s nitrogen to the C8 position of guanine (152). Thus it was concluded that the key DNA damaging reaction proceeded via a relatively straightforward electrophilic aromatic substitution mechanism. This contrasts with a proposal by Humphreys et al.,192 which held that the initial adduct was between the N7 of guanine and the N of the nitrenium ion (153). The latter was supported by experiments on guanine derivatives wherein the C8 position was substituted. It was argued that 153 was converted into the stable adduct via deprotonation to give zwitterion 154, followed by a 1,2-shift to give final product 151.

Novak and Kennedy193 studied the reaction of N-acetyl-N-2-fluorenylnitrenium ion (150) with single and double-stranded oligomeric DNA (6-mer), as well as

 

150

 

 

 

 

Ac

 

 

 

 

 

 

 

 

 

 

 

 

 

Ac

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ar

 

 

 

 

 

 

O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

O

 

 

 

 

 

 

N

 

 

 

 

 

 

 

Ar

 

 

N

 

 

 

 

 

 

Ar

N

Ac

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NH

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N

 

 

 

 

 

NH

 

 

 

 

 

 

N

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N

 

N

 

NH2

 

 

 

 

 

 

N

N

NH2

 

 

 

O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R

 

 

 

N

 

 

 

 

NH

 

 

 

 

 

 

 

 

153

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

154

 

 

 

 

NH2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

O

N

 

 

 

N

 

 

 

 

 

 

 

 

 

O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ar

 

 

 

 

 

 

 

 

 

 

Ar

 

 

 

R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

−H+

 

N

NH

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N

 

NH

 

 

 

 

 

 

 

 

 

 

 

 

Ac

 

 

N

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H

N

 

 

N NH2

 

 

 

Ac

 

N

N N NH2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

152

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

151

Ar =

Figure 13.73. Mechanisms for addition to N-acetyl-N-2-fluorenenyl guanosine.

THE ROLE OF ARYLNITRENIUM IONS IN DNA DAMAGING REACTIONS

643

plasmid DNA. With the oligomeric DNA, virtually all of the trapping could be attributed to the fraction of the 6-mer considered to be in the single-stranded form. With double stranded plasmid DNA the reactivity with nitrenium ion was significantly attenuated, being 2% of the reactivity of the free nucleoside. Single-stranded DNA, on the other hand showed reactivity that was slightly lower ( 30%) than the monomeric nucleoside. Thus it was suggested that rapidly replicating cells, which have a higher fraction of single-stranded DNA, should be particularly susceptible to damage by activated mutagens.

More recently attention has turned to the reactions of heteroarylnitrenium ions,166,173,194 These intermediates are derived from the activation of heterocyclic

amines such as 155, shown in Figure 13.74.166 These amines are in turn created in cooked foodstuffs through the high-temperature thermolysis of amino acids or

NHAc

N

N NMe

N

155

enzymatic reactions

NAc

N

N NMe

dG

N

156

 

NHAc

 

 

 

 

 

 

 

 

 

N

 

 

 

 

 

 

 

 

 

 

 

 

 

N

NMe

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N

 

 

 

O

N

 

 

 

 

R

 

 

 

N

Ac

N

NH

HN

N

 

 

 

N

N

 

 

 

 

 

 

 

 

 

 

 

N

N

N NH2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NMe

HN

N

 

 

 

R

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

157

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

158

 

 

 

 

 

 

 

 

 

 

 

Figure 13.74. Reactions of heteroarylnitrenium ions with guanosine.

644 NITRENIUM IONS

proteins. The nitrenium ion 156 is trapped by deoxyguanosine to give a C8 adduct (157) analogous to those observed for the carbocyclic nitrenium ions. An adduct connecting the guanine’s N2 with the ring carbon of the heterocycle (158) is also observed. Interestingly, this nitrenium ion also decays, presumably through hydride abstraction, to give the 155, along with some dimeric products. It was also found that similar reactions could occur through the less reactive conjugate base of this nitrenium ion.

7. CONCLUSION AND OUTLOOK

The past two decades have witnessed remarkable progress in the understanding of nitrenium ions. It is now widely agreed that these intermediates have a discrete existence. Several examples have been detected and characterized by direct means, including LFP. Numerous decay pathways for singlet-state nitrenium ions have been established both through product studies as well as by direct detection. The vast majority of studies have focused on the behavior of singlet arynitrenium ions. In a few cases, the chemical reactions of triplet arylnitrenium ions have been characterized, although few triplets have been studied by direct means. More information about the reactivity of alkyl and other non-arylnitrenium ions would be desirable.

Density functional-based theoretical methods have been demonstrated to give highly accurate predictions of nitrenium ions structures, singlet–triplet energy gaps, and vibrational spectra. Future challenges in the area of theoretical predictions include the accurate modeling of nitrenium ion reactions and reaction rates.

More recently, time-resolved vibrational spectroscopy has been applied to nitrenium ions. These techniques hold much promise for future research as they can provide more detailed structural information about nitrenium ions. In addition, the vibrational frequencies measured in this way can be used to assess the accuracy of theoretical calculations.

Nitrenium ions have been applied to the synthesis of macrocycles and other medicinally interesting compounds. The most successful reactions have been in cases where the nitrenium ion is covalently tethered to its intended target. Further efforts aimed at modulating the selectivity of these intermediates would increase their synthetic utility.

ADDITIONAL READING

R.A. Abramovitch and R. Jeyaraman, ‘‘Nitrenium Ions,’’ in Azides and Nitrenes, E. F. V. Scriven, Ed., Academic Press, New York 1984, p. 297.

M. Novak and S. Rajagopal, ‘‘N-Arylnitrenium Ions,’’ Adv. Phys. Org. Chem. 2001, I36, 167.

R.A. McClelland, ‘‘Flash Photolysis Generation and Reactivities of Carbenium Ions and Nitrenium Ions,’’ Tetrahedron, 1996, 52, 6823.

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