Classification of Carbamate Insecticides

Carbamates are usually classified according to the substituent on the NH-moiety, and they fall into two major groups: N-methyl and N-arylcarbamates, which exhibit insecticidal (both contact and systemic) and herbicidal activities, respectively. A more precise classification of the insecticidal N-methyl carba-mates is indicated below, and some significant examples from a commercial point of view are depicted in Figure 2.6:

- Aryl N-methylcarbamate insecticides (e.g. 66, 67)

- N,N-dimethylcarbamate insecticides (e.g. 68, 69)

- Benzofuranyl N-methylcarbamate insecticides ( e.g. 70, 71)

- Oxime carbamate insecticides (e.g. 72, 73)

Another group of carbamate derivatives is comprised of dithiocarbamates, sulfur-containing carbamates which have little or no esterase-inhibiting action and which are often, but not exclusively, used as fungicides and herbicides. These compounds are not discussed in any further detail in this section.

2.3.3.2.1 Aryl N-methylcarbamate Insecticides. Aryl N-methyl carbamates became one of the first efficient groups of insecticides in the 1950s, and currently comprise roughly 20 commercial derivatives.199 They are suitable against small sucking insects such as planthoppers and leafhoppers, and at the same time they lack toxicity against spiders, which act as predators against such insects.200

It was demonstrated that the presence of halogens and alkyl residues in either ortho or meta positions of the phenyl ring led to an increase in insecticidal activity.201 By contrast, the replacement of the N-methyl group with a longer alkyl chain improved the recovery of the insect, and therefore lead to an impairment of insecticidal activity.

One of the most popular carbamates in this groups is carbaryl 66 (1-naphthyl methylcarbamate), commercialized by Bayer under the brand name Sevin®. Carbaryl is widely used (granular, liquid, wettable power or dust formulations) to protect food crops, but also for home applications such as gardens,202 due to its low toxicity in mammals (oral LD50 of 500-850 mgkg-1 in rats).203 Propoxur 67 (Baygon®) is also quite effective as a home insecticide against cockroaches and mosquitoes,204 and was introduced as a replacement for DDT. Nevertheless, Propoxur possesses a relatively high toxicity against mammals (LD50 of 100mgkg_1 in rats, oral).

As a remarkable example of the lack of an eco-friendly methodology for the production of carbamates, we can mention the accident that occurred on December 3rd, 1984, at the Union Carbide factory in Bhopal (India), where both carbaryl 66 and aldicarb 72 were being synthetized.205 A gas leak of methyl isocyanate, one of the chemicals used in these syntheses, took place. Methyl isocyanate is slightly heavier than air, so it escaped into the atmosphere and stayed low to the ground. Approximately 40 tons of methyl isocyanate escaped into the air, spreading over a city of nearly 900 000 people. At least 3000 people died in the immediate aftermath, and over half a million people were seriously injured, and since then at least another 20 000 other deaths have occurred from gas-related diseases.

Environmental degradation studies on some of the carbamate insecticides proved that chemical hydrolysis was the major degradation pathway,189 especially at basic pH values. In the case of phenyl N-methylcarbamates, such degradation leads to the formation of methylamine and substituted phenols;

the presence of electron-withdrawing substituents increased the hydrolysis rate. The in vitro metabolism of some aryl N-methylcarbamates by the fetal tissues of Sprague Dawley rats was also investigated.206

Prousalis et al. developed199 a preparation of antibodies from chicken egg yolk to bind aryl N-methylcarbamates, which were used as immunosorbent preparations for improving the analytical quantification of environmental residues containing carbamates. Such antibodies exhibited a high binding capacity to carbaryl, trimethacarb, metolcarb, aminocarb and promecarb.

2.3.3.2.2 N,N-Dimethylcarbamate Insecticides. The N,N-dimethylcarbamate family of compounds is generally more stable than N-methylcarbamates, although their insecticidal properties are considerably lower.201 Dimetilan 68 and pirimicarb 69 are important examples used in baits for housefly control or as an aphicide on grain crops, respectively.207

Most insecticidal carbamates are esters of aromatic and heterocyclic hydroxyl derivatives of N-methylcarbamic acid. Reports on aryl N,N-dime-thylcarbamates are scarce however. In this context, Zhao et al.208 designed and synthesized a series of phthalimido alkyloxyphenyl N,N-dimethylcarbamates, in which the phthalimido and phenyl moieties are connected with an alkoxy chain. The variation of the AchE inhibitory activity upon modifying the length of the alkoxy chain and the position of the carbamato moiety on the phenyl ring were analyzed.

Concerning environmental degradation, Huang and Stone209 studied the hydrolytic cleavage of dimetilan 68 (see Figure 2.7), which can occur via nucleophilic attack at the carbamate group or at the substituted ureido moiety. This study showed a synergic effect between hydrolysis rates and the presence of + 2 transition metal ions, e.g. Ni(n), Cu(ii) and Zn(n), but not Pb(n).

2.3.3.2.3 Benzofuranyl N-Methylcarbamate Insecticides. Benzofuranyl-based insecticides are structurally related to aryl N-methylcarbamates, where a substituted furan ring is fused to the aromatic residue in the 2,3 positions. Carbofuran 70 is one of the most remarkable examples of benzofuranyl

Figure 2.7 Hydrolytic pathways for environmental degradation of dimetilan.

methylcarbamate insecticides, and many aspects of the compound have been studied: characterization of its AChE inhibition,210 analysis of its influence on mycorrhizal development,211 stimulation of nitrogenase and the populations of nitrogen-fixing bacteria associated with rice rhizosphere,212 and its leaching potential.213

Carbofuran (with an oral LD50 value for rats of 14mgkg-1) is one of the most toxic among the carbamate insecticides, and the toxicities of carbofuran and some derivatives have been investigated.214 Honeybees and earthworms are particularly sensitive to carbofuran, and numerous bird kills have been linked to direct ingestion of carbofuran while sifting sediment.215 For this reason its granular form has been banned in the USA. Nevertheless, carbofuran does not bioaccumulate to any noteworthy extent.

In 2008, following an announcement by the US EPA banning carbofuran,216 the only US manufacturer (FMC Corporation) announced that it had voluntarily requested the cancellation of six of the previously-allowed applications of the insecticide (on maize, potatoes, pumpkins, sunflowers, pine seedlings and spinach grown for seed).217 In May 2009, the EPA cancelled all uses of car-bofuran involving crops grown for human consumption.218

Much effort has been devoted to the study of the environmental behavior of carbofuran. Base-catalyzed hydrolysis to carbofuran phenol is considered to be the major degradation pathway of this insecticide in both water and sediments.219 Again, a substantial increase in the hydrolysis rate with increasing pH was observed both in soil and water.220 By contrast, the slower degradation in acidic and neutral soils was dominated by microbial mechanisms.221 Thus, a number of hydroxylated carbofuran metabolites produced in soils and in bacterial cultures have been isolated and identified, indicating that hydrolysis is not the only degradation mechanism. Breakdown products in soil include carbofuran phenol,222 3-hydroxycarbofuran and 3-ketocarbofuran.223

There is a general agreement which establishes that microbial degradation of a soil-applied pesticide may occur when a population of soil microorganisms is repeatedly exposed to a chemical and adapts by developing the ability to catabolize that chemical.224 226 In particular, for carbofuran, Getzin and Shanks found227 that enhanced degradation could take place with as little as just one or two applications.

Persistence of carbofuran in several media has also been reported. For instance, Caro et al.228 reported a soil dissipation half-life of 117 days in a cornfield. A low soil pH of 5.3 and low soil moisture content may explain the relatively slow rate of dissipation.

Photolysis is not generally considered a significant degradation pathway in water or soil, like oxidation and volatilization which are generally considered insignificant dissipation pathways for carbofuran in water.229 On the other hand, because of its water solubility (351 ppm at 25 °C) carbofuran is relatively mobile in soil and in surface runoff. Consequently it has the potential to contaminate lakes, streams and groundwater.

Although carbofuran possesses a low vapor pressure, it has been reported that its volatilization rate is much more rapid under flooded soil conditions than under non-flooded conditions, probably due to co-evaporation with the water of the soil surface.230

In addition, several studies have also been reported on the benzofuranyl methylcarbamate-related carbosulfan 71 concerning its degradation,231 toxicity232 and metabolism.233

2.3.3.2.4 Oxime Carbamate Insecticides. Oxime-based insecticides are the most recent carbamates, whose characteristic feature is the presence of an oximino ester scaffold.234 The double bond C=N confers rigidity, acting in a similar fashion as the aryl ring in N-aryl carbamates. Furthermore, two isomers (syn and anti) can be obtained which can sometimes be separated.234 The Z configuration was found to confer significantly more activity than the E counterpart. Compounds such as aldicarb 72 and methomyl 73, quite resembling acetylcholine, are commercially-available examples of oxime-based insecticides applied as plant systemic or foliar sprays, respectively.201

Aldicarb 72 was the first example of oxime N-methylcarbamates, with high water solubility, non-volatility, which was relatively stable under acidic conditions and easily degraded under alkaline conditions. These properties are important determinants of its systemic action in plants and of its problematic environmental behavior. Possible environmental hazards involving this chemical include groundwater contamination and excessive terminal residues in certain foods.235

Aldicarb has been registered worldwide to control a wide variety of insect, mite and nematode pests in agriculture. Due to its widespread use, many studies have been devoted to this insecticide. With an oral LD50 value for rats of 0.9mgkg-1 and classified by the EPA in the highest toxicity category, a strict control for its delivery and use has been established by this environmental agency.236

Several reviews concerning the toxicological effects of aldicarb have been published.237 The first review of aldicarb poisoning circumstances associated with clinical and analytical findings dates from 2000.238 In this sense, the toxicity and biochemical impact of several oxime carbamates have been tes-ted.239 Despite its acute toxicity to humans and laboratory animals, aldicarb is readily absorbed through both the gut and the skin, but is rapidly metabolized and excreted in the urine almost completely within twenty-four hours. Moreover, it is not known to be carcinogenic, teratogenic, or to produce other long-term adverse health effects.

Several experiments have been conducted to investigate the degradation of aldicarb, for instance, in sterile, non-sterile and plant-grown soils, and the capability of different plant species to accumulate the compound.240 This study showed that microorganisms play an important role in the degradation of aldicarb in soil. Likewise, the development of different methods for the separation of aldicarb and its degradation products is also of great interest,241 allowing for the routine monitoring of aldicarb and its soil derivatives in water at concentrations less than 1 mgl-1.242

Given that neither a toxicokinetic model nor an estimate of the target tissue dose of aldicarb and its metabolites in exposed organisms was available, a physiologically based toxicokinetic model was recently developed in rats and humans.243 The model describes the time-course behavior of the chemical in blood, liver, kidney, lungs, brain and fat. Perkins and Schlenk244 have also described the in vivo acetylcholinesterase inhibition, metabolism and toxicokinetics of aldicarb in Channel catfish.

The use of another important oxime carbamate, methomyl 73, is also restricted because of its high toxicity to humans.245 It has been object of several studies related to: the fatal poisoning caused by its inhalation and transdermal absorption;246 its penetration and fate in insects and two-spotted spider mites;247 its degradation by reduction in the presence of Fe(ii)248 and Cu(ii);249 as well as chromatographic methods for its quantification in blood.250

Recently, the risk assessment and chemical decontamination of methomyl from eggplants was investigated, in which it was found that the insecticide was neither appropriate nor effective for application in this vegetable.251

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