Preemergence activity, herbicides

Figure 2d. Three-dimensional perspective plot of the Hansch equation describing preemergence activity of 3-substituted TFMS herbicides on Foxtail grass (0.1% Tween 80 present)...

Although the synthesis program did not result in any commercial herbicides, two types of pyridazines were discovered which produced unexpected results, both in the level and the type of activity. The first series, which includes the 3-methoxy- and the 3-dimethylaminopyridazines, resulted in a high level of postemergence activity not observed in the lead or in the other 3-sub-stituted-pyridazines. Secondly, based on a comparison with the N-oxides of other pyridazines, the N-oxide of the 3-hydropyridazine resulted in unexpectedly high preemergence activity, yet without phytotoxicity to cotton. 

Herbicidal Activity. The subject furanones are primarily active as preemergence and preplant incorporated (PRE/PPI) materials and are moderately active when applied postemergence. The structure-activity relationships discussed below pertain to the preemergence activity. 

Since the meta-substituted epoxides were the most active herbicides, we next chose to look at the 3,5-disubstituted compounds. The first compound prepared was the 3,5-dichloro derivative, tridiphane, Its precursor, a-(2,2,2-trichloroethy 1)-3,5-dichlorostyrene, showed very little preemergent... 

Fluazifop-butyl has a preemergence activity too, lasting about 3-6 weeks according to soil type. This activity is only 25-50% of the herbicidal effect of postemergence application. Its selectivity is believed to be due to rapid degradation followed by conjugate formation in broad-leaved plants (Plowman et ai, 1980). 

Alachlor is a selective preemergence soil herbicide with action similar to that of propachlor. With its poorer solubility in water it needs more soil moisture than propachlor to exert its action, and irrigation must be used in a dry spring to obtain a satisfactory herbicidal action. On a molar basis, alachlor is twice as efficient as propachlor, and its activity lasts 2-4 weeks longer (Evans et al., 1968). 

Naptalam is a preemergence soil herbicide with systemic action and some hormone action. It can be used selectively at rates of 4.5 kg active ingredient/ha in cucurbits, soybean, potatoes and peanuts. It is not persistent, being degraded within 3-8 weeks in the soil. 

Prosulfalin (EL 131, 15) is a selective preemergence experimental herbicide effective against annual grass weeds and a few broad-leaved weeds on turf at application rates of 1.5-3.0 kg active ingredient/ha. On established turf it is used 3 weeks before the shooting of the weeds. Bermuda grass is sensitive to prosulfalin. Rain or irrigation is needed for the activation of the herbicide. 

As a starting point for structural optimization of the second lead, we divided the core structure into three subsections, the phenyl (X), the imino group (Y), and the thiazoline nucleus (R and R ) (Figure 5 ). First, we synthesized the 2-(N-ethoxycarbonylimino)-l,3-thiazolines that possessed a variety of substituents at each position of the phenyl ring to examine the effect on herbicidal activity. Preemergence activities are summarized in Figure 6. Substitution on the phenyl moiety was preferred in the meta position and we observed a progressive... 

When 3 was treated with two equivalents of sodium cyanide in DMF and heated to 100°C for two hours the 5-cyano-pyrazole ester 9 was produced and converted to the corresponding carboxamide 11 (Figure 3). Compound 11 was active at 0.5 Ib/acre in the greenhouse, and this derivative was pursued as a preemergent cereal herbicide (, 2). ... 

Cineole, 13, is a natural herbicide [73]. Its hydroxy derivative 14 (2-hydroxy-1,4-cineole 1,4-epoxy-p-menthane-2-ol) is a constituent of oil from rhizomes of Ferula jaeschkeana [74]. Its 2-methylbenzyl ether 15 (cinmethylin) is a preemergence grass herbicide [75, 76]. Alcohol 14 can be prepared by microbial hydroxy-lation of 13 [77]. This also produces ketone 16 and its enantiomer [78]. The fragrance of ketone 16 and isomeric l-isopropyl-4-methyl- 7-oxabicyclo[2.2.1] heptan-2-one is very similar to that of 14 and menthone [79]. Mullilam diol 17, a dihydroxy derivative of 13, has been isolated from Zanthoxylum rhetsa, a plant that exhibits antibiotic activity which is prescribed in dyspepsia and diarrhea. The eight-carbon system rengyoxide has been found in Forsythia suspensa fruits [80] (Fig. 4). 

Amino-2-bromoimidazoles (77) have been prepared by the action of hydrogen bromide on a-cyanoalkylcyanamides (74) and were found to display preemergent herbicide activity (64JOC153). These 4-aminoimidazoles (77) are unstable in the presence of water and undergo rapid decomposition on treatment with base. 

The sulfonylureas described here are new herbicides with unprecedented activity. Activity at extraordinarily low rates of application has been demonstrated by both preemergence and postemergence applications (Ref. 1). These rates are best describee in grams per hectare compared to kilograms per hectare for most herbicides currently marketed. Furthermore, the combination of low application rates, half lives of generally less than two months in the soil (Ref. 2) and excellent safety to mammals [LD5o s of generally >5000 mg/kg for technical material in male rats] (Ref. IC) makes these compounds attractive products from an environmental viewpoint. 

From the earliest greenhouse studies, it was obvious that AC 252,214 was an interesting compound. Remarkably, it was well tolerated by legumes, especially soybeans, yet showed a broad-spectrum of herbicidal activity, killing broad-leaved, grassy and perennial weeds. Further it was shown that AC 252,214 could be used both as a preemergent and postemergent herbicide. Rates for field application are 125-250 g/ha (12). 

Several polymeric acyl-transfer reactants have been used to give am-ide/ester products in the solution phase. The excess polymer-bound acyl-transfer reactants and polymer-bound nucleofuge byproducts are easily removed after completion of the reactions. One such application involved the activated nitrophenyl esters 25 (reaction 8).40 A mixture of 10 acid chlorides was converted to an equimolar mixture of 10 amide products a potent preemergent herbicide was discovered using this parallel synthetic approach.41... 

Triazine herbicides, principally simazine, have been an integral part of this change in management practice. Simazine is used alone and in combination with other preemergence herbicides for weed control in the plant row. It has low water solubility, low volatility, long residual activity, and gives a broad spectrum of annual weed control. 

There are three potential solutions for annual and perennial weed problems with drip irrigation. One is the use of preemergence herbicides that have residual weed control properties, particularly under continuous moisture. Another is the use of contact herbicides on the emerged weeds in the wet spot or persistent herbicides with contact activity. A third approach is the use of herbicides through the drip system when allowed by the product label (i.e., the use of herbicides capable of killing weeds as they germinate). With all three approaches it is essential that perennial weeds... 

A variety of herbicides have been used in combination with simazine and other triazines. These herbicides have preemergence and postemergence activity and are primarily effective on grass and perennial weeds. The preemergence herbicides used in combinations with simazine include oryzalin, pendimethalin, prodiamine, norflurazon, oxy-fluorfen, and diuron. Glyphosate, oxyfluorfen, paraquat, and amino triazole (used in some parts of the world) are... 

Diuron was introduced by du Pont in 1954 (Fig. 3). It is one of a series of substituted urea herbicides. Diuron is applied preemergence to crops such as cotton, alfalfa, grapes, fruit and nut crops. Foliar activity is enhanced when a surfactant is added to the spray. 

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