Phenylurea and Related Compounds

Phenylurea and A, A-dimethylphenylurea (380 and 381) reacted easily with hot chlorosulfonic acid (four equivalents) at 60-80 C (2 hours) to give the corresponding ureidobenzenesulfonyl chlorides 382 and 383 (Equation 120). 

When the substituents were in the m- or / -positions, the chlorosulfonation was more difficult and much lower yields of the sulfonyl chlorides were obtained. For instance, 3-methylphenylurea reacts with chlorosulfonic acid (four equiva- 

In the chlorosulfonation of 4-methylphenylurea 384, electronic factors favour sulfonation ortho to the ureido group giving 5-methyl-2-ureidobenzenesulfonyl chloride 385, while steric considerations favour attack ortho to the methyl group leading to 2-methyl-5-ureidobenzenesulfonyl chloride 386. NMR spectroscopy indicated that the major product of the chlorosulfonation was compound 386, suggesting that steric factors were paramount.  

Attempted chlorosulfonation of 3- and 4-chlorophenylureas failed, presumably as a result of a combination of unfavourable stereoelectronic factors. Sulfonylureas are an important group of highly potent herbicides which may be used for selective weed control in cereals at very low dose rates. Herbicidal (p-sulfamoyl)phenylureas were synthesized from the appropriate phenylureas by mixing them with chlorosulfonic acid at 12-15 C, followed by heating at 60 C and the resultant p-(chlorosulfonylphenyl)ureas were condensed with the amine. ° 

Phenylthiourea 387, by warming with chlorosulfonic acid (three equivalents) at 60 °C (3 hours), gave an abnormal product considered to be the iminosulfonic acid 388 which did not react with phosphorus pentachloride (Equation 122). 

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