ACTIVATION OF ORGANOMAGNESIUM REAGENTS BY CROWN ETHERS - UNUSUAL ETHER CLEAVAGE OF (2-METHOXY-1,3-XYLYLENE)-15-CROWN-4

The reaction of (2-methoxy-1,3-xylylene)-15-crown-4 (4a) with diphenylmagnesium at room temperature resulted in the cleavage of the methyl-oxygen bond with unprecedented ease, leading to (2-hydroxy-1,3-xylylene)-15-crown-4 (10) and toluene. In order to rationalize this unusually high reactivity, we suggest a transition state (8 or 9) with both an electrophilic and a nucleophilic component: the cationic magnesium atom is involved in an electrophilic attack on the oxygen, while the magnesate counterion attacks the methyl group in an S(N)2 process.