Phosphorins, electrophilic substitution

This type of X -phosphorin reaction opens up new possiblities of preparing 4-substituted X -phosphorins by other electrophilic substitutions. They are now being studied by us. 

Phosphinine and its derivatives are clearly aromatic however, they are considerably more reactive than benzene. The most significant influence on the reactivity of these molecules is the presence of the lone pair on phosphorus, and two significant reactions are its complexation with a variety of metals, and nucleophilic attack to form (ultimately) A5-phosphorins. The 71-system can undergo [4+2] cycloadditions, under milder conditions than benzene. Electrophilic substitution reactions on carbon are considered to be impossible <2001CRV1229>. 

Electrophilic substitution reactions at C-4 of phosphorins have been described. For example (104) reacts with diazonium salts in the presence of water (Scheme 7), to give an azo-compound, which can be protonated on the /S-nitrogen atom. In the presence of polarizable anions phosphinic acid derivatives (105) are formed. 

The P-substituted phosphorin anions 133 are ambident. The site of electrophilic addition depends upon the reaction conditions ... 

These X -phosphorins 720 a-c also fail to react with carbonyl compounds. However, they are attacked by electrophiles (H or alkyl cations) at the C—2 position. In this manner new 1,1-diphenyl-2,3-benzo-X-phosphorins which are sustituted at positions C—2 (and C-4) can be prepared. Diazonium ions attack at C—4 to form azocompounds if an excess is used, C—2 is also substituted Hydrolysis with hot water affords 747. The reaction with ortho-formic acid ester forms a cyanine dye having a bridge at the C—4 positions 142 The experimen-... 

Very useful transformations of the 4 -methylcarbenium salts (49) are possible in the presence of electrophiles and trace amounts of water. The 4-CH2+ group first gives the unstable 4-CH2OH derivatives (62), which with electrophiles, e.g. the carbenium salts (49) themselves or aryldiazonium salts, give the 4-E substituted A -phosphorin derivatives (63) or (64), respectively. The CH2OH+ group leaves position 4, and stable 4-E substituted A5-phosphorins are formed (equation (36)) (72TL843). 

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