Dichlorocarbene catalytic scheme

Compared with primary and secondary amines, tertiary amines are virtually unreac-tive towards carbenes and it has been demonstrated that they behave as phase-transfer catalysts for the generation of dichlorocarbene from chloroform. For example, tri-n-butylamine and its hydrochloride salt have the same catalytic effect as tetra-n-butylammonium chloride in the generation of dichlorocarbene and its subsequent insertion into the C=C bond of cyclohexene [20]. However, tertiary amines are generally insufficiently basic to deprotonate chloroform and the presence of sodium hydroxide is normally required. The initial reaction of the tertiary amine with chloroform, therefore, appears to be the formation of the A -ylid. This species does not partition between the two phases and cannot be responsible for the insertion reaction of the carbene in the C=C bond. Instead, it has been proposed that it acts as a lipophilic base for the deprotonation of chloroform (Scheme 7.26) to form a dichloromethylammonium ion-pair, which transfers into the organic phase where it decomposes to produce the carbene [21]. 

Optically pure P-ethanolamines react with dichlorocarbene under phase-transfer catalytic conditions to produce epoxides of high configurational retention [30]. Initial reaction occurs at the tertiary nitrogen centre (Scheme 7.29) with subsequent cleavage of the C-N bond. The reaction is configurationally controlled, as shown by the reaction of the conformationally rigid cyclic systems epoxide formation occurs with the equatorial hydroxyl system (50%), but not with the axial hydroxyl compound. 

Tetrahydrophosphinine oxide 166 reacts with dichlorocarbene under phase-transfer catalytic conditions giving 7,7-dichloro-1-methyl-3-phenyl-3-phosphabicyclo[4.1.0]heptane (167) in 71% yield. Heating the phosphabi-cyclo[4.1.0]heptane 167 at 220 °C makes the bicyclic ring opening and the simultaneous elimination of hydrogen chloride gives 4-chloro-3-methyl-l-phenyl-2,7-dihydrophosphepin 1-oxide (168), a seven-membered phosphorus heterocycle, in 67% yield (Scheme 54) [66]. 

In the above cases, dichlorocarbene was generated from chloroform under liquid-liquid phase transfer catalytic conditions. Alternatively, sodium trichloroac-etate was also used as the precursor of dichlorocarbene as shown by the 6 -> 7 transformation (Scheme 3)[16, 18]. 

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