Halocyclopropane alkene

Halocyclopropanations are found in Section 74F (Alkyls from Alkenes). 

Both phase-transfer and crown ether catalysis have been used to promote a-elimination reactions of chloroform and other haloalkanes.90 The carbene is trapped by alkenes to form halocyclopropanes. 

However, the ready availability of halocyclopropanes has led to extensive studies of their 1,2-dehydrochlorination, and amines are now rarely used as cyclopropene precursors. Although the reaction of 1,1-dichlorocyclopropanes with strong base does in certain situations lead to cyclopropenes, it is frequently the case that the initially formed 1-halocyclopropene does not survive under the reaction conditions, undergoing either addition of a nucleophile to the alkene bond or prototropic shifts followed by further dehydrohalogenation. Two main variations on this method are available which proceed under conditions where further reaction does not, in general, occur, that is 1,2-dehalogenation and 1,2-dehalosilylation. Each of these three alternatives will be considered in turn. 

Synthesis of 1-aryl-l-halocyclopropanes via addition of arylhalocarbenes to alkenes has been described in Houben-Weyl, Vol.4/3, pp228-233 and Vol.E19b, pp979-980, 982-986, 989, 994-996, 998 and 1001. 

The following methods for the preparation of 1-aryl-l-halocyclopropanes, based on the generation of arylhalocarbenes and their cycloaddition to alkenes, are known ... 

Alkynyl-l-halocyclopropanes are prepared by the addition of an alkynylhalocarbene (car-benoid) to an alkene. The process is realized via ... 

A mixture of l-(dihalomethyl)alkyne and an alkene is treated with potassium tert-hwi-oxide/ or powdered potassium hydroxide in the presence of benzyltriethylammonium chloride as a catalyst, to give 1-alkynyl-l-halocyclopropanes 1 in reasonable to good yields (Table 35). 

Table 35. 1-Alkynyl-l-halocyclopropanes from Alkynylhalocarbenes and Alkenes... 

Photolysis or thermolysis of a mixture of a 3-alkoxy(or -aryloxy)-3-halo-3//-diazirine and an alkene affords 1-alkoxy(or-aryloxy)-l-halocyclopropanes 3 (Houben-Weyl, Vol. E19b, pp 1654-1661). 

Table 37. l-Alkoxy(or -Aryloxy)-l-halocyclopropanes from Carbenes, Generated from Diazirines, and Alkenes... 

For halocyclopropanations, see Section 74F (Alkyls from Alkenes Cylclopropanations). cat. S-BINAP, H20/THF... 

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