Dichlorocarbene, reactions with alkynes

Alkynes tend to be much less reactive than alkenes. For example, 1,2-diphenylethyne produces only 23% of the dichlorocyclopropene from its reaction with dichlorocarbene, compared with 96% of the dichlorocyclopropane obtained from rrans-stilbene under analogous conditions [4]. Conjugated eneynes react preferentially at the C=C bond with dihalocarbenes [18-20, 22, 38] and with dimethylvinylidene carbene [158],... 

The addition of dihalocarbenes to alkynes is again a rather inefficient process and usually leads, to the isolation of the cyclopropenone rather than the 3,3-dichlorocyclo-propene. In a rather unusual example, however, 2-butyne is reported to be converted to (67). This product is apparently derived by addition of dichlorocarbene to the corresponding methylenecyclopropene, derived in turn by elimination of HC1 from the primary adduct (68). The cyclopropene (67) does not appear to ring open to a vinylcarbene, but can be trapped in Diels-Alder reactions with cyclopentadiene 60). A related addition of dichlorocarbene to ethyl 2-butynoate also leads to a low yield of the 3,3-dichlorocyclopropene, which may be hydrolysed to the cyclopropenone 6l). 

The reactivity of dichlorocarbene towards carbon-carbon triple bonds was systematically investigated by Dehmlov in the sixties, and this work has been reviewed". Dehmlov generated the carbene e.g. by decomposition of sodium trichloroacetate in 1,2-dime thoxy ethane. In additions to ene-yne substrates, sometimes the triple bond, whereas in other cases the double bond accepted the carbene. He observed, for instance, that reaction of the carbene with (E)-l,4-diphenylbutene-l-yne-3 gave the triple-bond addition product 20 exclusively. In reaction with 2-methylpentene-l-yne-3, however, the carbene preferred the double bond, leading to 21. Dichlorocarbene generated from potassium f-butylate and chloroform does not add to electron-poor alkynes, indicating electrophilic behavour of the carbene. 

SoHd-liguid phase-transfer catalysis. Crown ethers have commonly been used as catalysts for reactions between a solid-liquid interface, and quaternary ammonium and phosphonium salts have been used only as catalysts for reactions in two-phase liquid liquid reactions. However, several laboratories have reported that the latter catalysts are also satisfactory for two-phase solid liquid reactions. Thus dichlorocarbene can be generated from chloroform and solid sodium hydroxide under catalysis from benzyltriethylammonium chloride in yields comparable to those of the classical Makosza method. Another example of this type of catalysis is the oxidation of terminal and internal alkynes by solid potassium permanganate in CH2CI2 with Adogen 464 as catalyst. Aliquat 336 has been found to be as satisfactory as a crown ether for certain displacement reactions with NaOAc, KSCN, KNOa, and KF in CH3CN or CHaCla. ... 

The dibromocyclopropanes are frequently preferred to their less reactive dichlorides, although the latter are often prepared in higher yields from dichlorocarbene additions to alkenes. Di-fluorocyclopropanes are not useful precursors to cyclopropylidenes with reactions leading to alkylated cyclopropenes and alkynes. ... 

Alkynes react with various dichlorocarbene reagents to give 3,3-dichlorocyclopropenes 1, which can be hydrolyzed to cyclopropenones 2 in situ. The carbene reagents used are (A) chloroform, 50% sodium hydroxide, and triethylbenzylammonium chloride (TEBAC) (B) (bromo-dichloromethyl)phenylmercury (C) chloroform and potassium tert-butoxide and (D) chloroform and butyllithium. Although the yields of these reaction generally do not exceed 20%, a variety of cyclopropenones have been prepared by this method. The yields of biscyclo-propenones from diynes are very low. 

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