Carbon-hydrogen bonds alkynylations

Activated unsaturated compounds containing highly acidic carbon—hydrogen bonds react selectively with sulfur under mild conditions for example, terminal alkynes (4) give the alkynyl thiols (5) (Scheme 2). 

The first step in this scheme is a Michael addition of the nucleophile to the j5-carbon of the alkynyliodonium salt to give the ylide 102. Loss of iodobenzene from 102 gives alkylidenecarbene 103, which rearranges to alkyne 104 in the absence of external traps. This mechanism is experimentally supported by the isolation of cyclic by-products 108 besides the major products, the alkynyl esters 107 in the reaction of alkynyliodonium salt 105 with nucleophiles (equation 67). These cyclic enol ethers are the result of the insertion of the intermediate carbene 106 into the tertiary-8-carbon-hydrogen bond. 

Aryl, allyl, alkyl, alkynyl-palladium species being formed under the various conditions stated above might be subject of a reduction to yield a carbon-hydrogen bond. In some cases, reduction is a serious side-product in cross-coupling reactions. The reduction of allyl complexes has been discussed on page 800. 

All contain an acyl group, R—c- bonded to another residue. The R substituent of the acyl group may be alkyl, aryl, alkenyl, or alkynyl the other substituent to which tlie acyl fragment is bonded may be a carbon, hydrogen, oxygen, halogen, nitrogen, sulfur, or other atom. 

Now that the properties of carbon-carbon double bonds have been examined in detail, it s time to have a brief look at their relatives, carbon-carbon triple bonds. Not surprisingly, what you will find will be very similar to what you ve just. seen. Addition reactions will mtike up the main portion of triple bond chemistry. An iinporlani feature is the hydrogen attached to triply bonded carbon It is unusually acidic, allowing ready removal by strong bases to form a new and synthetically useful class of carbanions called alkynyl anions. 

A triple bond a to a cyclopropane ring facilitates abstraction of a hydrogen at the carbon atom to which the alkynyl group is attached. The efficiency of the process depends on the group on the other side of the triple bond. When 1-cyclopropylpropyne was reacted with butyl-lithium, reaction occurred at the methyl group exclusively whereas l-cyclopropyl-3-methylbut-1-yne under the same conditions was attacked at the cyclopropane ring only and afforded a... 

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