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Metalation of terminal alkynes is a significant side reaction that occurs under these conditions. If metalation is desired, tertiary amine complexes of DIBAL-H are useful. [13] (5) The use of silyl acetylenes avoids the problem of competitive metalation of terminal alkenes.
Terminal alkynes have the formula RC≡CH, where at least one end of the alkyne is a hydrogen atom. An example is methylacetylene (propyne using IUPAC nomenclature). They are often prepared by alkylation of monosodium acetylide. [4] Terminal alkynes, like acetylene itself, are mildly acidic, with pK a values of around 25.
An alkynyldimethylaluminum compound is the reaction product of trimethylaluminum and a terminal alkyne. [1] Synthesis of an ynone. An alternative is the direct coupling of an acyl chloride with a terminal alkyne, using a copper-based nanocatalyst: [2] Synthesis of an ynone
The Crabbé reaction (or Crabbé allene synthesis, Crabbé–Ma allene synthesis) is an organic reaction that converts a terminal alkyne and aldehyde (or, sometimes, a ketone) into an allene in the presence of a soft Lewis acid catalyst (or stoichiometric promoter) and secondary amine.
Metal alkyne complexes are intermediates in the semihydrogenation of alkynes to alkenes: C 2 R 2 + H 2 → cis-C 2 R 2 H 2. This transformation is conducted on a large scale in refineries, which unintentionally produce acetylene during the production of ethylene. It is also useful in the preparation of fine chemicals. Semihydrogenation affords ...
Usually two equivalents of sodium amide yields the desired alkyne. Three equivalents are necessary in the preparation of a terminal alkynes because the terminal CH of the resulting alkyne protonates an equivalent amount of base. Hydrogen chloride and ethanol can also be eliminated in this way, [11] as in the preparation of 1-ethoxy-1-butyne. [12]
Vinyl silyl can be made from terminal alkyne or other methods. The Kishi's group reported a mild preparation of vinyl iodide from vinyl silyl using NIS in mixture of acetonitrile and chloroacetonitrile. [20] They observed retention of olefin geometry in some vinyl silyl substrates while inversion in others.
For terminal alkynes, the reaction generally proceeds with good regioselectivity (>90:10 rr) and complete syn selectivity, even in the presence of propargylic or homopropargylic heteroatom substituents. Unfortunately, extension of the zirconocene-catalyzed methylalumination to alkylalumination with higher alkyls results in lower yields and poor ...