Alkyl acetylenes

The first examples of NHC-Pd complexes applied to the Sonogashira reaction were reported to show a limited scope in the coupling of aryl iodides and activated aryl bromides with acetylene [23,33,52]. However, the use of A-carbamoyl-substituted heterocyclic carbene Pd(ll) complexes expanded the use to alkyl-acetylenes and deactivated aryl iodides and bromides [124] (Scheme 6.40). 

The reactivity of dichloro carbene towards acetylenic bonds was systematically investigated by Dehmlow19, 20 with respect to substitution of the acetylene, especially those containing additional C-C multiple bonds. It was shown that with aiyl alkyl acetylenes, e.g. 1-phenyl-butyne-l, often the normal cyclopropenone formation occurs only to a minor extent (to yield, e.g. 14), whilst the main reaction consists of an insertion of a second carbene moiety into the original acetylene-alkyl bond (giving, e.g. 15) ... 

Of these, we have studied the reaction between the halide complexes and alkyl acetylenes most closely. Addition of excess 2-butyne to W(CCMe3)(dme)Cl3 yields red, paramagnetic, soluble W(t)5-C5Me4Bu )-(MeC5CMe)Cl2, and orange, paramagnetic,... 

Copper(II) acetylide may be prepared by passing alkyl acetylene vapors over aqueous solution of ammoniacal copper salt. 

The polymerization of acetylene by Ziegler catalysts very likely involves metal alkyl-acetylene insertion reactions also 26). 

Alkylation of Alkynes. Organic halides can alkylate acetylenes in the presence of Lewis acids. In most cases, however, the products are more reactive than the starting acetylenes. This and the ready polymerization of acetylenes under the reaction conditions result in the formation of substantial amounts of byproducts. Allyl, benzyl, and tert-alkyl halides giving stable carbocations under mild conditions are the best reagents to add to acetylenes.44 56... 

The best copper reagents are RCu.MgHlg2 and R2Cu.MgHlg derived from Grignard reagents, and R2CuLi (R = primary alkyl). Acetylene is the most reactive alkyne,515 whereas internal acetylenes do not react. 

Both 1-pentyne and 2-pentyne can be prepared by alkylating acetylene. All the alkylation steps involve nucleophilic substitution of a methyl or primary alkyl halide. 

Experiments showed that protecting one end of the diol 30 as the THP derivative 31 (chapter 9) gave good yields and the monobromo compound 32 was used to alkylate acetylene with the methyl group added later. Reduction and deprotection gave the E -alcohol 26 and acetylation gave the pheromone 25. 

General Procedure for Coupling of Aryl Chlorides with Alkyl Acetylene According to Buchwald and Gelman [48]... 

It is generally considered that alkylacetylenes are inert while others, especially with conjugative substituents, are active in polarographic reduction. That is, the supporting electrolyte and/or the solvent are usually reduced in preference to an alkyl-acetylene. It has been estimated that, relative to the standard calomel electrode (SCE), 3T for cyclononyne, 5-decyne, 1-hexyne, 3-hexyne and 2,2,5,5-... 

Alpine-Borane is an effective, readily available reagent for the reduction of acetylenic ketones. This class of ketones is reduced somewhat slower than aldehydes but the reactions generally proceed at 25 °C. Straight chain alkyl acetylenic ketones are completely reduced after 8 hours at 25 °C using a 0.5 M solution of Alpine-Borane in tetrahydrofuran. More hindered internal acetylenic ketones may require 1 4 days under these conditions, but the process may be accelerated by performing the reaction in the absence of solvent. Acetylenic ketones are reduced faster than other ketones, and can also be reduced in the presence of a methyl ketone24. An example is the selective reduction of 7-nonyne-2,6-dione24. 

Acetylides and fulminates form highly explosive shock- and heat-sensitive salts with many metals. Acetylides are the metal derivatives of acetylene. Hydrogen attached to carbon atoms bearing a triple bond is acidic in nature. It can be substituted by a metal ion to form acetylides, with the general structure M-C=C-M. These substances are made from acetylene, HC=CH, or alkyl acetylene, RC=CH, by passing acetylene gas or aUcyl acetylene vapors over the aqueous solutions of ammoniacal metal salts, as shown in the following reactions ... 

Acetylene and disubstituted acetylene (such as 2-butyne) do not form copolymers with SO2. However, -alkyl acetylenes and phenylacetylene reaction to give a copolymer. 

In favourable cases, the Lewis acid catalysed addition of mono-alkyl acetylenes to olefins also gave useful yields of cyclobutenes. The yields were highest with the most highly alkylated oleflnic reactants. Bicyclo[2,2,0]hexenes were formed in a secondary reaction. The use of dialkyl acetylenes in this reaction did not give any [2+ 2] addition products. 

In the synthesis of propargylic alcohols, we saw the reaction of an alkynyl nucleophile (either the anion RC=CNa or the Grignard RC CMgBr, both prepared from the alkyne RC CH) with a carbonyl electrophile to give an alcohol product. Such acetylide-type nucleophiles will undergo Sn2 reactions with alkyl halides to give more substituted alkyne products. With this two-step sequence (deprotonation followed by alkylation), acetylene can be converted to a terminal alkyne, and a terminal alkyne can be converted to an internal alkyne. Because acetylide anions are strong bases, the alkyl halide used must be methyl or 1° otherwise, the E2 elimination is favored over the Sn2 substitution mechanism. 

Vinylallenes (88) may be alkylated in good yield to give the 1-alkylallene (89) with minimal formation of the corresponding alkylated acetylenic isomer. Factors that control the regioselectivity of this reaction have been identified as the addition of 1 mole of HMPA to the 1 1 ether-THF solvent mixture, the reaction being carried out at -70 °C, and with strict control of the time interval between formation of the lithiated allene and its reaction with the halide. 

The catalyst Fe(acac)3/AlEt3 will polymerize 1° or 2° alkyl-acetylenes (HC=CR) to soluble high-molecular-weight polymers [112]. However, when the R groups are either 3°-alkyl or aromatic or contain a heteroatom, only oligomers and insoluble polymers are obtained. In general, Ziegler-Natta catalysts are not known to polymerize disubstituted acetylenes (RC=CR ). 

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