Alkynides

R,8S)-(+)-Disparlure (12) is the female sex pheromone of the gypsy moth (Lymantria dispar). Advent of Sharpless asymmetric dihydroxylation (AD) allowed several new syntheses of 12 possible. Sharpless synthesized 12 as shown in Scheme 17 [27]. Scheme 18 summarizes Ko s synthesis of 12 employing AD-mix-a [28]. He extended the carbon chain of A by Payne rearrangement followed by alkylation of an alkynide anion with the resulting epoxide to give B. Keinan developed another AD-based synthesis of 12 as shown in Scheme 19 [29]. Mit-sunobu inversion of A to give B was the key step, and the diol C could be purified by recrystallization. 

The acetylenic hydrogen is weakly acidic (pKa 25) and can be removed with a strong base (e.g. NaNH2) to give an anion (called an alkynide anion or acetylide ion). 

An alkyne Sodium amide An alkynide anion R must be methyl or 1° and... 

The alkyl halide used with the alkynide anion must be methyl or primary and also unbranched at its second (beta) carbon. 

Preparation of the alkynide anion involves simple Bronsted-Lowry acid-base chemistry. 

The alkynide anion is a Lewis base and reacts with the alkyl halide (as an electron pair acceptor, a Lewis acid). 

Electrostatic potential maps illustrate the complementary nucleophilic and electrophilic character of the alkynide anion and the alkyl halide. 

Sodium alkynides can be prepared by treating terminal alkynes with NaNH2 in liquid ammonia. 

Sodium alkynides are useful intermediates for the synthesis of other alkynes. 

This synthesis fails when secondary or tertiary halides are used because the alkynide ion acts as a base rather than as a nucleophile, and the major results is an E2 elimination. 

Sodium alkynides react with aldehydes and ketones to yield alcohols. 

Generally, organocopper compounds can be prepared by transmetallation between copper salts and organometallic reagents such as RLi, RMgX, and RZnX.53,53a,53b Copper alkynides can be obtained by reaction of terminal alkynes... 

Ru-vinylidene complexes can be easily prepared by reaction of low-valent ruthenium complexes with terminal acetylenes. Treatment of the Ru(ii) complex 117 with phenylacetylene gave the Ru(iv)-vinylidene complex 118 in 88% yield (Scheme 41 ).60 The reaction of 118 with C02 in the presence of Et3N afforded selectively the Ru-carboxylate complex 120, probably via the terminal alkynide intermediate 119. 

The carbaalanes [8, 9] possess clusters formed by aluminum and carbon atoms. They represent a new class of compounds which, in some respects, may be compared to the important class of carbaboranes. Usually, they were obtained by the reaction of aluminum alkynides with aluminum hydrides (hydroalumination) and the release of trialkylaluminum derivatives (condensation). The first carbaalane, (AlMe)g(CCH2Ph)5H 3 [10], was synthesized by the treatment of dimethylalumi-num phenylethynide with neat dimethylaluminum hydride. The idealized stoichiometric ratio of the components is given in Eq. (2), which also shows a schematic drawing of the molecular structure. Compound 3 was isolated in the form of colorless crystals in 60% yield. While 3 is only slightly air-sensitive, the less sterically shielded propynide derivative 4, also shown in Eq. (2), is highly pyrophoric [11],... 

However, when it comes to the more important 2,2-diaryl derivatives (1.25), the routes illustrated in Figure 1.7 are not very useful. For these derivatives the almost universally adopted synthetic method involves the reaction of a l,l-diarylprop-2-yn-l-ol (1.24) with a substituted phenol or naphthol in the presence of an acid catalyst. The acid catalyst can be alumina, an acidic clay or Nafion for heterogeneous reactions, or trifluoroacetic acid, p-toluenesulfonic acid and dodecylbenzenesulfonic acid for reactions carried out in solution. The alkynols are prepared by reaction of a benzophenone (1.22) with a Na or Li derivative of an alkynide, such as the trimethylsilyl acetylide (1.23), (Figure 1.8). ... 

Alcohols are obtained from epoxides by acid-catalysed cleavage of H2O or base-catalysed cleavage by Grignard reagents (RMgX, RLi), metal acet-alides or alkynides (RC=CM), metal hydroxides (KOH or NaOH) and LiAlH4 (see Section 5.5.4). 

Terminal alkynes are acidic, and the end hydrogen can be removed as a proton by strong bases (e.g. organolithiums, Grignard reagents and NaNH2) to form metal acetylides and alkynides. They are strong nucleophiles and bases, and are protonated in the presence of water and acids. Therefore, metal acetylides and alkynides must be protected from water and acids. 

Metal acetylides or alkynides react with primary alkyl halides or tosylates to prepare alkynes (see Sections 5.5.2 and 5.5.3). 

Besides electrophilic addition, terminal alkynes also perform acid-base type reaction due to acidic nature of the terminal hydrogen. The formation of acetylides and alkynides (alkynyl Grignard reagent and aUcylnyllithium) are important reactions of terminal alkynes (see Section 4.5.3). Acetylides and alkynides undergo nucleophilic addition with aldehydes and ketones to produce alcohols (see Section 5.3.2). 

They react with alkyl halides to give internal alkynes (see Section 5.5.2) via nucleophilic substitution reactions. This type of reaction also is known as alkylation. Any terminal alkyne can be converted to acetylide and alkynide, and then alkylated by the reaction with alkyl halide to produce an internal alkyne. In these reactions, the triple bonds are available for electrophilic additions to a number of other functional groups. 

Acetylide (RC=CNa) and alkynide (RC=CMgX and RC=CLi) are good nucleophiles. They react with carhonyl group to from alkoxide, which under acidic work-up gives alcohol. The addition of acelylides and alkynides produces similar alcohols to organometallic reagents. 

We have already learnt that alkyl halides react with alcohols and metal hydroxide (NaOH or KOH) to give ethers and alcohols, respectively. Depending on the alkyl halides and the reaction conditions, both S l and Sn2 reactions can occur. Alkyl halides undergo a variety of transformation through Sn2 reactions with a wide range of nucleophiles (alkoxides, cyanides, acetylides, alkynides, amides and carboxylates) to produce other functional groups. 

Preparation of alkynes The reaction of primary alkyl halides and metal acetylides or alkynides (R C=CNa or R C=CMgX) yields alkynes. The reaction is limited to 1° alkyl halides. Higher alkyl halides tend to react via elimination. 

---