Arylalkynes, terminal alkyne reaction

Terminal alkynes undergo the above-mentioned substitution reaction with aryl and alkenyl groups to form arylalkynes and enynes in the presence of Cul as described in Section 1.1.2.1. In addition, the insertion of terminal alkynes also takes place in the absence of Cul, and the alkenylpalladium complex 362 is formed as an intermediate, which cannot terminate by itself and must undergo further reactions such as alkene insertion or anion capture. These reactions of terminal alkynes are also treated in this section. 

Pt-catalyzed hydration of various aliphatic and aromatic alkynes under phase transfer conditions in (CH2C1)2/H20 in the presence of Aliquat 336 led to either a Markovnikov product, mixtures of two ketones, or ketones with the carbonyl group positioned away from the bulky side.72 In the absence of the phase transfer reagent, Aliquat 336, hardly any reaction took place. Recently, a hydrophobic, low-loading and alkylated polystyrene-supported sulfonic acid (LL-ALPS-SO3H) has also been developed for the hydration of terminal alkynes in pure water, leading to ketones as the product.73 Under microwave irradiation, the hydration of terminal arylalkynes was reported to proceed in superheated water (200°C) without any catalysts.74... 

As described above (Section 5.2), the Stephens-Castro reaction of alkynylcopper with aryl and vinyl halides in boiling pyridine is a useful route to aryl and vinyl acetylenes. Direct cross-coupling of organic halides, such as sp halides, with terminal alkynes is a more convenient procedure. Such a reaction is not so easy, but it can be done using a Pd-complex catalyst [41]. Especially facile Pd-catalyzed cross-coupling of aryl and alkenyl halides with terminal alkynes proceeds smoothly under mild conditions in the presence of a cocatalyst of cuprous iodide in amine solvents [Eq. (28)] [42]. This methodology is now used widely for the constiuction of conjugated arylalkyne or enyne systems [43], as described below. It is attractive from a synthetic point of view because mild reaction conditions and simplicity of the procedure are associated with recent developments in modem acetylene chemistry [44]. 

Pd-catalyzed coupling reactions of terminal and internal alkynes 1 and 4 with halides are surveyed in this section. Reactions of alkynes with aryl and alkenyl halides are classified in to two types. The first one is the preparation of arylalkynes 2 and alkenylalkynes (1,3-enynes) 3 by the reactions of terminal alkynes 1 with aryl and alkenyl halides in the presence of Pd(0) and Cul as catalysts (section 3.4.2). The second one is insertion of internal alkynes 4 to aryl- and alkenylpalladium bonds formed by oxidative addition of halides, generating alkenylpalladiums 5, which are living species and undergo further transformations before termination. Terminal alkynes also undergo the insertion in the absence of Cul catalyst (section 3.4.3)... 

Direct introduction of sp carbon to alkynes by the reaction of Cu acetylides with aryl and alkenyl halides to form arylalkynes and alkenylalkynes is known as the Castro reaction [1]. Later it was found that coupling of terminal alkynes (1-alkynes) with halides proceeds more smoothly by using Pd catalysts. There are two methods of Pd-catalyzed coupling, hi 1975 direct coupling of 1-alkynes catalyzed by a phosphine-Pd(O) complex in the presence of amines was reported by Heck and Cassar as an extension of the Heck reaction to 1-alkynes [2,3]. In the same year, Sonogashira and Hagihara found that the addition of Cul as a co-catalyst gave better results, and the Pd(0)-CuI-catalyzed reaction is called the... 

Halofuran and halothiophene derivatives undergo photochemical reactions with arylalkenes and arylalkynes and with benzo[6]furan513,514. With the arylalkenes and aryl-alkynes, heteroarylation takes place at the terminal alkene or alkyne carbon atom, while benzo[6]furan is substituted at position 2. The experimental results are interpreted in terms of solvent-separated or contact radical ion pairs. Iodothiophene and iodofuran derivatives can also be used to synthesize derivatives of benzimidazole by means of photochemical coupling515. The reaction of 5-iodothiophene-2-carboxaldehyde (157) with benzimidazole (158) giving the coupling product 159 is illustrated in equation 131. 

He et al. described a PtCU-catalyzed intramolecular cyclization of o-isopropyl or o-benzyl arylalkynes 214, which yielded functionalized indenes 215 (Scheme 82) [69]. In contrast to a previous report [44], the terminal carbon of alkyne in this reaction was substituted with aryl substituent. Notably, (TuBr (2.0 equiv.) was indispensable to achieve high yield. Theoretically, platinum(II)-activated electrophilic alkyne initially abstracts a hydride from benzylic C-H in [1,4]-manner to generate a benzylic carbocation I, which subsequently intercepts nucleophilic alkenyl-platinum(II) to afford the 5-membered ring. 

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