Alkynylcoppers

Trialkylsilanes are usually formed by the addition of a lithium or Grignard reagent to the silyl chloride, and thus, discussions related to the formation of the silyl acetylene bond will be kept to a minimum. Silyl acetylenes are prepared from the alkynylcopper(I) reagents in the presence of PPh3, Zn or TMEDA in CH3CN at 100°, 36-98% yield. It is interesting to note that the... 

It is interesting to note that a copper(II)-mediated coupling reaction of alkenyldialkyl- or trialkylboranes with alkynylcopper compounds, generated in situ, in the presence of various solvents and a small amount of water, gives (E)-l,3-enynes (or disubstituted alkynes) with various functional groups in reasonable yields (Eq. 4.3).12... 

Alkynylcoppers constitute a class of compounds relevant to several synthetic organic reactions,47 where they have been proposed as key intermediates. The interest in this area has supposed that the number of structurally characterized alkynylcopper complexes has considerably expanded in the last few years. The most common route toward alkynylcoppers is based on the reaction of a terminal alkyne with a copper source, either a salt or an organocopper compound (Equations (8) and (9)). 

The reaction of 38 with nucleophiles gave a dinuclear species 39 in which each copper center is bonded to three C=C units, in contrast with the usual two-alkynyl-based bridge observed in many dinuclear alkynylcoppers.18 This... 

It is speculated that an alkynylcopper species, which undergoes the transmetalation process more readily, is generated during the reaction with the aid of an amine. The aliphatic amine also serves as a reducing agent to generate Pd(0). For recent reviews on the Sonogashira reaction, see references [60] and [61]. 

Heteromolecular seeding, in crystal engineering, 12, 581 Heteronuclear alkynylcopper complexes, characteristics,... 

Neutral alkynylcopper compounds are not prepared by transmetalation of alkynyllithium compounds. Rather, they are obtained by partially deprotonating terminal alkynes with amines and capturing the ammonium acetylide formed at equilibrium with Cul (—> R-C=C-Cu + R3NH I example Figure 16.7). Copper(I) cyanide couples with aryl iodides and -bromides in a similar fashion as alkynylcopper compounds (which may well be conceived as their carba analogs). 

As Figure 16.7 reveals, an aryl iodide reacts more rapidly with an alkynylcopper compound than an aryl bromide. The palladium-catalyzed C,C coupling reactions, which will be discussed later in the present chapter, also proceed more rapidly with an aryl iodide than with an aryl bromide (example Suzuki coupling in Figure 16.22) or an aryl chloride (example Stille reaction in Figure 16.27). There are still some details that are not fully understood one is inclined to assume that in accordance with the Hammond postulate the weaker C-I bond (dissociation energy DE = 51 keal/mol) breaks more rapidly with the initial oxidative addition... 

Lastly, for organocopper reagents, in a rather surprising reaction, 2,2 -diiodobiphenyls react with alkynylcopper reagents to give 78% yield of a cyclized product (equation 62)476. 

Coupling reactions between allylic halides and alkynylcoppers have been rather heavily investigated as routes to stereochemically defined polyenyl natural products. The CuCl-catalyzed reaction of acetylenic Grignard reagents with chlorine substituted allylic chlorides has been employed in the synthesis of stereochemically pure 1,3,5-undecatrienes... 

Alkynylsilanes can be converted into alkynylcoppers by treatment with Cu salts in an aprotic polar solvent such as l,3-dimethyl-2-imidazolidinone (DMI).62 The Cu-promoted coupling reactions via the Si-Cu transmetallation of alkynylsilanes are valuable for the syntheses of alkynyl ketones, 1,3-diynes, 1,3-enynes, and 1-aryl-l-alkynes from acid chlorides,62 alkynyl chlorides,63 alkenyl iodides,64 and aryl iodides,64 respectively (Equation (12)). In addition, symmetric 1,3-diynes can be prepared by the CuCl-mediated homocoupling of alkynylsilanes.65 653... 

Fully substituted triazoles were synthesized via the four-component coupling reaction of the unactivated silylacetylenes 50, two equivalents of allyl carbonates 5b, and trimethylsilyl azide 42 in the presence of a Pd(0)-Cu(I) bimetallic catalyst (Scheme 18) [54], Various trisubstituted 1,2,3-triazoles were obtained in good yields. The reaction most probably proceeds through the formation of alkynylcopper species 52, which on cross-coupling reaction with the 7r-allylpalladium complex 53 gives the products 51. 

Mixed cuprates containing alkynyl, thiophenoxy or t-butoxy groups can be readily prepared by the reaction of an organolithium with an alkynylcopper, CuSPh or CuOBu-t, respectively. 

Tlie cross-coupling of a terminal alkyne 9 with a 1 -bromoalkync 8 in the presence of an aliphatic amine and a catalytic amount of a Cu(I) salt affords unsymmetrically substituted diynes [10, Eq.(5)]. This useful reaction, discovered by Cadiot and Chodkiewicz [8], can be employed advantageously for the synthesis of several polyunsaturated systems. Generally the bromoalkyne is introduced dropwise to a mixture of the alkyne, ethylamine, and MeOH or EtOH in the presence of a catalytic amount of CuCl, and a small amount of NH OH-HCl. The reducing agent, NHjOH-HCl, is used to reduce the copper(TI) ion. The alkynylcopper(I) is assumed to be the reactive intermediate. The formation of the symmetrical diyne can be suppressed by maintaining the concentration of the bromoalkyne. This side reaction is particularly significant in the case of less acidic alkynes such as alkylalkynes [9J. 

The reaction of aryl halides 1 with alkynylcopper(I) 2 under reflux in pyridine to prepare internal arylacetylenes is known as the Stephens-Castro reaction [Eq. (1)] [2]. The reaction has proved to be particularly important in the synthesis of a wide range of tolan and heteroaromatic alkynes [3]. Vinyl and allenic halides can also be used and several reviews... 

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]. 

Insoluble CuCsH4NMe2-2 reacts with 4-tolylacetylene to give a soluble mixed-aryl-alkynylcopper species in 58% yield ... 

The Castro-Stephens reaction entails coupling of alkynylcopper(I) reagents with aryl or heteroaromatic halides in refluxing pyridine to furnish arylacetylenes. The cuprous alkynylides are prepared by adding the 1-alkynes to an aqueous ammonia solution of cuprous iodide. Since dry copper alkynylides have the tendency to explode, they must be handled with great care and should only be used in a slightly damp state. ... 

Osakada, K., Yamamoto, T. Alkynylcopper(l) complexes. The structure and chemical properties relevant to synthetic organic reactions. Trends In Organometallic Chemistry A999, 3, 219-225. 

More recently, the stereoselective formation of 1,3-enynes has been reported, via a new carbon-carbon bond-forming reaction, involving the direct coupling of alkenyliodinium tosylates (1) with alkenylcop-per(I) reagents. A wide variety of alkynylcopper reagents (2) are easily made by addition of alkylcop-... 

Alkynylstannanes react with copper(II) nitrate to give, via the alkynylcopper(II) derivatives, the corresponding 1,3-dialkynes108 and they show a high reactivity in Stille-type palladium-catalysed coupling reactions (Section 22.2).109... 

We have found that alkynylsilanes are smoothly converted into alkynylcopper compounds by treatment with CuCl in l,3-dimethyl-2-imidazohdinone (DMI) the copper reagents can be isolated in good yields [563]. This study was the first example of preparation and isolation of organocopper compounds by use of organosilicon reagents. The Si-Cu transmetalation is applicable to the synthesis of alkynyl ketones by Cu-catalyzed alkynylation of acid chlorides (Scheme 10.217). We have also shown that a Cu-mediated system is effective in the cross-coupling reaction between arylsilanes or heteroarylsilanes and aryl hahdes [564]. 

---