Diyne conjugation reaction mechanisms

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

As is clear from the mechanisms, these reactions cannot occur with methylmetals. Their extensions beyond ethylmetallation are possible, but are prone to various side reactions [201,202], In contrast to the widely observable Zr-catalyzed carboalumination of al-kynes discussed earlier, the alkyne version of the Zr-catalyzed ethylmagnesation has not been widely observable, the only successful examples being those of conjugated diynes [203], In this context, further investigation of the Zr-catalyzed carbozincation of alkynes reported as early as 1983 [204,205] appears to be very desirable. 

Convincing evidence for phase separation was obtained from the photopolymerization behavior of 6 in the mixed 6/DSPE monolayer films. Photopolymerization of diacetylenes is a topotactic process which requires the proper alignment of the 1,3-diyne moieties [35]. Thus diacetylenes typically polymerize rapidly in the solid state but not in solution. Polymerization is triggered by ultraviolet irradiation and proceeds via a 1,4-addition mechanism yielding a conjugated ene-yne backbone (Fig. 5). The reaction can be followed by the growth of the visible absorption band of the polymer. 

Reaction of vinylcopper reagents, 62, with alkynyliodonium tosylates results in conjugated enyne 63, [53] [Eq. (24)]. The reaction is stereospecific with retention of olefin geometry. By appropriate order of addition, either of the two possible isomeric trisubstituted olefin isomers, 63, can be obtained in good isolated yields and excellent (>99%) stereoselectivity. Likewise, conjugated diynes, 65, are obtained [54] in the reaction of dialkynylcuprates, 64, with alkynyliodonium tosylates [Eq. (25)]. This method may be used for the preparation of unsym-metrical diynes. The mechanism of these coupling reactions is not understood at present. 

Although a mechanism of the above palladium-catalyzed cross-benzannulation between conjugated enynes and diynes is not perfectly understood, the reaction may proceed via not a concerted mechanism but a stepwise mechanism [48]. 

Evidence supporting the Bohlmann mechanism was provided by the direct involvement of a dicopper(II)-substituted y-Keggin silicotungstate (1), shown in Fig. 1, in catalytic oxidative alkyne-coupling reactions the catalyst was shown to give excellent yields of the expected conjugated diynes using a wide variety of alkynes, an example is shown in Scheme 10 [89]. 

In the case of conjugated diynes, the hydroalkoxylation reaction in the presence of phenol afforded alkoxylated enyne products. Aliphatic alcohols do not add to the diyne system indicating the necessity of an acidic alcohol (phenols, naphthol or 2,2,2-trifluoroethanol). The mechanism supposed the formation of a a-cumulenyl palladium complex intermediate is consistent with the absence of the double addition of phenol to the starting diyne or the second addition of phenol to the resulting enyne [100] (Scheme 50). 

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