Complexes, alkyne-metal encounter

We see from these examples that many of the carbon nucleophiles we encountered in Chapter 10 are also nucleophiles toward aldehydes and ketones (cf. Reactions 10-104-10-108 and 10-110). As we saw in Chapter 10, the initial products in many of these cases can be converted by relatively simple procedures (hydrolysis, reduction, decarboxylation, etc.) to various other products. In the reaction with terminal acetylenes, sodium acetylides are the most common reagents (when they are used, the reaction is often called the Nef reaction), but lithium, magnesium, and other metallic acetylides have also been used. A particularly convenient reagent is lithium acetylide-ethylenediamine complex, a stable, free-flowing powder that is commercially available. Alternatively, the substrate may be treated with the alkyne itself in the presence of a base, so that the acetylide is generated in situ. This procedure is called the Favorskii reaction, not to be confused with the Favorskii rearrangement (18-7). ... 

The metal-catalysed hydrogenation of multiply unsaturated hydrocarbons is, of necessity, more complex than that of monoolefins. The problems encountered in alkyne and alkadiene hydrogenation are essentially similar and it is appropriate, therefore, that the two systems are considered together. 

There seems no reason why any of the mechanisms discussed in Sections 3.4-3.6 cannot function in the conversion of alkynes to alkenes. The alkene route of hydrogenation is frequently encountered because alkynes complex more strongly to transition metals than alkenes and their complexes are formed preferentially in competition with the oxidative addition of dihydrogen. Internal alkynes coordinate to bis(arylimino)acenaphthene complexes of palladium and the fricoordinate species activate molecular hydrogen. Transfer of both atoms of hydrogen forms... 

We first encountered alkyne metathesis in Chapter 10 in connection with reactions of metal-carbyne complexes. The mechanism of alkyne metathesis, first proposed by Katz,64 is analogous to that for alkenes, and it is shown in Scheme 11.9. 

While all of the substrates discussed above are not shown in Fig. 2, the same analysis can be performed with all of them (alkynes, substituted methanes). One caveat that we encountered was that many of these substituted derivatives proved to be very stable. Loss of alkane from the n-pentyl hydride complex has a half-hfe of about an hour at 25°C. Methane loss from 3 has a half-life of about 5 h. Loss of benzene from 2, however, is extremely slow (months), and therefore, the rate of benzene reductive elimination at 25°C was determined by extrapolation from the rate at higher temperatures. The Eyring plot of hi( /T) vs. 1/T gave activation parameters for reductive elimination of benzene A// = 37.8 (1.1) kcal/mol and = 23 (3) e.u., which can be used to calculate the rate at other temperatures. As mentioned above, the substituted derivatives are much more stable. Reductive elimination of the alkynyl hydrides was examined at lOO C, as was the elimination of many of the substituted methyl derivatives. In these cases, the rate of benzene elimination was calculated from the Eyring parameters at the same temperature as that where the rate of reductive elimination was measured, so that the barriers could be directly compared as in Fig. 2. The determinatimi of AG° for all substrates allows Eq. 7 to be used to determine relative metal-carbon bond strengths for these compounds. Table 1 summarizes these data, giving A AG, AG°, and Drei(Rh-C) for all substrates. 

The use of [RhCl(CO)2]2 in intermolecular [5+2] cycloadditions often require heating, which in turn promotes competing cyclotrimerization of alkyne starting materials, decomposition of the VCP, or formation of undesired secondary isomerization products. Such transition metal-catalyzed intermolecular cycloadditions pose particular chemo-and regioselectivity challenges as well as entropic penalties not encountered in intramolecular processes, as the latter benefit from tether-derived alignment and proximity of reactive functionalities not possible in the former. In this context, Wender et al. have recently demonstrated that the cationic rhodium(I) complex, [RhCCioHsKcod)]" SbFe, promoted the remarkably efficient intermolecular [5+2] cycloaddition of 1-alkoxy-VCP 37, and 1-alkyi-VCP 42... 

---