1.4- Enynes, allenylzincation

Wang s synthesis of enyne-allenes by cross-coupling of ene-allenic iodides with alkynes has already been mentioned in Sect. 14.2.1.1 (Scheme 14.12). In a continuation of this work, the same group developed an alternative coupling reaction of allenylzinc chlorides 74 with enyne iodides 75 catalyzed by Pd(PPh3)4, which provided the expected enyne-allenes 76 in high yield and with excellent Z/E selectivity (Scheme 14.17) [38],... 

TMS alkynes were also examined as precursors to allenylzinc bromides15. Pd-catalyzed coupling of these reagents with / -iodo acrylates afforded enynes as sole products (Table 13). However, although o-iodotoluene also gave the alkynyl product, both iodobenzene and 1-iodonaphthalene gave rise to mixtures of alkynyl and allenyl adducts. In the latter case, replacement of the TMS substituent with the bulkier TBS resulted in complete conversion to the alkynyl product, no doubt the consequence of a steric directing effect. 

When the 1,7-enyne 365 was metallated with s-BuLi in THF at —45 °C, the resulting propargyl-allenyl lithium failed to cyclize to the double bond upon wanning to room temperature. The same result was observed after transmetallation with MgBr2. However, transmetallation with ZnBr2 led to the allenylzinc 366 that subsequently underwent a smooth cyclization to the double bond at room temperature. This process was ascribed to... 

The high diastereoselectivity obtained from the reaction of the allenylzinc bromide with isobutyraldehyde to produce the y/i-homopropargyl alcohol is rationalized by the well-known six-membered ring transition state [34]. The scope of this reaction is quite broad a-mono-a,/S-disubstituted silylated enynes as well as -mono- and ,j5-disubstituted silylated enynols are easily carbometallated using this procedure [33]. 

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