Allylzincation metalated alkynes

It is also possible to achieve the formation of five-membered rings by coupling the allylzincation of a metallated alkyne with an intramolecular nucleophilic substitution. Thus, allylzincation of the ethylzinc alkynylide generated from 5-iodo-l-pentyne (66)45, or direct treatment of the latter with excess crotylzinc chloride177, led to the alkenyl... 

Transition metal-promoted allylzincation of disubstituted alkynes. . 901... 

For other , 1-disubstituted alkynes lacking an activating group, transition metal catalysts or promotors could be used to achieve allylzincation. 

The ability of allylzinc bromide to add to terminal alkynes was first observed with acetylenic amines as substrates. Thus, the propargylic amine 290 underwent addition of allylzinc bromide in refluxing THF and afforded after hydrolysis the allylic amine 291. This reaction occurred much more easily compared to the case of ethylenic amines (see Section III.A) and the regioselectivity was also noteworthy as a branched product was obtained predominantly82,105. It was suggested that metallation of the terminal alkyne was responsible for the observed regioselectivity. Indeed, the formation of an alkenyl... 

It was observed that the metallation of terminal alkynes by organomagnesium (or organolithium) reagents considerably accelerated the rate of the subsequent addition of allylzinc bromide. The products resulting from a bis-addition predominate even in the case of sterically more demanding alkynes (equation 140). 

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