Preparation and Reactions of Calcium Metallocycles

This cuprate also underwent the conjugate addition with acyclic enones, for example, 2-hexene-4-one, to give 5-methyl-3-tridecanone in 47% yield. We have not attempted further optimization. Reaction of this calcium cuprate with a sterically hindered enone, for example, isophorone, produced 3% of the desired compound in 24h. The isolated yield, however, increased to 84% when the additives BF3 etherate and chlorotrimethylsilane [12] were used. In the aryl case, / -tolylcalcium cuprate also underwent this transformation with 2-cyclohexenone to give 3-(p-methylphenyl)cyclohexanone in reasonable yield (Table 9.3). 

Entry Diene Li/Ar CaXj Electrophile Product %Yield  

Reduction of 1,4-diphenyl- 1,3-butadiene with 2.2 equiv of preformed lithium naphthalenide without the presence of Ca(II) salts, followed by the addition of 1,3-dibromopropane, also yielded the same cyclopentane derivative, but the yield vras substantially lower than that obtained in the presence of calcium salts. 

Also of note is the fact that in the absence of calcium salts, over 30% of the starting material was recovered. It is possible that electron transfer from the butadiene dianions to the organic halides was facilitated in the absence of calcium salts, and these resulting radicals and/or anions did not efficiently add to the 1,3-diene. A similar result vras also noted in the nonactivated diene system. 

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