Organotin compounds are good precursors for generating active organometallic agents through transmetalation. [1, For example, active allylic titanium complexes generated from allylic tin complexes have performed effective allylation of carbonyl compounds. [1] In contrast, generation and synthetic use of similar early transition metal complexes such as tantalum reagents [3] have not been reported so far, although TaΓC bonds are known to be moderately reactive to electrophiles. [4, 5] We report here on the preparation of active tantalum reagents by the transmetalation of organotin compounds with tantalum(v) chloride. Of particular interest is that certain tantalum reagents promote the conjugate allylation of enones (Scheme 1).
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-Bu 3 SnCl Nu: , Scheme 1. Generation of an active tantalum Β± nucleophile complex and subsequent addition to an enone.
Compared with the direct allylation of carbonyl groups, little has been reported on the selective conjugate allylation of enones. [6] The only choice for this purpose has been the Hosomi Β± Sakurai reaction (allylsilane and TiCl 4 ). [7a] Later, modified reagents such as allylbarium [7b] and allylcopper [7c] were developed to avoid strong acidic conditions. However, with these modified reagents the allylation of acyclic enones is far more difficult than that of cyclic ones. The present system could be the method of choice for conjugate addition of allylic nucleophiles including sterically hindered ones to enones. [8] The results of the conjugate allylation of enones, both acyclic and cyclic substrates, are given in Table . Under the conditions described in the Experimental Section, benzalacetone (1 a) was allylated to give the conjugate adduct 2 a in 91 % yield (entry 1, Table ). The yield decreased to 50 % M. Fukuoka, Chem. Pharm. Bull. 1978, 26, 2365. [20] The conversion of cyclopentanone into cyclopentenone is considered to be challenging to industry and prompted www.innocentive.com,