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The alkynyl−alkenyl complex ( 2 ) reacts with HCl to give the butadienyl derivative ( 3 ), which can be also obtained by protonation of 2 with HBF 4 ·OEt 2 to give ( 4 ) and subsequent treatment of 4 with NaCl. The protonation of 3 with HBF 4 ·OEt 2 affords the α,β-unsaturated carbene complex ( 5 ).
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Bohanna et al. (1999) studied this question.
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