Exploratory Studies on the Synthesis of Unsymmetrically Substituted Diacetylenes Bearing Trialkoxysilyl Groups and Development of a Method for the Preparation of 1-Lithio-4-(2,8,9-trioxa-5-aza-1-silabicyclo[3.3.3]undecanyl)-1,3-butadiyne: Synthetic and Mechanistic Aspects

( Z )-CH 3 OCH CHC⋮CSi(OCH 3 ) 3 ( 2 ), (( Z )-CH 3 OCH CHC⋮C) 2 Si(OCH 3 ) 2 ( 5 ), and ( Z )-CH 3 OCH CHC⋮CSi(OCH(CH 3 ) 2 ) 3 ( 16 ) have been synthesized from ( Z )-CH 3 OCH CHC⋮CH ( 1 ). Enynes 2 and 16 were subjected to a deprotonation−elimination−deprotonation sequence with 2 equiv of lithium diisopropylamide (LDA) in THF and the expected intermediates (RO) 3 SiC⋮CC⋮CLi (R = CH 3, CH(CH 3 ) 2 ) allowed to react with R‘ 3 SiCl (R‘ = CH 3, C 6 H 5 ) to produce the unsymmetrical butadiynes (RO) 3 SiC⋮CC⋮CSiR‘ 3 . Symmetrical butadiynes of the type R‘ 3 SiC⋮CC⋮CSiR‘ 3 were obtained instead of the expected unsymmetrical ones due to cleavage of the C sp −Si(OR) 3 bond by CH 3 OLi formed in situ. Cleavage of the latter bond can be avoided by using a silatrane moiety in place of the trialkoxysilyl group. Thus, (CH 3 ) 3 SiC⋮CC⋮CSi(OCH 2 CH 2 ) 3 N ( 26a ) and (C 6 H 5 ) 3 SiC⋮CC⋮CSi(OCH 2 CH 2 ) 3 N ( 26b ) were obtained in 61% and 45% yield, respectively, upon subjecting ( Z )-CH 3 OCH CHC⋮CSi(OCH 2 CH 2 ) 3 N ( 20 ) to a deprotonation−elimination−metalation sequence with 2 equiv of LDA followed by quenching of the intermediate lithium compound LiC⋮CC⋮CSi(OCH 2 CH 2 ) 3 N ( 25 ) with (CH 3 ) 3 SiCl and (C 6 H 5 ) 3 SiCl. The deprotonation−elimination−metalation sequence applied to 20 is best carried out in pyridine, and the role of pyridine in this reaction is discussed.