Enynes tetrasubstituted

Some of the most striking examples for an intramolecular Pauson-Khand reaction involving a tetrasubstituted double bond are the cobalt-mediated cyc-lizations of bicyclopropylidene derivatives 234 leading to the interesting spirocyclopropanated tricyclic products 235. The successful cyclizations even of the trimethylsilyl-substituted enynes 234 demonstrate the unique reactivity of the strained double bond in the bicyclopropylidene moiety of these molecules (Scheme 55) [145]. 

For synthetic purpose, the selective preparation of tetrasubstituted enynes was also investigated in the reactions of l-cydopropyl-2-propyn-l-ols bearing a substituent at the a-position in a cyclopropane ring with anilines (Scheme 7.37). As expected, the corresponding tetrasubstituted enynes were obtained in high yields with almost complete selectivity. 

Pt(II) to the alkyne of the substrate likely triggers all these events. The cycloisomerization might undergo a metallacyclic intermediate that proceeds to eliminate /3-H. The formation of cyclopropanes is presumably succeeded via alkenyl platinum carbene followed by platina(IV)cyclobutane intermediates. The extension using formal metathesis of the enynes includes two transformations, the formation of 1,3-diene moieties and the stereoselective tetrasubstituted aUcene derivatives via O C allyl shift, both leading to diverse structural motifs and serving as the key step in the total synthesis of bioactive targets (Scheme 83). 

Similar results have been reported for the closely related complex 91, which was an efficient catalyst for the RCM of di-, tri- and tetrasubstituted diene and enyne substrates. The reactions were performed in a homogeneous mixture of [bmim][PF6]/CH2Cl2 (1 9 v/v) as the solvent and in the presence of 1 mol.% of catalyst 91 at 45 °C (Scheme 1.54). Catalyst 91 combined the advantages of high reactivity and a high level of recyclability and reusability with only a very slight loss of activity. 

The same authors later expanded the concept and additionaly provided an imidazolium-tagged Howeyda-Grubbs ruthenium carbene catalyst for the RCM reaction [260]. The resulting system proved to be highly active for the conversion of di-, tri- and tetrasubstituted diene and enyne substrates. In the catalyst solvent system [BMIM][PF6]-CH2Cl2 (volume ratios 1 1 to 1 9) the catalyst could be recycled 17 times with only very slight loss in activity. Also in this work it was demonstrated that the imidazolium tag is essential to obtain a stable and recycleable catalyst. 

The aniline derivative 208 was obtained by cross-benzannulation of the amino-enyne 207 with dodeca-5,7-diyne (50) [64]. Cross-benzannulation of the 1,4-disubstituted enyne 209 with fte diyne 50 afforded the 1,2,3,4-tetrasubstituted benzene 210 [63]. Pentasubstituted benzene 212 can be synthesized by the coupling of the trisubstituted l-buten-3-yne 211 with the diyne 50 [63]. 

However, a recently discovered catalytic system [8] based on Af-heterocyclic carbene-ligated palladium complex in combination with electron-rich phosphine ligand 27 enabled this transformation for different 2,4-disubstituted enynes, including 2,4-dialkylenynes possessing protected alcohol (26f) or amine (26g) moieties (Table 14.1). However, no examples of successful homo-benzannulation of tri- or tetrasubstituted enynes have been reported to date. 

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