Stereoselective dimerization

The regio- and stereoselective dimerization of terminal alkynes into disubstituted enynes is efficiently catalyzed by rare-earth metal alkyl and hydride complexes, as reported independently by Bercaw et al. and Teuben et al. in 1987 [211,212]. Takaki and coworkers have shown that complexes Ln[N(SiMe3)2]3 when combined with an amine additive (typically, ArNH2 compounds) afford an active species for the... 

Attempts at deprotonation of epoxyketone 190 with EDA or n-BuLi at —95°C result only in reduction or addition to the carbonyl moiety. By contrast, deprotonation of the epoxy imine 191 leads to the aziridine 192 through a stereoselective dimerization/aza-Darzens//3-deprotonation process (Scheme 84). No example of reaction of this lithiooxirane with other electrophiles is reported. 

Figure 30. Schematic illustration of the packing of stilbazolium cations inside a magnesium-rich saponite clay. Stereoselective dimerization takes place to give the syn head-to-tail dimer. [Reproduced with permission from K. Takagi, H. Usami, H. Fukaya, and Y. Sawaki, J. Chem. Soc., Chem. Commun. 1174 (1989).]...

Recently, it was shown that the metathesis catalyst RuCl2(PCy3)2(=CHPh), where Cy is cyclohexyl, reacted in refluxing toluene with phenylacetylene to produce a ruthenium vinylidene species which promoted the regioselective dimerization of phenylacetylene into ( )-1,4-diphenylbutenyne [56]. The addition of 1 Eq acetic acid did not lead to enol esters but to a faster reaction and the stereoselective dimerization of phenylacetylene into the Z dimer. 

Ge, S.Z., Norambuena, V.F.Q., and Hessen, B. (2007) Highly efficient regio- and stereoselective dimerization of (hetero)aromatic terminal alkynes by organo rare-earth metal catalysts. Organometallics, 26, 6508. 

Cationic hydridoosmium complexes catalyze the regio- and stereoselective dimerization of terminal alkynes. [OsH(N2)(PP3)]BPh4 [PP3 = P(CH2CH2PPh2)3] is a catalyst precursor for the dimerization of 1-phenylacetylene to give (Z)-l,4-diphenylbut-3-en-l-yne [252]. 

Norbornene or norbornadiene-type substrates with nickel, ruthenium or cobalt catalysts undergo stereoselective dimerization or codimerizations with other substrates. Numerous examples have been reported15. Stoichiometric use of an iridium complex in the cyclodimerization of norbornadiene results in an isolable c.vo-rrans-c.ro-metallacyclopentane 7, formed from two norbornadiene units18. Upon heating in the presence of triphenylphosphanc, the exo-trans-e.YO-dimer 8 is liberated from complex 7. 

Aryl(alkyl)ketenes in the presence of the chiral NHC catalyst with CSjCOj undergo stereoselective dimerization (Scheme 7.50). A one-pot procedure with in situ generation of the ketene and dimerization was also successful. 

FIGURE 3.14 CB[8]-mediated stereoselective dimerization of reactant 28 to yield predominantly product 29. (Adapted from Ref [121], with permission of the Royal Society of Chemistry.)... 

NMR spectra of some dipeptides show doubling of the amide proton resonances as a result of aggregation of mixtures of l and D enantiomers. The effect has been ascribed to stereoselective dimerization of enantiomeric molecules (Cung et al, 1977). 

---