Enyne with trans-alkenes

Tetrahedrane (11) is the ruthenium analog of the much-studied tricobaltnonacarbonyl clusters Co3(CO)9CR see Cobalt Organometallic Chemistty). The substitution chemistry of (11) has been studied. A starting material is prepared from (11) by reaction with BX3 (equation 2), which gives the chloro and bromo compounds (12). In addition, (11) can also be treated directly with compounds such as diynes to yield interesting substitution products. For example, when (11) is refluxed in THF with diphenylbutadiyne, cis- and trans-alkene isomers of two alkyne insertion regioisomers are formed (equation 3). The product seems to arise from dehydrogenation of one end of the diyne to yield cis and trans enynes and an imsaturated monohydride cluster intermediate, which then reacts with the enynes to yield the allylic derivative products... 

In the case of the cyclic substrate 51, the intervention of a C-H insertion pathway reveals itself in terms of the diastereoselectivity, not regioselectivity. Thus, exposure of enyne 51 to the standard Ru catalyst at ambient temperature produced the transfused bicyclo[5.4.0]undecene 52 (Equation 1.60, path a) [55]. If a metallacycle mechanism was operative, coordination of the metal with both the alkene and alkyne must occur to form the cis-fused product. On the other hand, coordination of the Ru with the Lewis basic bridgehead substituent directs it to abstract an allylic C-H on the same face as the substituent, which then leads to the trans-fused product as observed. On the other hand, cycloisomerization using a Pd(0) precatalyst does indeed lead to the Z-fused bicycle (Equation 1.60, path b). 

This catalytic system provides high enantioselectivities for a range of epoxides (Figure 19.2), including those derived from trisubstituted and traus-1,2-disubstituted alkenes, with complete stereospecificity (retention of the alkene geometry in the epoxide product) [17]. The reaction has been shown to be chem-oselective for the alkene of enynes [18], provided monoepoxides upon reaction with conjugated trans-dienes [19] and afforded up to 93% ee for the asymmetric epoxidation of fluoro-olefins [20]. However, decreased enantioselectivity was observed for both cis- and terminal alkenes. The catalytic system has also been applied to the resolution and desymmetrization of cyclic trisubstituted alkenes [21]. 

The development of the oxazolidinone catalyst has allowed Shi to further increase the scope of the epoxidation to a range of new substrates. Using ketone catalyst 5 styrene is epoxidized with 24% ee, while the use of oxazolidinone 13a for the same transformation gave 86% ee, with further improvement in enantioselectivity seen with the inclusion of electron-donating substituents on the aromatic ring of the styrene substrates [43]. ds/trans-Dienes and trienes were epoxidized regioselectivity at the ds-alkene and no isomerization or further epoxidation of the products were observed [44], while epoxidation of conjugated cis-enynes resulted in chemoselective epoxidation of the alkene [45]. 

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