Trans alkene production

Scheme 2. Key intermediates for the four possible routes to the trans alkene products. Formation of the cis products occurs in the same manner, except via cis metallacyclobutanes. For the sake of clarity, the formation of non-productive metallacyclobutanes is not shown. For the same reason the reversible nature of all of these steps is also omitted... 

Other commonly used synthetic routes to alkenes are also affected by crown ethers. The Wittig (80TL4831), Wittig-Horner and Wadsworth-Emmons (81S117) reactions all give pure trans- alkene products in higher yields in the presence of crown ethers. 

When the diene is acyclic, [4+4] cycloaddition remains the primary reaction pathway even when the result is a highly reactive and unstable trans-alkene product, e.g., 150 and 151 (Sch. 35). With 1,3-butadiene, these intermediates are intercepted by an additional equivalent of the diene, to give the 2 1 adducts 152 and 153. When a diene is used that cannot achieve an s-cis conformation such as 154, Diels-Alder reaction with [4+4] adducts 155 and 156 is impossible and these compounds relieve strain via Cope rearrangement to give cyclobutanes 157 and 158, respectively [98]. An intramolecular version of this reaction has been reported [99]. 

O .. which abstracts another proton from ammonia solvent to yield the final trans alkene product. 

When an alkyne is then added to the solution, an electron adds to the triple bond to yield an intermediate anion radical—a species that is both an anion has a negative charge) and a radical (ha.s an odd number of electrons). This anion radical is a strong base, which removes from ammonia to give a vinylic radical. Addition of a second electron to the vinylic radical gives a vinylic anion, which abstracts a second from ammonia to give trans alkene product. The mechanism is shown in Figure 8.4. 

Sodium or lithium metal in ammonia causes a dissolving metal reduction of alkynes to give a trans alkene product. This reduction does not work for terminal alkynes, but other metals are available (such as Zn-Cu) to accomplish this transformation. 

Hsung and coworkers have shown that the BF3 OEt2 promoted hetero [2 + 2] reactions of 10-propynyl-9(10H)-acridone (39) and various aldehydes lead to a highly stereoselective synthesis of trisubstituted alkenes, favoring the trans alkene products (40) (Equation 25) [29]. 

---