Catalytic metathesis Subject

Considering the facility with which dimerization products 81 and 84 are obtained, we reasoned that, in catalytic ring closure of 77, the derived dimer is perhaps initially formed as well. If the metathesis process is reversible [17b], such adducts may subsequently be converted to the desired macrocycle 76. To examine the validity of this paradigm, diene 77 was dimerized (— 85) by treatment with Ru catalyst lb. When 85 was treated with 22 mol% 2 (after pretreatment with ethylene to ensure formation of the active complex), 50-55% conversion to macrolactam 76 was detected within 7 h by 400 MHz H NMR analysis (Eq. 8). When 76 was subjected to the same reaction conditions, <2% of any of the acyclic products was detected. Although we do not as yet have a positive proof that 85 is formed in cyclization of 77, this observation suggests that if dimerization were to occur, the material can be readily converted to the desired macrolactam, which is kinetically immune to cleavage. 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

Dendralenes can be rapidly synthesized by consecutive enyne RCM of allylsi-lyl propargyl ethers followed by rhenium oxide-catalyzed 1,4-elimination of the resultant siloxenes [21f]. Hence, an efficient preparation of [4]dendralene 122 was accomphshed via domino dienyne metathesis of the bis(silyl ether) 120 to give the bis-siloxene 121, which was subjected to catalytic amounts of rhenium oxide to induce a double 1,4-ehmination with the formation of 122 (Scheme 2.44). 

The affect of halogens on catalytic chemistry is also beginning to be developed. Olefin metathesis is the subject of Chapter 21, and one of the most studied and most active olefin metathesis catalysts contains chlorides as ancillary ligands. The complexes containing chloride as ancillary ligands are more active than catalysts containing iodide. As shown in Equation 4.122, the origin of this effect can be traced to the relative reactivity of the... 

The stereochemistry of the olefin metathesis has been the subject of numerous publications which tried to rationalize the behaviour of various catalysts with acyclic olefins [1-5]. It had been noticed during these studies that a cis olefin gave preferentially a cis olefin and that a trans olefin led essentially to a trans olefin. This was explained in terms of stability of the metallacyclobutanes involved in the catalytic cycle (Scheme 1) the most favoured metallacyclobutanes are those where the substituents in positions 1 and 3 are equatorial. This simple rule allowed the explanation of most experimental results and the configuration retention. 

Reddy and coworkers proved that a sequential enyne metathesis/CM/Diels-Alder/aromatization strategy could be accomphshed in a one-pot quadruple reaction sequence [44]. In this work, enyne 112 and aUcene 113 were subjected to catalytic amounts of 2-Ru, after which DMAD was added (Scheme 17.22). Finally, DDQ was used to aromatize the ring system into the corresponding benzene 114, obtained in a respectable yield of 42% over the four synthetic steps involved. This compound was then readily converted into the natural product isofregenedadiol 115. 

The preparation and study of metallacycles has been a subject of active investigation for organometallic chemists. We have just seen one example where metallacycle formation is a key step in a catalytic process and there are several others most notably, olefin metathesis. The metal acts as a geometrical and electronic template in these reactions. For unsaturated metallacycles there are interesting questions concerning delocalization [29]. Certain metal carbynes can react with acetylene to give metallacyclobutadienes as intermediates [30]. One such example of an insoluble molecule is the tungstenacyclobutadiene complex, 18.36 [31]. The compound is quite stable and not very reactive (in contrast to cyclobutadienes... 

---