Metathesis five-membered rings

A unique bis-silylation system, in which a bis(silyl)palladium intermediate is generated via recombination of two Si-Si bonds, has been developed.8,97 A bis(disilanyl)dithiane reacts with alkynes in the presence of a palladium/ isocyanide catalyst, giving five-membered ring bis-silylation products in high yield with elimination of hexamethyl-disilane (Scheme 14). The recombination, that is, bond metathesis, is so efficient that no product derived from direct insertion of acetylene into the Si-Si bonds of the bis(silyl)dithiane is formed at all. 

As the representative examples in Scheme 6.11 illustrate, similar stragies may be applied to the corresponding alkenyl ethers (vs. styrenyl ethers) [26], The Zr-catalyzed kinetic resolution/Ru-catalyzed metathesis protocol thus delivers optically pure 2-substituted di-hydrofurans that cannot be accessed by resolution of the five-membered ring heterocycles (see Scheme 6.8). It should be noted, however, that the efficiency of the Zr-catalyzed resolution is strongly dependent, and not in a predictable manner, not only on the presence but the substitution of the acyclic alkene site of the diene substrate. The examples shown in Scheme 6.11 clearly illustrate this issue. 

RCM, followed by dihydroxylation of the double bond, is one of the most used methods for the synthesis of these compounds. For two syntheses of (—)-swainsonine 230, the five-membered ring is formed by RCM, followed by cis-dihydroxylation. Blechert et al. <2002JOC4325> prepared the precursor 2,5-dihydropyrroline 234 by domino ring-opening/ring-closing metathesis of an amino cyclopentene derivative 233 (Scheme 49). 

Steve Martin of UT Austin has reported (J. Org. Chem. 68 8867,2003) a detailed study of the synthesis of bridged azabicyclic structures via ring-closing alkene metathesis. Some examples of his work include the conversion of 12 to 13, efficiently forming six, seven and eight membered rings. He also demonstrated five-membered ring formation with the conversion of 14 to IS, which has the cocaine skeleton. 

All the above cascade alkene metathesis reactions are based on the ROM of a cycloalkene moiety. Harrity and co-workers have described the synthesis of functionalized spiro cyclic systems by cascade selective olefin ringclosing metathesis reactions from an acyclic tetraalkene. The selectivity for five-membered ring closure over seven-membered ring closure would be the result of a kinetically favored cyclization process [42] (Scheme 20). The syn-... 

Cyclopentadienyl compounds of the form CpCo(CO)2 are also known to convert l,n-enynes into the tranditional 1,3-diene cycloisomerization products (486, 487). Remarkably, the metathesis reaction is independent of tether length between both unsaturated functional groups, and leads to five-membered ring carbocycles exclusively (Scheme 60). 

Figure 10-10 Polymerization of 1,6-diynes using a molybdenum alkylidene catalyst [Rp is (CFjljCHjC] [67]. The 1,6-diyne monomer is drawn in two different exaggerated conformations to illustrate that head-tail polymerization leads to six-membered rings, and tail-tail polymerization leads to five-membered rings. See Fig. 10-8 for a more mechanistic diagram of acetylene metathesis.

Olefin Metathesis and Metathesis Polymerization 12.3 Five-membered rings... 

Transition metal-catalyzed carbocyclizations of 1,6-enynes A [7-11] such as cycloisomerization [12-18], metathesis [19-30], skeletal reorganization [31-34], and ene reactions [35] are useful synthetic methods leading to five-membered rings (Scheme 4). Transition metal-catalyzed carbocyclizations of... 

In the articles on metathesis reactions, ruthenium compounds of organometallic intramolecular-coordination five-membered ring compounds are used for what are generally referred to as Hoveyda-Grubbs first-generation 12 and second-generation 1.3 catalysts (Fig. 1.1) [68, 69]. 

Abstract Applications of five-membered ring products as catalysts in cyclometalation reactions inclnde chiral reactions, metathesis reactions, cross-coupling reactions, and polymerization reactions. Other reactions include reductions, Michael addition reactions, dehydrogenations, Diels-Alder reactions, and hydrogenations. 

Many ruthenium-carbene five-membered ring compounds have recently been reported to show good activity for metathesis [51-71]. Many types of reactions, such as ring-closing metathesis, ring-opening metathesis, ctoss metathesis, enyne metathesis, and diyne metathesis, proceed with the help of these catalysts, as shown in Eqs. (8.15), (8.16), (8.17), (8.18), and (8.19) [58]. 

Metathesis reactions are reported in some reviews [72-75], a handbook [76], etc. [77-82]. The first well-defined metathesis-active mthenium-alkylidene complex is a compound, (Ph3P)2Cl2 Ru=CH-CH=CPh2), but Hoveyda-Grubbs first- and second-generation ruthenium-based catalysts are ruthenium-alkylidene five-membered ring compounds (see Chap. 1). 

Shibasaki and co-workers used a ring-closing metathesis approach to prepare a number of five-, six-, and seven-membered rings from electron-deficient olefins. Treatment of acyclic enol ether 18 with 7 mol % of 3 in refluxing benzene provided the corresponding cyclic enol ether 19 in 94% yield. Deprotection of the silyl ether 19 (not shown) resulted in the corresponding cyclic ketone, a valuable synthetic intermediate in natural products synthesis and a number of industrial processes. The authors reported additional examples of the synthesis of five-membered ring carbocycles as part of this study. 

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