Olefins, metathesis tetrasubstituted

Alienation Another olefin metathesis-carbonyl olefination sequence utilizing titanacyclobutanes 11 is employed for the preparation of tri- or tetrasubstituted allenes [56]. Titanocene-methylidene 4 generated from 11 reacts with 1,1-disubsti-tuted allenes to produce the a-alkylidenetitanacyclobutanes 25 with the hberation of an olefin. Simple treatment of 25 with ketones and aldehydes at room temperature affords substituted allenes. The vinylidene complexes 26 are formed as the active species in these transformations (Scheme 4.20). 

In 2001, Furstner reported the preparation and characterisation of the NHC-Ru complex 22 containing iV,iV -bis[2,6-(diisopropyl)phenyl]imidazolidin-2-ylidene (SIPr) [29] (Fig. 3.6), which is the congener of complex 20. Subsequently, Mol and co-workers revealed that complex 22 was a highly active metathesis initiator [30]. More recent comparative studies showed that catalyst 22 could catalyse the RCM of 1 faster than any other NHC-Ru catalyst, while it was not stable enough to obtain complete conversion in the RCM of 3 and was inefficient for the construction of the tetrasubstituted double bond of cyclic olefin 6 [31]. 

In the previous examples, disubstituted double bonds have been synthesized via ring-closing metathesis. As shown in Scheme 5, it is possible to extend the method to tri- or tetrasubstituted double bonds. Thus, bisallylic ether 33 leads to the trisubstituted olefin 34 and compound 35 gives the tetrasubstituted olefin 36 [3]. Here, the additional substituents are simple methyl groups, but silyl groups for example, can be introduced too. The volatile byproducts in these cases are not ethylene but butene or propene. 

The activity of the catalyst systems increases as the dissociation energy of the labile ligand is reduced. The catalytic performance of these systems is comparable with Schrock s highly active yet very sensitive molybdenum system [45]. So even tetrasubstituted olefins are accessible by ring-closing metathesis using these optimized air- and moisture-stable ruthenium systems [46, 47]. 

Both of these complexes can be used in ADMET polymerizations at temperatures up to approximately 55 °C, although decomposition certainly occurs over the time scale of a typical ADMET polymerization (days). A structure-reactivity study was performed on complexes 1 and 2 that revealed a number of features of these complexes [68]. Notably, 2 will polymerize dienes containing a terminal and a 1,1-disubstituted olefin, but never produces a tetrasubstituted olefin. One of the substituents of the 1,1-disubstituted olefin must be a methyl group. In contrast, complex 1 will not react with a 1,1-disubstituted olefin. The tungsten complex is more reactive towards internal olefins than external olefins [23, 63] indicating that secondary metathesis, or trans-metathesis, probably dominates the catalytic turnovers in ADMET with complex 1. 

Scheme 12.9 Ring-closing metathesis reactions to form tetrasubstituted olefins in CgFg. f-Bu...

Stewart IC, Ung T, Pletnev AA, Berlin JM, Grubbs RH, Schrodi Y. Highly Efficient Ruthenium Catalysts for the Formation of Tetrasubstituted Olefins via Ring-Closing Metathesis. Org Lett. 2007 9(8) 1589-1592. 

Berlin JM, Campbell K, Ritter T, Funk TW, Chlenov A, Grubbs RH. Ruthenium-Catalyzed Ring-Closing Metathesis to Form Tetrasubstituted Olefins. Org Lett. 2007 9(7) 1339-1342. 

Very reactive commercial prototype of the family of Schrock s metathesis catalysts, 1990 (can achieve RCM of tri- and tetrasubstituted olefins) - (2) Prototype of Schrock s alkyne metathesis catalysts, 1982 - (3) Basset s well-defined active catalyst for the metathesis of alkenes and alkynes, 2001. 

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