Initiators olefin metathesis

To examine the possibility of a more efficient catalytic olefin metathesis, we prepared chiral Mo-based catalysts, 4a and 4b [10]. This approach was not without precedence related chiral Mo complexes were initially synthesized in 1993 and were used to promote polymer synthesis [6]. We judged that these biphen-based complexes would be able to initiate olefin metathesis with high levels of asymmetric induction due to their rigidity and steric attributes. Chiral complexes 4a and 4b are orange solids, stable indefinitely when kept under inert atmosphere. 

C. The former are generally electron-deficient with respect to the metal (<18e) and very active, while the latter are initially 18-electron complexes and must lose a ligand in order to initiate olefin metathesis (Rappe 1982b Marynick 1985 Cundari 1992). 

In olefin metathesis (vide supra), we have observed the initiation products of the reaction of propene and [(=SiO)Re(= CfBu)(= CHfBu)(CH2fBu)j fBuCH = CH2and frans-fBuCH=CHCH3(no ds fBuCH=CHCH3 is detected), which are formed in a 3 1 ratio [32]. 

Note also that (1) d° Ta alkyhdene complexes are alkane metathesis catalyst precursors (2) the cross-metathesis products in the metathesis of propane on Ta are similar to those obtained in the metathesis of propene on Re they differ only by 2 protons and (3) their ratio is similar to that observed for the initiation products in the metathesis of propane on [(=SiO)Ta(= CHfBu)(CH2fBu)2]. Therefore, the key step in alkane metathesis could probably involve the same key step as in olefin metathesis (Scheme 27) [ 101 ]. 

In the case of olefin metathesis, the selectivity in initiation products can be understood in terms of minimization of the steric interactions in the metal-lacyclobutane intermediates (vide supra), which are governed by the relative position of the substituents the metallacyclobutane with substituents in pos-... 

Despite the numerous reports concerning NHC-Ru olefin metathesis initiators, a complex incorporating a carbene that has only one exocyclic substituent adjacent to the carbenic centre was not reported until 2008. Studies by Grubbs and co-workers led to the development of ruthenium-based catalysts bearing such carbene ligands, in this case incorporating thiazole-2-ylidenes [63] (Fig. 3.19). 

ROMP is without doubt the most important incarnation of olefin metathesis in polymer chemistry [98]. Preconditions enabling this process involve a strained cyclic olefinic monomer and a suitable initiator. The driving force in ROMP is the release of ring strain, rendering the last step in the catalytic cycle irreversible (Scheme 3.6). The synthesis of well-defined polymers of complex architectures such as multi-functionaUsed block-copolymers is enabled by living polymerisation, one of the main benefits of ROMP [92, 98]. 

Osborn and Green s elegant results are instructive, but their relevance to metathesis must be qualified. Until actual catalytic activity with the respective complexes is demonstrated, it remains uncertain whether this chemistry indeed relates to olefin metathesis. With this qualification in mind, their work in concert is pioneering as it provides the initial experimental backing for a basic reaction wherein an olefin and a metal exclusively may produce the initiating carbene-metal complex by a simple sequence of 7r-complexation followed by a hydride shift, thus forming a 77-allyl-metal hydride entity which then rearranges into a metallocyclobutane via a nucleophilic attack of the hydride on the central atom of the 7r-allyl species ... 

To summarize, experimental evidence has been advanced regarding hydride involvement in the initiation step of olefin metathesis with certain catalysts. One concept considers the source of the hydride to be external—that is, originating from a promoter or a cocatalyst. A second concept assumes a hydride being generated internally from the metathesizing olefin. It is quite possible that both concepts are operative. 

As pointed out in an earlier section, Katz and co-workers demonstrated that Casey s complex, (CO)5W=CPh2, could also be employed to initiate the metathesis of a-olefins (26) as well as the polymerization of certain cycloolefins (27, 63). 

Adducts of M(CH-f-Bu)(NAr)(OR)2 complexes were prepared and studied as models for the initial olefin adduct [66] in an olefin metathesis reaction [67]. PMe3 was found to attack the CNO face of yy -M(CH-f-Bu)(NAr)(OR)2 rotamers to give TBP species in which the phosphine is bound in an axial position... 

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

Grubbs has reported a similar tandem olefin metathesis-carbonyl olelination process for the preparation of cyclic olefins [31]. In this case, treatment of a keto-olefin with the molybdenum alkylidene 1 at 20°C generates an intermediate alkylidene complex. Under these conditions, competing intermolecular olelination does not occur. However, intramolecular carbonyl olelination of the initially formed alkylidene complex can occur and this results in the formation of a cyclic olefin. This tandem sequence is illustrated by the transformation of keto-olefins... 

Tandem carbonyl olefmation—olefm metathesis utilizing the Tebbe reagent or dimethyl-titanocene is employed for the direct conversion of olefmic esters to six- and seven-mem-bered cyclic enol ethers. Titanocene-methylidene initially reacts with the ester carbonyl of 11 to form the vinyl ether 12. The ensuing productive olefm metathesis between titano-cene methylidene and the cis-1,2 -disubstituted double bond in the same molecule produces the alkylidene-titanocene 13. Ring-closing olefin metathesis (RCM) of the latter affords the cyclic vinyl ether 14 (Scheme 14.8) [18]. This sequence of reactions is useful for the construction of the complex cyclic polyether frameworks of maitotoxin [19]. 

Ruthenium Carbene-Based Olefin Metathesis Initiators Catalyst Decomposition and Longevity M. Ulman, R.H. Grubbs,/. Org.Chem. 1999, 64, 7202— 7207. 

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