Oxanorbornene derivatives

A subsequent publication by Blechert and co-workers demonstrated that the molybdenum alkylidene 3 and the ruthenium benzylidene 17 were also active catalysts for ring-opening cross-metathesis reactions [50]. Norbornene and 7-oxanorbornene derivatives underwent selective ring-opening cross-metathesis with a variety of terminal acyclic alkenes including acrylonitrile, an allylsilane, an allyl stannane and allyl cyanide (for example Eq. 34). 

Despite being forbidden by the Baldwin rules (5-mdo-trig ring opening see Section 9.2), cyclohexadienoic acid derivatives such as that required for this synthesis can be prepared by base-induced ring scission of 7-oxanorbornene derivatives, presumably because of the high strain-energy of norbornenes. The required 7-oxanorbornene, in turn, should be readily accessible from furan and an acrylate via the... 

The ring-opening metathesis polymerization (ROMP) of 7-oxanorbornene derivatives initiated by Ru(H20)6(4-toluenesulfonyl)2 in aqueous media was reported by Novak and Grubbs [45] (Eq. 20). Compared with the same reaction carried out in organic solvent, the initiation time was greatly decreased. After the polymerization, the aqueous catalyst solution was not only reused but also became more active in subsequent polymerizations. 

An extremely efficient and high yielding catalytic asymmetric Diels-Alder reaction of furan was reported by Corey [17]. In the presence of 10 mol% of oxazaborolidinone 7, Fig. 2, the cycloaddition between furan and 2-bromoacro-lein proceeded to give the exo oxanorbornene derivative 8 in excellent yield and 92% ee, Eq. 5. Compound 8 can be efficiently converted to oxanorbornenone (+)-2, which is otherwise obtained by the Vogel methodology. 

The development of living ring-opening metathesis polymerization (ROMP cf. Section 6.10) catalysts was greatly influenced by the initial use of hydrated late transition metal salts. In the aqueous polymerization of 7-oxanorbornene derivatives, RuC13 and [Ru(H20)6]2+ were found to produce polymer with the latter having a smaller induction period [68], A key intermediate and product in the reaction is the alkene adduct, [Ru(H20)5(alkene)]2+ with the use of RuC13 and... 

Figure 9.1 Monomers and polymers based on oxanorbornene derivatives [33]...

ROMP and ROM are the two most frequently used methods in aqueous media. It was shown that the activity of the Ru catalyst is increased in the presence of small amount of water in the ROMP of 7-oxanorbornene derivatives. The most effective precursor was the aqua complex [Ru(H20)6](tos)2. After the polymerization was completed the aqueous catalyst phase could be reused and the catalytic activity increased in the subsequent reaction (Figure 26). An aqua-ruthenium (ii) olefin complex was isolated with higher catalytic activity than the adduct formed from the [Ru(H20)6](tos)2-complex and the 7-oxanorbornene derivative it was suggested that the Ru-carbene complex was formed from this compound. 

Figure 12 Precursor route to transpolyacetylene from a poly (7-oxanorbornene) derivative.

Subjecting the oxanorbornene derivative 77 to catalyst [Ru]-I (5 mol%) in CH2CI2 under an atmosphere of ethylene resulted in smooth ROM-RCM cascade, providing tetrahydrofuran 78 in 83% yield. 

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