Norbornene cyclobutene derivatives

The [2 -I- 2] cycloaddition of an alkene and an alkyne is a valuable route leading to cyclobutene derivatives. The ruthenium(0)-catalyzed [2 -1- 2] cycloaddition of a strained cycloalkene, norbornene 40, vith highly electron-deficient DMAD afforded the cyclobutene 74 (Scheme 4.28) [62]. As expected, the reaction took place at the exo face of 40 via the ruthenacyclopentene intermediate 75, that ivas formed by the oxidative cyclization of DMAD and norbornene. In addition to the parent 40, various norbornene derivatives can also be used as alkene components. When the Ru" precatalyst 17 ivas employed, electronically neutral alkynes participated in the [2 -1- 2] cycloaddition with norbornene and its derivatives [63]. A similar [2 -1- 2] cycloaddi-... 

On the other hand, triazolines prepared by the high-pressure dipolar cycloadditions of azides to norbornene derivatives were thermally more stable. For instance, cycloaddition reaction of azides to 2-azabicyclo[2.2.1]hept-5-en-3-one was accelerated by high pressure, leading to a mixture of regioisomeric triazolines in good yields [77]. Furthermore, triazolines formed by high-pressure cycloadditions (14 kbar) of azides to norbornene cyclobutene diesters could be handled without decomposition [78]. 

One of the first examples of ruthenium-catalyzed C-C bond formation afforded the synthesis of cyclobutenes, from norbornene derivatives with dimethyl acetylenedicarboxylate, and was reported by Mitsudo and coworkers [45, 46] by using various catalysts such as RuH2(CO)[P(p-C6H4F)3]3 or RuH2(PPh3)4. More recently, the complex Cp RuCl(COD) has shown to be an excellent catalyst for the [2+2] cycloaddition of norbornenes with various internal alkynes [45] (Eq. 33) and with a variety of substituted norbornenes and norbornadienes [47]. The ruthenacycle intermediate, formed by oxidative coupling, cannot undergo /1-hydride elimination and leads to cyclobutene via a reductive elimination. 

Dimethyl acetylenedicarboxylate has been added to norbornene and several of its derivatives using H2Ru(PPh3) as catalyst. The [2+ 2] adducts are formed in moderate yields all the adducts had the exo-fused cyclobutene ring. 

Cyclobutene/ cyclopentene/ " methylenecyclopentene/ cyclohexene/ " cycloheptene/ cyclooctene/ bicyclo[2.2.1]hepta-2-ene (norbornene)/ " " and some derivatives of norbornene " are also known to copolymerize with MA to give noncrystalline equimolar copolymers. 

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