Strained cyclic alkenes

The Pauson-Khand reaction was originally developed using strained cyclic alkenes, and gives good yields with such substrates. Alkenes with sterically demanding substituents and acyclic as well as unstrained cyclic alkenes often are less suitable substrates. An exception to this is ethylene, which reacts well. Acetylene as well as simple terminal alkynes and aryl acetylenes can be used as triple-bond component. 

Diboration of alkene is catalyzed by Pt(0),42,48-51 Rh(i),52-57 Au(i),52 and Ag(i)58 complexes. Phosphine-free platinum complexes such as Pt(dba)2 and Pt(cod)2 are efficient catalysts for diboration of alkene, whereas those with phosphine ligands show much lower catalytic activities (Equations (3) and (4)).48,49 A PtCl2(cod) complex, which may be readily reduced to Pt(0) species with diboron, also catalyzes the addition of bis(catecholato)diboron to alkenes.42 Platinum-catalyzed diboration has so far been limited to terminal alkenes and strained cyclic alkenes. 

The ring-opening cross-metathesis reaction is similar to the acyclic cross-metathesis reaction discussed above, except that one of the acyclic alkenes is replaced with a strained cyclic alkene (Scheme 5). 

One special case of cross metathesis is ring-opening cross metathesis. When strained, cyclic alkenes (but not cyclopropenes [818]) are treated with a catalytically active carbene complex in the presence of an alkene, no ROMP but only the formation of monomeric cross-metathesis product is observed [818,937], The reaction, which works best with terminal alkenes, must be interrupted when the strained cycloalkene is consumed, to avoid further equilibration. As illustrated by the examples in Table 3.22, high yields and regioselectivities can be achieved with this interesting methodology. 

The stereoselectivity of this hydration process has been studied on a number of alkenes. Acyclic alkenes and most cyclic alkenes give exclusively trans addition (equations 214 and 2 1 5).331,332 However, some strained cyclic alkenes, such as rrans-cyclooctene and trans-cyclononene, give cis adducts.332... 

Blechert and co-workers have also explored the use of cycloheptenes in RRM in processes where the thermodynamic driving force is the final RCM (Scheme 6). In one example of this novel process, ROM-RCM-RCM of 7 leads to an excellent yield of the substituted tetrahydrooxepin 8 <02T7503>. This strategy also featured significantly in the preparation of (+)-dihydrocuscohygrine 9 <02JOC6456>. The more common sequential reactions of strained cyclic alkenes have been reviewed <03EJO611>. 

When cyclic alkenes are utilized as starting materials the metathesis reaction will lead to long chain polymers and/or cyclic oligomers [103, 104, 107, 108]. If a strained cyclic alkene is employed the reaction is effectively irreversible. Industrially cyclooctene (polymer Vestenamer), 2-norbornene (polymer Norsorex), and dicyclopentadiene (polymer Telene, Metton, Pentam) are used as monomers. Upon polymerization cyclooctene and 2-norbornene yield straight chain polymers while dicyclopentadiene also allows cross-linking (Scheme 5.56). 

W(NAr)(=CHBuO(OR)2 (R = CMe(CF3)2, 50) are highly active catalysts for the metathesis of internal alkenes (equation 16), and also effect the stereoselective olefmation of hydroxy ketones (equation 17). The reactivity of these catalysts can be tuned by varying the aUcoxide ligands for example, when R = Bn, the complex acts only upon strained cyclic alkenes and is a highly effective ring-opening metathesis polymerization (ROMP) catalyst (see Metathesis Polymerization Processes by Homogeneous Catalysis). 

As was shown earlier with the Suzuki coupling reactions, organopalladium intermediates can show good stability to water and other protic sources. This stability has been exploited in the synthesis of polyacetylene under air- and moisture-stable conditions. It was found that simple palladium(II) salts (PdCl2, Pd(CH3C02)2, etc.) can be used to initiate the 1,2-insertion polymerization of strained cyclic alkenes in water (eq. (8)) [31]. Once formed, poly-8 can be converted to polyacetylene through a retro-Diels-Alder reaction. 

Schrock has prepared alkylidene complexes of Mo and W whose activity is tuned so that they are unreactive to ordinary internal alkenes but actively catalyze polymerization of strained cyclic alkenes, such as norbornene. See R. R. Schrock, Acc. Chem. Res., 1990, 23, 158, and references therein. 

Synthesis of alkyl azides.1 Terminal alkenes and strained cyclic alkenes react with the reagent to give a mercurial intermediate, which on reduction with sodium borohydride gives an azide. Internal olefins do not react. The method is an extension of the hydroxymercuration reaction of Brown (2,265-267). 

The Rh/TPPTS catalyst system is only applicable to the hydroformylation of terminal linear alkenes. With branched or internal alkenes as substrates only very low conversion rates are achieved. Exceptions include strained cyclic alkenes such as cyclopentene and norbornene, which are hydroformylated at moderate rates under Ruhrchemie/Rhone-Poulenc conditions. 

Alkene and alkyne r-complexes see Alkene Complexes and Alkyne Complexes are known both for Au and Au. They are prepared at low temperature from AuCl or AuCls with an excess of the alkene or alkyne in the absence of any other potential donor molecules. The products, for example, of the types (MeCH=CHMe)AuCl and MeC=CMe-(AuCl3)2, are generally of low stability, and the complexation is reversible in a vacuum or on heating. Representative examples have recently been structurally characterized.Strained cyclic alkenes and alkynes give the most stable products. Multiple coordination of monoalkenes or of dialkenes (like butadiene) is known, but information about the products is limited. Alkene coordination to neutral gold atoms has been studied by matrix-isolation techniques at very low temperature. The adduct (C2H4)Au appears to be stable only below 40 K. 

Ring-opening metathesis polymerization (Section 14.15) The double bonds of strained cyclic alkenes are cleaved by certain carbene complexes of tungsten and, in the process, undergo polymerization. 

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