Ruthenacyclopentadiene

From the mechanistic point of view, the observed competitive reactions can be explained by considering two different pathways (Scheme 114). The intermediacy of ruthenacyclopentadiene 453 or biscarbenoid 452, formed from the reaction of a diyne and a ruthenium(ll) complex, is postulated in the proposed mechanism. Cyclopropanation of the alkene starts with the formation of ruthenacyclobutane 456, which leads to the generation of the vinylcarbene 457. Then, the second cyclopropanation occurs to afford the biscyclopropyl product 458. Insertion of the alkene 459 into the ruthenacyclopentadiene 453 affords the ruthenacycloheptadiene 454. The subsequent reductive elimination gives the cyclotrimerization product 455. The selectivity toward the bis-cyclopropyl product 458 is improved with an increasing order of haptotropic flexibility of the cyclopentadienyl-type ligand. 

The assumed mechanism proceeds via ruthenacyclopentadiene intermediates of type 160 or structure 161 as possible intermediates. Subsequently, the common bicyclic intermediate 162 is formed through insertion of the C=N double bond into the C-Ru bond, and reductive elimination of the ruthenium fragment gives rise to the desired bicyclic pyridones 163. 

The catalytic cyclocarbonylations of diynes proceed efficiently to afford fused cyclohexadienes via trapping of the ruthenacyclopentadiene intermediate by an alkene component <2000JA4310>. Thus, the ruthenium-catalyzed cyclo-co-trimerization of 1,6-heptadiyne derivatives possessing a heteroatom at the 4-position affords heterotricycles in good yields (Equation 110). 

Figure 3.7 a Generic equations for cyclometallation b formation of a ruthenacyclopentadiene complex via cyclometallation of a diacetylene. 

The reaction of 1,6-heptadiynes with alkenes led to a [2+2+2] cyclotrimer-ization in the case of cyclic or linear alkenes possessing heteroatoms at the al-lylic position. Bicyclic cyclohexadienes were thus produced in good yields with RuCl(COD)C5Me5 [92,93] (Eq. 71). A ruthenacyclopentadiene is invoked as an intermediate in the mechanism. Insertion of the alkene becomes possible by a heteroatom-assisted reaction. 

With strained bicycloalkenes such as norbornene derivatives a ruthenium-catalyzed tandem cyclopropanation occurred together with common [2+2+2] cy-clotrimerization, showing a biscarbenoid hybride structure for the ruthenacyclopentadiene intermediate [92] (Eq. 72). 

Recently, a formal ruthenium-catalyzed [4+2+2] cycloaddition of 1,6-diynesto 1,3-dienes gave conjugated 1,3,5-cyclooctatrienes and vinylcyclohexadienes [94] (Eq. 73). Insertion of a double bond in the ruthenacyclopentadiene can lead to the formation of tetraenes or vinyltrienes which undergo a thermal elec-trocyclization. 

One of the two isomeric products formed from reactions between C2(C02Me)2 and 308 (Cp Cp, CpMe) was structurally identified as 309, containing a ruthenacyclopentadiene unit attached to two other Ru fragments.398 A likely route to this compound is by a formal 1,3-cycloaddition of the alkyne to an RuC2 moiety of the cluster. 

Cyclotrimerization of alkynes mediated by the cationic complex [(77-Cp)Ru(acetonitrile)3](PF6) was shown by the DFT methods to proceed via the ruthenacyclopentadiene intermediates in accord with experimental findings <2003JOM(682)204>. One illustration for the transformation of such an intermediate into the final product is illustrated... 

Interaction of [( 7 -Cp)Ru(PCy3)(MeCN)2] with deca-2,8-diyne gives the ruthenacyclopentatriene complex 229 <2001CC1996>. [( 7 -Cp")(OC)2Ru(/t-CO)Co(CO)2] reacts with HC CTol-/) in the presence of Me3N0-2H20 in THF to yield ruthenacyclopentadiene 230 coordinated by the Co(CO)2 moiety <2000JOM(596)121>. The metalla-cycle is not planar and the Ru-Co bond is retained in the process of coordination. 

Ruthenacyclopentane 331 has been postulated as an intermediate in the ruthenium-catalyzed cycloisomerization of lactones <2003TL2157>. Cycloisomerization of phenylsulfonylallenes to the cyclohexane derivatives catalyzed by a ruthenium benzylidene complex might proceed through the ruthenacyclopentane intermediate 332 <2006TL3971>. The [( 7 -Cp")RuCl( -COD)]-catalyzed cyclotrimerization of 1-octyne with dimethyl acetylenedi-carboxylate proceeds via a ruthenacyclopentadiene <2004JMO(209)35>. 

As mentioned above, the intermediary of metallacydopentadienes has been widely recognized in many [2 -i- 2 -i- 2] cydotrimerizations of alkynes and related cydocotri-merizations [9]. Metallacydopentadienes are generally produced by the oxidative cydization of two alkyne molecules on a low-valent metal center. Various ruthena-cyclopentadienes were synthesized by this method. For example, heating a decalin solution of Ru3(CO)i2 and diphenylacetylene at 200 °C gave rise to the dinudear mthenacyclopentadiene complex 1 (Scheme 4.1) [10]. The similar dinudear complex 2 was obtained from dimethyl acetylenedicarboxylate (DMAD) at lower temperature [11]. Dinudear ruthenacyclopentadienes were also obtained, when conjugated... 

In contrast to the above thermal reactions, [trans-Ru(CO)3 P(OMe)3 2[ was irradiated in the presence of excess hexafluoro-2-butyne to afford the mononuclear ruthenacyclopentadiene 5, which was further converted into the arene complex 6 upon irradiation with the alkyne (Scheme 4.2) [14[. Thus, the stoichiometric cyclotrimer-ization of hexafluoro-2-butyne was accomplished in a stepwise manner. By contrast, only 1 equiv. of the alkyne gave the ruthenacyclobutene 7 under similar conditions. 

The mthenacyde-phosphine complex 50 was prepared from the bis(phosphine) complex 49 under acetylene atmosphere at room temperature (Scheme 4.16) [32]. Its ruthenacyclopentadiene structure was unambiguously confirmed by X-ray analysis. The Ru-Ca bond distances of 2.092(4) and 2.059(5) A are longer than those of the typical Rr C double bonds (1.83-1.91 A), and the Ca-C/3 bonds (1.321(6) and 1.338(7) A) were obviously shorter than the Cfi-Cfi bond (1.414(8) A). The NMR... 

In the presence of catalytic amounts of [Ru3(CO)i2] and PCy3, the reaction of 1,6-diynes 20 and t-BuMe2SiH under 50 atm CO in CH3CN at 140°C afforded bicyclic catechol derivatives 62 in moderate to good yields (Scheme 4.23) [56]. This novel benzarmulation was claimed to proceed via the ruthenacydopropenone 63 and the ruthenacyclopentadiene(dialkoxyacetylene) complex 64, as shown in Scheme 4.23. 

The strong forward donation-back donation of electrons (i.e., the Chatt model) between alkynes and ruthenium makes such a bond a very good ligand for Ru. Hence it is not surprising that reactions involving ruthenacyclopentadienes as intermediates, notably in the trimerization of alkynes to benzenes, occur readily. Intercepting the ruthenacyclopentadiene prior to its reaction with an additional alkyne, however, is rather rare. A unique juxtaposition of functionality occurs when a propargyl alcohol is the alkyne partner which allows such a diversion as shown in Scheme 1.3. 

The ability of water to attack the ruthenium-carbon double bond suggested that the ruthenacyclopentadiene might add water as depicted in Scheme 1.4. Remarkably, heating a tethered diyne in aqueous acetone to 60 °C in the presence of the trisacetonitrile complex 16 gave a nearly quantitative yield of the hydrated cyclization product as depicted in Equation 1.29 [26], Unsymmetrical diynes showed exquisite regioselectivity wherein the water attacked the least sterically hindered alkyne carbon (Equation 1.30). 

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