Carbocyclic olefin complexes

Interestingly, Widenhoefer reported a similar palladium(II) catalyzed cycliza-tion of indoles onto alkenes (Scheme 58) [72]. This mild protocol for cyclization/ carboxylation of 2-alkenyl indoles makes possible catalytic addition of a carbon-nucleophile and carbonyl group across a C-C bond. The mechanism, however, is thought to involve outer-sphere attack of indole onto a palladium-olefin complex rather than the electrophilic C-H activation of the indole C(3)-H bond, exhibited by the Stoltz carbocyclization. 

Ruthenium vinylidene species can be transformed into small carbocyclic rings via carbocyclization reactions. Ruthenium vinylidene complex 2, generated from the electrophilic reaction of alkyne complex 1 with haloalkanes, was deprotonated with "BU4NOH to give the unprecedented neutral cyclopropenyl complex 3 (Scheme 6.2) [5]. Gimeno and Bassetti prepared ruthenium vinylidene species 4a and 4b bearing a pendent vinyl group when these complexes were heated in chloroform for a brief period, cyclobutylidene products 5a and Sb formed via a [2 + 2] cycloaddition between the vinylidene Ca=Cp bond and olefin (Scheme 6.3) [6]. 

Versatile [3 + 2]-cycloaddition pathways to five-membered carbocycles involve the trimethylenemethane (= 2-methylene-propanediyl) synthon (B.M. Trost, 1986). PaIladium(0)-induced 1,3-elimination at suitable reagents generates a reactive n - -methylene-1,3-propa-nediyl complex which reacts highly diastereoselectively with electron-deficient olefins. The resulting methylenecyclopentanes are easily modified, e.g., by ozonolysis, hydroboration etc., and thus a large variety of interesting cyclopentane derivatives is accessible. 

These reactions are driven by a combination of factors, including a loss of ring strain or the release of a volatile olefin such as ethylene. They can also be kinetically controlled by the formation of a less reactive carbene complex. An important feature of RRM is the catalytic transfer of stereocentres from the corresponding substituted carbocycles, i.e. the chirality embedded in the carbocyclic starting material is completely transferred to the product side chain. This allows chirality to be introduced by means of side chains at the carbocycle. Synthetically, this... 

Electrochemical carbocyclization reactions involving the preparation of 3-, 4-, 5-, or 6-membered rings have been described. The reaction involves complexing of olefinic compounds, such as dimethyl maleate 126 and a,co-dibromide, such as 1,3-dibromopropane 127 in an undivided cell fitted with a sacrificial aluminum anode, in A-methylpyrrolidone at constant current (equation 66)99. The reaction is of special interest for the preparation... 

Alkenes act as nucleophiles with alkynes in the presence of gold catalysts. In the most simple version of the reaction, enynes are converted with gold complexes or salts, and in the absence of nucleophiles, into rearranged dienes, cyclopropanated carbocycles, and/or bicyclic cyclobutenes. Depending on the length of the tether and the nature of the substituents, the olefin attack to the alkyne occurs in an endo or an exo fashion (equation 33). Besides, substitution at the alkene plays an important role on the regioselectivity of the nucleophilic attack. ... 

An intramolecular C-H/olefin coupling reaction can provide a cyclization product. Rhodium complexes involving [RhCl(coe)2]2-PR3 and ( -C5Me5)Rh(C2H3SiMe3)2 complexes are superior as catalysts. Some ruthenium complexes are also reasonably effective for cyclization reactions. Intramolecular olefmic C-H/olefm coupling with the aid of Ru(CO)2(PPh3)3, which is also effective for the reaction of aromatic ketones with olefins, yields the carbocyclic compounds in high yield (Eq. 9.11) [25]. 

The cyclopropane chemical reactivity, which closely resembles that of an olefinic double bond, stems from the electronic properties of this three-membered carbocycle Effectively, cyclopropyl and olefinic groups interact with neighbouring 7c-electron systems and p-electron centres they both add acids, halogens and ozone, undergo catalytic hydrogenation and cycloaddition, form metal complexes, etc. 

Olefin metathesis has emerged as a powerful tool for the preparation of cyclic organic compounds. Metathesis involves the redistribution of carbon-carbon double bonds in the presence of metal carbene complexes ([M]=CR2). The reaction of these metal carbenes with a,co-dienes leads to well-defined carbocyclic systems in what is termed ring-closing olefin metathesis (RCM). ... 

The transient zirconocene butene complex, 105, has proved to be useful in a number of organic transformations. For example, butene substitution of zirconocene alkene complexes with alkoxy-substituted olefins results in /3-alkoxide elimination to furnish the zirconocene alkoxy compounds (R = Me, 123 R = Bnz, 124) (Scheme 16).50,51 Addition of propargyl alcohols to the zirconocene butene complex, 105, affords homoallylic alcohols. These reactions are of limited utility owing to the lack of stereoselectivity or formation of multiple products. Positioning the alkoxide functional group further down the hydrocarbyl chain allows synthesis of cyclopropanes, though mixtures of the carbocycle and alkene products are obtained in some cases (Scheme 16).52... 

The 1 1 complex derived from phenyltungsten trichloride and aluminium trichloride is an effective catalyst for diene-cyclobutane metathetical interconversions. Thus, the tetracyclic compounds (291) and (292) were respectively isomerized to the dienes (293) and (294). Rather more surprising was the virtually quantitative formation of the cyclobutanoid compound (296) from (295). Reaction of norbomadiene with 2,2 -bipyridyl(cyclo-octa-l,5-diene)nickel at 25°C yielded the exo-trans,endo-metal o-carbocyclic (297) which, on treatment with an activated olefin (e.g. maleic anhydride), afforded the cyclo-dimer (298 predominantly exo-trans,endo) in good yield by displacement of the hydrocarbon moiety. Catalytic conversions can also be achieved. 

The proposed mechanism for this reaction is illustrated in (Scheme 46) (i) Regioselective insertion of one of the olefin moiety of 1,6-diene 33 to the Si-[Pd] complex E-I to form /3-silylalkyl-[Pd] complex E-III via E-II, (ii) exo-carbocyclization to give trans-intermediate E-V, and (iii) reductive elimination, probably by a-bond metathesis with a hydrosilane, to yield 34 and regenerate the active catalyst species E-I (Scheme 18). 

As shown in the mechanism depicted in (Scheme 18), the Pd-catalyzed silyl-carbocyclization of 1,6-dienes involves a reversible insertion of an olefin moiety into the [Pd]-Si bond (E-II to E-III). However, the coordination of the second olefin moiety to the Pd metal (forming E-IV) would fill the coordination site required for the j8-silyl-elimination and would therefore render the C—Si bond formation irreversible, which leads to the irreversible carbometalation to jdeld E-V. Accordingly, when the chiral Pd(pyridine-oxazoline) complex is used as the catalyst, the enantioselectivity should be determined at the first olefin insertion step forming -silylalkyl-[Pd] complex E-III. 

A proposed mechanism for this reaction is shown in (Scheme 24). Insertion of the less hindered olefin moiety of a diene into the [Y]-H bond forms G-I. Car-bocyclization of G-I gives G-II, and the subsequent cr-bond metathesis with a hydrosilane yields the product and regenerates Cp 2YH THF. The observed high catalyst activity of the Cp 2YMe and Cp 2LuMe complexes relative to Ni(0) and Rh(I) complexes for this carbocyclization is ascribed to the Lewis acidity of the metal center and the presence of an open coordination site. These features favor both /3-migratory insertion and a-bond metathesis over oxidative addition and reductive elimination processes that are preferred in the Ni(0) and Rh(I) catalyst systems. 

A mechanism proposed for this cycloisomerization is shown in (Scheme 26). Coordination of the diene to [Rh(PPhs)2HCl2] and insertion of one of the olefin moieties of the diene into the [Rh]-H bond gives complex G-V. Carbocyclization... 

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