Norbomene alkyne reaction

Insertion of aUcynes into aromatic C-H bonds has been achieved by iridium complexes. Shibata and coworkers found that the cationic complex [Ir(COD)2]BF4 catalyzes the hydroarylation of internal alkynes with aryl ketones in the presence of BINAP (24) [111]. The reaction selectively produces ort/to-substituted alkenated-aryl products. Styrene and norbomene were also found to undergo hydroarylation under similar condition. [Cp IrCl2]2 catalyzes aromatization of benzoic acid with two equivalents of internal alkyne to form naphthalene derivatives via decarboxylation in the presence of Ag2C03 as an oxidant (25) [112]. 

The meso-ionic l,3-dithiol-4-ones (134) participate - in 1,3-dipolar cycloaddition reactions giving adducts of the general type 136. They show a remarkable degree of reactivity toward simple alkenes including tetramethylethylene, cyclopentene, norbomene, and norbor-nadiene as well as toward the more reactive 1,3-dipolarophilic olefins dimethyl maleate, dimethyl fumarate, methyl cinnamate, diben-zoylethylene, A -phenylmaleimide, and acenaphthylene. Alkynes such as dimethyl acetylenedicarboxylate also add to meso-ionic 1,3-dithiol-4-ones (134), but the intermediate cycloadducts are not isolable they eliminate carbonyl sulfide and yield thiophenes (137) directly. - ... 

Much less information is available about [2 + 2]-cycloadditions. These allow the formation of cyclobutane derivatives in the reaction between two alkenes, or that of cyclobutenes from alkenes and alkynes. The reaction can be achieved thermally via biradical intermediates,543 by photoreaction,544 and there are also examples for transition-metal-catalyzed transformations. An excellent example is a ruthenium-catalyzed reaction between norbomenes and alkynes to form cyclobutenes with exo structure ... 

In a manner analogous to classic nitrile unmes, the additions of tnfluoro-methylacetomtnle phenylimine occur regiospecifically with activated terminal alkenes but less selectively with alkynes [39], The nitrile inline reacts with both dimethyl fumarate and dimethyl maleate in moderate yields to give exclusively the trans product, presumably via epimenzation of the labile H at position 4 [40] (equation 42) The nitrile imine exhibits exo selectivities in its reactions with norbomene and norbornadiene, which are similar to those seen for the nitrile oxide [37], and even greater reactivity with enolates than that of the nitrile oxide [38, 41]. Reactions of trifluoroacetomtrile phenyl imine with isocyanates, isothiocyanates, and carbodiimides are also reported [42]... 

For the intermolecular version, strained alkenes such as norbomene give satisfactory results. Cyclopentenones are obtained in good yields by the reaction of alkyne Co complex with alkene in the presence of silica. Wet alumina also gives good... 

Electronic effects on alkene regioselectivity in the Pauson-Khand reaction have also been observed. The regioselectivity observed in cycloadditions of norbomen-2-ones has been interpreted as arising from an electronic preference for attachment of the 5 C-S of the alkene to an alkyne carbon rather than cobalt in the bond-forming insertion step (equation 13). In these systems electronic and steric effects have been separated by carrying out identical reactions with the corresponding norbomen-2-ols, in which the... 

Best efficiencies under the standard quasi-catalytic PK conditions are gained with electron-poor alkynes, strained olefins (e. g., norbomene), and open-chain olefins with low substitution (e. g., ethylene). Ethylene has the advantage that it can be applied in large excess by pressurizing the reaction system. Equations (13) and (14) are standard examples. 

Instead of a second or third alkyne, an alkene C=C double bond may be incorporated into the cyclotrimerization reaction. Iron [65], rhodium [66], nickel [67], palladium [68], or cobalt [69] catalysts have been used to form cyclohexa-dienes. However, the preparative use of this catalytic co-cyclization is disturbed by consecutive side reactions of the resulting dienes such as cycloaddition or dehydrogenation. Itoh, Ibers and co-workers [70] have reported the straight palladium-catalyzed co-cyclization reaction of C2(C02Me)2 and norbomene (eq. (24)). 

Intermolecular hydroamination of alkynes, which is a process with a relatively low activation barrier, has not been used for the synthesis of chiral amines, since the achiral Schiff base is a major reaction product. However, protected aminoalkynes may undergo an interesting intramolecular allylic cyclization using a palladium catalyst with a chiral norbomene based diphosphine ligand (Eq. 11.9) [115]. Unfor tunately, significantly higher catalyst loadings were required to achieve better enantioselectivities of up to 91% ee. 

Under the reaction conditions, phenylacetylene was found to be a much more reactive coupling partner than arylboronic acids in the analogous Suzuki-Miyaura coupling, as in addition to the desired product (38), alkynylation and further addition reactions occurred with a variety of transient palladium(II) species (Scheme 27). Despite these undesired side reactions, Catellani was able to fine-tune the reaction conditions to form predominantly product 38 or 39. The formation of the desired product 38 (and suppression of product 39) is promoted by acceleration of norbomene carbopalladation by KOAc [47] and by using an excess of alkyl halide affording several structurally similar unsymmetrical alkyne products in good yields (Scheme 28). 

The significance of ruthenium-catalyzed reactions is also emphasized by a publication by Mitsudo et al., [16] who achieved [2+2]cycloadditions of norbomene 29 and norbomadiene 32 with a range of alkynes 30. These reactions are catalyzed by chloroC " -cyclooctadiene)(77 -pentamethylcyclopenta-dienyl)ruthenium(II) [Cp Ru(COD)Cl] (35) and give 31 and 33, respectively, in good yields (Scheme 10). 

The reaction of the alkyne 154 in the presence of norbomene as a relay generates 155 by insertion of norbomene. The cyclopropane 156 is formed by 3-exo-trig cyclization, and the lactam 157 was obtained by /S-H elimination [52]. 

An analogous reaction mode is followed in the 1 2 cross-coupling of 2-bromostyrene with acenaphthylene, which yields a bisannelated tetrahydrofulvene (Scheme 48)J Norbomene can favorably serve as a relay for cascade carbopalladations as the /S-hydride elimination is virtually impossible. The reaction always starts with an alkenyl-or arylpaUadium starter, generated either by oxidative addition of an alkenyl or aryl halide to a paUadium(O) species or by hydro- or carbopalladation of an alkyne, adding to the double bond. With /3-bromostyrene, norbomene can yield the same type of bisannelated tetrahydrofulvene derivative " as with acenaphthylene, but under different reaction conditions can also react with a 2 1 stoichiometry to give a cyclohexadiene-annelated norbomane derivative (Scheme 49). ... 

A rather efhcient three-component reaction starts with an iodoaUcene, uses norbomene as relay, and finishes with a aoss-coupling to an alkyne (Scheme 55).[ i... 

In this novel domino sequence, norbornene (or norbomadiene) acts as a promoter as well as a reaction partner. It enters and exits the catalytic cycle and is eventually incorporated in the product, and therefore the reaction requires an excess of this reagent. The standard Pd-catalyzed/norbomene-mediated ortho alkylation of the aryl iodide with the alkyne would form the arylpalladium species 146 (Scheme 3.36). Intramolecular carbopalladation onto the tethered alkyne unit gives vinylpalladiuml47. Intermolecularcarbopalladationwitha moleculeofnorbornene... 

As discussed in Chapter 9, the insertion of olefins and alk)nes into metal-amido complexes is limited to a few examples. Such insertion reactions are proposed to occur as part of the mechanism of the hydroamination of norbomene catalyzed by an iridium(I) complex and as part of the hydroamination of alkenes and alkynes catalyzed by lanthanide and actinide metal complexes. This reaction was clearly shown to occur with the iridium(I) amido complex formed by oxidative addition of aniline, and this insertion process is presented in Chapter 9. The mechanism of the most active Ir(I) catalyst system for this process involving added fluoride is imknown. 

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