Immobilized alkynes

SOLID-PHASE MANNICH REACTIONS OF A RESIN-IMMOBILIZED ALKYNE... 

We describe here Mannich reactions of a resin-immobilized alkyne and demonstrate the versatility of this methodology.3 Aryl-, alkyl-, aralkyl-aldehydes, and formaldehyde are suitable... 

On the other hand, even crosslinked polystyrene was used in the reduction of immobilized alkynes with the Lindlar catalystP The subsequent further hydrogenation to the alkane was achieved with palladium acetate as a precatalyst (Scheme 32). ... 

These developments were extended to multistep reactions on the solid support, for example, via immobilization of the alkene or the alkyne component as an ester. The immobilized alkyne 34 created the possibility for a cross metathesis with a soluble olefin however, as previously noted, undesired CM products generated from soluble alkenes can easily be removed by filtration and washing. After ene-yne cross metathesis to 35, the resin... 

Gibson et al. have used polymer-bound cobalt complexes for immobilizing alkynes [27]. Scheme 5 demonstrates the indirect loading approach, meaning that the alkyne-cobalt complex is formed prior to the attachment onto soUd phase. The alcohol moiety of 7a/7b, formed as a mixture, can then be subjected to a munber of transformations, and the alkyne is subsequently released from the polymer by aerial oxidation. The direct loading approach is also described, where the cobalt complex is formed on soUd phase before the alkyne complexation step. This approach gives a somewhat lower substrate loading, but the subsequent steps proceed with almost the same efficiency in both cases. 

The heterogeneous catalytic system iron phthalocyanine (7) immobilized on silica and tert-butyl hydroperoxide, TBHP, has been proposed for allylic oxidation reactions (10). This catalytic system has shown good activity in the oxidation of 2,3,6-trimethylphenol for the production of 1,4-trimethylbenzoquinone (yield > 80%), a vitamin E precursor (11), and in the oxidation of alkynes and propargylic alcohols to a,p-acetylenic ketones (yields > 60%) (12). A 43% yield of 2-cyclohexen-l-one was obtained (10) over the p-oxo dimeric form of iron tetrasulfophthalocyanine (7a) immobilized on silica using TBHP as oxidant and CH3CN as solvent however, the catalyst deactivated under reaction conditions. 

Rhodium also has been reported as a catalyst for [2+2+2] alkyne cycloaddition in water. Uozumi et al. explored the use of an amphiphilic resin-supported rhodium-phosphine complex as catalyst (Eq. 4.60). The immobilized rhodium catalyst was effective for the [2+2+2] cycloaddition of internal alkynes in water,113 although the yields of products were not satisfactory. 

Sun, X.-L., Stabler, C.L., Cazalis, C.S., and Chaikof, E.L. (2006) Carbohydrate and protein immobilization onto solid surfaces by sequential diels-alder and azide-alkyne cycloadditions. Bioconjugate Chem. 17, 52-57. 

The use of dispersed or immobilized transition metals as catalysts for partial hydrogenation reactions of alkynes has been widely studied. Traditionally, alkyne hydrogenations for the preparation of fine chemicals and biologically active compounds were only performed with heterogeneous catalysts [80-82]. Palladium is the most selective metal catalyst for the semihydrogenation of mono-substituted acetylenes and for the transformation of alkynes to ds-alkenes. Commonly, such selectivity is due to stronger chemisorption of the triple bond on the active center. 

A selective hydrogenation catalyst for alkynes was obtained with the PdCl2 complex of such immobilized pyridine. Diphenylacetylene was hydrogenated under 0.44 MPa H2 in ethanolic solution. At full conversion, the following selec-tivities were observed cis-stilbene 80.7%, trans-stilbene 16.1%, and only 3.2% 1,2-diphenylethane [90]. 

To fully use the advantages afforded by multicomponent reaction systems in solid-phase organic synthesis, strategies in which each component is immobilized on the resin must be devised. In this way, individual components can be explored in terms of diversity without the restrictions imposed by immobilization. We have described solid-phase Mannich reactions1 of a resin-bound alkyne (see chapter 5), and we show here that the diversity of products using this chemistry can be enhanced when a different component of the reaction system is immobilized. Specifically, a secondary amine, piperazine, is bound to a resin and then treated with... 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

In summary, it has been demonstrated, that structurally diverse functionalized alkenes and alkynes are subject to catalytic immobilization onto allylsilyl polystyrene under C,C-bond... 

After the optimization of these conditions, by adding an azide to the input stream it was possible to synthesize a range of substituted triazoles in a heterogeneously catalysed three-component reaction (Scheme 18). After the CFC, the stream was passed through a column containing a resin-immobilized copper-based catalyst, which was used in a previous work by the same authors to successfully catalyze the formation of triazoles from alkynes and azides [44]. An immobilized thiourea-containing cartridge was subsequently used to remove any leached Cu catalyst. In a similar way as for the alkynes production, the series of resins was used to purify the product. 

The application of immobilized heterobimetallic cobalt-rhodium in nanoparticles has also been reported. In the presence of water, CO, and amine, internal acetylenes 119 were converted to 3,4-disubstituted furan-2(5H)-ones 120 and 121 in high yields, in which an amine was necessary for the formation of furanone and a higher CO pressure was required for good yield (Equation (8)). It is important to notice that the catalyst has been easily recovered without loss of activity or formation of hydrogenated side-products. The reaction proceeded in good yield for the symmetric substrates (entries 1 and 2) while it always gave two regioisomers for asymmetric alkyne substrates (entries 3-8). The isomer ratio was dependent on the steric and electronic nature of the substituents. 

Selective reduction of alkynes to cis alkenes using hydrosilane functions immobilized on silica gel in acetic acid in the presence of a Pd(0) catalyst system has been reported.239... 

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