Alkynes interesting examples

Another interesting example involves the dihydrosilylation of alkynes [20], which is carried out at room temperature in two steps, the first one with platinum chloride (0.01%) and the second one with palladium-MOP catalysts (0.3%). The reaction can be done with solely the palladium catalyst, but in that case the yield for the first step is low. The work-up leads to the formation of a chiral diol in high optical yield when the MOP derivative shown is used (Figure 18.12). 

Another interesting example of an insertion reaction is found through the addition of benzophenone to complex 59 (Scheme 15). In this case hydrogen is abstracted from an amine group with addition of an alkyne unit across the carbonyl to produce a radical anion. 

A particularly interesting example is the synthesis of alkyne-bridged oligomers and polymers, which are attractive materials for optical and electronic applications. Bunz and coworkers were able to modify the Mortreux catalyst system [Mo(CO)6 and a suitable phenol]11 and reaction conditions to perform acyclic alkyne metathesis of 1,4-dipropynylated benzenes to produce high-molecular-weight poly(p-... 

Likewise, alkynyliodonium tosylates can be coupled with dialkyl- and diphenyl cuprates 136 to afford the respective alkyl- and phenyl-substituted alkynes 137 (Scheme 56) [114]. An interesting example of this reaction involves the coupling of (trimethylsilyl)ethynyl iodonium triflate with cubyl cuprate generated in situ from iodocubane 138 [116]. 

A preparatively interesting example of a combination of reductive coupling with subsequent reductive elimination is the formation of the alkyne 127 (Eq. (198) ) from 1,1-bis (4-bromophenyl)-2,2,2-trichloroethane 43 la. ... 

An interesting example of an intramolecular cycloaddition reaction has been observed in the case of the mesoionic 1,3-dithiolone (97) which contains a non-activated alkyne in the same molecule. Thus, on heating (97) at 40-45 or 100 °C, the tricyclic thiophene derivative (99) is obtained via the non-isolable primary adduct (98) (81LA347). 

The product distribution from simple alkynes (Scheme 1) is consistent with the hypothesis that a discrete vinyl cation is involved. In particular, the formation of both cis and trans adducts, under kinetic control, is significant since it indicates that a free linear vinyl cation is the actual intermediate. The formation of hexaethylbenzene is another point of interest. Its yield increases with increasing concentrations of 3-hexyne, thus supporting the hypothesis that the trimerization process involves in its early stages interaction between the vinyl cation and the alkyne. Other examples of polymeric products formed via vinyl cations, probably through a concerted electrocyclic process, have been reported (see infra) although generally dimers are the ultimate products. 

Although the detailed mechanism is not yet clear, this reaction is a synthetically highly interesting example for a free radical-mediated activation of O2 that enables transformation of alkynes into a-diketones under extremely mild and metal-free conditions. 

The best example of chemical activation in cluster chemistry is the use of Me3NO which results in CO replacement under mild conditions (129). In an interesting example taken from alkyne-cluster chemistry, when Me3NO is used dry a monosubstituted cluster derivative is obtained, but with damp Me3NO a tetranuclear vinylidene complex is isolated (130, 131), as illustrated in Eqs. (4) and (5), respectively. 

A further interesting example is the conversion of the alkynes 244 (Scheme 39) into the a-ketoacetals 247 [118]. The reaction with phenylselenyl sulfate affords the product of double addition 245, which is deselenenylated by the excess of persulfate to give the tetramethoxy alkane 246. During work up deprotection occurs and compounds 247 are produced. The reaction occurs equally well with internal alkynes. Interesting enough, methyl ketones 248 reacted, under the same experimental conditions, to afford the same a-ketoacetals... 

Further interesting examples of C-S bond formation involve the reaction of previously prepared (or in situ anodically generated) thianthrene or phenothiazine radical cations with alkenes or alkynes, to give l,2-bis(hetaryl) alkanes (or the respective alkenes) [82]. With cyclooctene a 1 1 adduct is obtained instead. Another valuable application is the smooth reaction with ketones (Scheme 26). The thian-threnium salts (40) now obtained are readily deprotonated to the corresponding ylides (41) [83]. The latter compounds are directly obtained when yff-dicarbonyls are used. 

Polynuclear assemblies can also be readily formed due to the ability of the alkyne ligands to form strong o-bonds with metals, as well as then-propensity for 71 coordination (t -fashion) (102) with electrophilic metal centers like Cu(I), Ag(I), and Au(I). Fornies and co-workers (44, 45, 103, 104) prepared interesting examples of di- and polynuclear alkynyl platinum complexes (Scheme 12). The [PtCu4(C=CPh)g] (105-107)... 

The formation of j -alkenyl products is reasonable for these complexes because NMR studies indicate that the alkyne is a four-electron donor in these starting materials (making them 18-electron species). Many other examples of reactions of this type that lead to jy -alkenyl complexes are reported An interesting example is ... 

Platinum catalysts of the preceding section activate only the nucleophile (a secondary phosphine), which then reacts with the electrophile (an activated alkene). There are, however, interesting examples of lanthanide complexes activating both the nucleophile and the electrophile towards intramolecular diastereoselective hydrophosphination/cyclisations. In 2000 Marks and co-workers found that certain lanthanide complexes catalyse the intramolecular hydrophosphination of alkenes and alkynes (Scheme 6.13). 

A variant of this reaction involves the formation of the enamine precursors for Heck cyclization via the hydroamination of alkynes [76]. Examples of this transformation are shown in Scheme 6.55. Notably, the regiochemistry of these reactions can be controlled in the hydroamination step. As such this provides an interesting alternative to Larock chemistry for generating 2,3-substituted indoles (Section 6.4). 

A few interesting examples of allylations of alkynes have been described in the past decade. An intermolecular [2+2+1] carbonylative cocyclization between alkynes 175 and allyl halides 176 shows the versatile use of Ni catalysis for the construction of cyclic enone systems of type 177 (Scheme 12.83) [184]. 

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