Terminal alkynes without palladium

On the other hand, Suzuki and co-workers have demonstrated that copper(I) species can promote the reaction of vinyl halides with terminal alkynes without the need for a palladium catalyst however, as in the Castro reaction, a stoichiometric amount of copper salt is needed. By contrast, Miura and coworkers found that aryl and vinyl iodides smoothly react with terminal alkynes in the presence of a catalytic amount of copper iodide using potassium carbonate as base when an appropriate amount of triphenylphosphine is added. Pyridine or 1,2-bis(diphenylphosphino)ethane (dppe) have been used as ligands in the case of Cul-catalyzed reaction of terminal alkynes with nitrones. ... 

Recently, Pal et al. found that (.S )-prolinol could facilitate the coupling reaction of terminal alkynes with 3-iodoflavone under palladium-copper catalysis in aqueous DMF to give 3-alkynyl substituted flavones of potential biological interest (Eq. 4.17). The coupling of iodobenzene with terminal alkynes at room temperature in water without any cosolvent was completed within 30 minutes, affording the desired product in good yield.36... 

Ranu and Banerjee developed a [bmim][OH] TSIL for oxidative homocoupling of terminal alkynes to 1,4-disubstituted 1,3-diynes in atmospheric conditions using Cu(ii) without using either palladium catalyst, amines, oxidants or organic solvents. Significant advantages stated by the authors include fast kinetics, high yields and mild reaction conditions. 

Furo[2,3- ]pyridines can be synthesized from alkynylpyridones and iodonium sources (Scheme 31) <20060L1113>. Iodine proved to be much more effective at promoting the iodocyclization reaction than other iodonium sources (ICl, A -iodosuccinimide (NIS)). The pyridinium triiodide salt, 104, can be converted into the corresponding pyridinone by treatment with an external source of iodide. In a variation of the reaction, a one-pot synthesis of the furopyridine derivatives 105 can be achieved, with overall yields of 79-92%, by treatment with iodine followed by sodium iodide without isolation of the triiodide salt. Another similar one-pot synthesis involves 3-iodo-2-pyridones, terminal alkynes, and organic halides in a series of two palladium cross-coupling reactions (Equation 45) <20030L2441>. This reaction could also be carried out in a two-step sequence, but the overall reaction yields were typically improved for the one-pot method. 

Cleavage with an ensuing Heck reaction was developed by using the T1 triazene linker [51] (Scheme 6.1.20). On cleavage with trifluoroacetic acid a diazonium ion is first formed this can couple to an added alkene under the action of palladium catalysis. The coupling proceeds well with simple terminal alkenes, styrenes, and di- and even trisubstituted alkenes. The advantage of this process is clearly the possibility of using volatile alkenes (and alkynes) without contamination by any salt or other less volatile by-product, particularly with the use of palladium on charcoal as the catalyst. 

The first direct coupling of terminal alkynes with aryl iodides or bromides without palladium was reported by Wang and Li in 2006.135 Silver iodide and triphenylphos-phane in polar solvents proved to be the best catalyst combination, while potassium carbonate proved to be the better base, giving diarylacetylenes in high yields (Table 10.8). 

Hydroboration is especially valuable for the synthesis of stereodefined 1- alkenyl-boronic acids. A general method is the hydroboration of terminal alkynes with cate-cholborane 7 [16, 17] (eq (14)). The reaction is generally carried out at 70 without solvent, but it is very slow in THF solvent. More recent results demonstrate that the hydroboration of alkenes or alkynes with catecholborane is strongly accelerated in the presence of palladium [18]. rhodium [19]. or nickel catalysts [20], thus allowing the reaction to proceed below room temperature. 

On the other hand, Chen and Li reported a facile and selective copper-palladium-catalyzed addition of terminal alkynes to activated alkynes in water without the competition of the homocoupling of the terminal alkynes (Eq. 4.79). Subsequently, a simple and highly efficient Pd-catalyzed addition of a terminal alkyne to a C—C double bond, such as a conjugated enone, either in water or in acetone, was also developed (Eq. 4.80). A variety of... 

The preparation of (alk-l-ynyl)thiophenes 154 is possible via palladium-catalyzed Sonogashira cross-coupling reactions between iodo- or bromothiophenes 152 and terminal alkynes 153 (Scheme 60, Table 38) [309, 360, 363, 364], Alkynylated thiophenes represent an important structural motif found in many ir-electronic systems such as molecular rods or conjugated macrocycles [163, 362, 365, 366], In the case of bromoiodo-substituted thiophenes, the more reactive iodine atom is replaced selectively [367]. Some coupling methods require stoichiometric amotmts of paUadium(0) [360, 368], whereas other methods proceed successfully tmder photochemical conditions without the need of a catalyst [369]. Recent... 

A cascade Heck reaction with termination by nucleophiles is considered to start with an oxidative addition of a heteroatom-carbon bond (starter) onto a palladium(O) species (startup reaction), followed by carbopalladation of a nonaromatic carbon-carbon double or triple bond without subsequent dehydropalladation (relay), a second and possibly further carbopalladation of a carbon-carbon double or triple bond (second etc. relay). The terminating step is a displacement of the palladium residue by an appropriate nucleophile. It is crucial for a successful cascade carbopalladation that no premature dehydropalladation takes place, and that can be prevented by using alkynes and 1,1-disubstituted alkenes (or certain cycloalkenes) as relay stations since they give kinetically stable alkenyl- or neopentylpalladium intermediates, respectively. In addition, reaction of haloalkenes with alkenes in certain cases may form rr-allyl complexes, which are then trapped by various nucleophiles. 

Domino or cascade reactions provide valuable approaches, especially to various carbo- and heterocyclic systems with three, four, or even more annelated rings. The Heck reaction has successfully been employed in various inter-inter-, intra-inter-, inter-intra-, as well as all-intramolecular reaction cascades, hi this section, the termination of these processes by alkenes, arenes, and related ir-bond systems such as alkynes and allenes will be described. A cascade Heck reaction is considered to consist of an oxidative addition of a heteroatom-carbon bond to palladium (starter), carbopalladation of a nonaromatic carbon-carbon double or triple bond without immediate dehydropalladation (relay), one, two, or more fimher car-bopalladation(s) of a carbon-carbon double or triple bond, and eventually ensuing dehydropalladation. Crucial for a cascade reaction of this kind to occur is the blockage or retardation of a dehydropalladation at one of the intermediate stages by using 1,1-disubsti-tuted alkenes and appropriately substimted cycloalkenes, bicycloalkenes, or alkynes as relays since they give kinetically stable alkyl- or alkenylpalladium intermediates, respectively. 

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