Cyclo terminal alkynes

The cyclo addition of the alkene to the ruthenium vinylidene species leads to a ruthenacyclobutane which rearranges into an allylic ruthenium species resulting from / -elimination or deprotonation assisted by pyridine and produces the diene after reductive elimination (Scheme 16). This mechanism is supported by the stoichiometric C-C bond formation between a terminal alkyne and an olefin, leading to rf-butatrienyl and q2-butadienyl complexes via a ruthenacyclobutane resulting from [2+2] cycloaddition [62]. 

Trost et al. [31] discovered that by using the RuCp(C0D)Cl/In(0CF3S02)3/ NH4PF6 catalyst system, where Cp is cyclopentadienyl and COD is cyclo-octadiene, the reaction of terminal alkyne, water, and a-vinyl ketone afforded the 1,5-diketone in DMF-H20. The reaction is highly selective and showed a tolerance to alkynes with various functional groups (Eq. 12). 

A stereospecific cyclo-cotrimerization of a terminal alkyne and two molecules of f-BuC=P to yield the i76-l,3-diphosphinine complexes 124 is achieved by reacting Fe(i76-toluene)(i72-C2H4)2 with the alkynes at -40°C to -30°C and exposing the mixture obtained to an excess of the phosphaalkyne, see Eq. (28).65... 

Trani-allylstannylation is possible with ZrC as a Lewis acid catalyst, reported by Yamamoto and coworkers, also in 1996 (Scheme 5.7.4). Varions terminal alkynes such as aryl-, alkyl-, alkenyl-, or non-substituted acetylenes undergo the reaction with non- or methyl-substituted allylstannanes. Recently, silver complexes were also found to be effective as catalysts for the intra-molecular allylstannylation of alkynes (Scheme 5.7.5). The silver complex is considered to activate the triple bond, giving a cyclo-propylmethylidene or an alkenyl complex as an intermediate. Similar intra-molecular allylstannylations, but with yn-selectivity are known to proceed in the presence of a Pd(0) or an InCls catalyst. 

Internal or terminal alkynes can be catalytically converted into 2-pyrones by reaction with CO2 in the presence of phosphine-Ni(O) complexes (5.7). In addition to 2-p5Tones, cyclo-oligomers may also form as by-products. 

In more complex reaction cascades an additional alkyne-insertion step can occur. Thus starting with intramolecular carbopalladation of a vinyl iodide to a carbon-carbon triple bond, followed by two intramolecular alkene-insertion steps and termination with dehydropalladation, a palladium-catalyzed synthesis of l-(5 -methylbicyclo[3.1.0]hex-T-yl)-5,5-bis(carboethoxy)cyclo-hexadiene (52) starting from l-iodo-4,4-bis(carboethoxy)-ll-methyldodeca-l,ll-dien-6-yne (51) is achieved. ... 

Finally, cerium(IV) ammonium nitrate can serve as the radical source by itself, generating NO3 radicals If by photolysis. The addition of such radicals to cyclo-alkynes 20 initiates an interesting tandem reaction [12]. Transannular hydrogen atom abstraction by the vinyl radical 21 affords the intermediate 22, which undergoes a 5-exo cyclization to the radical 23. In the last step, the ketone 24 is formed by elimination of NO2 in moderate yield thus, the overall sequence can be described as a self-terminating radical reaction (Scheme 7). 

---