Alkyne derivatives trapping

Recently, Oshima has reported a cobalt-catalyzed allylzincation of internal alkyne derivatives (Scheme 10). Optimization of the reaction leads to utilization of cobalt(ll) chloride in THF at room temperature. No traces of regio- and stereoisomers are obtained. The resulting alkenylzinc species 34 can be trapped with a large number of electrophiles in order to generate stereoselectively the tri- and tetrasubstituted alkenes 35-37. 

Alt and co-workers have prepared numerous alkyl (67) and acyl (68) tungsten(II) alkyne complexes. A definitive paper detailing these results was published in 1985 (69). The reaction sequence which converts CpW-(CO)3R and free alkyne to CpW(CO)(RC=CR)R and free CO is not simple. Low temperature photolysis (-30°C) of the reagents in pentane first yields acyl alkyne products [Eq. (15)]. These products result from trapping of the initial alkyl alkyne derivative which rapidly reacts with CO to form the observed acyl product [Eq. (16), L = CO] (69). 

In addition to alcohols, some other nucleophiles such as amines and carbon nucleophiles can be used to trap the acylpalladium intermediates. The o-viny-lidene-/j-lactam 30 is prepared by the carbonylation of the 4-benzylamino-2-alkynyl methyl carbonate derivative 29[16]. The reaction proceeds using TMPP, a cyclic phosphite, as a ligand. When the amino group is protected as the p-toluenesulfonamide, the reaction proceeds in the presence of potassium carbonate, and the f>-alkynyl-/J-lactam 31 is obtained by the isomerization of the allenyl (vinylidene) group to the less strained alkyne. 

Numerous studies aimed at the understanding of the mechanism of these processes rapidly appeared. In this context, Murai examined the behavior of acyclic linear dienyne systems in order to trap any carbenoid intermediate by a pendant olefin (Scheme 82).302 A remarkable tetracyclic assembly took place and gave the unprecedented tetracyclo[6.4.0.0]-undecane derivatives as single diastereomer, such as 321 in Scheme 82. This transformation proved to be relatively general as shown by the variation of the starting materials. The reaction can be catalyzed by different organometallic complexes of the group 8-10 elements (ruthenium, rhodium, iridium, and platinum). Formally, this reaction involves two cyclopropanations as if both carbon atoms of the alkyne moiety have acted as carbenes, which results in the formation of four carbon-carbon bonds. 

The alternative building scheme C2 + Q was used by Petasis and Hu [89], who reacted various aldehydes and ketones with alkenyltitanocene derivatives 172 to obtain the corresponding allenes 173 in high chemical yields (Scheme 2.54). The reaction probably proceeds via titanocene vinylidene complexes, which can also be trapped with alkynes and isocyanides to afford allenylketene imines [90],... 

The intramolecular thermal cyclotrimerization of dodeca-1,6,11-triyne (110) at 450-600 °C afforded l,2,3,6,7,8-hexahydro[a5]indacene (112) and dehydro derivatives. An exothermic cycloaromatization mechanism has been proposed. An initial formation of a single bond gives diradical (111) which is then trapped by an alkyne. °... 

Doyle et al. (39) expanded the rhodium-catalyzed generation of isomiinchnones from diazoacetacetamides and subsequent trapping with dipolarophiles (38). As shown in Scheme 10.12, in the case of diazoacetoacetyl urea (79) the derived isomtinchnone 80 reacts with methyl propiolate to give a 2 1 mixture of cycloadducts 81. The resulting regiochemistry is successfully rationalized using frontier molecular orbital (FMO) theory as being isomiinchnone-HOMO controlled. This result represents one of the few reactions in which the cycloadducts from isomiinchnones and alkynes are stable. 

Dehydrobromination of bromotrifluoropropene affords the more expensive trifluoropropyne [237], which was metallated in situ and trapped with an aldehyde in the TIT group s [238]synthesis of 2,6-dideoxy-6,6,6-trifluorosugars (Eq. 77). Allylic alcohols derived from adducts of this type have been transformed into trifluoromethyl lactones via [3,3] -Claisen rearrangements and subsequent iodolactonisation [239]. Relatively weak bases such as hydroxide anion can be used to perform the dehydrobromination and when the alkyne is generated in the presence of nucleophilic species, addition usually follows. Trifluoromethyl enol ethers were prepared (stereoselectively) in this way (Eq. 78) the key intermediate is presumably a transient vinyl carbanion which protonates before defluorination can occur [240]. Palladium(II)-catalysed alkenylation or aryla-tion then proceeds [241]. 

Novel practical methods using various reagents, such as [Co(OAc)Br],1355 sulfur trioxide,1356 or ds-dioxoruthenium complexes,1357 were developed to transform alkynes to 1,2-diketones. Radical-catalyzed aerobic oxidation using A-hydro-xyphthalimide combined with a transition metal (Co, Cu, or Mn) affords a,P-acetylenic ketones in good yields.1358 Oxidation by the HOF. acetonitrile complex yields diketones, ketoepoxides, or cleavage products.1359 Ozonolysis of acetylenes combined with trapping techniques affords to isolate various derivatives.1360,1361 New information for the ozonolysis of acetylene was acquired by quantum-chemical investigatons.1362... 

---