Alkynyl group transfer

Exchange reactions of aryl and iodo ligands and of aryl and alkynyl ligands are not observed at all. Analogous alkynyl ligand transfer from aryl(alkynyl)palladium complexes to aryl(iodo)platinum complexes shown in Eq. 5.52 occurs above 35°C to cause complete alkynyl group transfer from Pd to Pt [183]. 

The catalytic enantio-selective addition of zinc alkynylides to various trifluo-romethyl ketones with selectivities that surpass 94% ee has been reported. By using 0 pseudoenantiomeric cinchona alkaloids as chiral ligands (e.g. 22), both enantiomers of the trifluoromethylated products were synthesized. The first experimental and computational evidence has been provided in support of alkynyl group transfer from an intermediate complex formed by transmetallation reaction between the alkynylide and the titanium catalyst. 

A smoother transmetalation procedure should be ensured by the more electronegative character of aluminium, as first demonstrated by Wipf and Ireland [35]. Thus, hydroalumination of 1-hexyne with DIBAL-H, followed by addition of the cuprate 59, bearing non-transferable alkynyl groups, provides the copper intermediate 60. This adds smoothly to 2-cyclohexenone to produce the Michael adduct 61, in 72% yield (Scheme 2.18) [36],... 

Scheme 10.1S. Dummy ligands selective transfer of the methyl (or alkyl, alkenyl, aryl) group in preference to transfer of the alkynyl group.

The total of the reactions 2-5 results in reaction 1. The oxidative addition and formation of copper acetylide are well known. The intermetal transfer of the alkynyl group from copper acetylide (e.g., from Cu(I) to Pd(II) [8]) has been revealed by organometallic chemistry [8-11]. 

Woerpel and Clark identified silver phosphate as the optimal catalyst to promote di-ferf-butylsilylene transfer from cyclohexene silacyclopropane to a variety of substituted alkynes (Scheme 7.25).95 While this silver salt exhibited attenuated reactivity as compared to silver triflate or silver trifluoroacetate, it exhibited greater functional group tolerance. Both di- and monosubstituted silacyclopropenes were easily accessed. Terminal alkynes are traditionally difficult substrates for silylene transfer and typically insert a second molecule of the starting acetylene.61,90 93 Consequently, the discovery of silver-mediated silylene transfer represents a significant advance as it enables further manipulation of monosubstituted silacyclopropenes. For enyne substrates, silylene transfer the alkynyl group was solely observed. The chemoselectivity of the formation of 99f was attributed to ring strain as theoretical calculations suggest that silacyclopropenes are less strained than silacyclopropanes.96 97... 

Alkenylboronic acids and esters underwent conjugated addition to ct,/ -unsaturated ketones in the presence of trifluoroborane etherate589 or cyanuric fluoride (Equation (129)).590,591 Alkadienyl trifluoromethyl ketones were stereoselectively prepared from (2-alkoxyvinyl) trifluoromethyl ketones (Equation (130)).592 Alkynyl boronates can transfer the alkynyl groups regioselectively and enantioselectively to enones (Equation (131 )).593... 

In another reaction, alkynyl groups were transferable in the presence of a catalytic amount of Me3Al (Equation (78)).279 Preference for the Ph group transfer has been well featured in the competitive alkyl transfer process leading to selective imine arylation (Equation (79)). Higher organic chains such as ethyl and butyl are... 

A catalytic amount of CuCl was found to activate alkynyl(trimethyl)silanes in the palladium-catalyzed coupling reaction with aryl triflates (Eq. 7) [12]. The catalytic cycle is considered to involve the transfer of an alkynyl group from an alkynylsilane to Cu(I) and then to palladium(II). A sequential palladium-catalyzed reaction of trimethylsilylacetylene gives unsymmetrical diarylacet-ylenes (Eq. 8). 

One characteristic feature of the MPV alkynylation is the chemoselective transfer of functionalized alkynyl groups to aldehyde carbonyls. Reaction of chloral with functionalized A1 reagent 28 in CH2CI2 proceeds nicely at room temperature to furnish alcohol 29 in good yield, leaving the keto functionality intact (Sch. 13) [34]. Such transformation is not easily realized by use of ordinary alkynylation procedures because of the difficulty of generating functionalized alkynylmetal reagents. 

One of the characteristic features of this approach is the successful fert-alkyl-al-kynyl coupling with dialkylaluminum alkynides which enables the introduction of a quaternary carbon in a position adjacent to an alkynyl group. Such transformation was previously achieved by the cross-coupling of ferf-alkyl chlorides with trialkynyl-aluminums as already described in this section [92]. The reaction of 98 with dimethyl-aluminum phenylacetylide (1.5 equiv.), readily prepared from lithium phenylacetylide and Me2AlCl, in toluene at -78 °C for 30 min resulted in formation of a cross-coupling product in 70 % yield. This result indicates the efficient and selective transfer of the alkynyl group from the aluminum center in dialkylaluminum alkynides as depicted in Sch. 65. 

Kuhn and Neumann reported examples of Stille coupling in which the tin moiety was chemically fixed on a polymer support [181] vinyl, allyl, and alkynyl groups were transferred in reactions with acid chlorides, a cycloalkenyl iodide, and a cycloalkenyl triflate. 

The reactivity of tin-carbon bonds allows the creation of new sp-sp carbon to carbon bonds through substitution reactions. General reviews of the organotin route for carbon-carbon bond formation are available. " With Pd catalysis, alkynyl groups are more readily transferred than alkyl groups to the aryl or vinyl iodides to afford sp-sp carlwn bonds (equations 13 and... 

Suzuki couplings. Pd-catalyzed reactions of B-allyl borate complex derived from 6-methoxy-9-borabicyclo[3.3.1]nonane with aryl triflates give allylarenes. This technique broadens the scope of the Suzuki coupling to allow transfer of Me, TMSCHj, and alkynyl groups which has eluded conventional manipulations. 

When we want to transfer a group from tin we do not want to waste three of them. The rule is that tin transfers the best anion to palladium (strengthening our view that the organotin compound is the nucleophile). Alkynyl groups are transferred best and the order for other groups is roughly as below. 

Similar results are obtained using alkynyldiethylalanes, (C,H5)2(RC C)A1, in which the alkynyl group is selectively transferred. When R is phenyl, it is known that the cyclic nitroate ions adopt a conformation in which the phenyl group is axial in order to avoid 1,3-allylic strain between it and the nitroate group130. Kinetic protonation of these ions occurs opposite to the phenyl group giving m-l,2-disubstituted products. 

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