Dimerisation alkyne

QCfimHBFJ [Ir(H)2(NCCH3)3(P/Pr3)]+ Alkynes Dimerisation/hydrogenation, depending on the substrate toluene as co-solvent product isolated by distillation. [5]... 

In the classical oxidative alkyne-dimerisation reaction large quantities of solvent are often required which makes alternative procedures attractive. With 20 mol% CuCl and 20 mol% TMEDA, the dimerisation product of phenylacetylene, viz. 1,4-diphenyl-1,3-butadiene, was obtained at ambient temperature in [C4Ciim][PF6] in 95% y ield.1 2 Products were isolated by extraction of the ionic liquid phase with toluene and reuse of the catalyst was possible, but addition of further catalyst was necessary every two runs to keep reaction rates more or less constant. 

The dimerisation of alkynes has also been catalysed by Pd-NHC complexes. The dimerisation reaction afforded enynes with diverse functionality and was also tolerant to substrate steric bulk. High TONS for [4+2] benzan-nulation were also reported. Other catalysts tested for alkyne dimerisation featured expanded ringNHCs, although the five-membered imidazole-based carbene appeared to operate as well or better. ... 

The amidouranium tetraphenylborate [U(NEt2)3][BPh4] (66) was an effective catalyst for the selective dimerisation of terminal alkynes, as shown in Scheme 5.23 a suggested key intermediate was [U(C=CR)(NEt2)2(r 2-CH=CR)][BPh4]. ,io3... 

The double-addition products of type XVI are surprisingly stable. Complex 36 i, e.g., can be crystallized from CH2Cl2/MeOH in the presence of hydrochloric acid without substantial decomposition. The most striking property of 36i is the conversion into the cyclobutadiene complex 33 (R = Ph), which is more conveniently prepared from 28a and PhC2Ph at 150 °C. As already mentioned, the reaction proceeds by a stepwise mechanism through complexes XIV—XVI. This reaction offers a new, facile preparation of 33. The cyclo-dimerisation of alkynes other than diphenylacetylene could not been substantiated, however. 

Trapping electrophilic phosphinidene complexes with alkynes has been used in the synthesis of the new phosphirene systems (341) and (342). In the latter, the phosphirene rings are coplanar and conjugated. The diphosphinines (343) are formed unexpectedly in a head to head dimerisation of l.ff-phosphir-... 

Numerous systems have been examined for photochromic activity and many of these involve the isomerisation of aromatic ring systems a monograph has been published which surveys many of these rearrangements. A short review, written in Chinese, on the photo-dimerisation of aromatic compounds and the photoaddition of enones and alkynes to aromatic rings has been reported. ... 

Catalytic Head-to-head Z Dimerisation of Terminal Alkynes... 

Our subsequent investigations have led to the finding that the diruthenium(iii) complex [Cp RuCl(p-SMe)2RuClCp ] 7a with bridging MeS ligands catalyses the head-to-head (Z)-dimerisation of various terminal alkynes stereoselectively (Equation 4). ... 

These dimerisation reactions of terminal alkynes have been further extended to the catalytic cyclisation of a,co-diynes. For example, treatment of 1,15-hexa-decadiyne with 10 mol% of 7a affords the endo-msLCTOcydic product, (Z)-l-cyclohexadecen-3-yne with complete stereoselectivity (Equation 5). This novel cyclisation is of particular utility, because synthetic routes to endo-cyc ic (Z)-l-en-3-ynes are extremely limited. A related palladium-catalysed cyclisation of a,co-diynes to give the corresponding exo-cyc c l-en-3-ynes has been reported by Trost and co-workers. 

The iminophosphine (134) with 1-alkynes gave various products of 1,1- or 1,2-additions. A new synthesis of diphosphenes (135) from aryldichlorophosphines and a tungsten complex has been described the tungsten complex catalysed an exchange between two diphosphenes to give the unsymmetric diphosphene. A ferrocenyl-substituted diphosphene (136) has been prepared which seems to represent a borderline case with respect to dimerisation thus the dimer (137) readily dissociates to (136) upon heating in xylene. 

The cyclization-dimerisation of allenylketones 29 to furans 28 and/or 2,4-disubstituted furans 30 is well known. By making the choice of either PdCl2(MeCN)2 or rac-31 as catalyst, the allenylketones 29 could be preferentially converted into 28 or 30. Since the catalyst tolerates several functional groups (terminal alkynes, a-halogenketones, alkyl halides) this catalyst is an important improvement of Marshall s Ag(l)-catalyzed isomerisation of 29 to 28 <97CB1449 97CB1457>. 

Cyclotrimerisation, Dimerisation and Nucleophilic Substitution. These three reactions are promoted by a class of catalysts where the bimetallic mechanism is generally very well understood. In this section the diversity of alkyne activation processes is highlighted through the characterisation of intermediate structures or their analogues. 

One of the most well-studied bimetallic catalysts used for the C-C coupling of alkynes are the thiolato-bridged diruthenium complexes 7 (Scheme 5) [22]. In the presence of NH4BF4 these complexes catalyse the head-to-head dimerisation of a number of terminal alkynes to selectively yield Z-enynes [23]. In contrast, related monometallic Ru catalysts typically yield a mixture of E- and Z-isomers, with the E-isomer more commonly favoured [24-26]. Previous work has shown that diruthenium complexes such as 7 are exceptionally robust due to the strong bridging ability of the thiolate ligands, which results in retention of the dinuclear core during reaction [27]. The proposed mechanism for the dimerisation reaction involves a concerted activation process where both Ru centres activate one alkyne each via the catalytic cycle shown in Scheme 5. Initial coordination of the first alkyne yields the vinylidene intermediate 8. The second alkyne then coordinates to... 

Scheme 5 Bimetallic Ru thiolate complex catalysing the dimerisation of alkynes...

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