2- Butyne transition metal complexes

Noncarbonyl transition metal complexes catalyze dimerization and aromatic cyclo-trimerization of ethynylcyclopropane. The product composition depends on the catalyst and the reaction conditions. Thus, Co(acac)2 in the presence of phosphines and AIEt2Cl afforded either the dimer 1,3-dicyclopropyl-1 -butyn-3-ene or a mixture of 1,2,4- and 1,3,5-tris(cyclopropyl)benzenes, whereas Pd(0 Ac)2 gave the same dimer in the presence of PPh3 but only a tris(cyclopropyl)fulvene in the absence of phosphines (equation 176)245. 

Although stoichiometric amounts of transition metal complexes were employed, the cross-[2+2+2] cycloaddition of three different alkynes has been achieved using zirconium and nickel complexes. The reaction of 2-butyne, 4-octyne, and Cp ZrEt selectively afforded unsymmetrical zirconacyclopentadiene. Subsequently, it reacted with 3-hexyne in the presence of NiBr fPPhjlj to give the desired hexasubstituted benzene (Scheme 21.6) [9]. 

In addition to complex-formation, the interaction of transition-metal atoms with organic substrates at low temperatures can result in rearrangement of the organic moiety without complexation. Two such reactions have already been briefly mentioned, namely, the polymerization of hexafluoro-2-butyne by Ge and Sn atoms (72) and the polymerization of styrene by Cr atoms (i 1). In this section we shall briefly summarize some of these transition-metal-atom-promoted, organic rearrangements. 

Acetylenes are well known to undergo facile trimerizations to derivatives of benzene in the presence of various transition metal catalysts 23). A number of mechanisms for this process have been considered including the intervention of metal-cyclobutadiene complexes 24). This chemistry, however, was subjected to close examination by Whitesides and Ehmann, who found no evidence for species with cyclobutadiene symmetry 25). Cyclotrimeri-zation of 2-butyne-l,l,l-d3 was studied using chromium(III), cobalt(II), cobalt(O), nickel(O), and titanium complexes. The absence of 1,2,3-trimethyl-4,5,6-tri(methyl-d3) benzene in the benzene products ruled out the intermediacy of cyclobutadiene-metal complexes in the formation of the benzene derivatives. The unusual stability of cyclobutadiene-metal complexes, however, makes them dubious candidates for intermediates in this chemistry. Once formed, it is doubtful that they would undergo sufficiently facile cycloaddition with acetylenes to constitute intermediates along a catalytic route to trimers. 

Hexafluoro-2-butyne shows unique complexing abilities with the later transition-metal atoms, Co, Ni, Pd and Pt. Each metal-CFaC CCFj pair yields an organometallic that is stable but tends toward decomposition to butyne trimer and metal particles. With Ni and Pd, 1 1 metalibutyne complexes are isolated as solvent adducts. The structures are unknown, but with Pd, a dimer is formed . 

Cycloaddition reactions of alkynes aided by transition metals were reviewed Various trimerization processes of acetylenic compounds have been reported. Titanium chloride catalyses the trimerization of acetylenic compounds, by way of intermediate complexes that can be isolated and characterized. This is shown in Table 2 for TiCU and 2-butyne. Acetylenes activated by ether groups in the propargyl position undergo trimerization catalysed by NiBri/Mg. Acetylenes without activation also undergo the same reaction, but with lower yields. Iron 7i-complexes can catalyse stepwise polymerization of alkynes ... 

The fact that quinones may form tt complexes with transition metals was first recognized by Sternberg et al. 53), who found that butyne reacts with iron pentacarbonyl in sunlight to afford duroquinone-iron tricarbonyl (XIX). These authors also reported that manganese pentacarbonyl hydride yields durohydroquinone under similar conditions whereas nickel carbonyl did not react 53a). However, more recent work has established that duro-quinone and some other substituted quinones are capable of forming Ni(0) complexes, most of which are surprisingly stable. 

The interaction of aromatic hydrocarbons with iron cations has been studied in the gas phase. The results of these studies provide information useful in the proposal that polycyclic aromatic hydrocarbons undergo efficient reactions with iron and other transition metals in the interstellar medium. Fe(r/-G2H4)2(r7-PhMe) 87 is a useful precursor for a range of arene iron complexes. Thus, reaction of 87 with the desired naphthalene derivative and three molecules of butyne or hexyne leads to the (hexaalkylbenzene)naphthalene iron complexes, as shown in Scheme 15. ... 

In contrast to many studies on cycloaromatization via transition metal-vinylidene complexes as key reactive intermediates, only one example of such a reaction via transition metal-allenylidene complexes has been reported to date. In 2008, Yada et al. reported the formation of substituted fiirans 78 from 3-butyne-l,2-diols 77 in the presence of a catalytic amount of thiolate-bridged diruthenium complex (Scheme 21.33) [45]. This methodology was also applied to the formation of a substituted pyrrole 80 from l-amino-2-butyn-2-ol 79. It is noteworthy that thiolate-bridged diruthenium complexes worked as effective catalysts toward cyclization involving both ruthenium-allenylidene and ruthenium-vinylidene complexes as key reactive intermediates. 

---