Cyclo trimerization

From the foregoing discussion, it is evident that a variety of isomers are possible for unsymmetrically substituted cyclophosphines P NMR spectra are informative for distinguishing between these isomers. For example, two isomers of the cyclic trimers cyclo-(PR)2(PR ) are observed. The symmetrical isomer with identical substituents on the same side of the ring exhibits an A2B spin system, whereas the asymmetric isomer gives rise to an ABC spin system (Figure 11.5) (see Section 3.4.3). 

In the chloro series, the compounds to be considered are N=S-C1, cyc/o-N3S3Cl3, cyclo-N3S3CI3O3, and cyc/o-N4S4Cl2 the ionic compounds [S4N3]" "C1 and [cyc/o-N2S3Cl]+Cl and [c<2/ <2-N(SCl)2] [BCl4] together with various isomeric oxo- and fluoro-chloro derivatives. Thi-azyl chloride, NSCl, is best obtained by pyrolysis of the trimer in vacuum at 100°. It can also be made by the reaction of CI2 on NSF (note that... 

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

Cycloaddition of cyclopropenes is catalyzed by transition metal complexes. 1-Methylcyclopropene 118 undergoes a facile PdCl2(PhCN)2-catalyzed cyclodimerization to dimethyltricyclo[3.1.0.02,4]hexanes 119. In contrast, cyclo-trimerization of 3,3-dimethylcyclopropene 120 occurs in the presence of a catalytic amount of Pd(PPh3)4 to give hexamethyl-frans-ff-trishomobenzene 121... 

Monoalkynes and acetonitrile also reacted in cyclo-co-trimerizations to afford the corresponding pyridines however, almost stoichiometric amounts of the ruthenium(O) catalyst were required in that particular case.369... 

It is noteworthy that the starting tf -benzene complex was prepared by cyclo-trimerization of acetylene by IrCl(C2H4)(triphos) [16]. All of the attempts to react the fragment [Ir(triphos)]+ with benzene were unsuccessful, which reflects the difficulty met by a transition-metal fragment to overcome the energy barrier to /4-benzene coordination. 

Zirconium hydride(s) supported on oxide materials catalyze several classical reactions such as hydrogenation, reductive cyclizahon, cyclo-trimerization and olefin polymerization [43-47]. Notably, it also catalyzes the hydrogenolysis of polyolefins... 

Transition metal-catalyzed intermolecular [2 + 2 + 2] cyclotrimerization of alkynes to benzene derivatives has been extensively studied. In this section, the focus is on the cyclo-trimerizations of the substrates bearing three independent unsaturated bond components. The key issue with this type of process usually involves the challenge of controlling regioselectivity [1—1]. However, 1,3,5-trisubstituted benzene 44 can be obtained as the sole product in good yield when 3-butyn-2-one 43 is used as the substrate for the cyclotrimerization catalyzed by Rh2(pfb)4 (pfb=perfluorobutyrate) in the presence of EtsSiH under a CO atmosphere (Eq. 11) [30]. 

The catalytic cyclocarbonylations of diynes proceed efficiently to afford fused cyclohexadienes via trapping of the ruthenacyclopentadiene intermediate by an alkene component <2000JA4310>. Thus, the ruthenium-catalyzed cyclo-co-trimerization of 1,6-heptadiyne derivatives possessing a heteroatom at the 4-position affords heterotricycles in good yields (Equation 110). 

In D O solution, the cyclic trimeric derivative of azetidine-2-carboxylic acid [cyclo(Aze)3] displayed (78MI2) more than one interconverting conformation, with peptide bonds slightly deviated from planarity. Circular di-chroism in methanol showed (78MI3) absorption very similar to that of... 

The effect of relative permitivity D on the cyclo-trimerization of phenyl isocyanate was studied in the solvent system acetonitrile (AN) - ethylacetate (EA) using cyclic sulfonium zwitterion VI as catalyst. It was found that the rate constant increased with the increase of relative permitivity D of the solvent system, the experimental data correlate well with the Kirkwood equation ... 

The novel trimeric ylide (86) is formed through attack of trimethylphosphine on chloromethylsilane. Base gives the unstable ylide from the phosphonium salt which cyclo-trimerizes (Scheme 142) (77ZN(B)603). 

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. 

It is advisable that the epoxide resin/hardener component has high Tg. A crosslinked IPN was obtained e.g. from epoxynovolak resin, pyromellitic dianhydride, triethanolamine (epoxide — acid anhydride reaction catalyst), BPA/DC and Zn acetate (cyclo-trimerization catalyst) [78],... 

Insertion and condensation reactions of alkynes have been well known for many years (1,191). Alkyne dimerization to form either cyclobutadiene (192) or metallocyclopentadiene products (193), trimerization to arenes (194), condensation of two alkynes and carbon monoxide to cyclo-pentadienones (195), and condensation of alkyne and isonitriles to cyclo-pentadienimines (196) are common reactions. This section surveys alkyne insertion products originating from monomeric Mo(II) and W(II) reagents. 

Some insight into the nature of the coordination and addition steps can be deduced from the cyclo-oligomerization work reported by Wilke (316). Butadiene can be converted into several isomers of cyclododeca-triene using (C2H5)aAlCl/TiCl4, AlEta/Cr02Cl2 and AlEta/nickel acetyl-acetonate catalysts. With the nickel catalysts, open chain, jr-complexed intermediates have been isolated for both the butadiene trimer and the dimer. The open chain dimer structure is shown below, where Do = a bulky Lewis base and the nickel is apparently zero valent. 

An example which we have studied only cursorily, but which nevertheless reveals much about the importance of molecular distortions in chemical reactions, is the trimerization of acetylene to form benzene (Fig. 9). This reaction was brought to our attention by the work of Vollhardt, who found that a cyclic triacetylene, 1,5,9-cyclo-dodecatriyne, undergoes reaction only at elevated temperature to give products suggestive of the intermediacy of tris-cyclobutenanenzene25. ... 

---