Alkyne-metallocene complexes

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

As mentioned earlier, many Lewis base metallocenes have been synthesized and structurally characterized. An examination of their structures shows that they are all predictably bent, more so than their base-free congeners, and have one two-electron, two two-electron or one four-electron Lewis base donor. Ca[l, 3-(SiMe3)2C5H3]2(THF),89 shown in Fig. 17, is an example of a Lewis base monosubstituted Ca metallocene complex, whereas Sr(t-BuC5H4)2(THF)2,88 shown in Fig. 18, is an example of a disubstituted metallocene. Even weak donor base such as alkynes, Me3SiCCCCSiMe3, will bind to the alkaline-earth metal centers (Fig. 19).104 In this case, the weak donor is easily displaced by stronger bases. 

This reaction can also proceed intermolecularly samarium and lutetium metallocene complexes catalyze the reaction of internal alkynes with primary amines to give enamines as precursors to ketimines.97... 

Other late transition metals used in special cases of hydrosilation include cobalt, iron, ruthenium (vide infra for reactions with alkynes), osmium, chromium, molybdenum, tungsten and copper. Metallocenes (see Metallocene Complexes) of early transition metals and lanthanides have also been found to catalyze the hydrosilation of a number of unsaturated compounds including alkenes and esters (vide infra). 

As already mentioned in the introduction, anionic early transition metallocene complexes of the type [Cp2Zr(R1)(R2)(R3)] with R, R2, R3 = alkyl, alkenyl, or alkynyl, are unstable. Most of them exhibit electrostatic anion-cation pairing resulting in dimer, trimer, oligomer, or polymeric structures [21-25]. In marked contrast, stable 18 electron-zirconate complexes were prepared via a formal [3 + 2] cycloaddition reaction between 2-phosphino-zirconaindene 16a and alkyne derivatives (Scheme 5) [26,27]. 

Codimers with alkynes can also be obtained from 1,2-diphenylcyclopropene when the trimethylphosphane-stabilized metallocene complexes 20 (M = Ti, Zr) of this cyclopropene derivative are used as substrates. The titanocene complex is prepared from bis(>/ -cyclopen-tadienyl)bis(trimethylphosphane)titanium (18), while the zirconocene complex is most conveniently formed from (f7 -but-l-ene)bis(f7 -cyclopentadienyl)(trimethylphosphane)zirconium (19) via a substitution reaction of the butene ligand. When 20 (M = Ti) is reacted with but-2-yne, the titanacycle 21 can be isolated as blue crystals in 71% yield. The zirconacycle 21 (M = Zr) is obtained in 78% yield as an orange powder. ... 

The metallocene complex 27 containing a M=X double bond undergoes overall [2 + 2] cycloaddition with an internal alkynes to give heterometallacyclobutenes (28) [77], A formal [2 + 2] cycloaddition of CpjZr (=N Bu)(thf) with imine affords a 2,4-diazametallacyclobutane, whose further reaction with imines results in an imine metathesis reaction [78] azametallacyclobutene is an intermediate in the Cp2Zr(NHR)2-assisted hydroamination of alkynes and allene [79],... 

Synthetic access to in situ generated metallocenes, especially of the titanium triad but also its neighbors, has given a growing number of reactive alkene and alkyne n-complexes that readily transform according to Eq. (b)i5-2o,23,96-98 chemical... 

Metallocene complexes of o-benzyne will also add to alkenes or alkynes, yielding 1-metallaindans and 1-metallaindenes, respectively (243). The zirconacy-clopentadienes formed from the dimerization of alkynes are convenient synthons for phospholes, siloles, and germoles (244). 

Zimgast M, Marschner C, Baumgartner J (2008) Group 4 metallocene complexes of tris (trimethylsilyl)silylacetylene and related alkynes. Organometallics 27 2570... 

Carbon Dioxide 0=C=0 The reactions of the metallocene sources 1-6 with carbon dioxide depend strongly on the metal and ligands used. Complex 1 gives, by elimination of half of the alkyne, the dimer 93, which forms the titanafuranone 94 after aerial oxidation [49]. 

The latter transformation requires the use of a small amount of an acid or its ammonium salt. By using [Cp2TiMe2] as the catalyst, primary anilines as well as steri-cally hindered tert-alkyl- and sec-alkylamines can be reacted.596 Hydroamination with sterically less hindered amines are very slow. This was explained by a mechanism in which equlibrium between the catalytically active [L1L2Ti=NR] imido complex and ist dimer for sterically hindered amines favors a fast reaction. Lantha-nade metallocenes catalyze the regiospecific addition of primary amines to alkenes, dienes, and alkynes.598 The rates, however, are several orders of magnitude lower than those of the corresponding intramolecular additions. 

An interesting new entry into metallocene-containing dithiolato and diselenolato complexes (see also Section 16.5.2.4.3) are the reactions of titanocene pentasulfide (57) or pentaselenide (58) with activated alkynes.83 Titanocene dithiolates (59) and diselenolates (60) with X = CF3 and C02Me were obtained. The authors also performed a number of exchange reactions with the Se compound (60 X = CQ2Me). 

---