Allyl bridged transition metal complexes

The more elegant route—which we call for simplicity the method of [1 + l]-addition —may also be used to prepare other binuclear complexes containing either two allyl groups, or one allyl group and one halogen in the bridging positions. The results so far obtained are summarized in Eqs. (9) and (10). Compounds 45-47 are the first allyl-transition metal complexes in which not one metal atom but a dimetal unit is coordinated sandwich-like by two i73-bonded allylic ligands (75, 79). 

The first organometallic compound of the transition metals to be characterized (1827) was Zeise s salt, K[(C2H4)PtCl3]-H20 (Fig. 18.1). It forms when K2[PtCl4] in aqueous ethanol is exposed to ethylene (ethene) a dimeric Pt—C2H4 complex with Cl bridges is also formed. In both species, the ethylene is bonded sideways to the platinum(II) center so that the two carbon atoms are equidistant from the metal. This is called the dihapto-or T]2 mode. A ligand such as an allyl radical with three adjacent carbons directly bonded to a metal atom would be trihapto- or t 3, and so on. 

Transition-metal-catalysed epoxidations work only on allylic alcohols, so there is one limitation to the method, but otherwise there are few restrictions on what can be epoxidized enantioselectively. When this reaction was discovered in 1981 it was by far the best asymmetric reaction known. Because of its importance, a lot of work went into discovering exactly how the reaction worked, and the scheme below shows what is believed to be the active complex, formed from two titanium atoms bridged by two tartrate ligands (shown in gold). Each titanium atom retains two of its isopropoxide ligands, and is coordinated to one of the carbonyl groups of the tartrate ligand. The reaction works best if the titanium and tartrate are left to stir for a while so that these dimers can form cleanly. 

As far as we are aware, no examples of bimetallic complexes of the structural type G exist, where a C, allyl ligand acts as an bridging ligand with two transition metals but without metal-metal bonds. Methods for the preparation of such complexes are not obvious and may pose a synthetic challenge. In contrast, many examples of /A-allyl (B), p-... 

Erker and co-workers (13,206-224) have introduced a new synthetic route for the synthesis of Ficher-type metallocene oxocarbene complexes. This method, whereby a terminal carbonyl ligand is converted to a carbene by a second, early transition metal center, involves C-C coupling of a coordinated alkene with an 17 -coordinated terminal carbonyl at an early transition metal center [Eq. (30)]. Review articles (13,207-209) have appeared in the literature that extensively cover this area of chemistry, so only a short summary concerning bimetallic complexes with cr-carbene and ir-allyl bridging ligands is presented. 

Books have been published on cyclic polyolefin complexes and metal vapour synthesis, while a review on metal vapour cryochemistry also contains information relevant to the latter. Reviews have also been published on organometallic derivatives of alkenes and ketenes, cationic Rh diolefin complexes, and nucleophilic addition to cationic hydrocarbon complexes. More personalized reviews have appeared by Maitlis on Pd -acetylene chemistry, by Jonas and Kruger on alkali-metal-transition-metal w-complexes, and by Werner on bridged allyl and cyclopentadienyl complexes. Articles by Wilke on homogeneous catalysis, by Vollhardt on metal-mediated approaches to steroid synthesis, by Schrock > on organotantalum chemistry, and by Grubbs on nickel metallocycles also contain material of relevance to r-complexes. 

Similar bridging hydrogen exchange processes have been observed for neutral and cationic (zj -allyL/i-IIjMD, complexes 25 (Scheme 3)23.58,63 j ny of these complexes are prepared by the protonation of the corresponding anionic or neutral (/j4-dicnc)MLn complexes 26 (see Section V.C.l). Migration of the metal about a cyclic ) -allyl ligand (i.e. 25a to 25b) is proposed to involve an (/j" -diene)MLn hydride intermediate/transition state 27. 

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