Jt-Complex

L-idose 293, 311 ff. imine formation 57 imine-enamine tautomerization 467 iminium-RhH jt complex 351 f. imipenem 348 indoline ligands 681, 684 indolizomycin 47 Iff. -.retrosynthetic analysis 472ff. 

How can we tell if 10 is present on the reaction path If it is present, there are two possibilities (1) The formation of 10 is rate determining (the conversion of 10 to 11 is much faster), or (2) the formation of 10 is rapid, and the conversion 10 to 11 is rate determining. One way to ascertain which species is formed in the rate determining step in a given reaction is to use the stability information given in Table 11.1. We measure the relative rates of reaction of a given electrophile with the series of compounds Usted in Table 11.1. If the relative rates resemble the arenium ion stabilities, we conclude that the arenium ion is formed in the slow step but if they resemble the stabilities of the Jt complexes, the latter are formed in the slow step. When such experiments are carried out, it is found in most cases that the relative rates are similar to the arenium ion and not to the n complex stabilities. For example,... 

Antimonyflll) chloride forms colored Jt-complexes with double bond systems (e.g. vitamin A). 

Another way in which Pt could bind to DNA is through the formation of intercalation compounds. The parallel here is with the hydrocarbon carcinogens and the nucleic acid stains, the acridines. It has been shown that metal chelates will form this same type of jt-complex. For example, palladium oxinate will form exactly the same type of -complexes as anthracene (88). 

FIGURE 1.10 Various possible surface species on a Pt or Pd (111) surface. A and B represent possible locations of 1,2-di-a-Cj 2-cyclohexane, and C, D, and E represent possible locations of Jt-complexed Jt-C -cyclohexene. Full complements of hydrogens are assumed at each angle and terminal that is not either a- or Jt-bonded to a surface site as indicated by a small circle. Half-hydrogenated states, which are mono-a-C -adsorbed species (where n is the number of the carbon attached to the surface), would be represented by one small circle at the carbon bonded to a surface site. F, G, and I represent possible locations of Jt-C -cyclohexene. F shows the three carbons of the Jt-allyl moiety adsorbed in three adjacent three-point hollow sites and G shows it over one three-point hollow site, whereas I shows adsorption over the approximate tops of three adjacent atoms. (Note Label H is not used to avoid confusion with hydrogen, which is not shown.)... 

Four kinds of adsorbed species are postulated to result from the adsorption of alkenes on noble metals, Jt-com plcxcd double bonds, Jt-complexed jt-allyl species, mono-O-bonded alkanes, and di-o-bonded alkanes. Distinguishing among these is difficult and has been the subject of much speculation. For example, alkenes may adsorb either by Jt-complexing or by di-C-adsorption, and 7t-allyl species are thought to occur readily on Pd but not so readily on Pt.55... 

Some years ago we were successful in our attempts to synthesize a Jt-complex with divalent silicon as the central atom. Starting from dihalogeno(pentamethylcyclopentadienyl)silanes, we have been able to prepare decamethylsilicocene (1) by reductive elimination processes [1]. Characteristic data concerning the synthesis, structure, and bonding of 1 have been published elsewhere together with preliminary results concerning the chemistry of this compound [2]. Here we describe some further progress in this field. 

We have recently prepared some new and very thermolabile CO- and N2 comPlexes derived from titanocene [1] or decamethyltitanocene [2], and have characterized them by their vibrational spectra. As well as "classical" matrix spectroscopy, we have used spectroscopy in liquid xenon (LXe). The application of chemistry and methodology indicates the decamethylsilicocene structure, which represents the first example of a stable jt-complex of divalent silicon [3]. Reaction with CO or N2 leads to the two title complexes [4] ... 

Several reaction systems have been reported in which phosphorus was added to carbon of substrates in which the organic portion was jt-complexed with a transition metal.24 27 Although interesting, these approaches have not proven to be of general utility for synthetic purposes. 

Fischer, E. O., Werner, H. Metall jt- complexes. Vol. 1. Complexes with di- and oligo-olefinic ligands. Amsterdam Elsevier 1966 ... 

In developing his theory of the polymerisations by ionising radiations Plesch now distinguishes between the mono-alkenes and other monomers, thus if a monomer, such as styrene, contains two donor groups, then both of these can be involved in the complex formation with the carbenium ion, but only the Jt-complex involving the double-bond can propagate. This means that Equation (40) must be replaced by... 

Similar models explain the 1,8-, 1,10- and 1,12-addition reactions to the extended Michael acceptors 91, 93 and 95, respectively (Schemes 2.32 and 2.33). Again, these transformations start with the formation of a cuprate Jt-complex at the double bond neighbouring the acceptor group [61a]. Subsequently, an equilibrating mixture of a-copper(III) intermediates is presumably formed and the regioselectivity of the reaction may then be governed by the different relative rates of the reductive elimination step of these intermediates. Consequently, the exclusive formation of allenic prod-... 

Interesting direct evidence for the reaction pathway of these metal-catalyzed reactions was obtained from stoichiometric reactions of allenylcarbinols such as 61 with the [j7S-CsHsFe(CO)2]+ (Fp+) complex. From the isobutene adduct of Fp+ and 61 the jt-complex 65 was obtained. The complex 65 was suspected to be the product of an acid-catalyzed rearrangement indeed, in the presence of N,N-dimethylaniline as a base, 63 was isolated (Scheme 15.14) [30],... 

In general the syn,syn complex is the most stable isomer but it is in equilibrium with the other three diastereomers (Scheme 17.3). The interconversion between the jt-complexes proceeds via the less stable rj-complexes and is rather slow. Usually more than one Jt-allyl intermediate is present in the reaction mixture, which makes chirality transfer from an enantiomerically enriched allene to the product complicated. 

From alkyl complex p0, the olefin can be captured to form the Jt-complex TCo, and inserted via 1,2- or 2,1-insertion route. In the model applied here we consider the olefin capture and its insertion as one reactive event i.e. we assume a pre-equilibrium between the alkyl and olefin complexes, described by an equilibrium constant Kcompi = [icq] / [Pol = exp (AGcomp . / RT). This... 

If olehn metathesis is to be conducted in solution, solvents of low Lewis-basicity will generally give the best results (CH2CI2 > toluene > THF). As discussed above, metathesis is initiated by the formation of a jt-complex between the metal and the alkene. Hence, other nucleophiles will compete with the alkene for these coordination sites and in some systems even THF can lead to complete deactivation of the catalyst [786]. Tungsten-based catalysts which can even metathesize allyl thioethers have, however, been described [787]. 

The JT-complexes formed between chromium(O), vanadium(O) or other transition metals, and mono- or poly-fluorobenzene show extreme sensitivity to heat and are... 

Another convenient method of preparing this Jt-complex of iron is a two-step process in which the first step involves preparation of cyclopentadienyl Grignard reagent, such as 2,4-cyclopentadienylmagnesium bromide CsHsMgBr which may then be combined with ferric chloride to yield dicyclopen tadienyl iron ... 

---