Complexes to a metal

Premetallized acid dyes are similar to the simple acid dyes but are already complexed to a metal. 

Attempts to produce vinylidene in the free state result in rapid reversion to ethyne, with a lifetime of 10 ° s [1]. As with many reactive organic intermediates, however, vinylidene can be stabilized by complexation to a metal center, using the lone pair for coordination and thus preventing the reversion to ethyne. Most 1-aIkynes can be converted into the analogous vinylidene complexes by simple reactions with appropriate transition metal substrates (Equation 1.2) ... 

In vinyl cations (Figure 3.13), as in trisubstituted carbenium ions, the positive charge is stabilized by electron donating substituents or by aryl or vinyl groups via jt-conjugation. Further stabilization may be achieved by c participation— that is, by hyperconjugation of a substituents, by complexation to a metal, or by the (3-silyl effect. 

Once an alkyne moiety is introduced into a peptide, e.g., as propargyl glycine (Pgl) in place of a simple glycine in Enk, it can of course be used directly for complexation to a metal complex. Along those lines, we have reacted Co2(CO)g with the peptide Ac-Tyr-Gly-Pgl-Phe-Leu-NH2, to yield the Co2(CO)6 complexed to the Pgl alkyne side chain [32]. In a real SPPS scheme with a pre-formed metal-alkyne complex, the amino acid building block [Tp WI(CO)(ri2-Fmoc-Pgl-OH)] was incorporated to yield the side-chain substituted Enk derivative H-Tyr-Gly-[Tp W(I)(CO)(r 2-Pgl)]-Phe-Leu-OH (Tp tris(3,5-dimethylpyrazolyl)-borate) [44]. 

Bicyclo[3.3.0]octa-l,3,5,7-tetraene (2), trivially called pentalene [26, 27], is the second member in the series of fully unsaturated oligoquinanes. Huckel MO theory predicts that this planar hydrocarbon with its 8 r-electron system should be an antiaromatic species [25]. 2-Methylpentalene (37) has been generated by a retro-Diels-Alder reaction and deposited as a film at —196 °C on a NaCl or quartz plate for its spectroscopic characterization. It rapidly dimerized upon warming the cold plates to temperatures above —140 °C [26]. Only two stable derivatives of pentalene not complexed to a metal [29], the hexaphenyl- (38) [30] and 2,4,7-tri-tert-butylpentalene (39) [31], have hitherto been reported (Figure 4). 

The formation of dihalocarbene is one of the most useful phase-transfer processes developed in organic chemistry. The dihalocarbenes, once generated from haloform and base, can undergo addition or insertion reactions (3, 11, 12). If the double bond is complexed to a metal, addition by the carbene will not occur, but rather insertion into a saturated carbon-hydrogen bond will take place. 

The platinum catalyst is effective in very small amounts, and can be introduced as H2PtCl6 or as elemental platinum on an inert support. A particularly active catalyst is the soluble platinum complex of divinyltetramethyldisilox-ane, CH2=CHSiMe2-0-SiMe2CH=CH2. The hydrosilyla-tion reaction operates through the Chalk-Harrod mechanism or one of its variants. bz jn these mechanisms, the first step involves the conversion of a metal alkene complex to a metal alkene silyl hydride complex. In addition to platinum, recently ruthenium, rhodium, palladium, copper, and zinc complexes are being studied as hydrosilation catalysts. " ... 

A number of linear quadridentate ligands have been made in situ and complexed to a metal ion without prior isolation of the ligand. However, most have been synthesized quite separately from complexation reactions, and these are listed in Table I. A number of ligand S3mtheses of general application will be outlined here. It must be emphasized that these are not complete syntheses and that many of the precursors used are themselves prepared by quite arduous routes. 

Complexation to a metal activates a double bond towards the addi- opening-... 

In a reaction to produce two or more products, complexation to a metal ion selectively stabilizes one component of the equilibrium mixture, favouring its production as the metal complex (Figure 5.12). This has the effect of shifting the equilibrium distribution of products so that instead of B being favoured over A, A becomes... 

Much of the work in this area concentrates on metal-free macrocycles, often with the nitrogen atoms of the macrocycle functionalized as amides [82], Reduction of these amides to facilitate complexation to a metal ion destroys the mesomorphism. 

Complexation to a metal activates a double bond towards the addition of a nucleophilic species (Scheme 10.16). The metal has modified the behaviour of the alkene, which would normally undergo addition reactions with electrophiles. 

The abbreviations for ligands used in this paper are shown in Figure 1. It should be noted that for the complexes described in this paper, the ligands become deprotonated at the N(l) position when complexed to a metal and hence acquire a formal charge of —1. 

Cyanides do form metal complexes, and are important as linking groups between metalsAn important reaction of cyanides is with a molecule already complexed to a metal. This reaction is quite similar to the reaction reported by Hogeveen between aluminium halide a complexes of cyclobutadiene and cyanides, yielding a mixture of pyridines. 

Collman and associates ((56) also have carried out extensive investigations of the chemistry of acetylacetonate anion when complexes to a metal. They find that the ring behaves similarly to aromatic compounds. Specifically, they find that ... 

Dicarboxylic acid 5 -monoesters of thymidine and of 5-(2-thienyl)-2 -deoxy-uridine have been prepared as triphosphate mimics. The ester of d4T with citric acid has also been made with the same objective, and more lipophilic bioisosteres 237 and the AZT equivalent have been reported, these being designed to mimic the conformation of the nucleoside triphosphate when complexed to a metal ion. Neither of these last two classes were effective, since all the activity against HIV could be shown to be due to hydrolysis to the parent nucleoside. 

Before we discuss the chemistry of this ligand we should note that it carries a number of protons which, with the power of modem nmr techniques, are almost all potential sources of stmctural information we have already mentioned the use of diastereotopic methylene groups, and in this ligand H15 and H15 and H7 and Hg are potentidly diastereotopic. Other protons, such as those in position 4 of the pyridines (H2, H13), H9, H20, and Mei are shifted upon complexation to a metal. Finally Nuclear Overhauser effects (NOESY, NOE difference spectra) can be used to obtain information on conformations a strong NOE between H15, and H14 confirms a cis conformation about the interannular bond when the tridentate site is complexed, and this is also tme for Me3 and H12 and Me2 and H3. The conformation about the spacer may be determined by study of the NOE between H5, and H9, Hjo- It is therefore possible to obtain rather detailed stmctural information in solution. 

Solvent effects on metal resonances are routinely tens of ppm, and changes in temperature during a measurement result in large enough shifts (often in the range 0.1-0.5 ppm °C ) that fine structure on the resonance is readily lost. Isotope effects (e.g., Cl vs. Cl, vs. 0, or vs. H) on metal resonance positions " are sufficiently large that the different chemical shifts from the individual isotopomers are often well resolved. These effects are not so marked in donor-atom NMR spectra, i.e., for N, or P complexed to a metal center, solvent effects are normally a few ppm or less. 

A key step in metal-induced olefin polymerization has the olefin tt face complexing to the metal center. The two faces of the propylene double bond are enantiotopic. Isotactic polypropylene forms when only one face of the propylene monomer consistently reacts to make polymer. Thus, a chiral catalyst is needed to distinguish enantiotopic faces of an olefin. But, how do we ensure that only one face reacts It is a complicated problem, because when an olefin like propylene complexes to a metal center in a typical chiral environment, not only will both faces complex to some extent, but many orientations are possible for each complex. This leads to many different reaction rates, and a mixture of Stereochemistries. A key to the solution, then, was to develop a catalyst that is chiral but not asymmetric. In particular, the C2-symmetric metallocene shown below was prepared. The metal is chirotopic but non-... 

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