Chromiumtricarbonyl

Figure 5.49 The structure of the benzene-chromiumtricarbonyl CT complex (C3 symmetry).

An interesting application of the Pd-catalyzed reduction was reported for the preparation of the chromiumtricarbonyl complexed phenylallene 27 (Scheme 3.14), which was obtained from the internal alkyne 26 with good regioselectivity [45]. 

Asymmetric induction of between 30 and 66% has been reported for the addition of perfluoroalkyl iodides to chiral arene/chromiumtricarbonyl complexes using ultraso-nically dispersed zinc at room temperature (Scheme 3.19) [101], Photochemical decomposition of the organometallic intermediate affords a chiral alcohol product. The reaction is carried out in DMF as solvent and high overall yields are reported e.g.80 % (R = Et). Only a small excess of the perfluoroalkyl iodide is required and the conversion is complete in under 1 h. 

Not all carbon nucleophiles will add to arene chromiumtricarbonyl complexes. For example, alkyllithium reagents and simple ketone enolates do not give adducts.198... 

The general synthesis of the Daniphos ligands starting from enantiomerically pure [(R)-l-(phenylethyl)dimethylamine]chromiumtricarbonyl 1, is depicted in Scheme 1.4.1 [15]. A directed ortho-metallation (DOM) and subsequent quench with a chlorophosphine leads to an enantiomerically pure planar-chiral complex, which after chlorination using ACE chloride (1-chloroethyl chloroformate) is transformed into the desired diphosphine by a nucleophilic substitution without any loss of optical purity (Scheme 1.4.1) [6, 10]. 

This indicates an ideal setup for the abstraction of the desired pro-R proton. The chromiumtricarbonyl moiety also facilitates the two consecutive nucleophiic substitutions of the reaction pathway, which both proceed with retention of configuration, by shielding the bottom of the molecule sterically and also by stabihzing the benzylic carbocationic intermediate [6, 7]. 

Schulte, J. L. Laschat, S. Kotila, S. Hecht, J. Frohlich, R. Wibbeling, B. Synthesis of j 6-(octahydroacridine)chromiumtricarbonyl complexes with non-polar tails via molecular sieves-catalyzed cydization of N-arylimines and subsequent diastereoselective complexa-tion. Heterocydes 1996, 43, 2713-2724. 

One of the most fascinating developments in borazine chemistry has been the recent isolation of hexaalkylborazine chromiumtricarbonyl complexes. A reaction was observed when hexamethylborazine was reacted Mo(CO)e but no definite products could be isolated 1S8>. However, with extremely pure starting materials using (CO)3Cr(CHaCN)3 or (CO)3(NH3)3Cr as reactants, the preparation of a borazine-metal complex has been effected iss.ieo). Solvents other than dioxane did not lead to the desired product, and it has been suggested that the reaction proceeds via a dioxane-chromium tricarbonyl complex. The reaction appears to proceed by altering the existing equilibria, indicated in the reaction as depicted in Eq. (43). 

Examples of non-platinum metal hydrogenation catalysts include (arene)-chromiumtricarbonyls which will hydrogenate dienes, alkynes, and so on, while ReH7(PCy3)2 will selectively hydrogenate acenaphthalene. Lanthanides and early transition metals are discussed later. Those catalysts operate via non-radical processes, but a few systems are known to involve radical reactions. The complex [CoH(CN)5]3 is a water-soluble catalyst that is selective for the hydrogenation of ajS-unsaturated compounds. 

Vinyl-substituted benzo[c]furans can be prepared by reaction of n-alkynylbenzaldehydes with chromium Fischer carbene complexes. Initially a benzo[c]furan chromiumtricarbonyl complex is believed to be formed which is converted into an alkylidenephthalan derivative or can be trapped with electron-deficient dienophiles with excellent ( 3 ti-selectivity (Equation 128) <20000L1267>. More elaborate vinylidene Fischer carbene complexes yield dienyl benzo[c]furans that undergo [8-1-2] cycloaddition with DMAD to furnish furanophane derivatives <2003JA12720>. An equilibrium between 7] -(o-ethynylbenzoyl)rhenium complexes and rhenium benzo[f]furyl carbene complexes has been observed. These species behave like other benzo[< ]furans in the reaction with DMAD <20040M4121>. 

In order to account for these observations, structural changes were proposed as shown in Scheme 1 the attraction of this scheme lies in the avoidance of 19-electron metal centers in the monoanion 12 and dianion 13. Data on the bis(chromiumtricarbonyl) complex of stilbene were interpreted similarly (28). 

Alkylation of (a-tetralone)chromiumtricarbonyl 37 with MeLi and subsequent ionic hydrogenolysis with excess EtsSiH and CF3CO2H gave (1-endo-methyltetralin)-chromiumtricarbonyl (39) via stereoselective exo-hydride displacement on 38. On the other hand, the endo-acstate complex 40 was converted into (1-exo-methyltetralin)-chromiumtricarbonyl (41) via exo-methyl attack to the carbocation by treatment with MejAl (Sch. 23) [47]. 

The reduction of the chromiumtricarbonyl complexes of indanones and tetralones 3.42 and 3.43 [JMl] are interesting in terms of steric hindrance. The hindered organometallic group blocks the attack of the hydride on the same face of the molecule. Because it is possible to obtain the corresponding ketones in nonracemic form, one has access to enantiomerically pure stereoisomers after decomplexation (Figure 3.16). 

Weak hydrogen-bonding involving a terminal alkyne function has been found in the substituted (cycloheptatriene)chromiumtricarbonyl derivative t] -7-exo-liC=... 

Diphenylhydantoin is a drug used in the treatment of epilepsy. The tracer 24 for this substance was synthesized by esterification of diphenylhydantoin 3-acetic acid 22 by (2-phenylethanol)chromiumtricarbonyl 23 (Scheme 8.12) [26], the latter having been prepared in two steps by alkylation of the sodium salt of diphenylhydantoin by ethyl bromoacetate followed by acid hydrolysis of the ethyl ester. 

Metzler-Nolte et al. successfully created organometallic PNA hybrids to provide an analytically observable tag for PNA strands, including ferrocene-PNA for electrochemical detection, (benzene)-chromiumtricarbonyl-PNA for IR detection of the carbonyl groups, and a bipyridyl-ruthenium derivative for spectroscopic and electron transfer properties. Hybridization of these strands with ssDNA has been shown to produce duplexes with stabilities only sUghtly lower ( 2°C) than an unmodified PNA-DNA system, which allows these organometallic structures to act as markers for duplex formation.Functionalized PNAs should... 

Chromium, Molybdenum and Tungsten The intramolecular rotational behaviour of a series of highly substituted Ti -arene chromium and molybdenum compounds has been examined in detail. Dynamic processes in the solid state, using spin-lattice relaxation time measurements, have been studied for two structurally characterised polymethyl benzenechromium tricarbonyl complexes. The mechanism of migration of the Cr(CO)3 unit from the six to the five membered ring in indenyl anions has been reported. The synthesis and cyclic voltammetry of (Ti -phenylmethylsila-14-crown-5)chromiumtricarbonyl has appeared. ... 

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