Chromium complexes carbonyl anion

Some important reactions of chromium hexacarbonyl involve partial or total replacements of CO ligands by organic moieties. For example, with pyridine (py) and other organic bases, in the presence of UV hght or heat, it forms various pyridine-carbonyl complexes, such as (py)Cr(CO)5, (py)2Cr(CO)4, (py)3Cr(CO)3, etc. With aromatics (ar), it forms complexes of the type, (ar)Cr(CO)3. Reaction with potassium iodide in diglyme produces a potassium diglyme salt of chromium tetracarbonyl iodide anion. The probable structure of this salt is [K(diglyme)3][Cr(CO)4lj. 

Reactions of metal carbonyls with Zintl phases do not always yield the desired result. An example is the reaction of KSi with the metal carbonyls M(CO)6 (M = Cr, Mo, W) and the chromium complex Cr(CO)sNMe3. Instead of forming a structure with a Si4 anion, KSi reduces these transition metal compounds to form the anions, [M2(CO)io] . In addition, certain organometallic transition metal clusters have been synthesized that contain Zintl anions coordinated to the metal, but Zintl compounds are not used as the reagents. 

An example, developed by our group, has been presented 3-chloro-propenyl pivalate, the new synthon we propose as a formal a-hydroxy allyl anion, and chromium(II) chemistry allow to achieve the formal diastereo- and enantio-selective a-hydroxyallylation of carbonyl compounds by applying a catalytic cycle based on an in situ produced Salen-chromium complex (Table 8.3). 

The properties of the PPN cation have also facilitated the crystallization of salts of the dinuclear metal carbonyl anions, [M2(CO)10]2 (M = Cr, Mo, W), and permitted a crystal-structure determination7 of these remarkable complexes. The dinuclear chromium decacarbonyl anion was first prepared by Behrens and Vogl8 by the reaction of Cr(CO)6 with NaBH4 in liquid NH3 at 40°. More recently Hayter9 prepared dinuclear decacarbonyl anions of all three Group VI metals by a photochemical process and isolated them as the tetraethylammonium salts. A convenient synthesis involving isolation of the PPN salts of these anions is described in Sec. D. 

It is well known that the tricarbonylchromium-complexed benzylic anions and cations are stabilized due to overlapping between d-orbital of the chromium and p-orbital of the benzylic carbon [1]. Tricarbonylchromium complexes of a-te-tralone and a-indanone having a carbonyl group at the side chain underwent a deprotonation of the exo-benzylic protons by treatment with base to give the stereo-controlled tricyclic compounds (Eqs. 1 and 2) [2]. In these cases, Robinson annulation products were formed in less than 10% yield. Also, base treatment of benzyl ether chromium complex having a chlorine at the side chain 3 gave cyclization product as a diastereomeric mixture (Eq. 3) [3]. 

Dioxobenzocyclobutene has also attracted interest for organic synthesis. In particular, much attention has been paid to the addition of carbon nucleophiles to the carbonyl group for transformation to naphthoquinone or indanone derivatives. Double additions of excess vinyllithium to (1,2-dioxobenzocyclo-butene)chromium complex 52 at -78 °C gave the tricarbonylchromium complex of 5,10-dioxobenzocyclooctene 53 in 87% yield without formation of simple diadduct (Eq. 38) [37]. Obviously, the product 53 was formed by a double addition of vinyllithium and subsequent anionic oxy-Cope rearrangement. 

The anionic complexes (50) are obtained from the group VI carbonyl and the dilithium derivatives of the hydroxyalkynes HC=CCR2(OH) subsequent protonation or acylation affords propadienylidene (51) or carbene (52) complexes by complex cyclization and addition reactions (Scheme 3) (28). Attempts to obtain the dimethyl complex by reaction of the chromium alkynolate dianion with COCl2 gave only polymeric material. More stable complexes were obtained with R = aryl (73). 

Density functional theory studies arene chromium tricarbonyls, 5, 255 beryllium monocyclopentadienyls, 2, 75 chromium carbonyls, 5, 228 in computational chemistry, 1, 663 Cp-amido titanium complexes, 4, 464—465 diiron carbonyl complexes, 6, 222 manganese carbonyls, 5, 763 molybdenum hexacarbonyl, 5, 392 and multiconfiguration techniques, 1, 649 neutral, cationic, anionic chromium carbonyls, 5, 203-204 nickel rj2-alkene complexes, 8, 134—135 palladium NHC complexes, 8, 234 Deoxygenative coupling, carbonyls to olefins, 11, 40 (+)-4,5-Deoxyneodolabelline, via ring-closing diene metathesis, 11, 219... 

---