Chromium complexes formyl

Complexation of indoles with chromium hexacarbonyl, which reduces the electron density of the heterocyclic system, promotes nucleophilic attack at the 7-position and, to a lesser extent, also at the 4-position of the indole ring and provides a viable synthetic route to 7-formyl-l-methylindole (78CC1076). Curiously, although the benzenoid ring is rendered susceptible to nucleophilic attack, the reaction of the chromium complex with butyllithium results in abstraction of the proton from the 2-position. However, if this position is... 

Reaction of the (1 / ),(—)- or (KV),(+)-tricarbony l(2-su Instituted benzaldehyde)chromium complexes 181 with the dianion of /-butylmethanesulfonamide affords, after decomplexation and intramolecular cyclization, the enantiomeri-cally pure 3-(2-phenyl-substituted)/ -sultam derivatives 182. The reaction of the dianion on the pro-stereogenic formyl group is key to the diastereoselective formation of the new stereogenic center, and it is controlled by means of... 

Still another route for selective formylation makes use of the tricarbonyl-chromium complexes of calix[4]arenes. Reaction of the tetrapropyl ether of 4 with Cr(CO)3 produces a complex in which a Cr(CO)3 moiety is associated with a single aromatic residue of the calixarene. Treatment of the complex with BuLi and then D2O, Mel, or DMF followed by decomplexation with I2 yields mono-substituted calix[4]arenes carrying D at a p- or m-position. Me at a p-position, and CHO at a p-position, respectively." ... 

A 1,2 or 1,3 unsymmetrically disubstituted arene is prochi-ral and therefore the corresponding chromium tricarbonyl compounds are chiral. (Substituted arene) complexes with amine, carboxyl, and formyl groups at the ortho position are resolved into optically active chromium complexes through corresponding diastereomeric adducts (eq 25). Biocatalysts also perform the kinetic resolution of racemic chromium complexes (eq 26). The optically active chromium complexes can be prepared by di-astereoselective ortho lithiation of the chiral benzaldehyde or acetophenone acetal complexes, and diastereoselective chromium complexation of the chiral ort/io-substituted benzaldehyde am-inals (eq 27). Catalytic asymmetric cross-coupling of meso (1,2-haloarene)chromium complex produces chiral monosubstituted complexes. The chiral (arene)chromium complexes can be used as ligands in asymmetric reactions. ... 

Treatment of a polyfunctional chromium-tricarbonyl-complexed hydroxy aldehyde with an excess of Et3SiFFTFA for 4.5 hours gives an 82% yield of fully reduced product with both the formyl and hydroxy groups completely and selectively reduced (Eq. 193).352... 

Among the types of complexes found here are formyl, acyl, alkyl, and aryl carbonyls, carbonyl cyanides," " carbonyl isocyanides and acetylides," and thiocarbonyl and seleno-carbonyl complexes." Of the greatest significance, however, are the chromium carbenes, for example, (CO)sCr=C(OR)R". This chemistry has been thoroughly reviewed " nevertheless, these compounds will be briefly discussed here. 

Building on their earlier research into S-endo cyclisations of Sml2-generated ketyl-type radicals, Molander and co-workers [107,108] developed this process as a key step in the syntheses of several naturally occurring lignans of the dibenzocyclooctadiene type. For example, they prepared the biaryl-chromium tricarbonyl complex 128 containing orffio-formyl and butenolide... 

A hydride is one of the simplest nucleophiles, and Casey ° and Gladysz have prepared kinetically stable formyl complexes by the direct attack of hydride on a number of neutral chromium-, molybdenum-, and iron-carbonyl complexes (Equation 11.2). Although these complexes are relatively electron rich, because they possess zero-valent metal centers, the negative charge in the product can be stabilized by the remaining -ir-ac-cepting CO ligands. 

Reduction of the cobalt complex (10) is much faster than reduction of (9) and leads to a chelate chromium(m) product, identified as (11) on the basis of its visible-u.v. spectrum. It is presumed that the reaction involves attack of chromium(ii) at the formyl group of (10), followed by rapid ring closure. 

Reduction of the 4-formylbenzonitrile complex (18) also proceeds by remote attack. The subsequent aquation of the carbonyl-bound chromium(iii) complex (19) can be detected kinetically. The analogous reaction with the 3-formyl complex leads to [Cr(H20)eP+ as the only observable product, but the possibility of a labile... 

Recent examples of kinetic studies of substitution in organic ligands include a qualitative investigation of the halogenation of the copper(n) complex of 8-hydroxyquinoline by the halogens themselves or by iV-halogenosuccinimides, and the establishment of the relative reactivity of the 3-position of acetylacetone co-ordinated to chromium(m) and to cobalt(m) from the relative rates of nitration and of formylation of the complexes Cr(acac)s and Co(acac)3. ... 

Another route to vinylcarbene C-glycosides is based on a TiCl4-assisted aldol condensation of pentacarbonyl[(methoxy)methylcarbene]chromium (229) and formyl glycosides 230-232. This reaction is trans-selective and affords sugar carbene complexes 233-235 in good yields (Scheme 11.53) [111]. 

---