Boron chromium complex

S)-Tricarbonyl(2-methoxyacetophenone)chromium is a starting material which provides remarkable substrate-induced stereoselectivity. Thus, its conversion into a boron enolate and subsequent addition to aldehydes delivers the chromium complexes 7 and 8 with diastereomeric ratios of 92 8 to 95 559. 

Highly diasteroselective and chemoselective reductions may be performed on the hydroxy functions of (r/6-arene)-tricarbonylchromium complexes. Treatment of the chromium-complexed benzylic alcohol 29 with triethylsilane and boron trifluoride etherate in dichloromethane at —78° to 0° gives only diastereomer 30 in 75% yield (Eq. 40).181 In a similar fashion, treatment of the complexed exo-allyl-endo-benzylic alcohol 31 with an excess of Et3SiH/TFA in dichloromethane at room temperature under nitrogen produces only the endo-aflyl product 32 in 92% yield after 1.5 hours (Eq. 41). It is noteworthy that no reduction of the isolated double bond occurs.182... 

Sodium triacetoxyborohydride, 283 Tetrabutylammonium iodide-Boron trifluoride etherate, 287 Tributylborane, 325 Tributyl borate, 326 Triethylborane, 33, 292 Trimethyl borate, 218 Zinc borohydride, 167 Chromium Compounds Arene(tricarbonyl)chromium complexes, 19... 

Goodall [7] polymerized norbomene using the cationic chromium-palladium/ boron pair complex, (IV) to aM of roughly 1,200,000 daltons with a PDI index of 1.2. 

Fig. 6. "B-NMR data for 2-diethylboryl-5-methyl thiophene (a) and its pentacarbonyl-chromium complex (b). In the latter, donor-acceptor relationship between boron and the carbonyl group is postulated to account for its higher field nB shift (c).

The cycloaddition reactions of chiral 2-amino-l, 3-dienes with Fischer-type carbene complexes have been examined by Barluenga et al. Reactions with tungsten vinyl carbene complexes A and with boron-nitrogen-chelated chromium complexes B lead very selectively to cyclohexanone derivatives. However, when employing chromium vinyl carbene complexes in acetonitrile at room temperature in these reactions, cycloheptadienes were obtained selectively by cyclopropanation and subsequent Cope rearrangement. 

Pyridazines form complexes with iodine, iodine monochloride, bromine, nickel(II) ethyl xanthate, iron carbonyls, iron carbonyl and triphenylphosphine, boron trihalides, silver salts, mercury(I) salts, iridium and ruthenium salts, chromium carbonyl and transition metals, and pentammine complexes of osmium(II) and osmium(III) (79ACS(A)125). Pyridazine N- oxide and its methyl and phenyl substituted derivatives form copper complexes (78TL1979). 

Although most of the examples of [3S+2C] cycloaddition reactions with carbene complexes are referred to as 1,3-dipolar processes, we should include in this section another kind of non-dipolar transformation dealing with the reaction of pentacarbonyl(methoxymethylcarbene)chromium with a base followed by treatment with an epoxide in the presence of boron trifluoride. This reaction gives cyclic carbene complexes in a process that can be considered a [3S+2C] cycloaddition [44] (Scheme 14). 

Treatment of chromium (III) acetylacetonate with acetic anhydride and boron trifluoride etherate yielded a complex mixture of acetylated chelates but very little starting material. Fractional crystallization and chromatographic purification of this mixture afforded the triacetylated chromium chelate (XVI), which was also prepared from pure triacetylmethane by a nonaqueous chelation reaction (8, 11). The enolic triacetylmethane was prepared by treating acetylacetone with ketene. The sharp contrast between the chemical properties of the coordinated and uncoordinated ligand is illustrated by the fact that chromium acetylacetonate does not react with ketene. 

A drawback of the simple superacid mechanism is that it implies no particular specificity for manganese (III) any other tripositive cation which readily complexes with peractic acid should be a good catalyst for the reaction. Preliminary experiments with chromium (III), aluminum-(III), and boron (III) acetates indicate that none of these salts catalyzes... 

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