Chromium silyl complexes

The Dotz benzannulation reaction yields either arene chromium tricarbonyl complexes or the decomplexed phenols, depending on the work-up conditions. Because of the instability of hydroxy-substituted arene chromium tricarbonyl complexes, yields of the latter tend to be low. High yields of arene complexes can, however, be obtained by in situ silylation of the crude product of the benzannulation reaction [336]. Oxidative work-up yields either decomplexed phenols or the corresponding quinones. Treatment of the benzannulation products with phosphines also leads to decomplexed phenols [272]. 

The chromium tricarbonyl complex 429 of triphenylphosphine oxide is hthiated and silylated (in situ quench) by 360 and McsSiCl to give 430 (X = Si) in 90% yield and 73% ee (Scheme 175) °. Interestingly, the sense of asymmetric induction is reversed from the anisole result. With two equivalents of chiral base, bis-sUyl compounds 432 could be formed. 

Aromatic organosilicon compounds react with chromium hexacar-bonyl on prolonged heating to give the corresponding silylated rjfi-arene chromium tricarbonyl complexes (57, 150). The trimethylsilyl derivative, obtained in 20% yield, is air-stable as a solid ... 

Trimethylsilyldiazomethane, 327 Silyl substituted arenes Bis(trimethylsilyl)acetylene, 97 Chromium carbene complexes, 82 Titanium(IV) chloride-Diethylalu-minum chloride, 309 Other organosilanes Osmium tetroxide-Trimethylamine N-oxide-Pyridine, 223 Tributyltin chloride, 315 Di- x-carbonylhexacarbonyldicobalt, 99 Trimethylsilyl trifluoromethanesul-fonate, 329... 

Even Lewis acid adducts of the acyl metalate 11 (Section 8.3.1.1, Scheme 8) proved effective in generating benzannulation products. The reaction of a trimethylsilyloxy chromium carbene complex with 3-hexyne to give the respective semi-silylated hydroquinone tricarbo-nylchromium complex in 35 % yield [30a] and the use of titanoxycarbene complexes [30b] constitute such examples. Thiocarbene complexes form hydrothioquinones in a Lewis acid supported benzannulation reaction [32]. 

TRICARBONYL-(ii5-CYCLOPENTADIENYL)SILYL COMPLEXES OF CHROMIUM, MOLYBDENUM, AND TUNGSTEN... 

The only example of Si H M agostic interaction supported by a phosphorus bridge was discovered by Driess et al. in the complexation of a silylated triphosphine ligand to a chromium tricarbonyl complex [Eq. (20)].143,144 A bigger metal (M = Mo) or smaller substituent at silicon (R = Ph) leads to the normal triphosphine complexes. The presence of a Si H "M interaction in 59 is deduced from the lack of C3 symmetry (evident from NMR), the red shift of the IR band for the coordinated Si-H bond (1994 cm-1 vs. 2142 cm-1), and the observation of a reduced Si-H coupling (135.7 Hz for the coordinated Si-H bond vs. 210.9 and 228.6 Hz for the free Si-H bonds). The X-ray structure determination of 59 shows that one of the phosphine ligands is tilted away from the chromium atom so that the... 

Acetals and ethers can also be used to produce benzylic car-bocations by reacting with a Lewis acid. Treatment of acetals derived from chiral 2-substituted benzaldehyde chromium tricarbonyl complexes with trimethylsilyl triflate followed by reaction with a silyl enol ether give diastereomerically pure alkylation products (Scheme 98). Benzylic alcohols can be... 

The synthesis of an enantiomerically enriched chromium complex via asymmetric lithiation of a prochiral tricarbonyl(ri -arene)chromium complex using a chiral lithium amide base was first demonstrated in 1994 by Simpkins [88]. Arene complex 44 was treated with C2-symmetric chiral base ent-39 in the presence of TMSCl as an internal quench and silylated complex 45 was obtained in 84% ee (Scheme 24). 

For monosubstituted arenes, kinetically controlled discrimination between the two enantiotopic ortho hydrogens of the planar chiral benzene chromium tricarbonyl complex leads to nonracemic products. Asymmetric lithiation is more efficient when one or more oxygen atoms, such as ether linkages, are present in the starting prochiral complex (Scheme 26.14). Treatment of Cr(CO)j-anisole complex 52 with the chiral lithium amide 53, in the presence of TMSCl under ISQ conditions, affords (+)-orfho-silylated complex 54 with good chemical yield and ee value [143-145]. The isobenzofuran system 55 reacts as well to give a-sUylated product 56 [146]. 

Silver(I) complexes with Tol-BINAP (270) were used by Yamamoto and coworkers for mediating diastereoselective and enantioselective Mukaiyama aldol additions. According to the authors conclusion, the mechanism does not involve transmetallation to silver enolates but follows the usual carbonyl group activation [135]. Hoveyda and coworkers used silver(II) fluoride in the presence of a dipeptide-type ligand for enantioselective additions of silyl enol ethers to a-keto esters [136]. The reaction of 2-trimethylsilyloxyfuran with aromatic and aliphatic aldehydes was catalyzed with chromium salen complex in the presence of protic additives like isopropanol [137]. Various protocols of enantioselective Mukaiyama aldol reactions that use water as cosolvent have been elaborated ... 

HYDRIDO-SILYL COMPLEXES OF CHROMIUM WITH METAL-HYOROGEN-SILICON THREE-CENTER BONDS... 

Roddick DM, Tilley TD, Rheingold AL, Geib SJ (1987) Coordinatively imsaturated tris (trimethylsilyl)silyl complexes of chromium, manganese, and iron. J Am Chem Soc 109 945... 

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