Chromium complexes irradiation

Methylthiophene is metallated in the 5-position whereas 3-methoxy-, 3-methylthio-, 3-carboxy- and 3-bromo-thiophenes are metallated in the 2-position (80TL5051). Lithiation of tricarbonyl(i7 -N-protected indole)chromium complexes occurs initially at C-2. If this position is trimethylsilylated, subsequent lithiation is at C-7 with minor amounts at C-4 (81CC1260). Tricarbonyl(Tj -l-triisopropylsilylindole)chromium(0) is selectively lithiated at C-4 by n-butyllithium-TMEDA. This offers an attractive intermediate for the preparation of 4-substituted indoles by reaction with electrophiles and deprotection by irradiation (82CC467). 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

Rather complex structures are obtained by a novel chromium(O)-mediated three-component domino [6jt+2jt] cycloaddition described by Rigby and coworkers [315]. Irradiation of a mixture of the chromium complex 6/4-134 and the tethered diyne 6/4-135 with a Pyrex filter at 0 °C gave the polycyclic compounds 6/4-136 in medium to good yield (Scheme 6/4.34). 

The relative amounts of the products depend on irradiation time, and the formation of the bis (triphenyl phosphite) chromium complex may be almost completely avoided by following the reaction with thin layer chromatography. The diastereoisomers are air stable and reasonably soluble in most organic solvents. Their infrared spectra exhibit a metal carbonyl band at 1925 cm 1 (i>co ester at 1729 cm"1) and a metal thiocarbonyl band at 1925 cm"1 (CH2C12 solution). The NMR spectra. I 5ch3 at 1.87 ppm 5co3CH3 at 3.77 ppm II 6ch3 at 1.77... 

A further chiral auxiliary-based tactic exploited tricarbonyl( 76-arene)chromium complexes of aromatic imines 71, which reacted under ultrasound (US) irradiation with a-bromoesters in a predictable stereochemical course to give comparable amounts of /S-aminoesters and / -lactams, as outlined in equation 44127. Chromium decomplexation is eventually achieved by photochemical oxidation under air. 

The spiro carbon is a stereogenic center in spiropyrans, but because of the achiral structure of the open merocyanine form, the photochromic process will always lead to racemization unless additional chiral moieties are present. When a chiral substituent was introduced, remote from the spiro center, it was possible to isolate diastereo-isomers of the spiropyrans, but rapid epimerization at the spiro center occurred.1441 Diastereoselective switching was successful with 28, in which a stereogenic center was present close to the spiro carbon (Scheme 15).[45] Distinct changes in CD absorption at 250 nm were monitored upon irradiation with UV (250 nm) and with visible light (>530 nm) and a diastereomeric ratio of 1.6 1.0 was calculated for the closed form 28. Furthermore, a temperature-dependent CD effect was observed with this system it was attributed to an inversion of the diastereomeric composition at low temperatures. It might be possible to exploit such effects in dual-mode chiral response systems. A diastereoselective ring-closure was also recently observed in a photochromic N6-spirobenzopyran tricarbonyl chromium complex. 451 ... 

A number of substituted benzenes, naphthalenes, indans, pyridmes, and indoles form arene(tricarbonyl)chromium complexes upon thermolysis under an inert atmosphere, usually in a high boiling ether, or by irradiation of the arenes in the presence of chromium hexacarbonyl. The complexes are relatively air-stable and can usually be stored for long periods in the absence of light. Somewhat milder conditions can be used by transfer of the chromium tricarbonyl group from preformed naphthalene(tricarbonyl)chromium, tris(L)tricarbonyl chromium (L = acetonitrile, ammonia, pyridine), or tricarbonyl( -l-methylpyrrole)chromium. Enan-tiomerically pure arene(tricarbonyl)chromium complexes having two different substituents, either ortho or meta can be prepared conveniently by classical resolution of racemic... 

HS form thermally stable complexes with transition metals, among others with iron and chromium. On irradiation the Fe (III) complexes undergo photoreduction producing Fe(II) and organic radicals (6,14). 

The imine bond in 2-imidazolines reacts with Fisher carbenes under irradiation (Scheme 144). Low yields were obtained with monosubstituted imidazolines 608a-c due to thermal decomposition of the substrates under the reaction conditions. Optically active 4,4-disubstituted A -imidazolines 608d-e form azapenams (609d, 610e, 611) with (methoxymethylcarbene)chromium complex in good yields and high diastereoselectivity. The electronic nature of RVR clearly influences the sense of diastereoselectivity (609 vs. 610, 611) <1977JOC3586>. 

Steric interactions also influence the ground-state stability and barrier heights of the (R,R)-(S,S) and (R,S)-(S,R) diastereomers found in (rf-C5H5)Fe(CO)[Sn(CH3)3](CH3CH==CH2) (56). The olefin in one pair of diastereomers is significantly more labile. Diastereomers can also be detected by both IR and NMR in (Tj -CsHsIMfCOKNOKolefin) where M = Cr, Mo, and W (57, 60, 66). Approximately equal amounts of the diastereomers of the chromium complexes may be prepared readily by irradiation of the nitrosyl dicarbonyl complex in the presence of an olefin, such as propene or styrene (66). 

Irradiation of a solution of thiepine 1,1-dioxide chromium complex (15) in the presence of diene (99), followed by metal decomplexation with oxygen provided [6 -I- 4] cycloadducts (100) <92TL5873, 93JA1382). The relative stereochemistry of the products (100) was shown to be derived from an endo transition state. The electronic character of the diene (99) appears to offer a moderate influence on reaction efficiency. For example, yields were uniformly lower for reactions involving electron-deficient diene partners relative to those employing electron-rich dienes (Equation (17)). 

Chromium-complexed ketenes are obtained by irradiation of chromium aminocarbene complexes derived from chiral oxazolidines (CO pressure). These ketenes were found to react with imines diastereoselectively furnishing rram-P-lactams, exclusively. ... 

Amide hydrolysis was described in the case of the chromium complex shown in Eq. 27.84 It is noteworthy that no alteration of the organometallic moiety occurs. In addition to the relative stability of chromium compoimds under sonication, the low-energy irradiation conditions selectively allowed the biphasic hydrolysis to occur, without promoting the more energy-demanding metal group sonolysis. 

Metal carbonyl halides can also undergo photoredox reactions if an MLCT chromophoreofsuitableenergyispresentinthecomplex.ThecomplexCrI(CO)5, for example, has a characteristic long-wavelength absorption at 638 nm that is due to an I - Cr (LMCT) absorption. Irradiation into this chromophore for a solution of CrI(CO)5 in acetonitrile leads to the formation of iodine and the zerovalent chromium complex Cr(CO)5MeCN with a quantum yield of 0.01 (Ref. 65) ... 

In an interesting and novel organometallic approach to monocyclic and bicyclic p-lactam synthesis, it has been shown that pentacarbonyl(methoxy-methyl)chromium complexes will add to imines under sunlight irradiation (Scheme 38). ... 

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