Decomplexation chromium complex

The cycloaddition of several diazoalkanes with (2-arylvinyl)-[(—)-(8-phenyl-menthyloxy] methylene chromium complexes 72 gave the A -pyrazolines 73 with high diastereoselectivity. These compounds were converted into pyrazolinecarbox-ylates 74 by A-protection and metal decomplexation (98) (Scheme 8.17). It is... 

Some further examples of stereoselective deprotonation/alkylation reactions of tricarbonyl-chromium complexed (V-methyl tetrahydroisoquinolines have been reported27. Starting with the enantiomerically pure (35)-methyl tetrahydroisoquinoline reaction with hexacarbonyl-chromium led to a mixture of endo- (40%) and exo- (60%) complexes, which were deprotonated with butyllithium and subsequently methylated with iodomethane. In this way methylation occurred firstly at the 4- and secondly at the 1-position. In all cases, the methyl group entered anti to the chromium complexed face. After separation of the alkylated complexes by chromatography and oxidative decomplexation, the enantiomerically pure diastereomers (—)-(l 5,35,47 )-and ( + )-(17 ,35,45)-1,2,3,4-tetrahydro-l,2,3,4-tetramethylisoquinolme were obtained, benzylic amines such as tetrahydroisoquinoline to 2-amino-4,5-dihydrooxazoles. Deprotona... 

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. 

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... 

A further application of chromium complexes in aromatic chemistry allows the construction of a new aromatic ring. In the Dotz benzannu-lation, an alkyne adds to an unsaturated alkoxychromium carbene 11 to give a hydroquinone-chromium complex 12. Decomplexation yields the aromatic compound (Scheme 10.30). 

Generally, arene(alkoxy)carbene chromium complexes react with aryl-, alkyl-, terminal or internal alkynes in ethers or acetonitrile to yield 4-alkoxy-l-naphthols, with the more hindered substituent ortho to the hydroxyl group . Upon treatment with alkynes, aryl(dialkylamino)carbene chromium complexes do not yield aminonaphthols, but they form indene derivatives . Vinyl(dialkylamino)carbene complexes, however, react with alkynes to yield aminophenols as the main products The solvent is one of the many factors that affects this type of reaction, for which the most important is the polarity and/or coordinating ability of the solvent. The Dotz benzannulation reaction yields either arene chromium tricarbonyl complexes or the decomplexed phenols, depending on the work-up conditions. Oxidative work-up yields either decomplexed phenols or the corresponding quinones. 

Chiral 2-substituted benzaldehyde chromium tricarbonyl complexes have been reacted with chloroacetophenone in the presence of KO-fert-Bu [544], After decomplexation, the E-epoxyketone is obtained with a high selectivity (Figure 6.88). This Darzens reaction with ClCF COO-tert-Bu is poorly stereoselective. Condensation of the same aldehydes with methyl aaylate or acrylonitrile in the presence of DABCO, followed by decomplexation, also leads highly selectively to P-hydroxyesters or -nitriles 6.105 (Y = COOMe or CN) [547] (Figure 6.88). An anti aldol product is also obtained with a high selectivity from a chromium complex and the titanium enolate of PhCF OCF COS-tert-Bu at -78°C [1281, 1282], Chiral aminals of a-ketoaldehydes react with lithium or sodium enolates of ethyl acetate. After treatment with acid, compounds 6.106 are obtained with a high enantiomeric excess (Figure 6.88). 

Reaction of 1,4-benzodioxin with chromium hexacarbonyl gives a 41% yield of the > -arene chromium complex (62). Reaction of this complex with n-butyllithium forms a yellow solution of the aryllithium. Reaction of this with a range of electrophiles gives a series of 5-substituted derivatives in modest yield, which are readily decomplexed by iodine to form the useful 5-substituted 1,4-benzodioxins (63) <89TL5519>. 

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." ... 

Subsequent oxidation of the chromium-complexed isopicrosteganol 14 with PCC/NaOAc in CH2CI2 simultaneously deprotected/decomplexed the chromium species and oxidized the carbinol to the corresponding ketone (Scheme 7). Although Dess-Martin periodinane was utilized in... 

Direct nucleophilic addition of potassium enolates derived from bis(trimethylsilyl)ketene acetals to aromatic chromium-complexed aromatic ethers affords meta substituted products (Scheme 124). A very high degree of asymmetric induction is obtained upon reaction of chiral arene chromium tricarbonyl complexes. For example, alkylation of complex (80) gave (81) after decomplexation (Scheme 125). ... 

The corresponding benzaldehydes without chromium complexation resulted in lower selectivity. Decomplexation and LiAlH4 reduction afforded amino alcohols. Similarly, ethyl isocyanoacetate afforded the corresponding trans-oxazolines with high diastereoselectivity by the reaction with planar chiral benzaldehyde chromium complexes in the presence of LDA in THE at -78 °C (Eq. 17)... 

The reaction conditions using KCN instead of LDA as base (EtOH at room temperature) gave diastereomeric mixture of trans- and cz5-oxazolines. The trans oxazolines obtained from the planar chiral benzaldehyde chromium complexes afforded optically pure a-amino-p-hydroxy acids by decomplexation followed by treatment with acid. 

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)). 

Typical Experimental Procedure -BuLi (1.4 mmol, 0.88 mL in THE) was added to a solution of TMS acetylene (1.4mmol in 2mL of THE) at -78°C under an N2 atmosphere. After 5 minutes a solution of chromium complex (1.1 mmol) in THE (3mL) was added dropwise. The yellow mixture was stirred for 1 hour at -78°C and then quenched with saturated aqueous NH4CI. Workup followed by chromatography furnished the products in high yields (70 to 90%). The compounds in CH2CI2 were exposed to sunlight for decomplexation to generate the chiral compounds. 

In related synthetic approaches intermediate phthalidotetralins have been prepared through Friedel-Crafts alkylation of tetrahns with bromophthalides [24,25], or through reaction of specifically lithiated ring D [26] or AB [27,28] precursors with aldehydes. Thus addition of aldehyde 18 to the ortho lithiated chromium complex 19, decomplexation, and treatment with acid and then base gave the phthalides 20 which were converted to ( )-ll-deoxydaunomycinone 7 [28]. 

Decomplexation of complex 15 by exposure to sunlight in ethereal solution affords the optically active free amino alcohol 18 in 90% yield, as described in Protocol 8. It is worth noting that recomplexation of this compound 18 by a ligand-transfer reaction with Ti -naphthalene chromium tricarbonyl did not give rise to the precursor 15 but to its diastereoisomeric complex in which the opposite face of the ring is coordinated to the chromium moiety. In this... 

Nucleophilic Substitution. Some carbon nucleophiles add to tricarbonyl(arene)chromium complexes to yield anionic ii -cyclohexadienyl complexes (2) (eq 8), which give the substituted arenes via decomplexation by oxidation with iodine. Protolysis of the intermediate cyclohexadienylchromium complexes (2) generate cyclohexadienes, and reaction with electrophiles generates either the arene chromium complexes or produces the acylated species. 

The synthetic value of the Dotz reaction has for example been demonstrated by the synthesis of vitamin Ki(20) 10 (simplified structure). This natural product has been prepared synthetically from the chromium carbene complex 8 and the alkyne 9 in two steps the second step being the oxidative decomplexation to yield the free product 10 ... 

Complete diastereoselection is observed in the HDA reaction of Danishefsky s diene with o-substituted benzaldehyde chromium tricarbonyl complexes. Decomplexation is facile and good yields of 2-aryl-2,3-dihydropyran-4-ones result <96SL258>. Cis-2,3-disubstituted pyranones are accessible from the Lewis-acid catalysed HDA reaction between (triisopropylsilyloxy) dienes and aldehydes and dehydrogenation of the resulting dihydropyrans <96JOC7600>. 

A chromium(O) pentacarbonyl-methylene chloride complex 131 was formed under irradiation, which reacted with an alkyne to form a vinylidene complex 132. Complex 132 further reacted with an imine or a dialkylcarbodimide to afford /3-lactams after decomplexation of chromium (Scheme 58).233... 

The 774-vinylketene complex (85) could be oxidatively decomplexed with Ce(IV) to afford the lactone (87). Although no reaction was observed with methanol (unlike a postulated chromium analogue16,18 26), treatment with sodium methoxide produced the expected /3, y-unsaturated ester (88). Thermolysis of complex 85 afforded no trace of the naphthol that one would expect33 from a proposed chromium vinylketene complex with the same syn relationship between the phenyl group and the ketene moiety. Instead, only the furan (89.a) was seen. Indeed, upon exhaustive reaction of tricarbon-ylcobalt carbenes (84 and 90) with different alkynes, the furans (89.a-d) were isolated as the exclusive products in moderate to excellent yields. 

The enantiomerically pure complex (5 )-tricarbonyl(t 6-1-methoxy-2-methoxymethylben-zene)chromium(0), available from the enantiomerically pure 2-methoxybenzaldehydo complex34, led to the diastereomerically pure [j76-l-methoxy-2-[(JR)-r-methoxyethyl]benzene derivative which gave, after decomplexation, 1-methoxy-2-[(/ )-l -methoxyethyl]benzene. This is, as yet, the sole reported example. 

Complexes of Cr, W, Mo, Fe, Ru, V, Mn and Rh form stable, isolable arene if -complexes. Among them, arene complexes of Cr(CO)3 have high synthetic uses. When benzene is refluxed with Cr(CO)6 in a mixture of dibutyl ether and THF, three coordinated CO molecules are displaced with six-7r-electrons of benzene to form the stable i/fi-benzene chromium tricarbonyl complex (170) which satisfies the 18-electron rule (6 from benzene + 6 from Cr(0) + 6 from 3 CO = 18). Complex formation is facilitated by electron-donating groups on benzene, and no complex of nitrobenzene is formed. Complex formation has a profound effect on reactivity of arenes, and the resulting complexes are used in synthetic reactions. The metal-free reaction products can be isolated easily after decomplexation by mild oxidation using low-valent Cr. Cycloheptatriene also forms a stable complex with Cr(CO)3 and its synthetic applications are discussed below. 

The concurrent formation of two rings by chromium catalysis is demonstrate in Scheme 6 <95TL3027>. Conversion of the hydrazone (48) into the complex (49) and subsequent reaction with hex-6-yn-l-ol affords the annulated complex (50) which is decomplexed by irradiation in benzene. The stereochemistry of the intermediate complex (50) was determined by recomplexation of the final product (51), which gave a separable mixture of the two possible diastereoisomers. 

Delorme and coworkers have published a stereoselective route that is effective with a wide range of amines, including those without a stereocenter on the amine (Scheme 8) [43]. Chiral reduction of the appropriate benzophe-none (as a chromium tricarbonyl complex) using Corey s oxazaborolidine approach afforded the benzhydrol with 91% ee. Treatment with tetrafluo-roboric acid followed by the piperazine gave the desired benzhydryl piperazine without any erosion of stereochemical purity after decomplexation. In addition to simplifying analogue synthesis, these two complementary routes provide a useful base for the future development of stereoselective manufacturing routes. 

Activation of aromatic compounds by transition-metal complexes was initially studied with Cr(CO)3 complexes. Nucleophilic addition of 2-lithio-l,3-dithianes to arene-chromium(O) complexes 185 followed usually by iodine-promoted decomplexation affords the corresponding 2-arylated 1,3-dithianes 186. The reaction of //-(toluene)- and (anisole)tricarbonylchromium (185) with compound 161 gave mixtures (52 46 and 10 90, respectively) of ortho and meta substituted derivatives (186) (Scheme 54)244. The meta directing effect was also observed (mainly better than 95%) with amino and fluoro substituted complexes245. 

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