Chromium alkyl complexes

Organochromium reagents have found varied use in organic synthesis. Aryl-chromium complexes influence significantly the reactivity of the aromatic ring and have been used widely. Other unsaturated chromium complexes, alkyl-chromium species and chromium carbenes promote useful transformations and continue to attract attention. 

We have previously described a class of cationic chromium(III) alkyls of the type [Cp Cr(L)2R] X% which catalyze the polymerization of ethylene at ambient temperature and pressure.[3] The most efficient of these catalysts was the complex shown below. 

Arene(alkoxy)carbene chromium complexes react with aryl-, alkyl-, terminal, or internal alkynes in ethers or acetonitrile to yield 4-alkoxy-1-naphthols, with the sterically more demanding substituent of the alkyne (Rl Figure 2.24) ortho to the hydroxy group. Acceptor-substituted alkynes can also be used in this reaction (Entry 4, Table 2.17) [331]. Donor-substituted alkynes can however lead to the formation of other products [191,192]. Also (diarylcarbene)pentacarbonyl chromium complexes can react with alkynes to yield phenols [332]. 

The intermediate vinylketene complexes can undergo several other types or reaction, depending primarily on the substitution pattern, the metal and the solvent used (Figure 2.27). More than 15 different types of product have been obtained from the reaction of aryl(alkoxy)carbene chromium complexes with alkynes [333,334]. In addition to the formation of indenes [337], some arylcarbene complexes yield cyclobutenones [338], lactones, or furans [91] (e.g. Entry 4, Table 2.19) upon reaction with alkynes. Cyclobutenones can also be obtained by reaction of alkoxy(alkyl)carbene complexes with alkynes [339]. 

The reaction of alkoxy(alkyl)carbene chromium complexes with alkynes has been reported to give modest yields of cyclopentenones [368] and a few examples of intramolecular carbene C-H insertions of Fischer-type carbene complexes, leading to five-membered heterocycles, have been reported [369,370] (Table 2.22). 

Although slightly outside the scope of this review, an interesting case of stereoselection should be presented here. It has been observed by Gibson (nee Thomas) and coworkers during the deprotonation of tricarbonylchromium complexes of benzyl alkyl ethers by means of the chiral bis(lithiumamide) base 234 (equation 54) . The base removes the benzylic pro-R-H atom in 233 from the most reactive conformation to form the planary chiral intermediate 235. The attack of the electrophile forming 236 proceeds exclusively from the upper face in 235, because the bulky chromium moiety shields the lower face. Simpkins and coworkers extended the method to the enantioselective substitution of the chromium complexes of 1,3-dihydroisobenzofurans . 

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... 

B. Chromium(III) Alkyl Compounds Polynuclear Chromium(III) Complexes Polyaminocarboxylic Ligands... 

Stereoselective benzylic alkylation.6 Benzylic acetates of arenechromium tricarbonyls react with R3A1 or (QHs Zn-TiCU to form exo-alkyl chromium complexes. 

Bis(phosphoranimine) ligands, chromium complexes, 5, 359 Bis(pinacolato)diboranes activated alkene additions, 10, 731—732 for alkyl group functionalization, 10, 110 alkyne additions, 10, 728 allene additions, 10, 730 carbenoid additions, 10, 733 diazoalkane additions, 10, 733 imine additions, 10, 733 methylenecyclopropane additions, 10, 733 Bisporphyrins, in organometallic synthesis, 1, 71 Bis(pyrazol-l-yl)borane acetyl complexes, with iron, 6, 88 Bis(pyrazolyl)borates, in platinum(II) complexes, 8, 503 Bispyrazolyl-methane rhodium complex, preparation, 7, 185 Bis(pyrazolyl)methanes, in platinum(II) complexes, 8, 503 Bis(3-pyrazolyl)nickel complexes, preparation, 8, 80-81 Bis(2-pyridyl)amines... 

Arenes and heteroarenes which are particularly easy to metalate are tricarbo-nyl( 76-arene)chromium complexes [380, 381], ferrocenes [13, 382, 383], thiophenes [157, 158, 181, 370, 384], furans [370, 385], and most azoles [386-389]. Meta-lated oxazoles, indoles, or furans can, however, be unstable and undergo ring-opening reactions [179, 181, 388]. Pyridines and other six-membered, nitrogen-containing heterocycles can also be lithiated [59, 370, 390-398] or magnesiated [399], but because nucleophilic organometallic compounds readily add to electron-deficient heteroarenes, dimerization can occur, and alkylations of such metalated heteroarenes often require careful optimization of the reaction conditions [368, 400, 401] (Schemes 5.42 and 5.69). 

Chromium complex 53 was also shown to efficiently catalyze the inverse electron-demand hetero Diels-Alder reaction of a,(3-unsaturated aldehydes with alkyl vinyl ethers (Scheme 17.19).26 Although the uncatalyzed process required elevated temperatures and pressures to give dihydropyrans in good yields but poor endo. exo selectivities, the reaction proceeded at room temperature in the presence of 5 mol% of ent-53 and 4A molecular sieves in dichloromethane of tert-butyl methyl ether with excellent diastereoselectivity (endo. exo >96 4) and promising enantioselectivities (72-78% ee). Optimal results were achieved using a solvent-free system and excess vinyl ether. 

Other ligands giving >90% ee for the alkylation or amination of 19 have been reported but will not be described in detail. They include derivatives of 3 [36], ferrocene-based ligands [54-57], l.l -binaphthyl-based ligands [58-60], natural product-based ligands such as fenchone [61], cholesterol [62], or carbohydrates [63,64], chiral aryl chromium complexes [65,66], chiral sulfimides [67], newP,N-ligands [19,22,52,53,68-78], and others [79-82]. 

Alkylidene complexes are of two types. The ones in which the metal is in a low oxidation state, like the chromium complex shown in Fig. 2.4, are often referred to as Fischer carbenes. The other type of alkylidene complexes has the metal ion in a high oxidation state. The tantalum complex is one such example. For both the types of alkylidene complexes direct experimental evidence of the presence of double bonds between the metal and the carbon atom comes from X-ray measurements. Alkylidene complexes are also formed by a-hydride elimination. An interaction between the metal and the a-hydrogen atom of the alkyl group that only weakens the C-H bond but does not break it completely is called an agostic interaction (see Fig. 2.5). An important reaction of alkylidene complexes with alkenes is the formation of a metallocycle. 

Stereoselective a-alkylation of ketones. This reaction can be effected by reaction of silyl enol ethers with benzyl acetates complexed with Cr(CO), in the presence of ZnCh (I equiv.). This methodology is particularly useful because only the adduct anti to the metal is obtained. Use of an optically active chromium complex such as 1 results in 100% stereoselective alkylation. 

The first oxoaUcyl of chromium(V), ( -C5Me5)CrO(Me)2, was prepared via oxygen transfer from trimethylamine N-oxide to the Cr precursor ( -C5Me5)Cr(py)Me2. Its effective magnetic moment at 285 K, 1.76 J,b, is consistent with the presence of pentavalent chromium. At ambient temperature and pressure, this complex does not function as an ethylene polymerization catalyst however, it does react with the starting material to give the Cr dimer [( l -C5Me5)Cr(Me)2]2(Ai2-0). Another chromium(V) alkyl, namely, dinuclear [Cp Cr(0)Me]2(/tr-0), was produced by the reaction of [Cp Cr(/u.-Me)]2 with O2. 

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