Catalytic amounts of chromium

If the substituents on generated carbene complex 20 are the same as those on the alkene, this reaction must proceed by a catalytic amount of chromium carbene complex 2 (Scheme 5) [9]. 

There are two important experimental factors that must be accounted for if we are to be successful in running 15N experiments. The 15N nucleus tends to relax very slowly Tj s of greater than 80 seconds have been measured. Thus, either long pulse delays must be incorporated into our pulse sequence or, alternatively, we could provide another route for spin relaxation. A common procedure is to add a catalytic amount of chromium (III) acetylacetonate, a paramagnetic substance, whose unpaired electrons efficiently stimulate transfer of spin. In cases where Tt s are not known (and not intended to be measured), pulse delays and pulse angles must be considered carefully because the signal from one (or more) 15N resonance can accrue too slowly or be missed altogether. 

Cobalt chloride in diglyme is a useful catalyst for benzylic [22a] and allylic [22b] oxidation under mild conditions. The addition of sodium azide to oxidations catalyzed by transition metal acetylacetonates, heteropolyacids, phtha-locyanines, bis-(pyridylimino)isoindolines, porphyrins and Schiff bases significantly enhances the rates of the low-temperature catalytic oxidation of alkanes [22c]. Ethylbenzene is slowly oxidized by air in MeCN in the presence of catalytic amount of chromium trioxide [23]. Complexes of Fe(III) and Co(II)... 

The reagent prepared from a gem-dibromoalkane, samarium metal, and samar-ium(II) iodide in the presence of a catalytic amount of chromium(III) chloride was found to transform ketones into alkenes via a Wittig-type reaction. This method allows the alkylidenation of easily enolizable ketones, such as yS-tetralone (Scheme 5.26) [37]. 

This chapter will review the literature up to the end of 2011 on the stereoselective Nozaki-Hiyama-Kishi reaction and applications in natural product synthesis. The transformation of the NHK reaction from using stoichiometric to catalytic amounts of chromium will be addressed initially then the diastereoselective NHK, followed by the enantioselective version (with an emphasis on the different classes of chiral ligands that have been employed in a wide variety of carbon-carbon bond forming processes), and finally, selected examples of the application of the NHK reaction to natural product synthesis will be described. 

The Ni(ii)/Cr(n)-mediated coupling reaction employs an excess of chromium(ii) chloride and a catalytic amount of nickel(n) chloride (0.1 %). The preferred solvent for this reaction is usually DMF, although THF, DMF/THF, or DMF/Me2S may also be used. The ability to activate the vinyl iodide (or related) substrate at ambient... 

Irradiation of complex 6 in the presence of ethyl acrylate provides the [6 + 2] 7t-adduct 9 as the single enrfo-diastereomer,276 which may also be obtained by heating a mixture of methyl l//-azepine-l-carboxylate with the ester in the presence of a catalytic amount of tricarbonyl(>]6-naphthalene)chromium(O).277... 

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

At this point the catalytic process developed by Dotz et al. using diazoalkanes and electron-rich dienes in the presence of catalytic amounts of pentacar-bonyl(r]2-ds-cyclooctene)chromium should be mentioned. This reaction leads to cyclopentene derivatives in a process which can be considered as a formal [4S+1C] cycloaddition reaction. A Fischer-type non-heteroatom-stabilised chromium carbene complex has been observed as an intermediate in this reaction [23a]. 

The oxidation of both linear and cyclic ethers to the corresponding acids and lactones by aqueous H202 as catalyzed by TS-1 and TS-2 was reported by Sasidharan et al. (241) (Scheme 17 and Table XXXV). The titanosilicates exhibited significantly better activity (about 55% conversion) and selectivity (98%) than chromium silicates, although vanadium silicates totally failed to catalyze the reaction. Such conversions are usually accomplished using either stoichiometric amounts of chromium trioxide, lead tetraacetate, or ruthenium tetroxide as oxidants (242) or catalytic amounts of Ru04 in the presence of... 

High-density polyethylene (HDPE) is a commodity chemical that is produced on a very large scale in one of two catalytic processes the Ziegler-Natta and the Phillips process. The latter accounts for about one third of all polyethylene. It uses a catalyst consisting of small amounts of chromium (0.2-1.0 wt% Cr) on a silica support, developed by Hogan and Banks at the Phillips Petroleum Company in the early 1950s [84,85]. 

Figure 9.1 compares the synthesis of acetophenone by classic oxidation of 1-phenylethanol with stoichiometric amounts of chromium oxide and sulphuric acid, with an atom efficiency of 42%, with the heterogeneous catalytic oxidation with O2, with an atom efficiency of 87%, and with water as the only by-product. This is especially important if we consider the environmental unfriendliness of chromium salts the potential environmental impact of reactions can be expressed by the environmental quotient (EQ), where E is the E-factor (kg waste/kg product) and Q is the environmental unfriendliness quotient of the waste. If Q is... 

Dehalogenation. Barton et at. (1, 148) effected dehalogenation of steroidal /i-hydroxy halides with chromium(II) acetate and butancthiol as the proton donor in DMSO. The method is only useful with tertiary halides. A recent improvement that permits reduction of halides of all types uses the ethylenediamine complex of CrtCIOzh and the tetrahydropyranyl ethers of the /J-hydroxy halide. Catalytic amounts of the reducing agent can be used in "indirect electrolysis." The reaction is convenient for preparation of deoxynucleosides.1... 

Oxidations with chromium trioxide.6 Secondary alcohols can be oxidized to ketones in good yields by Cr03 in the presence of catalytic amounts of tetraalkyl-ammonium halides. Yields from oxidation of primary alcohols are moderate. 

In a modification of the original work, the amount of chromium (TOXIC) needed was reduced by making it R catalytic by a coupled q—/... 

A number of new methods for the preparation of cyclopropanols from carbonyl derivatives via 1,3-bond formation between the carbonyl and carbons have been developed. 7>ons-2-alkylcyclopropanols are stereoselectively produced from 2- or 3-substituted acrolein upon exposure to chromium(II) chloride in the presence of a catalytic amount of nickel chloride in DMF (equation 56)73. 2,3-Disubstituted acroleins are, in contrast, inert to the chromium reagent. Treatment of / -stannyl carbonyls with titanium(IV) chloride affords cyclopropanols in good yields when the substrates are ketones not bearing j -alkyl... 

A great effort is dedicated to the development of methodologies for the oxidation of alcohols, involving catalytic quantities of chromium compounds, which are re-oxidized with other oxidants present in excess.406 Using chromium compounds in catalytic amounts is environmentally sound, and often facilitates the work-ups. 

Bruns and Haufe have described the first examples of a transition metal complex mediated asymmetric ring opening (ARO) of both meso- and racemic epoxides via formal hydro-fluorination [23]. Initial attempts with chiral Euln complexes led to very low asymmetric induction. Opening of cyclohexene oxide 30 with potassium hydrogendifluoride in the presence of 18-crown-6 and a stoichiometric amount of Jacobsens chiral chromium salen complex 29 [24a] finally yielded two products 31 and 32 in a 89 11 ratio and 92% combined yield, the desired product 31 being formed with 55% ee. Limiting 29 to a catalytic amount of 10 mol% led to an increase in the ratio of 31, however, with the enantiomeric excess dropping to 11% (Scheme 5). 

Usanov and Yamamoto recently found that catalytic amounts of Co(TPP) 367 led to a dramatic rate acceleration of Nozaki-Kishi-Hiyama reactions catalyzed by chromium complex 368 (Fig. 101) [460]. The authors attributed the rate enhancement to initial reduction of 367 to a Co(I) complex. The latter is able to undergo an Sn2 substitution at propargyl bromide 366 giving an allenylCo(ffl) species. It was proposed that its homolysis leads to allenyl radical 366A, which couples to Cr(II) complex 368. The resulting allenyl Cr(III) complex adds in an SN2 process... 

Intramolecular Nozaki reaction. The addition of a vinyl-chromium compound to an aldehyde (12, 137 14, 96) is useful for macrocyclization, particularly if the CrCl2 reagent is activated by a catalytic amount of Ni(acac)2. Under these conditions, the vinyl iodide with a w-formyl group (1) cyclizes to (+ )-brefeldin C 12) and the 4-epimer as a 1 4 mixture in 60% yield.1... 

---