﻿Aikawa, K., et al. (2001). "Asymmetric hetero Diels-Alder reaction using chiral cationic metallosalen complexes as catalysts." Tetrahedron 57(5): 845-851.
	Chiral cationic (R,S)- or (R,R)-(salen)-manganese(III) and -chromium(III) complexes served as the catalysts for asymmetric hetero Diels-Alder reaction of Danishefsky's diene with aldehydes, achieving high enantioselectivity (up to 97% ee at 0 degreesC). The reactions of aldehydes bearing no precoordinating functional group were well effected by using (R,R)-complexes as catalysts, while those of aldehydes bearing a precoordinating functionality were better effected by using (R,S)-complexes. (C) 2001 Elsevier Science Ltd. All rights reserved.

Berkessel, A., et al. (2006). "Chiral chromium(III) porphyrins as highly enantioselective catalysts for hetero-Diels-Alder reactions between aldehydes and dienes." Advanced Synthesis & Catalysis 348(1-2): 223-228.
	Starting from enantiornerically pure 5,10,15,20-tetrakis[(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octa-hydro-1,4:5,8-dimethanoanthracene-9-yl]porphyrin, treatment with CrCl2 and subsequent air oxidation afforded the corresponding Cr(III) complex, with chloride as counterion, in 96% yield. Anion exchange with AgBF4 gave the corresponding tetrafluoroborate. These hitherto unknown chiral chromium porphyrins are efficient and highly enantioselective catalysts for the hetero-Diels-Alder reaction of aliphatic, aromatic, and heteroaromatic aldehydes with dienes of varying electron density. In the case of 1-methoxy-3-(trimethylsilyloxy)butadiene ("Danishefsky's diene"), enantiomeric excesses > 90% were achieved in a number of cases, with furfural affording the highest ee (97%). Metal-coordinating aldehydes such as pyridine-2-carbaldehyde do not inactivate the Cr(III) porphyrin catalyst. The cycloaddition of less electron-rich dienes (such as 1-methoxybutadiene) is effected as well, affording diastereoselectivities up to > 99: 1, and enantiomeric excesses > 80%.

Chaladaj, W., et al. (2006). "Sterically modified chiral (salen)Cr(III) complexes - Efficient catalysts for the oxo-Diels-Alder reaction between glyoxylates and cyclohexa-1,3-diene." Synlett(19): 3263-3266.
	A group of chiral [(salen)Cr(III)]+BF4- Complexes, with enhanced steric hindrance in 3,3'-positions of salicylidene moiety, has been synthesized and applied for the oxo-Diels-Alder reaction of alkyl glyoxylates with cyclohexa-1,3-diene. A readily accessible complex that bears bulky adamanthyl substiments revealed its potential, leading to the cycloadducts with excellent selectivity (up to endo/exo 99:1, 98% ee), considerably better than the classic Jacobsen catalyst.

Chatterjee, D., et al. (2007). "Asymmetric epoxidation of alkenes with aqueous t-BuOOH catalyzed by novel chiral complexes of chromium(III) containing tridentate Schiff-base ligands." Journal of Molecular Catalysis a-Chemical 271(1-2): 270-276.
	The [Cr-III. (alpha-TDL1,*)(bipy)(Cl)] (1) and [Cr-III. (TDL2*)(bipy)(Cl)] (2) complexes (where H-2,-TDL1*=N-3,5-di-(t-butyl)salicylidine-D-glucosamine, H2TDL2*=N-3,5-di-(tertiarybutyl)salicylidine-L-alanine, bipy=bipyridyl) have been synthesized and characterized by analytical, spectral (UV-vis and IR), molar conductivity, magnetic moment and electrochemical studies. Complexes 1 and 2 catalyzed the epoxidation of styrenes, stilbenes, l-methylcyclohexene and 1,2-dihydronapthalene using aqueous tert-butyl hydroperoxide (t-BuOOH) as terminal oxidant. The selected alkenes were converted to their corresponding epoxides exhibiting moderate enantioselectivity at ambient temperature. (C) 2007 Elsevier B.V. All rights reserved.

Darensbourg, D. J. and J. C. Yarbrough (2002). "Mechanistic aspects of the copolymerization reaction of carbon dioxide and epoxides, using a chiral salen chromium chloride catalyst." Journal of the American Chemical Society 124(22): 6335-6342.
	The air-stable, chiral (salen)(CrCl)-Cl-III complex (3), where H(2)salen = N,N-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexene diamine, has been shown to be an effective catalyst for the coupling of cyclohexene oxide and carbon dioxide to afford poly(cyclohexenylene carbonate), along with a small quantity of its trans-cyclic carbonate. The thus produced polycarbonate contained >99% carbonate linkages and had a M-n value of 8900 g/mol with a polydispersity index of 1.2 as determined by gel permeation chromatography. The turnover number (TON) and turnover frequency (TOF) values of 683 g of polym/g of Cr and 28.5 g of polym/g of Cr/h, respectively for reactions carried out at 80 degreesC and 58.5 bar pressure increased by over 3-fold upon addition of 5 equiv of the Lewis base cocatalyst, N-methyl imidazole. Although this chiral catalyst is well documented for the asymmetric ring-opening (ARO) of epoxides, in this instance the copolymer produced was completely atactic as illustrated by C-13 NMR spectroscopy. Whereas the mechanism for the (salen) Cr-III-catalyzed ARO of epoxides displays a squared dependence on [catalyst], which presumably is true for the initiation step of the copolymerization reaction, the rate of carbonate chain growth leading to copolymer or cyclic carbonate formation is linearly dependent on [catalyst). This was demonstrated herein by way of in situ measurements at 80 degreesC and 58.5 bar pressure. Hence, an alternative mechanism for copolymer production is operative, which is suggested to involve a concerted attack of epoxide at the axial site of the chromium(III) complex where the growing polymer chain for epoxide ring-opening resides. Preliminary investigations of this (salen)Cr-III-catalyzed system for the coupling of propylene oxide and carbon dioxide reveal that although cyclic carbonate is the main product provided at elevated temperatures, at ambient temperature polycarbonate formation is dominant. A common reaction pathway for alicyclic (cyclohexene oxide) and aliphatic (propylene oxide) carbon dioxide coupling is thought to be in effect, where in the latter instance cyclic carbonate production has a greater temperature dependence compared to copolymer formation.

Darensbourg, D. J. and J. C. Yarbrough (2002). "Mechanistic aspects of the copolymerization reaction of carbon dioxide and epoxides, using a chiral salen chromium chloride catalyst." Journal of the American Chemical Society 124(22): 6335-6342.
	The air-stable, chiral (salen)(CrCl)-Cl-III complex (3), where H(2)salen = N,N-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexene diamine, has been shown to be an effective catalyst for the coupling of cyclohexene oxide and carbon dioxide to afford poly(cyclohexenylene carbonate), along with a small quantity of its trans-cyclic carbonate. The thus produced polycarbonate contained >99% carbonate linkages and had a M-n value of 8900 g/mol with a polydispersity index of 1.2 as determined by gel permeation chromatography. The turnover number (TON) and turnover frequency (TOF) values of 683 g of polym/g of Cr and 28.5 g of polym/g of Cr/h, respectively for reactions carried out at 80 degreesC and 58.5 bar pressure increased by over 3-fold upon addition of 5 equiv of the Lewis base cocatalyst, N-methyl imidazole. Although this chiral catalyst is well documented for the asymmetric ring-opening (ARO) of epoxides, in this instance the copolymer produced was completely atactic as illustrated by C-13 NMR spectroscopy. Whereas the mechanism for the (salen) Cr-III-catalyzed ARO of epoxides displays a squared dependence on [catalyst], which presumably is true for the initiation step of the copolymerization reaction, the rate of carbonate chain growth leading to copolymer or cyclic carbonate formation is linearly dependent on [catalyst). This was demonstrated herein by way of in situ measurements at 80 degreesC and 58.5 bar pressure. Hence, an alternative mechanism for copolymer production is operative, which is suggested to involve a concerted attack of epoxide at the axial site of the chromium(III) complex where the growing polymer chain for epoxide ring-opening resides. Preliminary investigations of this (salen)Cr-III-catalyzed system for the coupling of propylene oxide and carbon dioxide reveal that although cyclic carbonate is the main product provided at elevated temperatures, at ambient temperature polycarbonate formation is dominant. A common reaction pathway for alicyclic (cyclohexene oxide) and aliphatic (propylene oxide) carbon dioxide coupling is thought to be in effect, where in the latter instance cyclic carbonate production has a greater temperature dependence compared to copolymer formation.

Dossetter, A. G., et al. (1999). "Highly enantio- and diastereoselective hetero-Diels-Alder reactions catalyzed by new chiral tridentate chromium(III) catalysts." Angewandte Chemie-International Edition 38(16): 2398-2400.
	
Finney, K. S. and G. W. Everett (1974). "Chiral Chromium (Iii) and Ruthenium(Iii)Complexes of Salicylaldimine Ligands." Inorganica Chimica Acta 11(3): 185-188.
	
Finney, K. S. and G. W. Everett (1974). "CHIRAL CHROMIUM (III) AND RUTHENIUM(III)COMPLEXES OF SALICYLALDIMINE LIGANDS." Inorganica Chimica Acta 11(3): 185-188.
	
Gigante, B., et al. (2000). "Assessment of the negative factors responsible for the decrease in the enantioselectivity for the ring opening of epoxides catalyzed by chiral supported Cr(III)-salen complexes." Catalysis Letters 68(1-2): 113-119.
	In order to determine the influence of the inorganic support on the asymmetric induction, different chiral chromium(III)-salen complexes have been incorporated within the cavities of zeolites Y, EMT and into the interlamellar region of K-10 montmorillonite. These heterogeneous catalysts are able to promote the asymmetric ring opening of epoxides with trimethylsilylazide to afford chiral azido trimethylsilyl ethers and azido alcohols with modest enantiomeric excess that varies depending on the inorganic support. The factors that have been found to play a negative influence diminishing the enantioselectivity of the supported Cr(III)-salen catalyst compared to the unsupported complexes are the following: (i) the presence of adventitious acid sites, (ii) the encapsulation of no sufficiently stereogenic ligands and (iii) the change in the reaction mechanism from bimetallic to a single metal reaction mechanism.

Kawasaki, T., et al. (2007). "Asymmetric autocatalysis induced by octahedral tris(acetylacetonato)chromium(III) complex as a homogeneous chiral initiator." Chemistry Letters 36(1): 30-31.
	Enantioselective addition of diisopropylzinc to 2-t-butylethynylpyrimidine-5-carbaldehyde in the presence of chiral Delta-and Lambda-tris(acetylacetonato)chromium (III) complex [Cr(acac)(3)] affords highly enantiomerically enriched (S)- and (R)-5-pyrimidyl alkanols, respectively, in conjunction with asymmetric autocatalysis. The absolute configuration of the corresponding 2-tbutylethynyl-5-pyrimidyl alkanol is dependent upon the chirality of the octahedral chromium(III) complex.

Kureshy, R. I., et al. (2010). "Reusable Chiral Dicationic Chromium(III) Salen Catalysts for Aminolytic Kinetic Resolution of trans-Epoxides." Advanced Synthesis & Catalysis 352(17): 3053-3060.
	A series of new recyclable chiral dicationic chromium(III) salen complexes 1-10 bearing different substituents, viz., hydrogen, methyl, tert-butyl, triphenylphosphinomethyl, triethylaminomethyl, methylimidazolium, methylpyridinium, methyl-N,N-dimethylpyridinium at the 3,3'-and 5,5'-positions of the salen unit with (1S,2S)-(+)-1,2-diaminocyclohexane, (1S,2S)-(-)-1,2-diphenyl-1,2-diaminoethane, and (S)-(-)-1,1'-binaphthyl-2,2'-diamine collars have been synthesized and characterized by various physico-chemical methods. These complexes were used as catalysts for the highly enantioselective aminolytic kinetic resolution of racemic trans-epoxides with different anilines as nucleophiles at room temperature. With the use of catalyst 3, anti-beta-amino alcohols were obtained in excellent yields (> 99% with respect to the nucleophile) and enantioselectivities (ee > 99%) with the concomitant recovery of corresponding epoxides in high optical purity (ee up to > 99%) and quantitative yields in 12 h. The catalyst 3 is recyclable in the aminolytic kinetic resolution process and worked well up to six cycles with retention of enantioselectivity.

Kwiatkowski, P., et al. (2003). "Enantioselective [4+2] cycloaddition of buta-1,3-dienes to alkyl glyoxylates catalyzed by the chiral (salen)chromium(III) complex." Advanced Synthesis & Catalysis 345(4): 506-509.
	Thermal and high-pressure [4 + 2] cycloadditions of buta-1,3-diene (3), cyclohexa-1,3-diene (5), and 2,3-dimethylbuta-1,3-diene (6) to alkyl glyoxylates of type 2 ( R = n-Bu, i-Pr, t-Bu), catalyzed by chromium complex (R,R)-1, leading to the corresponding cycloadducts 4, 7 and 8 with relatively high enantioselectivity, are described.

Kwiatkowski, P., et al. (2006). "Catalytic asymmetric allylation of aldehydes using the chiral (salen)chromium(III) complexes." Tetrahedron 62(21): 5116-5125.
	The enantioselective addition of allylstannanes to glyoxylates and glyoxals, as well as simple aromatic and aliphatic aldehydes, catalyzed by chiral (salen)Cr(III) complexes, has been studied. The reaction proceeded smoothly for the reactive 2-oxoaldehydes and allyl-tributyltin in the presence of small amounts (1-2 mol %) of (salen)Cr(III)BF4 (1b) under mild, undemanding conditions. However, in the case of other simple aldehydes, the use of high-pressure conditions is required to obtain good yields. Classic chromium catalyst 1b, easily prepared from the commercially available chloride complex la, affords homoallylic alcohols usually in good yield and with enantiomeric purity of 50-79% ee. The stereochemical results are rationalized on the basis of the proposed model. (c) 2006 Elsevier Ltd. All rights reserved.

Miesowicz, S., et al. (2010). "Oxo-Diels-Alder Reaction of Danishefsky's Diene with Aldehydes, Catalyzed by Chiral Tridentate Chromium(III)-Schiff Base Complexes." Synlett(9): 1421-1425.
	The enantioselective hetero-Diels-Alder reaction of Danishefsky's diene with simple aromatic and aliphatic aldehydes is catalyzed by chiral tridentate Schiff base-chromium(III) complexes. In many cases, 2,3-dihydropyran-4-ones are obtained in good yields (up to 99%) and high enantioselectivities (up to 97%).

Mitsunuma, H., et al. (2019). "Catalytic asymmetric allylation of aldehydes with alkenes through allylic C(sp(3))-H functionalization mediated by organophotoredox and chiral chromium hybrid catalysis." Chemical Science 10(12): 3459-3465.
	We describe a hybrid system that realizes cooperativity between an organophotoredox acridinium catalyst and a chiral chromium complex catalyst, thereby enabling unprecedented exploitation of unactivated hydrocarbon alkenes as precursors to chiral allylchromium nucleophiles for asymmetric allylation of aldehydes. The reaction proceeds under visible light irradiation at room temperature, affording the corresponding homoallylic alcohols with a diastereomeric ratio >20/1 and up to 99% ee. The addition of Mg(ClO4)(2) markedly enhanced both the reactivity and enantioselectivity.

Renehan, M. F., et al. (2005). "Unsymmetrical chiral salen Schiff base ligands - Synthesis and use in metal-based asymmetric epoxidation reactions." Journal of Molecular Catalysis a-Chemical 231(1-2): 205-220.
	Six novel unsymmetrically substituted chiral non-racemic salen ligands were prepared from resolved trans-1,2-diaminocyclohexane and two different salicylaldehydes. These were then used to construct cationic chromium(III) and manganese(III) complexes that were employed in studies relevant to catalytic asymmetric epoxidation of alkenes. In the case of the chromium complexes, the yields and enantioselectivities obtained were compared to those obtained with the analogous symmetrically substituted counterparts. In individual cases improved enantioselection and a very strong beneficial effect of phosphine oxide additive was found, but in general the selectivity varied in an unpredictable manner. The results are rationalised through the formation of a mixture of diastereomeric active oxidants. In the case of the manganese complexes, where one of the salicylaldehydes is the 3,5-di-tert-butyl case, the standard preparative method leads to substantial scrambling of the ligand and the isolation of mainly Jacobsen's catalyst. One of the preparative methods investigated for the ligand synthesis worked for racemic ligand but failed for the non-racemic version. (c) 2005 Elsevier B.V. All rights reserved.

Schaus, S. E., et al. (1998). "Asymmetric hetero-Diels-Alder reactions catalyzed by chiral (salen)chromium(III) complexes." Journal of Organic Chemistry 63(2): 403-405.
	
Zulauf, A., et al. (2012). "Electropolymerization of chiral chromium-salen complexes: new materials for heterogeneous asymmetric catalysis." New Journal of Chemistry 36(6): 1399-1407.
	Electrochemical oxidation is described as a very efficient polymerization procedure for the heterogenization of metallic chiral catalysts. From chromium chiral complexes based on salen-thiophene ligands, this methodology provided an efficient access to various polymers. Recovered as insoluble powders, these materials were tested in different enantioselective heterogeneous catalytic reactions. Structural modifications were introduced on the salen core in order to evaluate their influence on redox polymer properties and on the enantioselectivity of the catalysis. Electrochemical experiments showed the particular stability of these deposited materials at the electrode surface and SEM analyses suggested the influence of the electropolymerization conditions on their morphology.

Zulauf, A., et al. (2010). "Recoverable chiral salen complexes for asymmetric catalysis: recent progress." Dalton Transactions 39(30): 6911-6935.
	Chiral salen-type complexes have already been proven to be particularly useful asymmetric catalysts for the preparation of a wide range of enantioenriched products. Research for efficient recovery and recycling of such complexes is ongoing and has already demonstrated the value of these procedures in terms of atom economy and overall economical savings. Results reported in the near past (2006-2009) dealing with the use of recyclable chiral salen complexes are summarized here, classified according to the type of heterogenization procedures involved.

Zulauf, A., et al. (2009). "New Chiral Thiophene-Salen Chromium Complexes for the Asymmetric Henry Reaction." Journal of Organic Chemistry 74(5): 2242-2245.
	Chiral thiophene-salen chromium complexes were investigated in their monomeric form as soluble catalysts in the enantioselective Henry reaction of several aldehydes. The anodic polymerization of one complex led to an insoluble powder that was successfully used as a heterogeneous catalyst for the transformation of 2-methoxybenzaldehyde with enantiomeric excesses up to 77%. The polymerized catalyst was recovered and also recycled in an original multisubstrate procedure.

Zulauf, A., et al. (2010). "New Chiral Calixsalen Chromium Complexes: Recyclable Asymmetric Catalysts." Chemistry-a European Journal 16(36): 11108-11114.
	A chiral N,N'-bis(salicylidene)ethylenediamine (salen) polymer has been prepared by a condensation reaction between a thiophenedisalicyladehyde derivative and (S,S)-cyclohexane-1,2-diamine. This polymeric compound was demonstrated to possess a cyclic structure with two to five repetitive units. The addition of chromium(II) salts led to the generation of a chiral catalyst that could be recovered as an insoluble powder. The performance of this new calixsalen-type catalyst was examined in various transformations, particularly in its ability to promote nucleophilic epoxide ring opening under heterogeneous conditions. The target products were obtained in high yields and with improved selectivity compared with those obtained by using analogous linear polymers. The arrangement of the catalytic sites in the cyclic structure is probably more suitable for the necessary cooperative bimetallic pathway of this demanding reaction. The catalyst could be successfully recycled. This approach represents the first use of calixsalen complexes under heterogeneous catalytic conditions.

