Camphor-derived ligand

Lanthanide /3-diketonates containing fluorinated chiral camphor-derived ligands (61, M = Pr, Eu, Er, Yb, R = Mef, n-Prf) form highly coordinated 14 complexes with zwitterionic, unprotected phenylalanine, leucine, and other amino acids under neutral conditions. This allows extraction of the amino acids from their neutral aqueous solutions into dichloromethane phases . 

Asymmetric intermolecular Pauson-Khand reactions have been realized using a number of chiral auxiliaries chelating to the metal and/or attached to the alkyne. One example using a camphor-derived ligand is seen in Scheme 253. Moderate asymmetric induction has been observed using chiral amine A-oxides as the promoter. For example, (+)-indolizino[3,4-b]quinoline A-oxide gave up to 53% ee. 

A few of these ligands are illustrated, including the camphor-derived ligand (6.03) (which was the first one to give high enantioselectivies), the norephedrine-derived aminoalcohol (6.04), the praline-derived ligand (6.05), aminoalcohols with axial chirality such as binaphthyl (6.06) and ferrocene-based aminoalcohols such as (6.07). ... 

Scheme 1.33 Test reaction with (+ )-camphor-derived pyridine thioether ligand.

Camphor-derived disulfonamide ligand for additions of ZnEt2 aldehydes. 

In 2006, Wang et al. reported the synthesis of a new camphor-derived disulfonamide ligand based on L-tartaric acid that was employed in similar reactions to those described above, giving rise to enantioselectivities of up to 83% ee by using 5 mol% of catalyst loading (Scheme 3.43). ... 

Scheme 3.44 Ionic liquids containing camphor-derived sulfonamide ligand for addition of ZnEt2 to benzaldehyde.

Scheme 3.50 Camphor-derived A -sulfonylated amino alcohol ligand for additions of ZnEt2 to aldehydes.

Scheme 4.24 Addition of ZnEt2 to acetophenone with camphor-derived mesylamide ligand.

Scheme 9.8 Ir-catalysed reduction of acetophenone with camphor-derived amino sulfide ligands.

MeLi to cyclohexenone with p-amino sulfide ligand 88-9 MeMgl to acyclic enone with arene-thiolatocopper(l) complexes 90-1 mesylamide ligand (camphor-derived) 174, 176... 

ZnEt2 addition reactions to acetophenone withbis(snlfon-amides) diol ligand 157-8 to acetophenone with camphor-derived mesylamide hgand 174, 176... 

The binding of the diuretic amiloride 95 to DNA has been studied <06CC1185>. The chiral camphor-derived pyrazine ligand 96 showed monomeric coordination to Cu and Zn, in contrast to the bidentate polymeric behaviour of related earlier compounds <06ARK218>. 

Knochel and coworkers synthesized a series of camphor-derived pyridine and quinoline P,N ligands. The catalysts 30 (Fig. 29.17) were used to hydrogenate substrates 1 and 2 in up to 95% and 96% ee, respectively [33]. The selectivities were moderate for other unfunctionalized alkenes however, a high enantioselec-tivity was reported for the hydrogenation of ethyl acetamidocinnamate 10 [34]. 

Tolman and co-workers (67) investigated a series of pyrazolyl-derived ligands for this reaction. Initial investigations centered on the use of tris(pyrazolyl) phosphine oxide (95) as a ligand with chirality derived from camphor. Diastereoselectivities with ethyl diazoacetate are poor, slightly favoring the cis isomer, and enantioselectivities are modest, Eq. 50. The BHT esters greatly increase the diastereoselectivity of this process (96 4) at the expense of enantioselectivity (10% ee for trans isomer). 

The catalytic system described above has been further developed to an asymmetric catalytic complexation of prochiral 1,3-dienes (99% yield, up to 86% ee) using an optically active camphor-derived 1-azabutadiene ligand [56]. This method provides for the first time planar-chiral transition metal 7t-complexes by asymmetric catalysis. 

A-Aminophthalimide (118) can also be added to olefins in an asymmetric fashion. Thus, reaction of A -enoyl oxazolidinone 122 with 118 and lead tetraacetate in the presence of the camphor-derived chiral ligand 120 provides aziridine 123 in 83% yield and with 95% ee <020L1107>. Other useful chiral ligands include imine 121, derived from the condensation of 2,2 -diamino-6,6 -dimethylbiphenyl with 2,6-dichlorobenzaldehyde. The corresponding monometallic Cu(I) complex was found to be very efficient in chiral nitrogen transfer onto chromene derivative 124 using (Ar-(p-toluenesulfonyl)imino)phenyliodinane (PhI=NTs) to provide aziridine 125 in 87% yield and 99% ee <02JOC3450>. 

Functionalised carbenes can anchor free carbenes to the metal site, introduce hemilabil-ity, provide a means to immobilise transition metal carbene catalysts, introduce chirality, provide a chelate ligand or bridge two metal centres. NHC can be attached to carbohydrates and camphor, derived from amino acids and purines, and they can be used as organocata-lysts mimicking vitamin B1 or as weak solvent donors in lanthanide chemistry. There are many possibilities which are still only scarcely explored. 

The asymmetric conjugate addition of diethylzinc with chalcone was also catalyzed by nickel and cobalt complex (Eq. (12.31)) [71]. A catalytic process was achieved by using a combination of 17 mol% of an aminoalcohol 34 and nickel acetylacetonate in the reaction of diethylzinc and chalcone to provide the product in 90% ee [72, 73]. Proline-derived chiral diamine 35 was also effective, giving 82% ee [74]. Camphor-derived tridentate aminoalcohol 36 also catalyzes the conjugate addition reaction of diethylzinc in the presence of nickel acetylacetonate to afford the product in 83% ee [75]. Similarly, the ligand 37-cobalt acetylacetonate complex catalyzes the reaction to afford the product in 83% ee [76]. 

An enantioselective version of the cyclopentaannulation via [3 + 2] cycloaddition has been developed using cyclopent-2-enone and several different methylenecyclopropanes. Whereas chiral phosphane ligands, such as menthyldiphenylphosphane (21), or the camphor-derived sultam 22 only result in enantiomeric excesses of 31% at a maximum in nickel(0)-catalyzed reactions, the enantioselectivity dramatically increases when the bidentate azaphospholene ligand 23 is employed. The yields, however, are relatively low due to the competing formation of alkylation products. ... 

Enantioselective aziridination has also been achieved by use of an enantiopure ligand [70]. Reaction of a variety of N-enoyl oxazohdinones with N-aminophthalimide and lead tetraacetate in the presence of camphor-derived chiral ligands provided the N-phthahmidoaziridines in good to high enantiomeric excess (Scheme 13.49). The oxazohdinone moiety of the substrate played an indispensable role in this reaction. The use of aryl acrylates led either to low stereoselectivity or low chemical yield. Coordination of the hgand-mediated Lewis acid to the bidentate acyl oxazohdinone might account for these results. 

Recently, another cobalt(II)/camphor-derived complex was developed for performing the asymmetric cyclopropanation of olefins [38]. The complex 18 was prepared by reacting the ligand 17, synthesized by condensation of (lR)-3-hydroxymethylenebornane-2-thione and the corresponding diamine, with co-balt(II) dichloride hexahydrate in degassed ethanol (Scheme 11). The cyclopropane derivatives were obtained in 50-60% yield using 3 mol % of the catalyst 18 and ethyl diazoacetate in styrene or 1-octene as solvent. The diastereomeric ratios were low for both styrene and 1-octene. 

Fig. 5.4. Probable conformation of the magnesiated lelrahydroisoquinolyl oxazolinc. based on analogy to the X-ray crystal structure of a magnesiated tetrahyrdoisoquinoline pivalanride 143. The aldehyde probably coordinatc,s to one of the ligand sites cis to the isoquinolyl carbon prior to reaction, (a) Conformation of isopropyl in 5-unsuhstitutcd auxiliary (b) Steric crowding produced by rotation of isopropyl in the 5..5-diinclhyl derivative (c) Similar crowding in a camphor-derived auxiliary (enantiomer of that drawn in Table 5, 5. entries 12 and L5).

D-Amino acids (with R configuration) are only rarely found in nature and, because of their comparatively high price, their preparation may sometimes appear desirable. Resolution procedures can often be applied, but synthetic methods are also available. (/ )-Amino acids can be obtained by catalytic hydrogenation of dehydro amino acids by the appropriate choice of enantiomeric control ligands (Scheme 1, Section D.2.3.I.), and by alkylation with chiral glycine equivalents as auxiliaries, such as camphor derivatives (Scheme 2)15-16 or imidazolidinones (Scheme 3)11. Modified carbohydrates have been successfully applied in the asymmetric Ugi reaction to obtain many (R)-amino acids (Scheme 4)11. 

Knollmiiller et al. tested seven camphor-derived 1,4-amino alcohols according to Eq. (1) and achieved with 12 up to 32% ee of (l )-l-phenyl-l-pentanol (5.2 9.2 1 ratio, in Et20 at -78°C) [47].Diethyl ether was found to give for all ligands superior enantioselectivities than THF. 

Itsuno has described the first enantioselective allylation of aldehydes using crosslinked polymer-supported A-sulfonylated aminoalcohols as chiral ligands [146], Polymer-supported ligands 220 and 221 were prepared by copolymerisation of the norephedrine-derived monomer 218 or D-camphor-derived 219 with styrene and divinylbenzene (DVB) in a [chiral monomer]/[styrene]/[DVB] molar ratio of 1/8/1 (Scheme 90). 

Chen et al. first demonstrated that the asymmetric induction can be obtained by the aid of a chiral Lewis acid for the MBH reaction. By using camphor-derived chiral ligand 382, which can be prepared from the condensation of... 

Preparation of the diastereomerically pure platinum(II) complexes bearing tetrahydro-5,8-methanocyclohexa-[3, 2 4, 5][l,3]oxazolo[3,2-fc][l,2,4]oxadiazole ligands (7, Scheme 13.6) was accomplished via the intermolecular 1,3-DCA between enantiomerically pure camphor-derived oxazoline-A-oxides and the coordinated nitriles in frans -[PtCl2(R CN)2] [26], The reaction proceeds at 20-25 °C. Free heterocyclic species were liberated as single stereoisomers from the respective platinum(II) complexes by treatment with excess NaCN [26],... 

Guest-induced chirality within the cavity of hydrogen-bonded softballs 307, 308, 359, and 360 (a racemic mixtnre of the latter is formed as a resnlt of dimerization of nonsymmetric ligand syntone 148) after encapsnlation of chiral camphor derivatives (Scheme 3.37) has been examined [39] by H NMR method. 

In 2003, Rawal reported the use of TADDOLs 177 as chiral H-bonding catalysts to facilitate highly enantioselec-tive hetero-Diels-Alder reactions between dienes 181 and different aldehydes 86 (Scheme 6.29A) [82], and also BINOL-based catalysts 178 were found to facilitate this reaction with excellent selectivities [83]. TADDOLs were also successfully used as organocatalysts for other asymmetric transformations like Mukaiyama aldol reactions, nitroso aldol reactions, or Strecker reactions to mention a few examples only [84]. In addition, also BINOL derivatives have been employed as efficient chiral H-bonding activators as exemplified in the Morita-Baylis-Hilhnan reaction of enone 184 with different carbaldehydes 86 [85]. The use of chiral squaramides for asymmetric reactions dates back to 2005 when Xie et al. first used camphor-derived squaric amino alcohols as ligands in borane reductions [86]. The first truly organocatalytic application was described by Rawal et al. in 2008 who found that minute amounts of the bifunctional cinchona alkaloid-based squaramide 180 are... 

An asymmetric variant of this reaction can be carried out introducing a camphor-derived 1-azabutadiene ligand. Enantiomeric excess values of up to 86% can be achieved with this system to obtain planar chiral iron complexes. The photolytically induced reaction of pentacarbonyliron with prochiral cyclohexa-1,3-dienes can also be run enantioselectively using a chiral 1-azabutadiene catalyst. Quantitative yields and ee values up to 86% are possible under these conditions. Cyclic 1,4-dienes can also be complexed by pentacarbonyliron under concomitant rearrangement to the 1,3-diene... 

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