Lithium complexes, amino

The 1-t-butylphospholane sulfide intermediate to TangPhos was also used to prepare the P,N ligands 48 by reacting the lithium complex with C02 and then oxazoline formation with a range of chiral amino alcohols [69b, 74]. The Ir complexes of these ligands have been successfully used in the reduction of / -methylcinnamic esters (80-99% ee) and methylstilbene derivatives (75-95% ee), a particularly challenging class of unfunctionalized olefins [4 c]. 

The chelate formation in lithium complexes 17 or 20 contributes to stabilization. Enhancement of kinetic acidity arises from the formation of pre-complexes 16 and 19, respectively. Here, already a dipole is induced and, in addition, proton exchange can proceed intramolecularly via a five- or six-membered ring. Despite these favourable features, the acidity of alkyl carbamates 15 is lower than those of the 1-proton in butane n-BuLi does not lead to deprotonation. In order to suppress carbonyl attack, a branched amino residue NR2 such as diisopropylamino (in Cb) or 2,2,4,4-tetramethyl-l,3-oxazolidin-3-yl (in Cby) is essential. A study on the carbenoid nature of compounds 17 was undertaken by Boche and coworkers. ... 

A series of lithium complexes have been utilized as synthons in the preparation of aminoborane complexes. (IV-Lithiomethylamino)dimethylborane is used as a reagent for the preparation of borylamino(amino)boranes and diborylamines (50). Lithium benzyl-/ -butylamide reacts with BC13 to yield the bisaminoborane [91575-50-1] shown in equation 16 (51). 

A few years later Arnold and co-workers also reported the synthesis of lithium complexes of the neutral and anionic salts of a tridentate amino bis-carbene ligand (Scheme 2).13 Treatment of the cationic amino bis-imida-zolium salt with three equivalents of //-butyl lithium affords the lithium amino bis-carbene chloride complex (5). Deprotonation with four equivalents of n-butyl lithium affords the lithium amide salt (6). Although the complexes were not characterised in the solid state, characteristic shifts in the multinuclear NMR spectra and elemental analysis are consistent with the lithium complexes being formed. NMR spectra of 5 suggest formation of a cluster of lithium chloride ions with lithium-NHC bonds (13C NMR NCN 203.9 ppm) and NH-chloride bonding interactions. Following further deprotonation to form 6 the complex also retains lithium chloride and exhibits a similar C2 chemical shift (13C NMR NCN 203.4 ppm). 

The reaction of the amino imidazolium salt hydrobromide with silver(I) oxide yields the corresponding silver(I) carbene complex (see Figure 4.29) that has the same central structural unit as the lithium complex [35]. 

When the W-trimethylsilyltris(2-pyridyl)phosphinimine (22) was treated with one equivalent of methyllithium at - 78°C, a lithium complex (23) binding a dipyridyl moiety with a trivalent amino-methyl (2-pyridyl)phosphane was isolated. This compound results from a ligand coupling process taking place on the pentacoordinate phosphorus intermediate formed by addition of methyllithium on the imine bond. 

Catalytic asymmetric conjugate additions of heteroatoms such as nitrogen or sulfur nucleophiles provide access to important / -substituted carbonyl deri-vates, and a number of successful strategies have been devised (38, 39, 41). Tomioka developed enantioselective additions of thiophenols to ,/i-unsatu-rated esters (Equation 42) [164]. This reaction proceeds in the presence of the presumed lithium complex 220, which was suggested as the catalytically active species. As a control experiment, the chiral amino diether itself is not a catalyst for the addition of thiophenol 219 to 218. A variety of trans enoates were shown to undergo addition to give the products with high levels of induction, as illustrated by the formation of thioether 221 (99 % yield, 97 % ee). 

Quite a number of asymmetric thiol conjugate addition reactions are known [84], but previous examples of enantioselective thiol conjugate additions were based on the activation of thiol nucleophiles by use of chiral base catalysts such as amino alcohols [85], the lithium thiolate complex of amino bisether [86], and a lanthanide tris(binaphthoxide) [87]. No examples have been reported for the enantioselective thiol conjugate additions through the activation of acceptors by the aid of chiral Lewis acid catalysts. We therefore focussed on the potential of J ,J -DBFOX/ Ph aqua complex catalysts as highly tolerant chiral Lewis acid catalyst in thiol conjugate addition reactions. 

We came up with the idea of using a dummy ligand, as shown in Scheme 1.23 [34]. Reaction of dimethylzinc with our chiral modifier (amino-alcohol) 46 provided the methylzinc complex 62, which was subsequently reacted with 1 equiv of MeOH, to form chiral zinc alkoxide 63, generating a total of 2 moles of methane. Addition of lithium acetylide to 63 would generate an ate complex 64. The ate complex 64 should exist in equilibrium with the monomeric zincate 65 and the dimer 66. However, we expected that the monomer ate complex 64 and the mono-... 

The reduction of a-hydroxynitriles to yield vicinal amino alcohols is conveniently accomplished with complex metal hydrides for example, lithium aluminum hydride or sodium borohydride [69]. However, it is still worth noting that a two-step chemo-enzymatic synthesis of (R)-2-amino-l-(2-furyl)ethanol for laboratory production was developed followed by successful up-scaling to kilogram scale using NaBH4/CF3COOH as reductant [70],... 

Nonmetallic systems (Chapter 11) are efficient for catalytic reduction and are complementary to the metallic catalytic methods. For example lithium aluminium hydride, sodium borohydride and borane-tetrahydrofuran have been modified with enantiomerically pure ligands161. Among those catalysts, the chirally modified boron complexes have received increased interest. Several ligands, such as amino alcohols[7], phosphino alcohols18 91 and hydroxysulfoximines[10], com-plexed with the borane, have been found to be selective reducing agents. 

Further variations of the Claisen rearrangement protocol were also utilized for the synthesis of allenic amino acid derivatives. Whereas the Ireland-Claisen rearrangement led to unsatisfactory results [133b], a number of variously substituted a-allenic a-amino acids were prepared by Kazmaier [135] by chelate-controlled Claisen rearrangement of ester enolates (Scheme 18.47). For example, deprotonation of the propargylic ester 147 with 2 equiv. of lithium diisopropylamide and transmetallation with zinc chloride furnished the chelate complex 148, which underwent a highly syn-stereoselective rearrangement to the amino acid derivative 149. 

Figure 10.19 Amino acid analyser trace. Separation of a complex physiological standard mixture of amino acids in 3.5 hours using lithium citrate buffers and ninhydrin detection 10 nmol of each amino acid, including the internal standard, nor-leucine, were applied to the column (0.3 X 35 cm) in a total volume of 50 ml. 

Consequently, by choosing proper conditions, especially the ratios of the carbonyl compound to the amino compound, very good yields of the desired amines can be obtained [322, 953]. In catalytic hydrogenations alkylation of amines was also achieved by alcohols under the conditions when they may be dehydrogenated to the carbonyl compounds [803]. The reaction of aldehydes and ketones with ammonia and amines in the presence of hydrogen is carried out on catalysts platinum oxide [957], nickel [803, 958] or Raney nickel [956, 959,960]. Yields range from low (23-35%) to very high (93%). An alternative route is the use of complex borohydrides sodium borohydride [954], lithium cyanoborohydride [955] and sodium cyanoborohydride [103] in aqueous-alcoholic solutions of pH 5-8. 

Iron and acetic or dilute hydrochloric acid can be safely used for the reduction of nitro group to an amino group in nitro esters. The problem arises when a nitro ester is to be reduced to a nitro alcohol. Nitro groups are not inert toward the best reagents for the reduction of esters to alcohols, complex hydrides. However the rate of reduction of a nitro group by lithium... 

The enantioselective addition of the amino organolithium reagents consists of two stereo-controlled reactions, the asymmetric deprotonation (equation 14) and the following addition to electrophiles. The stereochemical course of the addition depends on the electrophile E. In the cases where heterocyclic enone or a,-unsaturated lactones are the electrophiles (entries 5-7), the addition proceeds with retention of configuration. In contrast, with the other electrophiles in Table 10 and trimethyltin chloride in equation 15, the addition proceeds with inversion of configuration. In the addition which proceeds with retention of configuration, a pre-complexation between the electrophiles and lithium may be involved (equation 16). 

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