Lithium amides, addition

The authors proposed a chelating transition state model to explain these results (Fig. 8.14). The thermodynamically more stable intermediate resulting from initial lithium amide addition should have the amino group on the face opposite to the bulky tert-butyl group. Due to the same steric effect, the HMPA ligand should also occupy a position on the p face. The electrophile approaches the enolate from the ot face and gives the trans product. For bulky amines, either the aza enolate does not form due to severe steric hindrance or the aza enolate is inactive for the same reason. 

Hate 1. To a suspension of 0.40 mol of lithium amide in 400 ml of liquid NH3 (see Chapter II, Exp. 11) was added 0.30 mol of HCECCH20-tert.-CitHg Subsequently 0.46 mol of CjHsBr was introduced in 30 min. After an additional 1 h the NH3 was removed by placing the flask in a water-bath at 40°C. Addition of water, extraction with diethyl ether and distillation gave C2H C=CCH20-tert.-C,H in more than 85% yield. 

To a vigorously stirred suspension of 2 mol of lithium amide in 2 1 of liquid atimonia (see II, Exp. 11) was added in 15 min 1 mol of propargyl alcohol (commercial product, distilled in a partial vacuum before use). Subsequently, 1 mol of butyl bromide was added dropwise in 75 min. After an additional 1.5 h, stirring was stopped and the ammonia was allovied to evaporate. To the solid residue were added 500 ml of ice-water. After the solid mass had dissolved, six extractions with diethyl ether were performed. The (unwashed) combined extracts were dried over magnesium sulfate and then concentrated in a water-pump vacuum. Distillation of the residue through a 40-cm Vigreux column afforded 2-heptyn-l-ol, b.p. 

The formation of the above anions ("enolate type) depend on equilibria between the carbon compounds, the base, and the solvent. To ensure a substantial concentration of the anionic synthons in solution the pA" of both the conjugated acid of the base and of the solvent must be higher than the pAT -value of the carbon compound. Alkali hydroxides in water (p/T, 16), alkoxides in the corresponding alcohols (pAT, 20), sodium amide in liquid ammonia (pATj 35), dimsyl sodium in dimethyl sulfoxide (pAT, = 35), sodium hydride, lithium amides, or lithium alkyls in ether or hydrocarbon solvents (pAT, > 40) are common combinations used in synthesis. Sometimes the bases (e.g. methoxides, amides, lithium alkyls) react as nucleophiles, in other words they do not abstract a proton, but their anion undergoes addition and substitution reactions with the carbon compound. If such is the case, sterically hindered bases are employed. A few examples are given below (H.O. House, 1972 I. Kuwajima, 1976). 

A mixture containing 186 g (0.20 mol) of 2-aminopyridine, 0.55 g of lithium amide and 75 cc of anhydrous toluene was refluxed for 1.5 hours. Styrene oxide (12.0 g = 0.10 mol) was then added to the reaction mixture with stirring over a period of ten minutes. The reaction mixture was stirred and refluxed for an additional 3.5 hours. A crystalline precipitate was formed during the reaction which was removed by filtration, MP 170°C to 171°C, 1.5 g. The filtrate was concentrated to dryness and a dark residue remained which was crystallized from anhydrous ether yield 6.0 g. Upon recrystallization of the crude solid from 30 cc of isopropyl alcohol, 2.0 g of a light yellow solid was isolated MP 170°C to 171°C. 

The aldol reaction of 2,2-dimethyl-3-pentanone, which is mediated by chiral lithium amide bases, is another route for the formation of nonracemic aldols. Indeed, (lS,2S)-l-hydroxy-2,4,4-trimethyl-l-phenyl-3-pentanone (21) is obtained in 68% ee, if the chiral lithiated amide (/ )-A-isopropyl-n-lithio-2-methoxy-l-phenylethanamine is used in order to chelate the (Z)-lithium cnolate, and which thus promotes the addition to benzaldehyde in an enantioselective manner. No anti-adduct is formed25. 

The conjugate addition of Grignard reagents to 2-cyclohexenone was promoted by catalytic amounts (2-4 mol %) of alkylcopper(I) complexes of the lithium amide prepared from N- (R)-1 -phenylethyl]-2-[(/ )-l-phenylethyliminojcycloheptatrienamine, Li[CuR(CHIRAMT)]52,11. However, 3-substituted cyclohexanones were obtained in very low ee (4-14%). 

By analogy, the acetylene aldehyde 500 gives, on addition of the chiral Li-enolate 501 [79-82], the chiral //-lactams 502 and 503 in 75% yield [80-82]. Similar (fhc-tam-forming reactions are discussed elsewhere [70, 83-88]. The ketone 504 affords, with the lithium salt of the silylated lithium amide 505, the Schiff base 506, in 74% yield (Scheme 5.27). The Schiff base 506 is also obtained in 25% yield by heating ketone 504 with (C6H5)3P=N-C6H4Me 507 in boiling toluene for 7 days... 

Asymmetric conjugate addition of lithium amides to alkenoates has been one of the most powerful methods for the synthesis of chiral 3-aminoalkanoates. High stereochemical controls have been achieved by using either chiral acceptors as A-enoyl derivatives of oxazolidinones (Scheme 4) 7 7a-8 chiral lithium amides (Schemes 5 and 6),9-12 or chiral catalysts.13,14... 

In 1994, lithium amide 23 was used in the conjugate addition of 2-cyclohexenone to afford optically active adduct with up to 97% ee (Scheme 13).28-29 A dimeric structure was proposed as the intermediate, where the phenyl group in 23 blocked the bottom face and the cyclohexenone substrate approached from the upper face. 

Alternatively, a chiral lithium amide was added regio- and diastereoselectively to an achiral 2,4-dienoate, and the 1,4-addition product formed could again be converted into the desired, stereochemically pure /J-lactam (equation 31)106. 

Diastereoselective 1,4- and 1,6-addition reactions of lithium amides to chiral naph-thyloxazolines were used by Shimano and Meyers108-110 for the synthesis of novel amino acids. For example, treatment of (S )-2-(l-naphthyl)-4-t-butyloxazoline with lithi-ated l,4-dioxa-8-azaspiro[4.5]decane and iodomethane provided the diastereomerically pure 1,4-addition product with excellent yield cleavage of the heterocyclic rings then gave the desired /3-amino acid (>99% ee/ds equation 32)108,109. In contrast to this, most acyclic lithium amides reacted with these oxazolines under 1,6-addition the products were transformed smoothly to 5-amino acid derivatives (equation 33)110. 

Among other enantioselective alkylations, a series of 3-aminopyrrolidine lithium amides (67 derived from 4-hydroxy-L-proline) have been used to induce high ee% in the addition of alkyllithiums to various aldehydes. Structure-activity relationships are identified, and the role of a second chiral centre (in the R group) in determining the stereochemistry of the product is discussed. 

Enantiomeric excesses of up to 76% have been obtained for alkyllithium-aldehyde condensations using 3-aminopyrrolidine lithium amides as chiral auxiliaries. Addition of organolithiums to imines has been achieved with up to 89% ee, in the presence of C2-symmetric bis(aziridine) ligands. ... 

Both diastereoisomers of -homothreonine derivatives (109) and their 2-deuteriated analogues have been synthesized by 1,4-addition of homochiral lithium amides (107) as nitrogen nucleophiles to y-alkoxyenoates (108) (Scheme 13). The product distribution of the 1,4-addition depends strongly on the nature of the substrate (110) vs... 

The 1,6-addition reaction of lithium amides to the naphthalene ring system (141) followed by the electrophilic alkylation has been reported (Scheme 17). ... 

Dioxolanones 100 and 101 were treated with two equivalents of lithium amide bases, whereupon various electrophiles were added in excess (Scheme 23 and Table 1). Unfortunately, addition of two equivalents of LDA... 

The reaction depicted in equation 43 between a nitrile and a lithium amide takes place as a 1,2-addition to the cyano group. The product crystallizes as a dimer (236) in which the lithium atoms are solvated by nitrile molecules and differently bonded to the amidine moieties, as shown by XRD analysis. Low-temperature H NMR spectrum in solution points to uniform chemical environments for both the aryl groups and for the Me—Si groups, and to rapid rearrangement of the Li—N coordination structures. Acidolysis of the dimer in solution yields the corresponding amidine (237) . The crystal structure of the THF-solvated analog of 236 shows dissimilar N—Li bond lengths for the two Li atoms... 

Successful lithiation of aryl halides—carbocyclic or heterocyclic—with alkyUithiums is, however, the exception rather than the rule. The instability of ortholithiated carbocyclic aryl halides towards benzyne formation is always a limiting feature of their use, and aryl bromides and iodides undergo halogen-metal exchange in preference to deprotonation. Lithium amide bases avoid the second of these problems, but work well only with aryl halides benefitting from some additional acidifying feature. Chlorobenzene and bromobenzene can be lithiated with moderate yield and selectivity by LDA or LiTMP at -75 or -100 °C . 

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