Lithium, coordination

As a result of the combination of intra- and extra-annular chirality transfer, a productive approach has been developed for the design of chiral enolate systems in which a structurally organized diastereofacial bias is established, as illustrated in the equations in Scheme 2-11. A lithium-coordinated five-membered or six-membered ring fixes the orientation between the inducing asymmetric center and the enolate20 ... 

Table 1. Comparison of the Hyperfine Shifts (in ppm) Observed for Lithium Coordinated to a Single Mn + Ion via an Intervening Oxygen Ion for a Series of Local Environments ...

The problem of auxiliary removal is overcome when acetals are replaced with the much more labile aminals, formed by reaction of benzaldehydechromium tricarbonyl 403 with diamines, and readily cleaved with mild acid. The best choice of diamine is 404, because the aminal 405 lithiates with good to excellent regioselectivity in the ortho position (Scheme 167) the same could not be said for diamines lacking further lithium-coordinating side-chains . Treatment of 405 with three equivalents of n-BuLi in... 

The general intransigence of the crystalline iminolithium hexamers toward further interaction with Lewis bases is not shown by the amorphous diaryliminolithiums. This may reflect their extensively stacked nature (Section II,A Fig. 10), which, while raising the lithium coordination number to four in all but the outer rings of the polymer, will presumably weaken individual N—Li bonds. These materials dissolve quite readily in several polar solvents, e.g., THF (66), pyridine (66, 78, 85), and HMPA (86). Crystalline complexes can be recovered from these solutions. Two of these, both derivatives of (Ph2C=NLi) , have been characterized structurally in the solid state. The tetrameric cubane (Ph2C=NLipyridine)4 (7) is depicted in Fig. 14 (78, 85). 

With lithium counterions, as compared with sodium cations, a similar explanation holds. The water-lithium interaction is stronger than the water-sodium interaction. Hence, there is a somewhat greater tendency for lithium-coordinated water molecules to reorient around the 0-M axis — hence the observed smaller value... 

G. Hofmeister, J.A. Leary, Chiral recognition of lithium-coordinated diols using tandem mass spectrometry. Org. Mass Spectrom. 26, 811-812 (1991)... 

Lithiated allyl sulfides (in common with allyl sulfoxides and selenides, but in contrast with allyl ethers and allyl amine derivatives) tend to react with alkyl halides at the a position, adjacent to S.1 The best a-selectivities are obtained with lithium-coordinating S-substituents such as pyridyl (114),85 imidazolyl (115), and dimethylaminocarbonyl (116).1... 

Exceptions are the few secondary benzyllithiums which show configurational stability on the microscopic (but not macroscopic) timescale - in other words, they racemise slower than they react with electrophiles, though they still cannot be maintained in stereoisomerically pure form for periods of minutes or more. These are the carbamates 288 and the sulfones 289, discussed in sections 5.1.4 and 5.I.7.6 It is significant that both of these compound classes contain powerful lithium-coordinating oxygen atoms, which may hold the lithium counterion close to one face of the benzylic system.134-120... 

Three types of secondary benzyllithium are known which are configurationally stable on the macroscopic timescale, and they too each contain powerful lithium-coordinating oxygen atoms. One is the N-Boc amino substituted 180 discussed above.80 Partial configurational stability is evident in a close analogue of 180.81... 

In the event that the alkylcarbamate contains further lithium-coordinating heteroatoms, enantioselective deprotonation becomes impossible because sparteine is displaced from the lithium in this reactive complex. So, for example, the A,A-dimethyIamine 424 (R = Me) is lithiated with only 10% ee, which its dibenzyl analogue is lithiated in 97% ee.162 Most oxygen-containing functional groups do not have this effect.178181... 

J. Seyden-Penne, Lithium coordination by Wittig-Horner reagents formed by a-carbonyl substituted phosphonates and phosphine oxide, Bull. Soc. Chim. Fr. 1988, 238. 

The authors commented that perturbation of conjugation due to stereochemical control of lithium coordination among compounds 67-77 appears to be a continuously variable effect49d. Thus, as the differences between the Ci— C2 and C2—C3 bond lengths, respectively, and 13C chemical shifts S3 — <>1 both increase, the angle the C—Li+ bond makes with the allyl plane decreases systematically (Table 12)49d. 

The Michael-aldol process with methacrylates described in Section II.B can be also applied to the synthesis of substituted tetrahydrofurans, 245. If the reaction is carried out in THF, the yield and selectivity of the sequence decrease. It was proposed that the lithium coordination with THF molecules hinders the formation of the product 245. The authors concluded that the Lewis acidity of naked lithium cation is the key driving force for the reaction to proceed successfully. The tandem reaction with lithium thiophenolate, fumarate ester and benzaldehyde constitutes an useful methodology for the preparation of y-butyrolactone (Scheme 75)89,90. 

SCHEME 51. Lithium coordination modes in acetaldehyde enolate and in lithium ester silenolate201... 

The stereocontrol arises from lithium coordination to the acyl oxygen and delivery of the dimethylamide to the unhindered face of the (Z)-cinnamoyl ligand in the s-tram conformation with the acyl oxygen anti to the carbon monoxide ligand. These arguments were verified by an X-ray crystal structure analysis of the corresponding ( )-crotyl iron complex 29119. 

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