Cryptands reduction

Alder and Sessions have reported that reductive cleavage of hydrazinium dications is a useful approach to macrobicyclic amines of the type developed by Park and Sim-mons , but the method does not appear to have been successfully applied to any cryptand syntheses. 

Coxon and Stoddart have directed their attention to the formation of penta-erythritol-derived cryptands. With these molecules, the strategy was to block one pair of hydroxyl groups as an acetal and form a crown from the remaining diol. In the first of the two reports cited above, this was accomplished by treating the 0-benzylidine derivative of pentaerythritol with base and diethylene glycol ditosylate. The crown was then treated with a mixture of UAIH4 and BF3 which gives partial reduction of the acetal as shown in (8.9), above. The monoprotected diol could now be treated in a fashion similar to that previously described and the benzyloxy cryptand (77) would result. The scheme is illustrated below as Eq. (8.10). 

The in-out bicyclic amines prepared by Simmons and Park bear a remarkable semblance to the cryptands but lack the binding sites in the bridges. As a result, these molecules interact with electrophiles in a fashion similar to other tertiary amines and generally do not exhibit strong interactions with alkali or alkaline earth metal ions. The in-out bicyclic amines are prepared by reaction of the appropriate acid chlorides and amines in two stages to yield the macrobicyclic amine after reduction of the amidic linkages. A typical amine is shown above as compound 18. 

The tin homolog of 49, distannene anion radical 50, was successfully prepared employing the same synthetic protocol the direct reduction of distannene (t-Bu2MeSi)2Sn=Sn(SiMer-Bu2)2 with potassium mirror in THF in the presence of [2.2.2]cryptand (Scheme 2.38). ... 

The reduction of the stannyl radical (t-Bu2MeSi)3Sn with alkali metals produces a variety of structural modifications depending on the solvent used (Scheme 2.55). Thus, in nonpolar heptane, a dimeric stannyllithium species [58c Li ]2 (E = Sn) was formed, whereas in more polar benzene, the monomeric pyramidal structure 58c [Ti -Li (C6H5)] was produced. In the latter compound the Li+ ion was covalently bonded to the anionic Sn atom being at the same time n -coordinated to the benzene ring. A similar monomeric pyramidal CIP 58c [Li (thf)2] was prepared by reduction in polar THE the addition of [2.2.2]cryptand to this compound resulted in the isolation of the free stannyl anion 58c K+([2.2.2]cryptand), in which the ion lacked its bonding to the Sn atom. ... 

A typical synthesis. The preparation of (213 m, n = 1) serves to illustrate the general synthetic strategy used to obtain this class of polyether cage. Starting from diaza-18-crown-6 and the required diacid chloride, condensation followed by reduction yields the corresponding cryptand, 2.2.2 (Dietrich, Lehn Sauvage, 1970 Dietrich, Lehn, Sauvage Blanzat, 1973)-see [4.8],... 

Pierre and Handel (1974) observed that cyclohexanone was cleanly reduced to cyclohexanol by LiAlH4 in diglyme solution. On addition of one equivalent (based on Li+) of the strong Li+-complexing agent [2.1.1]-cryptand, all the LiAlH4 was solubilized, but the reduction was completely inhibited. In the presence of additional amounts of either Lil or Nal, the ketone was reduced in the normal way. On the basis of these results, the authors concluded that the... 

The effect of cryptands on the reduction of ketones and aldehydes by metal hydrides has also been studied by Loupy et al. (1976). Their results showed that, whereas cryptating the lithium cation in LiAlH4 completely inhibited the reduction of isobutyraldehyde, it merely reduced the rate of reduction of aromatic aldehydes and ketones. The authors rationalized the difference between the results obtained with aliphatic and aromatic compounds in terms of frontier orbital theory, which gave the following reactivity sequence Li+-co-ordinated aliphatic C=0 x Li+-co-ordinated aromatic C=0 > non-co-ordinated aromatic C=0 > non-co-ordinated aliphatic C=0. By increasing the reaction time, Loupy and Seyden-Penne (1978) showed that cyclohexenone [197] was reduced by LiAlH4 and LiBH4, even in the presence of [2.1.1]-cryptand, albeit much more slowly. In diethyl ether in the absence of... 

The preparation of novel phase transfer catalysts and their application in solving synthetic problems are well documented(l). Compounds such as quaternary ammonium and phosphonium salts, phosphoramides, crown ethers, cryptands, and open-chain polyethers promote a variety of anionic reactions. These include alkylations(2), carbene reactions (3), ylide reactions(4), epoxidations(S), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and displacement reactions(10) to name only a few. The unique activity of a particular catalyst rests in its ability to transport the ion across a phase boundary. This boundary is normally one which separates two immiscible liquids in a biphasic liquid-liquid reaction system. 

The addition of a cryptand to some polyelectrolytes leads to significant increases in conductivity and in some cases IR and Raman spectroscopy demonstrate that the cryptand breaks up the ion-ion interactions (Chen, Doan, Ganapathiappan, Ratner and Shriver, 1991 Doan, Ratner and Shriver, 1991). Apparently the reduction of ion association more than offsets the reduction in mobility of the cation-crypt complex, which has a larger effective radius than the simple cation. It is also possible that the cryptand-ion complex is rendered more mobile by the reduction of polymer-cation complex formation, but this point has not been investigated in any detail. 

Ellipsoidal cryptands can also be synthesized by direct alkylation procedures <77AG(E)720,80CB1487), obviating the need for a diborane or lithium aluminum hydride reduction step. In the case of [l.l.l]cryptand (15a) yields of the final amine alkylation step are enhanced by the amine proton itself acting as a template (81CC777). 

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