Lithium-containing four-membered rings

The lithium as well as the sodium derivative of 1,1,3, 3-tetra-tert-butyl- l-fluoro-3-hydroxydisiloxane should be predestined for the formation of four-membered rings by intramolecular salt elimination. The results of X-ray structural analysis seem to confirm this assumption. Each molecule of the lithiated derivative forms a six-membered ring showing intramolecular LiF-contact three molecules form a trimer, containing a central Li-O six-membered ring with three-coordinated lithium [8]. The sodium analogue of this compound reveals a surprisingly similar structure. 

The reaction of SiF-coupled six-membered rings with lithium organyls first produces ring contraction and subsequently eliminates LiF. As discussed in Section II, we have now obtained an intermediate ylide that contains silicon atoms with the coordination numbers three and five. This gives rise to a nucleophilic migration of a methanide ion. A fused bicyclic compound is formed (Scheme 24). This compound has also been characterized by an X-ray analysis and the structure is shown in Fig. 13. Both the four-membered rings are planar.42,46,50... 

A preliminary approach to understand the mechanism of the enantioselective protonation and the role of lithium bromide, based on TSs containing one hthium atom, failed to explain the selectivity enhancement by hthium bromide, and the calculated energies of the TSs did not account for the experimentally observed selectivity. Asensio, Domingo and coworkers studied the molecular process associated with the proton transfer at the semiempirical PM3 level . Based on hterature data , they defined the structure of a mixed dimer enolate 234 consisting of a four-membered ring where the bromide anion and the oxygen atom of the enolate were connected by two hthium cations. These bridging... 

For the reaction of labile ylides with aldehydes, the formation of cis-oxaphosphetanes is favored, unless lithium ion-containing bases are used. The four-membered ring system of cis-39 dissociates in a [2+2]-cycloreversion and forms triphenyl phosphonium oxide and the desired olefin Z-13. Using labile ylides in Wittig reactions that contain no electron-withdrawing group except the phosphonium moiety, leads predominantly to the formation of c/ -oxaphosphetanes. Hence, Z-olefins are typically formed with > 90 % selectivity. ... 

Treatment of 25 with an excess of lithium did not afford the digermenyllithium compound 28, but instead we obtained dark-red crystals of the ionic compound 27, which contains an allyl-like Gcj anion as part of a four-membered ring. Within the planar ring, the Ge-Ge multiple bond lengths (236.79 pm) differ substantially from the Ge-Ge single bond lengths (251.16 pm).P l... 

The lithium derivative (Fig. 12) crystallizes from n-hexane as a dimer. The crystal structure shows some irregularities. The dimer is formed via a four-membered (Li-N)2 ring, which is not entirely planar but folded by 10.7° across the li-Ii line. The (Si-O-Si-N)-rings are at an angle of 86° to the central (LiN)2 ring and at 5.8° to each other. As far as we know this is the smallest four-membered ring containing two Si atoms. The transannular Si "Si distance is found to be 237.2 pm. 

A rare example of a heterometallic imido complex is the triple-stacked Li4Na2[N=C(Ph)-(Bu )]6, prepared from the reaction of PhLi and PhNa with (Bu )NC (see Figure 34). The molecule has six metal sites in a triple-layered stack of four-membered M—rings, with the outer rings containing lithium and the central ring containing sodium the latter is four-coordinate. ... 

This general procedure has also been extended to allow for the insertion of a methoxy-containing carbon. The uncatalyzed addition of the lithium anion of methoxymethyl enyl sulfone to ketones proceeds readily at low temperature in dimethoxyethane to form the intermediate adduct. Addition of a Lewis acid (ethylaluminum dichloride or diisobutylaluminum diisopropylamide) directly to the reaction mixture effects the rearrangement reaction to produce the ring-expanded a-methoxy ketone. This sequence, illustrated by the example in Scheme 28, is limited to the expansion of four- and five-membered ring ketones. ... 

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