Free alkali metals

Alkali-Metal-Free Solutions. Films of CD PbS are usually p-type as deposited. One early suggestion to explain this was that the alkali metal ions used in the deposition solution (as NaOH or KOH) act as a p-type dopant [33]. Based on this supposition, Bloem deposited PbS from a solution of PbAci, hydrazine hydrate, and thiourea (free of Na or K). The as-deposited films were initially n-type but changed to p-type on exposure to air. Attempts to dope the films permanently n-type by adding trivalent ions to the deposition solution were unsuccessful. However, by depositing the films on a substrate coated with trivalent ions (such as Al, In, Ga), n-type behavior could be maintained for a considerable time. PbS p-n junctions were fabricated using this approach (see Chap. 9). 

The earliest of these studies was on PbS. PbS can have either p- or n-type conductivity, although CD PbS is usually p-type. Based on the belief that the p-type conductivity may be due to alkali metal cations from the deposition solution, an alkali metal—free deposition, using lead acetate, thiourea, and hydrazine hydrate was used [33]. While initially n-type, the film converted to p-type in air. Attempts to stabilize the p-type material by adding trivalent cations to the deposition solution were unsuccessful. However, deposition of the PbS on a trivalent metal, such as Al, did stabilize the n-PbS, at least for a time. In this way, p-n junctions were made (the PbS close to the trivalent metal was n-type, while the rest of the film was p-type). Photovoltages up to 100 mV were obtained from these junctions at room temperature and almost 300 mV at low temperatures (90 K). 

In contrast, asymmetric Michael reactions are promoted by alkali metal free La-BINOL ester enolate complexes (Scheme 28) [251]. The catalyst is best prepared by successive addition of the Michael donor and (S)-BINOL to La(0 Pr)3. 

II. Chiral, alkali metal free-lanthanoid-BINOL derivative complexes... 

The first part of this chapter describes recent advances in the use of novel, chiral, alkali metal free-lanthanoid-BINOL derivative complexes for a variety of efficient, catalytic, asymmetric reactions. For example, using a catalytic amount of chiral Ln-BINOL derivative complexes, asymmetric Michael reactions and asymmetric epoxidations of enones proceed in a highly enantioselective manner. 

II. CHIRAL, ALKALI METAL FREE-LANTHANOID-BINOL DERIVATIVE COMPLEXES... 

Catalytic, Asymmetric Epoxidations Using Alkali Metal-Free Lanthanoid Complexes... 

Isolable alkali metal-containing and alkali metal-free intermediates are involved in the course of the formation of the new alkali metal-cobalt-olefin complexes (27) (yield 70-90%) (43, 47). 

Subsequently, the alkali metal free systems (CsMes)2NdCl(thf) and... 

It was proposed that a Lewis acid lanthanum center controls the direction of the carbonyl function and activates the enone while the sodium alkoxide forms enolate intermediates and regenerates the catalyst by hydrogen abstraction (Scheme 6). Other Ln/alkali metal combinations, including La/Li, show negligible asymmetric induction, yet give almost racemic products in excellent yield. In contrast, alkali-metal free BINOL ester enolate complexes catalyze Michael reactions with high enantioselectivities, albeit at lower temperatures. 

The catalytic asymmetric epoxidation of a,/5-unsaturated ketones with hydroperoxides such as tert-butyl hydroperoxide (TBHP) and cumene hydroperoxide (CMHP) can be carried out at ambient temperature by using alkali-metal free Ln-BINOL complexes (eq. (22)) [184]. The oligomeric structure of the catalyst is assumed to play a key role that is, the Ln alkoxide moiety acts as a Brpnsted base, activating a hydroperoxide molecule, while another Ln metal ion acts as a Lewis acid, both activating and controlling the orientation of the enone. 

In the presence of the sodium-containing heterobimetallic catalyst (R)-LSB (10 mol%), the reaction of enone 52 with TBHP (2 equiv) was found to give the desired epoxide with 83% ee and in 92% yield [56]. Unfortunately LSB as well as other bimetallic catalysts were not useful for many other enones. Interestingly, in marked contrast to LSB an alkali metal free lanthanoid BINOL complex, which was prepared from Ln(0- -Pr)3 and (R)-BINOL or a derivative thereof (1 or 1.25 molar equiv) in the presence of MS 4A (Scheme 17), was found to be applicable to a range of enone substrates. Regarding enones with an aryl-substitu-ent in the a-keto position, the most effective catalytic system was revealed when using a lanthanum-(.R/)-3-hydroxymethyl-BINOL complex La-51 (l-5mol%) and cumene hydroperoxide (CMHP) as oxidant. The asymmetric epoxidation proceeded with excellent enantioselectivities (ees between 85 and 94%) and yields up to 95%. 

Preparation of Phosphines from Metallated Phosphines. - Alkali metal-free phosphide anions have been shown to be formed under synthetically useful conditions in the equilibria between primary or secondary phosphines with the Schwesinger bases (23). Techniques have been developed for the preparation of alkali metal diphenylphosphide reagents in high purity, as evidenced by P nmr studies. The same paper reports a study of the course of the reactions between potassium diphenylphosphide and a series of aryl-, n-alkyl- and neopentyl-halides. The results provide the first evidence of the involvement of single electron transfer (SET) processes in the reactions of alkyl halides. This pathway is dominant in the case of neopentyl-type iodides, but plays only a minor role in the related reactions of neopentyl-type bromides and chlorides. No evidence was adduced of the involvement of SET processes in the reactions of unhindered... 

The change from strength stabdity below 300 °C can be attributed to the thermodynamics of the above reaction. As with any chemical reaction, the rate of hydrolysis is a function of the presence of catalysts. Since the alkali metal glass modifiers (sodium—potassium) catalyse hydrolysis, alkali metal-free glasses are expected to show more resistance to high temperatures. 

By comparing with the alkali metal free La-BINOL catalyst, the (/ )-LSB-catalyzed Michael reactions of malonate to cyclic enone proceeded smoothly with high enantioselectivities even at room temperature. The reaction between 2-cyclohexen-l-one and dibenzyl methylmalonate completed in 12 hours at room temperature to afford adduct in 96% yield and 90% ee. When dimethyl malonate was used, 98% yield with 83% ee of the product was obtained. Switching the dimethyl analog diethyl malonate also furnished similar product yield and enantioselectivity (97% yield and 81% ee). 

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