Calcium alkenes

Both objectives have been met by designing special hydrogenation catalysts The most frequently used one is the Lindlar catalyst, a palladium on calcium carbonate combi nation to which lead acetate and quinoline have been added Lead acetate and quinoline partially deactivate ( poison ) the catalyst making it a poor catalyst for alkene hydro genation while retaining its ability to catalyze the addition of H2 to the triple bond... 

Lindlar catalyst (Section 9 9) A catalyst for the hydrogenation of alkynes to as alkenes It is composed of palladium which has been poisoned with lead(II) acetate and quino line supported on calcium carbonate... 

Reaction of HOCl, formed from calcium hypochlorite and CO2, with highly substituted alkenes in CH2CI2 is a convenient route to aHyUc chlorides (111). Ketones are chlorinated to a-chloroketones by reaction with HOCl Acetone initially gives CH2COCH2CI (112). Methyl ethyl ketone gives 78% CH3CHCICOCH3, 15% CH3CH2COCH2CI, and 7% dichlorides (113). 

N,N-Dimethy1aniline from Nakarai Chemicals was dried over calcium hydride and freshly distilled. Three molar equivalents of N,N-dimethylaniline are used to achieve complete conversion of the n-butyllithium, because In the present particular case free n-butyllithium, if present, causes the isomerization of the (Z)-alkene to the (E)-isomer. 

Complete reduction to the alkane occurs when palladium on carbon (Pd/C) is used as catalyst, but hydrogenation can be stopped at the alkene if the less active Lindlar catalyst is used. The Lindlar catalyst is a finely divided palladium metal that has been precipitated onto a calcium carbonate support and then deactivated by treatment with lead acetate and quinoline, an aromatic amine. The hydrogenation occurs with syn stereochemistry (Section 7.5), giving a cis alkene product. 

Griffin, D.S., and SegaU, H.J., 1987, Role of cellular calcium homeostasis in toxic liver injury induced by the pyrrolizidine alkaloid senecionine and the alkenal trans-4-OH-2-hexenal,... 

Palladium catalysts are more often modified for special selectivities than platinum catalysts. Palladium prepared by reduction of palladium chloride with sodium borohydride Procedure 4, p. 205) is suitable for the reduction of unsaturated aldehydes to saturated aldehydes [i7]. Palladimn on barium sulfate deactivated with sulfur compounds, most frequently the so-called quinoline-5 obtained by boiling quinoline with sulfur [34], is suitable for the Rosenmund reduction [i5] (p. 144). Palladium on calcium carbonate deactivated by lead acetate Lindlar s catalyst) is used for partial hydrogenation of acetylenes to cw-alkenes [36] (p. 44). 

A few years later, Hass reported an alternative synthesis of racemic amphetamine, which exemplifies the use of a nitroalkane as the source of the nitrogen atom (Scheme 17.4) (Hass et al., 1950). In this route, calcium-hydroxide-promoted Henry reaction of nitroethane and benzaldehyde (12) afforded an 86% yield of the nitro alkene 13. Simultaneous hydrogenation... 

Silver(I) fluoride dispersed on the surface of finely powdered calcium fluoride shows improved fluoride nuclcophilicity for addition to alkenes in halofluorination reactions.56 For example, hex-1-ene with l,3-dibromo-5,5-dimethylhydantoin and silver(I) fluoride/calcium fluoride in dichloromethane at 20 C gives, within 1 hour, l-bromo-2-fluorohexanc in 90% yield while with silver(I) fluoride, under similar conditions, the yield is only 39%. 

Studies on acetylenic molecules have been less extensive than those on alkenes diphenylacety-lene is converted at — 78 °C in Freon in the presence of calcium oxide with 2.5 equivalents of trifluoromethyl hypofluorite into l,l,2-trifluoro-l,2-diphenyl-2-(trifluoromethoxy)ethane (31) in 75% yield.37... 

Allylic chlorides Actually the reaction of HOG with highly substituted alkenes is a convenient route to allylic chlorides if CH2G2 is used as the organic cosolvent. The reagent is prepared by addition of dry ice to calcium hypochlorite (70%) in water. The reaction of 1-methylcyclohexene is typical (equation I). Chlorohydrins arc the main products only in the case of 1-alkenes and 1,2-disubstituted alkenes. 

Under oxidation with PCC, migration of alkenes into conjugation with aldehydes or ketones can be avoided by the addition of calcium carbonate (see page 47). 

With most hydrogenation catalysts of the platinum and nickel groups (Sections 4.2.61, p. 459 and 4.2.50, p. 450), a mixture of products is obtained even if an attempt is made to stop the reaction at the half-way stage. An alkene may only be obtained in good yield if particular attention is paid to the selection of a deactivated hydrogenation catalyst. An early, highly effective formulation of a deactivated palladium catalyst is Lindlar s catalyst (Section 4.2.54, p. 453), but palladium-on-barium sulphate in the presence of quinoline, or palladium-on-calcium carbonate, is also recommended. In these cases it is advisable to... 

Mono(cyclopentadienyl) complexes alkenes and tantalum, 5, 157 alkyne and niobium, 5, 80 calcium, strontium, barium, 2, 133... 

Lithium ferf-butylperoxide has been used effectively as an epoxidising agent with electron deficient alkenes (Scheme 40) [91, 92]. However, application of this methodology to systems containing fluorine has only recently been explored, and it is now established that this can be a very successful procedure with fluoroalkenes. Indeed, the Lithium tert-butylperoxide system worked in some cases where the calcium hypochlorite reaction was ineffective [93,94]. 

This pure Z-alkene was needed for studies on the mechanism of a rearrangement reaction. In Chapter 24 you met catalytic hydrogenation as a means of reducing alkenes to alkanes, and we introduced Lindlar s catalyst (palladium and lead acetate on a support of calcium carbonate) as a means of controlling chemoselectivity so that alkynes could be reduced to alkenes. What we did not empha-... 

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