Bases. elimination with

The phosphorus ylides of the Wittig reaction can be replaced by trimethylsilylmethyl-carbanions (Peterson reaction). These silylated carbanions add to carbonyl groups and can easily be eliminated with base to give olefins. The only by-products are volatile silanols. They are more easily removed than the phosphine oxides or phosphates of the more conventional Wittig or Homer reactions (D.J. Peterson, 1968). 

There also exists an acidregioselective condensation of the aldol type, namely the Mannich reaction (B. Reichert, 1959 H. Hellmann, 1960 see also p. 291f.). The condensation of secondary amines with aldehydes yields Immonium salts, which react with ketones to give 3-amino ketones (=Mannich bases). Ketones with two enolizable CHj-groupings may form 1,5-diamino-3-pentanones, but monosubstitution products can always be obtained in high yield. Unsymmetrical ketones react preferentially at the most highly substituted carbon atom. Sterical hindrance can reverse this regioselectivity. Thermal elimination of amines leads to the a,)3-unsaturated ketone. Another efficient pathway to vinyl ketones starts with the addition of terminal alkynes to immonium salts. On mercury(ll) catalyzed hydration the product is converted to the Mannich base (H. Smith, 1964). 

FIGURE 8 11 When a Lewis base reacts with an alkyl halide either substitution or elimination can occur Sub stitution (Sn2) occurs when the Lewis base acts as a nu cleophile and attacks carbon to displace bromide Elimi nation (E2) occurs when the Lewis base abstracts a pro ton from the p carbon The alkyl halide shown is iso propyl bromide and elimi nation (E2) predominates over substitution with alkox ide bases... 

Many carbamates have been used as protective groups. They are arranged in this chapter in order of increasing complexity of stmcture. The most useful compounds do not necessarily have the simplest stmctures, but are /-butyl (BOC), readily cleaved by acidic hydrolysis benzyl (Cbz or Z), cleaved by catalytic hy-drogenolysis 2,4-dichlorobenzyl, stable to the acid-catalyzed hydrolysis of benzyl and /-butyl carbamates 2-(biphenylyl)isopropyl, cleaved more easily than /-butyl carbamate by dilute acetic acid 9-fluorenylmethyl, cleaved by /3-elimination with base isonicotinyl, cleaved by reduction with zinc in acetic acid 1-adamantyl, readily cleaved by trifluoroacetic acid and ally], readily cleaved by Pd-catalyzed isomerisation. 

The 2,2 -bis(phenylthiomethyl) dispiroketal (dispoke) derivative is cleaved by oxidation to the sulfone, followed by treatment with LiN(TMS)2. The related bromo and iodo derivatives are cleaved reductively with LDBB (lithium 4,4 -di- -butylbiphenylide) or by elimination with the P4- -butylphosphazene base and acid hydrolysis of the enol ether. The 2,2-diphenyl dispiroketal is cleaved with FeCl3 (CH2CI2, rt, overnight)." The dimethyl dispiroketal is cleaved with TFA, and the allyl derivative is cleaved by ozonolysis followed by elimination. ... 

Dioxepanes 63 were hydrolyzed with aqueous hydrochloric acid to the starting diol. A thionyl chloride promoted ring-opening of dioxepane 63 to intermediate 64 has been reported. When treated with base, compound 64 can be transformed into vinylic ether 65 in 58% yield (81ZOR1047) (Scheme 31). 3-Methylfurazan-4-acetic acid was converted to the vinyl derivative 66 via an esterification, reduction, mesylation, and base elimination sequence (81JHC1247) (Scheme 31). 

The emulsion blocking mechanism involves formation of emulsion in the pores either by self-emulsification of water-based filtrate with the crude oil, or oil filtrate from an oil-based fluid emulsifying formation water. The emulsions are viscous and can block the pores. The remedial design is to prevent emulsification either by eliminating oil from completion fluid or by the use of demulsifiers. 

We said at the beginning of this chapter that two kinds of reactions can happen when a nucleophile/Lewis base reacts with an alkyl halide. The nucleophile can either substitute for the halide by reaction at carbon or cause elimination of HX by reaction at a neighboring hydrogen ... 

Bromination of 43 (R = H) also gave a tetrabromo product initially, but base treatment induced elimination with the formation of the 4,7-dibromo derivative (70RCR923), as well as about 20% of the 4,6- and a trace of the 4,5-isomers (70JHC629). With iron as catalyst, one molar proportion ofbromineinameltof43(R = H) resulted in almost exclusive formation of... 

Julia and Paris120 described an olefin synthesis, based on the use of a sulphonyl group which directs the formation of a carbon-carbon bond. Subsequent reductive elimination with sodium amalgam leads to the alkene, as outlined in equation (50). The reaction sequence is similar in principle to an olefin synthesis first developed by Cornforth121. The yields of all steps are generally above 80%. 

Anionic metal bases react with group-IIIB halides by eliminating halide anions, forming neutral, c-bonded IIIB-metal compounds ( 6.5.2.2) ... 

A second reason for the larger isotope effect observed by Jones and Maness (140) might be that in the less polar acetic acid solvent, there might be a small degree of E2 elimination (with solvent acting as base) superimposed on the dominant Sn 1 mechanism. Such an elimination would involve a primary kinetic deuterium isotope effect with a kn/ko s 2 to 6, and hence even a 1 to 5% contribution from such a pathway would have a significant effect on the experimentally observed kinetic isotope effect. 

Enol (9) comes from available dkione (10) and the synthesis was performed on the enol acetate (11) rather than enol (9). Elimination with strong base gave (6) and the synthesis was completed by Wolf-Kishner removal of the carbonyl group. 

It is particularly effective to pair structure-based design with array synthesis. A relatively larger virtual library of possible analogs can be docked into the structure and the analogs prioritized for synthesis or eliminated if scored low. 

As the vast majority of LC separations are carried out by means of gradient-elution RPLC, solvent-elimination RPLC-FUR interfaces suitable for the elimination of aqueous eluent contents are of considerable use. RPLC-FTTR systems based on TSP, PB and ultrasonic nebulisa-tion can handle relatively high flows of aqueous eluents (0.3-1 ml.min 1) and allow the use of conventional-size LC. However, due to diffuse spray characteristics and poor efficiency of analyte transfer to the substrate, their applicability is limited, with moderate (100 ng) to unfavourable (l-10pg) identification limits (mass injected). Better results (0.5-5 ng injected) are obtained with pneumatic and electrospray nebulisers, especially in combination with ZnSe substrates. Pneumatic LC-FI1R interfaces combine rapid solvent elimination with a relatively narrow spray. This allows deposition of analytes in narrow spots, so that FUR transmission microscopy achieves mass sensitivities in the low- or even sub-ng range. The flow-rates that can be handled directly by these systems are 2-50 pLmin-1, which means that micro- or narrow-bore LC (i.d. 0.2-1 mm) has to be applied. 

A variety of cleavage conditions have been reported for the release of amines from a solid support. Triazene linker 52 prepared from Merrifield resin in three steps was used for the solid-phase synthesis of aliphatic amines (Scheme 22) [61]. The triazenes were stable to basic conditions and the amino products were released in high yields upon treatment with mild acids. Alternatively, base labile linker 53 synthesized from a-bromo-p-toluic acid in two steps was used to anchor amino functions (Scheme 23) [62]. Cleavage was accomplished by oxidation of the thioether to the sulfone with m-chloroperbenzoic acid followed by 13-elimination with a 10% solution of NH4OH in 2,2,2-trifluoroethanol. A linker based on l-(4,4 -dimethyl-2,6-dioxocyclohexylidene)ethyl (Dde) primary amine protecting group was developed for attaching amino functions (Scheme 24) [65]. Linker 54 was stable to both acidic and basic conditions and the final products were cleaved from the resin by treatment with hydrazine or transamination with ra-propylamine. 

Attempts to initiate formation of a nitrene, and its rearrangement to the iminooxo-phosphorane 80, by subjecting l-chloroamino-2,2,3,4,4-pentamethylphosphetane 1-oxide to a-elimination with sodium methoxide proved unsuccessful48). In contrast, however, the phosphorylhydroxylamides 88 rearrange in the presence of tert-butyl-amine to the heterocumulene 89 and then add base to give the phosphonic diamides 90 (>90%)49). The reaction is reminiscent of the well-known Lossen degradation. 

The proportion of Hofmann elimination is also found to increase with increasing branching in the alkyl group of the substrate (constant Y and base), and with increasing branching in the base, e.g. with (43), a bromide where preferential Saytzev elimination would normally be expected ... 

It should however, be emphasised that in protic solvents, with the common bases, and with substrates containing fi-H atoms 1,1-elimination occurs to only a small extent if at all. 

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