Imidates addition reactions

The fluorination of P-diketones and p-ketoesters with N-/luorobis(trifluo-romethanesulfonyl)imide (Table 3a, B) can be controlled to give either mono-fluorination or difluorination. Monofluorination occurs when the strong acid, bis(trifluoromethanesulfonyl)imide, a reaction product, is removed by addition of water, which prevents further enolization and fluormation of the monofluoro adduct [83] (equation 38)... 

Besides addition reactions, azides or hydrazoic acid can also yield tetrazoles through displacement reactions. Thus, halide displacement in imide chloride (78) yields 1,5-disubstituted tetrazoles (79), and in 2-chloro-pyridine (80), yields tetrazolopyridine (81) (Eq. 16a,b).141 143 Vinylogous... 

From the results of the 1,3-diene addition reaction, the metal-catalyzed reaction of unactivated alkenes was examined, and it was found that the palladium complex effectively catalyzed the a rt-Markovnikov addition of triarylphosphines and bis(trifluoromethanesulfonyl)imide (Tf2NH).24... 

Crosslinking of amine- or hydroxy-terminated PAMAM dendrimers using cyclic anhydride - amine or cyclic anhydride - hydroxy addition reactions was employed for preparation of crosslinked thin films of very low permeability [73], Polyanhydrides, such as maleic anhydride-methyl vinyl ether copolymers, were used as crosslinking components. In the case of amine-terminated PAMAM, crosslinking and chemical stability were further increased by imidization of the maleamic acid groups retro-Michael eliminations were followed by Michael additions to further crosslink the film. 

The main intermediate of the rearrangement may be a nitrilium ion (225) in some cases or an imidate (226) in others. The resulting intermediate reacts with water to produce the amide (218) after tautomerization. If other nucleophiles (Nu ) are present, they can intercept the reactive intermediates (both inter- or intra-molecularly) and several different imino-substituted derivatives (227) can be formed. These rearrangement-addition reactions will be analysed later in this chapter as they can effectively broaden the scope of the Beckmann rearrangement reaction (Sections VI.D.2 and VI.E.2). 

The preparation of imides from reaction of isocyanates with anhydrides dates back to the early days of organic chemistry [96]. With the advent of polyimide chemistry in the early 1960s, this chemistry was soon explored for the synthesis of polyimides. However, in contrast to the preparation of polyimides via thejr poly(amic acid) intermediate, the reaction of aromatic dianhydrides with aromatic diisocyanates is much less understood. The reaction of aromatic dianhydrides with aliphatic or aromatic diisocyanates is believed to form a cyclic seven-membered intermediate which then splits out C02 to form the polyimide [97], see Scheme 27. The addition of water, which has been reported to accelerate the anydride/isocyanate reaction, can result in several transformations of either the anhydride or the isocyanate reagent, see Scheme 28... 

The Michael addition reaction of dimercaptodiphenylether with N-(3-ethynyl phenyl) maleimide allowed the synthesis of ethynyl-terminated imido-thioether as shown in Fig. 50 (139). This acetylene terminated imidothioether was blended with acetylene terminated polyarylene ether oligomers of different molecular weights and tested as composite resins (140). Blends of functionalized thermoplastics such as the acetylene terminated polyarylene ethers with brittle high-Tg imide resins are finding increased attention for tough high-Tg composites. 

Imides. Although the photochemistry of imides is quite diverse (203), there exists solid reason to believe that electron transfer can control the reactivity of some imide/olefin pairs. For example, addition reactions analogous to those discussed above for iminium salts can also be observed for imides, either in an intermolecular sense, eq. 67 (204),... 

As electron-rich olefins are more reactive, vinyl-sulfones are the most reactive species and are capable of reacting with thiols, amines, and even with small nucleophilic alcohol groups. Less reactive are acrylamides and acrylates, which are reactive towards amines and thiols. Maleimides are the least reactive of the mentioned species and allow selective addition of thiols in the presence of amines in the pH range 6.5-7.5. However, hydrolysis of the imide, especially at elevated pH values [35], may be a concern for certain applications. The mentioned Michael addition reactions do not require organic solvents and can be carried out at physiological temperature and pH [36], In acidic conditions, the reaction is either very slow or does not proceed because protonation removes the nucleophilic form in the case of amines, and the thiolate anion is usually the active species in Michael additions involving thiols [25],... 

The best results are obtained with trisyl azide, which again leads to high yields of the azide transfer product 2, especially if the enolate 1 is added to trisyl azide (see entries 1 and 2). Interestingly, the best chemoselectivity and, in addition, identical yields of azide (73%) result from the reaction of the lithium enolate with trisyl azide (entry 3). The reaction of the ester enolate 1 with trisyl azide is less sensitive to the nature of the enolate metal than is the corresponding imide enolate reaction (see Section 7.1.1.). Acetic acid quench, on the other hand, again proved to be useful. Unfortunately, bis-azidation to 3 and diazo transfer to 4 are also observed. 

A few 6- and 8-cyanopurines have been prepared and undergo characteristic nitrile addition reactions rather readily. Thus, alkaline hydrolysis produces carboxamides, then carboxylic acids, alcoholysis leads to imidates, ammonolysis to amidines, hydrazinolysis to amidhydrazines, hydroxylamine to amidoximes, and hydrogen sulfide to thioamides. Acid hydrolysis tends to give the decarboxylated acid derivative. Reduction either by sodium-ethanol or, preferably, by catalytic hydrogenation affords aminoalkylpurines and addition of Grignard reagents produces, in the first place, acylpurines. As with aldehydes, most of the compounds examined have been relatively non-polar derivatives. Table 28 lists some reactions and relevant literature. 

Chiral oxazolidinones have also been used to induce chirality in TiCU-mediated allylsilane addition reactions to a-keto imides (eq 14). ... 

Non-Evans Aldol Reactions. Either the syn- or onri-aldol adducts may be obtained from this family of imide-derived eno-lates, depending upon the specific conditions employed for the reaction. Although the illustrated boron enolate affords the illustrated jyn-aldol adduct in high diastereoselectivity, the addition reactions between this enolate and Lewis acid-coordinated aldehydes afford different stereochemical outcomes depending on the Lewis acid employed (eq 35). Open transition states have been proposed for the Diethylaluminum Chloride mediated, anti-selective reaction. These anfi-aldol reactions have been used in kinetic resolutions of 2-phenylthio aldehydes. ... 

Coixjugate Addition Reactions. a,3-Unsaturated N-acyloxazolidinones have been implemented as Michael acceptors, inducing chirality in the same sense as in enolate alkylation reactions. Chiral a,3-unsaturated imides undergo 1,4-addition when treated with diethylaluminum chloride (eq 55). Photochemical initiation is required for the analogous reaction with Dimethylaluminum Chloride. ... 

One of the most successful and widely used methods for diastereoselective aldol addition reactions employs Evans imides 17 and the derived dialkyl boryleno-lates [8J. The 1,2-svn aldol adducts are typically isolated in high diastereoisomeric purity (>250 1 dr) and useful yields. More recent investigations of Ti(IV) and Sn(II) enolates by Evans and others have considerably expanded the scope of the aldol process [9], In 1991, Heathcock documented that diverse stereochemical outcomes could be observed in the aldol process utilizing acyl oxazolidinone imides by variation of the Lewis acid in the reaction mixture [10]. Thus, for example, in contrast to the, l-syn adduct (21) isolated from traditional Evans aldol addition, the presence of excess TiCL yields the complementary non-Evans 1,2-syn aldol diastereomer. This and related observations employing other Lewis acids were suggested to arise from the operation of open transition-state structures wherein a second metal independently activates the aldehyde electrophile. 

A rhodium-mediated carbene addition has been employed as the key step in a synthesis of furans. The precursors were synthesized on TentaGel-NHi resin, which was transformed into an amide (135). Subsequent formation of imides 136 with malonic ethyl ester chloride and reaction with tosyl azide gave solid-phase-bound diazo imides 137. Reaction with Rh2(OAc)4 in the presence of electron-deficient alkynes produced substituted furans 139 via the intermediate isomiinch-none 138 through a sequence of a [2-i-3]-cycloaddition to the alkyne and subsequent cycloreversion. The yields of the reaction varied in the range 50-70% (Scheme 36) [52]. 

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