And formation of imines

Also other reaction types have been dealt with in CHEC(1984) and CHEC-II(19%) like reduction to alcohols (e.g., sodium borohydride), Wolff Kishner reduction, nucleophilic addition via reaction with Grignard reagents or organo-lithium compounds, and formation of imine type functional groups (e.g., hydrazones). New examples are the reaction of... 

The only other commonly reported technique is the acid-catalysed evolution of nitrogen gas. Typically, sulphuric acid is used as the catalyst . This is not totally reliable, however. Coombsreports that if carbon-nitrogen cleavage will yield a stable carbonium ion, then formation of HN3 predominates over loss of N2 and formation of imine. This is contradicted, however, by the report that i-butyl azide in the presence of sulphuric acid yields acetone-A-methylimine which subsequently hydrolyses to give acetone, isolated as its 2,4-dinitrophenylhydrazone. 

A number of anomeric D-hex-2-ulopyranosyl azides have been synthesized and their photochemistry examined. For both a- and P-azides, the major photoproducts arise from cleavage of the C-2C-3 bond and migration of the C-3 carbon to the nitrene centre. Decomposition reactions of a glycidyl azide polymer have been induced by pulsed laser infrared pyrolysis and UV photolysis of thin films at 17-77 K and monitored by IR spectroscopy. The initial step is elimination of N2 and formation of imines, which decompose on warming, possibly with secondary polymerization. 

Carbinolamines are formed by nucleophilic addition of an amine to a carbonyl group and are intermediates in the formation of imines and enamines. 

A related enamine alkylation is seen in the rearrangement of an ethylene imine vinylogous amide, which was heated with sodium iodide in diglyme. The presumed internal enamine alkylation constitutes a critical step in an oxocrinane synthesis (265). Use of an ethylene imine urethane for alkylation of an enamine and formation of the hexahydroindole system has also been reported (266). 

Apparently, aminobutenyne A, the intermediate of the pyrrole synthesis, is fixed in an advantageous eonfiguration by eoordination to the Cu eation, whereas the absenee of eatalyst may result in the formation of imine B having an aetive methylene group whieh attaeks the aeetylene bond to form dihydropyridine C and then pyridine 2 (by dehydrogenation). 

Formation of C—Nu The second mode of nucleophilic addition, which often occurs with amine nucleophiles, involves elimination of oxygen and formation of a C=Nu bond. For example, aldehydes and ketones react with primary amines, RNH2, to form imines, R2C=NR. These reactions proceed through exactly the same kind of tetrahedral intermediate as that formed during hydride reduction and Grignard reaction, but the initially formed alkoxide ion is not isolated. Instead, it is protonated and then loses water to form an imine, as shown in Figure 3. 

Scheme 4.15 Carbodiimidization of isocyanates and formation of uretone imines.

The head-to-tail-coupling reactions described above are potentially useful in the design of dynamic combinatorial libraries. Features of these reactions include the rapid and reversible formation of carbon-carbon bonds, multifunctional ene-imine building blocks, and formation of stereo centers upon ene-imine linkage. Support for template-directed synthesis utilizing ene-imine building blocks is the formation of a poly ene-imine species that could recognize 3 -GGA-5 sequences of DNA.48 It is noteworthy that some polyene-imines are helical and could form a triple helix with DNA. 

Amines can react with various carbonyl compounds and their derivatives in aqueous media to give the corresponding imine derivatives. These reactions have been discussed in related chapters. The synthetically most useful reaction of this type is the formation of imines and imine derivatives from the condensation of amines with aldehydes and ketones. Water is an excellent solvent for such condensation reactions. For example, water was found to be an ideal solvent for a high-yield, fast preparation of easily hydrolyzable 2-pyrrolecarbaldimines.23 In the presence of Cu2+, the reaction afforded the corresponding Cu(II) chelates (Eq. 11.19). 

TiCl4 also effectively promotes formation of imines and enamines from carbonyl compounds (Scheme 31). The combination of imine formation using TiCl4 and reduction leads to reductive alkylation of an amine moiety.113,114... 

Catalytic amounts of I fCl4-AgC104 and Hf(OTf)4 are used for activation of acid halides and acid anhydrides for Friedel -Crafts acylation (Scheme 42) 178 the reactions of both reactive and unreactive aromatic substrates proceed smoothly in the presence of Hf(OTf)4. Furthermore, the Fries rearrangement179,180 and direct C-acylation of phenolic compounds181,182 take place using Hf(OTf)4. Formation of esters and Mannich-type reactions and allylation of imines have been also reported.152... 

Similarly, imines can be formed by the condensation of ketones and suitable primary amines under microwave conditions (Scheme 6.148). Since the formation of imines is an equilibrium process, these transformations are best carried out under... 

Stable 3,5-dichloro-2,4,6-trimethylbenzonitrile oxide reacts with 2- and 4-aminopyridines in their imino forms, if acids are present to promote the formation of imine, to give cycloadducts such as 168 (337). 

Oxidation of primary amines with DMD or other oxidants leads to the formation of a complex mixture of nitroso, oximes, and nitro compounds (76). Utilization of DMD in acetone affords dimethyl nitrone (22). This is likely to be a result of the initial oxidation of primary amine (19) to hydroxylamine (20) with the subsequent condensation of acetone and oxidation of imine (21) (Scheme 2.9) (77). 

One of the possible ways to stabilize the amine-halonium complexes is to increase the basicity of the amine, bearing in mind that an appropriate one must also not have easily removable P-hydrogens which will lead to oxidation of the amine and formation of an imine. Quinuclidine (pKa of quinuclidinium ion is 11.3 (55)) is 105-106-fold more basic than the pyridines and both the bromonium (10 (36)) and iodonium (11 (57)) BF4 salts have been made and characterized by X-ray crystallography. Interestingly, although the reaction must generally occur as outlined in Figure 7, neither of these ions shows any observable reaction... 

If the hydroformylation of olefins is conducted in the presence of aromatic hydrazines and Bronsted or Lewis acids indoles can be obtained directly in one pot [91-93,95]. Hydroformylation of the olefin gives an intermediate aldehyde, which is trapped immediately by the present aromatic hydrazine as an aromatic hydrazones similar to the formation of imines under hydroformylation conditions. Under acid mediation these aromatic hydrazones undergo a Fischer indolization, consisting of a [3,3]-sigmatropic rearrangement followed by a cyclization and elimination of ammonia (Scheme 38). 

The reductive amination reaction (Scheme 12.10) has two steps the formation of imine and the reduction of imine. The single bead FTIR results (Fig. 12.14) show the combined result of both steps. The aldehyde IR bands at 1682 and 2769 cm disappeared in the final product. However, this did not suggest that the... 

Scheme 6.104 Key intermediates of the proposed catalytic cycle for the 100-catalyzed Michael addition of a,a-disubstituted aldehydes to aliphatic and aromatic nitroalkenes Formation of imine (A) and F-enamine (B), double hydrogen-bonding activation of the nitroalkene and nucleophilic enamine attack (C), zwitterionic structure (D), product-forming proton transfer, and hydrolysis.

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