Reaction with Electrophiles

Reactions with other Electrophiles. Reaction with trifluoroacetic acid leads to the slow formation of a ring-opened product trifluoroacetic anhydride provides a his(trifluoroacetoxy)cyclopropene [110], Triethyloxonium tetrafluoroborate ethylates the oxygen atom, giving an ethoxycyclopropenium salt [110]  

Cyoloaddition Reactions, With a variety of conjugated dienes cyclo-propene undergoes [4+2] cycloaddition across its alkene bond [13, 109]. Some examples are as follows  

Oxidation, When diphenylcyclopropenone is oxidised by m-chloroper-benzoic acid a mixture of benzil, diphenylacetylene and benzophenone (50 40 10) is formed [114]. Di-t-butylcyclopropenone provides only a trace of alkyne and the main product is 4,5-eDoxy-2,2,4,5-tetra-methylhexan-3-one, which is thought to arise by further oxidation of alkyne [114]. Alkaline hydrogen peroxide oxidises diphenylcyclopropenone to the same products, plus benzylphenyl ketone [114]. 

Reduotion. Catalytic reduction of cyclopropenone commonly leads to reduction of both the double bond and the three-membered ring with formation of a dialkyl ketone  

Examples are provided by the catalytic reduction of diphenyl- [26], di-n-propyl- [88], di-t-butyl- [111] andn-pentyl- [100] cyclopropenones. However, when palladium replaced platinum as catalyst, the product from di-n-propylcyclopropenone was instead 2-propylhex-2-enal [88]. 

4-Dioxin and 1,4-dithiin both undergo easy electrophilic addition reactions, e.g. of halogens to the double bonds. Alcohols under acid catalysis form ketal addition products. 

4-Dithiins react readily at sulfur with peracids, alkyl halides and hydroxylamine 0-sulfonic acid to give sulfoxides, thiinium salts and sulfilimines, respectively. Similar reactions are known for 

Synthetic and mechanistic studies have been reported on the reaction of alkyl halides with the anionic allyl complex [Fe(CO)3(iy -C3H5)] (78). Treatment of (78) with alkyl halides in THF followed by addition of PPh3 gave a,j3-unsaturated 

1 uses immediate formation of the acyl complexes (81), which 

Homochiral niolybdenum ry -methylallyl complexes appear to have considerable potential in the asymmetric synthesis of homoallylic alcohols. For example, reaction of optically-active (5)-(82) with benzaldehyde gives (R)-3-methyl-l-phenyl-3-buten-l-ol in 98% ee [Eq. (28), NM = neomenthyl]. In contrast, (R)-(82) yields the opposite enantiomeric product. The mechanism presumably involves 

Protonations of (83) and (84) give agostic complexes. Exo addition to the cycloheptatriene ring of (85) was established from experiments in D2O using H and NMR for product identification. Similar exo addition of (D ) occurs 

The regioselectivity of electrophilic addition to the 3-oxaplatina(II)cyclo-butane complex (89) is very dependent on the nature of the electrophile. Treatment of (89) with the strong electrophile acetyl chloride leads to exclusive attack at the hard metallacycle oxygen [Eq. (29)], to give the ring-opened product 

In contrast to reactions with nucleophiles, only few reactions of Fischer-type carbene complexes with electrophiles are known. 

Reactions with electrophiles also give alkenylidenecarbene complexes  

Unlike Fischer-type compounds, alkylidene derivatives of niobium and tantalum have nucleophilic carbene carbon atoms. Therefore, they react with Lewis acids. 

3-Pyridazine triflates are readily prepared in high yield by reaction of triflic anhydride either with 3(2//)-pyridazinones in pyridine at room temperature or with the lithium salts in THE at — 78°C (Equation (26)). These triflates, which are white crystalline solids stable at 0°C, are useful in palladium catalysed coupling reactions 94H(38)1273 . 

The methylene carbon atom in a condensed 4-thiazolidinone flanked by a sulfur atom and a carbonyl group possesses enhanced nucleophilic activity and attacks an electrophilic center with ease. If the structure permits, the reaction product loses a molecule of water, and an unsaturated derivative is formed. The reaction is carried out in the presence of a base which abstracts a methylene proton. It is the anion thus formed that attacks the electrophilic center. Generally, the anion condenses with aromatic aldehydes, nitroso compounds, aryldiazonium salts, and ethyl orthoformate, as well as undergoing Vilsmeier-Haack and Mannich reactions. 

While 1,2-diazinones usually give N-alkylation, O-alkylation can be achieved by first transforming them into the trimethylsilyloxy-l,2-diazines or silver salts 1996CHEC-II(6)1 . This approach has been used by El Ashry to attach sugar moieties to the oxygen atom of phthalazin-l(2//)-one 2003CAR2291 . 

Although the reports of chemical reactivity of coordinated C02 ligand in well-characterized complexes are still very limited, several examples have been described showing that the oxygen of the r 2- or ri -bonded coordinated C02 can undergo an electrophilic attack by protons or other similar reagents. 

For example, the C02 complex of iron(O), Fe(C02)(depe)2, which has a trigonal bipyramidal geometry with a side-on bonded C02, reacts with electrophiles such 

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The pyridine-like nitrogen of the 2H-pyrrol-2-yiidene unit tends to withdraw electrons from the conjugated system and deactivates it in reactions with electrophiles. The add-catalyzed condensations described above for pyrroles and dipyrromethanes therefore do not occur with dipyrromethenes. Vilsmeier formylation, for example, is only successful with pyrroles and dipyrromethanes but not with dipyrromethenes. 

Section 12 1 On reaction with electrophilic reagents compounds that contain a ben zene ring undergo electrophilic aromatic substitution Table 12 1 m Section 12 1 and Table 12 3 m this summary give examples... 

Although less researched than the 2-position, modifications at the 6-position of intact penems have been reported. Generation of the dianion of the penem (52, R = CH ) using a strong base such as / -butyUithium or lithium diisopropylamide, followed by reaction with electrophiles yields 6-substituted 2-methylpenems in moderate yield (128). The enhanced acidity of the 6-proton in the bromopenem (88) [114409-16-4] h.a.s been exploited to prepare the... 

Carboxylic acid derivatives on pyridopyrimidine rings appear to undergo normal reactions with electrophilic reagents, e.g. the 6-amide (70) is dehydrated to the 6-nitrile with phosphorus oxychloride. 

The high reactivity of pyrroles to electrophiles is similar to that of arylamines and is a reflection of the mesomeric release of electrons from nitrogen to ring carbons. Reactions with electrophilic reagents may result in addition rather than substitution. Thus furan reacts with acetyl nitrate to give a 2,5-adduct (33) and in a similar fashion an adduct (34) is obtained from the reaction of ethyl vinyl ether with hydrogen bromide. 

In view of the overall increased reactivity of furan compared with thiophene it would be anticipated that furan would be less regioselective in its reactions with electrophiles than thiophene. Possible reasons for the high regioselectivity of furan in electrophilic substitution reactions include complex formation between substrates and reagents and the ability of heteroatoms to assist in the stabilization of cationic intermediates (80CHE1195). 

Methylthiophene is metallated in the 5-position whereas 3-methoxy-, 3-methylthio-, 3-carboxy- and 3-bromo-thiophenes are metallated in the 2-position (80TL5051). Lithiation of tricarbonyl(i7 -N-protected indole)chromium complexes occurs initially at C-2. If this position is trimethylsilylated, subsequent lithiation is at C-7 with minor amounts at C-4 (81CC1260). Tricarbonyl(Tj -l-triisopropylsilylindole)chromium(0) is selectively lithiated at C-4 by n-butyllithium-TMEDA. This offers an attractive intermediate for the preparation of 4-substituted indoles by reaction with electrophiles and deprotection by irradiation (82CC467). 

Reactions with electrophiles (except nitrous acid)... 

Mesomeric shifts of the types shown in structures (82) and (83) increase the electron density on the nitrogen atom and facilitate reaction with electrophilic reagents. However, the heteroatom Z also has an adverse inductive effect the pK, of NH2OH is 6.0 and that of N2H4 is 8.0, both considerably lower than that of NH3 which is 9.5. 

These compounds generally exist in carbonyl forms. The oxygen function can be converted into halogen by phosphorus halides. Reactions with electrophiles are quite complex. Thus urazole (511) reacts with diazomethane quickly to yield (512), which is more slowly converted into (513). 1-Phenylurazole gives (514) however, 4-phenylurazole yields (515). Oxadiazolinones of type (516) can be alkylated at both O- and N-atoms. 

Perfluoroacetylenic lithium compounds on reaction with electrophiles yield perfluoroacetylenic functional derivatives [78, 79] (equation 43)... 

The reactions of NSF3 have been investigated in considerable detail. They can be classified under the following categories (a) reactions with electrophiles (b) addition to the SN triple bond and (c) reactions with nucleophiles. Some examples of these different types of behaviour are discussed below. 

C. Reaction with Electrophiles Directed to Atoms OF THE 1,2,4-Triazine Ring... 

The highly 7r-deficient character of the 1,2,4-triazine ring increases the nucle-ophilicity of the methyl group in methyl-1,2,4-triazine A-oxides in reactions with electrophilic alkenes and aldehydes. Thus treatment of the 6-methyl-3-phenyl-1,2,4-triazine 4-oxide 113 with l-(dimethylamino)-l-ethoxyethylene leads to the... 

The reactivity of Ce, C7, Cg aromatics is mainly associated with the benzene ring. Aromatic compounds in general are liable for electrophilic substitution. Most of the chemicals produced directly from benzene are obtained from its reactions with electrophilic reagents. Benzene could be alkylated, nitrated, or chlorinated to important chemicals that are precursors for many commercial products. 

Heterocyclic amines are compounds that contain one or more nitrogen atoms as part of a ring. Saturated heterocyclic amines usually have the same chemistry as their open-chain analogs, but unsaturated heterocycles such as pyrrole, imidazole, pyridine, and pyrimidine are aromatic. All four are unusually stable, and all undergo aromatic substitution on reaction with electrophiles. Pyrrole is nonbasic because its nitrogen lone-pair electrons are part of the aromatic it system. Fused-ring heterocycles such as quinoline, isoquinoline, indole, and purine are also commonly found in biological molecules. 

Lone-pair electrons (Section 1.4) Nonbonding valence-shell electron pairs. Lone-pair electrons are used by nucleophiles in their reactions with electrophiles. 

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