Intermediates, unsymmetrical

Similar results were obtained when o-phenylenediamine was reacted with mb. A range of low boiling compounds were separated from the reaction mixture after 10-12 hr heating at 220°C. These have been identified as diazadimethylsilaindane Vb (m.p. 76-78°C lit. (321 m.p. 75-83°C) and bis(phenylamino)dimethylsilane VIb (m.p. 46-48°C lit. (331 m.p, 45 2°C). They are probably formed by thermal decomposition of the intermediate unsymmetrical disilazane IVb". At 190-225°C/0.3 torr, a crystalline fraction with... 

Friedel-Crafts acylation of t-butanol (Scheme 6) followed by treatment with ammonia gives a mixture of 2,7-naphthyridines (379 R = Me, Et, Pr", Pr, Bu) and pyridines (380) (38-91% yield) <84BSF449>. Heterocycles (379) are the major products in these reactions and pyrylium salts have been proposed as reaction intermediates. Unsymmetrical naphthyridines (382) have been obtained as the major products of the reaction of alkenones (381) under similar conditions (Scheme 6) <84BSF454>. 

The first step of the reaction is the formation of the intermediate species E (Scheme 7) containing a single iridium-iridium bond. The migration of the methylene group leads to the formation of the final Ir111 111 symmetric complex. The intermediate unsymmetrical species E was isolated in the reaction of CH2I2 with 115. 

In 1968, detailed kinetic and stereochemical research showed Goering that while 2-bicyclo[2,2,2]octyl esters and 2-exo-bicyclo[3,2,l]octyl esters form an intermediate unsymmetrical nonclassical ion 158 maintaining the optical activity and retaining... 

Unsymmetrically substituted dipyrromethanes are obtained from n-unsubstitued pyrroles and fl(-(bromomethyl)pyiToIes in hot acetic acid within a few minutes. These reaction conditions are relatively mild and the o-unsubstituted pyrrole may even bear an electron withdrawing carboxylic ester function. It is still sufficiently nucleophilic to substitute bromine or acetoxy groups on an a-pyrrolic methyl group. Hetero atoms in this position are extremely reactive leaving groups since the a-pyrrolylmethenium( = azafulvenium ) cation formed as an intermediate is highly resonance-stabilized. 

With the catalysis of strong Lewis acids, such as tin(IV) chloride, dipyrromethenes may aiso be alkylated. A very successful porphyrin synthesis involves 5-bromo-S -bromomethyl and 5 -unsubstituted 5-methyl-dipyrromethenes. In the first alkylation step a tetrapyrrolic intermediate is formed which cyclizes to produce the porphyrin in DMSO in the presence of pyridine. This reaction sequence is useful for the synthesis of completely unsymmetrical porphyrins (K.M. Smith, 1975). 

The aryl- and heteroarylfluorosilanes 541 can be used for the preparation of the unsymmetrical ketones 542[400], Carbonylation of aryl triflate with the siloxycyclopropane 543 affords the 7-keto ester 545. In this reaction, transme-tallation of the siloxycyclopropane 543 with acylpalladium and ring opening generate Pd homoenolate as an intermediate 544 without undergoing elimination of/3-hydrogen[401],... 

The issue of regioselectivity arises with arylhydrazones of unsymmetrical ketones which can form two different enehydrazine intermediates. Under the conditions used most commonly for Fischer cyclizations, e g. ethanolic HCI, the major product is usually the one arising from the more highly substituted enehydrazine. Thus methyl ketones usually give 2-methy indoles and cycliz-ation occurs in a branched chain in preference to a straight chain. This regioselectivity is attributed to the greater stability of the more substituted enhydrazine and its dominance of the reaction path. 

Let s compare the carbocation intermediates for addition of a hydrogen halide (HX) to an unsymmetrical alkene of the type RCH=CH2 (a) according to Markovnikov s rule and (b) opposite to Markovnikov s rule (a) Addition according to Markovnikov s rule... 

The organoborate intermediates can also be generated from alkenylboronic esters and alkyllithium or Grignard reagents, or from ttialkylboranes and alkenyllithium compounds. Conjugated symmetrical and unsymmetrical diynes (289—291), stereochemically pure 1,3-dienes (292,293), and 1,3-enynes (294) including functionali2ed systems can be prepared (289,295). 

BenZotrichloride Method. The central carbon atom of the dye is supphed by the trichloromethyl group from iJ-chlorobenzotrichloride. Both symmetrical and unsymmetrical triphenyhnethane dyes suitable for acryhc fibers are prepared by this method. 4-Chlorobenzotrichloride is condensed with excess chlorobenzene in the presence of a Lewis acid such as aluminium chloride to produce the intermediate aluminium chloride complex of 4,4, 4"-trichlorotriphenylmethyl chloride (18). Stepwise nucleophihc substitution of the chlorine atoms of this intermediate is achieved by successive reactions with different arylamines to give both symmetrical (51) and unsymmetrical dyes (52), eg, N-(2-chlorophenyl)-4-[(4-chlorophenyl) [4-[(3-methylphenyl)imino]-2,5-cyclohexadien-l-yhdene]methyl]benzenaminemonohydrochloride [85356-86-1J (19) from. w-toluidine and o-chloroaniline. 

Mechanisms in Chlorine Dioxide Generation from Chlorite. The reactions between sodium chlorite and chloriue-based oxidizers and acids are complex and involve the formation of the proposed unsymmetrical intermediate, [CI2O2] (16,18,22,36,108,135—140). 

This mechanism explains the observed formation of the more highly substituted alcohol from unsymmetrical alkenes (Markownikoff s rule). A number of other points must be considered in order to provide a more complete picture of the mechanism. Is the protonation step reversible Is there a discrete carbocation intermediate, or does the nucleophile become involved before proton transfer is complete Can other reactions of the carbocation, such as rearrangement, compete with capture by water ... 

A recent paper by Singh et al. summarized the mechanism of the pyrazole formation via the Knorr reaction between diketones and monosubstituted hydrazines. The diketone is in equilibrium with its enolate forms 28a and 28b and NMR studies have shown the carbonyl group to react faster than its enolate forms.Computational studies were done to show that the product distribution ratio depended on the rates of dehydration of the 3,5-dihydroxy pyrazolidine intermediates of the two isomeric pathways for an unsymmetrical diketone 28. The affect of the hydrazine substituent R on the dehydration of the dihydroxy intermediates 19 and 22 was studied using semi-empirical calculations. ... 

Nucleophilic addition of the base to the intermediate 2 leads to ring opening. With a symmetrically substituted cyclopropanone, cleavage of either Ca-CO bond leads to the same product. With unsymmetrical cyclopropanones, that bond is broken preferentially that leads to the more stable carbanion 5 ... 

U Electrophilic addition to an unsymmetrically substituted alkene gives the more highly substituted carbocation intermediate. A more highly substituted carbocation forms faster than a less highly substituted one and, once formed, rapidly goes on to give the final product. 

Aikene chemistry is dominated by electrophilic addition reactions. When HX reacts with an unsymmetrically substituted aikene, Markovnikov s rule predicts that the H will add to the carbon having fewer alky) substituents and the X group will add to the carbon having more alkyl substituents. Electrophilic additions to alkenes take place through carbocation intermediates formed by reaction of the nucleophilic aikene tt bond with electrophilic H+. Carbocation stability follows the order... 

When an unsymmetrically substituted vinyl monomer such as propylene or styrene is polymerized, the radical addition steps can take place at either end of the double bond to yield either a primary radical intermediate (RCH2-) or a secondary radical (R2CH-). Just as in electrophilic addition reactions, however, we find that only the more highly substituted, secondary radical is formed. 

In addition to its effect on stability, delocalization of the unpaired electron in the allyl radical has other chemical consequences. Because the unpaired electron is delocalized over both ends of the nr orbital system, reaction with Br2 can occur at either end. As a result, allylic bromination of an unsymmetrical alkene often leads to a mixture of products. For example, bromination of 1-octene gives a mixture of 3-bromo-l-octene and l-bromo-2-octene. The two products are not formed in equal amounts, however, because the intermediate allylic radical is... 

Arylisoxazol-5(4//)-oncs 21 react with benzene-1,2-diamines to yield 4-aryl-l,5-benzodiaze-pinones 22 by elimination of hydroxylamine from the intermediate oximes. Unsymmetrically substituted benzene-1,2-diamines are attacked at the more nucleophilic amino group. Thus, 4-methylbenzene-1,2-diamine gives 7-methylbenzodiazepinones 22f-h, whereas 4-nitrobenzene-1,2-diamine gives 8-nitro compounds 22k-n. The benzodiazepinones are accompanied by minor amounts of 2-methylbenzimidazoles 23. Selected examples are given.275... 

The regio- and diastereoselectivity in carbonyl addition of unsymmetrically di- or higher substituted allyltitanium intermediates depend largely on the type of attached groups. 

When m-RuCl2(DMSO)4 is stirred in methanol containing traces of water (to catalyse the formation of intermediate aqua species) Ru2C14(DMSO)5 isformed this has the unsymmetrical structure (DMSO)2C1RuQl(-C1)3Ru(DMSO)3 based on face-sharing octahedra [110]. 

Benzyl radicals and a- and 3- substituted derivatives also undergo unsymmetrical coupling through the aromatic ring (Section 2.5). The formation of the (i-o and a—p coupling products is reversible. Consequently, these materials are often only observed as transient intermediates. 

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