Addition reactions isomerization

Mcthixls of preparation of nitroalkcncs Recent reactions of formatirin of nitroalkcncs C hcmical properties of nitroalkancs Addition reactions Isomerization... 

The common types of chemical reactions found in living cells are nucleophilic substitution, elimination reactions, addition reactions, isomerization reactions, and oxidadon-reduction reacdons. 21. In addition to being an important energy source, carbohydrates are important structural molecules in organisms and have a role in intracellular and intercellular communication. 

Methi>ds of prcparaiion of nilroalkenes Recent reactions of formation of nilroalkenes C hemical properties of nitroalkanes Addition reactions Isomerization... 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

Reactions of the double bonds include isomerization and conjugation, cyclization, various addition reactions including hydrogenation, pyrolytic and... 

Addition reactions with tetracyanoethane have provided access to 2,5-diamino-3,4-dicyano-thiophene and -selenophene (58JA2775,81ZOR1958). Base catalyzed rearrangement gives the isomeric pyrrolethiol (Scheme 53). 

Because the integrity of the dihydrothiazine ring and its C-4 carboxyl substituent is crucial to useful antimicrobial activity, reactions involving this part of the cephalosporin molecule are usually undesirable. The possibilities for sulfur oxidation or alkylation, substitution at C-2 which is adjacent to both sulfur and a double bond, double bond isomerization and addition reactions, and the influence of a free carboxylic acid must all be considered in designing reactions to selectively modify other cephalosporin functionalities. 

Heating the 9-methyl-9aH-quinolizine (83) with 2-methylpyridine in methanol causes isomerization to the corresponding 9-methyl-4H-quinolizine (84), but with 3-methylpyridine the isomeric 7-methyl-4H-quinolizine (86) is obtained. Similarly the 9-methyl compound (83) and the corresponding 7,9-dimethyl derivative (82) with pyridine yield tetramethyl 47f-quinolizine-l,2,3,4-tetracarboxylate (78) with loss of the original alkylated pyridine. The mechanism of these reactions has not been established but the addition-elimination isomerization sequence for 3-methylpyridine accounts for the known cases of exchange of the pyridine (see Scheme 1). 

In alkaline solution, the phenol 1 is deprotonated to the phenolate 4, which reacts at the ort/zo-position with dichlorocarbene 3. The initial addition reaction product 5 isomerizes to the aromatic o-dichloromethyl phenolate 6, which under the reaction conditions is hydrolyzed to the o-formyl phenolate." ... 

The industrial reactions involving cis- and trans-2-butene are the same and produce the same products. There are also addition reactions where both 1-butene and 2-butene give the same product. For this reason, it is economically feasible to isomerize 1-butene to 2-butene (cis and trans) and then separate the mixture. The isomerization reaction yields two streams, one of 2-butene and the other of isobutene, which are separated by fractional distillation, each with a purity of 80-90%. Table 2-3 shows the boiling points of the different butene isomers. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Glycolysis is a ten-step process that begins with isomerization of glucose from its cyclic hemiacetal form to its open-chain aldehyde form—a reverse nucleophilic addition reaction. The aldehyde then undergoes tautomerixa-tion to yield an enol, which undergoes yet another tautomerization to give the ketone fructose. 

Scheme 5-14 may be called a two-dimensional system of reactions, in contrast to Scheme 5-1 which consists of a one-dimensional sequence of two acid-base equilibria. In Scheme 5-14 the (Z/E) configurational isomerism is added to the acid-base reactions as a second dimension . The real situation, however, is yet more complex, as the TV-nitrosoamines may be involved as constitutional isomers of the diazohydroxide. In order not to make Scheme 5-14 too complex the nitrosoamines are not included, but are shown instead in Scheme 5-15. The latter also includes the addition reactions of the (Z)- and ( )-diazoates (5.4 and 5.5) to the diazonium ion to form the (Z,Z)-, (Z,E)- and (2 2i)-diazoanhydrides (5.6, 5.7 and 5.8) as well as proto-de-nitrosation reactions (steps 10, 11 and 12). This pathway corresponds to the reverse reaction of diazotization, as amine and nitrosating reagent (nitrosyl ion) are formed in this reaction sequence.

When the substituent groups in the polyphosphazenes were azobenzene [719] or spiropyran [720] derivatives, photochromic polymers were obtained, showing reversible light-induced trans-cis isomerization or merocyanine formation, respectively. Only photocrosslinking processes by [2+2] photo-addition reactions to cyclobutane rings could be observed when the substituent groups on the phosphazene backbone were 4-hydroxycinnamates [721-723] or 4-hydroxychalcones [722-724]. 

Bis(imino)pyridine iron complex 5 acts as a catalyst not only for hydrogenation (see 2.1) but also for hydrosilylation of multiple bonds [27]. The results are summarized in Table 10. The reaction rate for hydrosilylations is slower than that for the corresponding hydrogenation however, the trend of reaction rates is similar in each reaction. In case of tra s-2-hexene, the terminal addition product hexyl (phenyl)silane was obtained predominantly. This result clearly shows that an isomerization reaction takes place and the subsequent hydrosilylation reaction dehvers the corresponding product. Reaction of 1-hexene with H2SiPh2 also produced the hydrosilylated product in this system (eq. 1 in Scheme 18). However, the reaction rate for H2SiPh2 was slower than that for H3SiPh. In addition, reaction of diphenylacetylene as an atkyne with phenylsilane afforded the monoaddition product due to steric repulsion (eq. 2 in Scheme 18). 

Photolysis of complex (189), R = mesityl, causes geometric isomerization to (190), with the structure (190) established by single-crystal diffraction studies.354 Reaction of Ph2P(CH2)2Si(Me)2-Si(Me)2(CH2)2PPh2 with t/Y// ,v-[Ir(PPh3)2(CO)Cl] yields complex (191) via an oxidative addition reaction to the Si-Si linkage.355... 

When the reactions of alkane molecules larger than the butanes or neopentane are studied, and in particular when the molecule is large enough to form a Cs or a Ce ring, the complexity of the reaction pathway is considerably increased and an important feature is the occurrence, in addition to isomerization product, of important amounts of cyclic reaction products, particularly methylcyclopentane, formed by dehydrocycliza-tion this suggests the existence of adsorbed cyclic species. The question is whether the reaction paths for dehydrocyclization and isomerization are related. There is convincing evidence that they are. Skeletal interconversions involving n-hexane, 2- and 3-methylpentane may be represented. 

A diverse group of organic reactions catalyzed by montmorillonite has been described and some reviews on this subject have been published.19 Examples of those transformations include addition reactions, such as Michael addition of thiols to y./bunsatu rated carbonyl compounds 20 electrophilic aromatic substitutions,19c nucleophilic substitution of alcohols,21 acetal synthesis196 22 and deprotection,23 cyclizations,19b c isomerizations, and rearrangements.196 24... 

Reactions a and b in Scheme 8 represent different ways of coordination of butadiene on the nickel atom to form the transoid complex 27a or the cisoid complex 27b. The hydride addition reaction resulted in the formation of either the syn-7r-crotyl intermediate (28a), which eventually forms the trans isomer, or the anti-7r-crotyl intermediate (28b), which will lead to the formation of the cis isomer. Because 28a is thermodynamically more favorable than 28b according to Tolman (40) (equilibrium anti/syn ratio = 1 19), isomerization of the latter to the former can take place (reaction c). Thus, the trans/cis ratio of 1,4-hexadiene formed is determined by (i) the ratio of 28a to 28b and (ii) the extent of isomerization c before addition of ethylene to 28b, i.e., reaction d. The isomerization reaction can affect the trans/cis ratio only when the insertion reaction d is slower than the isomerization reaction c. 

Alkylperoxyl radicals produced by addition reactions can be destructively isomerized with the formation of epoxides [13,139,154,155] ... 

Rhodium species in oxidation states I and III are involved in the process. Rhodium-catalyzed hydrogenations generally involve oxidative addition reactions, followed by the reverse process of reductive elimination in the final step. Another common elimination process is the so-called (l-elimination, which accounts for the frequent side reaction of isomerization of alkenes, according to Eq. (1) ... 

Several other types of photochemical reactions involving unsaturated carbohydrates have been reported. One of these is38 photochemical, E -Z isomerization of the groups attached to a double bond (see Scheme 5). A second is the internal cycloaddition between two double bonds connected by a carbohydrate chain.39-41 Although the carbohydrate portion of the molecule is not directly involved in this cycloaddition, its presence induces optical activity in the cyclobutane derivatives produced photochemically. Finally, a group of acid-catalyzed addition-reactions has been observed for which the catalyst appears to arise from photochemical decomposition of a noncarbohydrate reactant.42-44... 

Cationic complexes are key intermediates in a great variety of organic transformations such as isomerizations, rearrangements, addition reactions, aromatic substitutions, polymerization and others. Long-lived cationic complexes are important structural models for these intermediates. Studies of such complexes by modem physical methods provide valuable insight regarding their structure and reactivity. 

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

In general, sulfur-substituted allenes are accessible starting from alkyne precursors by a variety of transformations such as isomerization, rearrangement or addition reactions. The standard method for the synthesis of donor-substituted allenes is again the base-induced isomerization of alkynes. This very first method was applied for the preparation of achiral [11, 162, 163] and chiral [164] S-functionalized 1,2-dienes (Scheme 8.78). 

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