Elimination reactions intramolecular

Note that for 4.42, in which no intramolecular base catalysis is possible, the elimination side reaction is not observed. This result supports the mechanism suggested in Scheme 4.13. Moreover, at pH 2, where both amine groups of 4.44 are protonated, UV-vis measurements indicate that the elimination reaction is significantly retarded as compared to neutral conditions, where protonation is less extensive. Interestingy, addition of copper(II)nitrate also suppresses the elimination reaction to a significant extent. Unfortunately, elimination is still faster than the Diels-Alder reaction on the internal double bond of 4.44. 

Mechanistically, the reaction of diketosulfides and glyoxal likely proceeds via an initial aldol reaction to provide 22. A second intramolecular aldol reaction and the elimination of two equivalents of water produce the thiophene 23. The timing of the elimination reactions and the ring-closing, carbonyl condensation reaction is not completely understood. However, 2,5-disubstituted thiophenes 23 are available in good yields via this process. 

Another helpful test is the so-called crossover experiment. Thus, a mixture of di-Pd(CH3) (PR3)2 and cA-Pd(CD3)2(PR.3)2 yields only C2D6 and C2H6 C2H3D3 is absent.20 This result shows that the elimination reaction is intramolecular. 

Dialkyl esters of cystine (39) and lanthionine (40) undergo a surprising thermolysis reaction at between 25 C and 80 °C to afford cis and trans methyl 2-methylthiazolidine-2,4-dicarboxylates (43) in protic solvents. A two stage process is proposed for this transformation. An initial i-elimination reaction gives the thiol (41) and the enamine (42). Thiol addition to the imine tautomer of (42) is then followed by loss of ammonia and an intramolecular cyclisation to give (43) <96CC843>. 

Another important family of elimination reactions has as its common mechanistic feature cyclic TSs in which an intramolecular hydrogen transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.20 depicts examples of these reaction types. These are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic TS dictates that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are often referred to as thermal syn eliminations. 

Fig. 29 EM-profiles for competing intramolecular elimination and substitution from o- 0C6H40(CH2) 4Br [1] in 99% Me2SO as a function of the size n of the cyclic transition states. The point for the elimination reaction where n = 6 is an estimate for the upper reactivity limit. (Reproduced with permission from Dalla Corte/ al., 1983)...

A detailed review of the ortho-effect in mass spectrometry was published by H. Schwarz [22]. He classified the processes related to the ortho-effect, gave examples of unusual elimination reactions, processes of intramolecular cyclization, exchange and reduction processes. 

An unusual one-pot intramolecular sulfoxide alkylation-elimination reaction was found by Gibson et al. <2001SL712>. These authors found that treatment of 459 with potassium bis-trimethylsilylamide resulted in a ring closure to 460 in acceptable yield. Furthermore, Batori and Messmer found an effective method for preparation of [l,2,3]triazolo[l,5- ]pyrimidinium salts <1994JHC1041> oxidative cyclization of hydrazones 461 by 2,4,4,6-tetrabromo-2,5-cyclohexadienone gave rise to the quaternary salts 462. Under certain reaction conditions, the formation of 6-bromo-salts 462 (R6 = Br) was also experienced. As neither the starting compound nor the quaternary triazolopyridinium salt underwent bromination in this position, the authors assumed that this bromination process occurred on one of the intermediates in the course of the above-mentioned cyclization reaction. 

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

As we have seen (Section 4, p. 191) the range of effective molarities associated with ring-closure reactions is very much greater than that characteristic of intramolecular general acid-base catalysis the main classification is therefore in terms of mechanism. By far the largest section (I, Tables A-D) gives EM s for intramolecular nucleophilic reactions. These can be concerted displacements (mostly at tetrahedral carbon), stepwise displacements (mostly addition-elimination reactions at trigonal carbon), or additions, and they have been classified in terms of the nucleophilic and electrophilic centres. 

Azetidones (p-lactams) are generally obtained in high yield from (3-halopropion-amides (Table 5.18) and the low yield from the reaction of N-phenyl (3-chloropropi-onamide can be reconciled with the isolation of A-phenyl acrylamide in 58% yield [34]. The unwanted elimination reaction can be obviated by conducting the cyclization in a soliddiquid system under high dilution [35, 36]. Azetidones are also formed by a predominant intramolecular cyclization of intermolecular dimerization to yield piperazine-2,5-diones, or intramolecular alkylation to yield aziridones. Aone-pot formation of azetidones in 45-58% yield from the amine and P-bromocarboxylic acid chloride has also been reported [38]. 

Methyleneazetidones have been obtained [39, 40] under liquiddiquid and solidrliquid basic conditions (Table 5.19) from an intramolecular cyclization and elimination reaction of 3-bromo-2-(bromomethyl)propionamides (Scheme 5.9). Traditional methods for the preparation of such compounds are either not particularly adaptable for general use, or involve lengthy and vigorous reaction conditions. In... 

A few examples of ester prodrugs that are activated by intramolecular reactions have been mentioned in Sect. 8.3.1, 8.5.1, and 8.5.2. Here, we discuss the special case of some carboxylic acid esters of active alcohols or phenols that are released following an intramolecular cyclization-elimination reaction [168], The general reaction scheme of such reactions is shown in Fig. 8.8. 

The mechanism of this remarkable a-elimination reaction has been scrutinized by several research groups [17,49,51,396-404]. From the experimental data obtained this process is best described as an intramolecular deprotonation of one neopentyl ligand by another, the latter being released as neopentane (Figure 3.4). 

In order to strengthen evidence in favour of the proposition that concerted inplane 5n2 displacement reactions can occur at vinylic carbon the kinetics of reactions of some /3-alkyl-substituted vinyliodonium salts (17) with chloride ion have been studied. Substitution and elimination reactions with formation of (21) and (22), respectively, compete following initial formation of a chloro-A, -iodane reaction intermediate (18). Both (17) and (18) undergo bimolecular substitution by chloride ion while (18) also undergoes a unimolecular (intramolecular) jS-elimination of iodoben-zene and HCl. The [21]/[22] ratios for reactions of (18a-b) increase with halide ion concentration, and there is no evidence for formation of the -isomer of (Z)-alkene (21) iodonium ion (17d) forms only the products of elimination, (22d) and (23). 

X = Cl, Br, or F. Ar = aryl), which were shown to undergo intramolecular ligand-metal H transfer and elimination reactions. ... 

Another interesting example of Ugi-Michael process is represented by the synthesis of pyridones 145 (Fig. 28), which originate from an intramolecular domino addition-elimination reaction of the active methylene group proceeding through intermediate 144 [120]. 

The same authors did not observe intramolecular cyclization by the coupling reaction of the ammonium cation and the carboxylate anion, at least in their work [81]. The presence of elimination reactions of the ammonium salt should not be ruled out, at least at high temperatures. 

Similar facile cycloaddition-elimination reactions are observed when the nitrile group is replaced by an alkyne or alkene group. Thus, thermolysis of thiatriazolines (56) in refluxing benzene for 3 days provided the intramolecular adducts (57) (Equation (5)) <9314439 >. 

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

The Gould-Jacobs sequence (Scheme 4.1) commences with an addition-elimination reaction between aniline 30 and substituted ethylenemalonate derivative 31 to yield malonic ester 32. Subsequent intramolecular cychzation delivers the 4-hydroxy-3-carboalkoxy-quinolone 33. In the presence of an alkylating agent, 33 is converted to 34. Saponfication of the ester affords quinolone core 35. 

An aza analog of phthalazine 240 (pyrido[3,4-r/]pyridazine skeleton) was obtained via intramolecular addition-elimination reaction in azaphthalohydrazide 239 with the loss of hydrazine (Equation 56) <1997T8225>. In a similar approach also the 5,6-dihydro[l,2,3]triazolo[4,5-r/]pyridazin-4,7-dione skeleton was constructed <2002JHC889>. 

The cyano-substituted nitrile ylides 123 have been generated via 1,1-elimination reactions. For example, the benzyhdene derivative 122 (R=Ph) eliminated benzene on vapor phase pyrolysis to give 123 (R=Ph), which reacted via 1,5-electrocycli-zation [see also (66)] to give the isoindole 124 (41%) (67). In a similar way, 122 [R=(CH2)3CH=CH2] gave the corresponding nitrile yhde that reacted via intramolecular cycloaddition to give the pyrroline derivative 126. 

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