Intramolecular reactions Isomerism

As supporting evidence, rapid isomerization of the ds- and maui-Tr-allylpal-ladium complexes 27 and 28 is catalyzed by Pd(Ph3P)4 in THF even at -15 C to give a 45 55 equilibrium mixture from either 27 or 28[29-31].. Actually, in the intramolecular reaction of soft nucleophiles of 29 and 30, a trans-ds mi.xttire (31 and 32) (1 1) was obtained from /raiw-allylic acetate 29. On the... 

The reverse reaction (formation of metal alkyls by addition of alkenes to M-H) is the basis of several important catalytic reactions such as alkene hydrogenation, hydroformylation, hydroboration, and isomerization. A good example of decomposition by y3-elimination is the first-order intramolecular reaction ... 

The stereoselectivity of these intermolecular reactions between 1-alkoxyallylstannanes and aldehydes induced by boron trifluoride-diethyl ether complex is consistent with an open-chain, antiperiplanar transition state. However, for intramolecular reactions, this transition state is inaccessible, and either (Z)-.yyn-products are formed, possibly from a synclinal process105, or 1,3-isomerization competes113. Remote substituents can influence the stereoselectivity of the intramolecular reaction114. 

The reaction was applied to an acyclic system for the synthesis of furanoid terpenes (Scheme ll)51. The palladium-catalyzed intramolecular reaction of 47 afforded 48 which was transformed to the target molecule. The latter product was obtained as a 1 1 mixture of marmelo oxide A and B, which is the isomeric mixture found in nature. 

For many years, intramolecular reactions such as conformational changes, bond cleavage, bond formation, and valence isomerizations have been observed only when hydrocarbons were reduced with alkali metals in ethereal solvents. In most electrochemical experiments, these reactions were dominated by the electrophilic processes already described. However, progress in experimental techniques [8, 9, 27-29] has made these reactions accessible to electroanalytical investigations, providing new mechanistic insight. 

It is impossible to specify in detail all the isomeric structures which occur as a network is forming. However, with regard to one-membered loop formation, two extreme types may be delineated, namely, linear and symmetric isomers. They are illustrated in Figure 9 for an RA polymerisation. Linear isomers are able to form the smallest number of one-membered loops and symmetric isomers the largest number. In an RAf polymerisation, for linear isomers of n units, the total number of pairs of unreacted ends for intramolecular reaction is... 

Table III. Total numers of loops and numbers and fractions of one-membered loops formed by random intramolecular reaction within linear and isomeric gel structures of different numbers of units(n) and generations (m). The fractions marked agree with the experimentally deduced concentrations of inelastic junction points or chains on the basis of one-membered loops...

This catalyst system was the first to utilize both terminal alkynes and olefins in the intramolecular reaction. Although a mechanistic rationale for the observed stereoselectivity was not offered, the formation of the single stereoisomer 26 may be rationalized through the diastereotopic binding of the rhodium complex to the diene moiety (Scheme 12.3). This facial selective binding of the initial ene-diene would then lead to the formation the metallacycle III, which ultimately isomerizes and reductively eliminates to afford the product [14]. 

Although metal-olefin complexation can be a source of enandoselection, reactions exploiting this mechanistic motif have not been developed much. Due to the facile enantioface interconversion process, the origin of the enantioselection often reverts back to Type C alkylation (Figure 8E, 1). To transfer chiral recognition of the coordination process to the ee of the product, kinetic trapping of the incipient 7t-allyl complex is required prior to any isomerization process. For this reason, few successful examples have come from the use of more reactive heteroatom nucleophiles (N, O and S) and/or intramolecular reactions. 

An important and frequently observed phenomenon in alkene pyrolysis is the ready equilibration of E and Z isomers at FVP temperatures above 500 °C. The apparently contrathermodynamic conversion of the E into the Z isomer has been quantified over the range 500-900 °C for stilbene, cinnamyl alcohol and cinnamonitrile37. In the last case, the proportion of Z isomer increases to 38% at 900 °C. In certain cases the diradical implicit in the isomerization process can be trapped by an intramolecular reaction and this is exemplified by the formation of 2-phenylindane in low yield from FVP of 56 at 700 °C37. The cis cyclobutene diester 58 is assumed to be formed as an intermediate in the FVP of the bicyclic sulphone 57 at 775 °C by loss of SO2 and ethylene. Under these conditions, however, it reacts further to give equal proportions of the E diesters 59... 

The photochemical cycloaddition of two different alkenes leads to multiply substituted cyclobutanes, and allows for a general access to this class of compounds. More specifically, ever since Ciamician observed the light-induced isomerization of carvone (1 —> 2) (Scheme 6.1) in 1908 [1], the inter- and intramolecular reaction between an a,(3-unsaturated carbonyl compound and an alkene has become the most intensively studied and most widely used class of [2 + 2]-photocycloaddition reactions [2-9]. 

The transfer of hydrogen to peroxyl radicals may proceed intra- or inter-molecularly. Intramolecular transfer reaction (isomerization) of peroxyl macroradicals of polypropylene occurs during the oxidation of the polymer in a solution of inactive solvent [75] while the intermolecular transfer is preferred during the oxidation in reactive solvent or in the crystalline state [76]. 

Titanocene(n) species promote the conversion of unsaturated thioacetals to cyclic compounds. This cyclization proceeds with the loss of the terminal alkene carbon. Treatment of the thioacetal 83 with the low-valent titanium species Cp2Ti[P(OEt)3]2 (3 equiv) in refluxing THF afforded benzoxocines 86 and 87 (by isomerization of 86) in 61% yield (Scheme 14) <1999SL354>. Using 4 equiv of the titanocene(n), the yield is higher (70%) but the selectivity is lower (the ratio 86 87 becomes 82 18). The mechanism or the reaction probably involves the formation of the titanium carbene complex 84, its intramolecular reaction with the double bond to form titanocyclobutane 85, and the subsequent elimination of methylidenetitanocene <1999SL354>. 

In this part, we will summarize some of our results on the investigation of the toluene intramolecular isomerization pathways." " Both cluster approach and periodic approach methods have been employed which allow giving an illustration of the consequence of the simplistic model in the cluster approach. H-Mordenite (H-MOR) zeolite is used for the periodic calculations. The toluene molecule does not have a problem to fit within the large 12-membered ring channels of this zeolite. Furthermore, the intramolecular transition states do not suffer from steric constraints. It is known that intramolecular aromatics isomerization can proceed via two different reaction pathways (see Figure 7). The first route proceeds through a methyl shift isomerization, whereas the second route involves a dealkylation or disproportionation reaction which results in the formation of a methoxy species and benzene as intermediate. 

Grieco has used his aqueous imnicmium Diels-Alder procedure to effect a number of intramolecular reactions. In one case, diene aldehyde (83) was treated with ammonium chloride to afford a 2.2 1 mixture of isomeric Diels-Alder adducts (87) and (85) (Scheme 10). Since intermediate immonium ions (84)/(86) cannot participate in secondary orbital effects as is the case with A-acyl imines (c/. 80), these results are probably due to steric factors. It was suggested that adduct (85) derives from conformation (84) and adduct (87) comes from (86). Conformer (84) is favored since there is a severe eclipsing of Ha b in (86). A more detailed account of the stereochemical aspects of intramolecular Diels-Alder reactions can be found in Chapter 4.4. 

Allenylsilanes and -stannanes combined with a titanium salt are versatile reagents for propargylation of aldehydes (Eq. 115) [297], ketones (Eq. 116) [298], (A,0)-acetals (Eq. 117) [299], and a,/ -unsaturated ketones in a conjugate fashion (Eq. 118) [300]. Intramolecular reaction has also been reported (Eq. 119) [301] in which a Bu3Sn-carbon bond was cleaved exclusively in the presence of a TBS-carbon bond. That the isomeric starting material, propargylstannane, did not give the desired product (Eq. 120) demonstrates that the direct scission of the carbon-Sn bond by the electrophile under these reaction conditions is not a feasible path [301]. 

Ketones and secondary amines furnish enamines in the presence of TiCl4 [580,581]. The preparation of a functionalized enamine shown in Eq. (251), in which the acetal moiety is retained in the product, illustrates the applicability of this reaction [582]. Enamines prepared by this method are summarized in Table 24. Application to an intramolecular reaction is also found in Table 24. If formation of the enamine is thermodynamically preferred to formation of the isomeric imine, the former becomes the product even in the reaction of a ketone, a primary amine, and TiCl4, as shown in Eq. (252) [583], in which the resulting enamine was, after acetylation, isolated as the enamide. 

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