1,3-Dienes stepwise

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

Isonitriles and some nitriles can be viewed formally as having a nucleophihc center and an electrophilic center at the same skeleton atom Tnmethylsilylcyanide and cyanoformates add to hetero-1,3 dienes in a stepwise process to give five-mem bered rmg systems with the same stmcture as the isonitnle adducts [41],... 

The reactions of bis(trifluoromethyl)-subsatuted hetero 1,3 dienes are predomi nantly LLJMO controlled processes [238] With polar or highly polarizable dieno philes, the tendency to undergo stepwise cycloaddition reactions is considerable Notably these hetero-1,3-dienes react with a,(l unsaturated hetero multiple bond systems across the hetero multiple bond exclusively [243, 246 248] (equation 53)... 

There have been few mechanistic studies of Lewis acid-catalyzed cycloaddition reactions with carbonyl compounds. Danishefsky et ah, for example, concluded that the reaction of benzaldehyde 1 with trans-l-methoxy-3-(trimethylsilyloxy)-l,3-di-methyl-1,3-butadiene (Danishefsky s diene) 2 in the presence of BF3 as the catalyst proceeds via a stepwise mechanism, whereas a concerted reaction occurs when ZnCl2 or lanthanides are used as catalysts (Scheme 4.3) [7]. The evidence of a change in the diastereochemistry of the reaction is that trans-3 is the major cycloaddition product in the Bp3-catalyzed reaction, whereas cis-3 is the major product in, for example, the ZnCl2-catalyzed reaction - the latter resulting from exo addition (Scheme 4.3). 

The major developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds with conjugated dienes have been presented. A variety of chiral catalysts is available for the different types of carbonyl compound. For unactivated aldehydes chiral catalysts such as BINOL-aluminum(III), BINOL-tita-nium(IV), acyloxylborane(III), and tridentate Schiff base chromium(III) complexes can catalyze highly diastereo- and enantioselective cycloaddition reactions. The mechanism of these reactions can be a stepwise pathway via a Mukaiyama aldol intermediate or a concerted mechanism. For a-dicarbonyl compounds, which can coordinate to the chiral catalyst in a bidentate fashion, the chiral BOX-copper(II)... 

The thermal addition of dimethyl acetylenedicarboxylate to indoles, unlike the photocycloaddition (see Section 3.2.1.4.1.1.), proceeds via a polar stepwise process to yield, initially, 3,4-benzo-2-azabicyclo[3.2.0]hepta-3,6-dienes which in some cases are isolable,13 141 but which, in general, ring open in situ to give the indolylacrylates 3 and/or undergo electrocyclic ring expansion to 1-benzazepines.21... 

The orbital interaction depicted in Scheme 1.15 shows that the two cr-bonds form at the same time but do not develop to the same extent. The Diels-Alder cycloaddition of unsymmetrical starting materials is therefore concerted but asynchronous. A highly unsymmetrical diene and/or dienophile give rise to a highly unsymmetrical transition state and a stepwise pathway can be followed. 

Conjugated dienes can be dimerized or trimerized at their 1,4 positions (formally, [4 4- 4] and [4 4-4 4-4] cycloadditions) by treatment with certain complexes or other transition metal compounds. " Thus butadiene gives 1,5-cyclooctadiene and 1,5,9-cyclododecatriene. " The relative amount of each product can be controlled by use of the proper catalyst. For example, Ni P(OC6H4—o-Ph)3 gives predominant dimerization, while Ni(cyclooctadiene)2 gives mostly trimerization. The products arise, not by direct 1,4 to 1,4 attack, but by stepwise mechanisms involving metal-alkene complexes. " ... 

According to the calculations at high levels of theory, the [4+2] cycloaddition reactions of dienes with the singlet ( A oxygen follow stepwise pathways [37, 38], These results, which were unexpected from the Woodward-Hoffmann rule and the frontier orbital theory, suggest that the [4+2] cycloadditions of the singlet ( A oxygen could be the reactions in the pseudoexcitation band. 

This reaction has been studied computationally with Zn2+ as the metal cation.32 The calculations indicate that a stepwise reaction occurs, beginning with electrophilic attack of the complexed dienophile on the diene. 

Stepwise formation constants have been determined in the system Hg2+/Cl-/diethylenetriamine (dien) and related systems by potentiometry. Thermodynamic parameters have been calculated and the contribution of the entropy term to complex stability discussed.208... 

Itoh and coworkers [223] have shown that fullerene derivatives as 6/2-113, which to date have been prepared in a stepwise procedure, can be obtained in a three-component domino process by treatment of diynes 6/2-109, dimethylphenylsilane 6/2-110 and fullerene (C60) in the presence of a Rh-catalyst [223]. Interestingly, using maleic anhydride as dienophile failed to give the desired cycloadduct, whereas Cso -in spite of its strong tendency to form complexes with various transition metals [224] - never suppressed the catalytic silylative cyclization step to give the diene 6/2-112 (Scheme 6/2.24). 

Thus BCP seems to follow two competitive pathways in the cycloaddition with dienes (i) a stepwise diradical process giving the [2 + 2] adduct, or (ii) a concerted pathway giving the [4 + 2] adduct. Accordingly, the proportion of the latter increases with the reactivity of diene in Diels-Alder reactions. Conversely, the reaction with 2,3-dicyanobutadiene (529), generated in situ by electrocyclic ring-opening of 1,2-dicyanoeyelobutene [142], furnishes selectively the [2 + 2] cycloadduct 530 (Table 42, entry 4) due to the presence of substituents able to stabilize the diradical intermediate [13b],... 

In an attempt toward electron-rich and electron-deficient multiple bonds as well as 1,3-dipoles, the triafulvene system may develop functionalization of a dipolarophilic, dienophilic, and diene component. Rigorous proof for a concerted or a stepwise mechanism, e.g. via dipolar intermediates, for any of the numerous reactions investigated cannot be presented. Therefore the following classification has been chosen from a more or less formal point of view. 

Such a conclusion is, nevertheless, connected with the synchronous character of the mechanism. If a stepwise process is involved (nonsimultaneous formation of the two new bonds), as for unsymmetric dienes and/or dienophiles or in hetero Diels-Alder reactions, a specific microwave effect could intervene, because charges are developed in the transition state. This could certainly be so for several cycloadditions [47, 48] and particularly for 1,3-dipolar cycloadditions [49]. Such an assumption has... 

The anodic oxidation of 1,2-dienes in methanol takes place stepwise at each double bond yielding a tetramethoxylated compound as one of the products (equation if. This result is reasonable since a 1,2-diene is not a conjugated diene. 

In general, allenyl cations 38 attack at the sp2-carbon atom of 1,3-dienes and form vinyl cations 39 and 40 (R = H, alkyl) or (R = aryl). Although a concerted cycloaddition mechanism is possible, a stepwise mechanism is preferred34. If a nucleophilic attack at the sp-carbon atom of the allenyl cation takes place, then cation 41 and the resulting cations 42 and 43 are formed. Some examples of bicyclic products obtained from cyclic 1,3-dienes and propargyl chlorides are given in equation 1534. 

Burgess and coworkers investigated the hydrogenation of the conjugated diene 87 (Scheme 30.1) [48]. Kinetic studies showed that the reaction occurred mostly stepwise via 2,3-diphenyl-l-butene, while only a small part of the diene was converted directly to 2,3-diphenylbutane, without dissociation of the catalyst from the intermediate mono-alkene. The first hydrogenation step was found to proceed with low enantioselectivity, whereas the second step was characterized by strong catalyst and strong substrate control. 

Af-Acyliminium ions are known to serve as electron-deficient 4n components and undergo [4+2] cycloaddition with alkenes and alkynes.15 The reaction has been utilized as a useftil method for the construction of heterocycles and acyclic amino alcohols. The reaction can be explained in terms of an inverse electron demand Diels-Alder type process that involves an electron-deficient hetero-diene with an electron-rich dienophile. Af-Acyliminium ions generated by the cation pool method were also found to undergo [4+2] cycloaddition reaction to give adduct 7 as shown in Scheme 7.16 The reaction with an aliphatic olefin seems to proceed by a concerted mechanism, whereas the reaction with styrene derivatives seems to proceed by a stepwise mechanism. In the latter case, significant amounts of polymeric products were obtained as byproducts. The formation of polymeric byproducts can be suppressed by micromixing. 

In view of the stereochemical behavior in the additions to alkenes and dienes, the authors suggest that the reaction proceeds via a stepwise electrophilic addition126. However, in this case the two sulfur atoms of the dithioether dication are positively charged. In the reaction with multiple bonds, therefore, one of these sulfur atoms should be an electrophilic center whereas the other one should simultaneously be a nucleophilic center. In... 

The Diels-Alder reaction is the best known and most widely used pericyclic reaction. Two limiting mechanisms are possible (see Fig. 10.11) and have been vigorously debated. In the first, the addition takes place in concerted fashion with two equivalent new bonds forming in the transition state (bottom center, Fig. 10.11), while for the second reaction path the addition occurs stepwise (top row, Fig. 10.11). The stepwise path involves the formation of a single bond between the diene (butadiene in our example) and the dienophile (ethylene) and (most likely) a diradical intermediate, although zwitterion structures have also been proposed. In the last step, ring closure results with the formation of a second new carbon carbon bond. Either step may be rate determining. 

Fig. 4 (a) Stepwise dehydrocyclization of -hexane (21, 62). (b) Temperature programmed desorption of benzene originating from various adsorbates over Pt-AljOs. Temperature of adsorption 25°C. Rate of heating 23°C per minute. Detector monopolar mass spectrometer, the ordinate corresponds to the I intensity of mass number 78, in arbitrary units. For clarity, the thermodesorption curves for other compounds (starting hydrocarbon) hexene from hexa-dienes and hydrogen have not been shown (62c). 

The cation radical Diels-Alder cycloadditions of cis- and franx-prop-1-enyl aryl ethers to cyclopenta-1,3-diene catalysed by tris(4-bromophenyl)aminium hexachloroantimonate are stepwise processes involving an intermediate distonic cation radical in which the carbocationic site is stabilized by the electron-donating functionality (Scheme 9). " ... 

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