Cycloadditions thermal reaction

An (SiNCO)-unit arises as intermediate product as result of a [2 + 2]-cycloaddition. Thermal reaction with LiF leads to a cyclohexane derivative. An alternative pathway might be the formation of an alcoxide starting from the aldehyde which attacks the silicon atom. 

The oxidation of o-substituted phenols with lead tetra-acetate in the presence of aP-unsaturated acids leads to the formation of o- and p-quinol esters of the acids since only the former can undergo internal [4 + 2] cycloaddition, thermal reaction... 

Whereas the cycloaddition of arylazirines with simple alkenes produces A -pyrrolines, a rearranged isomer can be formed when the alkene and the azirine moieties are suitably arranged in the same molecule. This type of intramolecular photocycloaddition was first detected using 2-vinyl-substituted azirines (75JA4682). Irradiation of azirine (54) in benzene afforded a 2,3-disubstituted pyrrole (55), while thermolysis gave a 2,5-disubstituted pyrrole (56). Photolysis of azirine (57) proceeded similarly and gave 1,2-diphenylimidazole (58) as the exclusive photoproduct. This stands in marked contrast to the thermal reaction of (57) which afforded 1,3-diphenylpyrazole (59) as the only product. 

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

Oxepin, 2-acetoxy-2,3,4,5-tetrahydro-thermal reactions, 7, 559 Oxepin, 3-chloro-synthesis, 3, 725 Oxepin, 2,3-dihydro-cycloaddition reactions, 7, 563 nucleophilic reactions, 7, 562 reduction, 7, 563 Oxepin, 2,5-dihydro-synthesis, 7, 578, 580 Oxepin, 4,5-dihydro-formation, 7, 579 reduction, 7, 563 synthesis, 7, 579 Oxepin, 2,7-dimethyl-NMR, 7, 552... 

The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the same reaction types discussed in Chapter 11 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2ti + 2ti] cycloaddition of two alkenes can serve as an example. This reaction was classified as a forbidden thermal reaction (Section 11.3) The correlation diagram for cycloaddition of two ethylene molecules (Fig. 13.2) shows that the ground-state molecules would lead to an excited state of cyclobutane and that the cycloaddition would therefore involve a prohibitive thermal activation energy. 

Tlie thermal reaction of dithiiranes is of particular interest in relation with the dithiirane/thioketone 5-sulfide manifold. Heating 5-oxodithiiranes (4) in solution led to both isomerization to 6,7-dithia-8-oxabicyclo[3.2.1]-octanes 74 and desulfurization to 5-oxothiones 75, the ratio of which was dependent on the reaction conditions employed. The intramolecular [3 + 2] cycloaddition of the thioketone 5-sulfide 76, generated by ring-opening, provides a straightforward explanation for the formation of 74. Meanwhile, 75 is probably formed by a nucleophilic attack on the sulfur atom by another molecule of 4 and/or by elemental sulfur formed during the reaction. 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

Trienone 124 underwent intramolecular cycloaddition affording hydrobenzo-suberone 125. Thermal reaction was poorly diastereoselective (62 38 cis trans stereoisomers). When the cycloaddition was carried out in the presence of LiBF4, trienone 124 was converted into cA-adduct 125 quantitatively and stereo selectively [118] (Equation 2.35). 

Diels-Alder reactions of vinylpyrazoles 45 and 46 only occur with highly reactive dienophiles under severe conditions (8-10 atm, 120-140 °C, several days). MW irradiation in solvent-free conditions also has a beneficial effect [40b] on the reaction time (Scheme 4.11). The indazole 48, present in large amounts in the cycloaddition of 45 with dimethylacetylenedicarboxylate, is the result of an ene reaction of primary Diels-Alder adduct with a second molecule of dienophile followed by two [l,3]-sigmatropic hydrogen shifts [42]. The MW-assisted cycloaddition of 46 with the poorly reactive dienophile ethylphenyl-propiolate (Scheme 4.11) is significant under the classical thermal reaction conditions (140 °C, 6d) only polymerization or decomposition products were detected. 

More functionalized 5,6-dihydro-2H-pyran-derivatives 71 and 72 have been prepared [26] by cycloaddition of 1 -methoxy-3-trialkylsilyloxy-1,3-butadienes 69 with t-butylglyoxylate (70) (Scheme 5.6). Whereas thermal reactions did not occur in good yields because of the decomposition of the cycloadducts, application of pressure (10 kbar) allowed milder conditions to be used, which markedly improved the reaction yields. The use of high pressure also gives preferentially en Jo-adduct allowing a stereocontrolled synthesis of a variety of substituted 5,6-dihydro-2H-pyran-derivatives, which are difficult to prepare by other procedures. 

The thermal reaction between two molecules of aUcene to give cyclobutane derivatives (a 2 + 2 cycloaddition) can be carried out where the aikenes are the same... 

The nonsymmetrical quinolizidine 373 was obtained from the acyclic symmetrical precursor 372 by means of a reaction sequence comprising azide formation, intramolecular 1,3-dipolar cycloaddition, thermal triazoline fragmentation to a diazoalkane, and Michael addition individual steps, as shown in Scheme 85 <2005CC4661>. 

Some examples of the lateral cyclization of suitable O-allyl and O-propargyl derivatives were discussed in CHEC-11(1996) <1996CHEC-II(8)747>. Thermal reaction of silyl diazoacetate 303 in xylene provides unspecific decomposition and a minor amount (about 2%) of a colorless solid can be precipitated with ether. The X-ray diffraction analysis identified the structure 305, which is a product of the lateral criss-cross cycloaddition of primarily formed azine 304 (Scheme 43) <2000T4139>. 

A second category of silene reactions involves interactions with tt-bonded reagents which may include homonuclear species such as 1,3-dienes, alkynes, alkenes, and azo compounds as well as heteronuclear reagents such as carbonyl compounds, imines, and nitriles. Four modes of reaction have been observed nominal [2 + 2] cycloaddition (thermally forbidden on the basis of orbital symmetry considerations), [2 + 4] cycloadditions accompanied in some cases by the products of apparent ene reactions (both thermally allowed), and some cases of (allowed) 1,3-dipolar cycloadditions. 

Aryl and vinyl nitriles have been prepared very efficiently from the corresponding bromides by palladium-catalyzed reactions under microwaves. This energy source has been employed for the conversion of these nitriles into aryl and vinyl tetrazoles by cycloaddition reactions with sodium azide (Scheme 9.66). The direct transformation of aryl halides to the aryl tetrazoles in a one pot procedure could be accomplished both in solution and on a solid support [115], The reactions were complete in a few minutes, a reaction time considerably shorter than those previously reported for the thermal reactions. The cydoadditions were performed with sodium azide and ammonium chloride in DMF and, although no explosion occurred in the development of this work, the authors point out the necessity of taking adequate precautions against this eventuality. 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

Eq. 17 is meant to represent the possibility for a concerted formation of oxetane product. A problem that always exist in cycloadditions is the question of whether the reaction takes place by a two-step biradical reaction pathway or through a concerted mechanism. Such questions have not even been resolved for purely thermal reactions. 4> A recent speculation on this point proposes almost universal concertedness for all cycloaddition reactions. 79> In that work, mixed stereochemistry in the products of [2+2] cycloaddition reactions is generally attributed to a mixture of two concerted reactions, suprafacial-suprafacial, and supra-facial-antarafacial. It will be seen later that the PMO calculations generally do not support this idea. A mixture of biradical and concerted reactions is in better agreement with experimental facts. 

The enone system itself is usually part of a five- or six membered ring, although acyclic a,(3-unsaturated ketones and enols of P-diketones are also found to undergo cycloadditions under certain conditions. For seven- and higher membered rings the primary photochemical event is Z—E isomerization around the C—C double bond, the E-isomer then eventually undergoing further thermal reactions. 

Transition metal-catalyzed [4+ 2]-cycloadditions ofdiene-allenes 247 can lead to different results. With a nickel catalyst Wender et al. isolated the anellated system of two six-membered rings 248 with a rhodium catalyst the anellation of a five- and a six-membered ring 249 was possible (Scheme 15.78) [149]. Both transformations proceed readily at low temperatures whereas the uncatalyzed thermal reaction requires 185 °C. Even an anellation of a six- and a seven-membered ring was achieved. 

Because of their previous findings that a,/l-unsaturated thioesters were more reactive than their ester counterparts in Diels-Alder reactions85, Hart and coworkers86 performed a systematic study of the cycloaddition reactions of a,/l-unsaturated thioesters and a,ft-unsaturated selenoesters with several dienes. Thermal reactions were compared with Lewis acid catalyzed reactions at room temperature (equation 28 and Table 2). The results clearly demonstrated that use of a Lewis acid enhanced the regioselectivity (entries 1 vs 2, 3 vs 4, 5 vs 6 and 7 vs 8) as well as the endo (with respect to the thioester or selenoester group) selectivity (entries 5 vs 6 and 7 vs 8). 

The [3 -f 2] cycloaddition of aziridines and dipolarophiles, like dimethyl acetylenedicarboxylate or dimethyl fumarate and maleate, was investigated by Gaebert and Mattay. Via C—C and C—bond cleavage five-membered heterocycles are formed in moderate yields. The different product ratios dependent on the reaction conditions (PET/direct excitation/thermal reaction) gave insights to the reaction details and are summarized in the proposed mechanism (Scheme 52) [84],... 

Our preliminary experiments have provided the first example of Lewis acid promoted C-C bond heterolysis of epoxides and productive cycloaddition (eq 7). Under the influence of TiCl4-(THF)2 (2 equiv), epoxide 26 reacts with methyl pyruvate to provide acetal 27 (52% isolated yield), along with C-O cleavage product 28 (23 °C, 3 h). The diaste-reoselectivity for formation of 27 is 2.3 1. We have performed the analogous reaction in the absence of a Lewis acid the thermal reaction requires several days at 110 °C and gives a diastereomer ratio (dr) of ca. 1.3 1... Although not optimized from the standpoint of chemoselectivity, these results are promising because of the relatively low reaction temperature and potential for enhanced diastereocontrol. 

Our initial studies focused on the transition metal-catalyzed [4+4] cycloaddition reactions of bis-dienes. These reactions are thermally forbidden, but occur photochemically in some specific, constrained systems. While the transition metal-catalyzed intermole-cular [4+4] cycloaddition of simple dienes is industrially important [7], this process generally does not work well with more complex substituted dienes and had not been explored intramolecularly. In the first studies on the intramolecular metal-catalyzed [4+4] cycloaddition, the reaction was found to proceed with high regio-, stereo-, and facial selectivity. The synthesis of (+)-asteriscanoHde (12) (Scheme 13.4a) [8] is illustrative of the utihty and step economy of this reaction. Recognition of the broader utiHty of adding dienes across rc-systems (not just across other dienes) led to further studies on the use of transition metal catalysts to facilitate otherwise difficult Diels-Alder reactions [9]. For example, the attempted thermal cycloaddition of diene-yne 15 leads only... 

Intermolecular Allylboration. A tandem aza[4+2] cycloaddition/allyl-boration three-component reaction has been designed based on the prece-dented carbocyclic [4- -2] cycloaddition/allylboration and a snbsequent one-pot variant. Thns, the thermal reaction between hydrazonobutadienes 138, A-substitnted maleimides, and aldehydes provides polysnbstituted a-hydroxy-alkylpiperidines 141 via the cyclic allylboronate intermediate 139 and the proposed chairlike transition stmctnre 140 (Eq. 103). Monoactivated dienophiles like acrylates fail to react with heterodienes 138 bnt the scope of aldehydes is very broad both ahphatic and aromatic aldehydes are snitable, inclnding electron-rich ones. An inverse electron-demand variant to access the corresponding dihy-dropyran derivatives via the intermediacy of enantiomerically enriched pyranyl allylic boronate 76 has been snbsequently developed (see Eq. 64). ° ... 

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