Olefins from -cycloreversion

Certain 1,5 diazabicyclo[3 3 0]oct-2-enes can be transformed unexpectedly into 4//-5,5-dihydro-l, 2 diazepines on heating [209] 1,5-Dipoles formed on heating of l,5-diazabicyclo[3 3 0]oct-2-enes [210] can be trapped with olefins to give [3+2] cycloadducts At elevated temperatures, they undergo a [3+2] cycloreversion Tins reaction sequence offers a simple route to dienes with interesting substitution patterns, for example, 1,1 bis(trifluoromethyl)-l,3-butadiene [211] The [3+2] cycloadducts that arise from the reaction of the 1,5 dipoles with acetylenes undergo... 

In addition there are certain other methods for the preparation such compounds. Upon heating of the thionocarbonate 2 with a trivalent phosphorus compound e.g. trimethyl phosphite, a -elimination reaction takes place to yield the olefin 3. A nucleophilic addition of the phosphorus to sulfur leads to the zwitterionic species 6, which is likely to react to the phosphorus ylide 7 via cyclization and subsequent desulfurization. An alternative pathway for the formation of 7 via a 2-carbena-l,3-dioxolane 8 has been formulated. From the ylide 7 the olefin 3 is formed stereospecifically by a concerted 1,3-dipolar cycloreversion (see 1,3-dipolar cycloaddition), together with the unstable phosphorus compound 9, which decomposes into carbon dioxide and R3P. The latter is finally obtained as R3PS ... 

If the cycloaddition and cycloreversion steps occurred under the same conditions, an equilibrium would establish and a mixture of reactant and product olefins be obtained, which is a severe limitation to its synthetic use. In many cases, however, the two steps can very well be separated, with the cycloreversion under totally different conditions often showing pronounced regioselectivity, e.g. for thermodynamic reasons (product vs. reactant stability), and this type of olefin metathesis has been successfully applied to organic synthesis. In fact, this aspect of the synthetic application of four-membered ring compounds has recently aroused considerable attention, as it leads the way to their transformation into other useful intermediates. For example aza[18]annulene (371) could be synthesized utilizing a sequence of [2 + 2] cycloaddition and cycloreversion. (369), one of the dimers obtained from cyclooctatetraene upon heating to 100 °C, was transformed by carbethoxycarbene addition to two tetracyclic carboxylates, which subsequently lead to the isomeric azides (368) and (370). Upon direct photolysis of these, (371) was obtained in 25 and 28% yield, respectively 127). Aza[14]annulene could be synthesized in a similar fashion I28). 

Triazolines bearing three electron-withdrawing groups (Scheme 85) undergo complex thermolysis reactions. Aziridine formation is observed but sometimes the azide cycloreversion operates pyrrolidines are thus formed by reaction of the olefins with the azomethine ylides from the aziridines. The aziridines also dimerize to piperazines under the conditions of thermolysis.446... 

The key intermediate is a metallacyclobutane, which can undergo cycloreversion either towards products or back to starting materials. When the olefins of the substrate are terminal, the driving force for RCM is the removal of ethene from the reaction mixture. 

However, only limited experimental evidence is available concerning the key step of the dimerization, i,e. the addition of the radical cation to the parent olefin. Does this addition occur stepwise or in concerted fashion Does the radical cation serve as a the diene component ([3 + 2]cycloaddition) or as dienophile ([4+ l]cy-cloaddition) The observed retention of dienophile stereochemistry and orbital symmetry arguments (Fig. 7) favor the [4 + l]cycloaddition type. Although it is difficult to distinguish the [3 + 2] from the [4 + l]addition type, a stepwise component for the cycloaddition and the complementary cycloreversion has been established in at least one system, viz., spiro[2.4]heptadiene. 

The authors proposed that initial bond formation occurs from the less substituted olefin carbon to the a-carbon of the vinylogous amide to yield diradical intermediate 127 (Scheme 32, path A). This diradical can then undergo carbon-nitrogen bond homolysis to give the observed product, 126. However, the formation of 126 is also consistent with the formation of 128 by a straight cycloaddition followed by cycloreversion as outlined in Scheme 32, path B. 

Metallacycloalkanes are proven key intermediates in metal-catalyzed cycloadditions and cycloreversions of alkenes. The relationship of some iron metallacyclopentane derivatives with bis(olefin) complexes has been investigated theoretically. Scheme 1 shows a general route from bisalkene... 

In summary, the reactivity of strained olefins is different from that of simple alkenes. The wealth of transformations initiated by strained double bonds has recently been reviewed in great detail (276). In this chapter some structural and strain-related features of reactivity observed for olefins with nonplanar double bonds have been presented. It is apparent that in polar reactions as well as cycloadditions of these molecules the release of strain may contribute to enhanced rates, but it is certainly not the controlling factor. In view of the operation of microscopic reversibility, it may be questioned whether the buildup of strain in reverse reactions—eliminations and cycloreversions—is the controlling factor. 

The cycloreversion of the cyclobutane (278) to the olefin occurs from the singlet state on irradiation at 265 nm. Triplet-state reactivity is reported for cycloreversion using A = 347 nm. The isomerization of 1,2-diphenylcyclobutanes has been used as a means of establishing the efficiency of electron-transfer processes in the phenanthrene-dicyanobenzene system. 

There are a number of processes that can conpete with the di-n-rearrangement and related reactions. As indicated earlier, in the case of the parent DPM rearrangement, unproductive isomerization of the olefins within the substrate can occur when triplet-mediated conditions are applied to acyclic substrates. Electrocyclic processes can also conpete. For exanple, while acetophenone-sensitized irradiation of benzobarrelene (44) affords benzosemibullvalene (8), direct irradiation of the same substrate yields benzocyclooctatetraene (45) via a [2+2] photocycloaddition/cycloreversion sequence. Such electrocyclic processes tend to proceed preferentially from the singlet excited state and take place exceptionally rapidly. In broad terms, cyclic substrates are more likely to suffer from conpeting electrocyclic reactions by conparison with their acyclic counterparts, one reason being the entropic advantage conferred on such processes by more conformationally rigid frameworks. 

In the proposed mechanism, for the olefin metathesis, the W-bis-carbene is generated via hydrogen transfer from the methyl to the W-methylidyne in the presence of an olefin. This further undergoes [2+2] cycloaddition with external olefin followed by cycloreversion to give ethylene and W-alkylidene, which reacts... 

While in alkane metathesis mechanism (Scheme 20, b), the n-decane undergoes o-bond metathesis to generate methane and the W-bis-decyl species which, upon P-H elimination, produces the W-H with a coordinated olefin. Further, the a-hydrogen transfer from the alkyl to alkylidyne forms the hydrido W-bis-carbene [55, 76]. This upon [2-1-2] cycloaddition and cycloreversion gives an internal olefin and hydrido W-bis-carbene. Successive insertion/elimination steps (by chain walking) [77] give the terminal alkene, which reacts to a new W-alkylidene. The CH activation of the pendant W-hydride with -decane followed by p-H elimination provides 1-decene. A second metathesis between 1-decene and newly formed W-alkylidene followed by hydrogenolysis produces the alkane. 

Aromatic aldehydes and ketones react with diphenylketene at 120-160 °C to form aromatic olefins 227 . In these reactions, diphenylketene can be generated in situ from its 2 1 cycloadduct with quinoline, because cycloreversion occurs above its melting point (121 °C). 

Cycloadditions IV-Sulfonylurethane, Me00C-N=S02 78, which is obtained in a [4-1-2] cycloreversion reaction from 1,4,3,5-oxathiadiazines, reacts with olefins, like styrene and 1,1-diphenylethylene, to give a mixture of [2-1-2] cycloadducts 79 and [4-1-2] cycloadducts 80 . 

The selectivity exhibited by an olefin metathesis catalyst for the production of -olefins or Z-olefins is a result of both kinetic and thermodynamic factors. Kinetic selectivity results from preferential formation of either syn- or anri-metallacyclo-butanes following olefin binding iy -metallacycles will undergo a cycloreversion to produce Z-olefins, whereas -olefins are derived from anfi-metallacycles. Thermodynamic selectivity arises as a result of secondary metathesis processes, in which the product olefins continue to react with the propagating catalyst. 

Cheletropic reactions are special type of concerted cycloadditions or cycloreversions in which two bonds are formed on or fissioned from same atom. For example, reaction of a singlet carbene with olefin. 

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