Elimination reactions cycloreversion

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 ... 

A number of studies dealing with a-oxo sulfines has appeared. The stabilities of the attractive molecules usually do not allow isolation. The strategy used for the manipulation of unstable thioaldehydes was applied here. A precursor is submitted to an elimination reaction (or a cycloreversion) to generate the sulfine in the presence of a diene, and it is the product of... 

Functionalized alkynes are made via rDA cycloreversion of cycloadducts that act as protected alkynes. Substituted alkynes (93a-h) are obtained from adducts (92a-h) via FVP-induced rDA cycloreversion as shown in equation (40). This method, based on the rDA reaction, provides an alternative route to the alkynic bond, which is more commonly formed via elimination reactions. 

In principle, rearrangement/elimination reactions via ortho effect may occur always as a multistep reaction if the molecular part Y (1) is able to act as a hydrogen acceptor (e.g., a carbonyl function). In such a case, the hydrogen primarily can be transferred onto this function, whereas the elimination of H-Z would be the result of a four center cycloreversion. It cannot be stated with certainty that the arene elimination from ortho hydroxyphenylarylketones belongs to this class of stepwise reactions6,18> 19. ... 

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 retro Diels-Alder reaction (cycloreversion) under mild conditions can be used for the preparation of pyridazines. Bridged 1,4-epimino-naphthalenes 172 react readily with 3,6-di(2-pyridyl)-s-tetrazine 173 in an exothermic reaction involving nitrogen elimination to yield pyridazines 176 (2003COR1423 Scheme 31). 

Interestingly, in the inverse-electron-demand Diels-Alder reactions of oxepin with various enophiles such as cyclopentadienones and tetrazines the oxepin form, rather than the benzene oxide, undergoes the cycloaddition.234 236 Usually, the central C-C double bond acts as dienophile. Oxepin reacts with 2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dienone to give the cycloadduct 6 across the 4,5-C-C double bond of the heterocycle.234 The adduct resists thermal carbon monoxide elimination but undergoes cycloreversion to oxepin and the cyclopenta-dienone.234... 

In the last 5 years, catalytic antibodies have been generated for several reaction types, including the various types of hydrolysis, transesterification, amide bond formation, /3-elimination, cycloreversion, transacylation, redox reactions, E-Z isomerization, epoxidation, and Diels-Alder reactions. For more information on these and other recent developments, such as semi-synthetic antibodies, site-directed mutagenesis, and the bait-and-switch strategy, the reader should consult the appropriate authorities (Schultz, 1988, 1989a,b Benkovic et al., 1990 Janda et al., 1990, 1991 Janjic and Tramontano, 1990 Lerner et al., 1991). 

In addition to the reaction of vinylcarbene complexes with alkynes, further synthetic procedures have been developed in which Fischer-type carbene complexes are used for the preparation of benzenes. Most of these transformations are likely to be mechanistically related to the Dbtz benzannulation reaction, and can be rationalized as sequences of alkyne-insertions, CO-insertions, and electrocycli-zations. A selection of examples is given in Table 2.18. Entry 4 in Table 2.18 is an example of the Diels-Alder reaction (with inverse electron demand) of an enamine with a pyran-2-ylidene complex (see also Section 2.2.7 and Figure 2.36). In this example the adduct initially formed eliminates both chromium hexacarbonyl ([4 -I- 2] cycloreversion) and pyrrolidine to yield a substituted benzene. 

Similar to their reaction with phosphaalkenes, l-diazo-2-(oxoalkyl)silanes 29 react with various heterophospholes by [3 + 2] cycloaddition of the diazocumulene system 30 (which is in equilibrium with 29) across the P=C bond. With 2-acyl-1,2,3-diazaphospholes 119 (R = Ac, Bz no reaction with R = Me, Ph up to 60 °C), the expected cycloaddition products 120 (Scheme 8.27) could be isolated (186). Elimination of N2 from these bicyclic A -pyrazolines occurred upon heating at 100 °C and furnished the tricyclic systems 122 when SiRs was a trialkylsilyl group. Apparently, the thermolysis of 120 generates the 5-aIkenylidene-l,2,5-diazaphosphole 121 (by N2 extrusion) as well as diazaphosphole 119 (by a [3 + 2] cycloreversion process), which recombine in an intermolecular cycloaddition to furnish 122. When SiRa = SiPhaf-Bu, a formal intramolecular [3 + 2] cycloaddition of the C=P=C unit with an aromatic C=C bond occurs and the polycyclic compound 123 is obtained (187). 

Intramolecular azomethine ylide cycloaddition to the C—O double bond of an aldehyde was reported in 197369 and cycloaddition to the C—C double bond was first reported in 1975.70 Competition between 1,1- and 1,3-cycloaddition is observed in intramolecular reactions, although intermolecular reactions give only 1,3-cycloaddition. Photolysis of 2//-azirines is one generation method of nitrile ylides applicable to intramolecular cycloaddition.70 Another method involves the base-catalyzed 1,3-elimination of hydrogen halide from alkenyl imidoyl halides. Still other procedures involve thermolytic and photolytic cycloreversions of oxazolinones and dihydrooxazaphospholes. 

Keywords me thane thiol, [2+2]-cycloreversion, [l,2,(3)4]-elimination, melt reaction... 

The acid-catalyzed Peterson olefination is presumably an E2-elimination, that is, a one-step reaction. On the other hand, the base-induced Peterson olefination probably takes place via an intermediate. In all probability, this intermediate is a four-membered heterocycle with a pentavalent, negatively charged Si atom. This heterocycle probably decomposes by a [2+2]-cycloreversion just like the oxaphosphetane intermediate of the Wittig reaction (Section 4.7.3). 

Heating these products (endo and exo mixtures) was also a source of thiocarbonyl compounds, by cycloreversion [213], and led, in the presence of a diene, to different adducts (exo rich equilibrium mixtures). These reactions were applied in the field of morpholine alkaloids and thioshikimic acid synthesis [5]. Another effective source of thioaldehydes are sulfenyl phthalimides [214], which are very prone to elimination, as described hereunder in the work of Capozzi. 

You will learn about the reaction of a-metallated phosphonic add esters with aldehydes in Section 9.4. This reaction also seems to give a fraus-configured oxaphosphetane (Figure 4.41). A. vyu-selective /3-elimination by a [2+2]-cycloreversion of a compound containing a P=0 double bond follows this compound is (EtO)2 P(=0)0. The second elimination product is an olefin. It is predominantly or exclusively traus-configured. 

The nitrogen source for the aziridination of alkenes, a nitrene or nitrenoid, can be generated in various ways (1) oxidation of a primary amine (2) base-induced -elimination of HX from an amine or amide with an electronegative atom X (X = halogen, O) attached to the NH group or by -elimination of metal halides from metal A-arenesulfonyl-A-haloamides (3) metal-catalyzed reaction of [A-(alkane/arenesulfonyl)imino]aryliodanes (4) thermolytic or photolytic decomposition of organyl azides and (5) thermally induced cycloreversion reactions . 

The formation of the 3-aza-l,2,4,6-tetraphosphaquadicyclanes 62a-62e was rationalized by a sequence of [2+2] cycloadditions, insertion reactions, reductive eliminations, [4+2] cycloaddition, and cycloreversion processes (Scheme 25) <1998AGE1233, 2001ZN951, 2000CEJ4558>. 

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