Thermally allowed intermediate

The direct connection of rings A and D at C l cannot be achieved by enamine or sul> fide couplings. This reaction has been carried out in almost quantitative yield by electrocyclic reactions of A/D Secocorrinoid metal complexes and constitutes a magnificent application of the Woodward-Hoffmann rules. First an antarafacial hydrogen shift from C-19 to C-1 is induced by light (sigmatropic 18-electron rearrangement), and second, a conrotatory thermally allowed cyclization of the mesoionic 16 rc-electron intermediate occurs. Only the A -trans-isomer is formed (A. Eschenmoser, 1974 A. Pfaltz, 1977). 

The higher strain energy in thiirene dioxides (19) compared to thiirane dioxides (17) is obvious. Yet, the elimination of sulfur dioxide from the latter is significantly faster than one would expect for a thermally allowed concerted process. Consequently, either aromatic-type conjugative stabilization effects are operative in thiirene dioxides2,12 or the relative ease of S02 elimination reflects the relative thermodynamic stability of the (diradical )99 intermediates involved in the nonconcerted stepwise elimination process. 

Similarly, enamino vinyl sulfones (345) can undergo a thermally allowed electrocyclic reaction between the termini of the enaminic double bond and the allyl sulfonyl portion in the intermediate anion (346) to afford a, /1-unsaturated thiene dioxides (348) as shown in equation 126335. 

In a photochemical cycloaddition, one component is electronically excited as a consequence of the promotion of one electron from the HOMO to the LUMO. The HOMO -LUMO of the component in the excited state interact with the HOMO-LUMO orbitals of the other component in the ground state. These interactions are bonding in [2+2] cycloadditions, giving an intermediate called exciplex, but are antibonding at one end in the [,i4j + 2j] Diels-Alder reaction (Scheme 1.17) therefore this type of cycloaddition cannot be concerted and any stereospecificity can be lost. According to the Woodward-Hoffmann rules [65], a concerted Diels-Alder reaction is thermally allowed but photochemically forbidden. 

A full development of the rate law for the bimolecular reaction of MDI to yield carbodiimide and CO indicates that the reaction should truly be 2nd-order in MDI. This would be observed experimentally under conditions in which MDI is at limiting concentrations. This is not the case for these experimements MDI is present in considerable excess (usually 5.5-6 g of MDI (4.7-5.1 ml) are used in an 8.8 ml vessel). So at least at the early stages of reaction, the carbon dioxide evolution would be expected to display pseudo-zero order kinetics. As the amount of MDI is depleted, then 2nd-order kinetics should be observed. In fact, the asymptotic portion of the 225 C Isotherm can be fitted to a 2nd-order rate law. This kinetic analysis is consistent with a more detailed mechanism for the decomposition, in which 2 molecules of MDI form a cyclic intermediate through a thermally allowed [2+2] cycloaddition, which is formed at steady state concentrations and may then decompose to carbodiimide and carbon dioxide. Isocyanates and other related compounds have been reported to participate in [2 + 2] and [4 + 2] cycloaddition reactions (8.91. 

The reported proposed sequence also offers two additional alternative mechanisms for the cyclodimerization of BCP (3), involving either intermediate 463 or 464 [6a, 13b]. However, they appear less likely, requiring successive three-membered ring fissions and formations. Alternatively, a thermally allowed concerted [jt2s + rt2a -I- pericyclic reaction involving the Walsh type molecular orbital of cyclopropane [124] has been proposed (Fig. 4) [13b]. 

Tius and co-workers investigated a number of cationic cyclopentannelations of allenyl ethers [113] and found that 1-lithio-l-alkoxyallenes 180 react with a,/3-unsatu-rated carbonyl compounds 181 leading to highly functionalized cyclopentenones 182 (Scheme 8.44). The primary products are a-allenyl ketones 183, which form pentadienyl cations 184 by protonation. The latter undergo a thermally allowed 4jt-conrotatory ring closure to intermediates 185, which with elimination of R1 finally lead to the expected products 182 (Scheme 8.45). 

Attack by the carbene on a suitably placed aryl ring (Scheme 58152) gives an intermediate 164, which undergoes a thermally allowed suprafacial [l,5]-hydrogen shift to give the indene 165.151,152 The intramolecular nature of the hydride shift was established by lack of exchange with deuterium in the solvent.152... 

Thus, nucleophilic attack of the nitrogen of the a, 3-unsaturated imine (21) (Scheme 6) on the electrophilic carbon atom of ketenes (2) leads to the formation of zwitterionic intermediates (22) in the (3 and 8 conformations. The thermally allowed [n4c] reaction (22(3) leads to the formation of (3-lactams (23), whereas the [n6d] electrocyclization of (228) leads to the formation of the corresponding 8-lactams (24) (Scheme 6). 

Sulfines are obtained by oxidation of thioketones and, after some days, elemental sulfur is formed and the corresponding ketones are produced quantitatively. A possible mechanism is the thermally allowed electrocyclization of sulfines to give an intermediate oxathiirane 141 which, upon sulfur extrusion, affords the corresponding ketones409. 

The potential of IR ellipsometric spectroscopy (IRES) for investigating surface processes and reactions relevant to gas-solid heterogeneous catalysis is examined, both for single crystal and model dispersed catalytic systems. With it, structural and chemical changes can be followed over a wide range of temperature and gas pressure, allowing one to thermally stabilize intermediates for investigation, and study surface species under conditions close to those in practical catalytic reactions. 

This reaction has been shown to occur in two thermally allowed steps. Show the structure of die intermediate in the reaction and explain why each step is allowed. 

Concerted formation of 7-4 would be a [tt/ -l- tt/] cycloaddition, which is not thermally allowed. Thus, formation of this product probably involves a diradical intermediate ... 

The dipolar intermediate 35 is pivotal in further reactions. The formation of 36 requires a 1,2-hydrogen shift and formally at least such a shift would be a thermally forbidden pericyclic reaction. An alternative to the forbidden process would involve a thermally allowed 1,5-shift to give, for example, 37... 

The substitution pattern of the cyclopentenones requires the loss of the oxo group rather than of the carbonyl of the carboxyl group. The mechanism is believed to consist of the formation of an acylketene with subsequent thermally allowed tc 2s -f tc 2a electrocyclic reaction leading to an intermediate cyclopropanone which then undergoes carbon monoxide elimination ... 

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