Electrocyclic rearrangements compounds

Several examples pertaining to the photochemical electrocyclization of compounds possessing an aromatic moiety and an olefinic substituent are also known [69, 70]. For example, one of the earliest reports in this series was concerned with the photocyclization-rearrangement of a-(N-methylanilino) styrene (139) which, on irradiation in the absence of oxygen, produced l-methyl-2-phenyl-2,3-dihydroindole (140) in 73% yield, as depicted in Scheme 8.40. 

Walk Rearrangement Electrocyclic Ring Compound AG (kcal/mol) Opening AG (kcal/mol) Ref. 

Irradiation of 1 in the presence of A -methylpyrrole furnished the bicyclic product 9 in almost quantitative yield. On longer heating at 100°C the bicyclic product 9 underwent an electrocyclic rearrangement to afford the cyclohexadiene derivative 10. Renewed attack of 1 (2) on 9 finally led to the seven-membered heterocyclic product 11 which was characterized by X-ray crystallography [7]. This example again illustrates that exchange of the heteroatom in these five-membered ring compounds can result in a eompletely different product spectrum. 

Methylvinyldiazirine (199) rearranges at room temperature in the course of some days. Formation of the linear isomer is followed by electrocyclic ring closure to give 3-methyl-pyrazole. The linear diazo compound could be trapped by its reaction with acids to form esters, while the starting diazirine (199) is inert towards acids (B-71MI50801). 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

Bromination of bicyclopropenyl system 369 at ambient temperature in absolute CHCI3 leads either to diene 372 (15%) and trienes 374-376 (15%, 35% and 10%, respectively) when R = H, or to the stable cyclopropenium salt 371 (95%) when R = Ph (equation 134)188. The electrophilic attack of bromine on compounds 369 creates the cationoid intermediates 370 which undergo either fragmentation to salt 371 (path a) or an electrocyclic ring opening (path b). When diene 372 is heated at about 150 °C in the solid state it rearranges to 1,2,3,5-tetraphenylbenzene 373 with concomitant loss of bromine. 

The (diphenylmethylene)aminocyclobutenecarboxylates 109 obtained by rearrangement of the DMPA-H adducts of 1-Me, 2-Me, contain a 2-azadiene unit and a cyclobutene moiety. Indeed, the parent compound 109 a reacted with 4-phenyl-l,2,4-triazoline-3,5-dione (PTAD, [80]) at room temperature in a [4-1-2] cycloaddition mode to yield the tricyclic tetraazaundecene 132 in almost quantitative yield (Scheme 44) [8]. As substituted cyclobutenes, compounds 109 should be capable of opening up to the corresponding butadienes [1, 2b, 811. When compounds 109 were subjected to flash vacuum pyrolysis, the dihydro-isoquinolines 135 were obtained, presumably via the expected ring-opened intermediates 133, which subsequently underwent bn electrocyclization followed by a 1,5-shift, as is common for other 3-aza-l,3,5-hexatrienes [82]. 

Thus in the N-silyl substituted series, 17 and 18, which rearrange thermally to the corresponding diazo compounds, the stability increases through the series R=Me, Ph, i-Pr. As discussed below, these compound undergo the usual cycloaddition and electrocyclization reactions of nitrile imines and are not simply overstabilized curiosities. The usefulness in synthesis of those with P—C bonds is probably limited since these bonds are not easily broken, but products derived from those with C—Si and C—B bonds (e.g., 21 and 22) should be capable of further... 

Thiocarbonyl ylides are both nucleophilic and basic compounds (40,41,86). For example, adamantanethione (5)-methylide (52) is able to deprotonate its precursor, the corresponding 2,5-dihydro-1,3,4-thiadiazole, and a 1 1 adduct is formed in a multistep reaction (40,86). Thioxonium ion (56) (Scheme 5.22) was proposed as a reactive intermediate. On the other hand, thiofenchone (S)-methylide (48) is not able to deprotonate its precursor but instead undergoes electrocyclization to give a mixture of diastereoisomeric thiiranes (41,87,88). The addition of a trace of acetic acid changes the reaction course remarkably, and instead of an electrocyclization product, the new isomer 51 was isolated (41,87) (Scheme 5.18). The formation of 51 is the result of a Wagner-Meerwein rearrangement of thioxonium ion 49. 

---