Patai S., Rappoport Z. 2000 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 2 / 0739 884

Ar = Ph, p-MeOC6H4

In a search of -donor systems for the preparation of compounds having a metallic conductivity, the bis-thioxanthene cumulene 56 was obtained. It was oxidized by conc. H2SO4 to the acetylenic dication 57 rather than undergoing the expected protonation of the multiple bonds (equation 22)36.

The rearrangement of arylethynyl carbinols 58 that occurs via allene intermediates 59 and 60 in the presence of a polymeric silylvanadate catalyst37 (equation 23) is noteworthy.

In the 1980s an extensive investigation of allenyl cations 62, which were generated from haloallenes 61 and reacted with various nucleophilic reagents, was carried out (for reviews of previous work, see References 16, 18, 38 and 39). The conditions under which stepwise and concerted cycloaddition reactions take place were studied. For example, a treatment of chlorotriphenylallene 61 with silver trifluoroacetate in the presence of cyclopentadiene in pentane and subsequent work up with KOH/EtOH gave a mixture of products 63–66, 68–70 in 95% total yield (equations 24 and 25)40. An analysis of the reaction products has shown that the dienes 65 and 66 can be formed via a stepwise [2 C 2]-cycloaddition while compounds 68 and 69 were produced by a concerted [4 C 2]- cycloaddition through the intermediate allyl cation 6740,41. A change of the reaction conditions resulted in the isolation of triphenylallenium hexachloroantimonate 71 which can be easily hydrated to the ethynyl carbinol 72 and the unsaturated ketone 70 in a ratio of

+

S

(57)

R = H, Me; Ar = Ph, 4-i-PrC6H4, 4-t-BuC6H4, 4-MeOC6H4, 4-MeC6H4, 2,4-Me2C6H3

+

H2O

−H+

Me

(75) Me

III.REARRANGEMENTS OF CONJUGATED DIENES AND POLYENES A. Vinylcyclopropanes and Related Systems

This wide range of transformations includes many reactions which are one way or another connected with cyclopropane derivatives. The cyclopropane moieties can be part of the structure of both the linear dienes or of annulated polycyclic unsaturated systems as well as being part of a spiro compound.

One such typical transformation is the thermal isomerization of the spiropentane derivative 76 into triene 80 which is assumed to occur via the diene intermediate 78 with the intermediate participation of the cyclopropyl –trimethylenemethane (TMM) 77 and the vinyl –TMM 79 diradicals (equation 29)44. It was shown by using deuterium labels that the diradical 79 forms the triene 80 by 1,6-hydrogen shift. The pathway 76 ! 80 which occurs via tetramethylene – ethane diradical was recognized as a less probable route.

Tetramethylene – ethane (TME), or 2,20 -bis-allyl diradical 81, was suggested as an intermediate in the thermal dimerization of allene, as well as in the interconversions of 1,2-dimethylenecyclobutane 82, methylenespiropentane 83, bis-cyclopropylidene 84 and other bicyclic systems (equation 30)45. The isolation of two different isomeric dimethylene cyclobutanes 87 and 88 (in a ca 2 : 1 ratio) after the thermal rearrangement of the deuteriated 1,2-dimethylene cyclobutane 85 suggests that the rearrangement proceeds via a ‘perpendicular’ tetramethyleneethane diradical (2,20 -bisallyl) 86 (equation 31)45.

The participation of the above mentioned trimethylenemethane (TMM) diradicals in the thermal rearrangements of methylenecyclopropanes was also investigated by using systems containing an additional vinyl group which was part of rings fused with the cyclopropane (equations 32 and 33)46,47. The diene 91 was obtained from dichloride 89 (in 50% yield) via the diene intermediate 90 which undergoes the so-called methylenecyclopropane rearrangement to diene 91.

However, bicyclic dichloride 95 under the same conditions was converted into diene 96 and not to the rearrangement product 94 (equation 33). This result is explained by the larger size of the ring, which is far less strained than that in diene 9046. The gasphase thermolysis of diene 91 at 126 –186 °C afforded an almost equimolar mixture of bicyclic diene 92 and triene 93 which are formed, presumably, via the TMM-diradicals

98a and 98b (equation 34). Heating of diene 96 in DMF gave the triene 97 as a result of hydrogen shift (equation 33)46. The intermediate 99 was isolated by using flash pyrolysis of compound 91 at a temperature of 200 (š5) °C and a pressure of 10 3 Torr. As expected, this cis-2-vinyl-1,3,5-hexatriene 99 rearranges smoothly at 220 °C to yield only the triene 9348.

•

•

(29)

• •

••

(30)

(81)