The aza-di-n-Methane Rearrangements

Photochemical rearrangement reactions of 1-aza-1,4-dienes and 2-aza-1,4-dienes from their triplet states to form corresponding cyclopropylimines are known as the aza-di-n-methane rearrangements [62]. 

The rearrangement is analogous to the di-n-methane rearrangement and is considered as 1,2-shift of imino group from C(3) to C(4), followed by a o bond formation between C(3) and C(5). For example, the photorearrangement of P,y-unsaturated imine 55 gives cyclopropyl imine 56, which on hydrolysis gives cyclopropane aldehyde 57 [62]. 

This reaction is useful for synthesis of cyclopropane carboxylic acids. Similarly, the p,Y-unsaturated oxime acetate 58 gives 59 in high yield [63]. 

The presence of an electron-withdrawing group in the oxime ester improves the yield of the reaction. For example, oxime 60 gives 61 in 90 % yield [64]. 

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Me) cyclizes efficiently to give only (43, R = Ts, = Me) in 75% yield. In competition with this cyclization mode the aza-di-n-methane process occurs to yield the cyclopropanes (44) as shown in Scheme 3. Remarkably, the hydrazone derivatives (45) only undergo the aza-di-n-methane rearrangement to yield (46) efficiently. 

Armesto, D. and Ramos, A., Photochemical synthesis of oxime acetates derivatives of 1-carbalde-hydobicyclo[n.l.O]alkanes by the aza-di-Ji-methane rearrangement. Tetrahedron, 49, 7159-7168, 1993. 

I, 743,1987 (b) Armesto, D., Horspool. W. M., Langa, R, and Perez-Ossorio, R., Substitution effects on the aza-di-7t-methane rearrangement of imines, /. Chem. Soc., Perkin Trans. 2, 1039,1987 (c) Armesto, D., Horspool, W.M., and Langa, R, The aza-di-Jt-methane rearrangement of l-aryl-4,4-dimethyl-6,6-diphenyl-2-azahexa-2,5-dienes. The influence of substituents on the N-benzyl group, /. Chem. Soc., Perkin Trans. 2, 903, 1989 (d) Armesto, D., Horspool, W.M., Mancheno, M.J., and Ortiz, M.J., Chemically efficient aza-di-Jt-methane photoreactivity with novel stable derivatives of P,y-unsaturated compounds, /. Chem. Soc., Perkin Trans. 1, 2325, 1992, and papers cited in Reference 37. 

The hetera-substituted di-n-methane rearrangement is currently an active area of investigation. So far, most examples have been reported for oxa and aza substitution. From the available data, it appears that the rearrangement is operative only when the heteroatom is in the 1-position. Replacement of the sfP-carbon at the 2-position or the spi carbon at the 3-position by heteroatoms such as O, N, S, Se, B, P, etc. entails systems which have not been investigated or which have proved incapable of giving di-n-meth-ane products. 

The di-7t-methane rearrangement has also been observed in other 1,4-unsaturated systems, such as nitrogen-containing compounds.630,642 The corresponding aza-di-n-methane triplet-sensitized (sens) rearrangement of the ( , /-unsaturated imine 91 produces, inefficiently ( 0.01), a single product, cyclopropylmethanimine (92), which can subsequently be easily converted to an aldehyde by acid hydrolysis (Scheme 6.36).661... 

Aza-di-n-methane (ADPM)- rearrangement reactions of both 1-aza- and 2-aza-1,4-dienes are known but not so common at the present time. Nevertheless, and largely due to the work of Armesto et a/., the potential utility of such processes is becoming clearer. In the case of ADPM rearrangement reactions involving 1-aza-1,4-dienes, regioselective conversions are observed with, for exanple, the oxime 23 being converted into cyclopropane 24 in 74% yield after irradiation under triplet-sensitized conditions for just 30 min fScheme 9.S1. 

The production of 69 and 70 in this photoreaction is in accord with the operation of mechanistic pathways involving the generation and competitive cleavage of aziridinyl-dicarbinyl biradical 71 (Scheme 12). Thus, the major photoproduct 69 is formed by C—N bond cleavage in 71 (path a). The alternative fragmentation of the C—C bond in 71 affords intermediate 72, which cyclizes yielding aziridine 70 (path b). As such, this represents the first example of a 2-aza-di -rr-methane (2-ADPM) rearrangement that occurs via a three-membered-... 

In spite of the large number of studies carried out on the DPM and the ODPM rearrangements since 1966, ten years elapsed before the reaction was extended to other 1,4-unsaturated systems, particularly to C-N double bond derivatives. The first example of a 1 -aza-di-Ti-methane (1-ADPM) rearrangement was reported by Nitta et al. in a study on the photoreactivity of the tricyclic oximes 5 [10]. Direct irradiation of compound 5a brought about the formation of the DPM product 6 and the 1-ADPM derivative 7a, in the first example of competition between these two processes. However, the methyl substituted derivative 5b yielded the 1-ADPM photoproduct 7b, exclusively (Sch. 4) [10a]. 

In summary, our results on the triplet-sensitized photochemistry of a series of 2-aza-1,4-dienes have uncovered the first examples of 2-ADPM rearrangements yielding cyclopropyl imines. In the case of azadiene 1, the N-vinylaziridine 3 was also obtained in the first example of a di-Jt-methane rearrangement... 

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