Oxa-di- ir-methane

Irradiation of the salt crystals of benzocyclohexadienone carboxylic acid 72 with optically pure amines caused enantioselective oxa-di-ir-methane photorearrangement to give the chiral product 73 [82]. The best results were obtained with (S)-( —)-1-phenylethylamine in around 80% ee (Scheme 16). The other amines gave optical purities of 20-50% ee. 

The chemistry of the oxa-di-ir-methane rearrangement is covered in Chapter 2.6, so that here, for the sake of completeness, only a few remarks on its more salient features are presented. 

The 3-oxa-di- i -methane rearrangement is far less common. As a symmetrical system, the product should be a vinyl epoxide (equation 23). So far, no 3-oxa-di-T -methane products have been observed from the divinyl ethers depicted in the equation. It was shown that the solution photochemistry of (21a) derives from its singlet i , i -state and parallels that of the unsubstituted divinyl ether (21d) and of furan in the gas phase. All products could be rationalized in terms of the initial formation of a singlet vinyl-vi-nyloxy radical pair (equation 24). Triplet sensitization brings about cis-trans isomerization and consequent deactivation. Had the 3-oxa-di- ir-methane product, i.e. the vinyloxirane (22), been formed, it would have decomposed via Griffin fragmentation as confirmed by irradiation of an authentic sample. 

An explanation for the lack of formation of 3-oxa-di- ir-methane products is offered in terms of Zimmerman s bond order control. Thus, while the 1,4-diene is a four-electron system with bonding between... 

Of mechanistic interest in this context is the independent generation of di-ir-methane diradicals of 3-oxa-di-TT-methane systems which do not give the rearrangement, in order to probe whether such diradicals at least in principle are prone to give di-ir-methane products. For this purpose, the cyclopropyldicarbinyl diradical in equation (25) was generated via photochemical carbon monoxide extrusion. Indeed, the expected oxa-di-ir-methane product 2,5-dimethyl-4,5-epoxy-2-hexene was formed, as well as other products. 

This photochemical transformation, shown in equation (42), is a process that occurs especially frequently in the oxa-di-ir-methane rearrangement. This isomerization may proceed via a Norrish type I reaction. 

Adam and his coworkers have studied the oxa-di-ir-methane rearrangement of the norbornenone (209). They approached the problem by the synthesis of precursors of the putative biradicals such as (210). The oxa-di-ir-methane rearrangement of the P,7-unsaturated enones (211, 212) has been reported by Schultz and his coworkers. 

When only one of the two double bonds is part of a ring, there are many possible combinations these have already been reviewed. A recently reported representative example involves the photochemistry of (7), which contains both the aryl and the acetyl chromophores (equation 7). The observed products arise both from the standard di-ir-methane and oxa-di-ir-methane rearrangements. 

The l-aza-S-oxa-di-ir-methane rearrangement of (25) does not occur. Instead, cis-trans isomerization and photoelimination of methanol are observed for this oxime (equation 21) Finally, the l-oxa-4-aza-di-iT-methane rearrangement of compound (26) also did not take place. This substrate gave rise to the photo-induced l,S-benzoyl migration shown in equation (28). ... 

Most 3,y-unsaturated ketones rearrange upon triplet sensitization to cyclopropyl ketones via the so-called oxa-di-ir-methane (ODPM) reanangement, the result of which is an overall 1,2-acyl shift as schematically shown for (1) -+ (2). 3,y-Enone arrangements in which the alkene moiety is further conjugated to either phenyl or sites also undergo this transformation upon direct excitation, i.e. via the singlet excited state. 

Oxa-di- ir-methane Photoisomerizations I (cf. also Section 2.6.3.4.1) II (this Section)... 

Formation of Four-membered Heterocycles Photochemical Cycloadditions The Patemo-Bttchi Reaction Di-ir-methane Photoisomerizations Oxa di-ir methane Photoisomerizations... 

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