Di-Jt-methane rearrangements,

A simple example serves to illnstrate the similarities between a reaction mechanism with a conventional intermediate and a reaction mechanism with a conical intersection. Consider Scheme 9.2 for the photochemical di-tt-methane rearrangement. Chemical intnition snggests two possible key intermediate structures, II and III. Computations conhrm that, for the singlet photochemical di-Jt-methane rearrangement, structure III is a conical intersection that divides the excited-state branch of the reaction coordinate from the ground state branch. In contrast, structure II is a conventional biradical intermediate for the triplet reaction. 

Now let us return to our discussion of the conical intersection structure for the [2+2] photochemical cycloaddition of two ethylenes and photochemical di-Jt-methane rearrangement. They are both similar to the 4 orbital 4 electron model just discussed, except that we have p and p overlaps rather than Is orbital overlaps. In Figure 9.5 it is clear that the conical intersection geometry is associated with T = 0 in Eq. 9.2b. Thus (inspecting Figure 9.5) we can deduce that... 

Let us summarize briefly at this stage. We have seen that the point of degeneracy forms an extended hyperline which we have illnstrated in detail for a four electrons in four Is orbitals model. The geometries that lie on the hyperline are predictable for the 4 orbital 4 electron case using the VB bond energy (Eq. 9.1) and the London formula (Eq. 9.2). This concept can be nsed to provide nseful qualitative information in other problems. Thns we were able to rationalize the conical intersection geometry for a [2+2] photochemical cycloaddition and the di-Jt-methane rearrangement. 

Nair, V., Nandakumar, M.V., Anilkumar, G.N., Maliakal, D., Vairamani, M., Prabhakar, S., and Rath, N.P. (2000) Cycloaddition of 2-oxo-2H-cydohepta[b] furan derivatives with arylacetylenes and di-Jt-methane rearrangement of... 

Zimmerman, H.E. and Schissel, D.N. (1986) Di-Jt-methane rearrangements of highly sterically congested molecules inhibition of free-rotor energy dissipation. Mechanistic and exploratory organic photochemistry. Journal of Organic Chemistry, 51 (2), 196-207. 

Armesto, D., Ortiz, M.J., and Agarrabeitia, A.R. (2004) Novel di-Jt-methane rearrangements promoted by photoelectron transfer and triplet sensitization, in CRC Handbook of Organic Photochemistry and Photobiology, 2nd edn (eds W. M. Horspool and... 

Sadeghpoor, R., Ghandi, M., Najafi, H.M., and Farzaneh, F. (1998) The oxa-di-jt-methane rearrangement of p.y-unsaturated ketones induced by the external heavy atom cation effect within a zeolite. Chemical Communications, (3), 329-330. 

Steric and electronic effects on the photochemical reactivity of oxime acetates of p/y-unsaturated aldehydes. Journal of the Chemical Society, Perkin Transactions 1, 163-169 (b) Armesto, D., Horspool, W.M., Mancheno, M.J., and Ortiz, M.J. (1990) The aza-di-jt-methane rearrangement of stable derivatives of 2,2-dimethyl-4,4-diphenylbut-3-enal. Journal of the Chemical Society, Perkin Transactions... 

Unsaturated ketones undergo a formal 1,2-acyl shift from the lowest excited triplet state to afford cyclopropyl ketones (for some early examples cf. Houben-Weyl, Vol. 4/5b, p 794), a reaction which has been termed oxa-di-Jt-methane rearrangement.Acyclic ketones normally do not undergo this reaction, however, an exception is in systems where the C-C double bond is further conjugated, e.g. to a phenyl group. 

Erythrolide A (86) has been thought to be produced naturally from the diterpenoid 87 by a light-induced di-Jt-methane rearrangement (Scheme 6.33), because both compounds were isolated from the Caribbean octocoral Erythropodium caribaeorum.656 In order to confirm this hypothesis, the potential precursor 87 was irradiated under various conditions and 86 was obtained in relatively high chemical yields. 

Oxa-di-Jt-methane rearrangement leads to cyclopropane derivatives, compounds that are otherwise difficult to synthesize. The diphenylenal 88, for example, is converted to the cyclopropyl aldehyde 89 by triplet sensitization (Scheme 6.35).660 The photoproduct can be further transformed to other compounds, for example a diphenylvinylcyclopropane derivative 90. 

Zimmerman, H. E., Mariano, P. S., Di Jt methane Rearrangement. Interaction of Electronically Excited Vinyl Chromophores. Mechanistic and Exploratory Organic Photochemistry 41,7. Am. Chem. Soc. 1969, 91, 1718 1727. 

Acetophenone-sensitized irradiation of (83) affords an 80% yield of the triquinane derivative (84). This product is the key intermediate in a new synthesis of (— )-hirsutene. The tricyclotridecenones shown in Scheme 3 are photoreac-tive. Irradiation with acetone as the sensitizer brings about an oxa-di- jt-methane rearrangement to yield (85) while direct irradiation in benzene gives (86) by a... 

The Di-jt-methane Rearrangement and Related Processes. - The crown ether annelated dibenzobarrelene (60) exhibits medium-dependent photochemistry. In solution irradiation affords the COT derivative (61), while in the crystal the di-ji-methane rearrangement occurs to yield the semibullvalene derivative (62). A modification of the normal di-Jt-methane reactivity has been proposed to account for the direct irradiative conversion of deuterium-labelled 7- and... 

The photoreactivity of the enone derivatives (110) has been studied. The outcome of the reactions appears to be dependent on substitution and on ring size. All the derivatives studied undergo trans,cis-isomerization to afford (111). Three of the derivatives undergo the di-Jt-methane rearrangement to yield (112). Only one example (110, R = Me, n = 1) undergoes (2 + 2)-cycloaddition to yield... 

Photochemical oxa-di-jt-methane rearrangement of bicyclo[3.2.1]octa-noid scaffolds affords multifunctional, donor-acceptor cyclopropanes (34). Application of this reaction to bicyclo[2.2.2]octenones has been used as a key step in the total synthesis the hirsutane-type sesquiterpenes (-)-connatusin A (35), (+)-connatusin B (36), and the marine sesquiterpenoid 2-isocyanoallopupukeanane (37). ... 

Not less useful is the photochemistry of non-conjugated dienes. Thus, 1,4-dienes undergo a typical photo-rearrangement to vinyl cyclopropanes, which may be a valid way for building a three-membered ring, particularly in a complex structure (this is known as the di-jt-methane rearrangement, see Scheme 2.2). p,y-Unsaturated ketones and imines rearrange in the same way to cyclopropylketones and imines respectively [9]. 

---