Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 0641 703

SCHEME 8

Considerably improved dimerization of medium small ring alkenes was obtained in copper(I) salt catalyzed irradiations. Interestingly, the dimerization of small37 and bridged bicyclic40 alkenes 40 and 46, respectively, afforded the corresponding cis anti cis dimers as major products, whereas dimerization of cyclohexene and cycloheptene37 afforded the trans anti trans dimers 43 and 51, respectively (Scheme 10). These results are consistent with the suggested mechanism for the cases of catalyzed photoisomerizations to the E-alkene that undergo stereoselective dimerization with a Z-alkene affording the trans anti trans geometry in the produced dimer.

B. Dimerizations of Vinylaryls

The photodimerization of trans-stilbene was first discovered by Ciamician41 at the beginning of the twentieth century; however, the structures of the major product 52 and the minor isomer 53 (Scheme 11) were determined much later42. The mechanism of the photoaddition was extensively investigated and recently discussed by Meier43. Due to the fact that intersystem crossing of alkenes and polyenes is generally inefficient from S1 to its corresponding T1, the reaction proceeds stereospecifically upon direct irradiation, via the singlet excited state by diffusion-controlled formation of nonfluoresent singlet excimers, which transfer to the minima D1 of the doubly excited singlet state for a pericyclic geometry of the intermediate and are rapidly deactivated to the ground state of the products44. Excimers and pericyclic minima determine the regioselectivity (H,H vs H,T) and the stereoselectivity (syn vs anti) of the cycloaddition reaction. However, predicting the regiochemistry is difficult as noted by Meier43, since according to the perturbation theory, the head-to-head adducts should give the most stable excimers and the head-to-tail adducts the most stable pericyclic minima44.

The photodimerization is a reaction competing with the previously discussed photoisomerization (Section III). The quantum yields of photodimerizations are affected by the concentration of the photosubstrate (c 0.1 mol L 1) and the polarity of the

Ph

SCHEME 12

Nishimura and coworkers49 have systematically examined the photodimerization of divinylnaphthalenes and found that 1,3-, 1,7-, 2,3-, 2,6- and 2,7-divinylnaphthalenes afforded the corresponding [2.2]naphthalenophanes, whereas 1,4-, 1,5- and 1,6- divinylnaphthalenes gave polymeric material with no detection of [2.2]naphthalenophanes. The best yields of this reaction were found in systems 61b and 61c, indicating the effect of substituents on the efficiency of the photoaddition (Scheme 13). Although no mechanistic work has been done yet, the observation of half-cyclized products was considered to indicate a stepwise mechanism49. Further application of this chemistry took place in the preparation of various [2.2]biphenylophanes of type 64 and its regioisomer 65. Intermolecular photocycloaddition of the corresponding divinylbiphenyls 63 afforded two

(59)

hν

SCHEME 13

hν

(63)

(65)

SCHEME 13. (continued)

X

hν

Birch reduction

(a)X = CH2

(b)X = Me2 Si

(68)

SCHEME 14

O

O

hν

out of the six possible dimers (endo,endo; endo,exo; exo,exo of each regioisomer) in ca 10% yield.

Intramolecular photocyclodimerizations of styrene systems of type 66 have been applied50 for the synthesis of cyclophanes (Scheme 14). The irradiation of compound 66a afforded straight photoproducts 67, subsequently transformed, upon reductive cleavage of the produced four-membered ring, to cyclophane 68. The approach seems successful for longer chains between the styrene units and allows introduction of heteroatoms such as oxygen51,52 (69) and silicon53 (66b).

V. [2 + 2] PHOTOCYCLOADDITIONS OF ALKENES TO ENONES

The interand intramolecular [2 C 2] photocycloadditions of CDC bonds, affording four-membered ring structures, became the method of choice for the synthesis of cyclobutane systems. Extensive investigation on the mechanistic aspects and synthetic applications54 58 were carried out during the past three decades. The successful control in the regioand stereoselectivity of the photocycloaddition, especially in the intramolecular cases56,57, prompted many research groups to apply this reaction as the key step for the formation of strained four-membered rings, which in many cases were followed by subsequent selective fragmentation to afford stereoselective synthesis of polycyclic or macrocyclic structures. It should be noted that there are still open questions on the nature of the excited chromophore, especially of conjugated systems (conformation, polarity etc.), and its effect on the stereoselectivity of the photocycloaddition process. Thus, the stereoselectivity of this reaction, which probably could be considered as the most successful contribution by far of photochemistry to organic synthesis, could not yet be predicted for any given case. However, in some cases, good predictions can be made based on current knowledge and experimental data available.

The photodimerization of unconjugated alkenes, discussed above, could be regarded as the most synthetically useful photocycloaddition of CDC double bonds in the intermolecular photocycloadditions. Mixed photocycloadditions between two different CDC double bonds generally give mixtures when both possess similar photoreactivity. In fortunate cases, one of the possible isomers may predominate. A special category of mixed alkene photoaddition that is generally selective is the reaction of ˛,ˇ-unsaturated ketones (enones) with unconjugated CDC double bonds via selective excitation of the enone chromophore. This has been investigated extensively for mechanistic studies and synthetic applications, especially after Corey’s elegant synthesis of caryophyllene and isocaryophyllene (Scheme 15) using this reaction as the key step in the synthetic sequence59.

A. Regioselectivity

1. Intermolecular photocycloadditions

Based on systematic investigation of this reaction, Corey and coworkers60 proposed the formation of an ‘oriented -complex’ (exciplex) between the excited enone and the alkene in its ground state. The reaction proceeds via formation of a 1,4-diradical that cyclizes to the corresponding four-membered ring (Figure 4a). Following some kinetic studies, Loutfy and de Mayo61 suggested that the proposed triplet exciplex is formed irreversibly and is short-lived. The exciplex gives rise to a 1,4-diradical that can either cyclize to produce a four-membered ring or revert back to the starting materials. Its modified mechanistic scheme is presented in Figure 4b.

The proposed exciplex orientation, suggested to dictate the regioselectivity of the photoaddition, is consistent with the orientation of the dipolar attraction between the electronically n, Ł excited triplet enone and the ground state alkene. Generally, electron acceptor substituents on the alkene provide preferential formation of the head-to-head (H,H) products, whereas electron donor substituents provide preferential formation of the head-to-tail (H,T) photoproducts55 (Scheme 16).