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

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13. Photochemistry of compounds containing CDC double bonds

691

The effect of substituents on the stereoselectivity of the intramolecular photocycloadditions of alkenes to cyclohexenones was systematically examined by Becker and coworkers84 who obtained high stereofacial selectivity in compounds 283a c. However, small changes in the position, geometry or steric effect of the substituents have dramatically affected the selectivity, indicating the complexity in predicting the stereoselectivity in such system (Scheme 61).

Becker and coworkers125 have examined the intramolecular photoaddition of chiral allenes 286 and 288 to cyclohexenone. Based on the quantum yields of the disubstituted allenes 286a (0.81), 286b (0.63) and 286c (0.6) and of the trisubstituted allene 286d (0.11) they concluded that the disubstituted allenes approach the cyclic enone in a highly stereoselective manner in which the hydrogen points toward the enone and the substituent points in the opposite direction. Preliminary investigations on the irradiation of the chiral allene 288 afforded poor chiral induction and low ee was obtained (Scheme 62).

	R2	R1		O	R1
					R2
O	C		hν
			hν		O	C
					O	C
	(286)				(287)	(288)
		R1	R2			(288)
		R1	R2	Quantum yield
	(a)	H	H	0.81
	(b)	Ph	H	0.63
	(c)	t-Bu	H	0.60
	(d)	Me	Me	0.11

SCHEME 62

Recently, Carreira and coworkers126 obtained very high asymmetric induction (83 100%) in intramolecular [2 C 2] photocycloadditions of 1,3-disubstituted allenes with enones 289 and 290 (Scheme 63).

Stereoselective intramolecular photocycloaddition of alkenes to enones with a chiral auxiliary located at the enone chromophore is not a well explored approach. The use of conformationally rigid chiral dioxinones, found to provide good diastereofacial selectivity in the intermolecular photoadditions, are described in Scheme 43. The ﬁrst successful example of very high stereofacial selectivity in the intramolecular photocycloaddition of alkenes to chiral dioxinones have recently been reported by the group of Sato127 on the irradiation of compound 291. The single product 292 was obtained in the irradiation of 291b in 90% yield via selective approach of the alkene from the more exposed side of the dioxinone. Interestingly, a 1:1 mixture of isomers was obtained when the side chain was reduced by one carbon. The authors attribute the lack of diastereofacial selectivity in the photoaddition of 291a to additional geometrical constraints imposed on the reaction sites which mainly determine the preferential site by kinetic control irrespective of the

692			Nizar Haddad
H	Bu-t		O
			O
O	C
		hν	Bu-t
		hν	n ( )
			n ( )
			O

( )n

(289)(a) n =2 89% yield, >99% ee

(b) n=1 72% yield, >83% ee

t-Bu

hν

88% yield, 100% ee

H O O

(290)

SCHEME 63

b-side

( )n

sofa-conformation

a-side

(291) ν h

	O	Me
		Me
	O		( )n
	O
	(292)
(a) n = 1		1	:
(b) n = 2		1	:

SCHEME 64

O O

	O
	( )n
	O
	O
	Me
	(293)
1	70% yield
0	90% yield

13. Photochemistry of compounds containing CDC double bonds

693

conformation of the dioxinone, whereas homologation of the alkenyl side chain (291b) reduces the geometrical constraint and the facial selectivity is strongly affected by the dioxinone conformation while the approach of the alkene is preferable from the more exposed side, as found in the intermolecular cases (Scheme 43). In this regard the authors suggest that the dioxinones exist as the sofa-conformation in the excited state, just like in the ground state (Scheme 64).

The fact that diradical intermediates formed in the intramolecular photocycloaddition of alkenes to cyclic enones does not cleave efﬁciently to the starting material, as pointed out by several research groups77,79,85, suggests that the conformation of the cyclic enone at the reactive triplet excited state and the order of the ﬁrst bond formation play an important role in the diastereofacial selectivity of the photocycloaddition of cyclic enones under kinetically controlled conditions. Alternatively to the sofa-conformation of dioxinones in the triplet excited state, suggested by the groups of Demuth97 and of Sato127, strong pyramidalization was found in the ab initio calculations of the triplet excited states of dioxinone conformations 294, 295 and 296, reported by Seebach and coworkers128 who suggested, in accordance with Wiesner’s model, that pyramidalization of ˇ-carbon in the triplet excited state of chiral 1,3-dioxin-4-ones is strong and could be the origin of the observed stereoselectivity (Figure 5).

Systematic study on the diastereofacial selectivity in the intramolecular photocycloaddition of alkenes to chiral dioxinones was recently reported by Haddad and coworkers129 on compounds of type 298. Preferred pyramidalization in the direction of the less exposed side (the axial methyl at the acetal center) described in structure 298b, and ﬁrst bond formation at this position (found to be the case in dioxinones 143 and 146, Scheme 31), are essential features for obtaining selective photocycloadditions of alkenes to chiral dioxinones from this side, leading to the kinetically favored products. In such cases the preferred approach is not necessarily from the more exposed side (Figure 6).

The preferred facial selectivity from the less exposed side (b-side) obtained in the irradiation of 301 under kinetically controlled conditions (entry 4) cannot be explained only on the basis of steric effect; however, it is consistent with the direction of pyramidalization in structure 299. The increase in the facial selectivity from the same side, upon reducing the steric effect in compound 301d, emphasizes that steric effects cannot be neglected in rationalizing the facial selectivity in these or related systems (Scheme 65).

General and stereoselective synthesis of spiroethers and less thermodynamically stable spiroketals have recently been developed by Hadded and coworkers129,130. The key step is the intramolecular photocycloaddition of chiral dioxinones of type 305 to dihydropyrones. Subsequent fragmentation of the produced four-membered ring provides, after oxidative enlargement of the cyclic ketone, the thermodynamically less stable spiroketal 310 (R D H) as was demonstrated on photoproduct 308 (Scheme 66).

FIGURE 5. 3-21G triplet states energies and related conformations of 1,3-dioxon-4-one system. Reprinted with permission from Ref. 128. Copyright (1988) American Chemical Society

694		Nizar Haddad
			R1		R1
			R1
			b-side
			O	O	O	O	O
		O		O		O
		O
O	O	O			R2
O	O	R2	H		R2
		R2	H			H
		R	a-side			H
		R	a-side
		Sofa-conformation
	(297)		(298)		(299)
(a) R1 =H, R2 =Me					1
					R
(b) R1 =R2 =Me
					O	O	O
					R2

(300) H

FIGURE 6. Possible triplet conformations of chiral 1,3-dioxin-4-ones

	O			O				O
				H				H
	O		O					O
			hν		R2	+			R2
R1	O	R2	R1	O			R1	O
		(	)n	(	)n			(	)n
	(301)			(302)				(303)
	Racemic				Racemic
(a) R1 = Me, R2 = H, n = 1; (b) R1 = R2 = Me, n = 1; (c) R1 = Me, R2 = H, n = 2; (d) R1 = R2 = H, n = 2
Entry Compound			Products (ratio)	T (°C)			O
Entry Compound			Products (ratio)	T (°C)				H
								H
1	301a	302a (1.0), 303a (2.0)		0		O
2	301b	302b(1.0), 303b(2.4)		0 or −70				H
2	301b	302b(1.0), 303b(2.4)		0 or −70
3	301c	302c (1.0), 303c (1.0)		0		H	O
3	301c	302c (1.0), 303c (1.0)		0		H
4	301c	302c (1.8), 303c (1.0)		−70			(304)
							(304)
5	301d	302d(3.3), 303d(1.5), 304 (1.0)		0
6	301d	302d(17), 303d(3.7), 304 (1.0)		−70

SCHEME 65

13. Photochemistry of compounds containing CDC double bonds

695

a: R1 =H, R2 =t-Bu

EtO2 C

CO2 Et

b: R1 =Me, R2 =Ph

hν

(305)

EtO2 C CO2 Et

(306)

(307)

R1 = H, R2 = t-Bu

1.8

85% yield

reference 130

R1 = Me, R2 = Ph

1.0

70% yield

reference 131

EtO2 C

CO2 Et

EtO2 C

CO2 Et

EtO2 C

CO2 Et

(308)

R= H, EtO

(309)

(310)

SCHEME 66

VI. DI- -METHANE REARRANGEMENT

One of the important photoreactions of CDC bonds is the di- -methane rearrangement. Because of space limitation, the azadi- -methane132 and the oxadi- -methane133 rearrangements are not reviewed in this chapter, and we brieﬂy summarize the principles and typical examples of the di- -methane rearrangement which was recently reviewed by Zimmermen134, the principal researcher of the process135.

The di- -methane rearrangement, takes place on structures that possess two -groups connected with a single carbon (the ‘central’ carbon), upon direct and/or sensitized irradiation of a CDC double bond chromophore, affording vinylcyclopropane products. Zimmerman proposed a stepwise cyclization of the excited -system via a 1,4-diradical of

696	Nizar Haddad

type 312 (Scheme 67) that rearranges to the corresponding 1,3-diradical 313, followed by cyclization to the cyclopropyl ring 314 as described in the typical examples in Scheme 67.

			•
hν
•	•		•
(311a)	(312a)	(313a)	(314a)
			•
hν			Ph
			Ph
	•	•	•
(311b)	(312b)		(313b)
			Ph
			(314b)

SCHEME 67

Generally, the di- -methane rearrangement proceeds efﬁciently via the singlet excited state136 in acyclic systems such as 315. Triplet excitation of such systems, using typical triplet sensitizers (benzophenone, acetophenone or acetone), reveals an alternative Z E isomerization, usually found to proceed more efﬁciently than the di- -methane rearrangement. However, inhibition of the free rotation in the Z E isomerization by either increasing the steric hindrance on the ‘central’ carbon or incorporating the di- -methane system in cyclic structures137 revealed an efﬁcient di- -methane rearrangement as shown with compounds 318 and 320. A similar effect was achieved upon substitution of the ‘central’ carbon by strongly odd-electron stabilizing groups such as cyano138, phenyl139 or carbomethoxy140 as illustrated in Scheme 68.

Substituents were found to play an important role in controlling the regioselectivity of the di- -methane rearrangement in compounds 324a and 324b141. Electron-withdrawing groups (W) and electron-donating groups (D) promote isomerization of the 1,4-diradical 325 via the (a) pathway, in which the (W) group ends up at the cyclopropyl part as shown by structure 327. This selectivity could be rationalized by formation of the more stable142 1,3-diradical 326 (Scheme 69).

Zimmerman and Baum143 obtained selective rearrangement in compound 324c, which was rationalized by preferred formation of the delocalized homoallylic radical 326c, followed by subsequent cyclization of this radical to the corresponding cyclopropyl product. Interestingly, this delocalization effect was also demonstrated in the di- -methane rearrangement of the bicyclic ˛-naphthobarrelene 330 labeled by deuterium at the bridged positions144. Sensitized irradiation of 330 afforded the more delocalized diradical 331 that underwent rearrangement to 332, followed by two possible cyclizations to give 335 and 336. The positions of the labeled carbons preclude the alternative mechanism via diradical 332a (Scheme 70).

The application of the di- -methane rearrangement in organic synthesis could be expected to increase. Pattenden and Whybrow145 have applied this rearrangement as a key step in the total synthesis of (š) desoxytaylorione 341 (Scheme 71).

Interestingly, much attention has been paid to the di- -methane rearrangement in solid state chemistry146.

13. Photochemistry of compounds containing CDC double bonds					697
		hν
				Ph
	Ph			Ph
Ph	Ph			Ph
Ph				Ph
(315)			(316)
			hν
	sensitizer

			Ph	Ph
			Ph
			(317)
		hν
		Benzophenone	Ph
		Benzophenone			Ph
					Ph
Ph		Ph		Ph	Ph
Ph	Ph	Ph
Ph	Ph
(318)					(319)
		hν
		A cetone
(320)					(321)
X	X				X
		hν			X
		Direct or	R
Ph		sensitized			Ph
Ph		Ph			Ph
R	Ph	Ph		R	Ph
R	Ph			R	Ph
	(322) (a) X = Ph, R= Me			(323)

(b)X = CN, R= Ph

(c)X = CO2 Me, R= Ph

SCHEME 68

698

Nizar Haddad

•

hν

•

favored

(324)

(325)

(326)

(a) D = p-Me2 NPh; W= Ph

disfavored

(b) D = Ph; W= p-CNC6 H4

•

D W

(329)

(328)

(327)

SCHEME 69

(340)

•D

•

(345)

	• D	•	D
	• D	•
hν	•		•
hν	D	D	•
	D	D
	(341)		(342)
	(341)
	• D	•	D
		•	•
	•	D
	D	D
	(343)		(344)
	D			D
	D			D
+
D	D		+	D
			+

	(346)	(347)		(348)
				were not formed

SCHEME 70

13. Photochemistry of compounds containing CDC double bonds			699
O	H	H
O	O
	hν
	n-C4 H9		n-C4 H9
	n-C4 H9
(339)	(340)	(341)
	SCHEME 71

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700	Nizar Haddad

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