Topochemical photodimerization of 7-acetoxycoumarin: the acetoxy group as a steering agent

Abstract

Among the various substituted coumarins investigated only 7-acetoxycoumarin is observed to photodimerize topochemically in the crystalline state and this observation may be of importance in the context of "crystal engineering".

Structure of ethyl 1,2,2-tricyano-3-(4-nitrophenyl)-cyclopropane-1-carboxylate

“…The other four substituents interact through the 3E'(b) orbital and this interaction will increase the bond length C(1)-C(2). The observed bond length C(1)-C(2), 1.557 (5)/k, agrees well with bond lengths found in similar compounds: ethyl 1,2, 2 -tricyano -3,3 -dimethylcyclopropane-1 -c arboxylate, 1.552 (3)/~ (Ramasubbu, Rajaram & Venkatesan, 1982); ethylenetetracarbonitrile, 1.559 (2)/k (Lemley, Skarstad & Hughes, 1976); 3-(2-methoxyphenyl)cyclopropane-l,l,2,2-tetracarbonitrile, 1.557 (3)/k (Usha & Venkatesan, 1980). If the nitrophenyl group had also been interacting, then the bond length C(1)-C(2) would be significantly shorter than 1.557 (5)/k.…”

“…Particularly noteworthy is the distance O(4)...C(5) which is as short as 2.83/k. Such interactions have also been found in the crystal structures of BrC(CN) 3, C1C(CN) 3, CHaC(CN) 3 (Witt et al, 1972), tetrahydroxy-p-benzoquinone (Klug, 1965), and ethyl 1,2,2-tricyano-3,3-dimethylcy clopropane-1 -carboxylate (Ramasubbu et al, 1982).…”

“…In other instances where an ethoxycarbonyl group is attached to a cyclopropane ring (Ramasubbu et al, 1982) the corresponding torSion angles are 177.5 (3) and 167.5 (5) °, and 178.2 (2) and 175.8 (2) ° for ethyl 1,2,2-tricyano-3,3-dimethylcyclopropane-1-carboxylate and ethyl 2 ',3 ',3 '-tricyanocyclohexanespirocyclopropane-2'-carboxylate respectively. The bond lengths in the ethoxycarbonyl and CN groups are normal.…”

Structure of 6-acetoxycoumarin: topochemical photodimerization and analysis of acetoxy.acetoxy interactions in the solid state

“…(12) 498 (I) -339 (7) 41 (2) 934 (12) 368 (1) -2320 (7) 42 (2) 470 (13) 555 (1) -4121 (7) 40 (2) 1644 (11) 1120 (1) --5944 (6) 37 (1) 3301 (11) 1442 (1) -5778 (6) 35 (1) 5453 (12) 1563 (1) --3880 (7) 40 (2) 5879 (12) 1351 (I) -2084 (7) 40 (2) 4205 (1 l) 1026 (1) -2238 (6) 35 (1) 2083 (ll) 902 (1) -4136 (6) 33 (I) 3707 (9) 352 (1) 1377 (5) 59 (1) 2973 (8) 1646 (1) --7648 (4) 42 (1) 1644 (12) 1981 (1) -7684 (7) 41 (2) 552 (9) 2102 (I) -6257 (6) 60 (1) 1739 (20) 2166 (2) -9687 (10) 59 (2) Geometry of the molecule. Bond lengths and angles in 6-acetoxycoumarin are in good agreement, within experimental error, with those observed in 7-acetoxycoumarin (Ramasubbu et al, 1982). The plane of the acetoxy group makes a dihedral angle of 60.9 (5) ° with the plane of the coumarin ring, whereas in 7-acetoxycoumarin this angle is 57.4 (3) °.…”

“…An analysis of the reactivity pattern of these compounds revealed that chloro and acetoxy groups are outstanding in their steering abilities; all the five chlorocoumarins investigated were photoreactive, while three (6-acetoxy-, 7-acetoxy-and 4-methyl-7-acetoxycoumarin) out of five acetoxycoumarins investigated dimerized upon irradiation yielding syn head-head dimers. The crystal structure of 7-acetoxycoumarin (Ramasubbu, Gnanaguru, Venkatesan & Ramamurthy, 1982) 0108-2701/88/020307-05503.00 revealed that the reactive molecules pack with the reactive double bonds separated by 3.833 (2)/~, along the a axis. To investigate further the details of the molecular packing of the other acetoxycoumarins, crystallographic investigations of 6-acetoxy-and 4-methyl-7-acetoxycoumarins were undertaken.…”