4248
The relative configuration of the two hydroxyl groups as d o the lower known members containing 12, 16, and in Sa was not determined. 20 out-of-plane T electrons ( n = 3, 4, and 5, respecTreatment of 5a with excess methanesulfonyl chloride t i ~ e l y ) . It~ ~will be of interest t o prepare and study the in pyridine at room temperature for 16 hr gave the dibehavior of a dehydro[28]annulene, the next higher methanesulfonate 5b [75% yield; mp 205 dec;'O member of this series ( n = 7), in order t o find the limitCH3CN ing size for the existence of a paramagnetic ring current. A,,,,, 227 mp ( E 1520), 240 (1140), and 254 (720)],1 which was then boiled with potassium hydroxide in (19) National Research Council (Canada) Postdoctoral Overseas aqueous methanol and dimethyl sulfoxide for 1 hr. Fellow, 1965-1967. This reaction effected elimination and rearrangement 1 2 Clifford C. Leznoff,lg Franz Sondheimer to give a mixture of products from which a tridehydroVnicersiiy Chemical Labomtory [26]annulene could be separated in ca. 2 % yield by Cambridge, Englutid preparative thin layer chromatography on silica gel.I3 Receiced June 12, 1967 The positions of the three acetylenes or the stereochemistry of the double bonds in this compound cannot be On Triplet States of Flavins assigned at present, and the 1,9,17-tridehydro structure Sir: 6 is one of many possibilities. The [26]annulene formed red-brown crystals (red in concentrated solution), deWe have observed anaerobically the phosphorescence composing on attempted melting point determination; of riboflavin (RF) at 6650 A (uncorrected) in a 0.8% mol wt 332.1566 (calcd for C26H20, 332.156); A*;: 237 glucose-water matrix,' while others reported the emismp ( E 23,000), 291 (33,700), 300(33,400), 383 (1 19,000),14 sion at shorter wavelengths (6030-6100 A) under aerobic and 451 (12,300); infrared bands (KBr) 2140 (w) conditions and in an organic m a t r i ~ . 2 - ~ (acetylene) and 988 (s) cm-' (trans double bond), no Recently, Steele and Cusachs5 proposed the spinallene band at ca. 1950 cm-'. The monocyclic nature conserved energy-transfer process given by eq 1 to acof the substance was established by catalytic hydrogencount for the apparent oxygen requirement in the ation in ethyl acetate over platinum, which led to cyclophosphorescence emission from RF. Ironically, oxyhexacosane (mol wt 364).6 h"(4500 .\) The nmr spectrum of tridehydro[26]annulene at room OZt+ RFn Op* + RFt (1) temperature (100 Mc/sec, in CDC13 or CD3COCD3, iiu(6030 A) TMS used as internal standard) exhibited a very broad gen is a rather well-known triplet quencher.6 multiplet at ca. 2.0-4.5, no discrete inner or outer proWe investigated flavin triplets theoretically in an attons being recognizable. Cooling to -60" caused estempt to establish singlet-triplet intervals in RF. sentially no change. The spectrum was similar to that Reasonably reliable values for triplet states can be comof tridehydr0[30]annulene"~and pentadehydro[30]anputed by means of the SCF-ASMO-CI method in the nulene,"" as well as that of a long-chain linear polyenePariser-Parr-Pople semiempirical framework.7cs polyyne. It appears therefore that dehydroannulenes Several flavins were investigated. Methyl groups containing (4n 2) out-of-plane K electrons show no were treated by means of the group orbital and heteroring current and resemble a linear analog when n 3 6. atom approximation^.^ Different sets of semiempirical As regards lower members of this series, the five known integrals were used and most of these were taken from dehydro[14]annuIenes (12 = 3) and the three known published sources, lo The assumed geometry of the dehydro[l8]annulenes (n = 4) have all been found t o basic isoalloxazine ring was the same as in ref 1 1. sustain a diamagnetic ring ~ u r r e n t . ~It is now of interest Results are shown in Table I. Several conclusions to synthesize and study the behavior of members becan be made from the results presented. (1) The sets of longing to the unknown dehydro[22]annulene ( n = 5) integrals used by Grabel' gave triplet states of considerseries. ably low energies. The calculated singlet-singlet transiRegarding dehydroannulenes containing 4n out-oftion energies were also too low to compare favorably plane K electrons, the largest member to be investigated with the experimental absorption spectrum. Notice is tetrahydro[24]annulene (12 = 6). This substance was found to possess a paramagnetic ring ~ u r r e n t , ~ ~ ,also ' ~ that the first triplet is below the singlet ground state in the case of C. Therefore, we conclude that the results for A, B, and C are questionable due to the use (12) The base treatment under similar conditions of a cyclic compound containing the 1,6-heptadiyn-4-01methanesulfonate grouping to of inappropriate integrals. give cyclic conjugated systems has been described by us previously (J. ( 2 ) Values obtained for D and E appear to be reasonMayer and F. Sondheimer, J . Am. Chem. SOC.,88, 602, 603 (1966)). (1 3) The only other highly colored substance eluted [ca. 2 Z yield; able with respect to the bond-order matrix and singlet-
-7 -
+
390 m p (e 131,000)] showed mol wt red-brown powder; main A"'"' mrl 428.6 The formula therefore appears to be CzH?i03S (calcd mol wt 428), derived from the dimethanesulfonate 5b by elimination of one molecule of methanesulfonic acid. It is possible that this substance is the methanesulfonate of a monohydroxyhisdehydro[26]annulene, but it has not been investigated further. (14) By comparison, tetradehydro[24]annulene showed the principal LnP 340 mp (E 225,000),"" and tridehydro[30]annulene showed the principal XjdP,","""' 397 mp (c 1 14,000).'5b ( 1 5 ) (a) F. Sondheimer and R. Wolovsky, J . A m . Chem. SOC.,84, 260 (1962); (b) F. Sondheimer and Y . Gaoni, ibid., 84, 3520 (1962). (16) W. H. Okamura and F. Sondheimer, unpublished observations. (17) A hexadehydro[36]annulene (n = 9) has been obtained in impure form, but the nmr spectrum was not determined.15" (18) See J. A . Pople and I
