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Scheme VI2efi factors.46 For the flavin-linked porphyrins the one-electron redox potentials are not correlated to the log k2nd, but we assume that geometrical factors play an important role. The methylene chain length considerably affects the chemical shifts in the 'H N M R spectra, which reflect the geometry of the flavin-linked porphyrin. In kinetic studies, we used the polar and protic solvent ethanol for practical reasons. In this solvent the flavin moiety is probably partly solvated, and the stability of the stacked conformation decreases with an increase of the flexible methylene chain length. The intermediate state clearly reflects the initial conformation. F1,xC,(TPP)MnlllC1, which has little freedom around the methylene spacer, is in a more folded conformation than l b and IC, and the reaction should proceed via a ternary complex [PyH,... F1,,-~(TPP)Mnllr+]. It has been well-known that the redox reaction between flavin and a 1,4-dihydropyridine proceeds via a preequilibrium charge-transfer-type complex [ PyH,.-FI,,] .46 In the proposed ternary complex, one electron can be rapidly transferred to the near manganese(II1) porphyrin. The reaction of lb, IC, Id, and l e proceeds only partly via this ternary complex and mostly via its open form. The proposed reaction mechanism is summarized in Scheme VI.Conclusion. Novel flavin-linked porphyrins have been synthesized. The key step, the condensation of the flavin carboxylic acid and the o-NH2TPPH2, which was carried out via the flavin acid chloride, proceeds in good yield. Spectrophotometric studies revealed that the flavin and the porphyrin moieties are in close proximity in the all-oxidized form. Electrochemical studies suggest an interaction of the chromophores in redox reactions. Especially, the potentials of the Flox/Fl'and Fl'-/F12couples are significantly shifted to more positive values. The flavin-catalyzed 2e/le electron-transfer reactions between NADH model compounds and (TPP)Mn"*Cl were investigated in intermolecular systems (FIox + PyH, + (TPP)Mn"'CI) as well as in intramolecular systems ( FIoxC,(TPP)MnlllC1 + PyH,). Reaction rates were accelerated by the intramolecular effect, and this acceleration was strongly affected by the methylene spacer length and the linking position. Abstract: Reaction of alkylidene complexes [(q5-C5H5)Re(NO)(PPh,)(=CHCH2R)]+PF6-(l-PF6-; a, R = H; b, R = CH,; C, R = CH2CH2CH3; d, R = CH2C6H5) with DBU or t-BuO-K+ gives vinyl complexes (E)-(q5-C5H5)Re(NO)(PPh3)(CH=CHR) ((E)-Za-d; 7246%). Complexes (E)-Zb-d equilibrate to 84-92:16-8

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1,2-disubstituted and 1,2,3-trisubstituted aromatic hydrocarbons.Acknowledgment. This research was supported in part by a grant from the National Cancer Institute (CA 13963). Mass spectra were obtained from facilities provided under grants from the National Institutes of Health (CA 11388 and GM 16864) and NMR spectra from the University of Illinois Midwest NSF Regional NMR Facility (CHE 79-16100). We thank Professor Peter Beak for helpful discussions.(19) Traditional lithiation procedures must employ lithiating agents basic enough to convert the carbon acid largely to its conjugate base. The in situ trapping procedure can be useful with weaker bases if the deprotonation, to produce a small concentration of trappable aryllithium, is sufficiently rapid to make this process competitive in rate with reaction of the hindered base with the in situ electrophile.

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