Bimolecular formation of radicals by hydrogen transfer, 9. Uncatalyzed transfer hydrogenation of α‐methylstyrene by 9,10‐dihydroacridine and N‐methyl‐9,10‐dihydroacridine

Abstract

Hydrogen is transferred from acridane (lc) and N-methylacridane (Id) to a-methyl-styrene (2) in a bimolecular uncatalyzed reaction at 260-300°C. On the basis of the activation parameters (for lc: AH* = 29.0 kcal/mol, A S 9 = -23.5 e.u. in triglyme) the kinetic isotope effect (kH/kD = 2.0, in triglyme at 3 0 0 T ) as well as the small solvent effects and from the observation of a n EPR signal of N-methylhydroacridyl radicals a three-step radical mechanism is prosposed which is initiated by a retrodisproportionation between 1 and 2. der to test if the homolytic retrodisproportionation reaction can be shifted to a true hydride transfer process by proper choice of the reactants we investigated the title reaction. Acridane (lc, X = NH) should be a much more efficient hydride donor than xanthene (1 b) or 9, lo-dihydroanthracene (la) because the acridinium ions (7c) are more effectively stabilized than the xanthylium ions (7b) or 9-hydroanthrylium ions (7a) which are formed from xanthene (lb) or DHA (la), respectively, by hydride expulsion.Scheme 2This is in contrast to current views on several similar dehydrogenationlhydrogenation reactions like quinoneL81 or nitrobenzenef91 aromatization reactions or even biochemical reductions by NADH or FADH ["]. There has been a longstanding discussion in the literature concentrating on the mechanisms of these processes, whether they are initiated by hydride transfer or electron transfer" ' 1. Initiation by a homolytic retrodisproportionation step as in Eq. (1) When a-methylstyrene (2) was heated with a 2 lofold excess of acridane (lc) to 260-300 "C in pyridine an almost quantitative yield of cumene and acridine was obtained (see Figure 1).The kinetics of the reaction followed by GC in the solvents pyridine, triglyme, p-tolunitrile, and N-methylacetamide, in which lc is sufficiently soluble (0.5-1 .O M), proved to be first order with respect to 2 and l c with standard deviations in k2 of 5 2 % and mass balances of 293% (see Table 1). When [9,9-D2]-lc, obtained from 9,10-dihydro-9-oxoacridine and LiAlD,, was used, no incorporation of deuterium into 2 was detected when 81% of the initially present 2 had reacted. This is conclusive evidence that the initiating retrodisproportionation step [analogous to step (1) in Scheme 11 is irreversible and accordingly rate-de-

On the Mechanism of Peripentacene Formation from Pentacene: Computational Studies of a Prototype for Graphene Formation from Smaller Acenes

“…Kinetic Analysis. The reactions described here are related to Rüchardt's studies of the kinetics of H atom transfer from dihydroaromatics (e.g., DHA , , dihydronaphthalene, xanthene, etc.) to hydrogen acceptors (e.g., styrenes, dienes, imines, etc.).…”

Disproportionation-induced solid-state fluorescence in 6,13-dihydropentacenes

“…2 It has been postulated that H-atom-transfer processes play a crucial role in such disproportion reactions. Indeed, H-atom transfer from 9,10-dihydroantharacene, 3,4 xanthene, 4 uorene, 5 acridan, 6 and other H donors (D-H) with weak C-H bonds 7 to unsaturated H acceptors has been widely explored.…”

“…, b ¼ 89.245(10) , g ¼ 84.00(6), V ¼ 785(4)Å 3 , Z ¼ 1,D calcd ¼ 1.241 g cm À3 , m ¼ 0.080 mm À1 , R 1 (I > 2s(I)) ¼ 0.0616, wR 2 (all data) ¼ 0.2120, T ¼ 103(2) K, GOF ¼ 1.051. 3a (v) : C 80 H 84 N 4 O 9 , MW ¼ 1245.51, triclinic, space group P 1 (#2), a ¼ 4.483(3)Å, b ¼ 16.529(1)Å, c ¼ 21.775(13)Å, a ¼ 85.587(10) , b ¼ 88.775(17) , g ¼ 87.293(12), V ¼ 1074.5(11)Å 3 , Z ¼ 1, D calcd ¼ 1.287 g cm À3 , m ¼ 0.074 mm À1 , R 1 (I > 2s(I)) ¼ 0.045, wR 2 (all data) ¼ 0.1036, T ¼ 103(2) K, GOF ¼ 1.031.…”