Radical-cation cyclodimerizations of electron-rich cyclic 1,3dienes and radical-cation Diels-Alder reactions .of these dienes with several electron-rich olefins have been investigated. In some cases the efficiency of the electron transfer could be increased if the electron acceptors were combined with LiC104 (special salt effect). The dimerization of 1,3-cyclohexadiene (74, as well as 1 -acetoxy-and l-methoxy-1,3-cyclohexadiene (7b, 7c) with several electron acceptors yielded endo-selectively the Diels-Alder dimers. The formation of the Diels-Alder products via radical-ion intermediates could be demonstrated by quenching experiments. In addition, cyclobutane dimers were also formed, mostly through triplet-reaction channels. Only in the case of I-acetoxy-1,3-cyclohexadiene (7b) photochemically induced electron transfer is involved as well, as shown by quenching experiments. Some of these Diels-Alder dimerizations are indicated by a characteristic concentration dependence, i. e. the endo [2 + 41 dimers were preferably formed at low diene concentrations, whereas high diene concentrations favored the ex0 [2 + 4) adducts. In the reaction sensitized by 1,rl-dicyanophthalene (2) these concentration effects could be emphasized by differently strong quenching of the different products. Concentration and quenching effects indicate the involvement of different radical-ion intermediates. Crossed Diels-Alder reactions usually ran endo-selectively. Here, quenching of product formation with lJ,4-trimethoxybenzene (TMB) indicated the involvement of radical intermediates as well. D CN CN A * D tion of LiClO,.

values of the absorption coefficients at less than 5 MHz may be necessary by other ultrasonic methods though they are impossible in our laboratory.In conclusion, the pulse frequency range ultrasonic absorptions in aqueous solutions of te/7-butylamine are characterized by two relaxation processes. One is due to the hydrolysis, and the other is related to the molecular aggregation of nonionized amine molecules.

Abstract2,5‐Dimethyl‐2,4‐hexadiene (1) dimerizes under Lewis acid catalysis or in the presence of tris(4‐bromophenyl)aminium hexachlorostibanate (4) mainly to give exo‐3‐isopropenyl‐exo‐6‐isopropyl‐1,endo‐2,4,4‐tetramethylbicyclo[3.1.0]hexane (2). Small amounts of the acyclic dimer 2,5,7,7,10‐pentamethyl‐2,4,8‐undecatriene (3) and traces of other unidentified dimeric and trimeric products are formed as well. The latter are the only products in the presence of Brønsted acids, whereas 2 and 3 were not detected under these conditions. The structural assignment of 2 has been carried out by means of 2D‐INADEQUATE, ‐HETCOR, and ‐COSY spectra.

2,5-Dimethyl-2,4-hexadiene (1) dimerizes under Lewis acid catalysis or in the presence of tris(4-bromophenyi)aminium hexachlorostibanate (4) mainly to give exo-3-isopropenyl-exo-6-isopropyl-l,endo-2,4,4-tetramethylbicyclo[3.l.O]hexane (2). Small amounts of the acyclic dimer 2,5,7,7,1O-pentamethyl-2,4,8-undecatriene (3) and traces of other unidentified dimeric and trimeric products are formed as well. The latter are the only products in the presence of Brmsted acids, whereas 2 and 3 ,were not detected under these conditions. The structural assignment of 2 has been carried out by means of 2D-INADEQUATE, -HETCOR, and -COSY spectra.2,5-Dimethyl-2,4-hexadiene (1) has been used as dienophile in radical cation-catalyzed Diels-Alder reactions with 1,3-cycIohexadiene'.*'. Although there is no report in the literature on dimerization reactions of 1 itself, we have recently found that 1 dimerizes in the presence of tris(4-bromophenyl)aminium hexachlorostibanate (4) even in the presence of 1,3-~yclohexadiene~~~). Since this aminium salt is reported to generate cationic reaction pathways as well" we have untertaken a systematic investigation of the acid catalysis in the dimerization of 1. Acid-Catalyzed Dimerization of 1The results of the dimerization of 1 to give exo-3-isopropenylexo-6-isopropyl-1 ,endo-2,4,4-tetramethylbicyclo[3.1 .O]hexane (2) (main product) and 2,5,7,7,10-pentamethyl-2,4,8-undecatriene (3) (ca. 10% of 2) by using either tris(4-bromophenyl)aminium hexachlorostibanate (4) or antimony pentachloride and other Brernsted acids are summerized in Table 1.The following aspects are noteworthy:(1) The bicyclic dimerization product 2 is only formed in the presence of 4, SbC15, or HSbCI6.(2) The radical cation catalyst 4 has no effect in the presence of 2,6-di-tert-butylpyridine (5)''. However, under Gassman's conditions (entry 3) small amounts of unidentified dimeric and trimeric products are formed.(3) Brernsted acids such as p-toluenesulfonic acid (entry 7 ) and gaseous HCI (entry 8) d o not work.These results clearly show that both 2 and 3 are formed in the presence of the Lewis acid SbCIS. Proton acids only lead to the formation of di-and trimeric products of 1, which have not been Chem.

Cs2S5 · H2O has been synthesized from aqueous solutions of Cs2S and stoichiometric quantities of sulfur. Cs2S solutions result from the reaction between Ba(OH)2·8H20, Cs2SO4, and H2S.Cs2S5-H2O crystallizes triclinicly, space group P1̄ , with Z = 2. The lattice constants are as follows:a = 6.974(3) Å, b = 7.870(4) Å, c = 10.413(4) Å, α = 108.72(4)°, β = 111,23(4)°, γ = 82,00(4)°.Characteristic features are the unbranched zig-zag-chains of S52- like in other known pentachalcogenides M2X5. Hydrogen bridges occur between the terminal sulfur atoms of the chains and the oxygen atoms of water molecules.