Cationic gold(I) promotes the retro-Buchner reaction of 7-substituted 1,3,5-cycloheptatrienes, leading to gold(I) carbenes that cyclopropanate alkenes.
Cationic gold(I) promotes the retro-Buchner reaction of 7-substituted 1,3,5-cycloheptatrienes, leading to gold(I) carbenes that cyclopropanate alkenes.
“…It can be noted that all compounds investigated herein, or derivatives thereof, have previously been synthesized, − and many of these compounds are found as ligands in organometallic complexes. − The results herein could thus have a direct impact on molecular design.…”
The singlet ground states and lowest triplet states of penta- and heptafulvene, their benzannulated derivatives, as well as the lowest quintet states of pentaheptafulvalenes, either the parent compound or compounds in which the two rings are intercepted by either an alkynyl or a phenyl segment, were investigated at the (U)OLYP/6-311G(d,p) density functional theory level. The influence of (anti)aromaticity was analyzed by the structure-based aromaticity index HOMA, the harmonic oscillator model of aromaticity. The extent of (anti)aromatic character was also evaluated in terms of the π-electron (de)localization as measured by the π component of the electron localization function (ELF(π)). The natural atomic orbital (NAO) occupancies were calculated in order to evaluate the degree of π-electron shift caused by the opposing electron-counting rules for aromaticity in the electronic ground state (S(0); Hückel's rule) and the first ππ* excited triplet state (T(1); Baird's rule). Pentaheptafulvalene (5) shows a shift of 0.5 π electrons from the 5-ring to the 7-ring when going from the S(0) state to the lowest quintet state (Qu(1)). The pentaheptafulvalene 5 and [5.6.7]quinarene 7 were also investigated in their 90° twisted conformations. From our study it is apparent that excitation localization in fulvalenes, but not in fulvenes, to a substantial degree is determined by aromaticity localization to triplet biradical 4n π-electron cycles. Isolated benzene rings in these compounds tend to remain as closed-shell 6π-electron cycles.
“…If the closo -icosahedral [C 2 B 10 H 12 ] carborane unit is viewed as chemically similar to the organic two-dimensional π-aromatic species benzene, a useful comparison can be made between the analogous organic [5.6.7]quinarene system ( 8 ) and our ultimate synthetic target cluster molecule ( 9 ), as illustrated in Scheme . , Indeed, there is quite a large amount of data in the literature to suggest that this comparison between closo -[C 2 B 10 H 12 ] and benzene is quite reasonable both from electronic and reactivity considerations. , Among other similarities, both are electron-delocalized species, both undergo predominantly electrophilic substitution reactions and not addition reactions, and both are very stable species. One major difference between the two analogues that impacts their intramolecular polarization, however, is that the bridging benzene unit is known to provide facile electronic communication between the tropylium and cyclopentadienide units leading to a fully delocalized system, while the carborane bridge should significantly restrict this electronic redistribution, potentially leading to a highly polarized cluster molecule. , While derivatives of 8 have been prepared in the literature by the synthetic pathway shown in Scheme , , the cluster analogue species is unknown. 1 Synthetic Scheme for Preparation of Cycloheptatriene- and Cyclopentadiene- Bis-substituted c loso -1,12-[C 2 B 10 H 10 ] Polyhedron, 7 a a Cage and olefinic ring hydrogen atoms have been omitted from the structures for clarity with the boxes indicating compounds for which X-ray crystallographic characterization has been completed 2 Literature Synthetic Route to 8 9,10 and Its Comparison to the Target Bis-substituted c loso -1,12-[C 2 B 10 H 10 ] Cluster, 9 …”
The synthesis and characterization of a series of new olefin-substituted closo-1,12-[C(2)B(10)H(12)] compounds is described. The reaction of deprotonated closo-1,12-dicarbadodecaborane with 7-methoxycycloheptatriene yields the cycloheptatriene-substituted carborane compound, closo-[1-(1-C(7)H(7))-12-(H)-C(2)B(10)H(10)] (3). Deprotonation of 3 with butyllithium and subsequent reaction with 3,4-dimethyl-2-cyclopenten-1-one was found to yield the bis-olefin-substituted carbinol compound [1-(1-C(7)H(7))-12-(C(5)H(4)-1-(OH)-3,4-(CH(3))(2))-C(2)B(10)H(10)] (5) in good yield. Acidic dehydration of 5 quantitatively converted it into the simple bis-olefin cage compound [1-(1-C(7)H(7))-12-(C(5)H(3)-3,4-(CH(3))(2))-C(2)B(10)H(10)] (6). Finally, thermal treatment of 6 in refluxing toluene was employed to prepare the isomerized cycloheptatriene compound [1-(4-C(7)H(7))-12-(C(5)H(3)-3,4-(CH(3))(2))-C(2)B(10)H(10)] (7) in high yield. These compounds represent the first metal-free cyclopentadiene and bis-olefin large cluster species reported in which the C(5) ring is attached directly to the cage. The complete characterization of products by multinuclear NMR ((1)H, (11)B, and (13)C), infrared, UV-visible, and mass spectroscopic analyses is presented. The X-ray crystal structures of 3 and 7 are also reported. X-ray data for 3: triclinic system, space group P&onemacr; with cell constants a = 6.6807(3) A, b = 10.2939(3) A, c = 10.3962(4) A, alpha = 89.342(2) degrees, beta = 74.610(2) degrees, gamma = 83.373(2) degrees, Z = 2, R1 = 0.0487 (wR2 (all data) = 0.1272). X-ray data for 7: monoclinic system, space group P2(1)/c with cell constants a = 7.7207(7) A, b = 15.8730(14) A, c = 15.5493(13) A, beta = 99.146(2) degrees, Z = 4, R1 = 0.0761 (wR2 (all data) = 0.2050).
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