Aluminum trichloride dimer

In a related study, 161 was reported from a dimerization of a 3-vinylindole precursor (61JOC4254). Previously, a compound with the structure 162 had been suggested as a product originating from a dimerization of an alcohol precursor under acidic conditions (63G238). It has recently been demonstrated that the alcohol 163, when treated with aluminum trichloride in the presence of acetic anhydride, produced the indolocarbazole 160, which could be isolated in 41% yield as an 83 17 mixture of cis and trans isomers (98JOC2909). 

Naming becomes more difficult if the molecular complexity of a compound changes when it is fused or vaporized. Aluminum chloride is an ionic solid but vaporizes to dimeric molecules the solid is then aluminum trichloride whereas the vapor is dialuminum hexachloride. Similar considerations hold also for FeCl3, P2O5, and a number of additional compounds. Likewise, phosphorus(V) chloride is an appropriate name for PCI5 in the vapor state, but an ideal system of nomenclature should find some way of indicating that the solid consists of equal quantities of PCl and PClJf ions. It is a little difficult to decide just how much information about the structure of a compound must be included in its name before this name is to be considered adequate. 

There seems to be even less structural similarity for many other metal halides as the crystalline systems are compared with the molecules in the vapor phase. Aluminum trichloride, e.g., crystallizes in a hexagonal layer structure. Upon melting, and then, upon evaporation at relatively low temperatures, dimeric molecules are formed. At higher temperatures they dissociate into monomers (Figure 9-58) [107], The coordination number decreases from 6 to 4 and then to 3 in this process. However, at closer scrutiny, even the dimeric aluminum trichloride molecules can be derived from the crystal structure. Figure 9-59 shows another representation of crystalline aluminum trichloride which facilitates the identification of the dimeric units. A further example is chromium dichloride illustrated in Figure 9-60. The small oligomers in its vapor have structures [108] that are closely related to the solid structure [109], Correlation between the molecular composition of the vapor and their source crystal has been established for some metal halides [110],... 

The cis addition of nitrosyl chloride to cyclic unconjugated polyenes was observed in dichloromethane28. From cyclooctadiene a single diastereomeric dimer 3 (either meso or dl) was produced due to conformational factors associated with the cis configuration. This product underwent transannular electrophilic reaction to give 4. From (Z,Z,Z)-l,5,9-cyclododecatriene the chloro nitroso adduct 5 was obtained as a mixture of the meso and dl forms and was converted to -chloro amine 6 by lithium aluminum hydride/aluminum trichloride reduction 6 was converted to the corresponding aziridine. 13C-NMR spectroscopy was extremely useful in determining the diastereomeric composition of the nitroso dimers. 

Structural assignments for the dimer of aluminum trichloride are based on the electron diffraction data reported by Shen (. These values are in good agreement with earlier electron diffraction studies by Palmer and Elliott (1 ) and Akishin et al... 

Whatever metal is used, homogeneous processes suffer from high cost resulting from the consumption of the catalyst, whether recycled or not. This is why two-phase catalytic processes have been developed such as hydroformylation catalyzed by rhodium complexes, which are dissolved in water thanks to hydrophilic phosphines (cf. Section 3.1.1.1) [17]. Due to the sensitivity of most dimerization catalysts to proton-active or coordinating solvents, the use of non-aqueous ionic liquids (NAILs) as catalyst solvents has been proposed. These media are typically mixtures of quaternary ammonium or phosphonium salts, such as 1,3-dialkylimi-dazolium chloride, with aluminum trichloride (cf. Section 3.1.1.2.2). They prove to be superb solvents for cationic active species such as the cationic nickel complexes which are the active species of olefin dimerization [18, 19]. The dimers. 

In general, cycloproparenes are thermally labile compounds, and they tend to decompose slowly at or above room temperature. When benzocyclopropene was heated to 100°C in a sealed tube, 9,10-dihydrophenanthrene was formed in 40% yield.The same product formed upon exposure of benzocyclopropene to copper(II) acetate (40%). In the presence of aluminum trichloride, however, benzocyclopropene dimerized to 9,10-dihydroanthracene and anthracene in 25% combined yield.Dihydroanthracene (45%) was also formed with silver(I) tetrafluoro-borate. Cyclopropa[A]naphthalene dimerized at 20°C in the presence of 1,3-diphenylisoben-zofuran or in its absence to give the linear 6,13-dihydropentacene (1) in ca. 85% yield.The angular isomer was not formed. The fact that 1,3-diphenylisobenzofuran did not react with cyclopropa[6]naphthalcne suggests that the dimerization does not proceed via biradicals. On the other hand, biradicals have been trapped with buta-1,3-diene under the conditions of the thermal dimerization of benzocyclopropene (80"C). ... 

Dehydroglaucine (45) may be readUy alkylated or acylated at C-7, and 7-benzoylde-hydroglaucine has been converted Into 7-benzylldeneglauclne. Reduction of 7-for-myldehydroglaucine with lithium aluminum hydride and aluminum trichloride leads to 7-methyldehydroglaucine. Alternatively, when dehydroglaucine was heated with formaldehyde, the dimeric species 7,7 -bisdehydroglaucinemethane was isolated. 

Figure 14.4 The dimeric structure of gaseous aluminum chloride. Despite its name, aluminum trichloride exists in the gas phase as the dimer, AI2CI6.

There seems to be even less structural similarity for many other metal halides when the crystalline systems are compared with the molecules in the vapor phase. Aluminum trichloride, for example, crystallizes in a hexagonal layer structure. Upon melting and then evaporation at relatively low temperatures, dimeric molecules are formed. At higher temperatures, they dissociate into monomers (Figure 9-60) [9-60]. The coordination number decreases from six to four and then to three in this process. 

Under closer scrutiny, even the dimeric aluminum trichloride molecules can be derived from the crystal structure. Figure 9-61 shows another representation of crystalline aluminum trichloride which facilitates the identification of the dimeric units. Correlation between the molecular composition of the vapor and the source crystal has been established for some metal halides [9-62]. 

Figure 9-61. The crystal structure of aluminum trichloride, after Ref. f9-61]. The dimeric unit with a four-niembered ring is discernible. Copyright (1993) John Wiley Sons. Used by permission.

Aluminum trichloride (AICI3) is an electron deficient molecule. It has a tendency to form a dimer (a molecule made of two AICI3 units). 

Chemistry of aqueous solution of aluminum hydroxide from simple aluminum salts, such as aluminum trichloride, has been studied extensively (Cotton et al., 1999). In an aqueous solution, aqua ion, [A1(H20)6], or deprotonated ion, [Al(H20)s0H], exists stably. Th deprotonated ion forms dimer with two hydroxyl groups bridging two aluminum ions. Furthermore, the aqua ion tends to polymerize with increasing pH. Polynuclear ions, such as [Al3(OH)n], [Al6(OH)i5] +, and [Alg(OH)22] are recognized. Under extremely basic conditions, soluble aluminum tetra hydroxide ion, [Al(OH)4] , forms. Therefore, alumina sol is prepared around neutral pH. The mechanism of the polymerization leading to alumina sols, however, is not perfectly clarified. 

Diphenylacetylene undergoes unusual dimerization forming 1,2,3-triphenyl-azulene upon treatment with aluminum trichloride and sulfenyl chloride (Scheme 7.39) [54]. Although the reaction mechanism is unclear, one benzene ring is expanded into a seven-membered ring. An analogous ring expansion process is observed in a platinum-catalyzed cyclization reaction of arylalkynes [55]. 

The formation of different anions is dependent on the chloride/AICb ratio. The addition of aluminum trichloride to the chloride initially results in the formation of the AlCU ion and this is essentially the only anion present at an aluminum trichloride mole-fraction of exactly 0.5. At x(AICl3)>0.5, multi-nuclear chloroaluminate anions are formed which are in equilibrium with one another, the AlCU ion and at very high AICI3 mole fractions with dimeric aluminum trichloride (Eq 2.27 and 2.28). Chloroaluminates are well known, however other ionic liquids may also be prepared from a halide and Lewis acid (Table 2.12). 

Boron trichloride, a colorless, reactive gas of BC13 molecules, behaves chemically like BF3. However, the trichloride of aluminum, which is in the same group as boron, forms dimers, linked pairs of molecules. Aluminum chloride is a volatile white solid that vaporizes at 180°C to a gas of Al2Cl6 molecules. These molecules survive in the gas up to about 200°C and only then fall apart into A1C13 molecules. The Al,CI6 molecule exists because a Cl atom in one AlCI, molecule uses one of its lone pairs to form a coordinate covalent bond to the Al atom in a neighboring AICI molecule (33). This arrangement can occur in aluminum chloride hut not boron trichloride because the atomic radius of Al is bigger than that of B. 

A titanium(Il) species formed from titanium trichloride and lithium aluminum hydride is a useful reagent for the reductive coupling of carbonyl compounds to olefins (McMurry, 1974 McMurry and Fleming, 1974). Both aliphatic and aromatic ketones can be converted to tetrasubstituted olefins in excellent yields. Reductive dimerization of retinal (CCLXXFV) affords j6-carotene (CCLXXV) in 85% yield. The course of the reaction can be accounted for by assuming pinacol formation followed by loss of titanium dioxide. 

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