Magnesium-anthracene adduct

Magnesium anthracene. Syntheses of organomagnesium compounds from the magnesium-anthracene adduct (see p. 40) proceed via dissociation of the adduct to give magnesium in a highly reactive form [9, 10]. Indeed, anthracene may be used to catalyse the reactions of other forms of magnesium (see below). 

An arbitrary selection of other (li-Grignard reagents 101-105 is given in Scheme I 1.30. While 101-104 (1611 164]. respectively) were obtained more or loss by the classical direct approach, 105 65a,b and other benzylic Grignard reagents 16.5c—e require the use of the magnesium-anthracene adduct (vide infra) which, in a way, may be considered as a soluble and activated form of magnesium, in order to obtain satisfactory results. 

Reactions of the magnesium-anthracene adducts at room temperature with CCU gi c somewhat... 

As is the case for plienylacelyletio-magnesium adducts, the yields of magnesium -anthracene adducts estimated from hydrolysis data are much lower than estimated trum reactions with CCl (Table 12.1 h. To investigate this discrepancy, the reaction of magnesium anthracene adducts with CVli was studied in solution at 29,3 K. The CIICl. yield, which at low temperature had been 40rv of... 

The cluster magnesium-anthracene adduct is extraordinarily active. Reactions are observed upon attempts to dissolve it even in such comparatively inert solvents as THF. CHyCH, toluene, or benzene. In these solvents, autohydrogenolysis takes places with the formation of dihydroan-thracenc and condensation products. The use as the solvent of CHyCH leads to polymers containing chlorine and use of THF leads to products of ring cleavage of the THF. 

As is the case for phcnylacetylene-magnesinm adducts, the yields of maguesium-aiuhracenc tidducts estimated from hydrolysis dtita arc much lower than estimated from reactions with CCIj (Table 12.11). To investigate this discrepancy, tlie reaction of magnesium-anthracene adducts witli CCI4 W as studied in solution at 298 K. The CHCl i yield, which at low lempeniture had been 40 if of... 

Anthracene-activated magnesium. An activated form of Mg powder is obtained by equilibration with the anthracene adduct. The method involves sonication of a mixture of Mg with anthracene (0.02 equiv.) and CH,I in I HF at 20° for 14 hours. This activated magnesium is particularly convenient for preparation of allylic Grignard reagents (11, 308)."... 

The crystallization of magnesium anthracene itself proved to be difficult, owing to its low solubility even in THF. Therefore, derivatives with substituents at the anthracene moiety were the first to be structurally characterized. Crystal structures were obtained for [9,10-bis(trimethylsilyl)an-thracene]magnesium (THF)2 (54) (56) and a 1 1 adduct of... 

During the investigation of magnesium anthracene, a remarkable complex of a radical anion was isolated. In [(THF)3Mg(/i-Cl)3 Mg(THF)j]+-[anthracenyl] (57) (55), an adduct of MgCH with magne-... 

Another route for isomerization of I-alkenes by magnesium - hydrocarbon adducts is initiated by addition of I-alkenes to the Mg-anthracene adduct tu low or room temperature. The combined yield ol 2-ociene, 3 ociene, and 4-octene formed from I-ocicne ranges from 2.1 to 4.5 moles per mole of Mg. 

Substituted allyl Grignard reagents were synthesized in high yields from the corresponding chlorides and Mg-slurry, obtained by evaporation, activated by equilibration with its anthracene adduct in THF [30]. The authors compared this magnesium with Rieke-magnesium and found, on careful experimentation, that also the Rieke-method gave satisfactory results. 

Magnesium reacts with naphthalene and higher polycyclic aromatics in liquid aimnonia or HMPA to form the radical ion. Adducts with anthracene and substituted anthracenes form in THF. Depending upon substitution and conditions, they have a variety of structures. The l,10-bis(trimethylsilyl) compound had structure (47) in the crystal, with magnesium bridging the 1,10-positions (C-Mg =... 

Dinuclear aromatics, such as naphthalene, biphenyl and fluorene (and likewise phenanthrene and anthracene), add Mg only in more basic solvents, such as HMPA or NHj. Magnesium dissolves in HMPA to give a blue solution that converts the aromatics to the adducts. It may be advantageous to prepare the adducts in NH3 first and then replace this solvent by HMPA The green adducts obtained in NH, are resistant to ammonolysis. On hydrolysis, the naphthalene adduct forms 1,4-dihydro-naphthalene (besides naphthalene, tetralin and bis-2-tetralyl) with 2 1 xs Mg, only tetralin and bis-2-tetralyl are formed . Similarly, the biphenyl-Mg adduct yields 1-, 3-and 4-phenylcyclohexene on hydrolysis . 

Altogether, there are live crystal structures known of the magnesium adducts 18 to the 0.10-position of anthracene and its derivatives. The siruciuie of 18.3 has been obtained from two independent investigations, one in the pure crystal 68J. the other in a 1 1 ratio with 18.4 J69 both structures of 18.3 are essentially identical, as are the magnesium-dihydroanihraccne partial structures of all four compounds. 

We have found unusual catalytic properties of the anthracene-magnesium adduct in various reactions that involve C -H and C C bonds — isomer i7.ation of alkenes, autohydrogenolysis. and D II exchange. Control experiments showed that these reactions cannot be observed in the presence of dispersed magnesium, classical (irignard reagents. 

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