The Lightest Metals: Science and Technology from Lithium to Calcium by Hanusa Timothy P.;

Author:Hanusa, Timothy P.; [Hanusa, Timothy P.]
Language: eng
Format: epub
Publisher: John Wiley & Sons, Incorporated
Published: 2015-10-28T00:00:00+00:00

2.5 Alkenylalkaline-Earth Metal Complexes (Ae–C(sp2) bonds)

Vinylmagnesium compounds were known and studied spectroscopically a half century ago;135 however, alkenylmagnesium reagents have remained a sporadically investigated substance class. In crystalline [(dme)2Mg(Vi)Br] [dme = 1,2-dimethoxyethane (ligand); Vi = vinyl], the magnesium atom is embedded in a distorted octahedral environment with a cis arrangement of the anions.136 Transmetalation of tetravinyllead with calcium in the presence of catalytic amounts of HgBr2 yields divinylcalcium; however, no physical properties have been provided in the patent that described the preparation.137

Despite the fact that alkenylmagnesium reagents have attracted only sporadic attention, there are still some reports on these organometallics. Besides the direct synthesis approach (reduction of alkenyl halides with magnesium turnings), the halogen–metal exchange reaction of a 1,1-dibromoalkene with n-octylmagnesium bromide yields a carbenoid (equation 21).138 Reduction of diphenylacetylene with cocondensed magnesium in THF at −100 °C leads to the formation of the tetranuclear cage compound [Mg(PhC=CPh)]4.139 Furthermore, the addition of magnesium bis[bis(trimethylsilyl)phosphanide] to one triple bond in diphenylbutadiyne gives an alkenylmagnesium derivative that crystallizes as a dinuclear complex that is stabilized by intramolecular coordination of the phosphanyl moieties to the magnesium atoms.140

Owing to the enormous reactivity of calcium-based organometallics, often leading to ether degradation reactions, alkenylcalcium derivatives represent an infrequently encountered substance class. As one example, the reduction of 1,2-dihydro-4-iodonaphthalene with calcium in THF yields tetrakis(thf) (1,2-dihydronaphth-4-yl)calcium iodide with an octahedrally coordinated metal atom and the anions in trans positions (equation 22).141

21

22

Another class of complexes with Ae–C(sp2) interactions are the carbene (especially substituted 2,3-dihydro-1H-imidazol-2-ylidene) adducts of alkaline earth metal-based organometallics.142 Imidazol-2-ylidenes with bulky N-bound substituents help to stabilize low coordination numbers at beryllium143 and magnesium, or to deaggregate oligomeric Grignard reagents.144–146 Furthermore, metallocenes of the alkaline earth metals also readily form adducts with imidazol-2-ylidenes.147, 148

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