Magnesiacyclopentadienes as Alkaline‐Earth Metallacyclopentadienes: Facile Synthesis, Structural Characterization, and Synthetic Application

Abstract

Metallacyclopentadienes have attracted much attention as building blocks for synthetic chemistry as well as key intermediates in many metal-mediated or metal-catalyzed reactions. However, metallacyclopentadienes of the alkalineearth metals have not been reported, to say nothing of their structures, reaction chemistry, and synthetic applications. In this work, the first series of magnesiacyclopentadienes, spirodilithio magnesiacyclopentadienes, and dimagnesiabutadiene were synthesized from 1,4-dilithio 1,3-butadienes. Singlecrystal X-ray structural analysis of these magnesiacycles revealed unique structural characteristics and bonding modes. Their reaction chemistry and synthetic application were preliminarily studied and efficient access to amino cyclopentadienes was established through their reaction with thioformamides. Experimental and DFT calculations demonstrated that these magnesiacyclopentadienes could be regarded as bis(Grignard) reagents wherein the two MgÀC(sp 2 ) bonds have a synergetic effect when reacting with substrates.

Structure and Reaction Chemistry of Magnesium Organocuprates Derived from Magnesiacyclopentadienes and Copper(I) Salts

“…We recently reported afacile synthesis of multisubstituted magnesiacyclopentadienes (1)asthe first example of alkalineearth metallocyclopentadienes. [4] Single-crystal X-ray structural analysis of these magnesiacyclopentadienes revealed an early coplanar ring skeleton. Although the lengths of the chemical bonds of the ring, including two Mg À C(sp 2 )b onds, are comparable to those reported, the bite angle of C1-Mg-C4 (91.548 8)ismuch smaller than the classic C-Mg-C angles in the usual tetracoordinate magnesium compounds (115-1308 8).…”

“…TheM ga toms are tetracoordinated by two alkenyl carbon atoms,one bromide atom, and one oxygen atom. Compared to that in the magnesiacyclopentadiene 1a-TMEDA, [4] However,the C-Cu-C bond angle [156.8(4)8 8]issignificantly less obtuse than that of diaryl lithium organocuprates (163-1788 8), which indicates that ahigher degree of covalencyinthe Mg À Cbond induces ac orresponding increase in the CuÀCÀMg three-center-twoelectron bonding character. [3f] TheC u ÀMg distance [2.709-(2) ]i ss horter than the sum of van der Waals radii of Cu (2.0 )and Mg (2.2 )atoms,which indicates aweak CuÀMg interaction.…”

“…It comprises three nonplanar magnesiacyclopentadiene units and two separated CuCl units (for the structure of 4b,see the Supporting Information). Compared to that in the corresponding magnesiacyclopentadiene 1b-TMEDA, [4] the Mg À C5 bond is longer [2.2872(17) (17) ]a nd C-Cu-C bond angles [164.11(7)8 8]a re both in the range expected for diaryl organocuprates. [2a] As the topological structure shows, dimeric 4a can be regarded as ac ombination of two [(MeTMS-vinyl) 2 Cu] À cuprate units and two [(MeTMSvinyl) Mg 1.5 Cl] + countercation units.…”

C,C- and C,N-Chelated Organocopper Compounds

“…Although neutral N,N-chelated spiro copper complexes are relatively common, the anionic C,N- or C,C-cheated spiro counterparts are sporadic. As mentioned above, Xi’s group found that the C,C-bidentate ligand is an excellent platform to build up diversified coordination complexes across the periodic table [ 32 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 ]. The chelating effect increases the thermal stability of these organometallic species, facilitating isolation and following characterization.…”

Lithium, Sodium, and Potassium Magnesiate Chemistry

“…The remaining two higher order magnesiates, have a subtly different structure and can be described as 'magnesiacyclopentadienes'. 17 By reacting substituted 1,4dilithio-1,3-butadienes with 0.5 molar equivalents of MgCl2 in the presence of TMEDA, the spiro-dilithio magnesiacyclopentadiene complexes (TMEDA)2Li2Mg[CR 1 2(CR 2 2)2CR 1 2]2 9 and 10 (for 9, R 1 = SiMe3; R 2 = Me; and for 10, R 1 = SiMe3; R 2 = Ph) are formed ( Figure 7).…”