1,3-Dibora-[3]ferrocenophanes; synthesis and characterization

Unlocking Heteroaromatic Ring Systems through Chalcogen Insertion into Boroles

In this work, we report the reactivity of various annulated borole derivatives toward chalcogen (O, S, and Se) insertion. Among a series of 9-borafluorenes with different boron substituents (Ph, Br, or o-carboranyl) and a mixed thiophene-benzene-fused derivative, only the 9-o-carboranyl-substituted 9-borafluorene yielded the complete set of chalcogen-containing heteroarenes, including the first 1,2-selenaborinine derivative. To evaluate the aromaticity of this heterocyclic analogue of phenanthrene, nucleus-independent chemical shift (NICS) values were computed and compared to those of its lighter group 16 congeners.

Dialkynyldiboranes(4) and the selectable reactivity of their C-H, C[triple bond, length as m-dash]C and B-B bonds

The synthesis and reactivity of dialkynyldiboranes(4), a little-studied family of diboranes, are presented herein. Three dialkynyldiboranes(4) were prepared via two different salt metathesis pathways. The three reactive sites of these dialkynyldiboranes(4) are then selectively addressed by judicious application of reagents: addition of an amine N-oxide leads to oxygen insertion into the B-B bond, dicobaltoctacarbonyl binds to the alkynyl C[triple bond, length as m-dash]C bonds, while Sonogashira-Hagihara cross-coupling conditions lead to double C-C bond formation at the alkynyl C-H groups.

[n]Ferrocenophanes (n = 2, 3) with Nitrogen and Phosphorus in Bridging Positions

The in situ prepared dilithio derivative of the known species 1-bromo-1'-(trimethylsilylamino)ferrocene (1) reacted with tBuPCl2 to form the first example of a [2]ferrocenophane ([2]FCP) bridged by nitrogen and phosphorus (2). Sulfurization of 2 followed by column chromatography on silica gel gave the expected [2]FCP with a tBu(S)PN(SiMe3) bridging moiety (3a) and its desilylated counterpart with a tBu(S)PNH bridging moiety (3b). The molecular structure of 3b was determined by single-crystal X-ray analysis (α = 18.40(11)°). Using a common synthetic methodology, two new 1-amino-1'-bromoferrocene derivatives were prepared, one species with a PhCH2 (6a) and another with a tBuCH2 group (6b) on nitrogen. Dilithiation of 6a followed by addition of tBuPCl2 gave a mixture of three constitutional isomers: the targeted [2]FCP (7a), the 1,1'-disubstituted ferrocene derivative (tBuPH)(PhCH═N)fc (8a), and the [3]FCP bridged by a (NH)(CHPh)PtBu moiety (9a). NMR spectroscopy revealed that 8a is the precursor for 9a. The salt-metathesis reaction of the dilithio derivative of 6b with tBuPCl2 exclusively gave the 1,1'-disubstituted ferrocene derivative (tBuPH)(tBuCH═N)fc (8b), which does not isomerize to the respective [3]FCP. DFT calculations at the M06/6-311+G(d,p) level were used to better rationalize these unexpected results.

Synthesis and oxidation of hexafluoroisopropoxyborylferrocenes

The ferrocenylboranes (BOhfip2)Fc and 1,1'-(BOhfip2)2Fc were prepared (Fc = ferrocenyl, Ohfip = (CF3)2CH-O); 1,1'-(BOhfip2)2Fc was further oxidized giving [1,1'-(BOhfip2)2Fc]+[Al(ORF)4]– by oxidation of 1,1'-(BOhfip2)2Fc with Ag+[Al(ORF)4]– (RF = C(CF3)3). Their X-ray crystal structures were determined and give further information on the Fe–B interaction through the dip angles α*. In [1,1'-(BOhfip2)2Fc]+[Al(ORF)4]–, this Fe–B interaction is enfeebled: the dip angles α* (0.3º and 5.4º) are significantly smaller than in (BOhfip2)Fc (7.8º) and 1,1'-(BOhfip2)2Fc (8.0º), and the hexafluoroisopropoxy moieties are twisted out of the Cp plane by up to 66.1º. NMR and vibrational spectra support the crystal structure assignments.

C5H4?BR2 Bending in Ferrocenylboranes: A Delocalized Through-Space Interaction Between Iron and Boron

A comparison of the molecular structures of mono-, di- and tetraborylated ferrocenes [Fc{B(R(1))(R(2))}] (R(1)/R(2)=Br/Br, Br/Fc, Br/Me, Me/Me, Me/OH, OMe/OMe), 1,1'-[fc{B(R(1))(R(2))}(2)] (R(1)/R(2)=Br/Br, Br/Me, OMe/OMe), and 1,1',3,3'-[Fe{C(5)H(3)(BMe(2))(2)}(2)] revealed the boryl substituent(s) to be bent out of the Cp ring plane towards the iron center. The corresponding dip angle alpha* decreases with decreasing Lewis acidity of the boron atom and with increasing degree of borylation at the ferrocene core. This trend is well reproduced by DFT calculations (including [FcBH(2)], not yet accessible experimentally). A Bader analysis of the electron density topology of [FcBH(2)] (alpha*=26.5 degrees ; BP86/TZVP) clearly showed that there is no direct iron-boron bonding in this compound. Instead, strongly delocalized orbital interactions have been identified that involve the boron p orbital, C(ipso) of the adjacent Cp ring, d orbitals at iron, and a through-space interaction with the second Cp ring. A second important factor is attractive electrostatic interactions, which are enhanced upon ligand bending. Cyclic voltammetric measurements on the series [FcBMe(2)], 1,1'-[fc(BMe(2))(2)], and 1,1',3,3'-[Fe{C(5)H(3)(BMe(2))(2)}(2)] indicate a substantial anodic shift in the oxidation potential of the central iron atom upon introduction of BMe(2) substituents. Addition of 4-dimethylaminopyridine (DMAP) does not just counterbalance this effect, but leads to a cathodic shift of the Fe(II)/Fe(III) redox transition far beyond the half-wave potential of parent ferrocene. In the Mossbauer spectra, a continuous decrease in the quadrupole splitting (QS) is observed upon going from parent ferrocene to [FcBMe(2)], to 1,1'-[fc(BMe(2))(2)], and to 1,1',3,3'-[Fe{C(5)H(3)(BMe(2))(2)}(2)]. In contrast, no significant differences are found between the QS values of ferrocene, [Fc(BMe(2)-DMAP)], and 1,1'-[fc(BMe(2)-DMAP)(2)].

Radical-Type Reactivity of the 1,3-Dibora-2,4-Diphosphoniocyclobutane-1,3-diyl

Despite through-space and through-bond B-B interactions, the stable 1,3-bora-2,4-diphosphoniocyclobutane-1,3-diyl displays some radical-type behavior, as illustrated by the spontaneous formation of the trans 1,3-adduct with trimethyltin hydride and a B-spiro derivative with bromotrichloromethane.