Insight into the Formation of Highly Strained [1]Ferrocenophanes with Boron in Bridging Position

Four planar-chiral, enantiomerically
pure ferrocene dibromides
(3R1R2; [(CHR1R2)­BrH3C5]2Fe) equipped with
two CHR1R2 groups in α position to bromine
were synthesized. From the four C2 symmetrical
species, two are already known [CHR1R2 = CHMe2 (3MeMe), CHEt2 (3EtEt)] and two are new compounds
[CHR1R2 = R-CHEtMe (3EtMe), S-CHMeEt (3MeEt)]. The dibromides 3R1R2 were in situ converted into
dilithio ferrocene derivatives and reacted with Cl2BNiPr2 resulting in mixtures of bora[1]­ferrocenophanes
(4R1R2) and 1,1′-bis­(boryl)­ferrocenes
(5R1R2). The aim of this investigation
was to test the hypothesis that the alkyl group that is oriented approximately perpendicular
to the Cp ring, i.e., R2, affects the outcome of the salt-metathesis
reaction. The obtained product ratios 4R1R2:5R1R2 were determined
by 1H NMR spectroscopy and revealed that systems with the
same R2 group gave similar 4R1R2:5R1R2 ratios (1.0:0.51
and 1.0:0.49 for R2 = Me; 1.0:0.30 and 1.0:0.27 for R2 = Et), confirming the hypothesis. Shown by DFT calculations
(B3PW91/6-311+G­(d,p)), reaction paths resulting in either product 4R1R2 or product 5R1R2 are both concerted steps.