[ArylNH2BBr2] + G, based on their relative LUMO energies (Fig. ). Both LUMOs have significant character on the boron centre involved in forming the C-B bond during borylation, but for the functional borenium equivalent F, the LUMO is much (ca. 0.7 eV) higher in energy than the bona-fide borenium cation G. This is consistent with previous studies where protonolysis at N generates stronger Lewis acids relative to analogues formed by coordination of Lewis acids at N in oxazaborolidines. It is also noteworthy to highlight the disparity between facile BBr3 mediated directed C-H borylation reactions (e.g. using N-heterocycles or pivaloyl as directing groups) versus the much more challenging directed borylation to make 1-Br using just BBr3. In other systems, e.g. imidazole or pivaloyl directed borylation, formation of the borenium cation via halide abstraction from the Lewis base→BBr3 adduct using BBr3 is calculated to be uphill by between +10 -+13 kcalmol -1 (Scheme 8, top). Thus these borenium cations are readily accessible at room temperature. In contrast, bromide abstraction from 3-Br using BBr3 to form G is significantly more endergonic, at +24 kcal mol -1 . This disparity is presumably due to the absence of π donation from the directing group (e.g. pivaloyl or an N-heterocycle) that will stabilise the borocation as previously discussed. In compound G the formally empty boron pz orbital only is stabilised by π donation from two bromides. The relatively high energy of the borenium cation presumably contributes to the failure to form 1-Br from 2 using just excess BBr3 at room temperature, as the barrier to C-H borylation mediated by G will be > 24.3 kcal mol -1 . Discussion of the structures of BN-helicenes 1-Br crystallised as a racemate with a single helicene molecule in the asymmetric unit. Metrics within the azaborine unit are unremarkable and are comparable to B-aryl substituted azaborines (no B-Br azaborine structure has been reported to date to our knowledge). Comparison of the structure of 1-Br to that of the all carbon [5]helicene C is noteworthy. In 1-Br the C9-C10-C11-C20 and C1-C10-C11-C12 torsion angles (Fig. ) are greater (34.6(5) o and 24.1(5)°, respectively) than the analogous angles in C (32.3(2) o and 20.2(2)°). Furthermore, the angles between the centroids A-B-C in 1-Br is 122.95°, while the comparable angle in C is 124.22°. In addition, the dihedral angle between the mean planes of the two terminal rings of 1-Br (58.88°) are significantly larger than that of the all carbon [5]-helicene C (47.30°). Note, similar trends are observed when comparing the metrics of the optimized geometries of 1-Mes and it carbo [5]helicene analogue (at the same level of theory), thus they are not due to packing effects. The origin of these disparities is attributed to the difference in key bond distances between the inner helix and the outer helix; for 1-Br C10-C11 (inner) = 1.472(5) Å, B1-N1 (outer) = 1.393(6) Å ( = 0.079 Å) while the respective distances in C are: (inner) = 1.451(2) Å and (outer) = 1.348(2) Å ( = 0.103 Å). The relatively long BN bond located at the centre of the outer helix in 1-Mes is a direct result of the weaker π bond in BN units versus that in CC, and this generates a slightly larger in-plane turn for the BN helicene relative to the carbon-analogue. The solid state structures of 6-Mes and 7-Mes also were obtained, although the former contained significant disorder thus is not discussed (Fig. ). Though it should be noted that inspection of the calculated structures of 6-Mes and its carbo [6]helicene analogue (at the same level) revealed comparable trends to that discussed above for the BN and CC [5]helicene isosteres. Furthermore, the structure of 7-Mes has comparable key metrics to that of 1-Br, including similar BN and C10-C11 distances (for 7-Mes 1.409(3) Å and 1.465(2) Å, respectively). The sum of the torsion angles of the inner rim for 7-Mes is 71.30°, which is smaller than the all carbon 6-helicene D (88.08°) but is comparable to that of an aza-6-helicene that also contains a five-membered pyrrole unit (67.71°). The packing structures of rac-1-Br and rac-7-Mes are shown in Figure . The extended structure of 1-Br contains adjacent columns of left handed and right handed helices, with the closest intermolecular distance between non-hydrogen atoms within a column being 3.248 Å. The adjacent P-and M-isomers are connected through intermolecular C-H⸱⸱⸱π interactions with a distance of 2.855 Å. In contrast to 1-Br, rac-7-Mes packed in an alternating fashion with P-and M-isomers interacting with each other through C-H⸱⸱⸱π and N-H⸱⸱⸱π interactions. To probe for consequences of the structural differences between the BN and CC helicene analogues discussed above the strain energy was calculated using a previously reported methodology. This was performed using the model complexes 1-H and 6-H to enable direct comparison to previously studied carbon analogues (with calculations performed at an identical level to previous work); note it has been shown previously that changing substituents on the outer helix (e.g. from 1-H to 1-Mes) has minimal effect on key steric modified parameters (e.g. strain energy/racemization barriers). These calculations (Scheme 9) showed that the BN- [5]-and [6]helicenes have approximately 1 kcal mol -1 greater strain energy than the carbon analogues. This is consistent with the greater torsion angles observed in 1-Br (large torsion angles relative to all carbon analogues also are observed along the inner helix for the calculated BN-helicenes) due to the smaller difference between inner and outer helix bond distances.