2,5-Bis-trimethylsilyl substituted boroles

Abstract

This manuscript includes a comprehensive study of the synthesis and spectroscopic features of 2,5-disilyl boroles. Reacting boron trichloride BCl₃ with 2,3-Ph*₂-1,4-(SiMe₃)₂-1,4-dilithiobuta-1,3-diene (Ph* = 3,5-t-Bu₂(C₆H₃)) allowed reliable access to 1-Chloro-2,5-(SiMe₃)₂-3,4-(Ph*)₂-borole in good yields (60%). Unlike 2,3,4,5-tetraphenyl haloboroles, this 2,5-bis-trimethylsilyl substituted chloroborole is thermally stable in solution to up to 130 °C. Metathesis reactions of the chloroborole with metal aryls or of the dilithiobutadiene with arylboron dihalides grant access to 1-Ar-2,5-(SiMe₃)₂-3,4-(Ph*)₂ boroles (Ar = Ph, Mes, Ph*, C₆F₅). Unlike the generally intensely blue-green 2,3,4,5-tetraaryl boroles, brightly orange/red 2,5-bis-trimethylsilyl substituted boroles reveal blue-shifted π/π*-transitions due to a lack of π-system interaction between borole and 2,5-bound aryls. Light is shed on the synthetic peculiarities for the synthesis of 2,5-disilyl-boroles. While direct treatment of the respective 1,1-dimethyl-stannole with ArBCl₂via otherwise well-established B/Sn exchange reactions fails, the selectivity of reactions of 2,3-Ph*₂-1,4-(SiMe₃)₂-1,4-dilithiobuta-1,3-diene with ArBCl₂ is solvent dependent and leads to rearranged 3-borolenes in hydrocarbons. Gutmann–Beckett analysis reveal reduced Lewis-acidity of disilylboroles compared to pentaphenyl borole.

---

Introduction

Among unsaturated five-membered main group heterocycles, 1H-boroles adopt a unique position. Isoelectronic to the elusive cyclopentadienyl cation, with four π-electrons in cyclic conjugation via the empty p-orbital of boron, these systems exhibit a weakly anti-aromatic character.^1–6 Free boroles are very reactive species that add dihydrogen or silane Si–H bonds across the π-system,^7–10 react as potent Lewis-acids also towards weak bases,^2,11–13 engage in various Diels–Alder and ring-expansion reactions^14–31 or readily accept two electrons to form dianionic 6π-electron systems.^32–35 Borolediides are isoelectronic to cyclopentadienyl anions and have thus been used and studied as ligands in transition metal coordination chemistry.^32,36–46 Only recently accounts of boroles as π-ligands for p-block elements (Al, Ge) have been reported.^47,48 Bearing rather small substituents, free boroles undergo Diels–Alder dimerization. These dimers can provide a source of monomeric borole synthons upon thermal treatment.^49–51

With free boroles being so reactive, to date, their synthesis and successful isolation is limited to relatively few substituents around the central C₄B cycle.⁵² Most reports on free borole chemistry discuss pentaaryl boroles^34,53,54 and particularly (PhC)₄BAr for which reliable synthetic protocols exist. Here, a key reaction involves the commercially available diphenyl acetylene which can be readily reductively coupled with lithium to provide 1,4-dilithiobuta-1,3-dienes as valuable precursor for further derivatization.^5,6 This allowed for the synthesis of tetraphenylboroles with varying boron-bound substituents in the past.^2,33,55–60

The 2,5-bis-trimethylsilyl-butadiene backbone has recently found application for s-/ and p-block element metaloles, mainly driven by the groups of Xi (groups 2 and 13), Saito and Müller (group 14).^61–81 However, only very few examples of 2,5-bis-trialkylsilyl-substituted boroles are known, all of which have been accessed via synthetically rather unusual routes (Scheme 1). Saito and coworkers obtained free fluoroborole Avia Pb/B exchange reactions starting from donor-stabilized plumbole.⁶⁹ Erker and coworkers reported formation of free 2,5-bis-trimethylsilyl boroles B by 1,1-carboborations after treatment of bis-alkynylboranes with tris(pentafluorophenyl)borane BPh^F₃.⁸² Other than these free boroles, the borole motive can be found in Sekiguchi's and Lee's dilithio borolediide obtained from reduction of chloro borapyramidane as well as Müller's and Albers' Ge(II) complex of a borolediide.^47,83

Scheme 1 Examples of 2,5-disilyl-boroles.

We are eager to extend the library of accessible free boroles with regards to the substituents attached to boron and carbon atoms of the central moiety and to study the electronic impacts of each individual substituent.⁸⁴ Here we report on the synthetic access to 2,5-bis-trimethylsilyl substituted boroles.

Results and discussion

Precursor synthesis

Free boroles (RC)₄BR’ are usually accessed via salt metathesis reaction of 1,4-dilithiobuta-1,3-dienes or B/Sn exchange reactions of the respective stannole with boron dihalides RBX₂. Stannoles are again either obtained from 1,4-dilithiobuta-1,3-dienes or Fagan–Nugent-type Sn/Zr transmetallations from the respective zirconacyclopentadiene.^85–87 1,4-Dilithiobuta-1,3-dienes are accessed by reductive coupling of acetylenes with lithium or from 1,4-diiodobuta-1,3-dienes. The latter are generally derived from zirconacyclopentadienes. However, access to any of these (cyclic) 1,4-dimetalla-1,3-butadiene precursors strongly depends on the nature of the substituents. Especially coupling of silylacetylenes over lithium is heavily depending on the substituents.⁸⁸

We are keen to provide systems that are soluble in apolar hydrocarbon solvents and therefore anticipated 3,5-di-t-butylphenyl-trimethylsilyl acetylene C Ph*CCSiMe₃ (Ph* = 3,5-t-Bu₂(C₆H₃)) to be a suitable building block to start from. Acetylene C however did not reveal any reaction with lithium metal and thus we had to take the detour via established zirconacyclopentadiene protocols using Negishi's or Rosenthal's reagents.^89,90 2,5-Disilyl-zirconacyclopentadiene 1 forms in excellent yields (>95%) (Scheme 2). However, likely due to steric pressure of the substituents 1 reversibly eliminates alkyne over time (see ESI†).⁹¹ Our attempts for direct Zr/Sn transmetallation of 1 with Me₂SnCl₂ to stannole 5 failed (see ESI†). Reactions of 1 with boron halides RBCl₂ neither produced boroles 6 or boryl-borolenes 7 (vide infra) but led to formation of yet unidentified products.

Scheme 2 Precursor syntheses.

CuCl-supported iodination of in situ generated zirconacyclopentadiene 1 reliably yields the anticipated (Z,Z)-isomer of 1,4-diiodo-butadiene 2.⁹² In our hands, iodination of previously isolated 1 has under identical conditions led to unfavourable mixtures of (Z,Z) and (E,Z)-isomers of 2 (see ESI†). 2 is conveniently transformed into the 1,4-dilithiobuta-1,3-diene 3 with tert-butyllithium in essentially quantitative yield (98%). 1,4-Dilithiobuta-1,3-diene 3 is obtained as an intensely orange crystalline solid which revealed a dimeric structure in the solid state (Fig. 1). The central distorted [Li₄]-tetrahedron reveals its longest Li–Li distances between Li-atoms that are connected to the same butadiene dianion. All lithium atoms feature a further short contact (Li–C_TMS 2.423(3)–2.472(3)) to one silicon-bound methyl group thus also providing steric protection of the reactive nucleus. A related structure was previously reported by Saito and coworkers.⁸⁸

Fig. 1 ORTEP plot of the solid-state structure of dimeric 1,4-dilithiobuta-1,3-diene (3)₂. Anisotropic displacement parameters are drawn at 50% probability level. Selected bond length [Å] are given: Li1–C1 2.105(3), C1–Li2 2.173(3), Li1–C4 2.169(3), Li1–C39 2.198(3), Li2–C4 2.112(3), Li2–C42 2.214(3), Li3–C39 2.114(3), Li3–C42 2.170(3), Li4–C42 2.100(3), Li4–C39 2.165(3), Li4–C4 2.195(3), C1–Li3 2.215(3), Li1–Li2 2.721(3), Li1–Li3 2.390(3), Li1–Li4 2.496(3), Li2–Li4 2.386(3), Li2–Li3 2.482(3), Li3–Li4 2.696(3), C1–C2 1.365(2), C2–C3 1.542(2), C3–C4 1.368(2), C39–C40 1.366(2), C40–C41 1.538(2), C41–C42 1.367(2), Li4–C10 2.423(3).

When Li/I exchange was attempted applying n-butyllithium followed by subsequent quenching of in situ generated 3 with Me₂SnCl₂ at 0 °C, rearrangements were observed to take place and we isolated and identified silole 4 as a major product.⁹³ Under these conditions, butyl iodide apparently reacts with dilithio-butadiene 3. Thus, using t-BuLi to produce 3 remained the more reliable approach. In the presence of HMPA (OP(NMe₂)₃), formation of 1,1-dimethylsiloles from rearrangements of 1,4-dilithio-1,4-disilylbuta-1,3-dienes, presumably via 2-lithiosilole and MeLi elimination, was described earlier by Xi and coworkers.^94,95

Reaction of 3 with a mild electrophile such as Me₂SnCl₂ yielded the 1,1-dimethylstannole 5 in moderate crystalline yields (58%).^71,96 Unfortunately however, the well-established boron-tin exchange reaction with arylboron dichlorides RBCl₂ (R = Ph, Xyl^F {Xyl^F = 3,5-(CF₃)₂(C₆H₃)}), that allows convenient and clean access to boroles with 2,3,4,5-tetraaryl-butadiene backbones^5,33,84 does not occur in the case of this 2,5-disilyl system (Scheme 3). No formation of Me₂SnCl₂ is observed in NMR screening reactions. Side reactions leading to intractable product mixtures likely involve methyl abstraction from the tin atom in 5. An abnormal B/Sn exchange reaction behaviour was recently reported by Braunschweig and coworkers.⁹⁷

Scheme 3 Attempted boron-tin-exchange Reactions.

Being short of one major synthetic pathway to boroles, we thus turned to direct treatment of 1,4-dilithiobutadiene 3 with organoboron dihalides. Reactions of 3 and 1 equiv. of ArBCl₂ in THF revealed to cleanly (>90% by NMR) form the aryl boroles and allowed isolation of 6-Ph and 6-Ph* in good yields (Scheme 4). However, quantitative removal of THF from the resulting Lewis-acidic boroles in vacuo (10⁻³ mbar) for several days was found to be tedious. The application of THF is therefore usually avoided when free boroles are targeted. While 6-Ph* was obtained free of donor solvent after drying in vacuo, 6-Ph always contained residual amounts of THF, despite their Lewis-acidities were found to be identical (Gutmann–Beckett method, see below). At mildly elevated temperatures (40 °C) THF is liberated from 6-Ph, however substantial decomposition occurs.

Scheme 4 Reaction of 3 towards boron electrophiles in THF (yields after crystallisation).

With MesBCl₂ the THF route fails completely. In Et₂O, borole formation from 3 with ArBCl₂ was much less selective and did not provide satisfyingly pure material. However, when 3 was analogously reacted with phenylboron dichloride in hexane or benzene in order to access free organoboroles, irrespective of the applied stoichiometric ratio of the reagents, a clean consumption of 2 equiv. PhBCl₂ per 3 took place yielding a 2-borylated 3-borolene 7-Ph. The structure of racemic mixtures of 7-Ph is confirmed by an X-ray structure (Fig. 2) and reveals a double bond between C2 and C3. We reason, that formation of 7-Ph involves the intermediate generation of the anticipated phenyl–borole and rapid subsequent addition of a second equivalent of PhBCl₂via an electrophilic attack of PhBCl₂ at one C_α atom in the borole (Scheme 5).

Fig. 2 (a) ORTEP plot of the solid-state structure of 2-boryl-3-borolene (7-Ph). Anisotropic displacement parameters are drawn at 50% probability level. Only one moleculare within the asymmetric unit is shown. Selected bond length [Å] are given: B1–Cl1 1.766(4), B1–C1 1.569(6), C1–C2 1.533(5), C2–C3 1.353(6), C3–C4 1.545(5), B1–C4 1.586(6), C4–B2 1.553(5), B2–Cl2 1.814(4).

Scheme 5 Reaction of 3 towards boron electrophiles in hydrocarbons.

These C_α positions should be considerably nucleophilic given they bear two electropositive atom substituents (–SiMe₃ and –BR₂) and the Si β-effect should further moderate the α-addition of the B-electrophile in these special cases of vinylsilanes. Within the rearrangement sequence, the B-bound Ph-residue then migrates to the other C_α when a chloride adds to the borole B-atom. The addition of a second equivalent ArBCl₂ to 6-Ar is facilitated by the removal of anti-aromatic cyclic 4π-electron delocalization and stabilizing lonepair π-donation from Cl into the empty p-orbital of boron. Addition of B–H moieties across a borole to give 2-boryl-3-borolenes where reported earlier.¹⁰ Somewhat related chemistry was described for the reaction of azobenzene with (PhC)₄BPh.⁹⁸ Similar reaction behavior consuming 2 equiv. of aryl boron dihalides per 3 are also observed for the reactions with ArBCl₂ (Ar = Ph*, Xyl^F). To support the proposed mechanism via formation of the free borole, 1 equiv. of isolated free borole 6-Ph* was treated with the respective Ph*BCl₂ to give the same asymmetric compound 2-boryl-3-borolene 7-Ph* as the reaction of 3 with 2 equiv. of Ph*BCl₂, thus corroborating the proposed mechanism (see ESI†). Notably, with Ar = Mes (Mes = 2,4,6-Me₃(C₆H₂), no reaction occurred at all, while (PhC)₄BMes is reported to be readily formed from (PhC)₄Li₂ and MesBCl₂.⁵⁸ Also when the electrophilicity of the boron species is reduced such as in i-Pr₂NBCl₂ or in the Lewis-adduct PhBCl₂(DMAP) (DMAP = 4-(N′,N′-dimethylamino)pyridine), dilithio-butadiene 3 did not reveal any reaction in C₆H₆ or Et₂O.

To our surprise, when a solution of 3 was treated with 1 equiv. of boron trichloride in hexane, chloroborole 8 was isolated in moderate yields of about 60% as an orange-red crystalline solid. Unlike in the previously discussed reactions of arylboron dihalides, feasibility of the isolation of 8 likely stems from stabilizing np-π-donation interactions reducing both the Lewis-acidity and anti-aromatic character of chloroborole 8. An X-ray crystallographic examination revealed the structure of 8 as only the third example of a crystallographically characterized haloborole (Fig. 3).

Fig. 3 (a) ORTEP plot of the solid-state structure of chloroborole (8). Anisotropic displacement parameters are drawn at 50% probability level. (b) Excerpt of the central substituted C₄B-ring. Selected bond length [Å] are given: B1–Cl1 1.7543(17), B1–C1 1.564(2), C1–C2 1.354(2), C2–C3 1.538(2), C3–C4 1.353(2), B1–C4 1.568(2), C1–Si1 1.8683(15), Si2–C4 1.8725(16).

Bond lengths within the C₄B-ring are essentially identical to fluoroborole A (Scheme 1) and (PhC)₄B–Cl with the B1–Cl1 bond of 1.7543(17) Å in 8 being slightly longer than in the latter (1.7433(13) Å).⁵⁵ Unlike for these other haloboroles, the substituents around the central C₄B-ring slightly bend out of the least-square plane through the borole atoms (Fig. 3b). While substituents at C4 and C3 essentially lie within the plane, substituents at B1, C1 and C2 alternatingly bend out above or below the central plane by ca. 11(1)° each.

The amount of known 1-haloboroles in total is limited to four stable examples. Eisch's and Braunschweig's previously reported 2,3,4,5-tetraphenyl-haloboroles either dimerize at 40 °C ((PhC)₄B–Cl)⁵⁵ or 55 °C ((PhC)₄B–Br)⁵⁷ or even decompose at ambient temperature over time. Marder reported on a transient chloroborole that readily dimerizes.⁹⁹ This 2,5-disilyl chloroborole 8 is remarkably thermally stable and even forcing conditions in benzene at 130 °C (in a Teflon-valve sealed NMR-tube) for several hours did not indicate any decomposition of 8. Its thermal stability thus more resembles Piers’ perfluorinated 2,3,4,5-tetraphenyl bromoborole (Ph^FC)₄B–Br (Ph^F = (C₆F₅)).⁵⁴ Stable haloboroles are key synthetic precursors to borolyl-substituted systems. By functionalization of (PhC)₄BCl, Braunschweig and coworkers previously accessed amino-boroles or even oxidatively added the B–Cl bond across the zero-valent metal in [Pt(PCy₃)₂].^2,56

The ¹¹B-NMR signal of 8 is found as a broad resonance (ω_1/2 = 1370 Hz) at δ_11B = 70.8 ppm. This is more lowfield shifted than in (PhC)₄BCl (δ_11B = 66.4 ppm) and (Ph^FC)₄B–Br (δ_11B = 67 ppm). Saito's structurally related example of 1-F-2,5-(SiMe₂t-Bu)₂-3,4-(Ph₂)-borole (A), however, features an ¹¹B-NMR resonance at δ_11B = 54.9 ppm in line with a pronounced B–F π-interaction.

To explore the synthetic potential of chloroborole 8, we probed the accessibility of boroles by salt-metathesis approaches starting from 8 (Scheme 6). When (Ph*Li)_n, (MesLi)₂ or Zn(Ph^F)₂ are added to toluene solutions of 8, NMR monitoring indicates the crudes to primarily contain the respective aryl boroles 6-Ar (>80%), however for 6-Ph* and 6-Ph^F isolated crystalline yields were lower. Notably, attempts to obtain 6-Ph from this approach with PhLi or Ph₂Zn in toluene failed and gave intractable product mixtures, that contained intensely colored side products. An intensely green side product (<1% NMR) of unknown constitution was also observed in case of 6-Ph* when prepared via the Ph*Li route. Only reactions of 8 with 0.5 equiv. MgPh₂ in THF led to clean formation of 6-Ph which was again contaminated by THF. X-ray structures were obtained for 6-Ar (Ar = Ph*, Mes, Ph^F) (Fig. 4). Crystals of 6-Ph were repeatedly found to be thin needles that diffracted poorly. Comparable bond lengths within the central C₄B ring of all boroles 6-Ar are virtually identical and are well in the range of Erker's related 1-Ph-2,5-disilylborole (B-Ph, Scheme 1)⁸² and pentaaryl boroles^34,54,84 (except for (PhC)₄BPh D).¹

Scheme 6 Formation of 6-Ar from 8 (yields after crystallization). ^#Repeatedly recrystallised product contained ca. 5–10% of an impurity that could not be separated.

Fig. 4 ORTEP plots of the solid-state structure of boroles (6-Ar). Anisotropic displacement parameters are drawn at 50% probability level. Lattice solvent molecules and disordered t-Bu groups are omitted for the sake of clarity. Top: 6-Ph*, middle: 6-Mes, bottom: 6-Ph^F. Key structural features are summarized in Table 1.

As observed for chloroborole 8, in all cases the silyl-groups, and in some cases but much less pronounced C_β–Ph*, bend out of the central C₄B-plane by 9–15°. Even for aryls (such as Ph or Ph*) that do not feature substituents in ortho-position that would directly govern this torsion angle, the boron-bound aryls reveal rather large torsion angles between the respective C₄B- and aryl-planes of >50°. This is likely owing to the bulky silyl groups. With pentaaryl boroles, this torsion angle usually lies between 15–30°. The torsion of the C_β-bound aryls is much less affected. For 6-Ar these torsions are found between 52–58° not much different from the torsions in (PhC)₄BAr (45–55°)^1,34 or (Ph*C)₄B–Ar (52–58°).⁸⁴

The ¹¹B-NMR resonances in 6-Ar are found at comparatively low field (Ar = Ph: 76.6 ppm; = Ph*: 77.1 ppm; = Mes: 79.9 ppm; = Ph^F: 77.4 ppm). Pentaaryl boroles ¹¹B resonances are usually found between 65 and 75 ppm (ref. 1, 30, 54 and 84) and Erker's B-Ph (Scheme 1) at 74.7 ppm.⁸² Previously, only sterically congested mesityl substituted (ArC)₄BMes boroles revealed ¹¹B resonances that low-field shifted (Ar = Ph: 79 ppm; Ar = thienyl: 77 ppm) indicating, that increasing perpendicularity of the B-bound aryl correlates with low-field shifts.

Properties of 2,5-disilylboroles

NICS^101–103 values for 6-Ar and 8 were calculated and NICS(0) and NICS(1) are tabulated in Table 1. The impact of the 2,5-disilyl substitution pattern on the (anti)aromaticity of boroles causes the NICS(0) values to be lower than the parent E (20.6), but higher (with a maximum of 16.7 for 6-Ph^F) than D (PhC)₄BPh (13.6). However the NICS-profiles of 6-Ar drop steeper and NICS(1) values do not differ significantly from D (see ESI†). The Lewis acidity of 2,5-disilylboroles was probed by means of the Gutmann–Beckett approach that correlates the ³¹P chemical shift of Et₃P=O interacting with Lewis acids with their acidity.^104–106 For steric reasons the Lewis-acidity of 6-Mes is poorly accounted for by this method but 8 (70.7), 6-Ph* (72.1), 6-Ph (72.1) and even 6-Ph^F (73.8) all reveal similar acceptor numbers (AN) well below those previously determined under identical conditions for D (78.7).¹⁰⁰ The bulky and electropositive silyl groups seem to lower the Lewis-acidity, however the method cannot provide insight which influence is dominating.

Table 1 Structural, spectroscopic and computational details of 2,5-(SiMe₃)₂-boroles and some references

Entry	B–Cα, Cα–Cβ, Cβ–Cβ ^a	C4B–Ar torsion^b	δ ^13C (Cα, Cβ)^c	δ ^11B^c	λ exp,^d (λcalc),^eελ^f	HOMO/LUMO/gap^g	NICS^h	δ ^31P^i AN
a In [Å]. b Torsion angle in [°]. c In parts per million, ppm. d Absorption bands of lowest energy in nm. e TD-DFT: RIJCOSX-CAM-B3LYP\def2-SVP\\RI-BP86-D3BJ\def2-TZVP. f In L mol^−1 cm^−1. g RI-BP86\def2-TZVP, energies in eV. h NICSiso(0) and NICSiso(1) GIAO PBE0\def2-TZVP. i Gutmann–Beckett Lewis-acidity scale parameters derived from mixtures with Et3PO in C6D6. Acceptor Numbers AN = 2.21 × (δ31P −41). j Bond lengths as reported in ref. 1. Note that C4B bond lengths in this structure markedly deviate from other known pentaaryl boroles.
8 chloroborole	1.568(2), 1.353(2), 1.538(2)	—	134.8, 182.1	70.8	447, (450)	−5.33/−3.55	14.0	73.0
	1.564(2), 1.354(2)				258	1.78	6.5	70.7
6-Ph	—	—	139.2, 183.1	76.6	480, (475)	−5.16/−3.50	15.1	73.6
					—	1.66	7.4	72.1
6-Ph*	1.587(3), 1.355(3), 1.540(4)	51	139.2, 182.8	77.1	473, (469)	−5.11/−3.41	14.7	73.6
					130	1.70	7.1	72.1
6-Mes	1.590(3), 1.360(4), 1.538(4)	85	138.8, 181.4	79.9	≈480, (462)	−5.17/−3.45	14.6	46.1
	1.594(4), 1.359(3)				310	1.72	6.8	11.3
6-Ph^F	1.577(2), 1.359(2), 1.539 (2)	59	137.6, 184.8	77.4	≈505, (510)	−5.37/−3.83	16.7	74.4
	1.577(2), 1.357(2)				121	1.54	8.4	73.8
D^1 (PhC)4BPh	1.526(2), 1.428(2), 1.470(2)	33	137.9, 162.1	65.4	560, (577)	−4.84/−3.64	13.6	76.6^100
	1.539(2), 1.425(2)^j					1.20	7.2	78.7
E (HC)4BH	1.585, 1.348, 1.520^g	—	—	—	—, (466)	−5.72/−3.95	20.6	—
						1.77	11.5

---

Since the isolation of deeply-blue (PhC)₄BPh (λ_max 560 nm),⁵ a striking feature of the hitherto well-characterized examples of pentaaryl boroles is their intense color with broad absorptions in the visible spectra roughly spanning from λ_max 530 nm (purple (Ph^FC)₄BPh^F)⁵⁴ to 635 nm (green (PhC)₄BPh^F).³⁰ Substitution with heteroaryls (e.g. thiophene) allowed even stronger shifts of λ_max.^53,60 The 2,5-disilyl-substituted boroles reported here are all orange to red both in solid state and in solution with absorption bands around 470 nm. We originally anticipated that having electropositive substituents, such as the SiMe₃ group, attached to the C_α-carbons in boroles should narrow the π/π* gap, which is dominatingly responsible for the intense colorization, and thus induce a red-shifted absorption. However, the opposite seems to be the case. We therefore computationally probed the photochemical properties of the elusive parent borole (HC)₄BH (E), the 2,5-disilyl-boroles 6-Ar and pentaphenyl borole (PhC)₄BPh (D) (Fig. 5).

Fig. 5 Difference density plots for the computationally predicted lowest energy excitations of a series of boroles at an isovalue of 0.001 a.u.; (a) (HC)₄BH E, (b) (PhC)₄BPh D, (c) 6-Mes, (d) 6-Ph* (positive: green; negative: magenta). TD-DFT RIJCOSX-CAM-B3LYP\def2-SVP.

TD-DFT calculations on 8 and 6-Ar reproduced the experimental absorption spectra very well. In all cases the lowest energy absorption is dominated by the π/π* transition between the borole C₄B based HOMO and LUMO. The absorption for the π/π* transition in parent (HC)₄BH E is predicted at 466 nm well in the range where the respective resonances for 6-Ar are found. The difference density plots for the respective excitations of E and 6-Mes are very similar and highlight only minor contributions from substituents (Fig. 5).

Pentaphenylborole (PhC)₄BPh however reveals major contributions both from the boron-bound (accepting) and C_α-bound (donating) phenyl π-systems. From comparison with our 2,5-disilylboroles we reason, that particularly the π-interaction of C₄B with C_α-bound aryls causes the red-shifted absorption that leads to the intense purple to green colors of (substituted) pentaphenyl boroles. Thus the 2,5-disilylboroles without perturbation of the C₄B π-system by arene ligands represent a much more accurate synthetic model of the parent, but elusive, (HC)₄BH E when it comes to direct comparisons of the frontier orbital situation. This is also reflected in the HOMO–LUMO energies (Table 1).

To shed further light on the influence of aryl π-system interaction in boroles, in a qualitative computational approach, we probed the individual influence of C₄B bound aryls on the computationally predicted π/π* absorption band (Fig. 6).

Fig. 6 TD-DFT predicted shifts in excitation wavelength of (PhC)₄BPh depending on the torsion angle between C₄B-plane and B-bound, C_α-bound or C_β-bound phenyls.

Starting from a hypothetical structure of (PhC)₄BPh with all phenyl groups perpendicular to the C₄B plane, TD-DFT predicts the absorption at 504 nm. Minimizing effective π-interaction thus apparently results in blueshifted absorptions closer to unperturbed (HC)₄BH (E). From there on, gradually reducing the (C₄B)–Ph_i torsion angle at B-, C_α- and (to a reduced extent due to eventual collapsing overlap) C_β-bound aryls to coplanarity allows a qualitative insight into the individual effects. While co-planarily bound B–Ph groups lead to only mildly blue-shifted absorptions likely due to a LUMO raise on account of more effective π-interaction, particularly co-planarily arranged C_α-bound phenyls reveal a strong red-shifting effect. This further corroborates the essential impact of C_α-bound aryls on the color and HOMO/LUMO gap of boroles that we identified from their substitution by silyl groups. The vast influence of the C_α aryl torsion may also be a reason for the broad bands (ω_1/2 ≈ 200 nm) commonly observed for these transitions in pentaphenyl boroles. Some previous studies on heteroaryl substituted boroles already suggested the importance of torsion-angle dependent π-interaction affecting the spectroscopic features.^53,60

Conclusion

In summary we presented various synthetic approaches to 2,5-disilyl boroles and shed light on their limitations. While traditional routes via boron-tin exchange reactions from stannoles fail, the reaction of the 1,4-dilithiobutadiene with aryl borondichlorides provides access to substituted boroles when the reaction is conducted in THF. In hydrocarbons two equivalents of arylboron dihalides react with 1,4-dilithiobutadienes to afford 2-boryl-3-borolenes, putatively via the free borole. Treatment of 1,4-dilithiobutadiene with BCl₃ grants access to a thermally robust chloroborole. The reactions of the chloroborole with common available aryl–carbon nucleophiles such as aryllithium, Grignard reagents and arylzinc reagents revealed to be very dependent on the substituent and often leads to colored product mixtures.

π/π*-absorption features of 2,5-disilyl-boroles are distinctive from pentaaryl boroles in that they do not reveal deeply blue colors. The differences were routed back to absence of π-interaction contributions stemming from C_α-bound aryl groups that are the foundation of the purple-to-blue color of pentaaryl boroles. Spectroscopically and regarding the frontier orbital situation, 2,5-disilylboroles are much closer to the parent, yet elusive borole (HC)₄BH. Given the field of free borole chemistry has been dominated by tetraphenyl-butadiene systems for 50 years, the new boroles and their access routes reported in this contribution represent a significant extension to the existing library of substitution patterns that allow the handling of free boroles. We are currently exploring the chemical potential of these boroles.

Experimental

Extensive synthetic and analytical details are given in the ESI.†

Conflicts of interest

Content of this manuscript is or will be included in the PhD/BSc theses of T. H. and L. N. There are no conflicts to declare.

Acknowledgements

Prof. Dietmar Stalke is most gratefully acknowledged for generous and supportive mentorship. We are grateful to the Fonds der chemischen Industrie for funding (fellowships to CPS and TH). The institute of inorganic chemistry is thanked for the provision of excellent conditions. We are grateful to LANXESS Organometallics GmbH for a chemical donation. MSc Paul Niklas Ruth is acknowledged for crystallographic advice and expertise. This contribution was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - 389479699/GRK2455.

Notes and references

1. H. Braunschweig, I. Fernández, G. Frenking and T. Kupfer, Angew. Chem., Int. Ed., 2008, 47, 1951–1954.
2. H. Braunschweig and T. Kupfer, Chem. Commun., 2008, 37, 4487–4489.
3. H. Braunschweig, I. Krummenacher and J. Wahler, in Advances in Organometallic Chemistry, ed. A. F. Hill and M. J. Fink, Academic Press, 2013, vol. 61, pp. 1–53.
4. J. O. C. Jimenez-Halla, E. Matito, M. Sola, H. Braunschweig, C. Horl, I. Krummenacher and J. Wahler, Dalton Trans., 2015, 44, 6740–6747.
5. J. J. Eisch, N. K. Hota and S. Kozima, J. Am. Chem. Soc., 1969, 91, 4575–4577.
6. J. J. Eisch, J. E. Galle and S. Kozima, J. Am. Chem. Soc., 1986, 108, 379–385.
7. C. Fan, L. G. Mercier, W. E. Piers, H. M. Tuononen and M. Parvez, J. Am. Chem. Soc., 2010, 132, 9604–9606.
8. A. Y. Houghton, V. A. Karttunen, C. Fan, W. E. Piers and H. M. Tuononen, J. Am. Chem. Soc., 2013, 135, 941–947.
9. H. Braunschweig, A. Damme, C. Hörl, T. Kupfer and J. Wahler, Organometallics, 2013, 32, 6800–6803.
10. B. C. Caputo, Z. J. Manning, J. H. Barnard and C. D. Martin, Polyhedron, 2016, 114, 273–277.
11. A. Fukazawa, J. L. Dutton, C. Fan, L. G. Mercier, A. Y. Houghton, Q. Wu, W. E. Piers and M. Parvez, Chem. Sci., 2012, 3, 1814–1818.
12. H. Braunschweig, C.-W. Chiu, T. Kupfer and K. Radacki, Inorg. Chem., 2011, 50, 4247–4249.
13. K. Ansorg, H. Braunschweig, C. W. Chiu, B. Engels, D. Gamon, M. Hügel, T. Kupfer and K. Radacki, Angew. Chem., Int. Ed., 2011, 50, 2833–2836.
14. J. J. Eisch and J. E. Galle, J. Am. Chem. Soc., 1975, 97, 4436–4437.
15. J. J. Eisch, J. E. Galle, B. Shafii and A. L. Rheingold, Organometallics, 1990, 9, 2342–2349.
16. S. A. Couchman, T. K. Thompson, D. J. D. Wilson, J. L. Dutton and C. D. Martin, Chem. Commun., 2014, 50, 11724–11726.
17. K. Huang, S. A. Couchman, D. J. D. Wilson, J. L. Dutton and C. D. Martin, Inorg. Chem., 2015, 54, 8957–8968.
18. K. Huang and C. D. Martin, Inorg. Chem., 2015, 54, 1869–1875.
19. J. H. Barnard, S. Yruegas, K. Huang and C. D. Martin, Chem. Commun., 2016, 52, 9985–9991.
20. S. Yruegas, D. C. Patterson and C. D. Martin, Chem. Commun., 2016, 52, 6658–6661.
21. K. Huang and C. D. Martin, Inorg. Chem., 2016, 55, 330–337.
22. J. H. Barnard, S. Yruegas, S. A. Couchman, D. J. D. Wilson, J. L. Dutton and C. D. Martin, Organometallics, 2016, 35, 929–931.
23. S. Yruegas and C. D. Martin, Chem. – Eur. J., 2016, 22, 18358–18361.
24. S. Yruegas, C. Wilson, J. L. Dutton and C. D. Martin, Organometallics, 2017, 36, 2581–2587.
25. J. J. Baker, K. H. M. Al-Furaiji, O. T. Liyanage, D. J. D. Wilson, J. L. Dutton and C. D. Martin, Chem. – Eur. J., 2019, 25, 1581–1587.
26. H. Braunschweig, C. Hörl, L. Mailänder, K. Radacki and J. Wahler, Chem. – Eur. J., 2014, 20, 9858–9861.
27. H. Braunschweig, F. Hupp, I. Krummenacher, L. Mailänder and F. Rauch, Chem. – Eur. J., 2015, 21, 17844–17849.
28. H. Braunschweig, I. Krummenacher, L. Mailänder and F. Rauch, Chem. Commun., 2015, 51, 14513–14515.
29. H. Braunschweig, M. A. Celik, F. Hupp, I. Krummenacher and L. Mailänder, Angew. Chem., Int. Ed., 2015, 54, 6347–6351.
30. H. Braunschweig, M. A. Celik, T. Dellermann, G. Frenking, K. Hammond, F. Hupp, H. Kelch, I. Krummenacher, F. Lindl, L. Mailänder, J. H. Müssig and A. Ruppert, Chem. – Eur. J., 2017, 23, 8006–8013.
31. F. Lindl, S. Lin, I. Krummenacher, C. Lenczyk, A. Stoy, M. Müller, Z. Lin and H. Braunschweig, Angew. Chem., Int. Ed., 2019, 58, 338–342.
32. G. E. Herberich, B. Buller, B. Hessner and W. Oschmann, J. Organomet. Chem., 1980, 195, 253–259.
33. H. Braunschweig, C.-W. Chiu, A. Damme, B. Engels, D. Gamon, C. Hörl, T. Kupfer, I. Krummenacher, K. Radacki and C. Walter, Chem. – Eur. J., 2012, 18, 14292–14304.
34. C.-W. So, D. Watanabe, A. Wakamiya and S. Yamaguchi, Organometallics, 2008, 27, 3496–3501.
35. H. Braunschweig and I. Krummenacher, in Organic Redox Systems, 2015, pp. 503–522,  DOI:10.1002/9781118858981.ch17.
36. G. E. Herberich, J. Hengesbach, U. Kölle, G. Huttner and A. Frank, Angew. Chem., Int. Ed. Engl., 1976, 15, 433–434.
37. G. E. Herberich, J. Hengesbach, U. Kölle and W. Oschmann, Angew. Chem., Int. Ed. Engl., 1977, 16, 42–43.
38. G. E. Herberich, B. Hessner, W. Boveleth, H. Lüthe, R. Saive and L. Zelenka, Angew. Chem., Int. Ed. Engl., 1983, 22, 996–996.
39. G. E. Herberich, W. Boveleth, B. Hessner, D. P. J. Köffer, M. Negele and R. Saive, J. Organomet. Chem., 1986, 308, 153–166.
40. G. E. Herberich, B. Hessner, H. Ohst and I. A. Raap, J. Organomet. Chem., 1988, 348, 305–316.
41. G. E. Herberich, U. Englert, M. Hostalek and R. Laven, Chem. Ber., 1991, 124, 17–23.
42. G. E. Herberich, T. Carstensen and U. Englert, Chem. Ber., 1992, 125, 2351–2357.
43. R. W. Quan, G. C. Bazan, W. P. Schaefer, J. E. Bercaw and A. F. Kiely, J. Am. Chem. Soc., 1994, 116, 4489–4490.
44. C. K. Sperry, W. D. Cotter, R. A. Lee, R. J. Lachicotte and G. C. Bazan, J. Am. Chem. Soc., 1998, 120, 7791–7805.
45. G. J. Pindado, S. J. Lancaster, M. Thornton-Pett and M. Bochmann, J. Am. Chem. Soc., 1998, 120, 6816–6817.
46. T. J. Woodman, M. Thornton-Pett, D. L. Hughes and M. Bochmann, Organometallics, 2001, 20, 4080–4091.
47. P. Tholen, Z. Dong, M. Schmidtmann, L. Albers and T. Müller, Angew. Chem., Int. Ed., 2018, 57, 13319–13324.
48. C. P. Sindlinger and P. N. Ruth, Angew. Chem., Int. Ed., 2019, 58, 15051–15056.
49. P. J. Fagan, E. G. Burns and J. C. Calabrese, J. Am. Chem. Soc., 1988, 110, 2979–2981.
50. X. Su, J. J. Baker and C. D. Martin, Chem. Sci., 2020, 11, 126–131.
51. J. J. Baker, K. H. M. Al Furaiji, O. T. Liyanage, D. J. D. Wilson, J. L. Dutton and C. D. Martin, Chem. – Eur. J., 2019, 25, 1581–1587.
52. This discussion does not include systems where the borole 4π-electron system is embedded in larger, delocalised aromatic framworks as for example in 9-borafluorenes.
53. T. Araki, A. Fukazawa and S. Yamaguchi, Angew. Chem., Int. Ed., 2012, 51, 5484–5487.
54. C. Fan, W. E. Piers and M. Parvez, Angew. Chem., Int. Ed., 2009, 48, 2955–2958.
55. H. Braunschweig, C.-W. Chiu, J. Wahler, K. Radacki and T. Kupfer, Chem. – Eur. J., 2010, 16, 12229–12233.
56. H. Braunschweig, C.-W. Chiu, K. Radacki and P. Brenner, Chem. Commun., 2010, 46, 916–918.
57. H. Braunschweig, C.-W. Chiu, A. Damme, K. Ferkinghoff, K. Kraft, K. Radacki and J. Wahler, Organometallics, 2011, 30, 3210–3216.
58. H. Braunschweig, V. Dyakonov, J. O. C. Jimenez-Halla, K. Kraft, I. Krummenacher, K. Radacki, A. Sperlich and J. Wahler, Angew. Chem., Int. Ed., 2012, 51, 2977–2980.
59. H. Braunschweig, C.-W. Chiu, D. Gamon, M. Kaupp, I. Krummenacher, T. Kupfer, R. Müller and K. Radacki, Chem. – Eur. J., 2012, 18, 11732–11746.
60. H. Braunschweig, A. Damme, J. O. C. Jimenez-Halla, C. Hörl, I. Krummenacher, T. Kupfer, L. Mailänder and K. Radacki, J. Am. Chem. Soc., 2012, 134, 20169–20177.
61. A. J. Ashe, J. W. Kampf and P. M. Savla, Organometallics, 1993, 12, 3350–3353.
62. L. Liu, W.-X. Zhang, Q. Luo, H. Li and Z. Xi, Organometallics, 2010, 29, 278–281.
63. S. Zhang, M. Zhan, W.-X. Zhang and Z. Xi, Organometallics, 2013, 32, 4020–4023.
64. J. Wei, L. Liu, M. Zhan, L. Xu, W.-X. Zhang and Z. Xi, Angew. Chem., Int. Ed., 2014, 53, 5634–5638.
65. Y. Zhang, J. Wei, W.-X. Zhang and Z. Xi, Inorg. Chem., 2015, 54, 10695–10700.
66. Y. Zhang, Y. Chi, J. Wei, Q. Yang, Z. Yang, H. Chen, R. Yang, W.-X. Zhang and Z. Xi, Organometallics, 2017, 36, 2982–2986.
67. B. Wei, L. Liu, W.-X. Zhang and Z. Xi, Angew. Chem., Int. Ed., 2017, 56, 9188–9192.
68. Y. Zhang, Z. Yang, W.-X. Zhang and Z. Xi, Chem. – Eur. J., 2019, 25, 4218–4224.
69. M. Saito, T. Akiba, M. Kaneko, T. Kawamura, M. Abe, M. Hada and M. Minoura, Chem. – Eur. J., 2013, 19, 16946–16953.
70. T. Kuwabara, J.-D. Guo, S. Nagase, T. Sasamori, N. Tokitoh and M. Saito, J. Am. Chem. Soc., 2014, 136, 13059–13064.
71. T. Kuwabara, J.-D. Guo, S. Nagase, M. Minoura, R. H. Herber and M. Saito, Organometallics, 2014, 33, 2910–2913.
72. T. Kuwabara and M. Saito, Organometallics, 2015, 34, 4202–4204.
73. M. Nakada, T. Kuwabara, S. Furukawa, M. Hada, M. Minoura and M. Saito, Chem. Sci., 2017, 8, 3092–3097.
74. M. Saito, T. Akiba, S. Furukawa, M. Minoura, M. Hada and H. Y. Yoshikawa, Organometallics, 2017, 36, 2487–2490.
75. Z. Dong, C. R. W. Reinhold, M. Schmidtmann and T. Müller, J. Am. Chem. Soc., 2017, 139, 7117–7123.
76. Z. Dong, O. Janka, J. Kösters, M. Schmidtmann and T. Müller, Angew. Chem., Int. Ed., 2018, 57, 8634–8638.
77. Z. Dong, C. R. W. Reinhold, M. Schmidtmann and T. Müller, Organometallics, 2018, 37, 4736–4743.
78. Z. Dong, M. Schmidtmann and T. Müller, Z. Anorg. Allg. Chem., 2018, 644, 1041–1046.
79. C. R. W. Reinhold, Z. Dong, J. M. Winkler, H. Steinert, M. Schmidtmann and T. Müller, Chem. – Eur. J., 2018, 24, 848–854.
80. Z. Dong, M. Schmidtmann and T. Müller, Chem. – Eur. J., 2019, 25, 10858–10865.
81. Z. Dong, L. Albers, M. Schmidtmann and T. Müller, Chem. – Eur. J., 2019, 25, 1098–1105.
82. F. Ge, G. Kehr, C. G. Daniliuc and G. Erker, J. Am. Chem. Soc., 2014, 136, 68–71.
83. V. Y. Lee, H. Sugasawa, O. A. Gapurenko, R. M. Minyaev, V. I. Minkin, H. Gornitzka and A. Sekiguchi, J. Am. Chem. Soc., 2018, 140, 6053–6056.
84. T. Heitkemper and C. P. Sindlinger, Chem. – Eur. J., 2019, 25, 6628–6637.
85. X. Yan and C. Xi, Acc. Chem. Res., 2015, 48, 935–946.
86. P. J. Fagan and W. A. Nugent, J. Am. Chem. Soc., 1988, 110, 2310–2312.
87. P. J. Fagan, W. A. Nugent and J. C. Calabrese, J. Am. Chem. Soc., 1994, 116, 1880–1889.
88. M. Saito, M. Nakamura, T. Tajima and M. Yoshioka, Angew. Chem., Int. Ed., 2007, 46, 1504–1507.
89. E.-I. Negishi and T. Takahashi, Acc. Chem. Res., 1994, 27, 124–130.
90. U. Rosenthal, A. Ohff, W. Baumann, A. Tillack, H. Görls, V. V. Burlakov and V. B. Shur, Z. Anorg. Allg. Chem., 1995, 621, 77–83.
91. A. D. Miller, J. F. Tannaci, S. A. Johnson, H. Lee, J. L. McBee and T. D. Tilley, J. Am. Chem. Soc., 2009, 131, 4917–4927.
92. C. Xi, S. Huo, T. H. Afifi, R. Hara and T. Takahashi, Tetrahedron Lett., 1997, 38, 4099–4102.
93. The crystal structure of 4 is documented in the ESI.†.
94. C. Wang, Q. Luo, H. Sun, X. Guo and Z. Xi, J. Am. Chem. Soc., 2007, 129, 3094–3095.
95. Q. Luo, C. Wang, L. Gu, W.-X. Zhang and Z. Xi, Chem. – Asian J., 2010, 5, 1120–1128.
96. Analytical and crystallographic data for 5 is documented in the ESI.†.
97. H. Braunschweig, M. Dömling, S. Kachel, H. Kelch, T. Kramer, I. Krummenacher, C. Lenczyk, S. Lin, Z. Lin, C. Possiel and K. Radacki, Chem. – Eur. J., 2017, 23, 16167–16170.
98. V. A. K. Adiraju and C. D. Martin, Chem. – Eur. J., 2017, 23, 11437–11444.
99. Z. Zhang, Z. Wang, M. Haehnel, A. Eichhorn, R. M. Edkins, A. Steffen, A. Krueger, Z. Lin and T. B. Marder, Chem. Commun., 2016, 52, 9707–9710.
100. S. Yruegas, J. J. Martinez and C. D. Martin, Chem. Commun., 2018, 54, 6808–6811.
101. P. V. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao and N. J. R. van Eikema Hommes, J. Am. Chem. Soc., 1996, 118, 6317–6318.
102. Z. Chen, C. S. Wannere, C. Corminboeuf, R. Puchta and P. V. R. Schleyer, Chem. Rev., 2005, 105, 3842–3888.
103. A. Stanger, J. Org. Chem., 2006, 71, 883–893.
104. U. Mayer, V. Gutmann and W. Gerger, Monatsh. Chem., 1975, 106, 1235–1257.
105. M. A. Beckett, G. C. Strickland, J. R. Holland and K. Sukumar Varma, Polymer, 1996, 37, 4629–4631.
106. I. B. Sivaev and V. I. Bregadze, Coord. Chem. Rev., 2014, 270–271, 75–88.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental and synthetic details, depiction of spectra and crystal structures, computational details. CCDC 1976852–1976861. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0dt00393j

---