A Boratafulvene

Abstract Structurally authenticated free B‐alkyl boroles are presented and electronic implications of alkyl substitution were assessed. Deprotonation of a boron‐bound exocyclic methyl group in a B‐methyl borole yields the first 5‐boratafulvene anion—an isomer to boratabenzene. Boratafulvene was structurally characterized and its electronic structure probed by DFT calculations. The pK a value of the exocyclic B−CH3 in a set of boroles was computationally approximated and confirmed a pronounced acidic character caused by the boron atom embedded in an anti‐aromatic moiety. The non‐aromatic boratafulvene reacts as a C‐centered nucleophile with the mild electrophile Me3SnCl to give a stannylmethyl borole, regenerating the anti‐aromaticity. As nucleophilic synthons for boroles, boratafulvenes thus open an entirely new avenue for synthetic strategies toward this highly reactive class of heterocycles. Boratafulvene reacts as a methylene transfer reagent in a bora‐Wittig‐type reaction generating a borole oxide.


Introduction
Replacement of carbon by isoelectronic heteroelement fragments in classic hydrocarbons has long been af ruitful synthetic challenge which led to fundamental structural motifs of E/CÀEb onding interactions. [1] Ap lethora of molecules and materials with altered properties resulted from these efforts,p articularly when more electropositive boron atoms are introduced. [2] Among classic hydrocarbon molecules,t he parent (penta)fulvene is ar eactive isomer of benzene featuring an unsaturated five-membered ring with a"cross-conjugate" exocyclic methylene group. [3] Compounds conventionally also considered heterofulvenes usually feature exocyclice lectronegative oxygen or imine nitrogen atoms (Scheme 1). [4] Heterofulvenes with endocyclich eteroatoms, except for ubiquitous N-atom containing rings as in dipyrromethene-based compounds,a re much scarcer and often transient. [5] Erker and Nçth reported on borata-(di)benzofulvene derivatives with exocyclic =BR 2 moieties. [6] We now present an anionic 5-boratafulvene,a ccessed by deprotonation of B-methyl 1H-boroles,w ith an exocyclic methylene group as anew entry into heterofulvene chemistry (Scheme 1). 1H-Boroles are unsaturated five-membered boron heterocycles with four cyclic conjugate p-electrons and reveal (weakly) anti-aromatic properties. [7] This results in high reactivity of the butadiene and pronounced Lewis acidity of the organoborane moiety.

Results and Discussion
Only few substitution patterns that sufficiently stabilize free boroles have been reported and our group has recently established reliable protocols towards 1-chloro-2,5-(TMS) 2borole (A). [7b, 8] When A was treated with ethereal methyl Grignard solutions,1 -methylborole 1 is formed and is obtained in ca. 80 %c rystalline yield as ab rightly orange solid. Boron-bound alkyl groups in free boroles are rare: (PhC) 4 BCH 3 ,prepared by Sn/B exchange from (PhC) 4 SnMe 2 and MeBX 2 ,i st he only derivative described in the literatur-  e. [7b,c] Related B-alkyl (di)benzoboroles, [9] and Me-borole derivatives,s ufficiently stabilized in transition metal complexes or as base adducts,are documented. [10] We were able to structurally characterize 1 and its molecular structure is depicted in Figure 1.
Localized single and double bonds are found within the central borole ring in 1 as to be expected for an anti-aromatic system. Theexocyclic methyl group is slightly bent out of the borole plane by ca. 78 8 (H 3 C-B-(C b C b ) centroid ca. 1738 8)with aB À CH 3 bond length of 1.559(2) in the typical range of B À C(sp 3 )s ingle bonds.T he characteristic p!p*t ransition of borole-based frontier orbitals is found at l max = 458 nm, slightly blue shifted to comparable B-aryl derivatives (l max % 475 nm). [7b, 8d] As to be expected for tri-coordinate boron, the 11 BNMR resonance is found lowfield-shifted at 80.0 ppm and the B-methyl group resonates at 1.32 ppm ( 1 H) and 11.9 ppm ( 13 C).
a-CH acidity of boranes is known. [6a,11] However,suitable diorgano alkyl boranes R 2 B(CHR' 2 )and conditions that allow selective deprotonation are scarce. [6b] Successful deprotonation of Ar 2 BCH 3 with suitable amides to yield borataalkenes is restricted to sterically demanding aryl groups (such as mesityl) and bases that prevent from adduct formation and quaternation at the boron atom. [11b, 12] This route granted access to the yet sole example of as tructurally characterized borataalkene with an unsupported terminal methylene unit in [Mes 2 BCH 2 ] À . [11b] Erker proposed intermediate formation of borataalkenes by tautomerization in an indane-bridged FLP. [11a] As af urther example,H erberich reported twofold deprotonation of endocyclic a-CH in 1-amino-3-borolene to yield the Hückel-aromatic borole dianion. [13] We reasoned that the exocyclic methyl protons of 1 bound to aLewis-acidic boron atom, which is embedded in an anti-aromatically destabilized borole moiety,m ight reveal an increased acidic character that would facilitate deprotonation. Along the lines of arecent computational approach by Erker and coworkers to estimate pK a values for a-CH bonds in boranes (with pK a (CpH) = 18.0 as ar eference), [14] we found the pK a of 1 with polar DMSO solvent model to be 22.6, slightly higher than, for example,( C 6 F 5 ) 2 BCH 3 (18.7, Figure 2). [11a, 15] Notably,aseries of substituted B-methyl boroles were probed and all revealed general, significantly increased acidity along with strong dependencyo nt he substituents [(MeC) 4 BMe 24.8; (PhC) 4 BMe 18.8;( HC) 4 BMe 18.6;( Ph F C) 4 BMe 11.1 (Ph F = C 6 F 5 ); (F 3 CC) 4 BMe 6.7].M ethyl boranes with comparably inductively active vinyl substituents,y et lacking the cyclic conjugation, reveal significantly higher computational pK a (vinyl:2 8.8;1 -silyl-2-phenylvinyl:3 2.2) than the respective boroles,c learly pointing at the remarkable acidity enhancement which results from the thermodynamic incentive that is the removal of anti-aromaticity upon deprotonation. Comparison with predicted pK a of five-membered B-methyl 2-or 3-borolene (32.6;3 1.7) also advocates against ring-strain effects to account for the increased C À Ha cidity in methylborole compared to acyclicd ivinyl derivatives.C ompared to parent methyl borole (HC) 4 BMe,b enzannulation as in 1boraindene (21.8) or 9-borafluorene (23.7) increasingly reduces the CÀHa cidity,p resumably due to reduced antiaromatic character.W ep ropose this (computational) acidity assessment to be auseful measure for anti-aromaticity-driven reactivity enhancement in boroles.P revious approaches to quantify this effect include shifts in CN-stretching modes of nitril adducts to boroles, [7b] and computational measures such as aromatic stabilization energy (ASE) or NICS. [16] Successful deprotonation of 1 (Scheme 2) and reliable isolation of boratafulvene anion 2 are very sensitive to base and solvation conditions.I nb enzene,t reatment with LiTMP (TMP = 2,2',6,6'-tetramethylpiperidine) leads to decomposition and intractable mixtures,w hile in [D 8 ]THF immediate clean conversion with LiTMP is indicated by NMR monitoring. However,i solation attempts fail as,a gain, intractable mixtures form. Tr eatment of orange solutions of 1 in toluene with K[N(SiMe 3 ) 2 ]f or 18 hy ields as paringly soluble yellow solid. This crude solid contained boratafulvene 2 and varying amounts (0 to ca. 30 %) of two side-products,ofwhich one was identified as the colorless amide adduct B (Scheme 2, see SI for structure). [17] Computational assessment (BP86/ def2TZVPP and benzene solvation model, see SI) of the reaction indeed reveals the adduct formation to be more exergonic (À28.8 kcal mol À1 )than the deprotonation reaction (À15.7 kcal mol À1 ). TheL ewis-acidic boron atom in 1 is significantly less sterically shielded than in previous cases of successful R 2 B À Me deprotonation (as in Mes 2 B À CH 3 with pK a = 29.0), where adduct formation is sterically impaired. In that respect, the successful deprotonation of 1 likely benefits from its increased CÀHa cidity (pK a = 22.6).
After work-up,boratafulvene 2[K(thf) 2 ]can be isolated in small to moderate yields from THF by fractional crystallization. These crystals have been probed several times by X-ray diffraction but only provided poor data which only allowed identification of the connectivity pattern as ac oordination polymer of {2[K(thf) 2 } 1 (see SI). ]) can be reliably isolated from toluene as crystalline material suitable for X-ray diffraction in moderate yields (ca. 44 %). Them olecular structure of 2a is shown in Figure 3. [18,19] Thestructure reveals B = Ccontacts to acrownether-coordinated K-cation. Compared to 1,the B1ÀC1 bond lies in the borole plane and is significantly shortened to 1.457(2) as to be expected for authentic C = Bi nb orataalkenes (C = Bi n[ Mes 2 B = CH 2 ] À :1 .444 (8) ). [1, 11b,20] The 11 BNMR resonance is found at 40.3 ppm highfield-shifted relative to 1 indicating involvement of the previously empty boron p-orbital in aB =C p-bond. Them ethylene signals are found at 4.48 ppm ( 1 H) and 96.1 ppm ( 13 C).
Anion 2 is isoelectronic to (penta)fulvene and thus arare case of ah eterofulvene with an endocyclich eteroatom. Due to resonance stabilization of an egative charge in aromatic cyclopentadienyl moieties,fulvenes are polar molecules with the dipole moment aligned along the polar exocyclicC =C double bond. [21] NBO analysis of boratafulvene reveals similar polarities for both, the BÀC s-a nd p-bonds with dominant contributions (65 %) of the more electronegative carbon atom. NRTa nalysis further suggests very similar resonance structure contributions compared to fulvene (Scheme 3). [22] Despite similar contributions of ac ationic exocyclicC H 2 group attached to an aromatic,d ianionic borole ring, the dominant polar double bond resonance structure results in an inverted directionality of the dipole moment in boratafulvenes compared to fulvenes.The electrostatic potential surface map of boratafulvene thus reveals an ucleophilic site along the B=Cb ond and in that respect, boratafulvene more resembles the profile of N-heterocyclic olefins (Figure 4), which have evolved as versatile C-nucleophilic ligands in coordination chemistry. [23] Respectively,(Nheterocyclic) borataalkenes were most recently shown to be suitable ligands. [12b, 24] Fulvene is ah igh energy isomer of benzene (by ca. 33 kcal mol À1 )a nd accordingly,p arent boratafulvene is 31 kcal mol À1 higher in energy than boratabenzene,o fw hich substitution and transition-metal complex derivatives are known. [25,26] We treated 2 with Me 3 SnCl as am ild electrophile.A s observed for borataalkenes before, [11c, e] 2 reacts as ac arboncentered nucleophile to selectively give the stannaneopentyl borole 3 (Scheme 4a). Remarkably,i nt he course of this reaction the unfavorable anti-aromatic character within the borole ring is regained from an on-aromatic precursor,a s supported by ac haracteristic NICS zz profile of 3 (see Supporting Information). [27] However,o nt he basis of the NICS(1) zz value,t he anti-aromatic character in 3 (13.0) is significantly reduced compared to 1 (24.8). Thus,b oratafulvenes allow for the synthesis of functionalized alkyl-substituted free boroles with the borole fragment being introduced   as anucleophilic reagent. It is important to stress that this new synthetic avenue can be very valuable as the known routes for the synthesis of highly reactive,a nti-aromatic free boroles, particularly the rare B-alkyl-substituted derivatives,a re very limited. Ad istinct difference to formally related NHCsupported borole anions (or rather B-imidazolium substituted borole dianions), featuring aB ÀC carbene single bond, must be noted as those are reported to react as one-electron reductants or boron-centered nucleophiles,y et for the latter not generating free boroles. [10f,28] This differences of a p-acidic formal methylene unit attached to the boron atom in borole anions compared to ad ominantly s-donating N-heterocyclic carbene is reflected in their electrostatic potential maps (Figure 4d).
Them olecular structure of stannaneopentyl borole 3 is depicted in Figure 5. The-CH 2 SnMe 3 group is notably bent out of the borole plane by ca. 168 8 (C2-B1-(C4/C5) centroid ca. 1648 8). Thesingle and double bond lengths within the ring are as to be expected but the exocyclic B1-C2 distance is relatively short (1.496 (7) )ranging between the single bond in 1 (ca. 1.56 )a nd the double bond in 2 (ca. 1.46 ). The methylene À Sn bond is notably elongated compared to the other SnÀCH 3 contacts and the B-C-Sn angle is found at 103(1)8 8.
The 11 BNMR resonance of 3 is found at 68.7 ppm, which is considerably highfield-shifted compared to other free 2,5disilylboroles. [8d] TheB -bound methylene group resonates at 2.27 ppm ( 1 H) and 24.9 ppm ( 13 C). 1 J SnÀC coupling amounts to 335 Hz for the CH 3 groups (almost identical to the coupling in SnMe 4 )b ut only 46 Hz for the CH 2 group.T his points at reduced s-orbital contributions involved in the C2ÀSn1 bond. [29] Indeed, NBO calculations suggest sp 3 hybridization for the Sn-atom and the methyl C-atoms attached but only af airly small s-orbital contribution (10 %) of methylene Catom to the C2 À Sn bond, thus reducing the Fermi contact (see SI for further details).
Afurther spectroscopic feature of stannaneopentyl borole 3 is its bright yellow color.The characteristic p!p*transition in boroles is observed at l max % 424 nm, notably blue-shifted compared to its related methyl derivative 1 (458 nm). This correlates with an increased HOMO/LUMO gap (2.04 eV in 3;1 .81 eV in 1)t hat mainly results from an energetically elevated LUMO level. [25] NBO analysis of 3 suggests aclassic Lewis structure as depicted in Scheme 4, however secondorder perturbation theory (SOPT) calculations suggest significant hyperconjugation of the C À Sn s-bond into the empty p-orbital of the boron atom (17.8 kcal mol À1 ). [22] When this hyperconjugation is probed computationally for as eries of boranes,t he exceptional Lewis-acidic character of antiaromatic boroles becomes apparent (Scheme 5). Acyclic boranes R 2 B(CH 2 SnMe 3 )including those with electron-withdrawing substituents such as C 6 F 5 groups reveal smaller respective hyperconjugation interaction energies from SOPT. Only CF 3 groups render boranes comparably Lewis-acidic to rival the hyperconjugation predicted in the parent borole (HC) 4 B(CH 2 SnMe 3 )i na ccordance with the pK a approximations.Reduced interaction in 3 compared to the parent borole may stem from steric hindrance preventing from smaller B-C-Sn angles.
We further probed the reactivity of the boratafulvene anion 2 towards benzophenone as am odel carbonyl compound and monitored the reaction by NMR spectroscopy (Scheme 4b). After several days at room temperature,c lean conversion to 1,1-diphenylethylene and anew borole species, borole oxide 4 (the molecular structure of its [K (18-crown-6)] + salt is shown in Figure 6) was observed, indicating that 2 serves as amethylene transfer reagent in aborata-Wittig-type reaction to form alkenes.
Such reactions were reported previously for borataalkenes and carbonyls. [11c, 12a,15, 30] Preliminary analysis of the reaction mixtures by NMR spectroscopy indicates the formation of an oxaboretane intermediate C as the dominant species of amixture after afew hours. [15] Themethylene CH 2 Scheme 4. Reactivities of Boratafulvene 2. signals are observed highfield-shifted at 2.82 ppm ( 1 H) and 24.3 ppm ( 13 Cvia HSQC), indicating asaturated species with the 11 Br esonance at 11.2 ppm strongly advocating for at etracoordinate boron atom and thus the four-membered cycle.A long with slowly increasing amounts of 4 and 1,1diphenylethylene,i ntermediate presence of species lacking mirror-plane symmetry and revealing two diastereotopic protons of the methylene group is observed, plausibly yet putatively assigned to ar ing-expanded oxaborolane D.T he observation of individual intermediates seems to be dependent on solvent (benzene vs.THF) and presence of 18-crown-6, yet in each case clean conversion to 4 and 1,1-diphenylethylene is reached eventually after two weeks.According to NBO analysis,b orole oxide 4 is an oxoborane best represented by the Lewis structure depicted in Scheme 4w ith a B=Od ouble bond and the short B-O distance of 1.281 (3) lies well in between those recently reported for neutral (1.2867(16) )o ra nionic (1.273 (8) )a cid-free azaborolederived oxoboranes that were discussed as "bora carbonyls", but longer than in amost recent entry (1.256 (3) )byXie and co-workers. [31] TheK1-O1 distance is found at 2.522 (2) ,in the range of distances observed in ar elated, yet dimeric potassium salt of ad iazaborole oxide (2.47-2.59 ). [32] Computationally (BP86/def2TZVPP and benzene solvation model, see SI), the overall reaction of 2 and benzophenone to form 4 and diphenylethylene is predicted to be exergonic (À27.3 kcal mol À1 ). In line with the proposed reaction progress,f ormation of oxaboretan C (À10.6 kcal mol À1 )and its putative subsequent rearrangement to oxaborolane D (À14.4 kcal mol À1 ), as well as their respective reactions to the final products 4 and diphenylethylene are exergonic (C: À16.7 kcal mol À1 ; D: À2.2 kcal mol À1 ).

Conclusion
In summary,w ep resented the first synthesis of ab oratafulvene by deprotonation of methyl borole.A ccording to computational pK a approximations,a nti-aromaticity of boroles increases a-CH acidity to as imilar extent as strongly electron-withdrawing fluorinated substituents.Afirst example for the suitability of boratafulvenes as nucleophilic reagents to generate free boroles is demonstrated. Borata-Wittig reactivity as methylene transfer reagent was observed that also leads to ay et unprecedented borole oxide.