Characteristic Reactivity of Highly Lewis Acidic Aryl

Characteristic Reactivity of Highly Lewis Acidic Aryl Substituted Diborane(4) toward Multiple─ Bonds ─

Makoto Yamashita ＊

＊ Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University Furo ─ cho, Chikusa ─ ku, Nagoya 464 ─ 8603, Japan

(Received August 5, 2018; E ─ mail: [email protected])

Abstract: This account focuses on the synthesis and reactivity of the diborane(4) compounds, pinB ─ BMes 2 and B 2(o ─ tol) 4 (pin = pinacolato, Mes = 2,4,6 ─ Me 3C 6H 2, o ─ tol = 2 ─ MeC 6H 4), which have been recently reported by the author’s group. Both compounds exhibit higher Lewis acidity than the most common diborane(4) B 2pin 2, which is due to the overlapping vacant p ─ orbitals of the two boron atoms. As a result, pinB ─ BMes 2 and B 2(o ─ tol) 4 exhibit a peculiar reactivity toward multiple bond compounds and small molecules such as CO, isocyanides, alkynes, nitriles, pyridine, and H 2. DFT calculations revealed that the combination of the high electrophilicity of these diborane(4)s, which facilitates the complexation of weak nucleophiles, and the reactive B ─ B bonding electrons should be responsible for the observed unique reactivity.

2 3 transformation, the transferred Bpin moiety from the sp ─ sp 1． Introduction diborane intermediate C can be considered as a “boron nucleo-

The incorporation of heteroatoms (p ─ block elements other phile”. A combination of B 2pin 2, MeOH, and a base also gen- 2 3 than carbon) into organic molecules affords access to a wide erates sp ─ sp diborane intermediate D, which exhibits reactiv- 6h,11 variety of characteristic properties. The lightest group ─ 13 ele- ity toward multiple bonds. DFT calculations have clearly 1 ment, boron, has a slightly larger atomic radius (0.83 Å) com- demonstrated that the B ─ B bonding electrons in D serve as a pared to its neighbor, carbon (0.77 Å) 1 due to the difference in nucleophile. This mode of the reaction can also be found in the 2 effective nuclear charge. The larger size of boron atom also transmetallation of the boryl group from B 2pin 2 to transition 12 contributes to longer boron ─ containing bonds in organic mol- metals in the copper ─ catalyzed borylation of unsaturated 13 ecules. For example, the B ─ C single bond, i.e., the sum of the bonds. A related reaction, i.e., the insertion of carbenoid spe- 1 covalent radii of boron and carbon atoms (1.65 Å) is longer cies using B 2pin 2 and diazoalkanes affords 1,1 ─ diborated 1 14 than the corresponding C ─ C single bond (1.54 Å). The products, in which the transition state E with a C ─ B ─ B three ─ Nobel ─ prize winning Suzuki ─ Miyaura cross ─ coupling reac- membered ring structure was postulated rather than a stable 2 3 tion uses this reactive B ─ C bond in boronic acid derivatives to sp ─ sp diborane intermediate. construct a new C ─ C bonds with a remarkably wide func- In contrast, diborane(4) containing no heteroatoms such as 3 tional ─ group tolerance. oxygen and nitrogen, being able to interact with the vacant p ─

The mononuclear hydride of boron, borane (BH 3), exists as its dimer, B 2H 6, via the formation of three ─ center ─ two ─ electron bonds. 4 According to the IUPAC nomenclature, 5 the

B 2H 6 molecule should be called diborane(6) as it consists of two boron atoms and six hydride ligands. Formal removal of two hydrogen atoms from B 2H 6 leads to B 2H 4 with a B ─ B single bond, which should be named diborane(4) due to its four hydride ligands. On account of the long B ─ B single bond (1.75 Å) and two vacant p ─ orbitals on the two boron atoms, diborane(4) exhibits characteristic reactivity. The most common diborane(4), bis(pinacolato)diborane(4) (B 2pin 2), has been widely used for the borylation of organic molecules, e.g. 6 the diboration of unsaturated bonds and the C ─ H borylation of alkanes and arenes. 7 2 3 The addition of Lewis bases to diborane(4) affords sp ─ sp diborane compounds, which can be applied as a boron ─ cen- 2 3 tered nucleophiles. The rst isolation of sp ─ sp diborane A was achieved by an addition of 4 ─ picoline to bis(catecholato) ─ 8 n diborane(4) (Figure 1). In the reaction of B 2pin 2 with BuLi 2 3 and cinnamyl bromide, an sp ─ sp diborane intermediate B with nucleophilicity on the boron center was postulated. 9 It has also been reported that the addition of a catalytic amount of NHCs (N ─ heterocyclic carbenes) to B 2pin 2 induces the β ─ 10 2 3 borylation of α, β ─ unsaturated carbonyl compounds. In this Figure 1. Hitherto reported sp ─ sp diborane compounds A ─ E.

Vol.76 No.11 2018 （ 87 ） 1223 orbital of the boron atom, exhibits a slightly different reactiv- and boryl ─ substituted azaallene 5, in which the isomeric ratio ity. In the absence of a catalyst, tetrauoro ─ and tetrachloro ─ depends on the reaction conditions (e.g. concentration of the t diborane(4) react with acetylene to generate syn ─ diborylethy- reaction mixture and stoichiometry of Bu ─ N≡C). Both 4 and 15 lene (eq. 1), which is probably due to the high electrophilicity 5 were structurally characterized by single ─ crystal X ─ ray dif- of the boron center. The addition of Lewis ─ base or nucleo- fraction analysis. philes to halogen ─ substituted diborane(4) induces an exchange 16 t 1 of substituents to form rearranged products (eqs. 2 and 3). Scheme 2. Reaction of 1 with CO and Bu ─ N≡C ( H NMR yield in parentheses).

Since it is dif cult to distinguish adjacent atoms from the same period in the periodic table based on single ─ crystal X ─ ray diffraction techniques, labeling experiments were carried 13 13 13 out using 4 ─ C and 5 ─ C, which were obtained from C ─ t 13 13 13 labeled Bu ─ N≡ C. The C NMR spectrum of 4 ─ C exhib- In this account, the author focuses on his recent work ited two large broadened signals, which were assigned to two 13 regarding the synthesis of two new diborane(4) compounds C ─ labeled carbon atoms bound to a quadrupolar boron 13 that exhibit high electrophilicity, and on their characteristic nucleus in the C ─ B=C skeleton of 4 ─ C [Figure 2(a)]. Con- 13 13 reactivity toward multiple ─ bond ─ containing compounds and versely, the C NMR spectrum of 5 ─ C showed two enhanced 1 small molecules. signals at δ C 89.1 and 168.0 ppm with a satellite ( J CC = 86 Hz),

which indicates that the δ C 89.1 signal is the carbon atom 2． Reactivity of the Unsymmetrical Diborane(4), 13 17 bound to the quadrupolar boron atom in 5 ─ C, according to pinB BMes ─ 2 the broadening observed [Figure 2(b)]. Treatment of B 2pin 2 with mesitylmagnesium bromide affords the unsymmetrical diborane(4), pinB ─ BMes 2 (1), as colorless crystals (Scheme 1). The twisted structure of 1, which was unequivocally determined by a single ─ crystal X ─ ray diffraction analysis, stands in contrast to the completely planar structure of B 2pin 2, which was independently con rmed by 17a ourself. The B ─ B single bond of 1.722(4) Å was comparable to that of B 2pin 2.

1 Scheme 1. Synthesis of pinB ─ BMes 2 ( H NMR yield in parentheses).

13 13 13 Figure 2. The C NMR spectra of C ─ labeled (a) 4 ─ C and 13 13 (b) 5 ─ C (broadened signals were assigned as C atoms bound to a quadrupolar boron nucleus). Exposing a benzene solution of 1 to one atmosphere of CO afforded CO ─ coordinated boraalkene 2 through incorpo- A reaction mechanism for the formation of 2 ─ 5 via the ration of two CO molecules (Scheme 2). The structure of 2 was characteristic intermediates 6 ─ 9 was proposed based on DFT determined by a single ─ crystal X ─ ray diffraction analysis that calculations (Scheme 3). Coordination of CO or isocyanide 2 3 revealed a π ─ conjugated O ─ C ─ B ─ C ─ O moiety. The formation affords sp ─ sp diborane intermediates 6a and 6b. In order to of the framework of 2 requires cleavage of B ─ B and B ─ Mes cancel the positive and negative charges in 6a and 6b, the t bonds in 1 during the reaction. A Bu ─ substituted isocyanide (pinacolato)boryl group engages in a nucleophilic 1,2 ─ migra- t ( Bu ─ N≡C), which is isoelectronic to CO, also reacted with 1 tion to the carbon atom to afford diboryl ─ ketone 7a or ─ imine and furnished boraindane 3 via cleavage of one B ─ B, two B ─ 7b. Subsequently, 7a and 7b rearrange to oxa ─ or aza ─ borake- 3 Mes, and a C(sp ) ─ H bonds. The structure of 3 was con rmed tene 8a and 8b via the migration of the (pinacolato)boryl by NMR spectroscopy and single ─ crystal X ─ ray diffraction group to the oxygen or nitrogen atom to form stable B ─ O or t analysis. Increasing the amount of Bu ─ N≡C added furnished B ─ N bonds. Since 8a and 8b can be considered as a stabilized t an isomeric mixture of Bu ─ N≡C ─ coordinated boraalkene 4 carbene species, the Mes group should be able to migrate to the

1224 （ 88 ） J. Synth. Org. Chem., Jpn. electrophilic carbene carbon atom to afford boraalkenes 9a The formation of a wide variety of products in Scheme 4 and 9b under concomitant cancellation of the positive and can be explained by a simple reaction mechanism (Scheme 5). t negative charges. A nucleophilic attack of the second Bu ─ In the absence of pyridine, or the presence of small amount of N≡C molecule onto the central carbon atom in 8a and 8b fur- pyridine, 1 should be converted into the common boraalkene nishes 5, while subsequent reactions from 9a and 9b result in intermediate 9c, which should lead to the formation of 10 ─ 12 the formation of 2 ─ 4. via a rearrangement, a reaction with Xyl ─ N≡C, or a reaction with pyridine. In the presence of a large excess of pyridine, 1 2 3 Scheme 3. A simpli ed reaction mechanism for the formation of 2 ─ 5 should be converted into sp ─ sp diborane 15, which Xyl ─ N≡C with key intermediates. attacks at the 2 ─ position of the coordinating pyridine ring, furnishing 13 and 14.

Scheme 5. A simpli ed reaction mechanism for the formation of 10 ─ 14 via the two common intermediates 9c and 15.

Diborane(4) 1 also exhibits characteristic reactivity toward xylyl (2,6 ─ Me 2C 6H 3) ─ substituted isocyanides (Scheme 4). Treating 1 with 1 equiv. of Xyl ─ N≡C afforded spiroborate 10, which contains a four ─ and ve ─ membered ring as the major product. Interestingly, this transformation from 1 into 10 can be interpreted as a ring ─ contraction of the Bpin ring, which is usually considered as a very stable substituent. Increasing the The reaction of 1 with Xyl ─ N≡C in the presence of substi- stoichiometry of the Xyl ─ N≡C to 2 equiv. led to the formation tuted pyridines affords a similar series of compounds of Xyl ─ N≡C ─ coordinated boraalkene 11, which exhibits a (Scheme 6). Treatment of 1 with 2 ─ Cl ─ and 2 ─ Me ─ substituted structure similar to that of 2. The presence of pyridine (10 pyridines in the presence of Xyl ─ N≡C leads to the formation equiv.) in the reaction of 1 with Xyl ─ N≡C furnished the pyri- of 10 and 11, respectively, and these compounds do not incor- dine ─ coordinated boraalkene 12. It should be noted that 12 porate 2 ─ substituted pyridines, probably due to the steric hin- exhibites blue color in hexane solution, despite its small π ─ drance of the substituents that inhibit the coordination to 1. conjugated system. TD ─ DFT calculations on 12 indicated that The corresponding reactions with 3 ─ OMe ─ , 3 ─ Cl ─ , and 4 ─ this characteristic absorption can be assigned to the intramole- OMe ─ substituted pyridines furnished pyridine ─ coordinated cular charge transfer from an energetically high B=C π ─ boraalkenes 12a ─ c or spiroborate 10, depending on the stoichi- orbital to the low ─ lying π ＊─ orbital of the boron ─ coordinating ometry of Xyl ─ N≡C. In these reactions, slightly electron ─ pyridine ring. Increasing the stoichiometry of pyridine in the withdrawing substituents may shift the equilibrium between 1 reaction of 1 with Xyl ─ N≡C afforded 13 through a C ─ H bond and 15 in Scheme 5 in favor of 1, leading to the formation of cleavage at the 2 ─ position of pyridine and dearomatization of the pyridine ring. The structure of the dearomatized pyridine Scheme 6. Reaction of 1 with substituted pyridine derivatives and 1 ring of 13 was con rmed by a single ─ crystal X ─ ray diffraction Xyl ─ N≡C ( H NMR yield in parentheses). analysis. In the presence of a large excess of pyridine, reaction of 1 with 2 equiv. of Xyl ─ NC afforded spiroborate 14 through a C ─ H bond cleavage at the 2 ─ position of pyridine, a dearomatization of the pyridine ring, and a C ─ C coupling between two Xyl ─ N≡C molecules. It was independently con rmed that the addition of Xyl ─ N≡C to 13 afforded 14 in high yield.

Scheme 4. Reaction of 1 with Xyl ─ N≡C in the presence/absence 1 of pyridine (Xyl = 2,6 ─ Me 2C 6H 3) ( H NMR yield in parentheses).

Vol.76 No.11 2018 （ 89 ） 1225 n non ─ C ─ H ─ cleaved products. The reaction of 3 ─ CF 3─ , 3 ─ 3 mol % of BuLi and 3 mol % of DME provided a satisfac- CO 2Me ─ , 4 ─ CF 3─ , and 4 ─ CO 2Me ─ substituted pyridines fur- tory yield of 21c (run 4). All the regioisomers 21a ─ c were iso- nishes dearomatized 13a ─ d and C ─ C coupled 16a ─ d. Electron ─ lated by recrystallization and structurally characterized by sin- withdrawing substituents should accelerate a nucleophilic gle ─ crystal X ─ ray diffraction analysis. In addition to attack of Xyl ─ N≡C on intermediate 15 (Scheme 5), which phenylacetylene, a variety of substituted phenylacetylenes and would lead to the formation of C ─ H cleaved products. In the internal alkynes are applicable to this metal ─ free diboration to case of very electron ─ rich pyridines, such as 4 ─ dimethylamino- give diborylalkenes, in which the yield and selectivity of each pyridine, ring ─ contracted product 17 or pyridine ─ coordinated isomers were varied depending on the substituents (see original boraalkene 11 were obtained. article for detail). 17d The combination of 1 and Xyl ─ N≡C also reacts with other nitrogen ─ containing heterocycles (Scheme 7). Reaction with Table 1. Reaction of 1 with phenylacetylene a (DME: pyrazine afforded 18 with diboration at two nitrogen atoms 1,2 ─ dimethoxyethane). and selective insertion of the isocyanide into the C ─ H bond at the 2 ─ position. In the case of the reaction with pyrimidine, spiroborate 19 was obtained as a major product. Simple N,N’ ─ diboration was observed in the reaction with pyridazine to form 20. Quinoline can also be a substrate for the reaction with 1 and Xyl ─ N≡C to selectively form C ─ H cleaved products 13e and 16e. In contrast, reaction with isoquinoline furnished the corresponding isoquinoline ─ coordinated boraalkene without the formation of the C ─ H cleaved product.

Scheme 7. Reaction of 1 with Xyl ─ N≡C in the presence of 1 N ─ heterocycles other than pyridines ( H NMR yield in parentheses).

There are two conceivable intermediates for the base ─ catalyzed diboration with 1 in the presence of ethereal additives (Scheme 8). The addition of lithium phenylacetylide to 1 in 2 3 THF afforded sp ─ sp diborane 22(thf) 2 through a nucleophilic attack of the phenylacetylide onto the more Lewis acidic

BMes 2 moiety. In the crystal structure, the lithium cation is coordinated by two THF molecules, one oxygen atom of the Bpin moiety, and the π ─ electrons of the alkyne. Subsequent

gentle heating of 22(thf) 2 to 70 ℃ led to a migration of the

Bpin group to furnish borataallene 23(thf) 2. In the crystal

structure of 23(thf) 2, the two characteristic bond lengths of the C=C [1.340(4) Å] and B=C [1.435(4) Å] bonds support a

borataallene structure. Although the boron atom of the BMes 2 moiety should formally carry a negative charge, the lithium cation was coordinated by the central carbon atom of the borataallene moiety, two THF molecules, and one oxygen atom of the Bpin moiety. This result indicates a carbanionic

character for the central carbon atom in 23(thf) 2. Due to the basicity and nucleophilicity of the central carbon atom in

23(thf) 2, the addition of a proton source led to the formation

Scheme 8. Synthesis and reactivity of intermediates 21(thf) 2 and 1 22(thf) 2 ( H NMR yield in parentheses).

Diborane(4) 1 can also react with alkynes (Table 1). Heat- ing a mixture of 1 and phenylacetylene in toluene provided an isomeric mixture of syn ─ diborated 21a (69%) and 21b (30%) (run 1). Addition of a catalytic amount of nBuLi changed the product ratio, whereby 21b was obtained as the major product (run 2). Further addition of DME furnished anti ─ diborated 21c as the major product (run 3). After optimization of the reaction conditions, it was found that a combination of

1226 （ 90 ） J. Synth. Org. Chem., Jpn. of anti ─ diborylalkene 21c as the major product, while the reac- (4 ─ bromophenyl)diphenylamine 27 to furnish an isomeric tion with MeI resulted in the formation of tetrasubstituted mixture of 28a and 28d. Both molecules obtained exhibit a diborylalkene 24c. The former result strongly supports the characteristic absorption around 600 nm in their UV ─ vis spec- unusual selectivity for the formation of anti ─ diborylalkenes in tra, as well as emission in solution and the solid state (see the the presence of nBuLi and ethereal additive. original report for details).

Two plausible mechanisms for the diboration of phenyl- To clarify the electrophilicity of pinB ─ BMes 2 (1), the acetylene with 1 are illustrated in Scheme 9. In the absence of a reduction potential of 1 was measured by cyclic voltammetry base, the π ─ electrons of the alkyne directly interacts with a (Figure 3). A reversible reduction wave was observed for 1 at + vacant p ─ orbital of the BMes 2 moiety to induce the migration -2.50 V (vs. FcH/FcH ) in THF. Increasing the scan rate from of the Bpin moiety to the alkynyl carbon atom. The steric 100 to 500 mV/s did not affect the reduction potential. It repulsion between the phenylacetylene and the Bpin moiety should be noted that a second reduction was not observed in should determine the regioselectivity of the diboration. In the the applied potential window and conditions. The reference presence of a base, the lithium phenylacetylide formed in situ compound, Mes 3B, exhibited a more negative reduction poten- n by a reaction between phenylacetylene and BuLi reacts with 1 tial ( ─ 2.70 V), which indicates that 1 is easier to be reduced 2 3 to form sp ─ sp diborane intermediate 22(solv) n. Subsequently, than Mes 3B. It should be emphasized that the most popular the Bpin group rearranges into an acetylenic carbon atom to diborane(4), pinB ─ Bpin, does not show any reduction wave afford borataallene intermediate 22(solv) n. In the presence of under the same conditions. excess phenylacetylene, the protonation of 22(solv) n at the central carbon furnishes syn ─ or anti ─ diborylalkene products 21b and 21c under concomitant regeneration of the lithium phenylacetylide.

Scheme 9. Synthesis and reactivity of intermediates 21(thf) 2 and 1 22(thf) 2 ( H NMR yield in parentheses).

Figure 3. Cyclic voltammogram for the reduction of 1 and Mes 3B n (solvent: THF; supporting electrolyte: [ Bu 4N][PF 6]).

As the reversibility of the reduction wave of 1 indicated that the radical anion derived from 1 is stable, the chemical one ─ electron reduction of 1 was examined (Scheme 11). Treat- The present diboration of alkynes with 1 was applied to ment of 1 with Na/K alloy in the presence of [2,2,2] ─ cryptand the synthesis of emissive molecules (Scheme 10). For example, afforded radical anion 29. Even though leaving the solution of treatment of 8 ─ ethynylquinoline with 1 furnished an isomeric 29 in THF at room temperature led to a decomposition into mixture of diborylalkene 26a and 26d, accompanied by an the corresponding hydroborate (30), 29 could be isolated as intramolecular coordination of the nitrogen atom in the quino- deep blue crystals. A single ─ crystal X ─ ray diffraction analysis line moiety to the BMes 2 moiety. This mixture was directly revealed that 29 is a separated ion pair with a short B ─ B bond subjected to Suzuki ─ Miyaura cross ─ coupling conditions with [1.661(5) Å] that exhibits a single ─ electron π ─ bond character similar to that reported for the radical anion and dianion of 18 Scheme 10. Two ─ step synthesis of the uorescent molecules 26a,d diborane(4). Reecting the π ─ bond character between two (1 H NMR yield in parentheses). boron atoms in 29, the two boron planes are almost coplanar with a C ─ B ─ B ─ O torsion angle of ─ 19.4(4)°. Radical anion 29 was also characterized by ESR spectroscopy. The obtained spectrum [Figure 4(a)] could be reproduced by a simulation based on four isotopomers, indicating that the unpaired electron is delocalized over the two boron atoms. The two different

Scheme 11. Synthesis of radical anion 29 from 1 and its decomposition into 30.

Vol.76 No.11 2018 （ 91 ） 1227 hyper ne coupling constants for the two boron nuclei exhibit a different population of spin density on each boron atoms, which is consistent with the spin ─ density distribution derived from DFT calculations [Figure 4(b)]. The presence of an unpaired electron was also con rmed by the UV ─ vis spectrum of 29 in hexane [Figure 4(c)]. Based on TD ─ DFT calculations, the characteristic absorption at ca. 600 nm was assigned to the SOMO ─ related transitions.

Figure 5. The (a) LUMO energy level (eV) and (b) relative energy (kcal/mol) of 1 as a function of the C ─ B ─ B ─ O torsion angle: calculated at M06/6 ─ 31G(d) level of theory with solvent effect of THF (SMD model).

the C ─ B ─ B ─ C torsion angle. A hexane solution of 31 was exposed to dihydrogen to furnish hydrogen ─ bridged diarylbo- rane dimer 32, which was structurally characterized by a single ─ crystal X ─ ray diffraction analysis. The two bridging hydrogen atoms were also characterized by solid ─ state IR spectroscopy. Since the full characterization of 32 in solution proved to be dif cult, due to equilibrium with its monomeric Figure 4. (a) Observed ESR spectrum of 29 in THF at 230 K (top) form and isomers, pyridine adduct 33 was isolated and charac- and simulated ESR spectrum (bottom). (b) Calculated spin terized. density of 29. (c) UV ─ Vis spectrum of 29 (hexane, 100 mM, 0 ℃). Scheme 12. Synthesis of tetra(o ─ tolyl)diborane(4) 31 and its DFT calculations also provided an insight into the origins reaction with H 2 (Ar = o ─ tolyl). of the high electrophilicity of 1 (Figure 5). The calculated LUMO level of 1 (-1.15 eV) at the ground state is higher than that of Mes 3B (-1.29 eV), which is inconsistent with the results of the electrochemical measurements that indicate a lower

LUMO level for 1 than for Mes 3B. Therefore, the relationship between the LUMO level and the C ─ B ─ B ─ O torsion angle was analyzed by DFT calculations. We discovered that a smaller DFT ─ calculations provided information on the reaction torsion angle leads to a lower LUMO level with a slight mechanism and characteristics of the transition state increase of the relative energy. This result indicates that the (Scheme 13). The interaction between 31 and H 2 led to the overlap of the two vacant p ─ orbitals on the two boron atoms formation of the transition state TS 31 ─ 32 (see the original paper decreases the LUMO level, which is also con rmed by the for details). The obtained energy barrier for TS 31 ─ 32 is consis- shape of the LUMO (two merged p ─ orbitals). Thus, the Bpin tent with the fact that the reaction is nished within two hours. group serves as a strong π ─ acceptor to lower the energy level At the transition state TS 31 ─ 32, two important interactions were of the p ─ orbital on the boron atom of the BMes 2 moiety. found. One is interaction between B ─ B bonding electrons and

19 the H A atom being trapped by left diarylboryl moiety. The 3． Reactivity of the Symmetrical Diborane(4), B 2(o tol) 4 ─ other one is interaction between B ─ H B bonding electron and Tetra(o ─ tol)diborane(4) (31) was obtained from the reac- the H A atom. This result means that the reaction can be con- tion of B 2cat 2 with o ─ tolylmagnesium bromide (Scheme 12). A sidered as a deprotonation by the left diarylboryl moiety, in single ─ crystal X ─ ray diffraction analysis of 12 revealed a which H 2 is activated through a coordination to the right dia- twisted structure with a torsion angle of 101.9(7)°. It should be rylboryl moiety. In this reaction, the left diarylboryl moiety noted that 12 exhibits a reduction potential of ─ 2.1 V (vs. FcH/ can be considered as an equivalent of a “diarylboryl anion”. + FcH ) in THF, which is much less negative than that of pinB ─ The highly Lewis acidic 31 can also react with CO in the

BMes 2 1. This result indicates a higher electron af nity for 12 absence of any additives (Scheme 14). When a benzene solu- than for 1. Similar to the case of 1, DFT calculations suggested tion of 31 was exposed to an atmosphere of CO (1 bar), a that the LUMO level and relative stability of 12 depends on mixture of boraindane 34 and tri(o ─ tolyl)boroxine 35 was

1228 （ 92 ） J. Synth. Org. Chem., Jpn. Scheme 13. A simpli ed reaction mechanism for the hydrogenolysis Scheme 15. Possible mechanism of for the cleavage of the C≡O of 31 [Ar = o ─ tolyl; calculated at the M06/6 ─ 31G(d) triple bond in the reaction between 31 and CO that level of theory; dashed lines in the structure of TS 31 ─ 32 affords 34 and 35 (Ar = o ─ tolyl). correspond to the formation/cleavage of bonds or 3 ─ center ─ 2 ─ electron bonds; relative free energies and electronic energies (in parentheses) are given in kcal/ mol.].

N≡C afforded 1 ─ azaallene 44 through a C ─ C coupling between obtained. The formation of 34 was con rmed by NMR spec- two isocyanide molecules, and the structure of 44 was con- troscopy and a single ─ crystal X ─ ray diffraction analysis, while rmed by single ─ crystal X ─ ray diffraction analysis. A VT 35 was characterized by comparison of its NMR and MS data 1 H NMR study suggested that 44 engages in a dynamic process t with literature values. Since the benzylic carbon atom in 34 involving restricted rotation around the (Ar 2B)N( Bu) ─ C bond 13 seems to come from CO, a C ─ labeling experiment was carried on the NMR timescale. Reaction of 31 with Xyl ─ N≡C (1 13 1 out to produce 34 ─ C, which exhibited characteristic H and equiv.) afforded 45, which contains a benzoannulated six ─ 13 C NMR signals (Figure 6) that con rm that the benzylic car- membered ring, as evident from a single ─ crystal X ─ ray diffrac- bon atom in 34 stems from CO. tion analysis. The formation of a benzoannulated [C 3B 2N] ring system in 45 should involve the cleavage of a C ─ H bond at the Scheme 14. Reaction of 31 with CO or 13 CO (yield in parentheses 6 ─ position of the o ─ tolyl group. Increasing the stoichiometry 1 estimated by H NMR spectroscopy). of Xyl ─ N≡C to 2 equiv. led to the formation of 46 via the coordination of Xyl ─ N≡C to 45, which was con rmed with an independent experiment. i.e., adding 1 equiv. of Xyl ─ N≡C to 45.

Scheme 16. Reaction of 31 with isocyanides (Ar = o ─ tol, Xyl = 2,6 ─ 1 Me 2C 6H 3; yield estimated by H NMR spectroscopy).

Figure 6. Schematic summary of the coupling in the 13 C NMR 13 spectrum of 2 ─ C.

A plausible reaction mechanism was proposed based on The DFT ─ derived reaction mechanism for the formation DFT calculations (Scheme 15). The rst four steps to produce of 44 ─ 46 could be rationalized based on one common reaction boraalkene intermediate 39 are identical to those for the reac- intermediate (Scheme 17). Coordination of isocyanide to 31 2 3 tion of pinB ─ BMes 2 (1) with CO. Subsequently, a highly elec- affords sp ─ sp diborane 47, which subsequently undergoes trophilic boraalkene moiety induces a migration of one of the migration of a BAr 2 moiety to the carbon atom of the isocya- two o ─ tolyl groups to form 41, which spontaneously dissoci- nide to furnish diborylimine 48, which contains an intramole- ates into an oxoborane and betaine ─ type borataallene 42. This cular N ─ B coordination bond. A second migration of the BAr 2 is the key step to cleave the C ─ O σ bond in CO. The oxoborane moiety to the nitrogen atom forms azaboraallene 49 as a com- t thus formed immediately trimerizes to form boroxine 35, while mon intermediate. In the case of using Bu ─ N≡C, the second t the latter undergoes a migration of an aryl group and a hydro- equivalent of Bu ─ N≡C coordinates to the central carbon gen atom to furnish 34. Thus, the highly electrophilic character atom, which can be considered as an electrophilic push ─ pull ─ of 31 enables the cleavage of the C≡O triple bond in CO. stabilized carbene, to furnish 44. In contrast, the common Moreover, reactivity of diborane(4) 31 toward isocyanide intermediate 49 (R = Xyl) undergoes a migration of the aryl t was also examined (Scheme 16). Treatment of 31 with Bu ─ group to the central carbon atom, which generates boraalkene

Vol.76 No.11 2018 （ 93 ） 1229 50. An adjacent o ─ tol group interacts with the electrophilic References boraalkene to afford the benzoannulated [C 3B 2N] ring of 45, 1） Emsley, J., The Elements. 3rd ed.; Oxford University Press: New York, followed by a coordination of Xyl ─ N≡C to furnish 46. 1998. 2） Clementi, E.; Raimondi, D. L. J. Chem. Phys. 1963, 38, 2686. 3） Suzuki, A. Angew. Chem. Int. Ed. 2011, 50, 6722. Scheme 17. Plausible mechanism for the incorporation of 4） Lipscomb, W. N. Angew. Chem. 1977, 89, 685. isocyanides in 31 and the C ─ H bond cleavage to 5） Connelly, N. G.; Damhus, T.; Hartshorn, R. M.; Hutton, A. T. generate 44 ─ 46 (Ar = o ─ tol, Xyl = 2,6 ─ Me 2C 6H 3). Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005. Royal Society of Chemistry: 2005. 6） (a) Ishiyama, T.; Matsuda, N.; Miyaura, N.; Suzuki, A. J. Am. Chem. Soc. 1993, 115, 11018. (b) Ishiyama, T.; Murata, M.; Miyaura, N. J. Org. Chem. 1995, 60, 7508. (c) Ishiyama, T.; Miyaura, H. J. Synth. Org. 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Soc. 2010, 132, 10630. (c) Wu, H.; Radomkit, S.; O’Brien, J. facetted reactivity toward compounds with multiple bonds and M.; Hoveyda, A. H. J. Am. Chem. Soc. 2012, 134, 8277. small molecules, including CO, isocyanides, alkynes, nitriles, 11） (a) Bonet, A.; Pubill ─ Ulldemolins, C.; Bo, C.; Gulyás, H.; Fernández, pyridine, and H 2. It can thus be concluded that the coexistence E. Angew. Chem. Int. Ed. 2011, 50, 7158. (b) Pubill ─ Ulldemolins, C.; of the high electrophilicity which allows coordination of weak Bonet, A.; Bo, C.; Gulyás, H.; Fernández, E. Chem. Eur. J. 2012, 18, 1121. (c) Pubill ─ Ulldemolins, C.; Bonet, A.; Gulyas, H.; Bo, C.; Fer- nucleophiles, and reactive B ─ B bonding electrons produces a nandez, E. Org. Biomol. Chem. 2012, 10, 9677. (d) Sanz, X.; Lee, G. broad spectrum of interesting reactivity. M.; Pubill ─ Ulldemolins, C.; Bonet, A.; Gulyas, H.; Westcott, S. A.; Bo, C.; Fernandez, E. Org. Biomol. Chem. 2013, 11, 7004. (e) Miralles, Acknowledgements N.; Cid, J.; Cuenca, A. B.; Carbo, J. J.; Fernandez, E. Chem. Commun. 2015, 51, 1693. (f) Cuenca, A. B.; Zigon, N.; Duplan, V.; Hoshino, M.; The author would like to thank all his coauthors and col- Fujita, M.; Fernández, E. Chem. Eur. J. 2016, 22, 4723. (g) Miralles, laborators, including Prof. Zhenyang Lin, Dr. Ka Ho Lee, and N.; Alam, R.; Szabó, K. J.; Fernández, E. Angew. Chem. Int. Ed. 2016, Ms. Linlin Wu at the Hong Kong University of Science and 55, 4303. 12） (a) Takahashi, K.; Ishiyama, T.; Miyaura, N. J. Organomet. Chem. Technology; Prof. Ko Furukawa at Niigata University; and 2001, 625, 47. (b) Zhao, H.; Dang, L.; Marder, T. B.; Lin, Z. J. Am. Mr. Hiroki Asakawa, Ms. Chiemi Kojima, Ms. Nana Tsuka- Chem. Soc. 2008, 130, 5586. hara, and Mr. Yuhei Katsuma at Chuo University. He is also 13） Ito, H.; Yamanaka, H.; Tateiwa, J.; Hosomi, A. Tetrahedron Lett. grateful to Prof. Tamejiro Hiyama at Chuo University for pro- 2000, 41, 6821. 14） (a) Li, H.; Shangguan, X.; Zhang, Z.; Huang, S.; Zhang, Y.; Wang, J. viding access to X ─ ray diffractometers, to Prof. Yoshiaki Org. Lett. 2014, 16, 448. (b) Cuenca, A. B.; Cid, J.; García ─ López, D.; Nishibayashi at the University of Tokyo for providing access Carbó, J. J.; Fernández, E. Org. Biomol. Chem. 2015, 13, 9659. to mass spectrometers, to Prof. Masa ─ aki Haga, Dr. Hiroaki 15） (a) Ceron, P.; Finch, A.; Frey, J.; Kerrigan, J.; Parsons, T.; Urry, G.; Schlesinger, H. I. J. Am. Chem. Soc. 1959, 81, 6368. (b) Rudolph, R. Ozawa, and Ms. Tomoko Odaka at Chuo University for the W. J. Am. Chem. Soc. 1967, 89, 4216. (c) Zeldin, M.; Gatti, A. R.; measurement of quantum yields and lifetimes, to Prof. Atsushi Wartik, T. J. Am. Chem. Soc. 1967, 89, 4217. (d) Siebert, W.; Wakamiya at Kyoto University for helpful discussions regard- Hildenbrand, M.; Hornbach, P.; Karger, G.; Pritzkow, H. Z. Natur- ing photophysical properties of uorescent molecules, and to forsch., B: Chem. Sci. 1989, 44, 1179. 16） (a) Höfner, A.; Ziegler, B.; Hunold, R.; Willershausen, P.; Massa, W.; Prof. Takahiro Sasamori at Nagoya City University for helpful Berndt, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 594. (b) Arnold, N.; discussions on the analysis of crystal structures. The author Braunschweig, H.; Damme, A.; Dewhurst, R. D.; Pentecost, L.; also gratefully acknowledges all the funding resources, the Radacki, K.; Stellwag ─ Konertz, S.; Thiess, T.; Trumpp, A.; Vargas, A. Grant in Aid for scienti c research (MEXT KAKENHI Chem. Commun. 2016, 52, 4898. (c) Arnold, N.; Braunschweig, H.; ─ ─ Dewhurst, R. D.; Hupp, F.; Radacki, K.; Trumpp, A. Chem. Eur. J. 24109012, JSPS KAKENHI 26288019, 17H01191), CREST 2016, 22, 13927. (d) Arnold, N.; Braunschweig, H.; Ewing, W. C.; 14529307 from the JST, The Science Research Promotion Kupfer, T.; Radacki, K.; Thiess, T.; Trumpp, A. Chem. Eur. J. 2016, Fund from The Promotion and Mutual Aid Corporation for 22, 11441. (e) Arrowsmith, M.; Braunschweig, H.; Radacki, K.; Thiess, T.; Turkin, A. Chem. Eur. J. 2017, 23, 2179. Private Schools of Japan, and the Asahi Glass Foundation. He 17） (a) Asakawa, H.; Lee, K. ─ H.; Lin, Z.; Yamashita, M. Nat. Commun. also appreciates computational resources provided by the 2014, 5, 4245; (b) Asakawa, H.; Lee, K. ─ H.; Furukawa, K.; Lin, Z.; Research Center for Computational Science, Okazaki, Japan. Yamashita, M. Chem. Eur. J. 2015, 21, 4267; (c) Katsuma, Y.; Asakawa, H.; Lee, K. ─ H.; Lin, Z.; Yamashita, M. Organometallics

1230 （ 94 ） J. Synth. Org. Chem., Jpn. 2016, 35, 2563. (d) Kojima, C.; Lee, K. ─ H.; Lin, Z.; Yamashita, M. J. PROFILE Am. Chem. Soc. 2016, 138, 6662. (e) Katsuma, Y.; Asakawa, H.; Yamashita, M. Chem. Sci. 2018, 9, 1301. Makoto Yamashita Professor of Chemistry at 18） (a) Berndt, A.; Klusik, H.; Schlüter, K. J. Organomet. Chem. 1981, Nagoya University. In 1997, he graduated 222, c25. (b) Klusik, H.; Berndt, A. Angew. Chem., Int. Ed. Engl. 1981, from Hiroshima University under the guid- 20, 870. (c) Klusik, H.; Berndt, A. J. Organomet. Chem. 1982, 234, ance of Prof. Kin ─ ya Akiba. In 2002, he re- C17. (d) Moezzi, A.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. ceived his Ph.D. from Hiroshima University 1992, 114, 2715. under the guidance of Prof. Yohsuke 19） (a) Tsukahara, N.; Asakawa, H.; Lee, K. ─ H.; Lin, Z.; Yamashita, M. Yamamoto. Between 2001 and 2004, he J. Am. Chem. Soc. 2017, 139, 2593. (b) Katsuma, Y.; Tsukahara, N.; worked as a JSPS research fellow, which in- Wu, L.; Lin, Z.; Yamashita, M. Angew. Chem. Int. Ed. 2018, 57, 6109. cluded research positions in Prof. John F. Hartwig’s group at Yale University (1.5 years) and in Prof. Takayuki Kawashima’s group at The University of Tokyo (0.5 years). In 2004, he accepted a position as a research associate in the group of Prof. Kyoko Nozaki (The University of Tokyo), where he was promoted to Assistant Professor and subsequently Lecturer. In 2011, he moved as an independent Associate Professor to Chuo Uni- versity, where he was promoted to Professor in 2015. In October 2016, he moved to Nagoya University. His research interests in- clude main ─ group chemistry, physical organic chemistry, organometallic chemistry, as well as homogeneous and heterogeneous ca- talysis.

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