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 com- mon 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 difcult 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 dif- fraction analysis, stands in contrast to the completely planar structure of B 2pin 2, which was independently conrmed 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 conrmed 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.