Chemistry of Boryl Anion-focusing on M. Yamashita’s work-

2014/12/13 (Sat.)
Literature Seminar

Yuya Morita (D1)1

Contents

1. Introduction

2. Boryllithium
3. Other Borylmetals
4. PBP Pincer Ligands

5. Summary2

1. Introduction

2. Boryllithium
3. Other Borylmetals
4. PBP Pincer Ligands

5. Summary3

Boron Chemistry

The boron atom is electrophilic in all cases shown above.

A boron-centered nucleophile has been a challenging target.
4Early Attempts to Prepare the Boryl Anion
Three examples had been reported asserting the existence of alkali metalsalts of anionic organoboron compounds as reactive intermediates.

1) Reduction of dialkylchloroborane
R. W. Auten, C. A. Kraus,
J. Am. Chem. Soc. 1952, 74, 3398.

K. Smith, K. Swaminathan,J. Chem. Soc., Dalton Trans. 1976, 2297.

5

Early Attempts to Prepare the Boryl Anion
2) Photolysis of sodium tetraphenylborate

J. J. Eisch, et al. J. Org. Chem. 1989, 54, 1627.
G. B. Schuster, et al. J. Am. Chem. Soc. 1991, 113, 2149.

3) Reduction of bromoborane

L. Weber, et al. J. Chem. Soc., Dalton Trans. 2001, 3459.

6Base-Stabilized Boryl Anion
Filling the vacant p-orbital of boron with Lewis base makes the boron atom
sp3, similar to carbon, to provide a good strategy for preparation ofnucleophilic boryl anions as carbanion can be made from haloalkanes.

1) NEt3-stabilized boryl anion

T. D. Parsons, et al. J. Am. Chem. Soc. 1967, 89, 3446.

2) Cy3P-stabilized boryl anion

first examples ofnucleophilic boron reagent

H. Schmidbaur, et al. J. Organomet. Chem. 1993, 462, 107.
T. Iwamoto, T. Hikosaka, J. Org. Chem. 1994, 59, 6753.

7

Borylcopper
1) Borylcopper intermediate generated from diboron
The catalyst system Cu(I)/diborane(4) could mediate nucleophilic borylation.

A. Hosomi, et al. Tetrahedron lett. 2000, 41, 6821.

N. Miyaura, et al. Chem. Lett. 2000, 982.
J. Organomet. Chem. 2001, 625, 47.

8

Borylcopper2) Isolated borylcopper spiecies
Borylcopper possessing NHC ligand was isolated.

Borylcopper have opened a new area of organoboron chemistry.
However the application of the boron nucleophile was still limited.
J. P. Sadighi, et al. J. Am. Chem. Soc. 2005, 127, 17196.
J. Am. Chem. Soc. 2006, 128, 11036.

9

Lithium Salts of Second-Row p-Block Elements
For most of the p-block elements in the second row of the periodic table,
anions can be prepared as lithium salts, such as LiF, LiOH, LiNH2 and H3CLi.
However, there had been no direct observations of the lithium salt of anionicboron atom, boryllithium.

10Isolated Nucleophilic Boryl Anions

Kyoko Nozaki

Makoto Yamashita
11Other Isolated Nucleophilic Boryl Anions12

1. Introduction

2. Boryllithium
3. Other Borylmetals
4. PBP Pincer Ligands

5. Summary

13

What is the Ground State of Boryllithium?
The H2B- anion is isoelectronic with CH2:, which has a triplet ground state.
What is the ground state of H2B-?

R. Schleyer, Inorg. Chem. 1995, 34, 607.

14

Rational Design for Boryllithium

R. Schleyer, Inorg. Chem. 1995, 34, 607.

15

Strategy for Generating a Boryllithium1) Deprotonation of hydroborane with base2) Reductive dehalogenation of haloborane with alkali metal

16

Strategy for Generating a Boryllithium3) Reductive cleavage of a B-B single bond of diborane(4)Synthesis of gallyl anion 25 was performed by reductive cleavage of theGa-Ga single bond of digallane(4) 24.

However the reduction of the diborane(4) 26 led to the formation ofisolable diborane(4) dianion 27, having a B=B double bond character withπ-coordination to lithium cations

17

Synthesis of Boryllithium

M. Yamashita, K. Nozaki, et al. Science 2006, 314, 113.
J. Am. Chem. Soc. 2008, 130, 16069.

18

Crystallographic Study: Solid State Structures(35a･DME)235a･(THF)2
M. Yamashita, K. Nozaki, et al. Science 2006, 314, 113.
J. Am. Chem. Soc. 2008, 130, 16069.

19

NMR Spectroscopic Study: Structure in Solution
To clarify the solution structure of boryllithium, several spectroscopic studies
were performed.

1) Comparison with carbene 47 and imidazolium salt 48

2) 7Li-NMR spectroscopic study

M. Yamashita, K. Nozaki, et al. Science 2006, 314, 113.
J. Am. Chem. Soc. 2008, 130, 16069.

20

DFT Study

M. Yamashita, K. Nozaki, et al. Science 2006, 314, 113.
J. Am. Chem. Soc. 2008, 130, 16069.

21

DFT Study: Lone Pair Character in the HOMO Orbital

M. Yamashita, K. Nozaki, et al. Science 2006, 314, 113.
J. Am. Chem. Soc. 2008, 130, 16069.

22

Reactions of Boryllithium
Reaction of boryllithium with various electrophiles

M. Yamashita, K. Nozaki, et al. J. Am. Chem. Soc. 2008, 130, 16069. 23

1. Introduction

2. Boryllithium
3. Other Borylmetals
4. PBP Pincer Ligands

5. Summary

24

Transmetallation of Boryllithium to Other Metals
Based on the high reactivity of boryllithium, transmetallation to other metals
enables synthesis of a series of borylmetals.

25

Borylmagnecium
Synthesis of borylmagnecium compounds
The discovery of boryllithium prompted them to synthesize new anionicboron nucleophile, borylmagnecium species.

Structure of 3, 4, and 5 in the crystals and solutions indicated the ionic character ofthe B-Mg bond.

M. Yamashita, K. Nozaki, et al. J. Am. Chem. Soc. 2007, 129, 9570.
26

Borylmagnecium
Reaction of borylmagnecium bromide to benzaldehyde

M. Yamashita, K. Nozaki, et al. J. Am. Chem. Soc. 2007, 129, 9570. 27

Borylcopper (and Borylzinc)
Synthesis of borylcopper and borylzinc compounds

M. Yamashita, K. Nozaki, et al. Angew. Chem. Int. Ed. 2008, 47, 6606. 28

Borylcopper (and Borylzinc)
1) Conjugate addition of borylcopper and borylzinc

Trapping the copper enolate intermediate with TMSCl

M. Yamashita, K. Nozaki, et al. Angew. Chem. Int. Ed. 2008, 47, 6606. 29

Borylcopper (and Borylzinc)2) One-pot carboboration of alkyne using borylcyanocuprate

M. Yamashita, K. Nozaki, et al. Angew. Chem. Int. Ed. 2011, 50, 920.
Eur. J. Org. Chem. 2011, 3951.

30

Boryltitanium and Borylhafnium
Synthesis of boryltitanium and borylhafnium
M. Yamashita, K. Nozaki, et al.
J. Am. Chem. Soc. 2009, 131, 14162.

The nature of the group-4 transition metal-boron bond31

1. Introduction

2. Boryllithium
3. Other Borylmetals
4. PBP Pincer Ligands

5. Summary

Background
Transition metal boryl complexes have been widely synthesized andapplied to catalytic borylation reactions.

J. F. Hartwig, et al. Science 2000, 287, 1995.
J. Am. Chem. Soc. 2005, 127, 2538.

T. Ishiyama, N. Miyaura, J. F. Hartwig, et al.
J. Am. Chem. Soc. 2002, 124, 390.

boryl ligands act as “reactive” ligands

33

Background
It is known that the boryl ligand has a greater σ-donor ability than othermonoanionic ligand of carbon, nitrogen and oxygen.

If the boryl ligand is stabilized and acts as “supporting” ligand, the strong
electron-releasing property of the boryl ligand would be applied tofunctionalization other than borylation.

PXP pincer ligands

34

Synthesis of PBP Pincer Iridium Complexes

M. Yamashita, K. Nozaki, et al. J. Am. Chem. Soc. 2009, 131, 9201. 35

Synthesis of PBP Pincer Iridium Complexes
Carbonyl complex

Ir-Cl: 2.5307(13) ÅIr-Cl: 2.475(2) ÅEthylene complex

Applications of these easily prepared PBP pincer complexes to catalytic
reactions are under investigation.
M. Yamashita, K. Nozaki, et al. J. Am. Chem. Soc. 2009, 131, 9201. 36

Other Metal Complexes Possessing PBP Pincer Ligand

Ir complex: M. Yamashita, K. Nozaki, et al. Organometallics 2009, 28, 6234.
Ru complex: M. Yamashita, et al. Organometallics 2014, 33, 6760.
Pt complex: M. Yamashita, et al. Dalton Trans. 2013, 42, 625.

37

PBP Rhodium Complex
Synthesis of PBP rhodium complex

M. Yamashita, K. Nozaki, et al. Angew. Chem. Int. Ed. 2012, 51, 6956. 38

PBP Rhodium Complex
Oxidative addition of a strained C-C bond at room temperature

An analogous decarbonylation reaction of cyclobutanone mediated byRh(I) complexes bearing PPh3 or NHC ligands generally requires reflux
conditions even when a stoichiometric amount of Rh(I) complex is used.
M. Yamashita, K. Nozaki, M. Murakami, et al. J. Am. Chem. Soc. 2013, 135, 7142. 39

PBP Rhodium Complex
Reaction with sec-Alcohols

PBP Rh(I) complexes possesses a high propensity for oxidative additionof nonpolar C-C σ-bonds as well as polar O-H bonds
M. Yamashita, K. Nozaki, M. Murakami, et al. J. Am. Chem. Soc. 2013, 135, 7142. 40

PBP Cobalt Complex
New approaches toward the generation of late first-row metal catalysts
that facilitate two-electron reductive transformations (e.g., hydrogenation)
more typical of noble-metal catalysts is important goal.

Synthesis of PBP cobalt complex and reversible H2 activation

RT
J. C. Peters, et al. J. Am. Chem. Soc. 2013, 135, 15310.

41

PBP Cobalt Complex
1) Hydrogenation of styrene with H22) Amine-borane dehydrogenatiojn

equilibrium between 2 and 4
in the presence of excess Et3N

J. C. Peters, et al. J. Am. Chem. Soc. 2013, 135, 15310.

42

PBP Cobalt Complex
3) Transfer hydrogenation reaction

Control experiments in the absenceof 2 showed no reactivity.

J. C. Peters, et al. J. Am. Chem. Soc. 2013, 135, 15310.

43

PBP Nickel Complex
Synthesis of PBP nickel complex

J. C. Peters, et al. J. Am. Chem. Soc. 2014, 136, 13672.

44

PBP Nickel Complex
Turnover frequency (TOF) of catalytic olefin hydrogenation

PBP Nickel complex 7 is a more activeolefin hydrogenation catalyst comparedwith its isoelectronic/isostructural phenyl
and amino analogues.

J. C. Peters, et al. J. Am. Chem. Soc. 2014, 136, 13672.

45

PBP Nickel Complex
Proposed mechanism

Ni-H = catalyst 7
S = olefin
P = hydrogenated product

Why PBP Nickel complex 7 is a more active olefin hydrogenation catalyst
compared with phenyl and amino analogues?

strong trans influence of the boryl group destabilizes the Ni-H and Ni-alkyl bondkinetically more favorable migratory insertion (k1) and σ-bond metathesis (k2) step
J. C. Peters, et al. J. Am. Chem. Soc. 2014, 136, 13672.

46

1. Introduction

2. Boryllithium
3. Other Borylmetals
4. PBP Pincer Ligands

5. Summary47

Summary
Boryllithium･First isolation and structural analysis of boryllithium was performed.
･Boryllithium reacts with various electrophile as alkyllithium does.

Borylmetals other than lithium･Transmetallation of boryllithium to other metals enables synthesis of a series ofborylmetals.
･However useful reactions taking advantage of isolable borylmetals were underdevelopment.

PBP pincer ligand･PBP ligand has higher σ-donor ability than PCP and PNP ligand.
･Some useful reactions taking advantage of higher σ-donor ability were developed.

48