Silicon Tetrachloride Promoted Cyclotrimerization Reactions: New

Route to Carbosilane Dendrons

Barada Prasanna Dash, Rashmirekha Satapathy, John A. Maguire & Narayan S. Hosmane

Department of Chemistry and BiochemistryNorthern Illinois University

Carborane Clusters in Materials Science

• Polymers– Linear/Branched Polymers– Coordination polymers (Metal-Organic frameworks)

• Dendritic Structures• Liquid Crystalline Substances

What makes carborane clusters attractive to make polymeric materials?

• High thermal stability– Insertion of carborane cage into polymer backbone

dramatically increases the thermal stability • 3-D cage structure• Almost same size as benzene

– A benzene ring can be replaced with a carborane cage– Ease of functionalization of a carborane cage

Linear/Branched Polymers

Conducting Polymers: Electropolymerization of carboranyl functionalized pyrroles and thiophenes (1 of 2)

• Heterocyclic conjugated polymers, such as polypyrrole and polythiophene are important class of conducting materials

• Incorporation of icosahedral o-, m- and p-carborane clusters into such polymers greatly enhances thermal and electrochemical stabilities in comparison with unsubstituted polythiophenes and polypyrroles

• The electropolymerization process is more favorable in the ortho carborane substituted thiophenes – Lower oxidation potential is required in this case

• Incorporation of Ortho carborane into thiophene polymer backbone dramatically increases the electrical conductivity of the polymer

Hao et al., Chem. Comm., 2007, 4387; Hao et al., Chem. Mater., 2007, 19, 6195; Fabre et al., Macromolecules, 2006, 39,112-119

Conducting Polymers: Electropolymerization of carboranyl functionalized pyrroles and thiophenes (2 of 2)

Hao et al., Chem. Comm., 2007, 4387; Hao et al., Chem. Mater., 2007, 19, 6195

Silyl-Carborane Hybrid Diethynylbenzene-Silylene Polymers

Kimura et al., Chem. Mater., 2003, 15, 355-362

Phenylene ether carboranylene ketone (PECK) polymer

Fox and Wade, J. Mater. Chem., 2002, 12, 1301–1306

Thermal stability upto 1000 oC

Coordination polymers (Metal-Organic frameworks)

Coordination Polymers with Carborane Anions

• Coordination polymers are highly ordered infinite 1D, 2D, 3D coordination complexes

• Such materials have substantial empty space where guest molecules or counter ions can be included

• Carboranes have acidic C-H groups capable of forming H-bonding interactions

• Many carboranes are anionic in nature and used as weakly coordination anions– Icosahedral mono carborane [CB11H12]-

– Cobalt (III) bis(dicarbollide) [Co(C2B9H11)2]-

Coordination Polymers with Carborane Anions: Silver Dinitrile Complexes

Westcott et al., Inorg. Chem. 2004, 43, 3663-3672, Hardie, M.J., Journal of Chemical Crystallography, 2007, 37, 69

Hydrogen Sorption Properties of Carborane BasedMetal-Organic Framework Materials

• Carborane-based MOFs, were made with Zn(II) coordination of the deprotonated form of 1,12-dihydroxycarbonyl-1,12-dicarba-closo-dodecaborane

• The solvothermal synthesis instead yielded a more complex MOF of the formula[Zn3(OH)(p-CDC)2.5(DEF)4]n (2) [DEF ) diethylformamide]

• The solvent free version (4) was prepared by heating 2 at 300 oC

Farha et al., J. Am. Chem. Soc. 2007, 129, 12680-12681

Hydrogen Sorption Properties of Carborane BasedMetal-Organic Framework Materials

Farha et al., J. Am. Chem. Soc. 2007, 129, 12680-12681

Hydrogen Sorption Properties of Carborane BasedMetal-Organic Framework Materials

• The solvent free version of the MOF shows remarkable hydrogen absorption capability

• At 1 atm it takes up about 6 additional H2 molecules per [Zn3(OH)]+5 cluster

Farha et al., J. Am. Chem. Soc. 2007, 129, 12680-12681

Dendrimers&

Liquid Crystalline Substances

Dendrimers

• Dendrimers are highly ordered, regularly branched, globular macromolecules prepared by stepwise iterative approach

• Dendritic structures have three distinct architectural regions– A core or focal moiety

– Layers of branched repeat units emanating from the core

– End groups on the outer layers of repeat units

Grayson, and Frechet, Chem. Rev., 2001, 101, 3819-3867

Liquid Crystals

• Liquid crystals are considered as the fourth state of matter.– Intermediate between

crystalline solid state and liquid state

• Liquid crystals based on Dendrimers have been commercially available– PAMAM [Poly(amidoamine)]

– PPI [Poly(propyleneimine)]

Binnemans, K. Chem. Rev., 2005, 105, 4148-4204

Abdallah, D.J., et al, Liq. Cryst., 2000, 27, 831; Kanazawa, A., et al., J. Am. Chem. Soc.,2001, 122, 3053 & 1997, 119, 7670; Kanazawa, A., et al., Angew. Chem. Int. Ed. Engl., 2000, 39, 612; Stebani, U., Angew. Chem. Int. Ed. Engl., 1996, 35, 1858; Stebani, U., Adv. Mater., 1995, 7, 578

General Applications

Dendrimers

• Dendritic Box- – Encapsulates small organic

molecules and control their release• Metallodendrimers show redox,

magetic, photo-optical properties• High turnover transition state

catalysts• Drug/ gene delivery agents• Hybrid materials, coatings,

lubricants, adhesives etc.

Grayson and Frechet, Chem. Rev., 2001, 101, 3819; Chem. Rev.,1999, 99, 845; Jensen, Science, 1994, 266, 1226; Piotti, M.E., J. Am. Chem. Soc., 1999, 121, 9471; Gene Ther., 1997, 4, 823; Nucleic Acids Res., 1996, 24, 2176.

Liquid Crystals

• Electronic display (LCDs)• Ion conductive materials• Organized reaction media

– Ionic liquids –Alternative to volatile organic solvents

• Functional nanostructured materials from ionic-self assembly

• Molecular electronics• Electrolytes in the dye-sensitized

solar cells

Binnemans, K., Chem. Rev., 2005, 105, 4148.,Kato, T., Science, 2002, 295,2414., Ohtake et al., Macromolecules, 2000, 33, 8109., Chem. Comm., 2000, 1911., Adv. Mater., 2003, 15, 673., Nature, 1991,

Advantages of Incorporating Boron

• High Thermodynamic and chemical stability– Flame retardant materials and coatings

• High neutron capture cross section of 10B (highest of all light elements)– Radiation shield coatings

• Medicinal applications (BNCT and BGdNCT)– Adequate boron concentration in the cell may be achieved

• 20-35 μg 10B/gm of tumor cell– Water solubility may be achieved

• By decapitating the boron cage (by making Na and K salts)• By enhancing the outer hydroxyl group ratio to the carborane to 8:1

• Unique steric and electronic molecular features – Molecular symmetry and conformational mobility plays significant role in

stabilization of liquid crystalline phases

Hawthrone et al., Angew. Chem. Int. Ed. Engl., 1995, 34, 1332; Parrot et al., J. Am. Chem. Soc., 2005, 127, 12081; Parrot et al., Langmuir, 2006, 22, 5251; Benhabbour et al., Macromolecules, 2007, 40, 5678, Gonzalez-Camp et al , Macromolecules, 2007, 40, 5644; Org. Lett., 2006, 8, 4549; Inorg. Chem., 2006, 45, 7815 & 2001, 40, 6622; Chem. Mater., 1998, 10, 2399

Carborane Containing Dendritic Structures

Newkome et al., Angew. Chem. Int. Ed. Engl., 1994, 33, 666; Qualmann et al., Angew. Chem. Int. Ed. Engl., 1996, 35, 909; Parrot et al., J. Am. Chem. Soc., 2005, 127, 12081; Parrot et al., Langmuir, 2006, 22, 5251; Benhabbour et al., Macromolecules, 2007, 40, 5678, Gonzalez-Camp et al., Macromolecules, 2007, 40, 5644; Nunez et al., Org.Lett., 2006, 8, 4549; Galie et al., Inorg. Chem., 2006, 45, 7815, Yao etal., Organometallics, 2003, 22, 4381

Synthetic Approaches

Core

Growth Growth

Branching UnitSurfaceGroup

Divergent approach- Dendrimers are built stepwise from a small polyfunctional core and through the reiterations of a sequence of reactions

Convergent approach- Dendrimers grow from the exterior of the molecule and progress inward. After sufficient repetition of the process, these dendrons finally attached to a polyfunctional core

Chem. Rev., 2001, 101, 3819 & 1999, 99,845

SiCl4 Mediated Cyclotrimerization of Aryl Ketones

Yang et al., J. Am. Chem. Soc.,2005, 3278; Bao et al., Chem. Eur. J., 2006, 3287; Cao et al., J. Org. Chem., 2004, 6050

C3 Symmetric Trimers

Mechanism for Triple Condensation of Ketones

R

CH3

OHH2C

OH

R

+OH OH

RRH3C + -H3O+ CH3 O

RR

R

CH3

OH+CH2 OH

RR

OH

R CH3

R

OH

R

H

-H3O+

CH3

R

O

R

H

R

HEnolise CH2

R

OH

R

H

R

H

R

H

R ROHHH

H

-H2O

R

H

R

H

H

R

Synthesis of 9-Benzyl derivatives of Carboranes

Dash, B. P.; Satapathy, R.; Maguire, J. A.; Hosmane, N.S. Org. Lett.2008, 10, 2247-2250

CO

CH3Br

1) AlCl3

2a R = H (82%)2b R = CH3 (75%)

2)

3) CH2Cl2, 50 oC

(1)

R

H

CO CH3

R

H

CO

CH3

H

H

Silicon Tetrachloride Promoted Cyclotrimerizations of 9-Benzyl derivatives of Carboranes

Dash, B. P.; Satapathy, R.; Maguire, J. A.; Hosmane, N.S. Org. Lett.2008, 10, 2247-2250

3a R = H (89%)3b R = CH3 (87%)

CO CH3

SiCl4, EtOH

R

H

R

H

R

HR

H

2a R = H, 2b R = CH3

Functionalization of Cyclotrimers1 of 2

Dash, B. P.; Satapathy, R.; Maguire, J. A.; Hosmane, N.S. Org. Lett.2008, 10, 2247-2250

1) n-BuLi

3a R = H, 3b R = CH3

R

H

R

H

R

H

R

R'

R

R'

R

R'

2)

4a R = n-C7H15, R' = n-C7H15 (85%)4b R = CH3, R' = n-C7H15 (91%)

I

Functionalization of Cyclotrimers2 of 2

Dash, B. P.; Satapathy, R.; Maguire, J. A.; Hosmane, N.S. Org. Lett.2008, 10, 2247-2250

3a R = H

R

H

R

H

R

H

Si

Si

Si

Si

Si

Si

SiCl2

1) 2 n-BuLi

2)

Convergent Approach

I

2 n-BuLi

2

AlCl3

CH2Cl2, 50 oC

C

O

CH3Br

CO CH3

R = R' = n-C7H15 (92%)

83 %

71%

RR'

RR'

RR'

SiCl4, EtOH

Unpublished Results

Metallacarboranes (1 of 3)

Unpublished Results

Water Soluble

H

H

H

H

H

H

MOH, MeOH

Reflux

HH

HH

HH

M+

M+M+

M = K; Na

HH

HH

HH

+NHMe3

+NHMe3+NHMe3

+NHMe3Cl-

Water Insoluble

Metallacarboranes (2 of 3)

HH

HH

HH

M+

M+M+

SnCl2; THF

M = K; Na = Sn

Unpublished Results

Metallacarboranes (3 of 3)

Unpublished Results

H

H

H

H

H

H

CsF, EtOH

Ref lux

HH

HH

HH

Cs+

Cs+Cs+

Synthesis of C-Fused Polyaromatic Compounds

IBr

CH3n-BuLi,DME

CH3I

CH3

CO

CH3

BHO

HO

PdCl2 (PPh3)2,K2CO3, H2O, Ref lux C

O

CH3

SiCl4

Ethanol, Toluene

CH3

CH3 H3C

73 %

91 %

83 %

Unpublished Results

B-Fused Vs. C-Fused

HH

HH

HH

CH3

CH3 H3C

CH3

CH3

CH3

Conclusions

• Polyhedral Carborane Clusters are useful building blocks in materials chemistry

• Acetyl aromatic moiety attached to the boron atom as well as carbon atoms of the carborane cages undergo trimerization reactions to generate Carborane–containing symmetric building blocks. – Synthesis of higher order dendrimers and liquid crystalline

substances, – Metallacarboranes and new BNCT agents.

Acknowledgments

• Prof. Hosmane (Research Advisor)• Dr Chong Zheng (For Crystal Structure)• Dr Rashmi, Dr Matt, Dr Vladimir, Sumathy, Patrick,

Sonam, Sunitha, Sri Harsha, Anila• National Science Foundation• Robert A Welch Foundation• Northern Illinois University