PD Research Report for the 2014 year
Name（Research group） GAYEN KRISHNANKA SHEKHAR（Professor T. Hamura’s group, Graduate School ofScience and Technology）
Research Theme
Synthesis of functionalized heptacenes
Research Period
1st May, 2014 ～ 31st July, 2014
Research Results
The synthesis of conjucated polyacenes has gained importance in the last few decades
owing to their semiconducting nature. The strategy that we are trying to develop involves
bis cycloaddition of benzyne with furan followed by successive cyclization andaromatization to give substituted heptacene molecules.
The target compound isIn these three months, the synthesis of the above heptacene skeleton was synthesized. At
first, bromanil was converted to tetrabromo-di-(triisopropylsilylacetylene)benzene bytwo-stepped reaction. At first the bromoanil was reacted triisopropylsilylacetylene anion,
which
wassubsequently
aromatized
togivetetrabromo-di-(triisopropylsilylacetylene)benzene. The yield of this process was 61.6%.

The next target was to convert compound (4) to bis cycloadduct by reacting it with furan.
This reaction leads to generation of three types of products. One is monocycloadduct, andother two are the diasteromers of bis-cycloaddition product – syn, syn and syn-anti. The
yield of diasteromers was 61 %. The reaction was optimized under different conditions asdepicted below:

Next step was the reaction of compound 5a and 5b with isobenzofuran moieties. For thisseveral isobenzofuran synthesis was targeted. The schemes are given below:
Preparation of 2-Formylmethylbenzoate
Phthalaldehydic acid (6, 5.0 g, 33.33 mmol) and potassium carbobate (6.0 g, 43.33 mmol)
was taken in acetone (110 mL) under argon atmosphere. To it, dropwise dimethylsulfatewas added and the reaction was stirred for 2 h at room temperature. The progress of thereaction was monitored by TLC. The reaction mixture was finally filtered through celite
bed and was concentrated in vaccuo. The pure product (7) was obtained by column
chromatography in a quantitative yield using hexane and ethyl acetate (8:2) as eluent

The compound (7, 3.0 g, 18.27 mmol) was dissolved in dry THF (15 mL) and was cooled
to -40 ºC under argon atmosphere. To this cooled solution, phenyl magnesium bromide inTHF (2 eq.) was added dropwise. The reaction was allowed to stir overnight at room
temperature. The reaction was cooled to 0 ºC and trifluoroacetic acid (5 eq, 7 ml) wasadded and the solution was stirred for 30 min. The reaction was quenched by adding
saturated NaHCO3 solution. The compound was extracted by ethyl acetate (x3), washedwith brine, dried over anhydrous Na 2 SO 4 and concentrated in a rotary evaporator. The
crude was subjected to column chromatography using silica gel. The desired compound
eluted using hexane: dichloromethane (8:2). The isolated yield of the diphenylisobenzofuran (8, 1.8 g).

The compound (7, 328 mg, 2.0 mmol) was dissolved in dry THF (5 mL) and was cooled to-40 ºC under argon atmosphere. To this cooled solution, p-trifluoromethyl phenyl
magnesium bromide in THF (2 eq.) was added dropwise. The reaction was allowed to stirovernight at room temperature. The reaction was cooled to 0 ºC and trifluoroacetic acid (5eq, 7 ml) was added and the solution was stirred for 30 min. The reaction was quenched byadding saturated NaHCO 3 solution. The compound was extracted by ethyl acetate (x3),washed with brine, dried over anhydrous Na2 SO4 and concentrated in a rotary evaporator.
The crude was subjected to column chromatography using silica gel. The desired
compound eluted using hexane: dichloromethane (8:2). The isolated yield of thedi-(4-trifluoromethyl phenyl) isobenzofuran (15, 25 mg).

The compound (7, 1.0 g, 6.09 mmol) was dissolved in dry THF (20 mL) and was cooled to-40 ºC under argon atmosphere. To this cooled solution, 3,5-bis-trifluoromethyl phenyl
magnesium bromide in THF (2.5 eq.) was added dropwise. The reaction was allowed to stir
to attain room temperature. The reaction was cooled to 0 ºC and trifluoroacetic acid (5 eq,2.32 ml) was added and the solution was stirred for 30 min. The reaction was quenched byadding saturated NaHCO 3 solution. The compound was extracted by ethyl acetate (x3),washed with brine, dried over anhydrous Na2 SO4 and concentrated in a rotary evaporator.
The crude was triturated using cooled hexane to give pure product. The isolated yield of theisobenzofuran (10, 3.2 g, 97 %).

Synthesis of heptacene precursora) The compound 5 (1eq.) was taken in different solvents like toluene or super dehydrated
dichloromethane (2 mL), to it isobenzofuran, 8 (2.2 eq) and was subjected to different
reaction conditions as enlisted in the above table. After the reaction was complete as
monitored by TLC, the solvent was removed in a rotor evaporator and the compound waspurified by PTLC. The NMR confirmed the formation of diasteromers of compound 7. The
isolated yields of the diasteromers are given in the table.

b) The compound 5 (1eq.) was taken in different solvents like toluene or super dehydrated
dichloromethane (2 mL), to it isobenzofuran, 10 (2.2 eq) and was subjected to different
reaction conditions as enlisted in the above table. After the reaction was complete as
monitored by TLC, the solvent was removed in a rotor evaporator and the compound waspurified by PTLC. The NMR confirmed the formation of diasteromers of compound 9. The
isolated yields of the diasteromers are given in the table

The other reactions performed are shown in schemes.