R. S. Kalgutkar, P. M. Lahti / Tetrahedron Letters 44 (2003) 2625–2628
2627
is unusual15 in p-conjugated open-shell molecules, so 5
provides a test of its effect. The results show that it acts
as expected based on simple connectivity (parity) con-
siderations. In addition, thanks to the structural rigidity
of 5, an unambiguous analysis of the quintet zfs is
possible that supports the dipolar model previously
been proposed as the major determinant of zfs in such
dinitrenes and dicarbenes.
dioxime tended to form small clumps that were broken
up using a ceramic spatula. Stirring the mixture at
100°C was discontinued after all the reactant had been
consumed and the mixture was completely dark brown
(approx. 45 min). The polyphosphoric acid was neutral-
ized using a 10% sodium hydroxide solution at 100°C
with stirring to prevent the acid from congealing
(CAUTION: highly exothermic reaction). After the
neutralization was completed, the mixture was made
alkaline by further addition of 10% sodium hydroxide
and was cooled down in an ice bath. After standing at
10°C for 2 h, the mixture was filtered and the residue
washed with water until the washings tested neutral.
The residue was vacuum dried, collected as a brown
powder and recrystallized from ethanol to yield a yel-
low powder (0.234 g, mp >200°C with decomposition).
Experimental
2,6-Diacetylbiphenylene (6).
A solution containing
biphenylene (0.230 g, 1.51 mmol) and acetyl chloride
(0.770 g; 9.81 mmol) was prepared in anhydrous carbon
disulphide (30 mL, Aldrich) by stirring at ambient
temperature under nitrogen. To this solution, fresh dry
aluminum chloride (approx. 1 g, 7.49 mmol, 99.99%)
was added all at once under nitrogen with constant
stirring. The yellow solution turned red upon stirring,
as a red flocculent precipitate formed. The solution was
stirred overnight at ambient temperature under nitro-
gen to yield a light yellow solution and a crystalline red
precipitate. Cold HCl (40 mL, 2 M) was added drop-
wise to the light product mixture with constant stirring.
The red crystalline precipitate decomposed into a yel-
low solid. The mixture was stirred for 20 min and
filtered to yield 2,6-diacetylbiphenylene 6 as a yellow
solid (0.317 g, 89%, mp 245–247°C, lit. mp 243–
1
A complex H NMR (200 MHz, CDCl3) indicated that
the product was a mixture of the 2,6-diacetamido-
biphenylene (8) and the 2,6-diaminobiphenylene (9)
that was adequate for use in the next step. IR (KBr, w¯
cm−1) 3400 (s), 3300 (s), 1650 (m).
The mixture of 8 and 9 from the previous step was
stirred with concentrated hydrochloric acid (10 mL 12
M), absolute ethanol (10 mL) and water (10 mL) at
100°C for 1 h. The color of the solution changed from
yellow–brown to purple during this period, and tiny
needle shaped crystals appeared. The heating was dis-
continued after 1 h and the solution was allowed to
stand for 90 min at ambient temperature. The solution
was made alkaline by further addition of the 10%
aqueous sodium hydroxide solution. The purple mix-
ture changed into a yellow precipitate and the mixture
was cooled in an ice bath for 1 h. The mixture was
filtered and the yellow residue was washed with water
until the washings tested neutral. The residue was dried
under vacuum overnight and then stirred in about 15
mL 10% aqueous sodium hydroxide solution for 1 h.
The mixture was neutralized with aqueous hydrochloric
acid (2 M) and then extracted with methylene chloride
(10×25 mL). The solvent was removed using a rotary
evaporator to yield 2,6-diaminobiphenylene (9) as a
yellow solid (0.134 g; mp 205–207°C, lit mp 218–220°C
1
245°C8a). H NMR (200 MHz, CDCl3): l 2.52 (s, 6H,
CH3), 6.84 (d, J=7.2 Hz, 2H, H4,8), 7.34 (s, 2H, H1,5),
7.58 (d, J=7.2 Hz, 2H, H3,7). IR (KBr, w¯ cm−1): 1660
(s), 1390 (s).
2,6-Diacetylbiphenylene dioxime (7). A solution of 6
(0.476 g, 2.03 mmol), hydroxylamine hydrochloride (1.0
g, 14.0 mmol) and sodium hydroxide (0.69 g, 17.3
mmol) was prepared by dissolving the reactants in
water (4–5 mL) and ethanol (25 mL, 100%) with con-
stant stirring. The mixture was gently refluxed for 16 h
with constant stirring. After 16 h of reflux the product
mixture was allowed to stand overnight. The solvent
was removed using a rotary evaporator and water was
added to the residue. The suspension was stirred for 20
min and filtered to yield 7 as a yellow solid (0.466 g,
86%, mp 289–291°C with decomposition and gas evolu-
tion, lit mp 290–291°C8b). 1H NMR (200 MHz,
CD3COCD3): l 2.14 (s, 6H, CH3), 6.80 (d, J=7.3 Hz,
2H, H4,8), 7.12 (d, J=7.3 Hz, 2H, H3,7), 7.19 (s, 2H,
H1,5), 10.36 (s, broad, NOH). IR (KBr, w¯ cm−1): 1720
(m), 920 (s). A small portion was recrystallized from
ethanol (95%) to yield 7 as a fine yellow powder (mp
1
after decomposition at 205°C8b). H NMR (200 MHz,
CDCl3): l 3.54 (s, broad, 4H, NH2), 5.89 (dd, J=1.4,
7.2 Hz, 2H, H3,7), 6.04 (d, J=1.4 Hz, 2H, H1,5), 6.39 (d,
J=7.2 Hz, 2H, H4,8). IR (KBr, w¯ cm−1): 3400 (br), 1690
(s), 700 (s). Note: Although 2,6-diaminobiphenylene
1
darkens upon prolonged exposure to air its H NMR
spectrum remains unchanged.
2,6-Diazidobiphenylene (4). A solution of 9 (0.015 g, 82
mmol) in 4 mL of THF was treated with an aqueous
solution of concentrated hydrochloric acid (0.03 mL,
0.36 mmol) with stirring at ambient temperature. A
purple–gray precipitate formed almost immediately. The
mixture was cooled to −12°C, and isoamyl nitrite (0.03
mL, 0.22 mmol; CAUTION: highly toxic) was added to
the cold reaction mixture with constant stirring. The
color of the reaction mixture darkened and a black
precipitate formed almost immediately. The mixture
was stirred at approximately −10°C for 15 min, then a
cold solution of sodium azide (0.016 g, 0.25 mmol in
1
>340°C). H NMR (200 MHz, CD3COCD3): l 2.14 (s,
6H, CH3), 6.80 (d, J=7.3 Hz, 2H, H4,8), 7.12 (d, J=7.3
Hz, 2H, H3,7), 7.19 (s, 2H, H1,5), 10.34 (s, broad,
NOH).
2,6-Diaminobiphenylene (9). Polyphosphoric acid (18 g,
Aldrich) was weighed into a 25 mL Erlenmeyer flask at
room temperature and heated to 100°C in an oil bath
with constant stirring. To the polyphosphoric acid was
added 7 (0.410 g, 1.54 mmol) as a fine powder with
constant stirring. The color of the mixture turned from
nearly colorless to dark brown. During the reaction, the