Preparation and properties of sterically protected diphosphene and fluorenylidenephosphine bearing the 2,6-di-tert-butyl-4-methoxyphenyl group

Article abstract of DOI:10.1246/cl.2003.430

A new bulky bromobenzene, 2-bromo-1,3-di-tert-butyl-5-methoxybenzene, was prepared and utilized to preparations of the corresponding diphosphene and fluorenylidenephosphine including low-coordinate phosphorus atom(s). An electronic perturbation of the p-m

Full text of DOI:10.1246/cl.2003.430

430
Chemistry Letters Vol.32, No.5 (2003)
Preparation and Properties of Sterically Protected Diphosphene and Fluorenylidenephosphine
Bearing the 2,6-Di-tert-butyl-4-methoxyphenyl Group
Kozo Toyota, Subaru Kawasaki, Akitake Nakamura, and Masaaki Yoshifujiꢀ
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578
(Received January 30, 2003; CL-030091)
A new bulky bromobenzene, 2-bromo-1,3-di-tert-butyl-5-
methoxybenzene, was prepared and utilized to preparations of
the corresponding diphosphene and fluorenylidenephosphine in-
cluding low-coordinate phosphorus atom(s). An electronic per-
turbation of the p-methoxy group in the system was indicated
by UV–vis spectra and 31P NMR chemical shifts.
reduction8 of 4d9 afforded 3d in good yield (70%).
Then we applied this methodology to the preparation of 3e
as
follows:
2-bromo-1,3-bis(bromomethyl)-5-methoxy-
benzene10 was allowed to react with KCN in acetonitrile (con-
taining ca. 10% of water) in the presence of 18-crown-6 at room
temperature for 24 h to give 5 in 66% yield.11 Methylation of 5
to 6 (quant.) followed by treatment with diisobutylaluminum
hydride gave 4e (96% yield), which was then reduced to 3e
(48% yield)11 by the Huang-Minlon method. Lithiation of 3e
with butyllithium followed by reaction with PCl3 gave 7 [31P
NMR (THF-C6D6) dP ¼ 153:3], which was then converted to
1e in 12% isolated yield.12 Reaction of 7 with 9-(trimethylsi-
lyl)-9-fluorenyllithium followed by treatment with n-Bu4NþFÀ
gave 2e12 (38% yield). Compound 2d was prepared from 3d by
a method similar to that for 2e (86% yield).12
Intramolecular interactions between a reactive site and the
neighboring functional group are of current interest.1 Sterically
protected diphosphenes include an essentially reactive –P=P–
bond in the molecule and they are kinetically stabilized by
bulky groups such as the 2,4,6-tri-tert-butylphenyl group.2 We
have studied modification of the bulky aryl substituents at the
o-position(s) and prepared 1,2-bis[2,4-di-tert-butyl-6-(dimethy-
lamino)phenyl]diphosphene (1b) as an o-amino analogue of
1a.3 It should be noted that an attempted preparation of 1,2-
bis(2,4-di-tert-butyl-6-methoxyphenyl)diphosphene (1c) has
been unsuccessful.4 In contrast, studies concerning the effect
of p-substituent(s) in diaryldiphosphenes have been limited.5
Although we have reported preparation of diphosphenes lacking
the p-tert-butyl groups, i.e., 1,2-bis(2,6-di-tert-butylphenyl)di-
phosphene (1d),6 the effect of introduction of an electron-donat-
ing or an electron-withdrawing substituent at the p-position of
the 2,6-di-tert-butylphenyl group has remained unclear. We re-
port here preparation and properties of p-methoxy substituted
diphosphene 1e, as well as fluorenylidenephosphines 2d and 2e.
Table 1 shows 31P NMR chemical shifts and UV–vis data
of 1a,d,e and 2a,d,e. As for the 31P NMR chemical shift, elec-
tron-donating substituent in 1a,e and 2a,e causes a shift to a
lower field, compared to that of 1d and 2d, respectively. The
order of the dP values is d<a<e, for both diphosphene and
fluorenylidenephosphine series. In the case of UV–vis spectra
of 1a,d,e, the p-substitution causes red shifts of absorption of
the longest ꢀmax attributable to n ! pꢀ transition (again, the or-
der is d<a<e). In contrast, p-substitution effect on the second
ꢀmax (p ! pꢀ transition) is small. These facts may indicate that
the p-methoxy group affects the electronic states of the phos-
phorus lone pair, although the effect on the –P=P– p system
is small, probably because the p-conjugation between the aro-
matic rings and the phosphorus double bonds is restricted, due
to steric hindrance that prevents coplanar conformation.2;13
Structure of 1e was unambiguously determined by X-ray
crystallography.16 Figure 1 shows a molecular structure of 1e.
First, we tried to prepare a bulky bromobenzene 3d.
Although the compound 3d was first prepared by Rundel, start-
ing from 2,4,6-tri-tert-butylaniline,7 the reported yield was not
good.6;7 Thus, we have sought an alternative synthetic route
and found that the Wolff-Kishner (the Huang-Minlon method)
ꢀ
Although the P–Cipso bond lengths for 1e [1.860(4) A for
ꢀ
P(1)–C(1) and 1.869(4) A for P(2)–C(7)] are very close to those
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.5 (2003)
431
Am. Chem. Soc., 103, 4587 (1981); M. Yoshifuji, I. Shima, N. Inamoto,
K. Hirotsu, and T. Higuchi, J. Am. Chem. Soc., 104, 6167 (1982). b) M.
Yoshifuji, J. Organomet. Chem., 611, 210 (2000).
M. Yoshifuji, M. Hirano, and K. Toyota, Tetrahedron Lett., 34, 1043
(1993).
3
4
5
M. Yoshifuji, D.-L. An, K. Toyota, and M. Yasunami, Chem. Lett.,
1993, 2069.
p-Phenylene-bridged bis(diphosphene) derivative has recently been re-
ported. See, S. Shah, T. Concolino, A. L. Rheingold, and J. D.
Protasiewicz, Inorg. Chem., 39, 3860 (2000).
M. Yoshifuji, T. Niitsu, D. Shiomi, and N. Inamoto, Tetrahedron Lett.,
30, 5433 (1989).
6
7
8
W. Rundel, Chem. Ber., 101, 2956 (1968).
K. Toyota, Y. Matsushita, N. Shinohara, and M. Yoshifuji, Heteroat.
Chem., 12, 418 (2001).
9
K. Toyota, A. Nakamura, and M. Yoshifuji, Chem. Commun., 2002,
3012.
10 L. A. van de Kuil, H. Luitjes, D. M. Grove, J. W. Zwikker, J. G. M. van
der Linden, A. M. Roelofsen, L. W. Jenneskens, W. Drenth, and G. van
Koten, Organometallics, 13, 468 (1994).
Figure 1. Molecular structure of 1e, showing the atomic label-
ing scheme with thermal ellipsoids (50% probability). Hydrogen
ꢀ
11 3e: mp 59–61 ꢁC; 1H NMR (400 MHz, CDCl3) ꢁ ¼ 1:63 (18H, s, t-
Bu), 3.85 (3H, s, OMe), 7.02 (2H, s); 13C{1Hg NMR (100 MHz,
CDCl3) ꢁ ¼ 31:2 (CMe3), 38.7 (CMe3), 55.5 (OMe), 112.7, 115.9,
151.0, 157.9; MS (70 eV) m=z 300 (Mþ+2; 100), 298 (Mþ; 100),
285 (Mþ–Me+2; 33), 283 (Mþ–Me; 33). Found: m=z 298.0934. Calcd
for C15H23BrO: M, 298.0932. 4e: mp 98–100ꢁC; 1H NMR (CDCl3)
ꢁ ¼ 1:54 (12H, s, CMe2), 3.88 (3H, s, OMe), 6.99 (2H, s), 9.78 (2H,
s, CHO); 13C{1Hg NMR (CDCl3) ꢁ ¼ 24:3 (CMe2), 53.2 (CMe2),
55.9 (OMe), 114.5, 115.7, 145.9, 159.6, 203.0 (CHO); MS m=z 328
(Mþ+2; 6), 326 (Mþ; 6), 247 (Mþ–Br; 100). Found: m=z 326.0521.
Calcd for C15H19BrO3: M, 326.0518. 5: mp 155–157ꢁC; 1H NMR
(CDCl3) ꢁ ¼ 3:86 (4H, s, CH2CN), 3.88 (3H, s, OMe), 7.10 (2H, s);
13C{1Hg NMR (CDCl3) ꢁ ¼ 26:1 (CH2), 56.2 (OMe), 115.2, 115.9,
116.9, 132.7, 159.8; MS m=z 266 (Mþ+2; 98) 264 (Mþ; 100). Found:
m=z 263.9905. Calcd for C11H9BrN2O: M, 263.9898. 6: mp 149–
151 ꢁC; 1H NMR (CDCl3) ꢁ ¼ 1:95 (12H, s, CMe2CN), 3.87 (3H, s,
OMe), 7.05 (2H, s); 13C{1Hg NMR (CDCl3) ꢁ ¼ 28:7 (CMe2), 38.6
(CMe2), 56.0 (OMe), 113.6, 114.8, 123.7, 142.2, 159.2; MS m=z 322
(Mþ+2; 99) 320 (Mþ; 100). Found: m=z 320.0520. Calcd for
atoms are omitted for clarity. Some selected bond lengths (A)
and angles (ꢁ): P(1)–P(2), 2.043(1); P(1)–C(1), 1.860(4); P(2)–
C(7), 1.869(4); O(1)–C(4), 1.372(4); O(2)–C(10), 1.370(4);
P(1)–P(2)–C(7), 98.7(1); P(2)–P(1)–C(1), 100.5(1).
Table 1. 31P NMR and UV-vis data of compounds 1 and 2
a
Compd. dP
ꢀ/nm (log e)
1a 490.0b 284 (4.19)c,d 340 (3.89) 460 (3.13)
1d 488.7e 290 (4.03)c,e 340 (3.85) 450 (3.00)
1e 495.9 291 (4.21)c 346 (3.75) 477 (3.12)
2a 256.5f 238 (4.48)g 266 (4.30) 275 (4.32) 365 (4.05)
2d 254.8 237 (4.55)g 266 (4.40) 275 (4.45) 362 (4.25)
2e 258.0 239 (4.61)g 266 (4.42) 275 (4.46) 369 (4.30)
aMeasured in CDCl3. bData taken from Ref. 14. cMeasured in
d
e
CH2Cl2. Data taken from Ref. 2a. Data taken from Ref. 6.
fData taken from Ref. 1b, see also, Ref. 15. gMeasured in
hexane.
C
15H17BrN2O: M, 320.0524.
12 1e: Orange prisms, mp 165–169 ꢁC (dec.); 1H NMR (400 MHz,
CDCl3) ꢁ ¼ 1:41 (36H, s, t-Bu), 3.56 (6H, s, OMe), and 7.14 (4H,
s); 13C{1Hg NMR (100 MHz, CDCl3) ꢁ ¼ 55:4 (OMe); 31P{1Hg
NMR (162 MHz, C6D6) ꢁ ¼ 498:8; MS (70 eV) m=z 500 (Mþ; 16)
and 249 (ArPþ–1; 100). Found: m=z 500.3000. Calcd for C30H46O2P2:
M, 500.2973. 2d: Yellow needles, mp 162–164 ꢁC; 1H NMR (CDCl3)
ꢁ ¼ 1:46 (18H, s, t-Bu), 5.29 (1H), 6.78 (1H), 7.17 (1H), 7.29–7.37
(2H), 7.45–7.59 (4H), 7.64 (1H), and 8.28 (1H); 13C{1Hg NMR
2a
ꢀ
for 1a [1.862(2) A], the P=P bond length for 1e [2.043(1) A]
ꢀ
ꢀ
is slightly longer than that for 1a [2.034(2) A]. The interplanar
angles between the average plane [P(1), P(2), C(1), C(7)] and
the aromatic rings [C(1)–C(6) and C(7)–C(12)] are 70.18(10)
and 108.08(10)ꢁ, respectively, whereas the corresponding angle
is 63.9 ꢁ in 1a. The interplanar angle between the two aromatic
rings of 1e is 70.7(1)ꢁ. The dihedral angle C(1)–P(1)–P(2)–C(7)
for 1e [165.0(2)ꢁ] is smaller than that for 1a [172.2(1)ꢁ], which
means that the deviation from the planarity of the –P=P– moi-
ety is larger in 1e than in 1a in the crystal.
In summary, we have developed a new bulky substituent
bearing a p-methoxy group. Some low-coordinated phosphorus
compounds bearing the substituent were prepared and the ef-
fects of the p-substituent were evaluated. The method described
here is promising for preparations of various p-functionalized
bulky bromobenzenes. Studies on the reactivities of 1e and 2e
are now in progress.
1
(CDCl3) ꢁ ¼ 170:3 (d, JPC ¼ 42:2 Hz, P=C); 31P{1Hg NMR (C6D6)
ꢁ ¼ 253:5; MS m=z 384 (Mþ; 27) and 219 (ArPþ–1; 100). Found:
m=z 384.2007. Calcd for C27H29OP: M, 384.2007. 2e: Yellow prisms,
mp 205–207 ꢁC (dec.); 1H NMR (CDCl3) ꢁ ¼ 1:44 (18H, s, t-Bu), 3.96
(3H, s, OMe), 5.56 (1H), 6.84 (1H), 7.14 (2H), 7.18 (1H), 7.28–7.37
(2H), 7.57 (1H), 7.64 (1H), and 8.27 (1H); 13C{1Hg NMR (CDCl3)
1
ꢁ ¼ 171:2 (d, JPC ¼ 43:2 Hz, P=C); 31P{1Hg NMR (C6D6)
ꢁ ¼ 257:2; MS m=z 414 (Mþ; 18), 249 (ArPþ–1; 100), and 165
(Fluþ+1; 33). Found: m=z 414.2104. Calcd for C28H31OP: M,
414.2113.
13 R. Gleiter, G. Friedrich, M. Yoshifuji, K. Shibayama, and N. Inamoto,
Chem. Lett., 1984, 313.
14 D.-L. An, K. Toyota, M. Yasunami, and M. Yoshifuji, J. Organomet.
Chem., 508, 7 (1996).
15 V. D. Romanenko, A. V. Ruban, M. I. Povolotskii, L. K. Polyachenko,
and L. N. Markovskii, Zh. Obshch. Khim., 56, 1186 (1986).
16 Recrystallized from CH2Cl2–hexane, measured at 153 K. C30H46O2P2,
M ¼ 500:64. Monoclinic, space group P21=n (#14), a ¼ 10:181ð7Þ,
This work was supported in part by the Grants-in-Aid for
Scientific Research (Nos. 13304049, 14044012, and
13640522) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
ꢁ
ꢀ
ꢀ 3
b ¼ 19:299ð4Þ, c ¼ 15:413ð3Þ A, ꢂ ¼ 104:75ð3Þ , V ¼ 2928ð2Þ A ,
Z ¼ 4, Dc ¼ 1:135 g cmÀ3, ꢃ (Mo Kꢄ) ¼1:72 cmÀ1. 3955 Unique re-
flections with 2ꢅ ꢂ 50:0 ꢁ. Of these, 3683 with I > 2:0 sðIÞ were used
for R1 calculation. The structure was solved by direct methods. Non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were in-
cluded but not refined. R1 ¼ 0:073, R ¼ 0:134, Rw ¼ 0:178. Crystallo-
graphic data have been deposited at the Cambridge Crystallographic
Data Centre (No. CCDC-202408).
References and Notes
1
a) J. G. Verkade, Acc. Chem. Res., 26, 483 (1993). b) K. Toyota, S.
Kawasaki, and M. Yoshifuji, Tetrahedron Lett., 43, 7953 (2002).
a) M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu, and T. Higuchi, J.
2
Published on the web (Advance View) April 9, 2003; DOI 10.1246/cl.2003.430