Synthesis and reactions of new diphosphenes bearing extremely bulky substituents

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY

J. Phys. Org. Chem. 2003; 16: 450–462

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.608

Synthesis and reactions of new diphosphenes bearing

extremely bulky substituents^†

Takahiro Sasamori, Nobuhiro Takeda and Norihiro Tokitoh*

Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

Received 30 September 2002; revised 2 December 2002; accepted 2 December 2002

ABSTRACT: In this paper, we present the synthesis of new diphosphenes TbtP=PTbt and BbtP=PBbt having

epoc

extremely bulky substituents, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) and 2,6-bis[bis(trimethylsilyl)-

methyl]-4-[tris(trimethylsilyl)methyl]phenyl groups (Bbt). Their x-ray crystallographic analysis revealed their

unique structures in the solid state. Furthermore, the conﬁgurations of these extremely overcrowded diphosphenes,

TbtP=PTbt and BbtP=PBbt, are twisted in different ways in spite of the close structural similarity between Tbt and

Bbt groups. DFT calculations were performed to estimate the energy difference between the two conﬁgurations of

diphosphenes. In addition, their structures and physical properties were compared with those of their heavier

congeners, distibenes (ArSb=SbAr, Ar = Tbt and Bbt) and dibismuthenes (ArBi=BiAr, Ar = Tbt and Bbt) having

the same substituents. Although the reactivities of the extremely hindered diphosphenes might be considerably

suppressed owing to the severe steric congestion, it was found that they can react with elemental sulfur and selenium

to give the corresponding thia- and selenadiphosphirane derivatives, respectively, in addition to the previously

reported diphosphenes. Copyright  2003 John Wiley & Sons, Ltd.

Additional material for this paper is available from the epoc website at http://www.wiley.com/epoc

KEYWORDS: diphosphenes; steric protection; kinetic stabilization; group 15 elements; distibene; dibismuthene;

multiple bond; chalcogenadiphosphirane

INTRODUCTION

Multiple-bond compounds between heavier group 15

elements have fascinated chemists for a long time.^1–3

Since the ﬁrst isolation of the diphosphene

Mes*P

using several steric protection groups on the chemistry of

variety of diphosphenes (RP

PR),¹diarsenes

(RAs

AsR)²and phosphaarsenes (RP AsR).³

=

PMes*,⁴intensive studies have been performed

a

=

Although the conventional steric protection groups

could not be applied to the synthesis of distibene and

dibismuthene, we have recently succeeded in the

syntheses and isolation of the ﬁrst stable distibenes and

Chart 1.

dibismuthenes, RE=ER (E = Sb or Bi, R = Tbt ot Bbt),

15 elements are no longer imaginary species but actually

exist.⁷

by taking advantage of the more effective steric

protection groups, 2,4,6-tris[bis(trimethylsilyl)methyl]-

phenyl (Tbt) and 2,6-bis[bis(trimethylsilyl)methyl]-4-

[tris(trimethylsilyl)methyl]phenyl (Bbt) groups.^5,6Now,

all the doubly bonded compounds between heavier group

Here, it was considered worth comparing the features

of Tbt- and Bbt-substituted diphosphenes 1a,b with those

of the distibenes 2a,b and the dibismuthenes 3a,b (Chart

1) to make a systematic comparison among a series of

heavier dipnictenes under the same effects of substitu-

ents. There is a question of whether extremely bulky

substituents such as Tbt or Bbt groups could be

applicable to kinetic stabilization of the lighter analogue,

diphosphene, regardless of severe steric repulsion. In this

paper, we present the details of the syntheses of new

diphosphenes having extremely bulky substituents, Tbt

*Correspondence to: N. Tokitoh, Institute for Chemical Research,

Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.

E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp

Contract/grant sponsor: Ministry of Education, Culture, Sports,

Science and Technology; Contract/grant number: 12CE2005; Con-

tract/grant number: 11166250.

Copyright  2003 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2003; 16: 450–462

DIPHOSPHENES BEARING EXTREMELY BULKY SUBSTITUENTS

451

P=

P double-bond character. Thus, the Tbt and Bbt

groups were found to be effective protection groups to

stabilize not only the heavier Sb Sb and Bi Bi double

bonds but also the lighter P P double bond.

=

Scheme 1.

X-ray crystallographic analysis of Tbt- and Bbt-

substituted diphosphenes

and Bbt groups, together with their physical and chemical

properties.

The molecular geometries and selected bond lengths and

angles of 1a,b revealed by x-ray crystallographic analysis

are shown in Figs 1 and 2. In the all cases of distibenes

2a,b and dibismuthenes 3a,b, their molecular structures

have a center of symmetry in the middle of their E=E

RESULTS AND DISCUSSION

(E = Sb, Bi) double bonds and have trans conﬁgurations

with the plane of the aryl rings of the Tbt or Bbt groups

perpendicular to their C—E—E—C (E = Sb, Bi)

Synthesis of TbtP

=PTbt and BbtP=PBbt

We examined the synthesis of new diphosphenes having

Tbt and Bbt groups via the most traditional method, i.e.

the simple reductive coupling reaction of the correspond-

ing dichlorophosphines. When ArPCl₂(4a, Ar = Tbt; 4b,

Ar = Bbt) prepared by the reaction of ArLi with PCl₃

were treated with metallic magnesium in THF at room

planes.^6aAlso, the diphosphene Mes*P

=PMes* (5) has

been reported to have similar features in its struc-

ture.⁴The molecular structures of the newly obtained,

extremely hindered diphosphenes 1a and 1b also have

trans conﬁgurations with their aryl substituents (Tbt

or Bbt, respectively), in addition to the distibenes and

dibismuthenes. However, in sharp contrast to 2a,b and

3a,b, the diphosphenes 1a and 1b do not have an

intramolecular center of symmetry, and the two Tbt and

Bbt groups in 1a and 1b are intertwined with each other.

temperature, stable red crystals of diphosphenes ArP

=

PAr (1a, Ar = Tbt; 1b, Ar = Bbt) were obtained in more

than 90% yield (Scheme 1). The characteristic low-ﬁeld

chemical shifts of 531.8 ppm for 1a and 529.1 ppm for 1b

in the ³¹P NMR spectra in C₆D₆strongly indicated their

The bond lengths of the P=P double bonds in 1a

Figure 1. ORTEP drawings of (A) TbtP

fragment of the solvated hexane was omitted for clarity

=

PTbt (1a) and (B) BbtP

=PBbt (1b) with thermal ellipsoid plot (50% probability). The

Copyright  2003 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2003; 16: 450–462

452

T. SASAMORI, N. TAKEDA AND N. TOKITOH

Figure 3. Side views of the diphosphenes 1a and 1b

group on P2 in Figs 1(B) and 2] is nearly parallel (23°) to

the C—P—P—C plane (see Table 1). Therefore, the C—

P—P angle at P2 atom [114.9(1)°] is wider than that at

the other phosphorus atom (P1) [97.8(1)°] in 1b owing to

the steric repulsion between the P=P double bond and

Figure 2. Selected bond lengths and bond angles of

diphosphenes 1a and 1b

one of the CH(SiMe₃)₂groups at the ortho-position of the

Bbt group. A diphosphene and a diarsene having bulky

m-terphenyl ligands [2,6-Mes₂C₆H₃or 2,6-Tip₂C₆H₃

(Tip = 2,4,6-triisopropylphenyl)] have been recently re-

ported to have a twisted conﬁguration similar to diphos-

phene 1b.^7c,10Although it was found that the bulkiness of

the substituents on the central atoms of the diphosphenes

causes the twisted structure, the reason for the difference

in conﬁgurations between 1a and 1b is not clear. The

crystals of 1a and 1b contain different solvents (1a,

˚

[2.051(2) A] and 1b [2.043(1) A] are very close to that in

4

˚

Mes*P=

PMes* (5; 2.034 A) and they are considerably

shorter by ca 8% than the typical P—P single-bond length

8

9

˚

[(PhP)₅, 2.217 A; (PhP)₆, 2.237 A ]. Thus, it was found

that the difference in geometries between diphosphenes

and their heavier analogues do not affect the double-bond

character of the central P=P bond in the solid state. It

appears that the bulkiness of the two Tbt or Bbt groups

does not allow them to be parallel owing to the steric

repulsion between the bulky CH(SiMe₃)₂groups at the

ortho positions accompanied by the shorter bond length

Table 1. Theoretically optimized and experimentally ob-

tained structural parameters for diphosphenes

Dihedral

of P

=

P double bonds than those of Sb

=

Sb [2a,

Bi [3a,

P—P—C

(°)

angles^a

5a

6a

˚

2.6422(7) A; 2b, 2.7037(6) A ] and Bi

˚

Compound

1a (TbtP

1b (BbtP

d (P

=

P) (A)

(°)

5b

6a

˚

2.8206(8) A; 3b, 2.8699(6) A ] double bonds. Here, it

was found that the P—C bonds between the phosphorus

and the ipso-carbon atoms of the extremely bulky Tbt or

Bbt group were rotated to release the severe steric

=

PTbt)^b

2.088

110.9

103.0

39

65

85

23

56

81

13

50

57

83

23

=

PBbt)^b

PAr)^b

2.068

101.7

114.4

repulsion without elongating the central P=P bond in the

1a' (ArP

1b' (ArP

1a (TbtP

1b (BbtP

=

2.055

105.9

solid state.

105.9

=

PAr)^b

PTbt)^c

PBbt)^c

2.065

98.4

The two planes of the aryl rings of Tbt or Bbt groups

are nearly perpendicular in both 1a and 1b, but the nature

of the distortion between the substituents and the C—P—

P—C plane is different in 1a and 1b. The difference is

clearly indicated by Fig. 3, in which the side views of 1a

and 1b are shown. As for the case of Tbt-substituted

diphosphene 1a, both of the two Tbt groups are twisted

from the C—P—P—C plane by 50° and 57°. By contrast,

in the case of 1b the plane of the aryl ring of one Bbt

group [Bbt group attached to P1 in Figs 1(B) and 2] is

almost perpendicular (83°) and that of the other [Bbt

114.7

=

2.051(2)

2.043(1)

106.4(2)

104.5(2)

97.8(1)

114.9(1)

=

a

The dihedral angles between aryl group and the C—P—P—C plane.

Ar = 2,6-[CH(SiMe₃)₂]₂C₆H₃. Structural parameters were obtained by the

b

theoretical calculations at the B3LYP/[TZ(2d) for P, 6–31G(d) for Si and 3–

21G for C and H] level as the local minimum point because it was difﬁcult

to ﬁnd the global minimum point owing to the immense number of atoms.

c

Structural parameters were obtained by x-ray crystallographic analysis

(see Figs 1 and 2).

Copyright  2003 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2003; 16: 450–462

DIPHOSPHENES BEARING EXTREMELY BULKY SUBSTITUENTS

453

Table 2. UV/visble spectroscopic data for homonuclear

doubly bonded systems between heavier group 15 elements

benzene; 1b, hexane), but they are not thought to affect

the molecular geometries of the diphosphenes.

In order to estimate the energy gap between the two

types of conformations observed for 1a and 1b, DFT

calculations were performed. When experimentally

obtained structures of 1a and 1b were used as initial

l₁(nm) (e) l₂(nm) (e)

Compound^a

Mes*P PMes*

No. (p → p*)

(n → p*)

Ref.

=

5

6

7

8

9

340 (7690)

353 (9474)

371 (8000)

393 (7270)

460 (1360)

484 (62.8)

455 (501)

502 (570)

4

13

10

7c

1c

TsiP

PTsi

PR¹

PR²

structures, the local minima of TbtP

BbtP PBbt were found, respectively. Next, the struc-

tural optimization were examined for two diphosphenes

1a' and 1b' (ArP PAr, Ar = 2,6-[CH(SiMe₃)₂]₂-C₆H₃),

=PTbt and

R¹P

=

R²P

Mes*P

=

PDis

PTbt

PBbt

325 (13000) 427 (370)

=

TbtP

BbtP

TsiAs

=

1a 405 (13000) 530 (2000) This work

1b 418 (12000) 532 (1000) This work

=

whose initial structures were obtained by replacing the

alkyl groups at para positions of the theoretically

optimized structures of 1a and 1b with hydrogen atoms,

respectively. As shown in Table 1, the optimized

structures of 1a, 1b, 1a' and 1b' have twisted conforma-

tions similar to the experimentally obtained structures of

1a and 1b, respectively. The results of theoretical

calculations show that the energy gap between 1a' and

1b' is only 0.8 kcal mol^À1(1 kcal = 4.184 kJ) and 1a' is

slightly more stable than 1b'. Thus, the difference

between the conformations of 1a and 1b may be

attributed to the subtle difference of bulkiness between

Tbt and Bbt groups.

=

AsTsi

10 380 (5000)

505 (10)

14

15

7c

Mes*As

=

AsDis 11 368 (6960)

449 (180)

462 (650)

504 (120)

599 (170)

594 (200)

R¹As

=

AsR¹

12 400 (6970)

13 409 (4060)

2a 466 (5200)

2b 490 (6000)

R²As

=

AsR²

=SbTbt

7c

5a

6a

7c

5b

6a

7c

TbtSb

BbtSb

=

SbBbt

R¹Sb

=

SbR¹

14 451 (5450) Not observed

15 470 (6570) Not observed

R²Sb

=

SbR²

TbtBi

=

BiTbt

3a 525 (4000)

3b 537 (6000)

16 505 (5010)

17 518 (6920)

660 (100)

670 (20)

970 (30)

913 (200)

BbtBi

BiBbt

R¹Bi

R²Bi

=

BiR¹

=

BiR²

a

Tsi = C(SiMe₃)₃; Dis = CH(SiMe₃)₂; Mes = mesityl; Tip = 2,4,6-triiso-

propylphenyl; Mes* = 2,4,6-tri-tert-butylphenyl; R¹= 2,6-Mes₂-C₆H₃;

R²= 2,6-Tip₂-C₆H₃.

Physical properties of Tbt- and Bbt-substituted

diphosphenes

n → p* transitions [530 nm (ꢀ = 2000) for 1a and 532 nm

(ꢀ = 1000) for 1b] of the P=P chromophore, respectively.

In the Raman spectra (measured in the solid state) of 1a

Although the UV/visible spectral data strongly supported

and 1b, strong lines attributable to the P

=

P stretching

the P P double-bond character of 1a and 1b in solution,

=

were observed at 609 and 603 cm^À1, respectively.

These values observed for the diphosphenes are higher

than the frequencies observed for diphosphanes (e.g.

the l_maxvalues of these absorption maxima are red

shifted compared with those of the previously reported

diphosphenes 5 and 6. The observed red shifts might be

due to the extremely bulky Tbt or Bbt group on the

phosphorus atoms. As described above, the two Tbt or

Ph₂P À PPh₂, 530 cm^{À1 11}

)

and very similar to the

PMes* (5,

experimentally observed value for Mes*P

=

610 cm^À1).¹²In addition, theoretical calculations for the

Bbt groups on the P=P double bonds in 1a and 1b are

harmonic vibrational frequency of HP=PH obtained at

intertwined with each other in the solid state to avoid

the MP2 (606.4 cm^À1), B3LYP (615.2 cm^À1) and QCSID

severe steric repulsion without elongation of the P=P

(626.4 cm^À1)/TZ(2d) [DZ(p) for H] levels^6aalso support

double-bond length. In solution, however, the ³¹P NMR

1

the assignment of 1a and 1b. Thus, the P

character in 1a and 1b in the solid state is supported by

not only their P P bond lengths but also the vibrational

information on their P P bonds.

=P double-bond

signals for the two phosphorus atoms and the H NMR

signals for the two Tbt and Bbt groups are observed

equivalently in both 1a and 1b. These NMR spectro-

scopic features of 1a and 1b suggest that there is no

restricted rotation of the substituents in solution.

Accordingly, in solution the severe steric repulsion of

Tbt or Bbt groups in 1a and 1b might slightly elongate the

=

The solutions of diphosphenes 1a and 1b in common

organic solvents (e.g. hexane, benzene, chloroform) are

red. Diphosphenes have two absorption maxima reason-

ably assigned to the symmetry-allowed p → p* electron

transitions and the symmetry-forbidden n → p* electron

transitions, which appear at wavelengths longer than

300 nm. In Table 2 are shown the absorption maxima and

molar extinction coefﬁcients in the UV/visible spectra of

diphosphenes 1a and 1b together with those for typical

examples of the homonuclear double-bond compounds

between heavier group 15 elements. The red solutions of

1a and 1b in hexane showed two absorption maxima,

which correspond to the p → p* transitions [405 nm

(ꢀ = 13000) for 1a and 418 nm (ꢀ = 12000) for 1b] and the

P=P double bonds compared with those of diphosphenes

5 and 6, and the overlapping of p-orbitals of the

phosphorus atoms might be decreased. Therefore, their

absorption maxima for the p → p* and n → p* transi-

tions were observed in the longer wavelength region in

solution, probably owing to the more weakened p bonds

between phosphorus atoms than those of 5 and 6. In

addition, the absorption maximum for the p → p*

transition of Bbt-substituted diphosphene 1b was slightly

red shifted compared with the Tbt-substituted compound

(1a). The bathochromic effect here observed might

Copyright  2003 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2003; 16: 450–462

454

T. SASAMORI, N. TAKEDA AND N. TOKITOH

Scheme 2.

Mes*P=PMes* (5) with elemental selenium activated

Figure 4. Absorption maxima for (A) TbtE

BbtE=EBbt

=ETbt and (B)

by triethylamine leading to the formation of the

corresponding selenadiphosphirane derivative 18 and

diselenoxophosphorane derivative 19 (Scheme 2).¹⁷

When diphosphene 1b was treated with gray selenium

in benzene-d₆at 120°C for 120 h in a sealed tube, a green

crude solution was obtained, indicating the formation of

the corresponding diselenoxophosphorane 20 (Scheme

3). The ³¹P and ⁷⁷Se NMR spectra of the crude mixture

indicate the formation of the selenadiphosphirane

indicate a slightly longer P=P double bond of 1b than

that of 1a in solution.

Since the UV/visible spectra of these double-bond

systems are sensitive to the substituents on the central

atoms, it is interesting to compare the absorption maxima

for the p → p* and n → p* transitions among a series of

1

derivative 21 (ꢁ_p= À55.3, ꢁ_Se= 14.6, J_SeP= 126 Hz)

the RE

=ER (E = P, Sb, Bi) systems having the same

and diselenoxophosphorane 20 (ꢁ_p= 260.6, ꢁ_Se= 1371.3,

¹J_SeP= 854 Hz), judging from their characteristic chemi-

cal shifts, coupling patterns and coupling constants,

which were similar to those of 18 and 19. Diselenoxo-

phosphorane 20 could not be isolated owing to its ex-

tremely high sensitivity to light as well as 19. On the

other hand, the selenadiphosphirane 21 is stable under

ambient conditions, and 21 was easily isolated and

puriﬁed as orange crystals in 57% yield by HPLC

separation. Counting backwards from the isolated yield

of 21 (57%), the yield of 20 could be estimated to be 12%

substituents. In Fig. 4 are shown the p → p* (squares)

and n → p* (circles) transitions for the series of Tbt- and

Bbt-substituted RE=ER systems (R = Tbt and Bbt,

E = P, Sb, and Bi), although the corresponding diarsenes,

TbtAs AsTbt and BbtAs AsBbt, are missing. In both

=

series of Tbt and Bbt derivatives, it can be seen that their

p → p* and n → p* transitions shift to the longer

wavelength region as the element row is descended.

These trends for the p → p* and n → p* transitions

indicate the weakening of the p bond as the principal

quantum number is increased, and it is consistent with the

results of calculations [HF/DZ(d,p)] on the frontier

1

on the basis of a comparison of the integrals of the H

NMR signals with those of 21 in the crude mixture.

Hence it was found that the diselenoxophosphorane 20

and selenadiphosphirane 21 were formed by the reaction

of the diphosphene 1b with elemental selenium as in the

case of 5. However, it was not necessary to use

triethylamine for the activation of gray selenium in this

case.

orbital energy gaps of HE=EH (E = P, As, Sb, and

Bi).¹⁶In addition, similar trends are also observed for the

absorption maxima of 2,6-Mes₂-C₆H₃and 2,6-Tip₂-C₆H₃

derivatives.^7c

Reactions of Tbt- and Bbt-substituted diphos-

phenes

Next, we examined the reaction of 1b with elemental

sulfur. Yoshifuji and co-workers have already reported

the reactions of Mes*P=PMes* (5) with elemental sulfur

activated by triethylamine leading to the formation of a

Kinetic stabilization (in other words, steric protection by

bulky substituents) has some problems caused by the

congestion of the substituents, e.g. difﬁculty in the

preparation of precursors of the desired substances and/or

low intermolecular reactivity of the stabilized compound.

In order to elucidate the reactivities of the diphosphene

1a and 1b having extremely bulky Tbt and Bbt groups,

their reactions with elemental sulfur and selenium were

examined. We examined the reaction of 1b with

elemental selenium ﬁrst because ⁷⁷Se NMR spectro-

scopic data would be helpful in analyses of the reaction

products.

Scheme 3.

Yoshifuji et al. have reported the reaction of

Copyright  2003 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2003; 16: 450–462

DIPHOSPHENES BEARING EXTREMELY BULKY SUBSTITUENTS

455

Scheme 4.

Scheme 6.

yellow intermediate 22, which underwent ready thermal

or photochemical isomerization to give thiadiphosphir-

ane 23 (Scheme 4).¹⁸When a benzene-d₆solution of 1b

and elemental sulfur (S₈, 5 equiv. as S) was heated at

120°C in a sealed tube for 120 h, the resulting red

solution showed characteristic signals at 529.1, 286.7

and À71.3 ppm by ³¹P NMR spectroscopy. The observa-

tion of these three signals indicates the presence of

remaining diphosphene 1b (529.1 ppm) and the forma-

tion of dithioxophosphorane 24 (286.7 ppm) and thiadi-

phosphirane 25 (À71.3 ppm), as inferred from analogy

with the reaction of 1b with elemental selenium (Scheme

5). Furthermore, when the solution was heated at 120°C

for 42 h, the ¹H NMR spectrum showed the peaks for 1b,

24, and 25 with the ratio of 6:4:5 and the reaction did not

go to completion. Thus, the sulfurization reaction of

diphosphene 1b proceeded more slowly than the

selenization reaction of 1b. In contrast to the sulfurization

protecting groups to avoid the oligomerization of the

reactive P P bonds.

=

The synthesis and isolation of Bbt-substituted thia- and

selenadiphosphirane derivatives by the reactions of 1b

with elemental sulfur and selenium naturally prompted us

to examine the reaction of 1b with elemental tellurium to

synthesize the heavier analogue, i.e. telluradiphosphirane

derivative 27, since there is only one example of

telluradiphosphirane derivatives¹⁹and no example of a

telluradiphosphirane derivative has been structurally

characterized. However, no change was observed by

1

³¹P and H NMR spectroscopy when a C₆D₆suspension

of 1b and elemental tellurium (5 equiv.) was heated even

at 150°C for 1 week in a sealed tube. The use of

Bu₃P=Te (2 equiv.) instead of elemental tellurium at

120°C for 1 week also resulted in the quantitative

recovery of 1b.

reaction of Mes*P

derivative 26 [BbtP(S)

=

PMes* (5), diphosphene-1-sulﬁde

PBbt] was not observed in this

Yoshifuji et al. reported that the phosphinidene 28 was

generated as an intermediate in the photolysis (100 W,

À78°C, THF) of 5, where 28 underwent immediate

intramolecular cyclization to give the corresponding

phosophaindane derivative 29 (Scheme 6).²⁰The inter-

mediate 28 could not be trapped intermolecularly by

MeOH, probably owing to the very fast intramolecular

cyclization of 28 and the extreme congestion around the

central phosphorus atom surrounded by the test-butyl

groups at ortho-positions. On the other hand, a Tbt or Bbt

group has enough space around the 1-position to avoid

intramolecular cyclization, in contrast to the Mes* group.

Indeed, the Tbt-substituted stibinidene (30; Tbt–Sb:),

which is a heavier congener of phosphinidene 31, was

successfully trapped by 2,3-dimethyl-1,3-butadiene via

[1 4] cycloaddition without any intramolecular cycli-

zation.²¹Taking into consideration of the features of 1a

and 1b in solution, we examined the dissociation reaction

of the diphosphene 1a having Tbt groups leading to the

formation of the corresponding phosphinidene derivative

31.

=

reaction. On the other hand, the reaction of 1b with an

excess amount of elemental sulfur (S₈, 10 equiv. as S) in

the presence of triethylamine (10 equiv.) at 120°C in

benzene-d₆for 40 h resulted in the formation of

thiadiphosphirane 25 as a main product. Since 1b

completely disappeared under the reaction conditions,

25 could be isolated by HPLC puriﬁcation as air- and

moisture-stable orange crystals in 88% yield. Here, it was

found that triethylamine would promote the sulfurization

reaction of diphosphene 1b.

Taking into account of these results, it can be

concluded that diphosphene 1b has enough reaction

space around the central P=P bond, although the Bbt

groups are extremely bulky and very effective steric

Although the thermolysis (85°C, 30 h) and photolysis

(100 W, 3 h) of a C₆D₆solution of 1a in the presence of

2,3-dimethyl-1,3-butadiene (10 equiv.) was examined in

expectation of trapping the intermediary phosphinidene

31, no phospholene derivative 32, i.e. the expected

[1 4] cycloadduct of 31 with 2,3-dimethyl-1,3-buta-

diene, was observed and diphosphene 1a was completely

Scheme 5.

Copyright  2003 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2003; 16: 450–462

456

T. SASAMORI, N. TAKEDA AND N. TOKITOH

Chart 3.

Scheme 7.

because the Bbt group has very little thermodynamic

inﬂuence on the central skeleton. Fortunately, single

crystals of [25Á0.5hexane] and [21Á0.5hexane] were

obtained by recrystallization from hexane at 0°C, and

their structures were determined by x-ray crystallo-

graphic analysis. In Figures 5–8 are shown the ORTEP

drawings and selected bond lengths and angles of the

thiadiphosphirane 25 and the selenadiphosphirane 21. In

both cases the two Bbt groups are oriented in a trans

conﬁguration with respect to the central chalcogenadi-

phosphirane skeleton.

recovered (Scheme 7). However, the generation of

phosphinidene 31 in these thermolysis and photolysis

reactions is not completely unlikely because the retro

[1 4] reaction of 32 might occur under these conditions

or the trapping reaction of 31 with 2,3-dimethyl-1,3-

butadiene might be much slower than the dimerization

reaction of 31 giving the starting material 1a. Therefore,

no conclusive result on the generation of the phosphini-

dene was obtained in these thermolysis and photolysis

reactions. Since it was conceivable that 29 was produced

not via phosphinidene 28 but via some other radical

intermediates and there has been no report on the

intermolecular trapping reaction of a phosphinidene

generated from a diphosphene so far, it is not clear

whether a phosphinidene is generated by the thermal or

Concerning the thia- and selenadisiliranes 35²⁴and

36,²⁵having a three-membered ring with two silicon and

one chalcogen (sulfur or selenium) atoms, a resonance

structure model (A and B) between a three-membered

ring (35a and 36a) and a p complex (35b and 36b) has

been proposed to account for their crystal structures (see

Chart 3).²⁶

photochemical cleavage of the P=P double bond of a

diphosphene.

Structures of Bbt-substituted thia- and selenadi-

phosphiranes

Although the syntheses of several types of thiadipho-

sphiranes have been reported,²²there is only one example

of structural characterization.^22aTherefore, the x-ray

crystallographic analysis of the newly obtained thiadi-

phosphirane 25 is interesting in view of the structural

analysis of thiadiphosphirane derivatives. On the other

hand, there have been two structurally characterized

selenadiphosphirane derivatives, 33^23aand 34.^23bHow-

ever, their three-membered ring skeletons are consider-

ably affected by the electronic perturbation due to their

peculiar substituents such as Cp* and R_f(Chart 2). From

this point of view, the structure of 21 is interesting

Figure 5. ORTEP drawing of the major part of thiadipho-

aphirane 25 with thermal ellipsoid plot (50% probability).

The fragment of solvated hexane was omitted for clarity

Chart 2.

Copyright  2003 John Wiley & Sons, Ltd.

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DIPHOSPHENES BEARING EXTREMELY BULKY SUBSTITUENTS

457

phosphorus and selenium (P—Se single bond, 2.27

26

˚

A ), respectively. Thus, it was found that thia- and

selenadiphosphiranes 25 and 21 have a three-membered

ring which consists of reasonable single bonds and almost

no character of a p complex, in contrast to the case of

chalcogenadisiliranes 35 and 36. Since the previously

reported chalcogenadiphosphiranes 23, 33 and 34 have

similar features to those of Bbt-substituted compounds in

the geometries of their three-membered rings, these

central three-membered ring skeletons may be little

affected by substituents. In addition, the structural

optimization at the B3LYP/6–311G(2d,p) level of 2,3-

dimethyl- and 2,3-diphenylchalcogenadiphosphirane

models 37–40 revealed that the structural parameters

for their thia- and selenadiphosphirane skeletons are very

similar to those obtained experimentally for 25 and 21,

respectively (Table 3), suggesting the reliable single

bond character of the chalcogenadiphosphiranes.

Figure 6. Selected bond lengths and bond angles of 25

(major part)

On the other hand, the bond lengths of the P—P bond

˚

[2.2349(12) A] and the P—S bond [2.1083(13) A and

2.1285(12) A] in the thiadiphosphirane 25 are almost

comparable to the corresponding typical single-bond

Summary

8,9

˚

lengths [P—P single bond, ca 2.2–2.3 A; P—S single

26

˚

bond, 2.14 A (sum of covalent radii) ]. In the case of

New stable diphosphenes having Tbt and Bbt groups, 1a

and 1b, were synthesized and their structures were

deﬁnitively determined by x-ray crystallographic analy-

sis. It was found that the extremely overcrowded

diphosphenes 1a and 1b have interesting structures with

selenadiphosphirane 21, the lengths of the P—P bond

˚

[fragment A, 2.250(3); fragment B, 2.239(4) A] and the

˚

P—Se bond [fragment A, 2.250(3) and 2.270(3) A;

˚

fragment B, 2.257(4) and 2.259 (4) A] are within the

range of the typical P—P single bond lengths (ca 2.2–2.3

intertwined substituents to preserve the central P

=P

˚

A) and the sum of single-bond covalent radii for

double bond lengths in the solid state. In solution,

Figure 7. ORTEP drawing of selenadiphoaphirane 21 with thermal ellipsoid plot (50% probability). The fragment of solvated

hexane was omitted for clarity

Copyright  2003 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2003; 16: 450–462

458

T. SASAMORI, N. TAKEDA AND N. TOKITOH

Table 3. Theoretically optimized structural parameters for

thia- and selenadiphosphiranes [B3LYP/6±311G(2d,p)]^a

˚

Bond lengths (A)

Bond angles (°)

P—P

P—Ch P—Ch—P Ch—P—P

Ch = S

R = Me 37 2.221

R = Ph 38 2.244

2.162

2.155

61.8

62.8

59.1

58.6

Ch = Se

R = Me 39 2.222

R = Ph 40 2.245

2.311

2.306

57.5

58.3

61.3

60.9

a

These models have a C2 rotational axis passing through the chalcogen

atom and the midpoint of the P—P bond.

solvents were dried by standard methods and freshly

distilled prior to use. ¹H NMR (400 or 300 MHz) and ¹³

C

NMR (100 or 75 MHz) spectra were measured in CDCl₃

or C₆D₆with a JEOL JNM AL-400 or JEOL JNM AL-

300 spectrometer. Signals due to CHCl₃(7.25 ppm) and

1

C₆D₅H (7.15 ppm) were used as references in H NMR,

and those due to CDCl₃(77 ppm) and C₆D₆(128 ppm)

were used in ¹³C NMR. Multiplicity of signals in ¹³C

NMR spectra was determined by the DEPT technique.

³¹P NMR (121 MHz) and ⁷⁷Se NMR (57 MHz) spectra

were measured in CDCl₃or C₆D₆with a JEOL AL-300

spectrometer using 85% H₃PO₄in water (0 ppm) and

diphenyl diselenide (460 ppm) as an external standard,

respectively. High- and low-resolution mass spectral data

were obtained on a JEOL JMS-700 spectrometer. HPLC

(high-performance liquid chromatography) was per-

formed on an LC-908 or LC-918 instrument (Japan

Analytical Industry) equipped with JAIGEL 1H and 2H

columns (eluent: chloroform or toluene). Preparative

thin-layer chromatography (PTLC) was performed with

Merck Kieselgel 60 PF254. Electronic spectra were

recorded on a JASCO V530 UV/visible or JASCO Ubest-

50 UV/visible spectrometer. Raman spectra were

measured at room temperature on a Raman spectrometer

consisting of a Spex 1877 Triplemate and an EG&G

PARC 1421 intensiﬁed photodiode-array detector. An

NEC GLG 108 He–Ne laser (632.8 or 1064 nm) was used

for Raman excitation. All melting-points were deter-

mined on a Yanaco micro melting-point apparatus and

are uncorrected. Elemental analyses were carried out at

the Microanalytical Laboratory of the Institute for

Chemical Research, Kyoto University. TbtBr²⁸and

BbtBr²⁹were prepared according to the reported

procedures.

Figure 8. Selected bond lengths and bond angles of 21

however, the NMR and UV/visible spectral data implied

that the entanglements between the substituents were

solved and the severe steric repulsion of Tbt or Bbt

groups of 1a or 1b might slightly elongate and weaken

the P=P double bond.

On the other hand, it was found that 1a and 1b can

react with elemental sulfur and selenium to give thia- and

selenadiphosphiranes, respectively, in spite of the

extreme bulkiness of Tbt and Bbt groups. While Tbt

and Bbt groups are very effective steric protecting groups

to prevent the oligomerization of reactive chemical

bonds, they still have sufﬁcient reaction space around

the 1-position for reactions with some small molecules.

The concept of kinetic stabilization with Tbt and Bbt

groups should certainly be of great use for the construc-

tion and elucidation of the physical and chemical

properties of unprecedented, reactive chemical bonds.²⁷

Furthermore, cooperative theoretical veriﬁcation of the

experimental outcome will become very important for

elucidating the intrinsic nature of new chemical bondings

between heavier main group elements.

Preparation of {2,4,6-tris[bis(trimethylsilyl)methyl]phe-

nyl}dichlorophosphine (4a). To a solution of TbtBr

(3.16 g, 5.00 mmol) in THF (45 ml) was added tert-

butyllithium (2.24 M pentane solution, 4.9 ml,

11.0 mmol) at À 78°C. After 30 min, PCl₃(0.70 ml,

8.0 mmol) was added. The reaction mixture was warmed

to room temperature and stirred for 17 h. The solvents

EXPERIMENTAL

General procedure. All experiments were performed

under an argon atmosphere unless stated otherwise. All

Copyright  2003 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2003; 16: 450–462

DIPHOSPHENES BEARING EXTREMELY BULKY SUBSTITUENTS

459

were evaporated under reduced pressure and 100 ml of

hexane were added to the residue. Insoluble inorganic

salts were removed by ﬁltration through Celite. After

removal of the solvent from the ﬁltrate, the residue was

reprecipitated from CH₂Cl₂–ethanol to afford TbtPCl₂

(4a, 2.65 g, 81%). 4a: colorless powder, m.p. 172.7–

174.9°C (decomp.). Found: C, 49.64; H, 9.11%. Calcd

for C₂₇H₅₉Si₆PCl₂: C, 49.57; H, 9.09%. ¹H NMR

(400 MHz, CDCl₃), ꢁ = 0.07 (s, 36H), 0.09 (s, 18H),

174.8–176.2°C (decomp.). Found: C, 49.50; H, 9.42%.

1

Calcd for C₃₀H₆₇Si₇PCl₂: C, 49.61; H, 9.30%. H NMR

(400 MHz, CDCl₃), ꢁ = 0.09 (s, 36H), 0.27 (s, 27H), 3.17

4

(d, ⁴J_PH= 11.6 Hz, 2H), 6.78 (d, J_PH= 4.6 Hz, 2H). ¹³

C

{¹H} NMR (100 MHz, CDCl₃), ꢁ = 1.70 (CH₃), 5.60

(CH₃), 23.68, 27.73 (d, J_PC= 25.5 Hz, CH), 127.83

3

1

(CH), 129.22 (d, J_PC= 67.5 Hz), 151.16, 151.30 (d,

²J_PC= 26.3 Hz). ³¹P NMR (121 MHz, CDCl₃), ꢁ = 162.6.

HRMS (FAB), found: m/z 725.2814 ([M H] ). Calcd

4

1.40 (s, 1H), 2.98 (d, J_PH= 10.7 Hz, 1H), 3.07 (d,

for C₃₀H₆₈³⁵Cl₂Si₇P ([M H] ): 725.2821.

⁴J_PH= 8.7 Hz, 1H), 6.33 (d, ⁴J_PH= 4.6 Hz, 1H), 6.47 (brs,

1H). ¹³C {¹H} NMR (100 MHz, CDCl₃), ꢁ = 0.77

Synthesis of bis{2,6-bis[bis(trimethylsilyl)methyl]-4-

[tris(trimethylsilyl)methyl]phenyl}diphosphene (1b). To

a THF solution (3 ml) of BbtPCl₂(4b) (726.3 mg,

1.00 mmol) was added Mg metal (243.1 mg, 10.0 mmol)

at room temperature. After the reaction mixture had been

stirred for 10 min, the solution turned deep red and then

deep violet. The solvent was evaporated under reduced

pressure and 100 ml of hexane were added to the residue.

Insoluble inorganic salts were removed by ﬁltration

through Celite. The removal of the solvent from the

ﬁltrate gave red crystals of diphosphene 1b (700.6 mg,

quant.). 1b: red crystals, m.p. >350°C. Found: C, 54.37;

H, 10.48%. Calcd for C₆₀H₁₃₄Si₁₄P₂: C, 54.98; H,

3

(CH₃Â 2), 1.18 (CH₃), 26.81 (d, J_PC= 18.1 Hz, CH),

27.04 (d, ³J_PC= 26.4 Hz, CH), 31.64 (CH), 123.20 (CH),

127.43 (d, ¹J_PC= 67.6 Hz), 127.81 (CH), 149.57, 151.72

(d, ²J_PC= 33.8 Hz), 152.35 (d, ²J_PC= 23.9 Hz). ³¹P NMR

(121 MHz, C₆D₆), ꢁ = 164.2. HRMS (FAB), found: m/z

654.2324 ([M] ). Calcd for C₂₇H₅₉³⁵Cl³⁷ClSi₆P ([M] ):

654.2318.

Synthesis of bis{2,4,6-tris[bis(trimethylsilyl)methyl]phe-

nyl}diphosphene (1a). To a THF solution (3 ml) of

TbtPCl₂(4a) (124.3 mg, 0.19 mmol) was added Mg

metal (54.6 mg, 2.24 mmol) at room temperature. After

the reaction mixture had been stirred for 30 min, the color

of the solution changed to deep red and then deep violet.

The solvent was evaporated under reduced pressure and

30 ml of hexane were added to the residue. Insoluble

inorganic salts were removed by ﬁltration through Celite

to give a red solution. Removal of the solvent from the

ﬁltrate followed by separation with GPLC (toluene)

afforded diphosphene 1a (106.7 mg, 96%). 1a: red solid,

m.p. 202.7–204.5°C (decomp.). Found: C, 56.94; H,

10.06%. Calcd for C₅₄H₁₁₈Si₁₂P₂Á0.5C₆H₁₄: C, 56.74; H,

1

10.30%. H NMR (300 MHz, C₆D₆), ꢁ = 0.31 (s, 72H),

0.40 (s, 54H), 3.49 (brs, 4H), 7.11 (s, 4H). ¹³C NMR

(75 MHz, C₆D₆), ꢁ = 2.12 (q), 5.70 (q), 22.81 (s), 32.08

(d), 127.61 (d), 139.57 (s), 147.70 (s), 148.53 (s). ³¹P

NMR (121 MHz, C₆D₆), ꢁ = 529.0. FT-Raman (Nd:YAG

laser, 1064 nm), 603 cm^À1(ꢂ_{P P}). UV/visible (hexane),

=

l_max532 nm (" 1000), 418 nm (12 000). HRMS (FAB),

found: m/z 1308.6736 ([M] ). Calcd for C₆₀H₁₃₄Si₁₄P₂

([M] ): 1308.6731.

1

10.19%. H NMR (300 MHz, C₆D₆), ꢁ = 0.21 (s, 36H),

Reaction of diphosphene 1b with elemental selenium.

0.27 (s, 72H), 1.55 (s, 2H), 3.22 (brs, 2H), 3.28 (brs, 2H),

6.74 (brs, 2H), 6.82 (brs, 2H). ³¹P NMR (121 MHz,

C₆D₆), ꢁ = 531.8. FT-Raman (Nd:YAG laser, 1064 nm),

A benzene-d₆suspension (0.7 ml) of BbtP=PBbt (1b)

(130.8 mg, 0.10 mmol) and elemental selenium (10.8 mg,

0.13 mmol) was degassed and sealed in an NMR tube.

When the suspension had been heated at 120°C for 120 h,

the signals of 1b disappeared and the signals which

correspond to diselenoxophosphorane 20 and selenadi-

phosphirane 21 were observed with a ratio of 2:9 in the

¹H NMR spectrum. The reaction mixture was separated

by HPLC (toluene) to give 77.4 mg of 2,3-bis{2,6-

bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)-

methyl]phenyl}selenadiphosphirane (21, 57%) and

18.5 mg of BbtH (30%). Pure orange crystals of

[21Á0.5hexane] were obtained by slow recrystallization

from hexane in a refrigerator at 0°C. 21: orange crystals,

m.p. 244.6–245.8°C (decomp.). Found: C, 52.84; H,

9.65%. Calcd for C₆₀H₁₃₄Si₁₄P₂SeÁ0.5C₆H₁₄: C, 52.81;

H, 9.92%. ¹H NMR (400 MHz, C₆D₆), ꢁ = 0.33 (s, 54H),

0.36 (s, 36H), 0.41 (s, 36H), 3.29–3.32 (m, 4H), 6.85 (s,

4H). ¹³C {¹H} NMR (100 MHz, C₆D₆, 50°C), ꢁ = 2.82

(CH₃), 2.85 (CH₃), 6.07 (CH₃), 22.81, 31.0–31.2 (m,

CH), 128.24 (CH), 133.3–134.3 (m), 146.29, 149.4–

149.6 (m). ³¹P NMR (121 MHz, C₆D₆), ꢁ = À55.3. ⁷⁷Se

609 cm^À1(ꢂ_{P P}). UV/vis (hexane), l_max530 nm ("

=

2000), 405 nm (13 000). LRMS (FAB), found: m/z 1166

([M H] ). Calcd for C₅₄H₁₁₉Si₁₂P₂([M H] ): 1166.

Preparation of {2,6-bis[bis(trimethylsilyl)methyl]-4-

[tris(trimethylsilyl)methyl]phenyl}dichlorophosphine

(4b). To a solution of BbtBr (5.63 g, 8.00 mmol) in THF

(80 ml) was added tert-butyllithium (2.3 M pentane

solution, 7.8 ml, 17.6 mmol) at À 78°C. After 30 min,

PCl₃(1.1 ml, 12.0 mmol) was added. After the reaction

mixture had been stirred at À 78°C for 3 h, LiCl (3.42 g,

80.7 mmol) was added. The reaction mixture was

warmed to room temperature and stirred for 13 h. The

solvents were evaporated under reduced pressure and

100 ml of hexane were added to the residue. Insoluble

inorganic salts were removed by ﬁltration through Celite.

After removal of the solvent from the ﬁltrate, the residue

was reprecipitated from CH₂Cl₂–ethanol to afford

BbtPCl₂(4b, 4.02 g, 69%). 4b: colorless powder, m.p.

J. Phys. Org. Chem. 2003; 16: 450–462

460

T. SASAMORI, N. TAKEDA AND N. TOKITOH

1

NMR (57 MHz, C₆D₆), ꢁ = 14.6 (t, J_SeP= 126 Hz).

Reaction of diphosphene 1b with Bu₃P

=

Te. A

PBbt (1b)

=Te (67.5 mg,

LRMS (FAB), found: m/z 1389 ([M] ). Calcd for

benzene-d₆suspension (0.7 ml) of BbtP

=

C₆₀H₁₃₄Si₁₄P₂Se ([M] ): 1389. 20: ¹H NMR

(134.2 mg, 0.10 mmol) and Bu₃P

(300 MHz, C₆D₆), ꢁ = 0.25 (s, 18H), 0.30 (s, 18H), 0.32

(s, 27H), 3.02 (d, ⁴J_HP= 4 Hz, 2H), 6.91 (d, ⁴J_HP= 6 Hz,

2H). ³¹P NMR (121 MHz, C₆D₆), ꢁ = 260.6. ⁷⁷Se NMR

(57 MHz, C₆D₆), ꢁ = 1371.3 (d, ¹J_SeP= 854 Hz).

0.20 mmol) was degassed and sealed in an NMR tube.

When the suspension had been heated at 80°C for 140 h,

monitoring by ¹H NMR spectroscopy showed no change.

After the additional heating at 120°C for 1 week, no

1

change was observed in the H NMR spectrum of the

Reaction of diphosphene 1b with elemental sulfur. A

suspension.

benzene-d₆suspension (0.7 ml) of BbtP=PBbt (1b)

(68.2 mg, 0.05 mmol) and elemental sulfur (S₈, 7.5 mg,

0.23 mmol as S) was degassed and sealed in an NMR

tube. When the suspension had been heated at 120°C for

120 h, the signals which correspond to diphosphene 1b,

dithioxophosphorane 24 and thiadiphosphirane 25 were

Thermolysis and photolysis of diphosphene 1a in the

presence of 2,3-dimethyl-1,3-butadiene. A benzene-d₆

solution (0.7 ml) of TbtP=PTbt (1a) (61.2 mg,

0.05 mmol) and 2,3-dimethyl-1,3-butadiene (0.06 ml,

0.53 mmol) was degassed and sealed in an NMR tube.

When the solution had been heated at 85°C for 30 h, no

1

observed with a ratio of 6:4:5 in the H NMR spectrum.

1

The reaction mixture was separated by PTLC (hexane) to

give 54.6 mg of a 6:5 mixture of diphosphene 1b (43%)

and 2,3-bis{2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(tri-

methylsilyl)methyl]phenyl}thiadiphosphirane (25, 36%)

and 29.4 mg of BbtH. 25: orange crystals, m.p. 258.2–

262.5°C (decomp.). Found: C, 53.58; H, 10.18%. Calcd

for C₆₀H₁₃₄Si₁₄P₂S: C, 53.66; H, 10.66%. ¹H NMR

(300 MHz, C₆D₆), ꢁ = 0.32 (s, 54H), 0.33 (s, 36H), 0.37

(s, 36H), 3.15–3.18 (m, 4H), 6.83 (s, 4H). ¹³C {¹H} NMR

(75 MHz, C₆D₆, 50°C), ꢁ = 2.20 (CH₃), 2.44 (CH₃), 5.74

(CH₃), 22.60, 30.2–30.4 (m, CH), 128.16 (CH), 133.2–

134.3 (m), 146.50, 149.5–149.7 (m). ³¹P NMR

(121 MHz, C₆D₆), ꢁ = À71.3. HRMS (FAB), found: m/z

change was observed in the H NMR spectrum. The

sealed tube was further irradiated with a medium-

pressure mercury lamp (100 W) at room temperature

for 3 h, but no exchange was observed.

X-ray crystallographic analyses of [1aÁbenzene-d₆],

[1bÁhexane], [21Á0.5hexane] and [25Á0.5hexane].

Crystal data for [1aÁbenzene-d₆], [1bÁhexane],

[21Á0.5hexane], [25Á0.5hexane] are summarized in Table

4 and have been deposited at the Cambridge Crystal-

lographic Data Centre as supplementary publication No.

CCDC 194236-194239. The intensity data were collected

on a Rigaku R-AXIS RAPID imaging plate detector with

graphite monochromated Mo Ka radiation (l = 0.71069

1341.6525 ([M H] ). Calcd for C₆₀H₁₃₅Si₁₄P₂S

([M H] ): 1341.6530. 24: ¹H NMR (300 MHz,

A) at À 180°C to 2ꢃ_max= 50° (55° for [25Á0.5hexane]).

˚

C₆D₆), ꢁ = 0.24 (s, 18H), 0.29 (s, 18H), 0.30 (s, 27H),

The structure was solved by a direct method (SIR-97)³⁰

or (SHELXS-97)³¹and reﬁned by full-matrix least-

squares procedures on F²for all reﬂections (SHELXL-

97).³¹

2.61 (d, J_HP= 4 Hz, 2H), 6.94 (d, J_HP= 6 Hz, 2H). ³¹P

4

NMR (121 MHz, C₆D₆), ꢁ = 286.7.

Reaction of diphosphene 1b with elemental sulfur in

the presence of triethylamine. A benzene-d₆suspension

(0.7 ml) of BbtP=PBbt (1b) (130.8 mg, 0.10 mmol),

Single crystals of [1aÁbenzene-d₆] suitable for x-ray

analysis were obtained by slow recrystallization from

C₆D₆in a sealed tube at room temperature. A deep-red,

prismatic crystal was mounted on a glass ﬁber. The

hexagonal skeleton of the solvated benzene was re-

strained using AFIX instructions. Two trimethylsilyl

groups of the CH(SiMe₃)₂group at the para position to

one of the Tbt groups were disordered. The occupancies

of methyl groups were reﬁned (0.70:0.30 and 0.62:0.38).

All hydrogen atoms were placed using AFIX instructions,

and all the other atoms were reﬁned anisotropically.

Single crystals of [1bÁhexane] suitable for x-ray

analysis were obtained by slow recrystallization from

hexane in a refrigerator at À40°C ﬁxed in a glove-box

ﬁlled with argon. A deep-red, prismatic crystal was

mounted on a glass ﬁber. All trimethylsilyl groups of the

C(SiMe₃)₃groups at the para position to the two Bbt

groups were disordered. The occupancies of trimethyl-

silyl groups were reﬁned (0.67:0.33 and 0.84:0.16). The

two Si—C bonds of the disordered trimethylsilyl groups

of the minor parts were restrained to have the same Si—C

distance using SADI instructions. The anisotropic

elemental sulfur (S₈, 32.1 mg, 1.00 mmol as S), and

triethylamine (0.15 ml, 1.0 mmol) was degassed and

sealed in an NMR tube. When the suspension had been

heated at 120°C for 40 h, the signals of 1b disappeared

and the signals corresponding to thiadiphosphirane 25

were observed in the ¹H NMR spectrum as a main

product. The reaction mixture was separated by HPLC

(toluene) to give 118.5 mg of thiadiphosphirane 25

(88%).

Reaction of diphosphene 1b with elemental tellurium.

A benzene-d₆suspension (0.7 ml) of BbtP=PBbt (1b)

(85.4 mg, 0.06 mmol) and elemental tellurium (41.6 mg,

0.32 mmol) was degassed and sealed in an NMR tube.

When the suspension had been heated at 120°C for 40 h

and monitored by ¹H NMR spectroscopy, no change was

observed in the ¹H NMR spectrum. Even after additional

1

heating at 150°C for 1 week, the H NMR spectrum of

the suspension showed no change.

J. Phys. Org. Chem. 2003; 16: 450–462

DIPHOSPHENES BEARING EXTREMELY BULKY SUBSTITUENTS

461

Table 4. Crystal data for [1aÁbenzene-d₆], [1bÁhexane], [21Á0.5hexane] and [25Á0.5hexane]

Parameter

[1aÁbenzene-d₆]

[1bÁhexane]

[21Á0.5hexane]

[25Á0.5hexane]

Formula

C₆₀H₁₂₄P₂Si₁₂

1244.61

C₆₆H₁₄₈P₂Si₁₄

1397.04

C₆₃H₁₄₁P₂SeSi₁₄

1432.92

C₆₃H₁₄₁SP₂Si₁₄

1386.02

Formula weight

Crystal color, habit

Crystal dimensions (mm)

Temperature (°C)

Crystal system

Deep red, prismatic

0.60 Â 0.30 Â 0.20

À180

Deep red, prismatic

0.60 Â 0.50 Â 0.40

À180

Orange, prismatic

0.30 Â 0.20 Â 0.10

À180

Orange, prismatic

0.50 Â 0.30 Â 0.20

À180

monoclinic

triclinic

Space group

P2₁/c (#14)

P2₁(#4)

P-1 (#2)

Lattice parameters:

˚

a (A)

11.9185(4)

25.5835(9)

27.0386(8)

90

100.3242(9)

90

8111.0(5)

4

1.019

13.4555(8)

21.8734(11)

30.4453(17)

90

96.400(3)

90

8904.7(9)

4

1.042

18.2260(15)

12.779(9)

36.468(3)

90

91.363(4)

90

12.9737(8)

17.7976(15)

19.0763(11)

86.809(10)

79.789(8)

87.278(10)

4325.1(5)

2

˚

b (A)

˚

c (A)

a (°)

b (°)

g (°)

3

˚

V (A )

8491.4(11)

4

Z

D_calc(g cm^À3

)

1.121

1.064

Independent reﬂections

No. of parameters

R₁[I > ꢄ(I)]

wR₂(all data)

GOF

13985

693

15514

853

27777

1426

18867

1299

0.087

0.096

0.094

0.068

0.246

0.231

0.196

0.179

0.929

1.103

1.08

1.089

temperature factors of four carbon atoms (in the minority

of disordered trimethylsilyl groups) were restrained not

to be non-positive values using ISOR instructions. All

hydrogen atoms were placed using AFIX instructions,

and all the other atoms were reﬁned anisotropically

except for one Si atom, which was in the minor part of the

disordered trimethyl silyl groups and reﬁned isotropi-

cally.

Single crystals of [21Á0.5hexane] suitable for x-ray

analysis were obtained by slow recrystallization from

hexane in a refrigerator at 0°C. An orange, prismatic

crystal of [21Á0.5hexane] was mounted on a glass ﬁber.

The selenadiphosphirane ring of the fragment B was

disordered. The occupancies of the disordered phos-

phorus and selenium atoms were reﬁned (0.86:0.14). The

minor part of the disordered selenadiphosphirane ring

was restrained to have the same structure as the major

part of the selenadiphosphirane ring using SAME

instructions. All hydrogen atoms were placed using

AFIX instructions. The disordered phosphorus and

selenium atoms of the minor part were reﬁned isotropi-

cally, and all the other non-hydrogen atoms were reﬁned

anisotropically.

instructions. All hydrogen atoms were placed using AFIX

instructions. Some carbon atoms and a silicon atom of the

disordered Bbt groups of the minor part were reﬁned

isotropically, and all the other non-hydrogen atoms were

reﬁned anisotropically.

Theoretical calculations. All theoretical calculations

were carried out using the Gaussian 98 program³²with

density functional theory at the B3LYP level.³³In

calculations for diphosphenes 1a, 1b, 1a' and 1b', the

triple zeta basis set ([3s3p])³⁴augmented by two sets of d

polarization functions for P (d exponents 0.537 and

0.153) was used with an effective core potential, and the

6–31G(d) (for Si) and the 3–21G (for C and H) basis sets

were used. The structural optimization of 1a, 1b, 1a' and

1b' were done by OPT keywords without force-constant

calculations. In calculations for chalcogenadiphosphir-

anes 37, 38, 39 and 40, the 6–311G(2d, p) basis set was

used for all atoms. Computation time was provided by the

Supercomputer Laboratory, Institute for Chemical Re-

search, Kyoto University.

Single crystals of [25Á0.5hexane] suitable for x-ray

analysis were obtained by slow recrystallization from

hexane in a refrigerator at 0°C. An orange, prismatic

crystal of [25Á0.5hexane] was mounted on a glass ﬁber.

The molecule of thiadiphosphirane 25 was disordered.

The occupancies of the disordered molecules were

reﬁned (0.74:0.26). The solvated hexane was disordered

and their occupancies were reﬁned (0.82:0.18). The

benzene rings of the Bbt groups of the minor part were

constrained to have right hexagonal structure using AFIX

Acknowledgements

This work was supported by a Grant-in-Aid for COE

Research on Elements Science (No. 12CE2005) and a

Grant-in-Aid for Scientiﬁc Research on Priority Areas

(A) (No. 11166250) from the Ministry of Education,

Culture, Sports, Science and Technology, Japan. We

thank Professor Y. Furukawa of Waseda University for

measuring the Raman spectra.

J. Phys. Org. Chem. 2003; 16: 450–462

462

T. SASAMORI, N. TAKEDA AND N. TOKITOH

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Article Doi

Synthesis and reactions of new diphosphenes bearing extremely bulky substituents

DOI: 10.1002/poc.608

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