Organometallics 1995, 14, 3141-3142
3141
4-Methyl-l,4=thiaborin,an Aromatic Boron-Sulfur Heterocycle Arthur J. Ashe, III,* Jeff W. Kampf, and Jack R. Waas Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109-1055 Received March 6, 1995@ Summary: The base-catalyzed hydrostannation of diethynyl sulfide with dibutylstannane affords4,4-dibutyl1,4-thiastannin, which reacts with methylboron dibromide to give I-methyl-l,4-thiaborin. The thiaborin has been converted to tricarbonyl(y6-4-methyl-l ,4-thiaborin)molybdenum, which has been characterized by singlecrystal X-ray diffraction. The boron-sulfur heterocycle 1,Cthiaborin (1)is a potentially six-n-electron aromatic compound. Comparison of 1 with the better known six-n-electron heterocycles borepin (2)l and thiophene (W should be intrinsically interesting. Although the synthesis of 4-butyl- and 4-bromo-2,6-diphenyl-l,44hiaborins (4a,b) was reported by Meller in 1982,3 the chemistry of thiaborins has been virtually unexplored. We report here on the synthesis of a C-unsubstituted thiaborin and on structural data which show that thiaborin can serve a s an @-aromatic ligand toward transition metals.
x 1
a,X=Me
x 2
3
4
a,X=Br b,X=Bu
The preparation of 1 relies on the general synthesis of boron heterocycles from diacetylenes via organotin intermediate^.^,^ The precursor diacetylene, diethynyl sulfide (6),5ais easily obtained by desilylation of bis((trimethylsilyUethyny1) sulfide6 (5) with Bu4NF in methanol. Since 6 is extremely labile, it is not isolated @Abstractpublished in Advance ACS Abstracts, June 15, 1995. (1) Ashe, A. J., 111; Klein, W.; Rousseau, R. Organometallics 1993, 12, 3225 and references cited therein. (2) (a) Gronowitz, S., Ed. Thiophene and Its Derivatives. In The Chemistry of Heterocyclic Compounds; Taylor, E. C., Weissberger, A., Eds.; Wiley: New York, 1986, 1987; Vol. 44. (b) Bird, C. W.; Cheeseman, G. W. H. In Comprehensive Heterocyclic Chemistry; Katritzsky, A. R., Rees, C. W., Eds.; Pergamon Press: Oxford, U.K., 1984; Vol. 4, pp 1-38. (3) Habben, C.; Maringgele, W.; Meller, A. 2.Naturforsch., E: Anorg. Chem. Org. Chem. 1982,37B, 43. (4) (a) Noltes, J. G.; van der Kerk, G. J. M. R e d . Trau. Chim. Pays.Bas 1962, 81, 41. Leusink, A. J.; Noltes, J. G.; Budding, H. A,; van der Kerk, G. J. M. Red. Trav. Chim. Pays-Bas 1964,83,1036. Leusink, A. J.; Drenth, W.; Noltes, J. G., van der Kerk, G. J . M. Tetrahedron Lett. 1967, 1263. (b)Ashe, A. J., 111; Shu, P. J.Am. Chem. Soc. 1971, 93, 1804. ( c ) Ashe, A. J., 111; Drone, F. J . Am. Chem. SOC.1987, 109, 1879; 1988, 110, 6599. Ashe, A. J., 111; Kampf, J. W.; Kausch, C. M.; Konishi, H.; Kristen, M. 0.;Kroker, J . Organometallics 1990,9,2944. (d) Sugihara, Y.; Yagi, T.; Murata, I.; Imamura, A. J.Am. Chem. Soc. 1992,314, 1479. (5)Also see: (a) Meijer, J.; Vermeer, P.; Verkruijsse, H. D.; Brandsma, L. Recl. Trav. Chim. Pays-Bas 1973,92,1326. (b)Schoufs, M.: Meiier, J.: Vermeer, P.; Brandsma, L. R e d . Trav. Chim. Pays-Bas 1974, 92, 244.
but is extracted with pentaneltoluene and the dried extracts are subject to base-catalyzed hydrostannation using dibutylstannane, powdered KOH, and 18-crown-6 ether at 25 "C.& The resulting 4,4-dibutyl-l,44hiastannin (7)is obtained as a stable distillable yellow oil in 20% yield from 5. The reaction of 7 with methylboron dibromide in pentane affords 4-methyl-l,6thiaborin (la)as an air-sensitive volatile oil in 57% yield.7 The most widely used criterion of aromaticity involves lH NMR spectroscopy. The protons of planar aromatic rings show low-field chemical shift values which are attributed to the presence of a diamagnetic ring current.8 By this measure l a seems to be aromatic. The chemical shift values for la (6 7.99 (Hz) and 7.40 (H3)) (6)Verboom, W.; Schoufs, M.; Meijer, J.; Verkruijsse, H. D.; Brandsma, L. R e d . Trav. Chim. Pays-Bas 1979,97, 245. (7) Experimental procedures and characterization of new compounds are as follows. (a) 4,4-Dibutyl-1,4-thiastannin (7): A solution of BudNF-V~HZO(12.6 g, 36.2 mmol) in 40 mL of methanol was added dropwise with stirring to a solution of bis((trimethylsily1)ethynyl) sulfide (5) (3.9 g, 17.2 mmol) in 80 mL of methanol under a nitrogen atmosphere. No discernible color change occurred. The mixture was then poured onto 150 mL of pentane and extracted twice with 200 mL of water. Then, 20 mL of toluene was added to the organic layer, which was thereafter dried over KzC03 and decanted. Careful concentration of the solution on a rotary evaporator (water bath temperature did not exceed 25 "C) gave 24.6 g of a light yellow solution. Analysis by 'H NMR spectroscopy indicated that the solution contained approximately 3.4%(830 mg, 59%yield) of diethynyl sulfide (6). lH NMR (C6H6, 300 MHz): b 3.22(s). To this solution was added dibutylstannane (2.62 g, 11.2 mmol), powdered KOH (285 mg, 5.07 mmol), and 18-crown-6 ether (134 mg, 0.51 mmol). The mixture was then stirred vigorouslv for 3 h under No. Removal of volatiles in vacuo and distillatiin (bp 68-70 W0.1 torr) gave 920 mg of 7; 20%yield based on 5. 'H NMR (C6D6, 200 MHz): d 6.66 (dd, J = 11.0, 2.0 Hz, Jl'SSnH = 96 Hz, 2H), 6.14 (dd, J = 11.0, 2.0 Hz, J l 1 9 S n H = 64 Hz, 2H), 1.450.86 (m, 18H). 13C NMR (CDC13, 50.3 MHz): d 137.2 ( J I I S = S ~9 C Hz), 122.4 (Jllus,~= 186 Hz), 28.7, 26.8, 13.6, 11.4. HRMS (EI, 70 eV): calcd for C12H22S lzoSn 318.0464, found 318.0477. Anal. Calcd for C12H22SSn: C, 45.46; H, 6.99. Found: C, 45.33, 45.29; H, 6.92, 6.81. (b) 4-Methyl-l,44hiaborin (la):Methylboron dibromide (132 mg, 710 ymol) was added dropwise with stirring to a solution of 7 (250 mg, 788 yL) in 1 mL of pentane at 25 "C. The solvent was removed under vacuum, and the product was pot-to-pot distilled at 25 "C under a high vacuum into a cold (-78 "C) receiver. The product (44 mg, 57%yield) was a colorless oil. 'H NMR (CDC13, 300 MHz): b 7.99 (d, J = 12.2 NMR (CDC13, Hz, 2H), 7.40 (dd, J = 2.5, 12.4 Hz, 2H), 0.82 (s,3H). 90.6 MHz): b 141.8, 136 (br),9 (br). IlB NMR (CDCls, 115.5 MHz): d 50.1. HRMS (EI, 70 eV): calcd for C5H711BS 110.0362, found 110.0363. ( c ) Tricarbonyl(p6-4-methyl-l,4-thiaborin)molybdenum ( 8 ) : As above, the reaction of 7 (330 mg, 1.04 mmol) and methylboron dibromide (193 mg, 1.04 mmol) in 0.5 mL of pentane afforded volatile products, which were taken up in 4 mL of ether. Tricarbonyltris(pyridine)molybdenum (520 mg, 1.25 mmol) was added with stirring at 25 "C, followed by BF3.OEt2 (0.64 mL, 5.2 mmol). The mixture was heated to reflux for 1 h, and after cooling to 25 "C volatile material was removed under high vacuum. The residue was extracted with pentane. After separation and removal of solvent, the product was recrystallized from hexane, yielding 124 mg (41%)of orange cubes, mp 116-119 "C. 'H NMR (C&, 300 MHz): 0 4.32 (d, J = 10.6 Hz, 2H), 4.01 (dd, J = 10.6, 1.6 Hz, 2H), 0.93 (s, 3H). I3C NMR (CeDs, 50.3 MHz): 201.0 (CO), 100.0, 95 (br), signal for the Me not observed. llB NMR (CsDs, 115.5 MHz): 22.3. IR (hexane): v(C0) 2002, 1936, 1931 cm-'. HRMS (EI, 70 eV): calcd for CaH,"BMo03S 291.9263; found 291.9268. Anal. Calcd for CaH7BMo03S: C, 33.14; H, 2.43. Found: C, 32.82, 32.96; H, 2.36, 2.28. (8)Garratt, P. J. Aromaticity. In Comprehensive Organic Chemistry; Barton, D., Ollis, W. D., Eds.; Pergamon Press: Oxford, U.K., 1979; Vol. 1, pp 215-240.
0276-7333/95/2314-3141$09.00/00 1995 American Chemical Society
Communications
3142 Organometallics, Vol. 14, No. 7, 1995
found in tricarbonyl(y5-2,5-dimethylthiophene)chromium (lO).'O The juxtaposition of the metal tricarbonyl group relative to the ring in 8 resembles that of 10, where one CO group is trans to sulfur,l0 and 9 , where a CO group eclipses boron'l This conformational feature has been found for other b ~ r ~ nand ~ ~ sul-J ~ l ~ ~ f ~ r ' ~heterocycles ,'~ and has been treated by MO studies1°J5.16which probably apply to 8.
s1
M$2 Me
,.Cr
02
Figure 1. Solid-state structure of 8 (ORTEP). Selected distances (A) and angles (deg): Mo-B, 2.523(4);Mo-C4, 2.339(4); Mo-C5, 2.414(3); Mo-C6, 2.426(4); Mo-Cv, 2.334(4); Mo-S, 2.512(1); B-C5, 1.520(6); B-C6, 1.523(4), B-C8, 1.578(7); C4-C5, 1.375(5); C6-C7, 1.369(6); C4-S, 1.754(3);C7-S, 1.750(4);C5-B-C6, 115.0(3);C4S-C7, 101.3(2). Scheme 1
' \SMe,
Me.$i
5
BU~SIIHL KOH 18-06
ether
0
MeBBra
Sn
I
Me
'Bu
Bu'
la
7
6
Scheme 2
1
Mo CO'," 'co
I
I
CO'' Q C'
co
co
9
10
It is striking that all corresponding ring bonds of 8-10 are essentially of the same length. Compare the C-C bond distances of 8 (1.37, 1.38 A) with the C-C bonds adjacent to the heteroatoms in 9 (1.39 A) and 10 (1.38 A). Similarly, the endocyclic C-B bonds of 8 and 9 (1.52, 1.53 A) and the C-S bonds of 8 and 10 (1.75, 1.76 A) are identical within experimental error. Thus the structural parameters of 8-10 indicate that the compounds are very similar n-complexes of aromatic rings. Finally we note that 8 is the only structurally characterized complex involving sulfur n-bonding to molybdenum. Mo n-bound thiophene complexes have figured prominently in mechanistic speculation about the mode of action of molybdenum-based hydrodesulfurization ~ a t a l y s i s . 'The ~ preparation of a related Mo n-bound sulfur complex may be relevant to these studies.
Acknowledgment. We thank Ms. Heather Crocker for performing the initial experiments on the desilylation of 5. We thank the National Science Foundation (Grant No. CHE-9224967) for financial support of our work. Supporting Information Available: Tables of crystallographic data, positional and thermal parameters, and interatomic distances and angles for 8 ( 5 pages). Ordering information is given on any current masthead page.
U O
OC.*''l 'co
OM95017 1 Q
co
19) Crystal data for 8: C ~ H ~ B M O Omonoclinic, ~S, P2dn (No. 141, a 8.634(2)A, b = 9.997(2) c = 12.471(3) A,/j = 104.80(2)",V = 1040.7(4)A3, Z = 4, D, = 1.85 g ~ m - T~ =, 298 K, p(Mo Ka) = 13.98 cm-'. Data were collected on a Syntex P 2 , d v diffractometer using the 20 scan range 5-52"; of the 2417 reflections measured, 2067 were unique (R,,, = 0.0146) and 1695 with F 2 4 d F ) were used in the refinement. The structure was solved by Patterson techniques and refined by full-matrix least squares. R = 0.0257, R, = 0.0356, and GOF = 0.93. (10) Sanger, M. J.; Angelici, R. J. Organometallics 1994, 13, 1821. ( 11 1 Ashe, A. J., 111; Kampf, J. W.; Nakadaira, Y.; Pace, J. M. Angew. Chem., Int. E d . Eng2. 1992, 31, 1255. Ashe, A. J., 111; Kampf, J. W.; Klein, W.; Rousseau, R. Angew. Chem., Int. E d . Engl. 1993,32, 1065. f 12) Huttner, G.; Krieg, B. Chem. Ber. 1972, 105, 3437. (13) Siebert, W.; Augustin, G.; Full, R.; Kruger, C.; Tsay, Y.-H. Angew. Chem., Int. E d . Engl. 1975, 14, 262. Siebert, W.; Full, R.; Edwin, J.;Kinberger, K.; Kruger, C. J . Organomet. Chem. 1977, 131, 1. (14) Bailey, M. F.; Dahl, L. F. Inorg. Chem. 1965, 4, 1306. (15)Albright,T. A,; Hoffmann, R. Chem. Ber. 1978, 1 1 2 , 1578. (16) Bohm, M. C.; Gleiter, R.; Herberich, G. E.; Hessner, B. J . Phys. Chem. 1985,89, 2129. ( 1 7 ) ( a )Rauchfuss, T. B . Prog. Znorg. Chem. 1991, 39, 259. (b) Angelici, R. J. Acc. Chem. Res. 1988, 21, 387; J . Coord. Chem. Rev. 1990, 105, 61. rc) References cited in ref 10. ( d )Blackman. A. Adv. Heterocycl. Chem. 1993, 58, 147-153. =
8
are comparable to those of the aromatic 1-methylborepin (2a;6 7.66 ( H d and 7.73 (H3)) and thiophene (3; 6 7.30 (Hz) and 7.10 (H3)). In addition, electron donation to the boron atom of la seems indicated by the observed "B chemical shift (6 50.1) upfield from that of 2a (6 54.8).' Surprisingly, the "B chemical shift of 4b (6 66.8) is a t much lower fields3 The reaction of la with tricarbonyltris(pyridine)molybdenum and BF3.OEt2 gives adduct 8 as dark red crystals in 40% yield. The X-ray structure of 8 illustrated in Figure l shows that the nearly planar thiaborin ring is $-coordinated to the molybdenum atom.g The boron and four carbon atoms lie in a common plane, but the sulfur atom is dis laced out of this plane away from the metal by 0.11 . This is a likely consequence of the larger size of sulfur and is analogous to the out-of-plane displacement (0.096 A)
x
A,
