equilibrium of the o-phenylene(5)/C=C(5) analogue 11 with the [6+2]cycloadduct 12 has already been observed, but only upon sensitized e ~ c i t a t i o n . ~ " ~

Received: October 17, 1985 [Z I500 IE] German version: Angew. Chem. 98 (1986) 193

[I] a) Preliminary communication: W. Berning, S. Hiinig, Angew. Chem. 89 (1977) 825: Angew. Chem. Int. Ed. Engl. 16 (1977) 777; b) W. Berning, S. Hunig, F. Prokschy, Chem. Ber. 117(1984) 1455.

[2) B. Albert, W. Berning, C. Burschka, S. Hiinig, H.-D. Martin, F. Prok- schy, Chem. Ber. 114 (1981) 423.

131 Cf. review: H.-D. Martin, B. Mayer, Angew. Chem. 95 (1983) 281: An- gew. Chem. Int. Ed. Engl. 22 (1983) 283.

[4] a) K. Beck, A. Hohn, S. Hiinig, F. Prokschy, Chem. Ber. 117 (1984) 517; b) S. Hiinig, F. Prokschy, rbid. 117 (1984) 534.

[ 5 ] B. Albert, W. Berning, C. Burschkd, S. Hiinig, F. Prokschy, Chem. Ber. 117 (1984) 1465.

[6] K. Beck, Dissertation, Universitat Wiirzburg 1986. (71 The structures of all the new compounds are consistent with their ele-

mental analyses and UV, IR, 'H-NMR, and '?C-NMR spectra. The lat- ter compare very well with those of the related systems already dis- cussed in detail in the literature [Ib, 4, 51.

[8] A mixture of 5 (500 mg, 1.74 mmol), trifluoroacetic acid (1.20 mL, 15.5 mmol) and 6 (802 mg, 5.20 mmol) was stirred for I d at RT. The crude product isolated with water/CHC13/KzC0, was, after Kugelrohr distil- lation (25"C/O 01 torr), separated several times (with heavy losses) by medium-pressure chromatography (petroleum ether/ethyl acetate (9 : I ) ; the products were sublimed at 60"C/0.01 torr. Yield: 63 mg (5%) 7, m.p. 147-148"C, and 16 mg (I%) 8, m.p. 158-159"C.

191 Cf. also P. S. Engel, D. W. Horsey, D. E. Keys, C. J. Nalepa, L. R. Sol- tero, J . Am. Chem. Soc. 105 (1983) 7108, and references cited therein.

[lo] An NMR tube containing 4, 7 or 8 in CDC13 was secured to an immer- sion cylinder cooled to -20°C and irradiated with a 150-W Hg-high- pressure lamp (Pyrex filter). On completion of reaction ( 'H-NMR mon- itoring), the solvent was removed. The crude product, after purification on a silica gel column, was sublimed in vacuo. 30 mg of 7 (0.12 mmol) furnished 20 mg (67%) of 9 , m.p. 177-178°C. 15 mg (0.06 mmol) of 8 yielded 8.6 mg (57%) of 10, m.p. 130°C (dec.).

1111 4 was discussed in a lecture delivered at a conference convened by the Arbeitsgemeinschaft Organische Chemie at Bad Nauheim (FRG), Octo- ber 4-6, 1984.

[I21 M. S . Raasch, J. Org Chem. 45 (1980) 856. [I31 Since the dehydrogenation even takes place with atmospheric oxygen in

boiling benzene, a dyotropic terminal-group transfer of hydrogen to the azo group could be at issue. This transfer has already been demon- strated in the case of a related system at 200°C with an ethano group as neighbor (D. Ginsburg, M. Koral, Tetrahedron 29 (1973) 2373.

[I41 N. J . Hales, H. Heaney, J. H. Hollinshead, Synthesis 1975, 707. [IS] A mixture of 1 (1.01 g. 3.96 mmol) and 2 (841 mg, 3.96 mmol) in chloro-

form (10 mL), which was allowed to stand for 7 d at room temperature, afforded 1.61 g (101%) of 3 ( 'H-NMR spectroscopically pure). Extrac- tion of the solid yielded 1.4Og (88%) of 3, m.p. 166-168°C. A weak stream of air was passed through a solution of 3 (1.40 g, 3.48 mmol) in boiling benzene (SO mL) for 2 d. The product obtained upon evaporating the solution to dryness was purified by extraction with acetic acid: there remained 1.10 g (79%) of 4[Cl,], m.p. 274-276°C. The crude product obtained from 4[CI,] (0.250 mmol), Na (100 mg), and tBuOH (0.30 mL) in T H F (4.00 mL) afforded, by the procedure given in ref. 1141, 43 mg (660/0) of 4 (m.p 197-199°C) after chromatography on silica gel and sub- sequent sublimation.

[I61 The '3C,'3C-coupling constants of the C atoms on the nitrogen bridges of the homologues of 4 and its azoxy derivative, which are accessible by means of our method, have recently been reported: H. Fritz, G . Fischer, W. Marterer, H. Prinzbach, Tefrahedron Lett. 26 (1985) 4427.

[I71 H. Prinzbach, C . Sedelmeier, C . Kruger, R. Goddard, H.-D. Martin, R. Gleiter, Angew. Chem. 90 (1978) 297: Angew. Chem. Inl. Ed. Engl. 1 7 (1978) 271.

Thianthrene 5-Oxide as Mechanistic Probe in Oxygen Transfer Reactions: Substrate Complexation in Oxidations with Transition Metal Peroxides** By Waldemar Adam* and B . Bhushan Lohray

Recently, we succeeded in determining quantitatively the electronic character of oxidizing agents by employing oxygen transfer to thianthrene 5-oxide (SSO).lll Thus, we showed that carbonyl oxides 1 and dioxiranes 2 are chem- ically different species.l2] Peroxo complexes of transition

w

1 2 3 4

metals can be represented, analogously to 1 and 2, in terms of their dipolar structure 3 and the cyclic valence structure 4, re~pectively.~~' Expectedly the terminal oxygen of 3 should be transferred more nucleophilically to thian- threne 5-oxide than by 4. Prior complexation of the sub- strate at the metal center would, however, mask the elec- tronic character of the oxygen transfer. A necessary condi- tion for the use of thianthrene 5-oxide as mechanistic probe for the electronic character of a species is that the oxygen transfer takes place directly at the peripheral ox- ygen without prior complexation of the oxidizing agent. However, the question whether complexation of the sub- strate takes place in oxidations with transition-metal per- oxides is presently being actively debated.[41 Consequently, the determination of the product pattern in the oxygen transfer by these peroxo complexes to thianthrene 5-oxide offers the unique opportunity of either gaining insight into the electronic character of these oxidizing agents or dem- onstrating prior complexation of the substrate at the transition metal center. Our present results (see a-d) sug- gest that thianthrene 5-oxide is first complexed by the me- tal a t the sulfide sulfur and that the oxygen is transferred subsequently. a) The diperoxo complexes (HMPT)Cr05, (HMPT)Mo05,

and (HMPT)( H20)W05 (HMFT = hexamethylphos- phoric triamide) were allowed to react with thianthrene 5-oxide (SSO) in CH2C12 in the ratio 1 : 10, respective- ly. Independent of the transition metal and ligands, the amount of oxygen transfer to the sulfoxide sulfur to

(HMPT),MoO,, (HMPT)(H,O)MoOS, (HMPT)WOS,

0

soso Q&J $b soso,

II I I 0 0

[*I Prof. Dr. W. Adam, Dr. B. B. Lohray ['I lnstitut fur Organische Chemie der Universitlt Am Hubland, D-8700 Wurzburg (FRG)

['I Alexander-von-Humboldt Fellow (1984/ 1985) [**I This work was supported by the Deutsche Forschungsgemeinschaft, the

Fonds der Chemischen Industrie, and the Stiftung Volkswagenwerk.

188 0 VCH Verlagsge,sellschafi mbH. 0-6940 Wemheim. 1986 0570-0833/86/0202-0188 $ 02 50/0 Angew Chem In!. Ed. Engl 25 (1986) No 2

give the thianthrene 5,5-dioxide ( S O 2 ) and to the sul- fide sulfur to give the thianthrene 5,10-dioxide (SOSO) was about equal (0.50 f 0.05). A competition experiment with an equimolar mixture of SSO, and SOSO showed that SOSO was oxidized about three times more rapidly than SSOz by

A ” P-N M R spectrum, recorded immediately after mix- ing SSO and (HMPT),MoO, in CDC13, showed the presence of bound HMPT ((63’P)=37.8) and free HMPT ((631P)=28.2) a t ca. 37°C. Precipitation of the metal peroxide by dilution with ether and immediate filtration afforded ca. 4% of oxidized thianthrene 5-ox- ide as a 1 : 1 mixture of SS02 and SOSO in the fil- trate. Only the cis-isomer of thianthrene 5,lO-dioxide (cis- SOSO) was formed when SSO was oxidized with (HMPT)MoO, in CH2CI2. As shown by control experi- ments, authentic trans-SOSO was stable under the con- ditions of the oxygen transfer and the HPLC analysis.

(HMPT)MoOS.

A plausible mechanism for these observations is out- lined in Scheme 1. The metal center is first complexed at the nucleophilic sulfide sulfur of SSO by ligand exchange. Subsequently, the oxygen is transferred transanriularly to the sulfoxide sulfur with formation of S O 2 . Oxidation at the complexed sulfide sulfur leads to cis-SOSO.

T

sso, sso cis- soso

Scheme I Mechanism of complexation for oxygen transfer to thianthrene 5-oxide SSO.

Unexpected is the finding that the products SS02 and cis-SOSO are formed in equal amounts. According to mo- lecular models, in the well-known folded conformation of SSO the transannular oxygen transfer can take place com- fortably. The steric shielding of the equatorial lone pair fa- vors the required axial complexation at the sulfide sulfur by the peri-hydrogen atoms. This very special arrangement of the oxygen donor and oxygen acceptor also promotes formation of cis-SOSO. Peripheral attack without com- plexation would lead to cis- and trans-SOSO. A further prerequisite is presumably the necessary distance between the sulfoxide sulfur and the complexed molybdenum oxid- izing agent. The following reactions support this concept: oxygen transfer with (HMPT)Mo05 in CH2C12 to I-(phe- nylsulfinyl)-4-(phenylthio)benzene 5 and to the ortho isomer 7 gave, besides traces of the trioxides (double ox-

ygen transfer), exclusively the disulfoxides 6 and 8, re- spectively. “Transannular” oxygen transfer in the case of the derivative 7 would lead to the sulfone. Hence, for transannular oxygen transfer to take place, the conforma- tional arrangement of the sulfide lone pair to be com- plexed and its steric accessibility are also important, as well as the proper distance between the sulfur centers.

0 I1

II 0

5 6 7 8

These conditions are apparently optimally fulfilled in the case of the rigid and folded thianthrene 5-oxide. This concept is further supported by the fact that a 1 : 1 mixture of cis- and trans-disulfoxide 8 was obtained in the oxida- tion of 7 with (HMPT)MOO,,[~I whereas only cis-SOSO was formed from SSO, alongside S O 2 . We presume that both sulfide lone pairs can be complexed in the case of 7, but only the axial lone pair in the case of SSO (Scheme I). Furthermore, the almost exclusive oxidation of the sub- strates 5 and 7 at the sulfide sulfur reflects the electro- philic character@’ of the transition metal peroxides.

We postulate that for the transannular oxygen transfer (only cis-SOSO; 1 : 1 ratio of SOSO : SSO,) to occur be- tween thianthrene 5-oxide SSO and (HMPT)MoOS one can presume that the sulfide sulfur of the oxygen acceptor SSO must be complexed at the metal center of the oxygen donor (HMPT)MoO5 during the oxygen transfer. Whether this complexation also takes place with other substrates (olefins, amines, phosphanes etc.) cannot be generalized.[41 The concept does, however, provide interesting possibili- ties for stereoselectively controlled oxygen transfer in syn- thesis.

Received: October 24, 1985: [Z 1508 IE]

German version: Angew. Chem. 98 (1986) 185 revised: December 6, 1985

[I] W. Adam, W. Haas, G. Sieker, J . Am Chem. Soc. 106 (1984) 5020 121 W. Adam, H. Diirr, W. Haas, B. B. Lohray, Angew. Chem. 98 (1986) 85;

Angew. Chem. I n f . Ed Engl. 25 (1986) 101. [3] H. Mimoun, Angew. Chem. 94 (1982) 750; Angew. Chem. In t . Ed. Enql. 21

(1982) 734. 141 a) R. C. Michalson, R. E. Palermo, K. B. Sharpless, J . Am. Chem. SOC. 99

(1977) 1990; b) P. Chaumette, H. Mimoun, L. Saussine, J Fischer, A. Mitschler, J. Organomef. Chem. 250 (1983) 291: c) R. Curci, S. Giannat- tasio, 0. Sciacovelli, L. Troisi, Tefrahedron 40 (1984) 2763; d) F. Di Furia, G. Modena, Pure Appl. Chem 54 (1982) 1852.

[S] G. Leandri, M. Pallati, Bull. SCL Fac Chim. Ind. (Bologna) 14 (1956) 54. [6] F. Di Furia, G. Modena, R. Seraglia, Synthesis 1984. 325.

Optically Active a-Chloro-(E)-crotylboronate Esters by Ally1 Rearrangement By Reinhard W. Hoffmann* and Stefan Dresely

Esters of a-chloroallylboronic acid“’ belong to the Q-

chiral allylmetal compounds, whose addition to aldehydes proceeds with high or complete transfer of chirality.121 Un-

[*] Prof. Dr. R. W. Hoffmann, DipLChem. S. Dresely Fachbereich Chemie der Universitat Mans-Meerwein-Str., D-3550 Marburg (FRG)

Anyew Chem. In f . Ed. Engl. 25 11986) No. 2 0 VCH Verlagsgesellrchali mbH, D-6940 Weinheim. 1986 0570-0833/86/0202-0189 $ 02.50/0 189