[1452247-44-5] · C9H6CrO5S · (278.201)
(reagent used for promotion of [6+2], [6+4], [6+2+2], and [6+2+2+2] higher-order cycloaddition reactions)
Physical Data: red solid; mp 173-174°C.
Solubility: THF, ethyl acetate, CH2Cl2, CHCl3, and 1,2-dichloroethane.
Form Supplied in: prepared from thiepin-1,1-dioxide and trisacetonitriletricarbonylchromium.
Purification: flash column chromatography on silica gel using hexanes/ethyl acetate.
Handling, Storage, and Precautions: the compound can be handled in air and is generally stable for several months when stored in the freezer.
Preparative Methods: as shown below, thiepin chromium complex 1 is readily prepared via standard methods1 from thiepin-1,1-dioxide2 2 and trisacetonitriletricarbonylchromium(0)3 3. The reaction proceeds in 12 h at room temperature to give an 86% yield of the desired chromium complex 1 (
Chromium complex 1 undergoes a variety of higher-order cycloaddition reactions.5 Thus, a reaction between complex 1 and disubstituted alkyne partners in the presence of UV light (uranium filter) produces [6p+2p] cycloadducts in good yields (
Surprisingly, unlike the tricarbonylcycloheptatriene chromium(0), complex 1 does not react with alkenes to give [6p+2p] cycloadducts.7
Rigby and coworkers have advanced the field of multicomponent cycloaddition reactions using thiepin chromium complex 1 and terminal alkynes.8 The reaction proceeds via a process that can be viewed formally as two consecutive [6p+2p] cycloaddition events to give tetracycle 5 (
Examples of the [6+2+2] reaction are included in Table
Furthermore, the 2 components can be tethered together so the new product will have one additional ring. Thus, diyne 6 is slowly added over 2 h to a solution of complex 1 in a photochemical reactor to provide a 57% yield of pentacycle 8. Likewise, slow addition of diyne 7 gives pentacycle 9 in a diminished yield, presumably due to competitive polymerization (
Recently, Rigby and coworkers observed another product during their studies on the three component cycloaddition reactions using tethered alkynes. Thus, a photochemically promoted reaction between thiepin chromium complex 1 and excess octa-1,7-diyne 10 gave pentacycle 12 in 45% yield. Likewise, nonadiene 11 provided pentacycle 13 in 38% yield (
The mechanism leading to pentacycles 12 and 13 has not been established; however, the reaction may proceed via two sequential [6p+2p] cycloaddition reactions followed by a what appears to be a [3+2] cycloaddition. If the final step involves a [3+2] cycloaddition reaction, then these examples would represent the first examples of a chromium-mediated [3+2] cycloaddition reaction.11
By far, the most extensively investigated reaction of chromium complex 1 is the [6+4] cycloaddition reaction.4,7a,12 The reaction between complex 1 and several dienes was examined, and it was determined that electron-rich 4 partners provided much higher yields than those of electron deficient partners. Thus, diene 14 reacts with chromium complex 1 under photochemical conditions to give an excellent yield of cycloadduct 15, whereas, diene 16 gives a 38% yield of the corresponding cycloadduct 17 (
Other more complex dienes have also been examined in this reaction and are listed below (eq
The [6p+4p] cycloaddition reaction can also be promoted thermally. For example, a tert-butylmethylether solution of chromium complex 1 and diene 14 was heated to reflux (110°C) for 48 h. Upon isolation via column chromatography on silica gel, compound 15 was again obtained but with a diminished yield (47%) (
The sulfur-containing products, which result from the higher-order cycloaddition reactions described above, have been submitted to various conditions to extrude the elements of sulfur dioxide. For example, photolysis (quartz filter) of cycloadduct 15 for 15 min promotes a cheletropic extrusion of sulfur dioxide to give a 54% yield of cyclodecatetraene 26 (
A very different sulfur dioxide extrusion protocol has been applied to the total synthesis of (+)-estradiol (30) by Rigby and coworkers.13 In the event, the trimethylsilyl-substituted chromium thiepin complex 27 participated in a [6p+4p] cycloaddition reaction to provide a 70% yield of the structurally complex dihydrothepin 28. Subsequent sulfur dioxide extrusion via a Ramberg-Bäcklund-type ring contraction gave tetracycle 29. The total synthesis of (+)-estradiol (30) was completed using a four-step sequence of standard reactions (
Wayne State University, Detroit, MI, USA