Sulfur heterocycles, ring contraction

Ring contraction of heterocycles by sulfur extrusion 96AHC(65)39. 

Ring contraction of heterocycles by sulfur extrusion including a chapter on four-membered rings from five-membered rings was recently reviewed by Bohle and Liebscher [96AHC(65)39]. 

Bohle, M., Liebscher, J., Ring Contraction of Heterocycles by Sulfur Extrusion,... 

Bodea, C., Silberg, L, Recent Advances in the Chemistry of Phenothiazines, 9, 321. Bohle, M., Liebscher, I., Ring Contraction of Heterocycles by Sulfur Extrusion,... 

Although reliable information on the mechanisms of ring contractions of heterocycles by sulfur extrusion are often difficult to obtain, some generally accepted concepts have been elaborated. Thus, fully conjugated seven-membered 34 and six-membered S-heterocycles 37 and 40 are antiaromatic... 

In Chapter 2, M. Bohle and J. Liebscher (Humboldt University, Berlin) review ring contractions of heterocycles by sulfur extrusion, which includes the elimination of various sulfur species all leading to a ring contraction containing one less member. This is the first comprehensive review of a subject that is of preparative as well as theoretical importance. 

In an earlier study the authors proposed a [3.2.0] bicyclic sulfonium salt 8 as the reactive intermediate in the trimethylsilyl iodide mediated ring contraction of 4-methoxythiephane <1996T5989>. Enantiomerically pure thio-lane derivatives were synthesized via a ring contraction of a seven-membered sulfur heterocycle by nucleophilic transannular substitution <2000TA1389>. The thiepane derivative 15, derived from d-sorbitol, was converted into the dimesyl derivative 16 following deprotection under acidic conditions. Treatment of 16 with sodium azide in DMSO at 120°C yielded the corresponding thiolane as a mixture of two diastereoisomers, 17a and 17b, in a 5 1 ratio (see Scheme 1). 

The present review deals with sulfur extrusions from heterocyclic compounds 5, giving rise to ring-contracted cyclic products 6 (Scheme 2). In the course of such ring contractions, the sulfur atom sometimes remains connected to the final ring system (formation of 7). This type of reaction is also considered to some extent in the following sections. 

Various methods, such as thermolysis, photolysis, interaction with triva-lent phosphorus compounds [e.g., PR3, P(NR2)3, P(OR)3] or bases, and reaction with nickel catalysts or oxidizing agents (e.g., H2O2, RCO3H), are useful for the ring-contraction reaction of heterocycles by sulfur extrusion. In a number of cases, the reagent adds to the sulfur atom in a first reaction step (Scheme 4). Hence, instead of elemental sulfur, its derivatives are eliminated in the subsequent ring contraction, such as SO2 in the presence... 

Reactions initiated by electrophilic or oxidative attack at the ring sulfur atom (Scheme 4) are considered in the present review only if intermediates 19, such as 5-oxides or 5-dioxides, are not isolated but are transformed to the sulfur-free ring contraction product without prior isolation. Thus, sulfur dioxide extrusion from such isolated 5,5-dioxides as thietane 5,5-dioxide 21 or the Ramberg-BScklund reaction of a-halosulfonyl compounds (e.g., of 23) (770R1) (Scheme 5) are not included (for reviews on SO2 extrusions from heterocycles, see 90MI1 92MI1). 

RING CONTRACTION OF SULFUR HETEROCYCLES WITH 2,2 -BIPYRIDYL AND (COD)2NI. 

The ease of interaction of Ni(0) complexes with organic substrates has been shown to depend upon both the ligands on nickel and the solvent. The presence of a,a-bipyridyl with the Ni(0) complex and the alkyne led to the isolation of a nickelacyclopropene, an observation in accord with the recently proposed metallocyclic pathway for the Ni(0)-cata-lyzed trimerization of alkynes. Allylic and benzylic ethers and epoxides have been observed to undergo oxidative insertion of Ni(0) into their C-0 bonds with solvent (TMEDA > THE > Et O > CeHe) and ligand (EtsP > PhsP a,a-bipy > COD) effects consistent with an electron-transfer attack by Ni(0). With such sulfur heterocycles as dibenzothiophene, phenoxathiin, phenothiazine, and thian-threne, a 1 1 admixture of (COD)2Ni with a,a-bipyridyl gave as the principal product the desulfurized, ring-contracted cyclic product. 

The ring contraction observed with all of these heterocycles (40 and 42), taken together with the inability of (COD)2Ni alone to eflFect desulfurization, indicates that bipy(COD)Ni operates principally by an electron-transfer mechanism rather than by insertion in a C-S bond (cf. low yield of 38). Since 2 equiv of 28 are more eflFective, prior coordination of one Ni(0) at sulfur seems to be important (Scheme 11). That biphenyl-... 

Ring contraction of other sulfur-containing six-membered heterocyclic compounds... 

Reports on reactivity of these eight-membered heterocycles are fairly limited and are predominantly reactions involving ring contraction, thermolysis or hydrolysis, alkylations at cyclic amides, and oxidation of ring sulfur atoms. 

Sulfur monochloride can add one sulfur atom to a molecule and these transformations usually suggest the extrusion of sulfur dichloride or another sulfur atom with contraction of the heterocycle to a more stable (often heteroaromatic) ring. 

---