Thiocyanates from alkenes

Thiiranes can be prepared directly from alkenes using specialized reagents. Thiourea with a tin catalyst gives the thiirane, for example. " Interestingly, internal alkynes were converted to 1,2-dichorothiiranes by reaction with S2CI2 (sulfur monochloride).It is noted that epoxides are converted to thiiranes with ammonium thiocyanate and a cerium complex. " A trans-thiiration reaction occurs with a molybdenum catalyst, in which an alkene reacts with styrene thiirane to give the new thiirane. 

Iron(n) salts mediated the coupling of trialkylboranes with KSGN to give RSCN via a radical process.553,554 Stereochemically pure ( )-l-thiocyanato-alkenes or ( )-l-azide-alkenes were obtained from alkynes when hydro-boration with disiamylborane was followed by reaction with potassium thiocyanate or sodium azide in the presence of copper(n) nitrate, copper(n) acetate, and small amount of water in polar aprotic solvent (Equation (116)).555... 

In contrast, the photolysis of these azides at 350 nm leads only to the corresponding nitriles and olefins in yields of 90 l-(Alkylthio)cyclopropyl azides (164), readily accessible from the corresponding chlorides and bromides, are smoothly decomposed at ITC with nitrogen evolution. The main process is a ring enlargement to 2-(alkylthio)azetines (165), accompanied by cleavage to an alkyl thiocyanate and an alkene (equation 114). ... 

Acetates of MBH adducts have also been established for the synthesis of sulfur-containing trisubstituted alkenes via a nucleophilic displacement protocol, in either S- 2 or S- 2 fashion with 5-centered nucleophiles, such as thio-lates, ° arenesulfinate and thiocyanate anions. Significantly, these reactions are highly stereoselective for both acrylonitrile/acrylate ester-derived MBH acetates, but with a reversed stereochemical directive effect, that is, acrylonitrile-derived MBH acetates afford ( )-sulfur-containing trisubstituted alkenes 336 as the sole product, while acrylate ester-derived MBH acetates afford (Z)-olefins 335 exclusively under the same reaction conditions. Both (Z)-and ( )-allyl sulfides have been stereoselectively obtained from MBH acetates in a one-pot with treatment with benzenethiol in the presence of catalytic amounts of 15% aqueous NaOH and TBAI in DM SO at room temperature (Scheme 3.146). ° The method has also been applied for the synthesis of (Z)-3-(4-methoxybenzylidene)thiochroman-4-one (337), a potent antifungal compound (Scheme 3.147). 

A series of papers detailing a continuation of the study of the synthesis and reactions of uic-iodothiocyanates and -iodoisothiocyanates has been published.Similarly, the treatment of alkenes with thiocyanogen bromide [from equimolar quantitites of bromine and thallium(i) thiocyanate] gives moder-ate-to-high yields of uic-bromothiocyanates. " Unlike the analogous vic-iodothiocyanates, the bromo-compounds are not readily isomerized to the corresponding isothiocyanates. 

Preparation of Thiocyanates.—Variations of standard methods for the synthesis of thiocyanates are illustrated in the addition of alkoxy- and thiocyanato-groups to alkenes using KSCN, CuClj (or other Cu salt), and an alcohol as solvent, to give a-alkoxy-alkyl thiocyanates and in the addition of pseudohalogens CISCN or (SCN)a to chalcones. A sulphonylthiocyanate RSOaSCN, prepared from (SCN)a and a sodium sulphinate, adds similarly to alkenes to give a-thio-cyanato-alkyl sulphones. > Aryl selenocyanates may be prepared from the... 

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