Allylic thiocarbamates

Haloarene chromium tricarbonyl complexes are activated to nucleophilic attack by thiolate anions [58, 59]. High yields of the thioethers are obtained under liquiddiquid two-phase conditions, but optimum yields are achieved under soliddiquid conditions. In many cases the thioether is produced directly but, where the reaction mixture contains thioether and its chromium complex, the thioether can be isolated by degradation of the complex with iodine or an excess of the thiol. Both 1,2- and 1,4-dichlorobenzenes yield only monothioethers, even when an excess of thiolate anion is used. In contrast, 1,3-dichlorobenzenes produce a mixture of the mono- and dithioethers [59]. Aryl allyl thioethers have been produced under catalysed Heck reaction conditions from S-allyl thiocarbamates and iodobenzene [60]. 

While the cyclohexenyl and cycloheptenyl derivatives roc-14aa-14ag and rac-14ba were stable at room temperature, the cyclopentenyl O-allylic thiocarbamates already suffered a partial rearrangement at ambient temperatures [19, 20] while the acyclic O-allylic thiocarbamates showed an intermediate thermal stability. 

Table 2.1.4.12 Pd(0)/BPA-catalyzed rearrangement of racemic acyclic O-allylic thiocarbamates.

The sense and degree of asymmetric induction of the Pd(0)-catalyzed rearrangement of the cyclic and acyclic O-allylic thiocarbamates in the presence of BPA are the same as, or similar to, those in the Pd-catalyzed substitutions of the corresponding cyclic and acyclic racemic allylic carbonates and acetates with sulfinates and thiols. It is therefore proposed that Pd(0)/BPA reacts with the racemic O-allylic thiocarbamate with formation of a jt-allyl-Pd(II) complex, which contains as counter ion the corresponding thiocarbamate ion (Scheme 2.1.4.19) [23, 24]. Substitution of the jt-allyl-Pd(II) complex by the thiocarbamate ion gives the S-allylic thiocarbamate and the Pd catalyst. 

Finally, the application of S-allylic thiocarbamates to the synthesis of allylic sulfides and thiols was investigated (Scheme 2.1.4.20). 

Scheme 2.1.4.20 Synthesis of allylic sulfides from allylic thiocarbamates.

This reaction was carried out on cyclic and acyclic allylic carbonates. The 5-allylic thiocarbamate products were hydrolyzed to the corresponding thiol or reacted with 2-chloropyrimidine in the presence of potassium hydroxide to provide the sulfide without any loss in stereochemical purity for either example. a-Acetoxysulfones can be regarded as acid-stable, but base-labile, chiral aldehyde equivalents. These can be accessed through the palladium(0)-cataly zed reaction of geminal esters with sodium benzenesulfinate under phase-transfer conditions (eq 12). ... 

Similar transformations can be achieved from y-methylthioallyl thiocarbamates or from allyl thio-carbamates (Scheme 32, entries c and d). Both reaction sequences take advantage of the easy sigma-tropic rearrangement of allyl thiocarbamates. Furthermore, -methoxyallyl thiocarbamates have played the role of an acetone enolate (Scheme 32, entry a). - ... 

Gais and Bdhme reported that Pd(0)-catalyzed enantioselective 0,5-rearrangement of the racemic O-allylic thiocarbamate 614 using Pd(0) and Trost L-1 as a chiral ligand provided the 5-allylic thiocarbamate 615 in 92% yield with 91 %... 

The palladium(0)-catalysed rearrangement of 0-allylic thiocarbamates in the presence of a chiral bisphosphane has been shown to proceed quantitatively to yield 5 -allylic thiocarbamates in high yield, while it has been reported that treatment of allylic alcohols with thiophosgene and pyridine gave thiolo chloroformates (104) directly, presumably via a rapid [3,3]-sigmatropic rearrangement of initially formed thiono chloroformates (103). 

Allylic thiocarbamates (145) are available from ketones by sequential condensation with vinyl-lithium species and dimethylthiocarbamoyl chloride followed by a [3,3] sigmatropic rearrangement. It has now been shown that these can be used as precursors to a,jS-unsaturated esters by bis-sulphenylation and mercury(ii)-promoted hydrolysis (Scheme 39). Only the B-isomers are produced when the final product (146) is unsubstituted i.e. = H) or carries an a-methyl... 

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