Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 1355 149

In situ combination of Lawesson reagent and anthracene allows reactive thioaldehydes to be formed and subsequently trapped as Diels-Alder cycloadducts, as illustrated in the case of pentafluorothiobenzaldehyde289 (equation 43), which is very reactive and polymerizes rapidly.

(43)

Several groups have developed a range of modified Lawesson reagents, such as 56 introduced by Sandstrom¨ and coworkers290 and employed in the synthesis of twisted push pull ethylenes290,291. Reagent 56 is somewhat more stable than 52 and these authors recommend DME as the solvent of choice. The groups of Heimgartner292 and of Nicolaou293 have modified Yokoyama’s reagents (54, 55) by preparing analogs 57 59 which have been used primarily for the thionation of amides292 and lactones293. Finally, Davy has reported a number of bis(alkylthio) derivatives of 57 which have found wide application in the preparation of dithioesters294,295.

S

S

R P P R

S

S

(56) R = SCH3

S

S

R S P P S R

S

S

(57)R = CH3 2 9 2

(58)R = F 2 9 3

(59)R = Cl2 9 3

d.Other reagents. In addition to phosphorus sulfides, boron sulfides such as B2S3 have been used as mild sulfuration reagents. This reagent is generated in situ by treatment of

BCl3 with bis(tricyclohexyltin) sulfide296,297 and it has been applied to the thionation of nonenolizable diketones298. On the other hand, dimethylthioformamide was employed in the preparation of naphthothiopyranopyranthiones299 (equation 44).

Another reagent with limited application in thionation reactions has been S2Cl2, which afforded intermediates leading to 1,3,4-oxadithiolanes300. However, among the different thionating reagents developed in recent years the most successful has been bis(trimethylsilyl) sulfide, which in the presence of a catalyst affords thioaldehydes

1416 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

and thioketones under mild conditions. The selection of the catalyst is important

and CoCl2.6H2O for thioaldehydes301,302 and trimethylsilyl triflate for thioketones are preferred302,303 (equation 45).

R1 = H,SiMe3 , catalyst: CoCl2 . 6H2 O

R1 = alkyl, vinyl, catalyst:TfOSiMe3

As usual, thioaldehydes were trapped in situ as Diels-Alder adducts and the intramolecular version of this strategy using a catalytic amount of butyllithium has also been reported304.

2. Acetals

Dimethyl acetals have been treated with hydrogen sulfide in the presence of zinc chloride to afford tautomerically pure aliphatic thioketones 60 (equation 46)305.

Enethiols 61 are tautomerically stable isomers of thioketones 60 and, in contrast to enols, enethiols have been isolated and characterized. This method provides a route for the selective synthesis of enethiols by deprotonation of 60 and quenching with trimethylchlorosilane, since under these conditions no isomerization takes place.

3. Imino derivatives

This approach is illustrated by the reaction of several immonium salts, obtained by Vilsmeier reaction with sodium hydrogen sulfide (equation 47) to afford a series of stable enamino thioaldehydes306.

1418 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

final products being the tricyclic thiones in 22 27% yield314 (equation 51).

••

DBU = 1,8-Diazabicyclo[5.4.0]undec-7-ene

E. Elimination Reactions

1. C,C cleavage

The reaction of thiiranes with BF3ž Et2O results in the formation of cyclopentenethiones, by a new type of cyclization via the initially generated thioallyl cations315,316 (equation 52).

(52)

S

TMS Ph

Ph

Photodissociation of thiophene yields several products among them thioketene, CH2DCDS, produced by C,C cleavage317. More synthetic utility possesses the thiocyanohydrins which, by vacuum-gas phase dehydrocyanation, yield the corresponding reactive thioaldehydes318 (equation 53). These thiocyanohydrins can be monoand

1420 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

In addition to pyrolysis or photolysis, S,C cleavage may be induced by nucleophiles (or bases) in substrates with an electrophilic (or acidic) center and a leaving group next to the sulfur1. This is exemplified in the treatment of a bridged thiocyanate with potassium tert-butoxide leading to an ˛-oxo thioketone which spontaneously dimerizes (equation 58)324.

H

S

SCN t-BuOK

O O

(58)

dimerization

S

S

O

O

100%

Similarly, the S CN bond in thiocyanates can be cleaved affording the corresponding thiolate anions which may be protonated to enethiols325 (equation 59).

This approach has also been applied to heterocyclic systems such as thiazolidine- 2,5-dithiones, which by treatment with base in the presence of a nucleophile afford a number of pyrroline thiones326. Thiopyrilium salts also undergo ring cleavage to yield thiobenzophenones in low yield327. Applications to thioquinanthrene salts have also been reported328. Finally, it is interesting to note that a thioketone has been invoked as intermediate in the W(CO)6-mediated desulfurdimerization of dithioketals329 which represents an unprecedented type of fragmentation of the dithiolane moiety. This result can be explained considering that group 6 metal carbonyls are thiophilic and C S bond cleavage is thus favoured329 (equation 60).

(E1cB mechanism)1. Thus, 9-thiofluorenone was prepared from the corresponding p- toluenethiosulfonate336 (equation 66).

A rSO2 SK

S

Thioacylthioketenes are formed by FVT of various 1,2-dithiole or dithietane derivatives337,338 (equation 67) via a free radical mechanism.

Another classical strategy involves the use of Bunte salts 65 to generate diethyl thioxomalonate339 (equation 68) or several transient thioaldehydes340, which undergo smooth cycloaddition with many dienes339, including the dienic system of thebaine340. In this case, transformation of the adducts gave rise to a variety of opiate agonists.

Disulfides treated with DABCO afforded thioketenes which were trapped with cyclopentadiene341 and photolysis of polysulfides produced a range of thione342 and dithione343 derivatives in low yields.

6. S,Se cleavage

An interesting reaction has been reported by Glass and coworkers344 consisting of the attack of a selenium heterocycle, Ebselen oxide 66, by ˛-toluenethiol (equation 69). In this reaction thiobenzaldehyde has been invoked as intermediate and could be trapped with cyclopentadiene. In the absence of a diene, dibenzyl disulfide is isolated instead of

1424 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

the thial along with selenenic acid 67.

SeOH

(67)

7. S,halogen cleavage

The elimination of HCl from the corresponding sulfenyl chlorides has been used in the synthesis of thioacetaldehyde182. The removal of HCl was accomplished by photolysis of ethanesulfenyl chloride in an argon matrix at 12 K (equation 70).

Related compounds such as alkanesulfonyl chlorides by treatment with base afford sulfene intermediates, which undergo attack by nucleophiles (H2O, alcohols, amines,

. . .) present in the reaction mixture. Although sulfenes have been the subject of intense studies345,346 it has been impossible so far to succeed in the isolation of even one of them.

Other labile thioaldehydes, such as EtO2CCHDS, have been prepared and trapped with anthracene347 (equation 71). On the other hand, a 1,6-elimination of HCl featured as key step in the generation of highly reactive monothioquinone S, S-dioxides 68 as depicted in equation 72348. the final products being the corresponding sulfonic acids.