| dc.description.abstract | This thesis deals with the chemistry of benzyltriethylammonium tetrathiomolybdate [PhC?H?NEt?]?MoS? (1) and its use in organic synthesis. Additionally, the synthesis of chiral tetrathiomolybdates 3 and 4 and their applications in organic synthesis are also presented. A general methodology for the conversion of chiral tosylates and chiral alcohols to the corresponding chiral disulfides is described in part 1 of chapter 1. A number of activated chiral alcohols, on treatment with benzyltriethylammonium tetrathiomolybdate 1, were converted to the corresponding chiral disulfides in good yields in a one-pot reaction. Chiral tosylates also provided chiral disulfides under mild reaction conditions with reagent 1 in good yields (Scheme 1).
Scheme 1:
OTs ? Ph (1) ? t-DCC/CuCl ? MoS?²? (1) ? C?H?CN ? 6 h, rt, 95%
Part 2 of chapter 1 describes a multicomponent reaction performed in one pot using benzyltriethylammonium tetrathiomolybdate, 1, as a key reagent. Here, results of tandem sulfur transfer-reduction-Michael addition of activated alcohols with ?,?-unsaturated systems assisted by tetrathiomolybdate 1 have been presented (Scheme 2). Additionally, this methodology was extended to the same tandem reaction involving a second Michael addition to furnish functionalized thiabicyclo [3.3.1] nonane systems and tricyclic systems in a five-component reaction starting from alcohols and halides as precursors. Some of the examples are highlighted in Scheme 3.
Scheme 2:
MoS?²? (2cq), 1 ? CH?CN/MS?A ? 12 h/80% ? COOEt ? H?
Scheme 3:
MoS?²? (2cq), 1 ? CH?CN ? 12 h, 88%
An interesting extension of this methodology is also presented in a tandem reaction involving activation of alcohols-sulfur transfer-reduction-Michael addition-aldol reaction in a single-pot operation using tetrathiomolybdate 1 as a key reagent (Scheme 4).
Scheme 4:
Michael-Aldol ? 1. DCC/CuCl ? 2. MoS?²? (2cq), 1 ? CH?CN/MS?A ? reaction ? BnS
Further, a few more tandem reactions, such as 1) Reduction of disulfide-Michael addition, 2) sulfur transfer-reduction-Michael addition starting from organic halides, and 3) reductive dimerization-cleavage of disulfide-Michael addition, were also performed using ammonium tetrathiomolybdate, 2, as the reagent in a water medium under sonochemical conditions, as shown in Scheme 5.
Scheme 5:
Cystine ? (NH?)?MoS? (1cq), 2 ? H?O ? 3 h, rt
A mild methodology for the synthesis of p-hydroxy disulfides from epoxides is presented in part 1 of chapter 2. A number of epoxides, on treatment with benzyltriethylammonium tetrathiomolybdate 1 at room temperature, afforded the corresponding p-hydroxy disulfides in excellent yield. Apart from this, cleavage of disulfides using tetrathiomolybdate 1 and opening of epoxides with the intermediate thus generated to produce p-hydroxy sulfides are also presented (Scheme 6).
Scheme 6:
MoS?²? (1cq), 1 ? CH?CN/EtOH ? 6 h, rt ? 84%
The application of induced internal redox reactions of tetrathiomolybdate 1 in the cleavage of diselenide bonds under appropriate reaction conditions is demonstrated in part 2 of chapter 2. Accordingly, results of tandem reactions involving activation of alcohols-formation of selenocyanates-diselenide bond formation-cleavage of diselenide and opening of the epoxide, mediated by tetrathiomolybdate 1, to produce p-hydroxy selenides are discussed (Scheme 7).
Scheme 7:
MoS?²? (1cq) ? Ph ? OH ? DCC/CuCl ? P h ? ScC N
Part 3 of chapter 2 presents an interesting multicomponent reaction performed using benzyltriethylammonium tetrathiomolybdate 1 as a key reagent. The ability of tetrathiomolybdate 1 to bring about sulfur transfer reaction and cleave the disulfide bond is exploited to open up epoxide derived from citral regioselectively to form a sulfur-containing 14-membered ring system (Scheme 8).
Scheme 8:
71%
Furthermore, the products obtained from opening epoxide rings with tetrathiomolybdate, 1, undergo reaction with Michael acceptors to furnish products of double Michael addition. The interesting part of the methodology is the consecutive reactions that take place involving epoxide ring opening-disulfide formation-cleavage of disulfide-Michael-Michael addition. All five reactions are carried out in a single pot without isolating any of the intermediates. The utility of this tandem reaction is demonstrated in the synthesis of a tricyclic system as presented in Scheme 9.
Scheme 9:
MoS?²? (2cq), 1 ? ElOOC ? Q+ 'O
Chapter 3 describes the synthesis and characterization of chiral tetrathiomolybdate, N-benzylquininium tetrathiomolybdate 3 and N-methyl ephedrinium tetrathiomolybdate 4. An attempt was made for asymmetric Michael addition of thiolates derived from organic disulfides to ?,?-unsaturated systems using chiral tetrathiomolybdates 3 and 4 under mild reaction conditions (Scheme 10). While the preliminary results are encouraging, the asymmetric induction is generally poor and further work needs to be done.
Scheme 10:
N-Me Eph-MoS? ? Asymmetric Michael addition ? PhSSPh + N-Bii Quin-MoS? ? CH?CN ? 85% | |
