Thioamides from amides

The ultrasonic preparation of thioamides from amides and phosphorus pentasulfide by Raucher(51) and of dichlorocarbene from chloroform and potassium hydroxide by Regen(52) are some of the more recent examples of nonmetallie applications. We were surprised to find that ultrasound greatly accelerates the reduction of haloaroma-tics by lithium aluminum hydride, permitting the reaction to be... 

Thioamides.2 The preparation of thioamides from amides and P4S,0 is improved by use of an ultrasonic laboratory cleaner. Lower temperatures and shorter reaction times can be used P4S10 is not required in large excess. Yields by use of ultrasound are 80-95%. 

Ionic liquids have been used both in solution and under homogeneous conditions. For example, Ley [37] described the preparation of thioamides from amides. Although the reaction under classical conditions occurs in excellent yield, the reaction time can be shortened by using MW irradiation (Scheme 5.4). The reaction was performed in toluene and, because this is not an optimum solvent for absorption and dissipation of MW energy, a small amount of polar solvent was added to the reaction mixture to ensure efficient heat distribution. 

Preparation of thioamides from amides was also shown to proceed faster and more efficiently under ultrasonic conditions (Scheme 88). Yields of between 77 and 97 % were obtained within 1-2 h [193]. In contrast, the standard reaction involves use of a large excess of reagent as a result of the insolubility of P4S q> results obtained using Lawesson s reagent tend to be unpredictable. 

When chloroacetaldehyde is condensed with higher thioamides prepared from amides and phosphorus pentasulfide according to Schwarz s method (222), 2-substituted thiazoles are obtained (4, 10,"22, 175). 

RCN From amide % yield from thioamide From amidine... 

A peroxyphosphorus intermediate 79 or 80 generated from phenylphosphinic dichloride and superoxide at —4°C desulfurizes thioamides to amides in excellent yields (equation 106)2. ... 

Table 13.21) or from amides by treatment with Lawesson s reagent. Thioamides can also be prepared on cross-linked polystyrene by the addition of H2S to nitriles (Entry 5, Table 13.21), by thiocarbamoylation of resin-bound organolithium compounds (Entry 6), or by the acylation of amines with reactive thio acid derivatives (Entry 7, Table 13.21). 

Amides being formally hydrated ynamines would constitute cheap starting materials for the latter and many stratagems have been devised to achieve this goal. It must be kept in mind, however, that direct dehydration of amides is not feasible. One indirect approach has already been exemplified, namely the prior conversion of amides into thioamides. Another classical method involves halogen chloride elimination from amide chlorides. These versatile salts have received only scant attention prior to 1960. 

Q or Br designates the corresponding a-chloro or a-bromo-aldehyde or ketone PjS5 designates thioamides formed in situ from amides. 

Another possibility to obtain thioamides from thioamides is via electrophilic attack in the a-position (Scheme 6). Thus, aldol-type reactions are feasible and proceed with high erythro selectivity providing (30), or even allowing optically active products (c/. Volume 2, Chapter 1.6). Heating with a form-amide 0,V-acetal gives an a,3-unsaturated thioamide (31) in a sequence of electrophilic attack and elimination. ... 

Adducts from Amides (Thioamides) and Acylating Reagents... 

Spending thioamides in the best yields. Thiolysis of iminium salts derived from amides and oxalyl chloride or phosphoric trichlorides with ammonium tetra-thiomolybdate 40 occurs below room temperature within 2 h (Eq. 10) [32]. It is advantageous that the by-products are easily separated, although H2S is needed in order to prepare 40. 

Thioamides and Related Compounds. Thioamides are the most stable thiocarbonyl compounds and have been prepared, for a century, from amides and P4S10 under rather drastic conditions. However, even for this purpose LR has turned out to be a superior reagent. High yields are obtained for all types of thioamides and -lactams including the elusive unsubstituted acrylo-thioamide (eq 18) and thioformamides or thioamides bearing sensitive substituents such as NO2, Z-NH, or OH (eq 19). ... 

Amino-oc,/9-ethylenecarboxylic acid thioamides from carboxylic acid thio-amides. 

Thiolactams react with Fe(CO)s in THF to give cyclic Schiff bases (H. Alper, unpublished). Nitriles are obtained from thioamides or amides (Alper and Edward, 1968). 

Thioamides and Related Compounds. Thioamides are the most stable thiocarbonyl compounds and have been prepared, for a century, from amides and P4S10 under ratber drastic conditions. However, even for this purpose LR has turned out to be a superior... 

Trifluoromethyl groups can be converted to carboxyl groups under mild conditions by photohydrolysis Phosphonitrile chloride has been recommended as carboxyl activator, particularly for the preparation of amides and hydrazides The preparation of amides from inactive esters can be facilitated by the use of bifunctional catalysts such as 2-hydroxypyridine The preparation of imidate hydrochlorides, including N-subst. ones, from amides and thioamides with chloroformate has been investigated. ... 

Thiazolium salts with alkyl (103, 722), arylalkyl (116), aryl (305), or heteroaryl (96) substituents on the nitrogen have been also prepared by this procedure. As in the thiazole series, N-substituted thioamides can be formed directly in the reaction mixture from phosphorus pentasulfide and N-substituted amides (127). These methods are important in the synthesis of thiamine 102 (vitamin Bj) (Scheme 45). 

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