Amines sulfenamides

A)Ai-dicyclohexyl-2-benzotliiazolesulfenamide (42) [4979-32-2], The cyclohexylamine derivative is preferred over /n/ butylamine [75-64-9] and morpholine sulfenamide analogues because of lower amine volatility and less nitrosamine risk respectively. 

Sulfenamides. Sulfenamides (4) are often produced by oxidising an equimolar mixture of MBT and an aliphatic amine. Alternatively, the /V-ch1oroamine can react with the sodium salt of MBT. One sulfenamide, OTOS (S), uses a thiocarbamyl functionaUty in place of the ben2othia2ole group. 

Guanidines. Guanidines (10) were one of the first aniline derivatives used as accelerators. They are formed by reaction of two moles of an aromatic amine with one mole of cyanogen chloride. Diphenylguanidine (DPG) has enjoyed a resurgence ia demand as an activator for sulfenamides and a co-accelerator ia tire tread compounds which employ siUca fillers for low rolling resistance. Guanidines alone show too Htde activity to be extensively used as primary accelerators. There were no U.S. producers as of mid-1996. 

It is common practice in the mbber industry for a compounder to use combinations of several accelerators in developing a cure system. Typically these cure systems are comprised of a primary accelerator and one or more secondary types. Primary accelerators are generally the thiazole and sulfenamide classes the secondary types (kickers) are the thiurams, dithiocarbamates, guanidines, and to a much lesser extent, certain amines and the dialkylphosphorodithioates (20). 

Sulfenamide accelerators generally requite less fatty acid because they release an amine during the vulcanization process which acts to solubilize the zinc. Guanidines and similar amine accelerators also serve to both activate and accelerate vulcanization. 

The thiophthalimide (CTP) and sulfenamide classes of retarders differ from the organic acid types by thek abiUty to retard scorch (onset of vulcanization) without significantly affecting cure rate or performance properties. Much has been pubUshed on the mechanism of CTP retardation. It functions particularly well with sulfenamide-accelerated diene polymers, typically those used in the the industry. During the initial stages of vulcanization, sulfenamides decompose to form mercaptobenzothiazole (MBT) and an amine. The MBT formed reacts with additional sulfenamide to complete the vulcanization process. If the MBT initially formed is removed as soon as it forms, vulcanization does not occur. It is the role of CTP to remove MBT as it forms. The retardation effect is linear with CTP concentration and allows for excellent control of scorch behavior. 

Six categories of 7V-hetero atom derivatives are considered N-M (M = boron, copper), N-N (e.g., N-nitro, A-nitroso), N-oxides (used to protect teriary amines), N-P (e.g., phosphinamides, phosphonamides), N-SiR3 (R = CH3), and N-S (e.g., sulfonamides, sulfenamides). 

Sulfenamides, R2NSR, prepared from an amine and a sulfenyl halide, " are readily cleaved by acid hydrolysis and have been used in syntheses of peptides, penicillins, and nucleosides. They are also cleaved by nucleophiles, and by Raney nickel desulfurization. ... 

Af,Af-dicyclohexyl-benzothiazole-sulfenamide (DCBS), cobalt naphthenate, and diaryl-p-diphenylene-amine (each 1 phr). 

Although ultra accelerators or sulfur donors can be used together with primary accelerator (such as sulfenamide, TBBS) to improve cure rate as well as the heat resistance [16-18], their use is restricted because of the associated nitrosamine issue [19]. Accelerators derived from secondary amines, for example, MBS, TMTD, TETD, TMTM, and OTOS fall into this category. The combination of sulfenamide, such as CBS or TBBS, and a thiuram, such as TMTD or TETD, shows high-cure rates but suffers from the adverse effects on scorch resistance and vulcanizate dynamic property [20]. Additionally as previously mentioned, the use of TMTD or Tetraethylthiuram disulhde (TETD) or A-oxidiethylene dithiocarbamyl-A -oxidiethylene sulfenamide (OTOS) or 4,4 -Dithiodimorpholine (DTDM) is undesirable [21] due to concerns over carcinogenic nature of the A-nitrosamines formed from the parent amines. The solution to this originated by introduction of nitrosamine safe ultra accelerator such as TBzTD [22,23]. 

Primary amine-based benzothiazyl sulfenamides (CBS, TBBS)... 

Mikolajczyk and coworkers have summarized other methods which lead to the desired sulfmate esters These are asymmetric oxidation of sulfenamides, kinetic resolution of racemic sulfmates in transesterification with chiral alcohols, kinetic resolution of racemic sulfinates upon treatment with chiral Grignard reagents, optical resolution via cyclodextrin complexes, and esterification of sulfinyl chlorides with chiral alcohols in the presence of optically active amines. None of these methods is very satisfactory since the esters produced are of low enantiomeric purity. However, the reaction of dialkyl sulfites (33) with t-butylmagnesium chloride in the presence of quinine gave the corresponding methyl, ethyl, n-propyl, isopropyl and n-butyl 2,2-dimethylpropane-l-yl sulfinates (34) of 43 to 73% enantiomeric purity in 50 to 84% yield. This made available sulfinate esters for the synthesis of t-butyl sulfoxides (35). 

A new stable sulfenylating reagent 3-phenylsulfenyl-2-GV-cyano-imino)thiazolidine 57 has been described. It reacts with amines or thiols to give sulfenamides or disulfides in excellent yields. a-Sulfenylation of carbonyl compounds also proceeds smoothly and if an optically active 4-diphenylmethyl substituent is attached to the thiazolidine ring (58), the cyclic (3-ketoester 59 can be sulfenylated in high yield with an ee of 96% to give the sulfide 60 <00SL32>. 

Thermolysis of the thiadiazole (164) leads to elimination of isocyanate and sulfur giving the triazine derivative (167). If the thermolysis is carried out in the presence of phenols 2-aryl-benzimidazoles (168) are produced <85JCS(P1)1007>. The S—N bond of (157) is readily cleaved with both N- and C-nucleophiles. Thus, treatment of (157) with an excess of amine gives the sulfenamide (169) (Scheme 39) and reaction of (157) with active methylene compounds leads to derivatives of type (170) (Scheme 39) which on heating furnish (171). Cyanide ion inserts into the S—N bond of (164), probably via the intermediate (172) which immediately recyclizes to give the thiadiazinone (173) (Scheme 40) <85JCS(P1)1007>. 

The condensation reaction of cyclic amidines with trichloromethylsulfenyl chloride yields sul-fenamides, which afford 5-arylimino-l,2,4-thiadiazolines on treatment with aromatic amines <84CHEC-I(6)463>. An example of this type of reaction starting from 2-amino-4-arylthiazoles (271) affords 3/f-thiazolo[2,3-c]-l,2,4-thiadiazoles (272), via the sulfenamide (270) (Scheme 60) <88IJC(B)501>. 

Trichloromethanesulfenyl chloride (98) converts 2-aminopyridine (97) into the intermediate sulfenamide (99) which with an aromatic amine cyclizes to (101), probably via the intermediate (100) (75JOC2600). 

The delay period exhibited with sulfenamide cures is explained in terms of the formation of intermediates by reaction with activated sulfur (Scheme 5) (80MI11508). The 2-mercap-tobenzothiazole (31) produced reacts rapidly with the sulfenamide providing a more facile pathway for vulcanization (equation 11). The amine (38) produced also acts as a catalyst, so that the cure, once started, becomes autocatalytic (64MI11503). 

In the case of sulfenamide accelerators, the inhibitor appears to function by reacting with 2-mercaptobenzothiazole (77MI11506) so suppressing the catalytic pathway (equation 11). This is illustrated by the reaction between 2-mercaptobenzothiazole and IV-cyclo-hexylthiophthalimide (39), one of the most effective PVIs currently in use (equation 12) (73MI11500). It is likely that the reaction between the PVI and amines (equation 13) also makes a contribution to the mechanism of inhibition (77MI11506). 

Amination of 2,2-disubstituted thioindoxyls followed by treatment with base results in ring opening to the sulfenamide (451). This can either cyclize to a benzisothiazole (when R=Me) or to a tetrahydro-l,2-benzothiazepin-5-one (when R = aryl), as shown in Scheme 153 (80JCS(Pi)2830). 3-Methoxybenzo[6]thiophene undergoes ring expansion to 3-chlorothiochromone on treatment with dichlorocarbene (70AHC(11)177). 

Their synthesis can also be carried out as a two-phase electrolysis, using ammonium salts 326). Sulfenamides can be produced by oxidizing tetraalkylthiuram disulfides in the presence of amines 321 ... 

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