Acylating cleavage

Scheme 5.1-15 The acylative cleavage of ethers in an ionic liquid.

The other mechanism involving acyl cleavage is the AacI mechanism. This is rare, being found only where R is very bulky, so that bimolecular attack is sterically hindered, and only in ionizing solvents. The mechanism has been demonstrated for esters of 2,4,6-trimethylbenzoic acid (mesitoic acid). This acid depresses the freezing point of sulfuric acid four times as much as would be predicted from its molecular weight, which is evidence for the equilibrium... 

This reaction, while rare, has been observed for various N-tert-huiyl amides in 98% sulfuric acid, where the mechanism was the AalI mechanism, and for certain amides containing an azo group, where a BalI mechanism was postulated. Of the two first-order acyl cleavage mechanisms, only the AacI has been observed, in concentrated sulfuric acid solutions. Of course, the diazotization of unsubstituted amides might be expected to follow this mechanism, and there is evidence that this is true. ... 

Drivers for Performing Acylating Cleavage in Micro Reactors... 

Beneficial Micro Reactor Properties for Acylating Cleavage... 

Acylating Cleavage Investigated in Micro Reactors Organic synthesis 11 [OS 11] Synthesis of piperazine... 

OS 11] [R 36] ]P 10] The feasibility of performing the acylating cleavage of T2-triazene resin (100 mg) in absolute tetrahydrofuran to give piperazine was demonstrated [75] 4 h of processing was required. 

Use of graphite-supported methodology has been reported for three types of reaction - the Friedel-Crafts acylation [15, 16, 27, 66], the acylative cleavage of ethers [15, 16], and the ketodecarboxylation of carboxylic diacids [67, 68], either with conventional heating (GS/A) or MW irradiation (GS/MW coupling) these are discussed below. First, however, we describe the analysis of two commercial graphites of different purity which are used for these experiments. 

The term acid catalysis is often taken to mean proton catalysis ( specific acid catalysis ) in contrast to general acid catalysis. In this sense, acid-catalyzed hydrolysis begins with protonation of the carbonyl O-atom, which renders the carbonyl C-atom more susceptible to nucleophilic attack. The reaction continues as depicted in Fig. 7. l.a (Pathway a) with hydration of the car-bonium ion to form a tetrahedral intermediate. This is followed by acyl cleavage (heterolytic cleavage of the acyl-0 bond). Pathway b presents an mechanism that can be observed in the presence of concentrated inorganic acids, but which appears irrelevant to hydrolysis under physiological conditions. The same is true for another mechanism of alkyl cleavage not shown in Fig. 7.Fa. All mechanisms of proton-catalyzed ester hydrolysis are reversible. 

Fig. 7.1. a) Specific acid catalysis (proton catalysis) with acyl cleavage in ester hydrolysis. Pathway a is the common mechanism involving a tetrahedral intermediate. Pathway b is SN1 mechanism observed in the presence of concentrated inorganic acids. Not shown here is a mechanism of alkyl cleavage, which can also be observed in the presence of concentrated inorganic acids, b) Schematic mechanism of general acid catalysis in ester hydrolysis. 

Fig. 7.2. a) The most common mechanism of base-catalyzed ester hydrolysis, namely specific base catalysis (HCT catalysis) with tetrahedral intermediate and acyl cleavage. Not shown here are an W mechanism with alkyl cleavage observed with some tertiary alkyl esters, and an 5n2 mechanism with alkyl cleavage sometimes observed with primary alkyl esters, particularly methyl esters, b) Schematic mechanism of general base catalysis in ester hydrolysis. Intermolecular catalysis (bl) and intramolecular catalysis (b2). c) The base-catalyzed hydrolysis of esters is but a particular case of nucleophilic attack. Intermolecular (cl) and intramolecular (c2). d) Spontaneous (uncatalyzed) hydrolysis. This becomes possible when the R moiety is... 

In the presence of a large excess of olefin, most of the HCo(CO)4 is quickly converted to the acyl compound and none is available for acyl cleavage to the final products hence, under these conditions aldehyde yield is very low and a 1 1 ratio of absorbed CO HCo(CO)4 is approached. On the other hand, in the absence of a large excess of olefin, the rate of acyl formation is slower, and sufficient HCo(CO)4 is available to cleave the... 

B.2.2. Dialkyl Ether Cleavage. Varying the mole fraction of AICI3 in an ionic liquid can optimize reactions that are sensitive to the strength of Lewis acidity. When the ionic liquids [EMIM]C1/A1C13 were applied for the acylative cleavage of dialkyl ethers (Scheme 6), the reaction was found to be sensitive to the bulk Lewis acidity of the ionic liquid solvent 138). 

The acylative cleavage of the cyclic ether tetrahydrofuran showed an excellent yield (95%) to 4-iodobutylbenzoate when fully acidic ionic liquid [EMIM]Al2Cl7 was used, but the yield to di-functionalized product (61%) suffered when the mildly acidic halogenoaluminate ionic liquid (which has a mole fraction of AICI3 (x) at a value of 0.52) was used as the solvent. In contrast, for 1,5-dimethyltetrahydrofuran and tetrahydropyran, a good yield to the cleavage products was obtained when the mildly acidic chloroaluminate was used. 

Scheme 2 Metal-catalyzed anionic ROP of lactide (a) deprotonation (b) acyl cleavage...

Cyclic esters of oo-hydroxycarboxylic acids can be polymerized by ring-opening to give linear aliphatic polyesters. According to the type of initiator and monomer the polymerization occurs either by alkyl or by acyl cleavage ... 

CHXOO Na + (R)-HOCH(CHj)CHjCH,. The alcohol is 2 we find O—acyl cleavage and no change in configuration of alcohol. RCOO Na forms in basic solution. 

CH,COOH + (R)-HOCH(CH,)CH,CH,. Again O—acyl cleavage occurs. 

---