- Ultrasound-Assisted Preparation of Di-tert-Butyl-, Di-1,1'-Adamantyl- and (1-Adamantyl)-tert-Butylketenes
-
Di-tert-butyl-, di(1-adamantyl)-, and (1-adamantyl)-tert-butylketenes were prepared in excellent yield from their corresponding acetyl chlorides with triethylamine under ultrasonic irradiation, showing a dramatic improvement over attempted conventional dehydrochlorination.
- Olah, George A.,Wu, An-hsiang,Farooq, Omar
-
-
Read Online
- Acylation mechanisms of DMSO/[D6]DMSO with Di-tert-butylketene and its congeners
-
Dimethyl sulfoxide (DMSO) and tBu2C=C=O in diglyme require heating to about 150 °C to furnish the Pummerer-type product tBu 2CHCO2CH2SCH3 through a novel mechanistic variant. The "ester enolate" tBu2C=C(O -)-O-S+(CH3)2 arising through the reversible addition of DMSO (step 1) to C-1 of tBu2C=C=O must be trapped through protonation (step 2) at C-2 by a carboxylic acid catalyst to form tBu2CH-C(=O)-O-S+(CH3)2 so that the reaction can proceed. The ensuing cleavage (step 3) of the O-S bond and one of the C-H bonds in the-S(CH3)2 group (E2 elimination, no ylide intermediate) results in the formation of tBu2CHCO 2- and H3CS-CH2+, whose combination (step 4) generates the final product. With a mixture of DMSO and [D6]DMSO competing for tBu2C=C=O in diglyme, the small value of the kinetic H/D isotope effect (KIE) kH/kD = 1.26 at 150 °C indicates that the cleavage of the C-H/C-D bonds (step 3) does not occur in the transition state with the highest free enthalpy. Therefore, the practically isotope-independent steps 1 and 2 determine the overall rate. The alternative slow initial protonation at C-2 of tBu2C=C=O generating the acylium cation tBu2CHC≡O+ can be excluded. Preparatory studies were undertaken to compare the mechanistic behavior of tBu2C=C=O with that of two related acylating agents: (i) The anhydride (tBu2CHCO)2O affords the same Pummerer-type product more slowly, again with an unexpectedly small KIE of 1.24 at 150 °C, which indicates that the overall rate is limited here by the almost isotope-independent initial O-acylation of DMSO in the addition/elimination (AE) mechanism. (ii) The acyl chloride tBu2CHCOCl affords ClCH 2SCH3 through a more common mechanistic variant involving neither the ketene nor the acylium cation tBu2CHC≡O +: The modestly enhanced kH/kD value of 2.4 at 55 °C shows that the C-H/C-D bond fissions contribute to the overall rate in cooperation with the retarded initial O-acylation. Deuterium labeling was quantified through 1H and 13C NMR integrations of deuterium-shifted signals.
- Knorr, Rudolf
-
experimental part
p. 6335 - 6342
(2011/12/05)
-
- Shorter and easier syntheses of Di-tert-butylketene and related gem-Di-tert-butyl compounds
-
The ketene tBu2C=C=O is prepared from tBu2C=O in three steps (performable as a two-stage operation) through elimination of HCl from the intermediate product tBu2CCl-CH=O. The acid tBu 2CH-CO2H, obtainable in two, three, or four preparative stages from tBu2C=O, adds slowly to the ketene to produce the anhydride (tBu2CH-CO)2O. Elemental lithium together with ClSiMe3 converts tBu2CCl-CH=O into tBu2C=CH- OSiMe3, which is a durable precursor of tBu2CH-CH=O, making this aldehyde easily and cheaply available from tBu2C=O. By exclusion of alternative mechanistic possibilities, the reduction of tBu 2CCl-CH=O by tBuMgCl is shown to involve at least one single-electron transfer, leading to the enolate tBu2C=CH-OMgCl, which can be converted into tBu2CH-CH=O (three steps from tBu2C=O) or into tBu2C=CH-OSiMe3. Hydride transfer from NaBH 4 to tBu2CCl-CH=O affords tBu2CCl-CH 2OH, the transformations of which provide an entertaining set of SN1-type reactions. Several other examples of carbenium-type behavior are encountered in this gem-tBu2 system; they are attributed to steric congestion, which also impedes bond rotations in the anhydride and in two esters. A convenient route to tBu2CH-C≡N (five steps from tBu2C=O) uses the conversion of tBu2C=CH-OSiMe3 into tBu2CH-CH=NOH. The slow thermal (Z)/(E) equilibration of tBu2CH-NH-CH=O reveals the ranking of ecliptic repulsions as H 3C 2CH. Copyright
- Knorr, Rudolf,Hennig, Karsten-Olaf,Schubert, Bernhard,Boehrer, Petra
-
body text
p. 6651 - 6664
(2011/02/26)
-
- HETEROCUMULENES IN ACYLATION REACTIONS II. REACTIVITY OF ALKYL-SUBSTITUTED KETENES IN SPONTANEOUS HYDROLYSIS REACTIONS
-
The structures and electronic states of six ketenes were investigated by the semiempirical MNDO, MINDO/3, and AM1 quantum-chemical methods.The kinetics of the spontaneous hydrolysis of mono- and dialkylsubstituted ketenes in water and water-acetonitrile mixtures at 20 deg C were studied.The induction and steric effects of the substituents on the process rate were evaluated quantitatively in terms of a ρ? analysis.An increase in the number, length, and branching of the hydrocarbon substituents at the terminal carbon atom of the ketenes leads to a large decrease in the hydrolysis rate.The results of the calculations were compared with experimental data.Possible alternatives for the mechanism of the process are discussed.
- Lyashchuk, S. N.,Skrypnik, Yu. G.
-
-
- Thioketene Syntheses, VI. Stable Thioketenes via Thionation of Sterically Hindered Acyl Chlorides
-
Sterically hindered acyl chlorides 11 are accessible via alkylation of the ester 1 or via chain elongation of the ketones 4 by one carbon atom in practicable, though multi-step reaction sequences.Action of phosphorus pentasulfide/pyridine on 11 leads to t
- Schaumann, Ernst
-
p. 2755 - 2765
(2007/10/02)
-
- SYNTHESE EN SERIE ENCOMBREE. PREPARATION DE L'ACIDE DITERTIOBUTYL-METHYLACETIQUE tBu2MeCCOOH ET DE QUELQUES COMPOSES CETONIQUES DERIVES DE CETTE STRUCTURE
-
The synthesis of tBu2MeCCOOH, which is among the most sterically hindered known acids is described.Only Newman's sequence via tBu2C=O, tBu2MeCOH, tBu2C=CH2, tBu2CHCH2OH, tBu2CHCOOH, tBu2CHCOCl and tBu2C=C=O, which has been optimized in this work by a direct access to tBu2CHCOOH, permits the preparation of tBu2MeCCOOH.The condensation of the corresponding chloride with a Grignard reagent yields new highly-hindered ketones tBu2MeCCOR wich by alkylation give more substituted structures.The limitations of each method have been studied in this work.
- Dubois, Jacques-Emile,Zhang, Ben Li,Lion, Claude
-
p. 4189 - 4194
(2007/10/02)
-