Catalysis in organosilicon chemistry. V. General-base catalysis of silicon-oxygen bond cleavage. Hidden isotope effects

1974 ◽  
Vol 96 (22) ◽  
pp. 6980-6982 ◽  
Author(s):  
Agnieska Modro ◽  
Richard L. Schowen
Keyword(s):  
Bond Cleavage ◽  
Isotope Effects ◽  
Base Catalysis ◽  
General Base ◽  
Organosilicon Chemistry ◽  
Oxygen Bond ◽  
Silicon Oxygen

Classical General Base Catalysis of Silicon-Oxygen Bond Cleavage1

1966 ◽  
Vol 88 (16) ◽  
pp. 3795-3797 ◽  
Author(s):  
Richard L. Schowen ◽  
Kenyon S. Latham
Keyword(s):  
Base Catalysis ◽  
General Base ◽  
Oxygen Bond ◽  
Silicon Oxygen

Multiple changes in rate-determining step in the acid and base catalyzed cyclizations of ethyl N-(p-nitrophenyl)hydantoates caused by methyl substitution

1999 ◽  
Vol 77 (5-6) ◽  
pp. 849-859
Author(s):  
Iva B Blagoeva ◽  
Anthony J Kirby ◽  
Asen H Koedjikov ◽  
Ivan G Pojarlieff
Keyword(s):  
Steric Hindrance ◽  
Acid Catalysis ◽  
Isotope Effects ◽  
Base Catalysis ◽  
Ethoxy Group ◽  
First Order ◽  
Tetrahedral Intermediate ◽  
General Base ◽  
Rate Determining Step ◽  
Acid And Base

The slopes of the pH-rate profiles for the cyclization of 2-methyl- and 2,3-dimethyl hydantoates 1-NPU and 2-NPU between pH 1 and 7 change from 1 to 0 and then back to 1. A reaction first order in H+ was observed with the latter compound. The 2,2,3-trimethyl derivative 3-NPU showed only one reaction first order in OH-, but complex acid catalysis is described by slopes 0, -1, 0, and finally -1 again. The cyclizations were general base catalyzed, with Brønsted β values of 0.5-0.6. The OH- catalysis at higher pH for 1-NPU and 2-NPU showed inverse solvent kinetic isotope effects and deviated from the Brønsted relationships, while that for 3-NPU showed a normal effect and complied with the Brønsted relationship. The accelerations due to the gem-dimethyl effect were lost with the OH- and general base-catalyzed reactions of 3-NPU. This behaviour is due to a change from the rate-determining formation of the tetrahedral intermediate with 1-NPU and 2-NPU to the rate-determining breakdown with 3-NPU, due to steric hindrance to protonation of the leaving ethoxy group. The OH- reaction at higher pH involves attack of the ureide anion with 1-NPU and 2- NPU, becoming concerted with deprotonation when catalyzed by general bases and changing to acid inhibition of the anion of the tetrahedral intermediate at low pH. With 3-NPU at higher pH, T- is in equilibrium and the conjugate acids of the general bases accelerate its breakdown by protonating the ethoxy group. Acid catalysis of the cyclization of 3-NPU at higher pH is also protonation of the leaving group from T0 changing to the rate-determining formation of T at lower pH. The latter mechanism is preferred for the cyclization of 2-NPU.Key words: gem-dimethyl effect, mechanism, general base catalysis, proton transfer, steric hindrance.

scholarly journals Tactics for probing aryne reactivity: mechanistic studies of silicon–oxygen bond cleavage during the trapping of (HDDA-generated) benzynes by silyl ethers

2014 ◽  
Vol 5 (2) ◽  
pp. 545-550 ◽  
Author(s):  
Thomas R. Hoye ◽  
Beeraiah Baire ◽  
Tao Wang
Keyword(s):  
Bond Cleavage ◽  
Mechanistic Studies ◽  
Silyl Ethers ◽  
Oxygen Bond ◽  
Silicon Oxygen ◽  
Clock Reaction

Mechanistic features of the trapping reaction of an HDDA-generated aryne by pendant silyl ethers were probed using, e.g., an intramolecular clock reaction (cf. k2).

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