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Vinylic Cations from Solvolysis. XX.l y 2 Ion Pairs and Free Ions in the Solvolysis and Isomerization of
 

Summary: 821
Vinylic Cations from Solvolysis. XX.l y 2 Ion Pairs and Free
Ions in the Solvolysis and Isomerization of
1,2-Dianisy1-2-phenylvinylHalides and Mesylates.
Use of Cis-Trans Isomerization as a Mechanistic Tool
Zvi RapPoport* and Yitzhak Apeloig
Contributionfrom the Department of Organic Chemistry, The Hebrew University,Jerusalem.
Israel. Received December 3, I973
Abstract: The acetolysis of cis- and trans- 1,2-dianisyl-2-phenylvinylbromides (5-Br and 6-Br) and the cis chloride (5-CI) in
unbuffered and buffered AcOH shows strong common ion rate depression within a run, or by added halide ion; >93% of the
products arises from the "dissociated" ion 7. The products are 54% of the cis and 46%of the trans acetates (5-OAc and 6-
OAc). Methods for evaluating the extrapolated titrimetric rate constants kr0 and the apparent selectivity constant aappof 7
are discussed. Capture of 7 by CI- gives a 1:l mixture of 5-CI and 6-CI. These reactions are accompanied by extensive cis-
trans isomerization of the unreacted halide, which is the main process in the presence of external halide ion. A mechanism
involving the ion pair 8 which gives internal return with isomerization and 7 which gives either external ion return with isom-
erization or solvolysis products fits the data and is verified by a simulation method: 8 from 5-Br gives 25.4% of 5-Br, 22% of
6-Br. and 52.6% of 7. The ionization rate constant k ,on and the true selectivity constant a of 7 were evaluated by several
methods. Both solvolysis and isomerization are accelerated by AgOAc, but only the isomerization is appreciably accelerated
by LiC104. Acetolysis of the corresponding mesylates 5-OMSand 6-OMS shows external ion return by OMS-, and the ion
pair 16 gives 13.6%of 5-OMs, 10.4% of 6-OMs, and 76% of 7. Nonheterolytic isomerization routes were excluded by using

  

Source: Apeloig, Yitzhak - Department of Chemistry, Technion, Israel Institute of Technology

 

Collections: Chemistry