Mechanism of Reaction of Geminal Dihalides with Lithium Aluminum Hydride. Evidence for Single Electron Transfer as the Major Reaction Pathway
摘要:
The reactions of a sterically hindered geminal dichloride 4a and the corresponding diiodide 4b with LiAlH4 (LAH) were found to involve an electron-transfer mechanism. Whereas the monochloro analog of 4a (8) is inert toward LAH, 4a was more reactive. Observation of radical-derived products in the reaction of 4a with LAH indicates that the presence of two chlorine atoms on the same carbon atom results in a favorable reduction potential for 4a, as compared to 8. These results show that LAH can function as a one-electron donor toward geminal dichlorides. It was found that the diiodo analog 4b is more reactive toward LAH than 4a due to the inherent favorable reduction potentials of alkyl iodides compared to chlorides. The reactivity of 4b toward LAH was also found to be greater than that of the monoiodo analog (Ib) in keeping with the more favorable reduction potential of the diiodide (4b) compared to the monoiodide (Ib). The rates of cyclization of the corresponding haloalkyl radicals generated from 4a and 4b were also determined and were found to be 7.4 x 10(5)/s for the chloroalkyl and 5.5 x 10(5)/s for the iodoalkyl radical at 25 degrees C. The formation of small amounts of the carbene-derived product, 5, in the reactions of both 4a and 4b with LAH appeared to be preceded by a radical intermediate.
Concerning the preparation of geminal digrignard reagents.
作者:Fabio Doctorovich、Abhay K. Deshpande、E.C. Ashby
DOI:10.1016/s0040-4020(01)90449-1
日期:1994.1
Mechanistic studies of the reactions of geminal dihalides with magnesium and sodium have been carried out and the possibility of preparing geminal DiGrignard Reagents for use in situ has been explored.
Electron Transfer in the Reactions of Geminal Dihalides with Ph2P-. Evidence for the Formation of a Carbene Intermediate from a Radical Precursor
作者:E. C. Ashby、Abhay K. Deshpande
DOI:10.1021/jo00127a014
日期:1995.11
The reactions of two sterically hindered geminal dihalides, 6,6-dichloro-5,5-dimethyl-1-hexene (5a) and 6,6-diiodo-5,5-dimethyl-1-hexene (5b), with Ph(2)P(-) have been found to involve a single electron transfer (SET) pathway. Since the corresponding monochloride reacts with Ph(2)P(-) by a S(N)2 pathway, it is clear that adding a second chlorine atom on a carbon atom produces a more favorable reduction potential resulting in a change in mechanism from S(N)2 to SET. Evidence is also presented that shows that the geminal dihalides produce carbene-derived products and that the carbene was derived from a haloalkyl radical precursor.
A Mechanistic Study of the Reactions of Geminal Dihalides with LDA. Evidence for a Single Electron Transfer Pathway in the Reaction of Geminal Diiodides
作者:E. C. Ashby、Abhay K. Deshpande、Girish S. Patil
DOI:10.1021/jo00108a032
日期:1995.2
While the reaction of the geminal dichloride, 6,6-dichloro-5,5-dimethyl-1-hexene (8a) with LDA afforded only one product, wh:ich was derived from a chlorocarbene intermediate, the reaction of the corresponding geminal diiodide 8b resulted in the formation of a number of products that were accounted for by a single electron transfer (SET) initiated reaction. This difference in the reactivity patterns of the two substrates shows that the reduction potential of 8b is more favorable than that of 8a and that LDA can function as a one-electron donor. Additionally, 8a was more reactive toward LDA than the already studied monochloro substrate (1a) with the same carbon skeleton. This enhanced reactivity is attributed to the increased acidity of the hydrogen atom attached to the carbon bearing the two chlorine atoms in 8a. The observation that 8b reacted with LDA at a faster rate, not only relative to 8a but also to the corresponding monoiodo compound 1c, indicates that the reduction potential of 8b is more favorable than that of 1c for SET reduction by LDA. Based on the nature of the products formed in the reactions of 8a and 8b with LDA, evidence is presented that supports the notion that, in addition to funtioning as a strong base, LDA can also function as a one-electron donor toward the geminal diiodide 8b.
Mechanism of Reaction of Geminal Dihalides with Magnesium. Evidence for the Formation of Carbenes from Radical Precursors. The Similarity in Reactions of Geminal Dihalides with Magnesium and LiAlH4
作者:E. C. Ashby、Abhay K. Deshpande、Fabio Doctorovich
DOI:10.1021/jo00100a024
日期:1994.10
Two geminal dihalides, 6,6-dichloro-5,5-dimethyl-1-hexene (2a) and 6,6-diiodo-5,5-dimethyl-1-hexene (2b) were allowed to react with Rieke activated magnesium (Mg*) in THF at 25 degrees C. Both radical and carbene intermediates were identified by product analyses, by radical trapping experiments using DCPH and THF-d(8), and by selective experiments in which isolated intermediates were shown to form the predicted products. Although carbene products predominated over radical products when the dichloride was allowed to react with Mg*, radical products predominated when the diiodide was allowed to react with Mg*. Evidence is presented that indicates that the carbene intermediate arises from a radical precursor. A mechanistic scheme (Scheme 5) is presented that is consistent with all of the observed data. A comparison of the reactions of 2b with Mg* and 2b with LiAlH4 shows that most of the products are common in both reactions thereby providing further evidence for LiAlH4 as a one-electron donor, as is the known behavior of Mg*.
The Mechanism of Reaction of Geminal Dihalides with Sodium Trimethyltin. Evidence for a Single Electron Transfer Initiated Reaction Which Produces Both Radical and Carbene Intermediates
作者:E. C. Ashby、Abhay K. Deshpande
DOI:10.1021/jo00103a030
日期:1994.12
The reactions of two geminal dihalides, 6,6-dichloro-5,5-dimethyl-1-hexene (2a) and 6,6-diiodo-5,5-dimethyl-1-hexene (2b) with sodium trimethyltin (NaSnMe(3)), a known one-electron donor toward alkyl halides, have been studied. Evidence for a single electron transfer (SET) pathway has been obtained for both reactions. Although the monochloro analog of 2a, namely 7, was inert toward NaSnMe(3), 2a afforded 11 products. The nature of the products clearly indicates that the presence of two chlorine atoms on the same carbon atom results in a more favorable reduction potential than when just one chlorine atom is present. Both 2a and 2b on reaction with NaSnMe(3) also produced carbene-derived products, and evidence is presented that establishes that the carbene intermediate was in fact preceded by a radical intermediate. In addition, trimethyltin-substituted products (open chain as well as cyclized) were also obtained from both substrates and were found. to be derived from radical intermediates.