Is Pinacol rearrangement the source for the ring expansion?

Edward Answered Jun 20 2021

Predict the major product of the given reaction

I have no idea what's happening as in the product (the answer is (a)) there is a new ring being formed on the left benzene. But as I studied after $\ce{H2O}$ leaves, a carbocation is then formed on stabilized by the right ring.

Can someone please explain the rest of the steps of this reaction? The thing I don't get is how does the three-membered ring opening happens. I attempted the question, but the answer that I came up with isn't the one matching in the book. Where did I go wrong with the mechanism?

attempted mechanism

Daley Answered Jun 20 2021

Your first and third structures are good but the arrow pushing in the second structure doesn't get the job done. As you have noted, diol 1 forms carbocation 2a the more stable of the two options. Now the lefthand cyclopropane bond with its pair of electrons migrates to the cationic center to form resonance-stabilized species 2b and on to ketone 3. This type of structure is a candidate for a Nazarov cyclization. The arrows in 4 may be drawn in the other direction as well. Rearomatization of 5 by loss of a proton and tautomerization leads to the product 6. [Note: Structures 2b and 4 are identical. Just a reprotonation of 3.]

ADDENDUM: To answer your original question, yes it is a pinacol rearrangement. The bond that migrates (2a --> 2b) just happens to be attached via the cyclopropane ring to the site to which it is migrating. So, 1 --> 3 is a pinacol rearrangement.

Valeria Answered Jun 20 2021

The first generated intermediate, an α-hydroxycarbenium ion, rearranges through a 1,2-alkyl shift to produce the carbonyl compound. If two of the substituents form a ring, the Pinacol Rearrangement can constitute a ring-expansion or ring-contraction reaction.

Pinacol Rearrangement

Pinacol Rearrangement In the conversion that gave its name to this reaction, the acid-catalyzed elimination of water from pinacol gives t -butyl methyl ketone. Mechanism of the Pinacol Rearrangement This reaction occurs with a variety of fully substituted 1,2-diols, and can be understood to involve the formation of a carbenium ion intermediate that subsequently undergoes a rearrangement. The first generated intermediate, an α-hydroxycarbenium ion, rearranges through a 1,2-alkyl shift to produce the carbonyl compound. If two of the substituents form a ring, the Pinacol Rearrangement can constitute a ring-expansion or ring-contraction reaction. Recent Literature Polymer-Mediated Pinacol Rearrangements C. Pavlik, M. D. Morton, M. B. Smith, Synlett , 2011 , 2191-2194. Asymmetric Addition of Alkylzinc Reagents to Cyclic α,β-Unsaturated Ketones and a Tandem Enantioselective Addition/Diastereoselective Epoxidation with Dioxygen S.-J. Jeon, P. J. Walsh, J. Am. Chem. Soc. , 2003 , 125 , 9544-9545. Applications of 1-Alkenyl-1,1-Heterobimetallics in the Stereoselective Synthesis of Cyclopropylboronate Esters, Trisubstituted Cyclopropanols and 2,3-Disubstituted Cyclobutanones M. M. Hussain, H. Li, N. Hussain, M. Ureña, P. J. Carroll, P. J. Walsh, J. Am. Chem. Soc. , 2009 , 131 , 6516-6524. Applications of 1-Alkenyl-1,1-Heterobimetallics in the Stereoselective Synthesis of Cyclopropylboronate Esters, Trisubstituted Cyclopropanols and 2,3-Disubstituted Cyclobutanones M. M. Hussain, H. Li, N. Hussain, M. Ureña, P. J. Carroll, P. J. Walsh, J. Am. Chem. Soc. , 2009 , 131 , 6516-6524. Visible-Light-Induced Aza-Pinacol Rearrangement: Ring Expansion of Alkylidenecyclopropanes W.-D. Liu, G.-Q. Xu, X.-Q. Hu, P.-F. Xu, Org. Lett. , 2017 , 19 , 6288-6291. Arylation and Vinylation of Alkenes Based on Unusual Sequential Semipinacol Rearrangement/Grob Fragmentation of Allylic Alcohols D.-Y. Yuan, Y.-Q. Tu, C.-A. Fan, J. Org. Chem. , 2008 , 73 , 7797-7799.

https://www.organic-chemistry.org/namedreactions/pinacol-rearrangement.shtm#:~:text=The%20first%20generated%20intermediate%2C%20an,expansion%20or%20ring%2Dcontraction%20reaction.

Share Tweet Email

Answer Follow+1