Possible nonclassical ion from a bicyclic system
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In this question a bicycling alcohol is reacted with acid to make what appears to be a tertiary carbocation, and the OP asked whether it could become aromatic. The given answer suggests it could be, but only if a nonclassical ion is formed. So, does that happen?
The substrate is 8a-methyl-1,3,4,8a-tetrahydronaphthalen-4a(2H)-ol.
organic-chemistry carbocation
asked May 28 at 15:12
Oscar LanziOscar Lanzi
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In this question a bicycling alcohol is reacted with acid to make what appears to be a tertiary carbocation, and the OP asked whether it could become aromatic. The given answer suggests it could be, but only if a nonclassical ion is formed. So, does that happen?
The substrate is 8a-methyl-1,3,4,8a-tetrahydronaphthalen-4a(2H)-ol.
organic-chemistry carbocation
asked May 28 at 15:12
Oscar LanziOscar Lanzi
19k2 gold badges32 silver badges58 bronze badges
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If classical carbocation is already heavily stabilised, it would be surprising if additional "nonclassical" stabilisation would be added. In this case it's a difference between sigma and pi complex between substituted benzene ring and methyl group - it would be rather a pi complex then nonclassical carbocation, even if it was an actual intermediate here.
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– Mithoron
May 29 at 21:20
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$begingroup$
In this question a bicycling alcohol is reacted with acid to make what appears to be a tertiary carbocation, and the OP asked whether it could become aromatic. The given answer suggests it could be, but only if a nonclassical ion is formed. So, does that happen?
The substrate is 8a-methyl-1,3,4,8a-tetrahydronaphthalen-4a(2H)-ol.
organic-chemistry carbocation
asked May 28 at 15:12
Oscar LanziOscar Lanzi
19k2 gold badges32 silver badges58 bronze badges
$endgroup$
In this question a bicycling alcohol is reacted with acid to make what appears to be a tertiary carbocation, and the OP asked whether it could become aromatic. The given answer suggests it could be, but only if a nonclassical ion is formed. So, does that happen?
The substrate is 8a-methyl-1,3,4,8a-tetrahydronaphthalen-4a(2H)-ol.
organic-chemistry carbocation
organic-chemistry carbocation
asked May 28 at 15:12
Oscar LanziOscar Lanzi
19k2 gold badges32 silver badges58 bronze badges
asked May 28 at 15:12
Oscar LanziOscar Lanzi
19k2 gold badges32 silver badges58 bronze badges
edited May 28 at 21:31
Oscar Lanzi
asked May 28 at 15:12
Oscar LanziOscar Lanzi
19k2 gold badges32 silver badges58 bronze badges
asked May 28 at 15:12
Oscar LanziOscar Lanzi
19k2 gold badges32 silver badges58 bronze badges
asked May 28 at 15:12
Oscar LanziOscar Lanzi
19k2 gold badges32 silver badges58 bronze badges
19k2 gold badges32 silver badges58 bronze badges
$begingroup$
If classical carbocation is already heavily stabilised, it would be surprising if additional "nonclassical" stabilisation would be added. In this case it's a difference between sigma and pi complex between substituted benzene ring and methyl group - it would be rather a pi complex then nonclassical carbocation, even if it was an actual intermediate here.
$endgroup$
– Mithoron
May 29 at 21:20
add a comment
|
$begingroup$
If classical carbocation is already heavily stabilised, it would be surprising if additional "nonclassical" stabilisation would be added. In this case it's a difference between sigma and pi complex between substituted benzene ring and methyl group - it would be rather a pi complex then nonclassical carbocation, even if it was an actual intermediate here.
$endgroup$
– Mithoron
May 29 at 21:20
$begingroup$
If classical carbocation is already heavily stabilised, it would be surprising if additional "nonclassical" stabilisation would be added. In this case it's a difference between sigma and pi complex between substituted benzene ring and methyl group - it would be rather a pi complex then nonclassical carbocation, even if it was an actual intermediate here.
$endgroup$
– Mithoron
May 29 at 21:20
$begingroup$
If classical carbocation is already heavily stabilised, it would be surprising if additional "nonclassical" stabilisation would be added. In this case it's a difference between sigma and pi complex between substituted benzene ring and methyl group - it would be rather a pi complex then nonclassical carbocation, even if it was an actual intermediate here.
$endgroup$
– Mithoron
May 29 at 21:20
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2 Answers
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I don't think it is necessary to consider a mechanism with a nonclassical carbocation. It would undergo normal rearrangement with a 1,2-methide shift within the same ring. See the mechanism I posted in previous question you were directing to:
Answer to this question would support for my mechanism.
answered May 28 at 18:29
Mathew MahindaratneMathew Mahindaratne
11.6k2 gold badges15 silver badges40 bronze badges
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So the answer seems to be "No". Fair enough.
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– Oscar Lanzi
May 28 at 21:32
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While there are examples of 1,2-methide shifts, this question has an uncanny resemblance to the dienone-phenol rearrangement whose mechanism was first elucidated by Woodward and Singh in 1950. Dienone 1 under acidic conditions undergoes rearrangement to phenol 6 and not, based on earlier speculation, to phenol 4. The direct 1,2-methide shift (2b --> 3) does not occur but rather the reaction proceeds through the spiro carbocation 5. continued 
Applying this approach to the carbocation 7 generated from the alcohol in this question, spiro carbocation 8 forms the predicted tetrahydronaphthalene 10. One way to distinguish between the two mechanisms is via a labeling experiment. All of the carbon label in 7 will retain its location as the red star in 10 if the 1,2-methyl shift mechanism applies. The spiro mechanism will partition the label ~50:50 between the two ring benzylic carbons. For related studies, see reference 2.
1) R. B. Woodward and T. Singh, J. Am. Chem. Soc., 1950, 72, 494.
2) A. J. Waring, J. H. Zaidi and J. W. Pilkington, J. Chem. Soc, Perkin Transactions I, 1981, 1454.
answered May 28 at 22:33
user55119user55119
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
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active
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$begingroup$
I don't think it is necessary to consider a mechanism with a nonclassical carbocation. It would undergo normal rearrangement with a 1,2-methide shift within the same ring. See the mechanism I posted in previous question you were directing to:
Answer to this question would support for my mechanism.
answered May 28 at 18:29
Mathew MahindaratneMathew Mahindaratne
11.6k2 gold badges15 silver badges40 bronze badges
$endgroup$
1
$begingroup$
So the answer seems to be "No". Fair enough.
$endgroup$
– Oscar Lanzi
May 28 at 21:32
add a comment
|
$begingroup$
I don't think it is necessary to consider a mechanism with a nonclassical carbocation. It would undergo normal rearrangement with a 1,2-methide shift within the same ring. See the mechanism I posted in previous question you were directing to:
Answer to this question would support for my mechanism.
answered May 28 at 18:29
Mathew MahindaratneMathew Mahindaratne
11.6k2 gold badges15 silver badges40 bronze badges
$endgroup$
1
$begingroup$
So the answer seems to be "No". Fair enough.
$endgroup$
– Oscar Lanzi
May 28 at 21:32
add a comment
|
$begingroup$
I don't think it is necessary to consider a mechanism with a nonclassical carbocation. It would undergo normal rearrangement with a 1,2-methide shift within the same ring. See the mechanism I posted in previous question you were directing to:
Answer to this question would support for my mechanism.
answered May 28 at 18:29
Mathew MahindaratneMathew Mahindaratne
11.6k2 gold badges15 silver badges40 bronze badges
$endgroup$
I don't think it is necessary to consider a mechanism with a nonclassical carbocation. It would undergo normal rearrangement with a 1,2-methide shift within the same ring. See the mechanism I posted in previous question you were directing to:
Answer to this question would support for my mechanism.
answered May 28 at 18:29
Mathew MahindaratneMathew Mahindaratne
11.6k2 gold badges15 silver badges40 bronze badges
edited May 28 at 19:21
answered May 28 at 18:29
Mathew MahindaratneMathew Mahindaratne
11.6k2 gold badges15 silver badges40 bronze badges
answered May 28 at 18:29
Mathew MahindaratneMathew Mahindaratne
11.6k2 gold badges15 silver badges40 bronze badges
answered May 28 at 18:29
Mathew MahindaratneMathew Mahindaratne
11.6k2 gold badges15 silver badges40 bronze badges
11.6k2 gold badges15 silver badges40 bronze badges
1
$begingroup$
So the answer seems to be "No". Fair enough.
$endgroup$
– Oscar Lanzi
May 28 at 21:32
add a comment
|
1
$begingroup$
So the answer seems to be "No". Fair enough.
$endgroup$
– Oscar Lanzi
May 28 at 21:32
1
1
$begingroup$
So the answer seems to be "No". Fair enough.
$endgroup$
– Oscar Lanzi
May 28 at 21:32
$begingroup$
So the answer seems to be "No". Fair enough.
$endgroup$
– Oscar Lanzi
May 28 at 21:32
add a comment
|
$begingroup$
While there are examples of 1,2-methide shifts, this question has an uncanny resemblance to the dienone-phenol rearrangement whose mechanism was first elucidated by Woodward and Singh in 1950. Dienone 1 under acidic conditions undergoes rearrangement to phenol 6 and not, based on earlier speculation, to phenol 4. The direct 1,2-methide shift (2b --> 3) does not occur but rather the reaction proceeds through the spiro carbocation 5. continued
Applying this approach to the carbocation 7 generated from the alcohol in this question, spiro carbocation 8 forms the predicted tetrahydronaphthalene 10. One way to distinguish between the two mechanisms is via a labeling experiment. All of the carbon label in 7 will retain its location as the red star in 10 if the 1,2-methyl shift mechanism applies. The spiro mechanism will partition the label ~50:50 between the two ring benzylic carbons. For related studies, see reference 2.
1) R. B. Woodward and T. Singh, J. Am. Chem. Soc., 1950, 72, 494.
2) A. J. Waring, J. H. Zaidi and J. W. Pilkington, J. Chem. Soc, Perkin Transactions I, 1981, 1454.
answered May 28 at 22:33
user55119user55119
5,4542 gold badges14 silver badges46 bronze badges
$endgroup$
add a comment
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$begingroup$
While there are examples of 1,2-methide shifts, this question has an uncanny resemblance to the dienone-phenol rearrangement whose mechanism was first elucidated by Woodward and Singh in 1950. Dienone 1 under acidic conditions undergoes rearrangement to phenol 6 and not, based on earlier speculation, to phenol 4. The direct 1,2-methide shift (2b --> 3) does not occur but rather the reaction proceeds through the spiro carbocation 5. continued
Applying this approach to the carbocation 7 generated from the alcohol in this question, spiro carbocation 8 forms the predicted tetrahydronaphthalene 10. One way to distinguish between the two mechanisms is via a labeling experiment. All of the carbon label in 7 will retain its location as the red star in 10 if the 1,2-methyl shift mechanism applies. The spiro mechanism will partition the label ~50:50 between the two ring benzylic carbons. For related studies, see reference 2.
1) R. B. Woodward and T. Singh, J. Am. Chem. Soc., 1950, 72, 494.
2) A. J. Waring, J. H. Zaidi and J. W. Pilkington, J. Chem. Soc, Perkin Transactions I, 1981, 1454.
answered May 28 at 22:33
user55119user55119
5,4542 gold badges14 silver badges46 bronze badges
$endgroup$
add a comment
|
$begingroup$
While there are examples of 1,2-methide shifts, this question has an uncanny resemblance to the dienone-phenol rearrangement whose mechanism was first elucidated by Woodward and Singh in 1950. Dienone 1 under acidic conditions undergoes rearrangement to phenol 6 and not, based on earlier speculation, to phenol 4. The direct 1,2-methide shift (2b --> 3) does not occur but rather the reaction proceeds through the spiro carbocation 5. continued
Applying this approach to the carbocation 7 generated from the alcohol in this question, spiro carbocation 8 forms the predicted tetrahydronaphthalene 10. One way to distinguish between the two mechanisms is via a labeling experiment. All of the carbon label in 7 will retain its location as the red star in 10 if the 1,2-methyl shift mechanism applies. The spiro mechanism will partition the label ~50:50 between the two ring benzylic carbons. For related studies, see reference 2.
1) R. B. Woodward and T. Singh, J. Am. Chem. Soc., 1950, 72, 494.
2) A. J. Waring, J. H. Zaidi and J. W. Pilkington, J. Chem. Soc, Perkin Transactions I, 1981, 1454.
answered May 28 at 22:33
user55119user55119
5,4542 gold badges14 silver badges46 bronze badges
$endgroup$
While there are examples of 1,2-methide shifts, this question has an uncanny resemblance to the dienone-phenol rearrangement whose mechanism was first elucidated by Woodward and Singh in 1950. Dienone 1 under acidic conditions undergoes rearrangement to phenol 6 and not, based on earlier speculation, to phenol 4. The direct 1,2-methide shift (2b --> 3) does not occur but rather the reaction proceeds through the spiro carbocation 5. continued
Applying this approach to the carbocation 7 generated from the alcohol in this question, spiro carbocation 8 forms the predicted tetrahydronaphthalene 10. One way to distinguish between the two mechanisms is via a labeling experiment. All of the carbon label in 7 will retain its location as the red star in 10 if the 1,2-methyl shift mechanism applies. The spiro mechanism will partition the label ~50:50 between the two ring benzylic carbons. For related studies, see reference 2.
1) R. B. Woodward and T. Singh, J. Am. Chem. Soc., 1950, 72, 494.
2) A. J. Waring, J. H. Zaidi and J. W. Pilkington, J. Chem. Soc, Perkin Transactions I, 1981, 1454.
answered May 28 at 22:33
user55119user55119
5,4542 gold badges14 silver badges46 bronze badges
answered May 28 at 22:33
user55119user55119
5,4542 gold badges14 silver badges46 bronze badges
answered May 28 at 22:33
user55119user55119
5,4542 gold badges14 silver badges46 bronze badges
answered May 28 at 22:33
user55119user55119
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5,4542 gold badges14 silver badges46 bronze badges
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If classical carbocation is already heavily stabilised, it would be surprising if additional "nonclassical" stabilisation would be added. In this case it's a difference between sigma and pi complex between substituted benzene ring and methyl group - it would be rather a pi complex then nonclassical carbocation, even if it was an actual intermediate here.
$endgroup$
– Mithoron
May 29 at 21:20