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So, in the previous class we were discussing about the sigma tropic reactions, right. So,
we broadly classify sigma tropic reactions as 1 n and as well as n number sigma tropic
reactions. One n we said that the residue will move from the first position to the end
position, and enum rearrangement we focus like it will be the movement of your sigma
bond across your pi system. Then, in 1 n system we again broadly classify that as hydrogen
movement system, and carbon migration system. So, we were dealing about hydrogen shift;
so what we studied there, we studied like reactions like 1 3 1 5 and 1 7, and then we
slowly got into their stereo chemistry.
So, stereo chemistry may we studied two things, right. One, it should be suprafacial or it
can be antarafacial. Then we had this Woodward Hofmann rule for your one n hydrogen shift. We said that we will take 4 n 4 n plus
2; I am talking about the number of electrons involved it will be pi system as well as sigma
electrons, then I said heating and in the light we said it will be antarafacial and
will be suprafacial, and just it will be opposite to this.
So, what I mean is that if I take 1 5 system - 1 5 hydrogen system, it will be 6 electrons
systems, so 6 electrons system if I heat it I might end up with a suprafacial, and on
the light it will be antarafacial. And if it is a 1 7 system, it will be other way round,
but the boss one thing you have to remember which has that 1 3 shift. In 1 3 normally
you do not observe any type of reactions on heating, when it is light then you can see
suprafacial, but on heating you do not see the any type of reactions we do not see 1
3 hydrogen shift, we will just see why it is?
R prime better to write it, so that I have a system like this hydrogen, now if I heat
this what happens? So by theoretically, I should expect a 1 3 hydrogen shift, right?
So, if you feel this, how this reaction goes just for an assumption that it is the bond
cleaves here, we think like it goes by a sort of radical it is not completely, because it
is pericyclic; goes by transition state. If you for assumption, if you feel that reaction
goes like this way, like get an cation and you can write as H plus, this can give you
the product. If you feel of course, now I am dealing with this system; this is what?
This is an allylic cation, so we are concerning about the heat. So, what we have to which
orbital we will be looking into it, we will be looking into the *** of allylic cation
right. Just you draw the *** of allylic cation,
how it looks like? One way of doing in a proper way is that you have an R R prime, so this
is my allylic cation which I have taken the *** of it 1 3, and this is my hydrogen. Now,
if you see this reaction what happens? It has to go and if has to shift it has to shift
like this, but if you see it is orbital not favored, so suprafacially it cannot do the
process, because this orbital is not favored that is the rule again the rule if you put
this in that Woodward Hoffman rule, it says yes, it should not be a suprafacial right
that also matches with your orbital one.
Now what I do is that same system, so this is your hydrogen so what the rule says it
can be antarafacial, so it can do an invertion to come here, so your hydrogen can do this
is because if you see orbitally it is favored, but what happens your hydrogen is very small
it cannot bind it cannot do this binding, because it is very small that is why antarafacially
also it does not occur even it is orbitally favored you can say geometrically it is unfavored
because hydrogen is very small, so it cannot bridge between this two orbitals.
That’s the only one exception that comes for this rule, otherwise it just everything
fits to that rule 1 1 5 1 7; all this fit exactly only in 1 3 if you go for your heating
both suprafacial and antarafacial does not occur, see if you see the same system photochemically.
So, what happens in light, so in light I will be taking the LUMO of I will be considering
the LUMO of allylic cation. So how to draw the LUMO, then
so this will be the LUMO of allylic cation, you have your hydrogen fine, now in light
it can easily do this it can move suprafacially, because you see orbitally it is favored it
can just move from this to this end suprafacially fine. So, same thae same thing you can do
for your 1 5 1 7 everything, so this is orbitaly favored as well as geometrically favored.
So, the reaction happens nice, so that is what 1 3 1 5 1 7 only one thing to remember
1 3 normally people ask this question all the time, yeah why 1 3 does not happen on
heating the reason is that 1 3 by if you heat it geometrically it is not favored, because
the hydrogen nucleus is very small to make a bridge between this. This point you have
to remember fine.
So that is about your 1 n hydrogen shift, now we will move to your 1 n carbon shift
if methyl moves, then usually it is methyl shift we will start with the simple example
read this. So, we will get a so this will be 1 2 and 3, so you can call this as 1 3
right because it moved from one to third position, so you can call it as 1 3, and here you can
call them as methyl shift 1 3 methyl shift. So, fine so same way you can draw for your
1 5 and 1 7, and as you did for your hydrogen shift, now then comes your stereo chemistry
into question whether this will be suprafacial or it will be antarafacial. See, if you just
take a methyl system, then you can write the same rule like for your Woodward Hoffman I
can write just like your hydrogen shift this also obeys your similar like your hydrogen
shift initially it will be antarafacial 4 n, then it will be supra 4 n plus 2 reverse
of it see just similar to your hydrogen shift when it is methyl keep that in mind when it
is methyl. any doubt up to this.
See the one interesting aspect comes in this carbon shift is that for example, I have a
system I have system like this now what happens your carbon is sort of chiral, I am taking
as carbon previously I said it is only a methyl shift where your carbon has 3 hydrogen’s.
Now, I am slowly introducing another concept where your carbon becomes sort of a chiral,
now what happens if you the if you look at the stereo chemistry. What it can happen?
First you will say that how it will be suprafacial, yeah what are the options you can write down
one I can say I can write suprafacial keeping in mind, now my chiral carbon can be in retention
one you should be in retention one option is there I can superficial retention can you
draw that, because it is a 1 5 hydrogen shift can you draw this what ibis have written 1
5 hydrogen just sorry a then you get A B. Then you get A B then you have your C just
try to draw this suprafacial retention.
Just I want to see just I will come back I will take a new paper you guys try it what
are the things you can draw C have A D suprafacial retention A B, because it has been shifted
from your then you write your C here D. Now, make sure that your carbon that you are writing
this is what now your carbon is having the same configuration that is retention, so another
option can I write suprafacial invertion you can think about suprafacial inversion can
also happen. Now, we are just saying that what are the possible options can be…
I can have an invertion
then what we can think about, so I have a suprafacial retention suprafacial invertion,
and then I can write anyhow I can write antarafacial yeah very good you can write antarafacial
retention, so antarafacial retention will be just in the downright with a carbon. So,
C and D will be up and your methyl will be in the down, and then you can write your antarafacial
invertion see once it comes to a carbon which is type of which is a chiral, then you have
to think about this 4 possibilities. So, one it can be suprafacial retention, suprafacial
invertion, antarafacial retention, and antarafacial invertion see most of the time, if you do
you get suprafacial retention when it is suprafacial most of the time it will be retention, if
it is antarafacial then it will be invertion, this suprafacial retention and antarafacial
retention more or less are same. So, you have cases where you can get suprafacial
invertion, this cases these are comes only in one case, if you take the rule most of
the time you will end up with suprafacial, if it is suprafacial it will be retention,
if it is antarafacial it will be invertion clear.
Now, what we will do we just take that Woodward Hoffman rule, which we wrote earlier for methyl
shift that is 4 n plus 2 what we said we said initially it will be antarafacial, then it
will be suprafacial - then suprafacial antarafacial so what we found out if it is a carbon with
a chiral, if it is antarafacial it should be invertion. So, I can put A plus i if it
is suprafacial retention, because it suprafacial there will be no change in your chirality,
so it will be S plus R. Fine now I just add one term into your carbon
shift, if it is a just a methyl shift no need to worry it will be similar to your hydrogen
shift, but if your carbon is a chiral. Then you have to understand whether is an invertion
or it is a retention fine, so the rule can be done like this for 4 n it will be A plus
i or S plus R 4 n plus 2 S plus R or A plus i clear, any doubt. Here see as we said in
hydrogen shift for 1 3 hydrogen shift, when you heat it there is an exception that it
does not undergo any reaction same way for carbon 4 1 3 heating, it does not follow this
rule whatever rule we have written.
So, we will just see what it happens in that case; for example I take our 1 3 just sigmatropic
carbon shift just taking on 1 3 for, we will just take methyl for simple representation
you will understand later why, so it is a 1 3 shift you know it goes by allylic cation.
As a earlier said if I break this shall we draw the *** of this, because I am doing
in heating, so I am just considering the *** of allylic cation, so how the *** looks it
should be just the opposite right. Now, I have to put my carbon sorry I will do put
this, now you have your I just dram a carbon here fine, now you know that this is not going
to happen, so what the rule says so it will be antarafacial invertion right. See the according
to rule it should be it should make a antarafacial invertion, yeah looks like to be, because
it says it can be but this methyl is… So, nice instead of doing this big thing what
it does it simply invert itself, here if you just see this how easy it is for this guy
to do this feels. So, nice to see how these guys work
so what it did is just inverted itself, now it can do easily suprafacial right why it
does not do now antarafacial, because the orbital is favored see actually by rule it
should go antarafacial invertion right, but what happen in this case you get suprafacial
invertion. So, this is the only one case I think you get suprafacial invertion most of
the case, if you say you will be dealing with suprafacial retention and antarafacial invertion
yeah, because antarafacial you get invertion suprafacial it maintains. So, this is the
only case where you get suprafacial invertion, so keep that in mind yeah yeah, see I am just
taking for or an example simple, if it is X Y Z it will be clear, but we are not seeing
the difference because but actually it happens suprafacially invertion, you are not it happens
like that you are not observing by the product, because yeah if you have a chiral carbon defiantly
you can observe it will do suprafacial invertion. And you can clearly observe that from the
product yeah, it is only for 1 3 sigmatropic shift, that also particularly in heating.
So this heating 1 3 heating in both the cases hydrogen as well as carbon does not fit exactly
with your rule that only point you have to remember, clear, any doubt with this no so
shall we get into problems.
So, we will do some problems on 1 n sigmatropic shift I can say examples or I say problems
see when you are going to write stereo chemistry is very important, because we are going to
talk about. So, I have a system like this just I am going to fortalize, now first thing
you have to find out whether what *** it is the second thing you have to find out the
stereo chemistry alright, because I have to find my hydrogen. So, shift you are going
to because this is fortalises I have defined the condition, so H mu which is possible 1
3 H mu 1 3 is very fast see 1 5 heating by other 1 7 again H mu, because these are all
orbitally favored one. So, you can do the 1 3 hydrogen shift, the second question is
that whether it will be antarafacial or suprafacial. So, what the rule says what system it is 4
n, so light it is S suprafacial, so first you write that then you write the product
once you finish it, so it is 1 2 3 right, so how your hydrogen will be in same phase
simple example, but tells you what happens clear, you have S system like this. Now, I
am saying that I am going to heat it, what you are expect think of heating this system,
see you have an option if you count have 1 3, and you have 1 5 that is a all should be
1 3 or 1 5, and I am giving heating you are just seeing which orbitally favored or 1 5
is favored, so it undergoes a nice 1 5 hydrogen shift, so 1 5 heating what it is antara or
suprafacial just keep that formulae into your mind.
So, then I can write the numbers 1 2 3 4 5 right, so it is a suprafacial right you can
get this any doubt? Again you can if you heat this, what happens? I can again heat this
you will end up same 1 5 hydrogen shift, but not the same product, but same chemistry 1
5 hydrogen shift supraficially write the product number it you can write the product, see 1
2 3 4 5 my hydrogen can come here very good examples to work on which due to no hydrogen
moves very, very fast compared to a its good. Then you can also move, so a we do not need
to go by one by examples, we just change otherwise it looks like easy to find out.
So, now I heat this what you look for see the hydrogen are on one side, so this is O
A C versitiale hydrogen, this is your hydrogen neutrium O A C and hydrogen, so what you observe
now I heat this guy or if I fertalize, whatever if it is just heat it what you can think about
there is no hydrogen you are thinking about hydrogen shift yeah, you can think about your
carbon shift 1 2 3 this carbon, so it will be 1 3 carbon shift, so what it is what the
rule says it will be an invertion. So can you write it, so it should be invertion keep
that in mind, so if I will be happy if you write the structure before me, because it
will I want to know how you see this particular carbon when it moves coz its undergoing invertion.
So, I know that you will write this part, but I will be much interested to see how you
write this so you have a O A C, and you have an hydrogen here that is fine, because it stays same like
that what happens this side. Now, what will happen to this hydrogen, and neutrium it will
be other way round, because your inverting right. Now, you can write your neutrium here,
and you can write your hydrogen see that is how you should know because this invertion,
and retention has very big in writing the stereo chemistry it is not only about hydrogen
shift or methyl shift or 1 3 or 1 7 it should be with your stereo chemistry fine, any doubt?
Take one another good example here, so whether you guys can write this the same one, but
O A C just I am taking the same example just want to see again how you write this the way
different way I am, yes, it is a same one right see that is what I am saying the way
of projection it is the same example the way of projection is like this. So, do not think
like when it is projected it is the different example, it is the same example that is what
I want to know, so when you write you can open up this. And then also you can draw your
stereo chemistry same way for this product, if you write in another way see I am just
it is can say this 3 same way I am writing it now whichever way you like can open it
up, and I am seeing people writing like this also that is also good way of representation
right. Same example different way of writing it out the same compound, so you should know
the many ways of seeing the molecule, that is why O just want to write this one right.
Now, we will take a good example for test of Woodward Hoffman, see you take this example
if you heat this for example, what you should get I take this example and heat it, so it
is 1 5, because where is 1 5 - 1 5 heating is what suprafacial right yeah. So, can you
draw the structure, so it is simple, so it is suprafacial, so I need to add hydrogen,
so it should be what if I number it R, this is R, Z, and E. See what happens is this compound
once you do the heating I get another product which is S Z and Z also along with this. So,
I get R, Z, E, I am getting S Z Z, so that means I am getting different stereo chemistry
stereo. So, how it is possible any idea 6 membered,
now if I just heat I get R Z that is you say as 1 5 hydrogen shift with a suprafacial or
you are thinking for getting this you are thinking like antarafacial, but antarafacial
never happens in 1 5, so what happens there? What happened to this molecule.
If you take in solution this molecule, so this molecule does very nice C-C bond rotation
single bond rotation, so you sorry this is, you can have other way round, so just it can
do a C-C bond rotation that is possible right. And this in 1 5 undergoes, 1 5 hydrogen shifts
suprafacially, it will give you the other product which will give you the other product
S, Z, E; that is what? See the nice way of writing it, so these are things you should
know I am taking a molecule it undergoes C-C bond rotation, so it can give this these are
all the options which are open when you do this type of examples. So, suddenly don break
the rule and say no sir it might also go in invertion I will get this product it does
not happen that.
We will take another molecule. This molecule you have to say, because where you have studied
you have studied this molecule in lot of places, any idea where you have sorry, you have methyl
there, 1 2 you have methyl there, you have studied this synthesis where you have studied
it has then it does a electro cyclic ring enclosure or you end up in sort of a vitamin,
you have studied this in vitamin D synthesis right. What shift you study there see this
if I do a fortalises, see when people ask they ask like this only fortalises what do
you expect this? Sis trans sigma tropic any sigmatropic reactions any sigamtropic that
we are dealing with sigamtropic only we are dealing right now with hydrogen shift on metals
shift anything comes in your mind. See if I write this hydrogen, then things will come
to your mind, if I write hydrogen like this that is what happens 1 7, see if I do not
add write that hydrogen then you have to think. See, if you have methyl just please write
the hydrogen then it is easy to find out, it will be nice 1 7 you can write one 2 3
4 5 6 7, they are nice 1 7 hydrogen shift antarafacial or suprafacial that is very important.
So, 1 7 hydrogen shift, so what your role says suprafacial 1 7 - 1 7 is what 1 7 is
what system what electron system 4 n system yeah, because this geometry is little bit
unfavored for suprafacial this should be just check it up I will get back to this regarding
the configuration, because this geometry is littile bit favored looks like to be in antarafacial
looks to be antarafacial geometry I will get back to you in next explanation.
You have S system like this now, now I say that I need a other way round I say that this
chemistry goes suprafacially retention, what it says I am saying it is suporafacially,
no you have double bond example 5 sorry yeah yeah sure. We need a double bond you shift
this you shift that here right, so I want this reaction this reaction goes by suprafacial
retention, so tell me what what condition it is other way round this is suprafacial
retention this geometry. So, what condition shall I use, because first
you find out what shift it will be what shift you want to do 1 2 3 4 5 right, so I can move
this carbon so it will be 1 5 shift so 1 5, if I want suprafacial retention what I have
to do heat it or fortalize. See, that is the other way of finding out see one way I can
ask you the that heat what you get otherwise I can ask you that this is the geometry I
got, so find out the condition you should do other way round also, now heating you just
draw the structure. So, it should be a methyl here, so further also you can do this same
way, then what you get again same right any doubt with this systems now.
So, you can do any examples then now I will give you one example which you guys should
try to do. I have a system like this; I have many combinations here. I say that I am just
triplating it for heating; what you want to do? Heating, so 1 5 what 1 5 there are 2 choices
- methyl or hydrogen. I can push my methyl; I can push my hydrogen; what you want to do?
Yeah, 1 5 hydrogen shift? Very good; so 1 2 3 4 5; see now, you know how it is, right?
You have your hydrogen here; so now you have you have conditions.
So, you see if an example is given, you have to find the condition. If the condition is
known then you find out whether it is 1 5 1 3 1 7; then you find out whether it is suprafacial
or antarafacial. Then you find out whether it is a retention or invertion. Any doubt
with these examples? So, we will stop your 1 n hydrogen shift; that is all on 1 n hydrogen
shift; we will be then concentrating on sigmatropic reactions.