[trinitrotoluene]

In this picture (Yeah, it's from wikipedia >.<) I can't figure out why one is endo and the other exo. I would have guessed that these are both exo cyclizations seeing as how the bonds being broken are both outside of the ring being formed.

It would be greatly appreciated if someone could shed some light on this.

Cheers

[discodermolide]

What about the attack of the nucleophile, what is different there?

[orgopete]

Exo refers to outside, exoskeleton, an outer skeleton. Endo refers to within, endoskeleton, like ours.

The endo opening incorporates the atoms into the ring and exo keeps them to the outside.

[clarkstill]

I think there's a mistake in the question (although I may be wrong).  As far as I can tell the first reaction can either be described as 6-exo-tet or 7-endo-tet, depending on which way you count around the rings. 

My understanding of the endo vs exo prefixes is that in endo reactions the bond that is broken lies within the ring that is forming, whereas exo refers to reactions where the bond that is broken lies outside this ring.

[trinitrotoluene]

Exo refers to outside, exoskeleton, an outer skeleton. Endo refers to within, endoskeleton, like ours.

The endo opening incorporates the atoms into the ring and exo keeps them to the outside.

When you say atoms, would it be fair to say that 2/3 of the C's that make up the cyclopropane ring are incorporated into the ring, and exo 1/3 C's of the cyclopropane get incorporated into the ring?

Thanks for the responses all, this has been driving me nuts for the past week or so.

[IA_khim]

TNT, you are absolutely right to be confused. The reason why you are confused is because you are smarter than the people who use Baldwin'd terminology to epoxide opening (unfortunately, this includes Baldwin himself).
Here is a quote from a paper that should appear soon and answer your questions:
"use of endo-tet and exo-tet for epoxide closures is a misconception as the breaking epoxide C-O bond is located outside the newly formed ring in both cases, each cyclization should be considered an exo-tet process". Another quote: "Only if both carbons of an epoxide are considered as one functional group similarly to an alkene is the endo/exo classification applicable, but then it should use the exo-trig and endo-trig (not tet!) cyclizations as the reference point ". In order to avoid this confusion, Tim Jamison suggested to use the terms of  “spiro” or “fused” TSs," respectively, for such reactions.
In other words, the terminology is incorrect and you are right to question it. All the best.

[trinitrotoluene]

TNT, you are absolutely right to be confused. The reason why you are confused is because you are smarter than the people who use Baldwin'd terminology to epoxide opening (unfortunately, this includes Baldwin himself).
Here is a quote from a paper that should appear soon and answer your questions:
"use of endo-tet and exo-tet for epoxide closures is a misconception as the breaking epoxide C-O bond is located outside the newly formed ring in both cases, each cyclization should be considered an exo-tet process". Another quote: "Only if both carbons of an epoxide are considered as one functional group similarly to an alkene is the endo/exo classification applicable, but then it should use the exo-trig and endo-trig (not tet!) cyclizations as the reference point ". In order to avoid this confusion, Tim Jamison suggested to use the terms of  “spiro” or “fused” TSs," respectively, for such reactions.
In other words, the terminology is incorrect and you are right to question it. All the best.

Thank you for the information, it is much appreciated.

It regards to the quote about using trig vs tet. That confuses me a bit. To my knowledge that nomenclature is related to the geometry of the electrophile. However, as I consider trends in the nomenclature that is used, seems that the geometry of the approach of the nucleophile and electrophile (the transition state?) is what truly governs the classification as trig or tet. Is this what is meant by the quote? I'm likely horribly misled here but, I'm having trouble wrapping my mind around this.

Cheers

[antimatter101]

Wait, aren't endo and exo used for naming carbon double rings formed in pericyclic Diers-Alder reactions?

[spirochete]

Wait, aren't endo and exo used for naming carbon double rings formed in pericyclic Diers-Alder reactions?

Yes, that is a completely different use of the terminology. With Diels-Alder reactions endo/exo refers to a particular aspect of diastereoselectivity of the reaction.

[IA_khim]

TNT, you are absolutely right to be confused. The reason why you are confused is because you are smarter than the people who use Baldwin'd terminology to epoxide opening (unfortunately, this includes Baldwin himself).
Here is a quote from a paper that should appear soon and answer your questions:
... Another quote: "Only if both carbons of an epoxide are considered as one functional group similarly to an alkene is the endo/exo classification applicable, but then it should use the exo-trig and endo-trig (not tet!) cyclizations as the reference point ". In order to avoid this confusion, Tim Jamison suggested to use the terms of  “spiro” or “fused” TSs," respectively, for such reactions.
In other words, the terminology is incorrect and you are right to question it. All the best.

Thank you for the information, it is much appreciated.

It regards to the quote about using trig vs tet. That confuses me a bit. To my knowledge that nomenclature is related to the geometry of the electrophile. However, as I consider trends in the nomenclature that is used, seems that the geometry of the approach of the nucleophile and electrophile (the transition state?) is what truly governs the classification as trig or tet. Is this what is meant by the quote? I'm likely horribly misled here but, I'm having trouble wrapping my mind around this.

Cheers

Yes, for nucleophilic cyclizations, the nomenclature is based on the nature of electrophile (tet for a single bond, trig for a double bond, etc. See: K. Gilmore and I. V. Alabugin, Chem. Rev., 2011, 111, 6513 for a recent discussion). What is confusing is how this nomenclature has been historically used for epoxides. Since the cyclization breaks a single bond, it is classified as "tet" but, when deciding on whether to use "endo" and "exo", the nomenclature does not look at the breaking bond. If it would, both cyclizations would be classified as "exo" since the broken bond is outside of the forming cycle in both cases. The reason why it has been classified as "endo" is because both carbon atoms of epoxide were taken as a "functional group" (akin to how this is done for alkene). This is what is the source of uncertainty. You are not alone in questioning this. Here is an article with a relevant discussion: I. Vilotijevic and T. F. Jamison, Mar. Drugs, 2010, 8, 771

[hinhthoi]

Endo means the attacking group attacks from the same side of the 3-member ring. Exo means attacking from the other side. In this case, it is sterically obvious that the first reaction must be Endo, the second one must be Exo.

[IA_khim]

Hi Hinthoi,
Here is the definition from the original  Baldwin paper where the cyclization rules were defined and the exo/endo classification was offered (free access at http://pubs.rsc.org/en/content/articlepdf/1976/c3/c39760000734): "I will describe a ring-forming process with the prefix Exo, when the breaking bond is exocyclic to the smallest so formed ring and Endo correspondingly, as in Scheme 1."
As you can see, it is different from your definition. If you read Baldwin's paper, you will understand wherethe original question in this discussion came from.  Also, if you read the paper, you will see that sterics is not a factor. The rules were based on stereoelectronic effects (which despite having "stereo" in their names have nothing to do with sterics).
Hope that this helps