[Jorriss]

So I have a general question. I find it difficult to recognize when your nucleophile is going to be a pi bond.

Specifically, I was looking at the synthesis of camphor.


http://upload.wikimedia.org/wikipedia/commons/2/2f/CamphorBiosynthesis.png


Does anyone have hints or a topic I could go read up on? I think I got a bad habit in my mind that they can't 'really' do that.

[orgopete]

One of the things I did in my book was to use the paradigm suggested by Jeremy Knowles that there is not such thing as a concerted reaction. (If you can divide time into infinitely small pieces, three billiard balls cannot collide at the same time. One will always collide before the other.) While this seems to be a controversial point of view, it can also be a useful one.

In the reaction of the linaloyl pyrophosphate, the curved arrows indicate three pairs of electrons all moving in a concerted manner. This understandably leads to your question, why should the pi electrons act as nucleophiles? If you considered the kinetics of the reaction, then the initial or slow step would be formation of the tertiary and allylic carbocation. The pi electrons now can understandably donate electrons to the electron deficient carbon atoms.

I would suggest a mechanism based book would probably answer your questions.

[Jorriss]

Thanks, I think you're right - I should just man up and spend the money on a mechanism book. I've heard Grossman's is very good.

[SVXX]

Well...organic chemistry for one, really likes stability, or so I believe. It may go through less stable states(RDSes of the respective reactions) to achieve that stability.
Here OPP is a good leaving group. It gives linaloyl pyrophosphate a chance to form a six-membered ring, which is thermodynamically miles ahead in stability than the open chain-form it was earlier. Looks like a cope-rearrangement to me, just that it isn't a 1,5-diene and is a 1,6-diene. One of the double bonds knocks the OPP out just as simultaneously the other double bond forms the six-member ring.
As someone said to me earlier in these very forums, "intramolecular reactions proceed much faster than intermolecular ones", provided there is scope for an intramolecular reaction. The double bond present in the same molecule acts as a nucleophile in the next step instead of OPP directly attacking as a nucleophile(simply because there is scope for intramolecular attack). Of course there might be by-products where OPP attacks straightaway after the rearrangement of linaloyl pyrophosphate. The intramolecular product would be major.
Now there is no scope for rearrangement. The methyl group cannot migrate to that position, as the bridgehead carbon must not be made planar. Hence OPP can attack.
PS : Just my 2 cents.

Edit : Another example is in the substrate CH₂=CH-CH₂CH₂NH₂, when it is diazotized, and then water is added. The double bond performs an intramolecular attack to form a cyclopropane ring (again here there is a good leaving group in the form of N₂). Then water relieves the carbocation. I figured while doing these types of questions(involving neighbouring group participation or anchimeric substitution assistance) that the group present in the substrate is a better nucleophile than the reagent which is going to attack. Debatably, a double bond is a better nucleophile than water(not hydroxyl ion, water) and hence I could apply NGP.
So I suppose that the double bond is a better nucleophile than OPP here? This is a NGP after all.



Note : There can be ring expansion if heat is provided.

[willug]

'One of the things I did in my book was to use the paradigm suggested by Jeremy Knowles that there is not such thing as a concerted reaction. (If you can divide time into infinitely small pieces, three billiard balls cannot collide at the same time. One will always collide before the other.) While this seems to be a controversial point of view, it can also be a useful one.'

Could you explain this idea in a bit more detail? I can't see why 3 billard balls cannot collide at the same time, and why balls colliding at the same, or similar, time has anything much to do with concerted reactions. (I am realitively new to organic chemistry, so I do not pretend to know that much about it!) Also, what about the reaction of a nitrone with an alkene? What intermediates would you propose there? I thought the 'reason' for concerted reactions was to avoid such high energy transition states? (ie, charged ones). Do you assert that the SN2 mechanism is not concerted?

This is the IUPAC definition of a concerted reaction;

concerted reaction
A single-step reaction through which reactants are directly
transformed into products, i.e., without involvement of any
intermediates.
1999, 71, 1930
IUPAC Compendium of Chemical Terminology 2006

[orgopete]

Re: billiard balls and concerted reactions
If you divide time into smaller and smaller segments, you would find that one ball always arrived before another (even though they may not have shown any perceptible movement). 

The model for an SN2 reaction implies that the timing of all SN2 reactions should be the same with equal amount of bond forming and breaking occurring. I suggest that a MODEL for SN1 and SN2 reactions respectively, could be thought of as bond cleavage occurring before bond formation or bond formation before bond cleavage and that all substitution reactions are somewhere between the two. You could think of the electron pairs representing billiard balls and the carbon nucleus a third ball. In realty you may find that some SN1 reactions have some inversion accompanying the carbocation model (bond formation occurring before complete bond cleavage) and some SN2 reactions have some carbocation character as well, such as rearrangements or eliminations. 

The idea of bond polarization implies electron movement (before bond formation) and enables attack by a pair of electrons. Changing leaving groups, solvents, or nucleophiles all can alter the rates or degree of bond formation or cleavage. Elimination reactions have an additional pair of electrons, so you may find it useful to consider how product preferences might change by faster or slower bond making and breaking processes. You may find it useful to think of Zaitsev eliminations as carbocation-like and Hoffman elimination reactions as E1cb-like. 

If you go back to your textbook and think about the implications of the Hammond Postulate (transition states being more starting material or product-like), we may philosophize  that Hammond was considering electron movements proceeding at different rates. 

You should recall that the original poster questioned why pi electrons should behave as nucleophiles in the biosynthesis of camphor in which a concerted mechanism was suggested. If bond cleavage or mere polarization (whichever is consistent with the reaction kinetics (not given)), then an SN1-like reaction may allow are more pleasing attack of the pi electrons. 

Organic chemistry is very abstract and many of the things that we know or think we know may offer more hypotheses than proofs. Also, because I am not in a class answering professor's questions, I am not bound to their thoughts.