[spirochete]

Is there a way to know when you will get 1,4 addition to a conjugated ketone and when you'll get 1,2 addition?  It seems like 1,4 would usually be more favorable thermodynamically because the product often contains a C=O pie bond.

[omegasynthesis999]

Conjugate addition is favored when weaker nucleophiles (like when enolates are used) while simple addition is favored when stronger nucleophiles are used (like CH3MgBr).

[macman104]

Hard nucleophiles (small, point charge, non-polarizable) favor 1,2.
Soft nucleophiles (larger, maybe resonate, more polarizable) favor 1,4

[agrobert]

Hard nucleophiles (small, point charge, non-polarizable) favor 1,2.
Soft nucleophiles (larger, maybe resonate, more polarizable) favor 1,4

Perfect

[spirochete]

Thanks for the good answers.  I'm still wondering the chemical reasoning, though.  Seems like it's probably kinetically related because it relates to nucleophile type.  Often kinetic arguments are fairly intuitive, but I'm not seeing anything obvious here.

[movies]

Read up on hard/soft theory.  As macman wrote, hard nucleophiles will add 1,2 while soft nucleophiles will add 1,4.  You can think of a conjugated ketone as an ambident electrophile with "soft" character at the beta-carbon and "hard" character at the carbonyl carbon.  You can rationalize this by thinking about the enone resonance structures and their relative contributions to the overall structure.

The basics of hard/soft theory are that hard interactions are like electrostatic interactions.  Simply bring together the "most negative" part with the "most positive" part.  For soft interactions the molecular orbital interactions play a much larger role so it is more like a thermodynamic process than a kinetic, electrostatic interaction.  There is more too it when you get into the orbitals, which I can explain, if you would like, but it might take a little while.

[spirochete]

Wow this is a bigger topic than I thought.  I did some googling on hard/soft theory and found a few lecture notes on the subject but I definitely need to find an advanced orgo book to really understand it.  Maybe I'll make another thread when I get that ambitious.