Due to conjugation, it is possible for 1,4-addition, or conjugate addition, to occur. I was thinking, since conjugation and resonance effects do no diminish with distance, unlike inductive effects, does that mean that we can indefinitely conjugate and even have 1,6-addition? And even 1,8-addition and so on? My chemistry teacher mentions that 1,6-additions are possible but is there really no limit as to the effects of conjugations? Perhaps, there is increasing "resistance" as the carbon chain lengthens, like how a longer wire would have a larger resistance? I wonder if that would be an appropriate analogy.
1,6-additions are indeed possible (1) but since you have competing 1,4-addition and 1,2-addition it becomes more difficult. To push it to the 1,6-addition bulky organometallic catalysts are used. Reaction pathways are also determined by the electrostatic interactions, and these diminish the further away you get from the carbonyl.