hey it's professor Dave let's talk about organocuprates
so organocuprates are organic compounds involving copper so that qualifies them as organometallic
reagents just like grignard reagents and also the organolithium reagents that we
looked at just before this and in fact they are prepared from organolithium
reagents so say we have two equivalents of this n butyl lithium and we're gonna
react with copper bromide and what we're gonna end up with here is we've got this
copper anion with both of the alkyl groups attached to it so we've got those
two butyl groups right here and then that lithium counter ion is going to
hang out near the copper anion and we also get LiBr so this is a little bit
different from the way we prepare both grignard and organolithium in fact we
need to do this it's called transmetallation because copper doesn't
insert itself into these but it doesn't do the kind of chemistry that lithium
and magnesium do so we have to we have to use something else first so the whole
thing if we wanted to start from an alkyl halide remember we could do what
we learned before reacting the alkyl halide with lithium we get the
organolithium reagent and then as we just said reacting with copper bromide
is gonna give us our organocuprate also known as a gilman reagent and so this is
the way we're gonna see it typically if we're just it's written out on a test or
something we're gonna see our two culi and we need to know what that means we
need to know that copper is the central atom there and that it bears a formal
negative charge that's very important in terms of what that region will do it's
got the two alkyl groups whatever they are R can be anything projecting from
that copper anion and then lithium serve simply as a counter ion that's a
positive counter ion there so that's how we're gonna make these organocuprates
so what are some things that organocuprates do so the most common
application for organocuprates is as follows say we have this one here where
the alkyl groups are methyl groups now what we can do we have this carbonyl
containing compound and we saw that grignard reagents like to attack
carbonyls we saw that organolithium reagents like to attack carbonyls but
instead organocuprates are gonna do something slightly different these are
these are not as reactive right organolithium compounds are extremely
reactive but these are much less so these are a bit softer and what they're
gonna do is they're actually going to attack the beta position when we have an
alpha beta unsaturated ketone so here's alpha here's beta this is very similar
to when we learned Michael addition we saw that an enolate would attack the
beta position of an alpha beta unsaturated ketone in a very similar way
what's going to happen is this methyl group with it's basically a methanide
we've got ch3 minus attacking there and the reason that that is favorable is because
the copper is negatively charged if this carbon leaves with both of the
electrons in that bond that neutralizes the copper atom so that's actually
favorable so we're gonna push that here and then we're gonna push this pi bond
up here and then we're gonna push that pi bond up there so it may seem a little
strange if you watched the Michael edition tutorial then it might make
sense but at first glance we may not see exactly why this could be an
electrophilic site but don't forget that resonance is involved here right we can
draw another resonance structure or we could with this pi bond up here this pi
bond up here it wouldn't be as strongly contributing because we'd have a formal
negative and positive charge but nevertheless it is a valid resonance
structure so there is one resonance structure where there's a formal
positive charge here that means on the composite there still is some electron
deficiency at that site right there so that explains why that's the site of
attack for this compound so the methyl the methaneide is going to attack here
kick that pi bond up there kick that pi bond up there and here we are and so
certainly the lithium counter a counter ion is gonna hang out by the oxyanion
but then all we need is some aqueous acidic workup I'm gonna reform the
carbonyl we're gonna do that you could also protonate right there but
then of course that would tautomerize right we know that emails quickly
tautomerize to ketones typically so but here
mechanistically we could just reform the carbonyl kick
that up grab the proton and we've got our ketone so notice that the alpha-beta
unsaturation is gone right that's not there any longer but what we have done
is we have attached this alkyl group in this case it was a methyl group but it
could have been any alkyl group including a very long alkyl fragment so
this is a great synthetic technique when we are trying to attach a large portion
of a molecule specifically at the position that is beta to a carbonyl
right all we need is that alpha beta degree of unsaturation and we are going
to be very certain that we are going to be able to attach that at that precise
position so if that is part of your synthetic strategy this is a very useful
technique that you can employ and that is also the primary purpose of using
organocuprates that is this conjugate addition right it's a 1,4 addition
rather than 1,2 with grignard or organolithium that would attack right
there right and then but also just to mention one alternate application here
we can do simple SN2 we can have out we can have the alkyl group attack here and
just displace the halogen and just attach that alkyl group on there just to
make a longer alkyl chain so this is just a very simple SN2 on an alkyl
halide but once again this is the primary application this kind of
conjugate addition on some carbonyl containing compound with an alpha beta
degree of unsaturation so that's a little bit about organic cuprates or gilman reagents.