[kkiddu]

Hi guys. This is my first post here, so please excuse me if I mess something up.

Here're my questions.

What's the role of a carbanion in the last step of Corey House Synthesis ?

Specifically:-

R₂CuLi + R'X  R-R'

And why does R' have to be 1^o ?

Thanks. A lot.

[SVXX]

First of all, dialkyl lithium cuprates are also known as Gilman reagents, which you must be knowing(R₂CuLi that is). In these types of organometallic reagents, the alkyl part acts as the anion, which you must also be knowing.
Gilman reagents are weakly polarised, i.e., the negative charge which appears on R is not strong enough(unlike in RLi where the negative charge is quite strongly polarised). Thus Gilman reagents cannot act on alkyl halides which are sterically hindered, as the carbanion so formed would not be strong enough to go through the nucleophilic substitution (S_N2). Hence the halides must be primary.

[g-bones]

First of all, dialkyl lithium cuprates are also known as Gilman reagents, which you must be knowing(R₂CuLi that is). In these types of organometallic reagents, the alkyl part acts as the anion, which you must also be knowing.
Gilman reagents are weakly polarised, i.e., the negative charge which appears on R is not strong enough(unlike in RLi where the negative charge is quite strongly polarised). Thus Gilman reagents cannot act on alkyl halides which are sterically hindered, as the carbanion so formed would not be strong enough to go through the nucleophilic substitution (S_N2). Hence the halides must be primary.

just a quick note,
In the strictest sense, the Gilman reagent is the dimethyl cuprate.  although it has become commonplace to simply refer to all organocuprates as Gilman reagents.  Just in case you run into someone quite picky about naming   

[SVXX]

Aight thanks for the tip

[kkiddu]

First of all, dialkyl lithium cuprates are also known as Gilman reagents, which you must be knowing(R₂CuLi that is). In these types of organometallic reagents, the alkyl part acts as the anion, which you must also be knowing.
Gilman reagents are weakly polarised, i.e., the negative charge which appears on R is not strong enough(unlike in RLi where the negative charge is quite strongly polarised). Thus Gilman reagents cannot act on alkyl halides which are sterically hindered, as the carbanion so formed would not be strong enough to go through the nucleophilic substitution (S_N2). Hence the halides must be primary.

Oh...totally forgot about my question. Thanks a lot.

[orgopete]

I don't know why primary halides are required, but I don't think it is due to steric hindrance. I think it is due to decreased reactivity. I have speculated on a mechanism here:
http://orgo.curvedarrow.com/punbb/viewtopic.php?id=337