Post on 01-Jun-2018
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Organometallic Compounds
Chapter 15
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Carbon Nucleophiles: Critical in making larger organic molecules.Review some of the ones that we have talked about.
C anide ion: CN ! " R# RCN RC$ %N$ %
&cet lide anions:
'nolate anions:
O't
O O base
O't
O O
$ $$
RX
O't
O O
R
RC C$
strong base
RC C:
RXRC CR
(r to see what factors promote the formation of the negative charge on thecarbon atoms: h bridi)ation* resonance.
Synthetic thinking: Disconnect
N$ % + CN-
+r
, nthetic (hinking: (his offers manopportunities provided ou can work with thetwo carbon straight chain segment. -h
-h
-h
-h
#
-h
-h
#or
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e e/amine two t pes of organometallics: R0g#* a rignard reagent* and R2i* an organolithium compound
-reparation " !
! "
,olvated b ether* aprotic solvent
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+asicitRecall that a carbanion* R 3C: !* is a ver strong base.
,o also rignards and alk l lithiums.
+ottom 2ine: rignards are destro ed b 4weak protic acids: amines*alcohols* water* terminal alk nes* phenols* carbo/ lic acids. (he
rignard* R0g#* is converted to a 0g salt eventuall and R$.
(he liberation of R$ can serve as a test for protic h drogens.
'thane* a gas.
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Reactivit patternsRecall the , N% reaction where the alk l group* R* is part of the electrophile.
Nu: - + R-X Nu - R + X -
'lectrophile
Nucleophile
6orming the rignard converts the R fromelectrophile to a potential nucleophile. & widerange of new reactions opens up with R asnucleophile.
R# " 0g R!0g!#! "
Nucleophile
'lectrophile
'lectrostatic potential
maps.
" !
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Recall Reactions of O/iraneswith Nucleophiles
Recall opening of o/irane with a strong* basic nucleophile.
O
C$ 3
$$
$
CH 3O-
C$ 3
$
O$
$ 3 CO
$$
(he ne/t slides recall the diversit of nucleophiles that ma be used.
Observe that there is limited opportunit of creating new C!C bonds* weldingtogether two R groups. We seem to be somewhat lacking in simplecarbon based nucleophiles.
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Recall , nthetic &pplicationsnucleophile
Onl reaction with the acet lideanion offers the means of makinga new C!C bond and a largermolecule. -roblem is that aterminal alk ne is needed.
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& rignard has a reactive* negative carbon. Now e/amine reaction ofrignard and o/irane ring.
Net results
The size of the alkyl group has increased by 2 . 2ook at this alcohol to alcoholse7uence
R!O$ R!# R!0g!# R!C$ %!C$ %!O$ .
(he functionalit 4O$ has remained at the end of the chain. e could make iteven longer b repeating the above se7uence.
Now a substituted o/irane
Note attack on lesshindered carbon
Newl formed bond
Newl formed bond
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, nthesis '/ample
O$
C$ %C$8C$ %
Retros nthesi)e the following
Recall reaction of a nucleophile with an4o/irane epo/ide to give a $O!C!C!Nu pattern. +ack side attack givesanti opening.
Trans geometr suggests tr ing ano/irane. hat should the nucleophile
be9
(he all l group should be thenucleophile. (his is done b using a
rignard 4or ilman .
O
CH 2=CH - CH 2Mg r
O$
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ilman Reagent 42ithiumdiorganocopper Reagents
R-X
!iR-!i
Cu"R 2Cu!i
ilman-reparation of ilman Reagents
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Reactions of ilman ReagentCoupling Reaction
sed to create new C ; Cbonds..
Overall result. R!# " R
Necessar details
R-X!i
R-!iCu"
R 2Cu!i &s before:
Ne/t step: R 2Cu!iR#-X
R - R#
Restrictions on the process. Caution.
R group which goesinto ilman ma be
meth l* 1 o 4best not % o or 3 o * all lic*vinylic
4unusual * ar l
&lk l 4not 3o * vin lic
nucleophile
electrophile
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-articularl useful* reaction withvin l halides to make an alkene.
Note that the stereochemistr of the alkene isretained.
trans
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ilman and o/iranes
R of the ilman reagent is the nucleophile* t pical of organometallics.
+ecause in basic media 4acid destro s ilman o/ gen of o/irane can notbe protonated. 2ess hindered carbon of o/irane is attacked.
O $% R 2Cu!i $O
R
2% H2O& HC'
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, nthetic &nal sis
O$% R 2Cu!i $O
R
2% H2O& HC'
Newl formed bond.Note its position
relative to the O .
,imilar to rignardanal sis.
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'/ample of Retros nthetic &nal sis
-h
O$
=esign a s nthesis using o/iranes
(he o/irane ring could beon either side of the O$.2ook at both possibilities.
-h
O$
or -h
O$
O
()hCH 2*2Cu!i
On the right* located here.Open o/irane here.Nucleophile makes this bond.
-h
O
!iCu(CH 2CH 3*2
% s nthetic routesavailable
Nucleophile can come in
on onl one position ofo/irane* on the C to whichthe O$ should not beattached
On the left * located here.Open o/irane here.Nucleophile makes this bond.
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, nthesis '/ampleCarr out the following transformation in as man steps as needed.+r O
OC$ 3
O
OC$ 3
O$
OC$ 3
O
+r target
Remembero/idation of asecondaralcohol canproduce aketone.
Note pattern of anucleophile4OC$ 3 then C!Cthen O$. sean epo/ide.
'po/idescan comefrom alkenesvia peracids.
&lkenes cancome fromhalides via'%.
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Carbenes* :C$ %1.
%.
carbene
-reparation of simple carbenes
0echanism of the elimination.
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Reactions of Carbenes* :C$ %4not fors nthesis
&ddition to doublebond.
>nsertion into C!$ bond
6ormation of lide 4later
li7uid
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,immons ,mith Reaction 4 for synthesis *addition to alkenes to ield c clopropanes
C$ %>% " ?n4Cu >C$ %?n>
Carbenoid* propertiessimilar to carbenes.
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'lectronic ,tructure
'lectrons paired* singlet
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(riplet and ,inglet 0eth lene
CH 2 N2
sing'et carbene
stereos+eci,ica--ition
+i e'ectrons
tri+'et carbene
C$%
-ira-ica'
+
non-stereos eci,ic
=ominant form
in solutionas phase
Rotation canoccur around this
bond.