Highly Selective Hydrogenation of Aromatic Ketones and Phenols

Jul 14, 2015 - Silylarene Hydrogenation: A Strategic Approach that Enables Direct ... Hydrogenation of Aromatic Ketones and Phenols Enabled by Cyclic ...
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Journal of the American Chemical Society

Highly  Selective  Hydrogenation  of  Aromatic  Ketones  and  Phenols   Enabled  by  Cyclic  (Amino)(alkyl)carbene  Rhodium  Complexes   Yu  Wei,†  Bin  Rao,†  Xuefeng  Cong,†  and  Xiaoming  Zeng*,†,‡     †

Center  for  Organic  Chemistry,  Frontier  Institute  of  Science  and  Technology,  Xi’an  Jiaotong  University,  Xi’an  710054,  China  



State  Key  Laboratory  of  Elemento-­‐organic  Chemistry,  Nankai  University,  Tianjin  300071,  China    

 Supporting  Information  Placeholder ABSTRACT:   Air-­‐stable   Rh   complexes   ligated   by   strongly   σ-­‐ donating   cyclic   (amino)(alkyl)carbenes   (CAACs)   show   unique   catalytic   activity   for   the   selective   hydrogenation   of   aromatic   ketones   and   phenols   by   reducing   the   aryl   groups.   The   use   of   CAAC  ligands  is  essential  for  achieving  high  selectivity  and  con-­‐ version.   This   method   is   characterized   by   its   good   compatibility   with   unsaturated   ketones,   esters,   carboxylic   acids,   amides,   and   amino  acids,  and  is  scalable  without  detriment  to  its  efficiency.  

yl   group   in   the   products,   thus   offering   a   straightforward   and   clean   route   to   cyclohexyl-­‐containing   ketones   and   cyclohexa-­‐ nones,   which   are   fundamental   feedstock   materials   and   widely   used  as  key  precursors  to  industrially  important  molecules  such   19 as  caprolactam  and  adipic  acid.  

Scheme  1.  Selective  Hydrogenation  of  Aromatic  Ketones   and  Phenols   (a) The selective hydrogenation of aromatic ketones (two pathways)

Selective  hydrogenation  is  one  of  the  most  powerful  and  useful   reactions  because  of  its  synthetic  significance  in  the  creation  of   pharmaceuticals,   materials,   and   fundamental   feedstock   chemi-­‐ 1,2 cals.  Despite  considerable  achievements,  chemoselectivity  has   long  been  a  prominent  obstacle  when  several  unsaturated  scaf-­‐ 3 folds  are  present  in  the  same  substrates.  Compared  with  polar   carbonyls,   the   aromatic   hydrocarbons   are   difficult   substrates   for   hydrogenation,  probably  because  of  the  stability  caused  by  aro-­‐ 4,5 maticity.   As   a   result,   the   hydrogenation   of   aromatic   ketones   often  leads  to  aryl-­‐containing  alcohol  products  via  a  preferential   6 carbonyl   reduction   (Scheme   1a,   path   a).   In   contrast,   switching   the  selectivity  to  realize  arene  hydrogenation  while  retaining  an   easily  reducible  carbonyl  is  more  challenging,  and  has  not  been   achieved   with   structurally   well-­‐defined   homogeneous   catalysts   7 to  date  (Scheme  1a,  path  b).  Another  key  problem  is  inhibiting   the   reduction   of   the   resulting   ketones   to   alcohols   or   dehydrox-­‐ ylated   products   (e.g.,   4a   and   5a).   Especially   for   naturally   abun-­‐ dant   phenols,   efficient   strategies   for   reducing   them   to   cyclohex-­‐ 8,9 anones  by  avoiding  a  carbonyl  reduction  are  rare  (Scheme  1b).   From  both  sustainable  and  synthetic  points  of  view,  developing   a  general  protocol  to  address  these  selectivity  challenges  would   help  advance  clean  chemical  syntheses.   Recently,   there   has   been   significant   progress   in   the   rational   design   of   effective   organometallic   catalysts   using   σ-­‐donating   N-­‐ 10,11 heterocyclic   carbenes   (NHCs),   and   a   number   of   elegant   ex-­‐ amples   of   NHC   ligand-­‐promoted   hydrogenation   have   been   re-­‐ 12 13 14 15 ported   by   Glorius,   Stephan,   Crabtree,   and   others.   Differ-­‐ ing   from   classic   NHCs,   cyclic   (amino)(alkyl)carbenes   (CAACs)   show  an  enhanced  nucleophilicity  because  of  a  σ-­‐donating  alkyl   substituent,  which  was  discovered  by  Bertrand  and  has  received   16,17 increasing   interest   recently.   However,   the   ligand   effect   of   18 CAACs   in   organometallic   catalysis   has   rarely   been   studied.   Herein  we  report  the  first  CAAC  ligand-­‐enabled,  highly  selective   hydrogenation   of   aromatic   ketones   and   phenols   with   rhodium   (Scheme  1).  This  method  retains  a  synthetically  valuable  carbon-­‐

Et

5a H2

Et

ketone hydrogenation path a

O

Et

OH

cat. Ru, Ir, Pd, Rh, Fe, FLP...

H2

H2

Et

O

1a

(CAAC)Rh(COD)Cl CF3CH2OH, r.t. arene hydrogenation path b

X

Et

4a

3a R1

N (b) The selective hydrogenation of phenols to cyclohexanones OH

H2

CF3CH2OH/H2O phenol hydrogenation

Cl

R2 Rh COD

CAAC-Rh

OH

O

(CAAC)Rh(COD)Cl

6a

OH

2a

X

isomerization A

OH

7a

8a

To  ensure  that  the  arene  hydrogenation  occurs  preferentially   over   that   of   ketones,   a   suitable   catalyst   that   allows   interaction   with   the   aryl   group   via   a   π-­‐coordination   is   required.   We   postu-­‐ lated   that   electron-­‐rich   CAAC   ligands,   with   their   strong   σ-­‐ electron  donation  to  the  metal  center,  might  enhance  the  inter-­‐ action   of   the   metal   with   the   aryl   by   the   back-­‐donation   of   elec-­‐ tron   density   from   the   filled   d   orbitals   of   the   metal   into   the   unoc-­‐ 12b cupied   π*aryl   anti-­‐bonding   orbital.   This   may   favor   the   arene   hydrogenation  with  H2.  Meanwhile,  the  electron-­‐rich  metal  may   20 be   expected   to   show   rather   low   oxophilicity.   Compared   with   other   transition   metals,   rhodium   shows   high   reactivity   towards   1 arene  hydrogenation.  At  the  outset,  the  impact  of  Rh  complex-­‐ es  on  the  selective  hydrogenation  of  propiophenone  was  evalu-­‐ ated   (Table   1).   In   the   presence   of   Wilkinson’s   catalyst,   a   trace   amount  of  the  aryl-­‐reduced  product  3a  was  detected  by  GC-­‐MS   analysis   (entry   2).   Similar   results   were   obtained   using   [Rh(cp*)Cl2]2   and   RhCl3•H2O   (entries   3   and   4).   A   complex   of   [Rh(COD)Cl]2  improves  the  reaction  to  give  3a  in  29%  yield,  but   with  various  by-­‐products  of  2a,  4a,  and  5a  (entry  5).  

Table  1.  The  Influence  of  Rhodium  Complexes  on  the  Aryl-­‐ a,b Selective  Hydrogenation  of  Propiophenone  

ACS Paragon Plus Environment

Journal of the American Chemical Society O

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O

Et

H2 (0.5 Mpa) rhodium complex (3 mol %)

Et

Et

3a

2a

+

OH

CF3CH2OH, 4 Å MS r.t., 24 h

1a

OH

Et

Et 5a

4a

Rhodium  complex  

Yield   (3a)  

1  

none  

nd  

nd  

c

nd  

c

nd  

2  

Rh(PPh3)3Cl