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Diastereospecificity in the formation of chelated organometallic complexes
c29
Journal of Organometall~c Chemrstry 139 (1977) C29-C33 o Ekevter Sequoia S A Lausanne -Printed m The Netherlands
Prelimmary communication
DIASTEREOSPECIFICITY IN THE FORMATION ORGANOMETALLIC COMPLEXES
PATRICK
LENNON,
and MYRON Department (Recewed
WIT’I’A PRIESTER,
ALAN
ROSAN,
OF CHELATED
MADDULA
MADHAVARAO
ROSENBLUM* of Chemistry
June 23rd
Branders Unwerslty
Waltham. Massacusetts
02154
(US
A)
1977)
Summary Three types of m&molecular cychzatlon reactions m organovon complexes having one or more chiral centers are shown to proceed with high dlastereospec&clty_
Diastereospeclficlty [ l] m chelate nng formation 1s a phenomenon commonly encountered m coordmatlon complexes derived from chval hgands [2] Its occurrence in smular reactions of organometalhc complexes 1s less well recognlzed We wish to report several examples of this important phenomenon m closely related organovon complexes in which the factors responsible for asymmetric induction may be ldentlfled We recently reported the first example of such di&ereospecfici@ in the conversion of the q* -ally1 alchohol complex Ia to the lactone IIa [S] . A closely related nng closure occurs when complex Lb 1s treated mth two equivalents of tri-n-butylamme. In each of these transformations a new chrral center, m addltzon to the one at the olefm, is created at the metal atom by chelation. In each of these reactions only a single dlastereomer IS formed as mdlcated by PMR spectral evidence* 133 _ The stereochemlcal factors which control these reactions are reatiy perceived Models** show that closure of Z to one of the two dlastereotoplc carbonyl groups constrains the olefm hgand to a plane either roughly parallel
*The PMR
rpcctrum
of Ilb
closely
a single dhstereomerue ready Yields **For
resembles that of IIa AU proton rc~~nan~cs corresponding to assignable Complexes IL%and IIb M obtained III 70 and 80%
rez&wctl~Y
Drciditu
= cee
mod&
based on molecuI~~ parameters for a CpFe(CO),
complcr
ref.
5.
R complex see ref 4 and for
GC
(IO,.? = Ib , Z =
OH,
NH,+1
(JIa.2
=
0,
TIb,Z
=
NHJ
with or perpendicular to that of the cyclopentadienyl ring and hence to a structure with minimal or maximal interaction between these groups (IIIa or IIIb).
Closely related to these transformations is the intramolecular process leadmg to the carbene complex IV (Scheme 1) [S] .
.--_ c) ’
oc
\ -__/ ‘I
ii’-
0
0
----. f \ -0 ---: oc
/Fe
I
oc
oc
Tl
-*’
0
0
.--w
I
;
\ -__’
-w
okFe 6
0--
5 6 SCHEME
1
c31
Epoxide opening by the complex anion creates chn-ahty on the hgand, and cychzatlon of the resulting alkoxlde mtroduces a second chiraI center at the metal. A PMR spectrum of the methylated complex (IV) provides little mformation about the number of dn&ereomers formed*_ However, a 13C NMR spectrum of this product shows it to be a single dlastereomer; (CD3N02) 6 (ppm) 26.14,29 32,33 88,39.73 (C(3,4,5,6)), 39.47 (C(2)), 57.09 (CH,), 84.79 (Cp), 97.47 (C(l)), 224.99 (CO), 265.70 (carbene C). Those steric factors controllmg the cycllzation step in the sequence leading to IV may readily be identrfied. Models show appreciable steric interactions between the ax& proton at C(3) on the cyclohexane rmg and cyclopentadlenyl protons in structure Via_ Such mteractlons are absent m VIb**_
q&
@e+ 0
\_:
co 0-
0-
The transformation of VII to the chelate IX provides perhaps the most stnkmg example of a dlastereospeclflc reaction (Scheme 2).
SCHEME 2
aIll)
Complex VII 1s obtained by condensiltlon of cyclohexanone
ux)
pyrrolidme
enamine with the ($-C, H, )Fe(CO), (s* -butadlene) cation, followed by hydrolysis 171. A PMR spectrum of VII shows two cyclopentadienyl proton *Complexa fV md V UC U&A obtemed in 40% yield from cycloherent lpoxide **The inta e of Cp and CO Hgamls in Vh end VXb does not exchange their bond vectors smce tbe conffgur~tion &out the iron atom IS not tetrahedral Provided the confipuratior;about the MP atom h not too differentfrom that found in CpPe
C32
resonances (6 (CS,) 4_76,4.78 ppm), indicatnre of the presence of the two anticipated diastereomers, m approxunately equal proportion. On bnef warming of this product in acetonitrile it is smoothly converted to the chelate IXA PMR spectrum of this product is uninformative as to the number of diastereomeric forms present. However, examination of the 13CNMR spectrum
reveakd the presence of three signals at 6 88 26,88
10 and 87.23 ppm (in-
tensity ratio 52/35/13)*, which were readily identified as cyclopentadienyl ring carbons by a single frequency proton dccouplmg cxperrment**- Thus, although two additional centers of clurahty are created m the conversion of VII to IX, only three of a posarbIe eight dmstereomeric pairs are formed, and one of these is a relatively minor component_ The steric factors controlling asymmetric induction at the iron atom in the chelation step VIII * IX may be identified with the configuration at C(3) in IX. Models show that for the chelate in which the olefm center has the preferred configuration (see IIIa), the ring may adopt a chairlike conformation in which the cyclohexanone substituent at C(3) is feed as either equatorial or axial (X)_ The latter configuration would clearly be disfavored on steric grounds since appreciabie nonbonded interactions exist between a large axial group at C(3) and &-protons of C(1) and C(4). Coordmation of the prochu-al iron atom in the conversion of VIII to IX simultaneously estabhshes the configuration at iron and the stereochemistry at the C(3) center Thus, provided this step is essentiahy irreverslhle under the reaction conditions, asymmetry at the iron atom is induced in a kinetically controlled chelation process, in which the configuration at C(3) determines the configuration at iron_ It seems unhkeIy that such asymmetry is induced in the conversion of VII to VIII since there is no apparent mechanism by which the distant chiral center at C(3) may control such a process. It follows, then that the actlvatron energy for the chelation step must be greater than that for interconversion of nonplanar, enantiomeric iron centers ‘Lnintermediate VIII. These conclusions do not, of course, provide any information regarding the preferred geometry at the coordinatively unsaturated iron atom, that is, whether the equilibrium configuration at iron in VIII is planar or nonplanar [ 81.
c33
These examples serveto &&rate the diverse forms which steric mteractzons may take in controlling the stereochemicalcourse of chelation reactionsat prochiralcenters in organometalliccomplexes. Diastereospecif~cityin these reactlonsshould prove to be an nnportant and generalphenomenon. Acknowledgement This work was supported by grantsfrom the NatIonal Institutesof Health (GM 16395) and by the National Science Foundation (CHE-09590). References 1
2 3
4 5 6
7 8
Angera_ Chem_ Intl Eda. 10 <1971) 871 K Bemauer m TOOICS III Current Chemhtw Spnneer Veriag. Berlm. 1976 Vol65. P 1 F Basolo and R Pearson Mechanfsms of Inorganic Chemutn 2nd cd Wiley. New York 1968. P 334-344 A. Cutler. D_ Ehntholt. P. Lennon. EC N~ckolas. D P. Marten. M_ Madhavarao. S R.a~hu. A Rosan and M. Rosenblum J Amer Chem Sot 97 (1975) 3149 M R Church and S W -Y_ Chang J Amer Chem Sot .95 <1973) 5931 BM_Forman. J Chem Sot Chem Commun <1975)221 P. Klemarczyk T_ Pnce W_ Pnesterand M. RosecbJum. J. Orwnometpl Chem 139 (1977X25 A Rosan. M Rosenblum and J Tancrcdc J Amer Chem Sot 95 <1973) 3062_ A Davison apd N. Martmez. J. Organometal Cnem .74 (1974) Cl7 T G Attxg P Retch-Rohn& and A WoSclck~ J OrgaaometaL Chem 51(1973) C21_ P. Ruth-RO~KWU and A WOfcicki, InolZ Y_ Izuxm
Chem
13 (1974)
2457
Journal of Organometall~c Chemrstry 139 (1977) C29-C33 o Ekevter Sequoia S A Lausanne -Printed m The Netherlands
Prelimmary communication
DIASTEREOSPECIFICITY IN THE FORMATION ORGANOMETALLIC COMPLEXES
PATRICK
LENNON,
and MYRON Department (Recewed
WIT’I’A PRIESTER,
ALAN
ROSAN,
OF CHELATED
MADDULA
MADHAVARAO
ROSENBLUM* of Chemistry
June 23rd
Branders Unwerslty
Waltham. Massacusetts
02154
(US
A)
1977)
Summary Three types of m&molecular cychzatlon reactions m organovon complexes having one or more chiral centers are shown to proceed with high dlastereospec&clty_
Diastereospeclficlty [ l] m chelate nng formation 1s a phenomenon commonly encountered m coordmatlon complexes derived from chval hgands [2] Its occurrence in smular reactions of organometalhc complexes 1s less well recognlzed We wish to report several examples of this important phenomenon m closely related organovon complexes in which the factors responsible for asymmetric induction may be ldentlfled We recently reported the first example of such di&ereospecfici@ in the conversion of the q* -ally1 alchohol complex Ia to the lactone IIa [S] . A closely related nng closure occurs when complex Lb 1s treated mth two equivalents of tri-n-butylamme. In each of these transformations a new chrral center, m addltzon to the one at the olefm, is created at the metal atom by chelation. In each of these reactions only a single dlastereomer IS formed as mdlcated by PMR spectral evidence* 133 _ The stereochemlcal factors which control these reactions are reatiy perceived Models** show that closure of Z to one of the two dlastereotoplc carbonyl groups constrains the olefm hgand to a plane either roughly parallel
*The PMR
rpcctrum
of Ilb
closely
a single dhstereomerue ready Yields **For
resembles that of IIa AU proton rc~~nan~cs corresponding to assignable Complexes IL%and IIb M obtained III 70 and 80%
rez&wctl~Y
Drciditu
= cee
mod&
based on molecuI~~ parameters for a CpFe(CO),
complcr
ref.
5.
R complex see ref 4 and for
GC
(IO,.? = Ib , Z =
OH,
NH,+1
(JIa.2
=
0,
TIb,Z
=
NHJ
with or perpendicular to that of the cyclopentadienyl ring and hence to a structure with minimal or maximal interaction between these groups (IIIa or IIIb).
Closely related to these transformations is the intramolecular process leadmg to the carbene complex IV (Scheme 1) [S] .
.--_ c) ’
oc
\ -__/ ‘I
ii’-
0
0
----. f \ -0 ---: oc
/Fe
I
oc
oc
Tl
-*’
0
0
.--w
I
;
\ -__’
-w
okFe 6
0--
5 6 SCHEME
1
c31
Epoxide opening by the complex anion creates chn-ahty on the hgand, and cychzatlon of the resulting alkoxlde mtroduces a second chiraI center at the metal. A PMR spectrum of the methylated complex (IV) provides little mformation about the number of dn&ereomers formed*_ However, a 13C NMR spectrum of this product shows it to be a single dlastereomer; (CD3N02) 6 (ppm) 26.14,29 32,33 88,39.73 (C(3,4,5,6)), 39.47 (C(2)), 57.09 (CH,), 84.79 (Cp), 97.47 (C(l)), 224.99 (CO), 265.70 (carbene C). Those steric factors controllmg the cycllzation step in the sequence leading to IV may readily be identrfied. Models show appreciable steric interactions between the ax& proton at C(3) on the cyclohexane rmg and cyclopentadlenyl protons in structure Via_ Such mteractlons are absent m VIb**_
q&
@e+ 0
\_:
co 0-
0-
The transformation of VII to the chelate IX provides perhaps the most stnkmg example of a dlastereospeclflc reaction (Scheme 2).
SCHEME 2
aIll)
Complex VII 1s obtained by condensiltlon of cyclohexanone
ux)
pyrrolidme
enamine with the ($-C, H, )Fe(CO), (s* -butadlene) cation, followed by hydrolysis 171. A PMR spectrum of VII shows two cyclopentadienyl proton *Complexa fV md V UC U&A obtemed in 40% yield from cycloherent lpoxide **The inta e of Cp and CO Hgamls in Vh end VXb does not exchange their bond vectors smce tbe conffgur~tion &out the iron atom IS not tetrahedral Provided the confipuratior;about the MP atom h not too differentfrom that found in CpPe
C32
resonances (6 (CS,) 4_76,4.78 ppm), indicatnre of the presence of the two anticipated diastereomers, m approxunately equal proportion. On bnef warming of this product in acetonitrile it is smoothly converted to the chelate IXA PMR spectrum of this product is uninformative as to the number of diastereomeric forms present. However, examination of the 13CNMR spectrum
reveakd the presence of three signals at 6 88 26,88
10 and 87.23 ppm (in-
tensity ratio 52/35/13)*, which were readily identified as cyclopentadienyl ring carbons by a single frequency proton dccouplmg cxperrment**- Thus, although two additional centers of clurahty are created m the conversion of VII to IX, only three of a posarbIe eight dmstereomeric pairs are formed, and one of these is a relatively minor component_ The steric factors controlling asymmetric induction at the iron atom in the chelation step VIII * IX may be identified with the configuration at C(3) in IX. Models show that for the chelate in which the olefm center has the preferred configuration (see IIIa), the ring may adopt a chairlike conformation in which the cyclohexanone substituent at C(3) is feed as either equatorial or axial (X)_ The latter configuration would clearly be disfavored on steric grounds since appreciabie nonbonded interactions exist between a large axial group at C(3) and &-protons of C(1) and C(4). Coordmation of the prochu-al iron atom in the conversion of VIII to IX simultaneously estabhshes the configuration at iron and the stereochemistry at the C(3) center Thus, provided this step is essentiahy irreverslhle under the reaction conditions, asymmetry at the iron atom is induced in a kinetically controlled chelation process, in which the configuration at C(3) determines the configuration at iron_ It seems unhkeIy that such asymmetry is induced in the conversion of VII to VIII since there is no apparent mechanism by which the distant chiral center at C(3) may control such a process. It follows, then that the actlvatron energy for the chelation step must be greater than that for interconversion of nonplanar, enantiomeric iron centers ‘Lnintermediate VIII. These conclusions do not, of course, provide any information regarding the preferred geometry at the coordinatively unsaturated iron atom, that is, whether the equilibrium configuration at iron in VIII is planar or nonplanar [ 81.
c33
These examples serveto &&rate the diverse forms which steric mteractzons may take in controlling the stereochemicalcourse of chelation reactionsat prochiralcenters in organometalliccomplexes. Diastereospecif~cityin these reactlonsshould prove to be an nnportant and generalphenomenon. Acknowledgement This work was supported by grantsfrom the NatIonal Institutesof Health (GM 16395) and by the National Science Foundation (CHE-09590). References 1
2 3
4 5 6
7 8
Angera_ Chem_ Intl Eda. 10 <1971) 871 K Bemauer m TOOICS III Current Chemhtw Spnneer Veriag. Berlm. 1976 Vol65. P 1 F Basolo and R Pearson Mechanfsms of Inorganic Chemutn 2nd cd Wiley. New York 1968. P 334-344 A. Cutler. D_ Ehntholt. P. Lennon. EC N~ckolas. D P. Marten. M_ Madhavarao. S R.a~hu. A Rosan and M. Rosenblum J Amer Chem Sot 97 (1975) 3149 M R Church and S W -Y_ Chang J Amer Chem Sot .95 <1973) 5931 BM_Forman. J Chem Sot Chem Commun <1975)221 P. Klemarczyk T_ Pnce W_ Pnesterand M. RosecbJum. J. Orwnometpl Chem 139 (1977X25 A Rosan. M Rosenblum and J Tancrcdc J Amer Chem Sot 95 <1973) 3062_ A Davison apd N. Martmez. J. Organometal Cnem .74 (1974) Cl7 T G Attxg P Retch-Rohn& and A WoSclck~ J OrgaaometaL Chem 51(1973) C21_ P. Ruth-RO~KWU and A WOfcicki, InolZ Y_ Izuxm
Chem
13 (1974)
2457