4766
Journal of the American Chemical Society
/ 100:15 / July
19, 1978
Lewis Acid Catalyzed [RFe(CO)$ Alkyl Migration Reactions. A Mechanistic Investigation James P. Collman,*’ Richard G. Finke, James N. Cawse, and John I. Braurnan*’ Contribution from the Department of Chemistry, Stanford Unicersity. Stanford, California 94305. Received January 9, 1978
Abstract: This paper is a kinetic and mechanistic study of alkyl migration reactions of (RFe(C0)4)-. Catalysis of these reactions by Lewis acid ion pairing is discussed. The study includes analysis of the several types of ion pairs derived from NaRFe(C0)4 and NaRC(O)Fe(C0)4 in THF and their interconnecting equilibrium constants, as well as IR and 13C NMR evidence implicating the acyl group as the cation binding site in (RC(O)Fe(C0)4)-. The overall rate law is determined, and kinetic data are presented showing the effect of various cations and crown ether complexed cations as well as CO and a variety of phosphines. Solvent and temperature effects upon this reaction are also given. A mechanism for (RFe(C0)4)- alkyl migration reactions is discussed in light of previous studies of alkyl migration reactions.
There is increasing interest in the use of organotransition metal reagents in organic synthesis. Since 1970 much of our work in this area has been directed toward understanding the reactions of NalFe(C0)4 and its organic derivatives. Previously, we described the use of Na2Fe(CO)4 to convert alkyl halides or sulfonates into aldehydes,2a unsymmetrical ketones,2bcarboxylic acids,2c esters,2Cand amides2c (Scheme I). The use of NazFe(C0)d for organic as well as inorganic synthesis has recently been reviewed.2d Central to these synthetic conversions is the alkyl migration “migratory insertion” reaction: (RFe(C0)d)RC(0)Fe(C0)3(L)-, Scheme I). Such reactions have played an important and unifying role in organotransition metal chemistry. W e have studied alkyl migration as well as many of the other reactions in Scheme I in some detail, with the hope that an understanding of the underlying mechanisms will promote practical applications of NazFe(CO)4 and other organotransition metal reagents. Early on, our interest in the mechanisms of the reactions in Scheme I was increased by the observation of dramatic ionpairing effects on the oxidative-addition reactions of the supern~cleophile,~ NazFe(CO)4, and on Lewis acid catalysis5 of alkyl migration reactions involving (RFe(C0)4)-. In this paper we present detailed kinetic and other mechanistic studies of alkyl migration reactions of (RFe(C0)4)-. The nature of the ionic species (tight and solvent-separated ion pairs, triple ions, and dissociated ions) derived from N a R Fe(C0)4 in T H F along with their interconnecting equilibrium constants and relative reactivities has been determined. The relative rates of alkyl migration in the presence of CO and a series of phosphines, along with the relative migratory aptitudes of benzyl and n-alkyl groups, have been measured. Solvent effects and temperature parameters have also been determined.
-
Results and Discussion The reaction of interest is alkyl migration with the binding of an additional ligand, L (L = CO, PR3, etc.), eq 1. For Na+ 0
(RFeiCO1,)-
f
L
I
-+
iRCFe(CO)IL)-
(1)
as the counterion and L = PPh3, this reaction in THF is slowed 170-fold if the N a + is complexed with the crown ether, dicyclohexyl- IS-crown-6. Replacement of N a + by Li+ accelerates the alkyl migration. In order to understand this ion-pairing (Lewis acid) catalysis, a knowledge of the ion-pairing6 properties of both the alkyl reactant and acyl product of eq 1 , (( RFe(C0)4)- and (RC(O)Fe(C0)4)-, respectively) was 0002-7863/78/1500-4766$01.00/0
Scheme I. Synthetic Organic Conversions Using Na,Fe(CO),
RFe(C0); \ -
Scheme 11. NaRFe(CO), Ion Pairing
Na’ (,ssociated, + RFe(CO),B)
~
NaRFe(CO), (tight, A )
-
‘/,INa’(RFe(CO),-)Na’i’
+
(KTT)”~
1
‘/,~RFe(CO),-(Na’)RFe(Co),-~-
\.
(triples, C) P
higher aggregates, D
required. In general, our studies have focused upon the sodium salt (NaRFe(C0)4 where R = n-CloH21). Determination of Ion-Pairing Equilibrium Constants Using Conductivity. The concave curve in T H F of molar conductance vs. NaRFe(C0)4 concentration, Figure 1, is typical7 of a system that forms ion pairs, dissociated ions, and triple ions in the concentration range studied (A, B, C, respectively, Scheme II).* These conductivity data are most conveniently analyzed’ in terms of the following equilibria for ion-pair dissociation, K D , and the equilibria for triple ion dissociation, K T D . The symbols C, a , and y3 have their usual meanings’ of the total concentration (e.&., [NaRFe(C0)4] total), the degree of dissociation, and the degree of association (of triples), respectively. Analysis9 of the conductivity data yields the estimatesgb M (for R = n-CloH21 KD= 9 X M and KTD= 6 X a t 25 “ C ) .These equilibrium constants can be used to calculategc the constant for the total triple ion equilibria (Scheme
0 1978 American Chemical Society
Collman, Brauman, et al.
/
4767
[RFe(CO)4]- Alkyl Migration Reactions
..
0
002
0 01
003
004
0
005
50
25
0
[ NaRFe ( C O ) 4 ] (M)
100
75
150
125
(EQUIV HMPA/ EQUIV No+)
Figure 1. Molar conductance vs. dilution for NaRFe(C0)4 in T H F at 25 OC (R = n-CloHz,).
Figure 3. I3C NMR chemical shift of the acyl peak in NaRC(O)Fe(C0)4 (R = CHlCH2) as a function of added HMPA. NaRC(O)Fe(CO)d with no HMPA, 6 279.7 ppm downfield from Me4Si used as a reference (A6 = 279.7 - 6). Table I. Relative Percentages of Initial NaRFe(C0)d or NaRC(=O)Fe(C0)4 Present as Ion Pairs, Triple Ions, or Dissociated Ions ( R = n-CloH21)
06
1 008
004
012
016
020
0
11), KTT = 2 M-'. Analysis9 of the conductivity data for NaRC(O)Fe(C0)4 (Figure 2) yields the important results that there is significantly tighter cation binding, K D = 5 X M, and that there is less tendency to form triple ions, K T D = 1 X lo-* M , KTT = 5 X M-I. Table I shows relative percentages of the various types of ions at selected concentrations.
C(1 - a - 3y3)
ca
+ RFe(C0)4-
Na+
CY3
ca
+
NaRFe(C0)4 C(1 - cy - 373)
(RFe(C0)4-( N a + ) R Fe( CO)4-)-
--
CY3
KTD
RFe(C0)4Ca
+
NaRFe(C0)4 C(1 - a
(2)
- 373)
Determination of Cation Binding Sites in RC(O)Fe(CO)d-. Based upon the recent work of DarensbourgIob in different but related systems, there is little doubt that the cation ( M + ) in M+(RFe(C0)4)- is associated with the oxygens of the carbonyl groups. More interesting and more pertinent to this work, however, is the cation binding site in M+(RC(O)Fe(C0)4)-. Because of the acyl oxy carbene resonance structure, we
-
%
dissocciated
2 x 10-1
alkyl acyl alkyl acyl alkyl acyl alkyl acyl
56 8 41 4 8 0.6 0.04 0.05
43 91 56 95 68 98 9 80
x
10-5
I 0.1 3 0.3 24 2 91 20
The percent of either [Na+(X-)Na+]+ or [X-(Na+)X-](X- = RFe(C0)4- or RC(=O)(C0)4-) is '/z of this value. Table 11. Cation Dependent Shifts in the Acyl Stretching Freauencv in RC(=O)Fe(COLacyl frequency, cm-'
cation
1610 1585 1560
anticipated that the acyl group was the cation binding site in (RC(O)Fe(CO)d)-. Consistent with this picture, the acyl 00
ca
(Na+(RFe(C0)4-)Na+J+
%
ion pairs
1
Figure 2. Molar conductance vs. dilution for NaRC(O)Fe(C0)4 i n T H F at 25 OC (R = n-Cl0H2~).
KD
%
tripleo ions
I x 10-3
024
[ NaRCFe (CO4](M)
NaRFe(C0)4 *Sa+
NaRFe(C0)d or NaRC(0)Fe(CO)?
5 x 10-2
J
0
selected initial concn, M
II
-
RC-Fe(
CO),
-I
RC=Fe(CO),
stretching frequency (Table 11) shifts to lower values with smaller, less polarizable cations such as Li+, suggesting increased importance of the oxy carbene resonance structure. Further evidence for the acyl as the cation binding site was obtained using I3C N M R . Although the positions of carbonyl 13C N M R peaks in (RFe(C0)4)- and (RC(O)Fe(C0)4)- are virtually independent of cation and solvent, the acyl 13C N M R peak of the acyl-iron anion is strongly affected by such changes. The acyl I3C N M R resonance in (CH3CH2C(O)Fe(C0)4)- is shifted upfield by 19.5 ppm by the addition of a large excess of HMPA (Figure 3). Changing the cation from S a + to (Ph3P)2N+ causes a shift of 18.2 ppm in the same direction. By comparison, the methyl I3C N M R signal in the propionyl iron complex shows a change of only -0.1 ppm and the carbonyls show a downfield shift of -0.7 ppm. Carbene
4768
Journal of the American Chemical Society
Table 111. Selected Equilibrium Constants for Anionic Metal Carbonvl Ion Pairs in THFO K~ comod
x 105, M
Na( Na Fe( C 0 ) 4 ) Na R Fe(CO), NaRC(O)Fe(C0)4 LiMo( CO) 5 ( C (0 ) P h ) LiMn(C0)s N a Mn( C 0 ) 5 (Na-crown) bMn(CO)5 (P~~P)ZN(WCO)~) All values at 25 f 1 OC.
-0.5 9 0.05 -0.01 4.3 2.0 3.9 9.4
K-ro X 103, M
ref 4 this work this work 1oc 10b 10b 10b 1Od
6 10
4.5 6.4 5.3
Crown = 15-crown-5.
ligands in complexes such as Cr(CO)s[C(OCH3)CH3] (6 362.3) exhibit" I3C resonances a t much lower field than acyl groups as in1' Fe(~5-CsHs)(CO)~[C(0)CH3] (6 254.4). These 13C N M R and IR results provide compelling evidence that the acyl group is the predominant cation binding site in ( R C ( 0 ) Fe(C0)4)-. Several additional general features important to this work can be gleaned from available literature data. The data in Table 111 clearly indicate that for a variety of cations, metal carbonyl monoanions in T H F generally have K D and K T D values in the range 10-7-10-4 (25 "C) and 10-3-10-2, respectively. Tight ion pairs are often present. (At 6.7 X M , 48% of N a M n ( C 0 ) s is present as tight ion pairslob.) The predominant effect of the addition of crown ethers or a few equivalents of dipolar aprotic solvents is to convert contact to solvent-separated ion pairs. In the acyl anions, the preference of the acyl group for small, nonpolarizable cations such as Li+ is reflected in their tighter binding (smaller K D values). The preference of the acyl in (RC(O)Fe(C0)4)- for N a + rather than the large (Ph3P)2N+ was confirmed by examining the following equilibrium. 0
KaEt Fe(COh
I1 + (Ph,P)2NEtCFe(COj4
-
K > 10
t P h P).NEtFe(CO),
0
It + SaEtCFe(CO)4
(3)
1
2 3 4 5 6 7 8
L Ph3P Ph3P Ph3P Ph3P Ph3P Ph3P Ph3P Ph3P
[L], M 0.344 0.127 0.0668 0.0668 0.0668 0.16 0.27 0.0668 0.020 0.15 0.10 0.25 0.17 0.13 0.09 0.10 0.20 0.50
[M+(RFe(C0)4)-],' M
100:15
/
July 19, 1978
Solutions (0.05 M ) of Na(EtFe(C0)p) and (Ph3P)2N(EtC(O)Fe(C0)4) were mixed and the relative sizes of the acyl peaks a t 1585 (Na+) and 1610 cm-' ((Ph3P)2N+) used to monitor the position of the equilibrium. The estimated equilibrium constant, K > l o 2 (eq 3), is predominantly a reflection of the tighter N a + binding by the acyl compound. Kinetic Studies of (RFe(C0)d)- Alkyl Migration Reactions. A compilation of our kinetic results is presented in Table IV. In addition to establishing the overall rate law and detailed ion-pairing effects, we have studied a variety of different phosphorus ligands, CO-promoted migrations, the migratory aptitude of n-alkyl vs. the electron-withdrawing benzyl group, and the temperature and solvent dependence of the alkyl migration reaction. The reaction is overall second order, first order each in [NaRFe(C0)4] and12 [PhjP], with kz(obsd) = 6.2 f 0.5 X M-' s-l (entries 1-7, Table IV). The rate of alkyl migration varies by about 20-fold over the series (0.0 O C , THF): Ph3P < (MeO)3P < n-Bu3P < CO < Ph2MeP < Et3P < PhMe2P < Me3P (entries 28, 10, 11, 22 (and ref 13), and 12-18, respectively). When this series is compared to either a steric bulk (cone angle)14 series or to an "electronic" ser i e ~ ' ~for , ' ~the phosphorus ligands, no simple correlation is observed. Thus, as one would expect, both steric and electronic effects seem to influence the relative reactivities of the various phosphorus ligands. The dramatic effect of ion pairing upon these reactions is evident upon examining Table V. Thus the reaction is slowed by > l o 3 if the Li+ counterion is replaced by (Na-crown)+ or (Ph3P)zN+ and varies Li+ > N a + > (Na-crown)+ > (Ph3P)N+. Since the predominant effect of the addition of crown ether is to convert tight ion pairs to solvent-separated ion pairs, this result and these rate variations (Table V) require that tight ion paired transition states are much more favorable than are more dissociated ones. The remaining data (Table IV) show that the apparent activation parametersl6 for this reaction in THF are AH* = 19.3 f 0.6 kcal/mol, AS* = - 19 f 2 eu (Table IV, entries 25-28), that benzyl migrates roughlyI7 60 times more slowly than n-alkyl (entry 29 and ref 17), that the reaction is slowed 35% by addition of 2.4 equiv of HzO/equiv NaRFe(C0)4, and that a change in solvent from T H F to N M P (n-methylpyrrolidinone) slows the reaction 413-fold (33 and 43).
Table IV. Kinetic Data for the Alkyl Migration Reaction:O M+(RFe(CO),)-
entry
/
+L
Temp, OC
-
M+(RC(=O)Fe(C0)3L)misc exptl info or comment
kz, M-I s-l
[NaRFe(C0)4] and [PPh3] Dependence 3.3 x 10-2 25.0 5.8 X 1.64 X 10-2 25.0 5.6 X IO-2 5.0 x 10-3 25.0 5.8 X 2.35 X 25.0 6.8 X 1.12 x 10-3 25.0 6.3 X 7.3 x 10-3 25.0 6.1 X 6.6 X 25.0 6.7 X 9.2 x 10-3 25.0 4.0 X lo-* Reactions with Different Phosphorus Ligands, L 4.0 x 10-3 25.0 22.0 x 10-2 2.5 X lo-* 0.0 0.35 x 2.0 x 10-2 0.0 0.57 X IO-* 1.9 x 10-2 0.0 1.7 x 10-2 0.0 1.2 x 10-2 Io-* 2.2 x 10-2 0.0 1.3 x 10-2 0.0 2.8 X IO-* 1.7 X IO-* 2.0 x 10-2 0.0 4.5 x 10-2 2.5 X 0.0 4.6 X 2.3 X 0.0 5.8 X
0.022 M degassed H2O added
1
k2/kz0 (using k1° for PPh3, entry 28) 1.3 2. I 4.8
4.8 10.4
-20
Collman, Brauman, et al.
/ (RFe(COJ41-Alkyl Migration Reactions
4769
Table IV (Conrinued) Reactions with CO entry
L
CO pressure, atm
19 20 21 22 23 24
co co CO co co co
1.o 1.o 1 .o 1.o 0.51 0.58
L
[LI M
[M+(RFe(CO)4)-11 M
entry
[M+(RFe(C0)4)-],' M
Temp, "C
0.01 0.025 0.025 0.025 0.01 0.01
25.0 25.0 25.0 0.0 25.0 25.0
Temp, "C
reaction monitored
k2,
atm-'
s-l
volumetrically IR GLC GLC volumetrically volumetrically
1.27 f 0.07 X 1.3 x 10-3 1.25 x 10-3 0.06 f 0.004 X 1.29 X 1.26 x 10-3
kz, M-I s-l
misc exptl info or comment
Temperature Dependenced 20.0 3.15 X lo-* 1.64 X 1OW2 14.8 7.90 0.75 X 0.0 0.274 X IO-*
25 26 27 28
Ph3P Ph3P Ph3P Ph3P
0.0668 0.0668 0.0668 0.0668
9.9 x 10-3 9.6 X 9.2 x 10-3 8.8 x 10-3
29 30 31 32
PhMe2P PhMezP PhMe2P PhMeZP
0.159 0.325 0.594 1.30
Data for Naf(PhCH2Fe(C0)4)6.2 X 25.0 0.105 X 8.0 x 10-3 25.0 0.114 X 6.7 X 25.0 0.126 X 5.6 X 25.0 0.159 X lo-*
33
Ph3P
34
Ph3P
0.127
8.7 X
25.0
35 36 37
Ph3P Ph3P Ph3P
0.20 0.040 0.067
10.0 X 6.0 X 6.7 X
25.0 25.0 22.5
38
Ph3P
0.067
6.7 X
22.5
0.74 X lo-*
39
Ph3P
0.073
4.5 x 10-3
22.5
0.13 X lo-*
40
Ph3P
0.080
3.0 X
22.5
0.59 X
41
Ph3P
0.080
3.0 X
22.5
0.23 x
42
Ph3P
0.080
3.0 X lo-*
22.5
0.056 x
43 44
Ph3P Me3P
1 .o
0.10 0.05
+
Ion Pairing: Effect of Varying Cation, of Added Crown Ether, of Adding the Polar Solvents HMPAfor NMPf average value for M+ = Na+, 25.0 6.2 X
g
entries 1-6 0.01 M dicyclohexyl-18-crown-6 (A isomer) added M+ = (Ph3P)2N+ M+ = Li+ 0.013 M HMPA added (2.0 equiv) 0.022 M H M P A added (3.3 equiv) 0.05 1 M H M P A added (1 1 equiv) 0.15 M N M P added (5 equiv) 0.30 M N M P added ( 10 equiv) 0.50 M N M P added (1 6 equiv)
0.036 x 0.015 x 37.0 X 1.3 X lo-*
Reactions in Pure NMPf 25.0 0.015 X 25.0 0.12 f 0.01 X
g
R = n-C5H, I used R = n-C5Hl, used
' R = either n-CloH21- or n-CsHls-, M+ = Na+, L = PPh3, in THFunless noted otherwise. k2 = ki(,,,/[L]. kl(av)is the averagepseudofirst-order rate constants for the disappearance of reactants and appearance of products (measured as R H and RCHO, respectively, after acid quench). Effective NaRFe(C0)4 concentration (see Experimental Section). Standard deviation of three experiments. e Only the disappearance of reactants was monitored. f H M P A = hexamethylphosphoric triamide; N M P = n-methylpyrrolidinone. g Four experiments were performed at (Me3P) = 0.026, 0.04, 0.05, and 0.07 M. The standard deviation for k z is given.
Table V. Effect of Cation for the Reaction
Mt( RFe(CO),)-
+ PhjP
-
0
II
THF
M+(RCOFe(CO)dPPh3))
M+
k2 (relative)O
M+
Li+ Na+ (Na*2HMPA)+ (Na.lONMP)+
1.0 X IO3 1.7 X lo2 36 6.4
(Na.lIHMPA)+ (Na-crown)+ (Ph3P)2Nf
a
Data from entries 33-42, Table IV.
k2 (relative)O 3.6
1 .o
0.42
Proposed Mechanism for (RFe(C0)d)- Alkyl Migration Reactions. W e have combined our results with some pertinent literature data in formulating a mechanism for (RFe(C0)d)alkyl migration reactions. Since comprehensive reviews of insertion reactions18 as well as a very recent reviewIg of CO insertion reactions are available, we cite only that literature which is relevant to our mechanism. This mechanism (Scheme 111) consists of NaRFe(C0)4 ion pairs in a rapid prior equilibrium with the tight ion-paired coordinatively unsaturated acyl iron tricarbonyl intermediate followed by rate-determining capture by L. W e have observed only overall second-order kinetics (k-3 >> k4[L], Scheme 111) despite the use of very active phosphines
4770
Journal of the American Chemical Society
/ 100:15 / July 19, 1978
Scheme 111. Proposed NaRFe(CO), Alkyl Migration Mechanism dissociated Na' + R F e ( C O ) , XK2
Ky
.Dlt
Na':S:RFe(CO),-
ion pairs
ll
t
{Na'(RFe(CO),)-Na'}'
I
(?)
- - - - --- - - - - 1
+
triples
(?)
-- - - -((RFe(CO),)-Na+(RFe(CO),)-JI
I
I
-1
4
03c
A
01 0
i
\
7---____ 001
002
---__ 003
004
005
INITIAL I NaRFe(C0l41 (M)
Figure 4. Calculated fraction of ion pairs, triple ions, or dissociated ions present as a function of the initial [NaRFe(C0)4]. (The - -, indicate the fraction, a , present as dissociated ions; -, the fraction, 73, present as triples; - - - - -, the fraction, (1 - a - 3y3), present as ion pairs. The calM, and K T D = 6 X culation used eq 2, K D = 9 X M.)
(Table IV, 10-18). Thus we have not been able to obtain the limiting conditions of k-3
