Chemoselective reactions of (E)-1,3-dienes: cobalt-mediated isomerization to (Z)-1,3-dienes and reactions with ethylene.

In the asymmetric hydrovinylation (HV) of an E/Z-mixture of a prototypical 1,3-diene with (S,S)-(DIOP)CoCl2 or (S,S)-(BDPP)CoCl2 catalyst in the presence of Me3Al, the (E)-isomer reacts significantly faster, leaving behind the Z-isomer intact at the end of the reaction. The presumed [LCo-H](+)-intermediate, especially when L is a large-bite angle ligand, similar to DIOP and BDPP, promote an unusual isomerization of (E/Z)-mixtures of 1,3-dienes to isomerically pure Z-isomers. A mechanism that involves an intramolecular hydride addition via an [η(4)-(diene)(LCo-H)](+) complex, followed by π-σ-π isomerization of the intermediate Co(allyl) species, is proposed for this reaction.

We recently reported a new protocol for a highly enantioselective Co(II)-catalyzed asymmetric hydrovinylation (HV) of unactivated 1,3-dienes that involves the use of a [1,n-bis-diphenylphosphinoalkane]CoCl2 and Me3Al (Scheme 1).[1,2]

Scheme 1

Ni(II)- and Co(II)-Catalyzed Hydrovinylation of 1,3-Dienes

In order to explain the improved selectivity in the Co-catalyzed reactions as compared to the corresponding Ni-catalyzed reactions[3] (Scheme 1), we invoked[1a] an η4-cobalt-hydride complex 4 that restricts the conformations of the reactive intermediates in the former (Scheme 2). Complex 4 subsequently forms an allyl-cobalt intermediate 5, which undergoes ethylene insertion and β-hydride elimination to regenerate the presumed [LCo–H]+ catalyst 3(4) to complete the catalytic cycle. When an E/Z-mixture of a prototypical 1,3-diene was subjected to our standard HV conditions (except for the presence of ethylene), the terminal (Z)-1,3-diene was found to be mostly unreactive at low temperature. The attendant implication is that the (Z)-isomer is unreactive toward [LCoH]+ or the equivalent catalyst. Since such hydride species are also known to be capable of isomerization of alkenes, we wondered if conditions can be found to maximize the Z-isomer from an E/Z-mixture under kinetic conditions.[5,6] The results of these studies are described in this paper.

Scheme 2

Working Hypothesis on the Mechanism of Co(II)-Catalyzed Hydrovinylation

The enantioselectivity in the asymmetric HV of a mixture of (E)-8 and (Z)-8 (Z:E = 53:47) using [(S,S)-DIOP]CoCl2/Me3Al (TMA) was found to depend on the conversion, with the (E)-isomer reacting at a significantly faster rate (eq 1, Table 1).[6,7] At low conversions (entries 1–3, Table 1), only E-8 undergoes hydrovinylation giving a maximum of ∼83% ee. As conversion increases, the proportion of Z-8 increases, leaving behind, at 23 h, essentially pure (Z)-8 (Z:E = 49:1, entry 5). A minor product (<5%), tentatively identified as a linear hydrovinylation product (10), is also formed at higher conversions. A similar behavior is observed with (S,S)-2,4-BDPP ligand, except a notable decrease in enantioselectivity (from 85% ee at 21% conversion to 73% ee at 61% conversion, entries 6–10) results. These results are most readily rationalized if one assumes that the (Z)-isomer is a reluctant partner in the HV reaction, while the (E)-isomer undergoes a fast reaction giving the (S)-9 [with (S,S)-DIOP] as the major product. With the more reactive BDPP complex, at higher temperatures (−15 °C), upon complete conversion of the (E)-isomer, the (Z)-isomer undergoes the reaction, giving enantiomeric product, along with 10. The (DIOP)CoCl2 complex shows much more discrimination in the reactions of the (Z)- and (E)-isomers of the starting diene, leaving behind almost all of the (Z)-isomer unreacted (46%, Z:E = 49:1, entry 5) at the end of the reaction (entry 5, compared to 25% in entry 10 using 2,4-BDPP). The lower enantioselectivity at higher conversions using the BDPP ligand (entries 9 and 10) also suggests that the 1,4-HV of the (Z)-8 gives predominantly the opposite enantiomer.[6] Implicit in these observations is also the striking absence of the isomerization of the (Z)-(8) into the thermodynamically favored (E)-isomer in the presence of the presumed Co(II)-hydride intermediates. Since (E)-8 appears to readily engage the [LCo–H]+ in the facile hydrovinylation, in the absence of ethylene, under proper conditions, kinetic control might prevail in the (E)–(Z)-isomerization of 8, and if so, using ligand effects could one make the (Z)-isomer?[5] These expectations have indeed been borne out, and here we report the details of these investigations. While there are excellent methods for the formation of the (E)-1,3-diene isomer,[8] there is a dearth of methods for the synthesis of isomerically pure (Z)-1,3-dienes,[9] and in many biologically important compounds this configuration is important.[10]

Table 1

Chemoselectivity in the Asymmetric HV of (Z/E)-8a

entry	time (h)	% (9)	ee %b (9)	% (8)	Z:Eb (8)	10
(S,S)-DIOPc
1	0.5	16	80	84	2.1:1.0	–
2	1.2	23	81	77	2.7:1.0	–
3	3	26	83	71	3.4:1.0	3
4	6	31	82	65	4.4:1.0	4
5	23	49	84	46	49:1	5
(S,S)-BDPPd
6	1	21	85	79	1.9:1.0	<1
7	2	30	85	70	2.6:1.0	<1
8	5.2	42	82	49	7.1:1.0	9
9	8	47	77	43	13.3:1.0	10
10	23	61	73	25	49:1	13

See eq 1 for procedure.

Determined by GC. See Supporting Information for chromatograms.

At −45 °C. (S)-9 major.

At −15 °C. (R)-9 major.

Before we describe our results, attention should be drawn to a related paper by Hilt et al., who documented a distinctly different protocol for the E/Z isomerization of 1,3-dienes.[5d] In this report (Scheme 3), a Co(I) complex prepared under reducing conditions from a tridentate pyridine-imine ligand (11), CoBr2, Zn, and ZnI2 gave moderate yields of (Z)-1,3-dienes (14) from a 1:1 mixture of (Z)- and (E)-dienes (13). Behavior of the 1,3-diene was found to be highly dependent on the ligand, with a bis-phosphine-CoBr2 under identical conditions (i.e., with Zn/ZnI2) giving modest yields of a product (15), arising via a 1,5-hydrogen migration. The difference between Hilt’s [bis-phosphine]Co-chemistry and the [bis-phosphine]CoCl2/Me3Al-mediated reactions described in this paper is highlighted by the total absence of the 1,5-hydrogen migration in the latter and the higher yields of Z-isomerization products obtained.

Scheme 3

Co(I)-Catalyzed Isomerization Reactions

The initial scouting experiments were carried out on a mixture of (Z)- and (E)-8 using CoCl2 complexes of chelating bis-diphenylphosphinoalkane ligands under conditions similar to the HV except for the presence of ethylene (eq 2). The composition of isomers and identification of the product(s) were determined by 1H and 13C NMR spectroscopy and gas chromatography.[6] The results are presented in Table 2.

Table 2

Isomerization of 1,3-Diene (Z/E)-8: Ligand Effectsa

entry	start mat (Z):(E)-8	ligand	bite angle	temp (°C)/time (h)	product (Z):(E)- 8
1	33:67	[DPPM]	72	–15/14	37:63
2	33:67	[DPPE]	85	–10/14	74:26
3	33:67	[DPPP]	91	–16/22	82:18
4	33:67	[DPPP]b	91	–4/16	33:67
5	33:67	[DPPB]	98	–15/14	>99:<1
6	33:67	[DPPpent]c	–	–15/14	>99:<1
7	33:67	(S,S)-DIOP	98	–10/12	100:0d

See eq 2 for procedure.

Using CoBr2.

Bis-1,5-diphenyphos-phinopentane.

Isomeric product not seen in GC or NMR.

As shown in Table 2, the Z/E composition of the products is highly dependent on the ligand. A Co-complex containing ligand with a small bite angle,[11] 1,1-bis-diphenylphosphinomethane (DPPM, bite angle β = 72), showed little tendency to effect the isomerization (entry 1), whereas complexes of large bite angle ligands, 1,4-bis-diphenylphosphinobutane (entry 5, β = 98), 1,5-bis-diphenylphosphinopentane (entry 6), and DIOP (entry 7, β = 98), gave quantitative conversion to the (Z)-isomer at low temperature. Bis-diphenylphosphinopropane (DPPP, bite angle 91) gave up to 82% of the (Z)-isomer at −15 °C (entry 3). The reaction is specific for the chloride complex; as shown in the entry 4, the corresponding bromide complex is ineffective for the isomerization reaction.

We have examined the isomerizations of the mixtures of a number of 1,3-dienes under the optimized conditions (eq 2), and the most significant results are listed in Table 3. An expanded list of complexes and their effect on the isomerization of each of the dienes is included in the Supporting Information. As can be seen in entries 1–9, the isomerization reaction is broadly applicable giving excellent yields of the (Z)-isomers of most dienes. As inferred from the scouting studies (Table 2), ligands with relatively large bite angles, DPPB and DPPPent, were found to be the most generally applicable for this reaction. For substrates where the diene is conjugated to an aromatic moiety (entries 5, 6, and 7), DPPPent is the ligand of choice, giving excellent conversion to the expected (Z)-isomer. DPPB leads to slightly lower selectivities. The isomerization reaction gives satisfactory results even in substrates that contain Lewis basic centers (entries 6 and 7). A preparative scale experiment (3 mmol) using 16 as the starting material gave 91% isolated yield of the expected product.[6]

Table 3

Co(II)-Catalyzed Isomerization of (Z)- and (E)-1,3-Dienesa

See eq 2 for procedure. For an expanded list of ligand effects for each substrate, see Supporting Information.

Yield of volatile products were estimated from NMR and GC.

Product contains an unidentified impurity from the starting material.

The isomerization reaction appears to be limited to terminal 1,3-dienes as illustrated by the examples shown in eqs 3 and 4. Substrates 24 and 25 failed to undergo the reaction under a variety of conditions using the ligands discussed previously. The starting material was recovered virtually unchanged.

The experiments listed in previous sections suggest that one reason for the poor reactivity/selectivity of the Z-substrates might be their reluctance to form an η4- complex.[12] A plausible explanation for the observed results, based on the assumption that the initial [LCo–H]+ addition to the 1,3-diene is reversible, is shown in Scheme 4. An intramolecular hydride delivery via an η4-complex 4E gives the syn-anti-Co(allyl)-complex 5as.[13] This species could undergo the familiar π–σ–π isomerization[14] to give, among others, an anti–anti complex (5aa). Hydride elimination from this species would generate a diene complex 4Z, which for steric reasons, might dissociate to give the (Z)-diene. The (Z)-diene, once formed, will most likely exist in the (s)-trans conformation, precluding any further η4-complexation with the Co(II)-catalyst.[15]

Scheme 4

Plausible Mechanism for the E- to Z-Isomerization of a 1,3-Diene

In summary, attempts to effect asymmetric hydrovinylation of a mixture of (Z)- and (E)-1,3-dienes using (P ∼ P)CoCl2/Me3Al reveal that there is a significant difference in the relative rates of ethylene incorporation, with the (E)-isomer reacting significantly faster. In the absence of ethylene, under otherwise identical conditions, this Co-catalyst promotes an unusual isomerization of an (E)/(Z)-mixture of 1,3-dienes almost exclusively to the (Z)-isomer. This result is strikingly different from the related reaction mediated by the reagent combination [(P ∼ P)CoBr2/Zn/ZnI2), where a product of 1,5-hydrogen shift is the major.[5d,16] A mechanism that involves an intramolecular hydride addition via an η4-complex and subsequent π–σ–π isomerization of the intermediate Co(allyl) species is proposed for this reaction.