Transfer hydrogenation process and catalyst

Details Transfer hydrogenation process and catalyst Transfer hydrogenation process and catalyst

2002-02-27

2003-04-08

Vollano, Jean F. (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Oxygen containing

C568S061000, C568S715000, C564S463000

Reexamination Certificate

active

06545188

ABSTRACT:

This invention relates to catalytic transfer hydrogenation, particularly in the presence of a complexed transition metal, to a catalyst for such hydrogenation and to a process of making optically active compounds.
Hydrogenation by hydrogen transfer using catalysts containing phosphorus- or nitrogen-ligands was reviewed at length by Zassinovich et al. in Chem. Rev., 1992, 92, 1051-1069. These authors concluded ‘in spite of excellent achievements, much work remains to be done’.
Transfer hydrogenation using catalysts in which the transition metal is coordinated to a benzenoid hydrocarbon have been explored. The following publications are of interest:
(1) Noyori et al., J. A. C. S., 1995, 117, 7562-7563: which discloses that use of chloro-ruthenium-mesitylene-N-monotosyl-1,2-diphenylethylenediamine as catalyst in the transfer hydrogenation of acetophenone to 1-phenylethanol by propan-2-ol gave up to a 95% yield of product having 97% enantiomeric excess. Similar results were obtained starting from other alkylaryl ketones. The efficiency of corresponding catalyst containing benzene in place of mesitylene was more sensitive to substituents on the aryl group of the starting ketone. Reaction times were generally rather long, typically 15 h; at longer reaction times stereoselectivity decreased, apparently owing to reverse hydrogenation. No turnover numbers are reported. The authors commented that ‘the overall catalytic performance is unable to rival that of the current best hydrogenation method’ as described in an earlier publication by themselves.
(2) Noyori et al., J. Chem. Soc. Chem. Commun., 1996, 233-234: which discloses that catalysts similar to those of Noyori et al. (1) above but containing other alkylbenzene ligands and various beta-amino alcohols in place of the diphenylethylenediamine were to differing extents effective in the hydrogenation of acetophenone. The beta-amino alcohol ligand gave greater catalyst stability. The preferred arene ligand was hexamethylbenzene. Turnover numbers were up to 227 moles of product per mole of catalyst per hour.
(3) Noyori et al., J. A. C. S., 1996, 118, 2521-2522: which discloses that to prevent reverse hydrogen transfer in the process of Noyori et al. (1) above, formic acid-triethylamine was used as hydrogen source. Reaction times mainly over 14 h and up to 90 h were used; no turnover numbers are reported.
(4) Noyori et al., J. A. C. S., 1996, 118, 4916-4917: which discloses that the process of Noyori et al. (3) above is effective for reduction of imines (especially cyclic imines) to enantioselected amines.
These processes appear to require relatively long cycle times. As well as involving uneconomic utilisation of chemical plant, such slow reaction can lead to decomposition of the catalytic complex and slow loss of product optical purity; also it affords limited scope for adjusting reaction conditions such as temperature and reactant concentration to maximise the difference in rate between enantiomerically wanted and unwanted reactions.
Besides the phosphorus-, nitrogen- and benzene-ligated transition metals, complexes based on pentamethylcyclopentadienyl (hereinafter Cp*) have been shown to be effective as catalysts in homogeneous hydrogenation of olefins by free hydrogen (Maitlis, Acc. Chem. Res., 1978, 11, 301-307; Maitlis et al., J. Chem. Soc. Dalton, 1978, 617-626); there was no disclosure of hydrogen transfer in the absence of free hydrogen or of catalysts containing a chelating or chiral-directing ligand.
Complexes of iridium with Cp* and acylated or sulphonylated alpha amino carboxylic acid have been described by Grotjahn et al. (J. A. C. S., 1994, 116, 6969-6970) but without evidence of catalytic activity. Complexes of rhodium with Cp* and 2,2,-bipyridyls and the use of these with formate to hydrogenate nicotinamide adenine dinucleotide (NAD) to NADH have been described by Steckhan et al. (Angew. Chem. Int. Ed. Engl., 1990, 29(4), 388-390). Turnover frequencies up to 67.5 per h are reported, but no activity after 100 catalytic cycles.
We have now found that stereoselective transfer hydrogenation can be efficiently carried out by means of a catalyst comprising a complex of a transition metal, a chelating ligand and a cyclopentadienyl group.
According to a first aspect of the present invention there is provided a process for the transfer hydrogenation of a compound of formula (1) to produce a compound of formula (2)
wherein:
X represents CR
3
R
4
, NR
5
, (NR
5
R
6
)
+
Q
−
, O or S;
R
1
, R
2
, R
3
, R
4
, R
5
and R
6
each independently represents a hydrogen atom, an optionally substituted hydrocarbyl, a perhalogenated hydrocarbyl or an optionally substituted heterocyclyl group, one or more of R
1
& R
2
, R
1
& R
3
, R
2
& R
4
, R
3
& R
4
, R
1
& R
5
, R
2
& R
6
and R
5
& R
6
optionally being linked in such a way as to form an optionally substituted ring(s); and
Q
−
represents an anion;
said process comprising reacting the compound of formula (1) with a hydrogen donor in the presence of a catalyst, characterised in that the catalyst has the general formula:
wherein:
R
7
represents an optionally substituted cyclopentadienyl group;
A represents —NR
8
—, —NR
9
—, —NHR
8
or —NR
8
R
9
where R
8
is H, C(O)R
10
, SO
2
R
10
, C(O)NR
10
R
14
, C(S)NR
10
R
14
, C(═NR
14
)SR
15
or C(═NR
14
)OR
15
, R
9
and R
10
each independently represents an optionally substituted hydrocarbyl, perhalogenated hydrocarbyl or an optionally substituted heterocyclyl group, and R
14
and R
15
are each independently hydrogen or a group as defined for R
10
;
B represents —O—, —OH, OR
11
, —S—, —SH, SR
11
, —NR
11
—, —NR
12
—, —NHR
12
or —NR
11
R
12
where R
12
is H, C(O)R
13
, SO
2
R
13
, C(O)NR
13
R
16
, C(S)NR
13
R
16
, C(═NR
16
)SR
17
or C(═NR
16
)OR
17
, R
11
and R
13
each independently represents an optionally substituted hydrocarbyl, perhalogenated hydrocarbyl or an optionally substituted heterocyclyl group, and R
16
and R
17
are each independently hydrogen or a group as defined for R
13
;
E represents a linking group;
M represents a metal capable of catalysing transfer hydrogenation; and
Y represents an anionic group, a basic ligand or a vacant site;
provided that when Y is not a vacant site that at least one of A or B carries a hydrogen atom.
The catalytic species is believed to be substantially as represented in the above formula. It may be introduced on a solid support.
Hydrocarbyl groups which may be represented by R
1-6
, R
9
, R
10
, R
11
and R
13-17
independently include alkyl, alkenyl and aryl groups, and any combination thereof, such as aralkyl and alkaryl, for example benzyl groups.
Alkyl groups which may be represented by R
1-6
, R
9
, R
10
, R
11
and R
13-17
include linear and branched alkyl groups comprising up to 20 carbon atoms, particularly from 1 to 7 carbon atoms and preferably from 1 to 5 carbon atoms. When the alkyl groups are branched, the groups often comprising up to 10 branch chain carbon atoms, preferably up to 4 branch chain atoms. In certain embodiments, the alkyl group may be cyclic, commonly comprising from 3 to 10 carbon atoms in the largest ring and optionally featuring one or more bridging rings. Examples of alkyl groups which may be represented by R
1-6
, R
9
, R
10
, R
11
and R
13-17
include methyl, ethyl, propyl, 2-propyl, butyl, 2-butyl, t-butyl and cyclohexyl groups.
Alkenyl groups which may be represented by R
1-6
, R
9
, R
10
, R
11
and R
13-17
include C
2-20
, and preferably C
2-6
alkenyl groups. One or more carbon—carbon double bonds may be present. The alkenyl group may carry one or more substituents, particularly phenyl substituents. Examples of alkenyl groups include vinyl, styryl and indenyl groups. When either of R
1
or R
2
represents an alkenyl group, a carbon—carbon double bond is preferably located at the position &bgr; to the C═X moiety. When either of R
1
or R
2
represents an alkenyl group, the compound of formula (1) is preferably an &agr;, &bgr;-unsaturated ketone.
Aryl groups which may be represented by R
1-6
, R
9
, R
10

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