Novel diazaphospholidine terminated polyhedral

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Table 1 Hydroformylation of alkenes with catalysis by rhodium complexa. Entry ...... lowed by 10 drops of Karstedt catalyst (Platinum(0)-1,3-divinyl-. 1,1,3 ...
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Novel diazaphospholidine terminated polyhedral oligomeric silsesquioxanes in styrene and vinyl acetate hydroformylation: Synthesis and molecular dynamics studies Nicolas R. Vautravers,a Pascal Andr´e*b and David J. Cole-Hamilton*a Received 13th November 2008, Accepted 19th January 2009 First published as an Advance Article on the web 27th March 2009 DOI: 10.1039/b820349k New diazaphospholidine POSS macromolecules have been synthesised and tested in styrene and vinyl acetate hydroformylation. Whilst some of them have shown activity, others precipitated upon mixing with the rhodium precursor preventing its efficient use. Molecular dynamics has been used to help understand these observations. Rigid and compact dendritic structures with phosphine groups engineered to have low mobility but high probability of sitting at distances favouring bidentate coordination with the rhodium precursors are necessary for the macromolecular ligands to be active. More flexible structures having lower probability of phosphine separations suitable for bidentate complex formation are more prone to form oligomeric dendritic species and hence to crosslink the macromolecules and precipitate.

Introduction Molecules having several similar functional groups at their periphery are of considerable interest, not only because their size may be such that they can be separated from smaller molecules by ultrafiltration, but also because interactions may occur between the groups at the periphery leading to effects that are not present in small molecule analogues. We have been studying polyhedral oligomeric silsesquioxane (POSS) based macromolecules, including dendrimers with phosphine ligands attached to their periphery and discovered that a dendrimer with 16 terminal diphenylphosphines gave much higher selectivity to the desired linear product leading to a linear to branched ratio (l:b) of 13.9 in oct-1-ene hydroformylation reactions than any of its small molecule analogues typically providing linear to branched ratios between 2 and 5.1,2 This was one of the very first positive dendritic effects to be discovered and was very specific for a dendrimer in which the diphenylphosphine groups were separated by a –(CH2 )2 Si(CH2 )2 – spacer. Shorter or longer chains, or replacing a carbon atom in the spacer by oxygen did not give enhanced regioselectivity. That discovery occurred by chance and we used molecular dynamics calculations to show that the increased selectivity probably arose because the phosphines were constrained by the particular macromolecular geometry to bind the rhodium atom in two equatorial sites of a trigonal bipyramidal intermediate,3 a geometry that is known to give high linear selectivity in hydroformylation reactions.4 Numerical simulations at various levels of theory can be used to explain electronic properties or to gain insight into molecular conformations.5–7 In the first case for instance, electronic properties a EaStCHEM, School of Chemistry, University of St. Andrews, St. Andrews, Fife, UK KY16 9ST. E-mail: [email protected]; Fax: +44-1334-463808; Tel: +44-1334-463805 b Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews, Fife, Scotland, UK KY16 9SS. E-mail: [email protected]

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of single POSS core derivatives have been calculated and compared with experimental results, providing increased insight into the properties of single hybrid organic–inorganic nanomaterials.8–11 The calculations include numerical simulations of hydrodynamic radii, extension and back-folding, crowding and arm flexibility as a function of dendrimer generation and solvent quality;3,12–15 parameters which are of special interest to predict and understand catalytic activities. Since it appears that steric crowding at the periphery of a dendrimer can be used to force bidentate coordination of ligands which would be unidentate when in their small molecule forms,1 we attempted to discover systems where such effects could be engineered by design. ESPHOS is a bidentate bis(diazaphospholidine) ligand which gives excellent activity, regio and stereoselectivity in the asymmetric hydroformylation of vinyl acetate. SEMIESPHOS, which is a unidentate ligand corresponding to half of ESPHOS is almost inactive, see Scheme 116,17

Scheme 1 Asymmetric hydroformylation of vinyl acetate catalysed by rhodium complexes of ESPHOS. SEMI-ESPHOS gives very poor results (yield < 15%, branched:linear (b:l) 70:30, enantiomeric excess (ee) < 2%)17 . C 2004 Elsevier B.V. Adapted from ref. 16 with permission. 

Dalton Trans., 2009, 3413–3424 | 3413

In this paper, we describe attempts to make SEMI-ESPHOS bound at the surface of macromolecules with polyhedral oligomeric silsesquioxane (POSS) cores behave like ESPHOS. Molecular dynamics studies have been used to help rationalise the behaviour observed. A preliminary account of some of these results has appeared.18

Results and discussion Synthesis of SEMI-ESPHOS decorated POSS cored macromolecules Fig. 1 illustrates the six ESPHOS decorated POSS cored macromolecules prepared for this work. The red arrows labelled “S” highlight an increase in the steric crowding at the periphery of

the dendrimer via an increase in the number of functional groups linked to one single silicon atom upon moving from G1–8SEMIESPHOS to G1-16SEMI-ESPHOS and G1-24SEMI-ESPHOS. The blue arrow labelled “E” shows a reduction of steric crowding and enhancement of the arm flexibility between G1-24SEMIESPHOS and G1-24SEMI-ESPHOS(ext) but operating via the arm extension. It should be noted that the arm extension leads to two silicon atoms being present on each arm in contrast to one and three silicon atoms per arm for G1 and G2, respectively. The purple arrows labelled “G” represent the relation between generation 1 and generation 2 POSS, G1-8SEMI-ESPHOS vs G2-16SEMIESPHOS and G1-16SEMI-ESPHOS vs G2-32SEMI-ESPHOS, where the number of silicon atoms per arm is now equal to three. The formation of a generation 2 dendrimer with a POSS core surrounded by 48 peripheral SEMI-ESPHOS was not attempted

Fig. 1 SEMI-ESPHOS modified POSS cored dendrimers. The left hand column illustrates the 1st generation dendrimers, with the arrows oriented from top-to-bottom and labelled “S” highlighting the increase of steric hindrance. The right hand column displays the 2nd generation dendrimers (top and middle macromolecules) and the extended G1 dendrimer (bottom), with the left-to-right arrows highlighting the rational evolution between the sets of macromolecules, “G” from G1 to G2 and “E” from G1 to G1 extended. Colours of the arrows are guides for the eyes.

3414 | Dalton Trans., 2009, 3413–3424

This journal is © The Royal Society of Chemistry 2009

as it was expected that steric hindrance at the periphery of the cube would have led to incomplete functionalisation.19 The four first generation SEMI-ESPHOS decorated POSS cored macromolecules G1-8SEMI-ESPHOS, G1-16SEMI-ESPHOS, G1-24SEMI-ESPHOS18 and G1-24SEMI-ESPHOS(ext) and two second generation dendrimers, G2-16SEMI-ESPHOS and G2-32SEMI-ESPHOS, see Fig. 1, have been synthesized by the sequence of reactions depicted in Scheme 2. Lithiation of 3-bis(dimethylamino)-phosphinobromobenzene (2, Scheme 2), prepared by a literature method,20 followed by reaction with chlorosilane terminated POSS (1, Scheme 2)21 led to bis(dimethyl-amino)phosphine terminated dendrimers (3, Scheme 2). The synthesis was completed by amine exchange with the chiral (S)-2-(phenylaminomethyl)pyrrolidine (4, Scheme 2)22 to form the desired SEMI-ESPHOS terminated cube (5, Scheme 2) driven by the thermodynamic displacement of gaseous dimethylamine.

The first generation dendrimer G1-24SEMI-ESPHOS(ext) represents an extended version of G1-24SEMI-ESPHOS in which an extra –Si(CH2 )2 – unit has been placed in the dendron arm. While already known chlorosilane terminated POSS21 were employed in the synthesis of G1-8SEMI-ESPHOS, G1-16SEMIESPHOS and G1-24SEMI-ESPHOS, new precursors—G1ethyl-24Cl(ext), G2-ethyl-16Cl and G2-ethyl-32Cl—were prepared following the literature method developed by Morris and coworkers21 to produce G1-24SEMI-ESPHOS(ext), G2-16SEMIESPHOS and G2-32SEMI-ESPHOS, see Fig. 2. Alkenylation of chlorosilane-terminated dendrimers with homemade vinylmagnesium bromide and hydrosilylation of vinyl terminated POSS led to the formation of 3 new chlorosilane terminated POSS molecules, which have all been characterized by multinucleus nuclear magnetic resonance (13 C, 1 H and 29 Si) spectroscopy and microanalysis.

Fig. 2 Representation of new chlorosilane terminated POSS G1-ethyl24Cl(ext), G2-ethyl-16Cl and G2-ethyl-32Cl.

Scheme 2 Synthetic strategy for the preparation of SEMI-ESPHOS decorated POSS core macromolecules.

This journal is © The Royal Society of Chemistry 2009

PNMe2 terminated compounds (3, Scheme 2) were only worked up with basic water to destroy any traces of remaining n-butyllithium and used in the next step without any further purification. SEMI-ESPHOS terminated POSS (5, Scheme 2) were precipitated in hexane to wash away the excess of small molecules (3-bis(dimethylamino)-phosphinobromobenzene) (2, Scheme 2) and (S)-2-(phenylamino-methyl)pyrrolidine) (4, Scheme 2) until no phosphorus signal attributed to these small molecules could be detected by 31 P NMR spectroscopy. Routine multinuclear NMR Dalton Trans., 2009, 3413–3424 | 3415

spectroscopic techniques (31 P and 29 Si) were used to assess the completeness of the transformations. 29 Si NMR spectroscopy was also very useful in monitoring this synthesis as 3 different signals—Si–O on the vertex of the cube (Si(T)), Si–Cl and Si–Phenyl—are present in the final product. The purity of the final product was shown by the presence of a single 29 Si NMR signal at -67 ppm. This resonance shows that the cube remains intact and that each corner has been substituted by the same group showing the presence of only one cube. Moreover, the nucleophilic substitution of the chlorine atoms on chlorosilane terminated cubes (1, Scheme 2) by the lithium aryl entity, was confirmed by the observation in the 29 Si NMR spectrum of downfield signals characteristic of the Si–Phenyl bond at -1.3 ppm, -1.6 ppm, -6.0 ppm, -6.3 ppm, -9.4 ppm and -10.1 ppm (respectively for G1-8SEMI-ESPHOS, G2-16SEMI-ESPHOS, G2-32SEMIESPHOS, G1-16SEMI-ESPHOS, G1-24SEMI-ESPHOS(ext) and G1-24SEMI-ESPHOS). In addition microanalysis was also carried out and testified to the high degree of purity of these compounds.

SEMI-ESPHOS decorated POSS cored macromolecules in vinyl acetate and styrene hydroformylation Summarised in Table 1, all the dendritic SEMI-ESPHOS species were then investigated in rhodium catalysed vinyl acetate hydroformylation. Catalytic solutions were prepared in toluene by mixing the dendritic ligand with [Rh(acac)(CO)2 ] (acac = acetylacetonate) at the desired rhodium to phosphine ratio, before being transferred to a batch Hastelloy autoclave via canula. The conditions and results are summarized in Table 1 (entries 1–22). Entries 1 and 2 show the low activity of monomeric SEMIESPHOS at different phosphine to metal ratio leading to about 11.5% conversion, a low branched to linear ratio of about 0.5 and no enantiomeric excess (ee). In contrast entry 3 emphasizes the very good behavior of ESPHOS in this reaction as total conversion is reached after 3 hours.16 Entry 4 shows an interesting fact namely the complete absence of activity of ESPHOS when a 6 to 1 phosphine to rhodium ratio is employed. This can be rationalized by the formation of an inactive complex containing

Table 1 Hydroformylation of alkenes with catalysis by rhodium complexa

Entry 1 2

Substrate Vinyl Acetatee

3 4 5 6 7 8 9 10 11 12 13

Aldehyde ee [%]

Alcohol yield [%]d

Alcohol ee [%]

P:Rh

Conc. [Mol/L]

SEMI-ESPHOS

3.0 6.0

0.010 0.010

10.2 12.9

2.5 2.3

0.3 0.4

1.5 f , g 6.0 23 3.0 6.0 6.0 3.0 6.0 6.0 6.0 f 3.0 6.0

0.010 0.010 0.010 0.010 0.001 0.010 0.010 0.005 0.010 0.010 0.005

100.0 -ppt -ppt

34.9 — —

15.9 — —

76.0 (S) — —

58.8 — —

84.0 (S) — —

-ppt











6.7 42.0 48.4 9.2

1.5 11.6 15.5 2.1

ESPHOS G1–8SEMI-ESPHOS

G1–16SEMI-ESPHOS

G1–24SEMI-ESPHOS

Conv. [%]

11.0 60.2 51.5 13.6

Aldehyde yield [%]b

Aldehyde b:lc

Ligand

0.0

0.0 0.0 0.0 0.0

0.0

0.0 5.9 0.0 0.0

0.0

0.0 0.0 0.0 0.0

14 15 16

3.0 6.0 6.0

0.010 0.010 0.001

-ppt











G1–24SEMI-ESPHOS(ext)

17 18 19

3.0 6.0 6.0

0.010 0.010 0.001

-ppt











G2–16SEMI-ESPHOS

3.0 6.0 6.0 2i 6i 1,5 6

0.010 0.010 0.001 0.010

-ppt











91.3 91.0 99.6 -ppt

85.0 86.0 98.9 —

1.5 1.6 3.5 —

0.0 0.0 0.0 —

1.1 0.9 0.0 —

0.0 0.0 0.0 —

6 6i 6

0.005 0.005 0.010

17.8 70.5 72.2

17.8 69.0 70.9

2.4 2.8 1.2

0.0 3.0 3.7

0.0 0.0 1.4

0.0 0.0 0.0

20 21 22 23 24 25 26 27 28 29

G2–32SEMI-ESPHOS Styreneh

SEMI-ESPHOS ESPHOS G1–16SEMI-ESPHOS G1–24SEMI-ESPHOS

0.010

Catalyst prepared in situ from [Rh(acac)(CO)2 ] and the phosphine in toluene (4 cm3 ) containing substrate (1 cm3 ); “P:Rh” is the phosphorus to rhodium ratio, “Conc.” is the molar concentration, “Conv.” stands for conversion, “b:l” is the branched to linear ratio, “ee” is the enantiomeric excess, “-ppt” indicates that the compound precipitated out of solution. Entries 1 to 22 and 23 to 29 refer to vinyl acetate and styrene, respectively. b For vinyl acetate reactions, the yield refers to 2-acetoxypropanal (1-acetoxypropanal decomposes under the reaction conditions to acetic acid and propenal) and for styrene reactions, the yield refers to 2-phenylpropanal + 3-phenylpropanal. c Refers to aldehyde formation before aldehyde decomposition/hydrogenation–for vinyl acetate reactions, determined from the ratio of branched-chain product (aldehyde + alcohol) to acetic acid and for styrene reactions, determined from the ratio of branched chain product (aldehyde + alcohol) to straight chain product (aldehyde + alcohol). d For vinyl acetate reactions refers to 2-acetoxy-1-propanol + 1-acetoxy-2-propanol and for styrene reactions refers to 2-phenyl-1-propanol plus 3-phenyl-1-propanol. e Conditions: 80 ◦ C, 40 bar CO/H2 (1/1), 20 hours. f Acetoxyacetone (0.3–1.3%) is also a product. g Reaction time = 3 hours. h Conditions: 80 ◦ C, 10 bar CO/H2 (1/1), 2 hours. i Acetophenone (