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Reports: ND352709-ND3: New Homoleptic Rare-Earth Metal Complexes for Catalytic Hydrophosphination

Joseph A. R. Schmidt, PhD, University of Toledo

Progress Report

Summary. The goal of this research project is the development of new lanthanide complexes with exceptional activity for the catalytic hydrophosphination of a wide range of unsaturated substrates. We have shown that homoleptic trialkyl rare-earth metal complexes are effective as precatalysts for the hydrophosphination of several heterocumulenes under mild reaction conditions. These lanthanide species, supported by alpha-metalated dimethylbenzylamine ligands, are relatively simple to synthesize and isolate, making them ideal for use as hydrophosphination precatalysts. During the current reporting period, we have developed optimized reaction conditions for the hydrophosphination of carbodiimides, isocyanates, and isothiocyanates. Some of the resulting phospha-ureas were also utilized as new ancillary ligands for tantalum-catalyzed hydroaminoalkylation reactions in tandem with our collaborators at the University of British Columbia. To date, this research project has resulted in the development of highly active catalysts for the hydrophosphination of a wide range of cumulenes, while ongoing investigations include the hydrophosphination of all-carbon unsaturated frameworks and other heteroatom containing species, especially those with triply-bonded moieties.

Hydrophosphination. The homoleptic alpha-metalated N,N-dimethylbenzylamine, (DMBA)3La, complex was synthesized following a simple procedure involving salt metathesis of lanthanum chloride with alpha-potassiated dimethylbenzylamine at -50 °C in THF (Scheme 1). The product precatalyst, alpha-La(DMBA)3, was isolated by recrystallization from THF/pentane at -25 °C in excellent yield.

Scheme 1. Synthesis of alpha-La(DMBA)3.

With this lanthanum complex in hand, hydrophosphination of a wide range of heterocumulenes with diphenylphosphine was undertaken (Table 1). Additionally, a couple of other commercially available phosphines were investigated. Hydrophosphination of unhindered carbodiimides was very efficient and the resulting phosphaguanidines were isolated in excellent yields. In contrast, attempted hydrophosphination of N,N’-di-tert-butylcarbodiimide with diphenylphosphine did not result in production of the desired phosphaguanidine, but rather only starting materials were observed. This is likely because this substrate was unable to react at the metal center due to the larger steric bulk of the tert-butyl groups. The hydrophosphination reaction also worked well with isocyanates and isothiocyanates and it was tolerant of both electron withdrawing and electron donating nitrogen substituents on these heterocumulenes. In fact, for most of the isocyanates, NMR spectroscopic observation of the crude reaction products indicated virtually quantitative conversion to the desired phospha-ureas. The reduced overall yield is generally reflective of losses upon isolation and purification. A decrease in reaction yield with 1-adamantyl isocyanate was attributed to the larger steric bulk of the adamantyl group, hindering insertion into the La-phosphide bond and consequently reducing conversion to the product. Hydrophosphination of the slightly larger tert-butylisocyanate was even worse, with the desired phospha-urea compound afforded in very low yield (<20%), again demonstrating the deleterious effect of steric hindrance on this reaction. Furthermore, scale-up of the hydrophosphination reaction with lower catalyst loading (1 mol%) was successful, giving gram-scale isolated yields of products, but requiring longer reaction times (7 days). Small amounts of (4-MeOC6H4)2PH and di-p-tolylphosphine were obtained from commercial sources, and in both cases, the related phospha-ureas were produced and isolated in very similar yields to those obtained with the unsubstituted diphenylphosphine, indicating little or no sensitivity to electronic effects at this position.

Table 1. Catalytic addition of phosphines to heterocumulenes.a

R

R’-N=C=X

Yieldb [%]

Ph

iPrN=C=NiPr

93

Ph

CyN=C=NCy

74

Ph

PhN=C=O

60

Ph

CyN=C=O

54

Ph

AdN=C=O

38c

Ph

1-NaphN=C=O

76c

Ph

PhN=C=S

91

Ph

(4-FC6H4)N=C=O

83d

Ph

(4-ClC6H4)N=C=O

83

Ph

(4-BrC6H4)N=C=O

49

Ph

(4-MeOC6H4)N=C=O

62

Ph

(4-F3CC6H4)N=C=O

88

4-MeOC6H4

(4-BrC6H4)N=C=O

45

4-MeC6H4

(4-F3CC6H4)N=C=O

65
a Conditions: phosphine (1.15 mmol), heterocumulene (1.00 mmol), catalyst (0.05 mmol), THF (3 mL). b Isolated yield. c Reaction stirred at 55°C. d Product isolated as a 3:1 ratio of phospha-urea and trimerized isocyanate.

In addition to the study of catalytic hydrophosphination, a series of stoichiometric reactions involving the La(DMBA)3 complexes was also undertaken. The triple insertion reaction of N,N’-diisopropylcarbodiimide directly led to a homoleptic amidinate complex (Scheme 2). Furthermore, when alpha-La(DMBA)3 was treated with three equivalents of diphenylphosphine in THF (15 mL), followed by three equivalents of N,N’-diisopropylcarbodiimide (Scheme 2), a homoleptic complex in which three phosphaguanidinate ligands were bound to the metal center through their chelating nitrogen atoms was formed.

Scheme 2. Stoichiometric formation of amidinate and guanidinate complexes.

Hydroaminoalkylation. The Schafer group at the University of British Columbia has been investigating catalytic hydroaminoalkylation using amidate-supported tantalum complexes for several years. As a means of furthering their study, we supplied them with samples of our phospha-urea products, generated through the catalytic hydrophosphination described above. These were then utilized as precursors for the formation of tantalum complexes of the form LTa(NMe2)­4 (Scheme 3). Ultimately, it was found that the new phospha-urea supported tantalum complexes produced only poor to moderate activity catalysts for hydroaminoalkylation of 1-octene, with more active catalysts resulting from bulky aryl-substituted amidates.

Scheme 3. Hydroaminoalkylation of 1-octene using phospha-urea supported tantalum complexes.

Human Resources Development. Many researchers have played a role in the chemistry accomplished through this PRF grant funding, including those research group members working on unrelated projects, as each of them has been involved in intellectual discussions, editing of manuscripts and posters, and refinement of presentations. The researchers most directly involved in this project include three graduate students (Andrew Behrle, PhD 2012, now a post-doctoral associate at the University of Missouri and Miriam Basiouny and Sreejit Menon, both PhDs in progress).

Conclusions. The research supported by the PRF New Directions grant has led to a new field of chemistry in the Schmidt group. With hydrophosphination catalysis using lanthanide complexes, we have expanded beyond the palladium-based hydroamination chemistry that was previously the primary specialty of our research group. This midterm report summarizes our early successes in this field. Recent results have shown that these catalysts are very effective at hydrophosphination of unsaturated substrates with our current experiments displaying catalytic activity to form several unprecedented reaction products. These results will be published within the next year and summarized during the following reporting period. Ultimately, these new reaction manifolds will form the foundation of future submissions for major federal grant funding.