Light-Driven, Zirconium-Catalyzed Hydrophosphination with Primary

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Light-Driven, Zirconium-Catalyzed Hydrophosphination with Primary Phosphines Christine A Bange, Matthew A Conger, Bryan Novas, Elizabeth R Young, Matthew Denis Liptak, and Rory Waterman ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01002 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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ACS Catalysis

LightLight-Driven, ZirconiumZirconium-Catalyzed Hydrophosphination with Primary Phosphines Christine A. Bange,† Matthew A. Conger,† Bryan T. Novas, † Elizabeth R. Young, ‡ Matthew D. Liptak,† and Rory Waterman*† †Department ‡Department

of Chemistry, University of Vermont, Discovery Hall, Burlington, Vermont, 05401, United States of Chemistry, Lehigh University, Seeley G. Mudd, Bethlehem, Pennsylvania, 18015, United States

Keywords: Hydrophosphination, primary phosphine, zirconium, photocatalysis, charge transfer ABSTRACT: Catalytic hydrophosphination using [κ5-N,N,N,N,C-(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH2]Zr (1) under photolysis substantially enhances activity and avails greater substrate scope. Quantitative conversions of alkenes to secondary phosphines are reached in as little as twenty minutes at ambient temperature with 1 under ultraviolet or visible irradiation. A larger class of unactivated alkenes are now facile substrates under photolysis conditions, and 1 can engage in a previously unknown tandem inter/intramolecular hydrophosphination of 1,4-pentadiene to give the heterocyclic phosphorinane product. Computational and spectroscopic data indicate that photoexcitation of 1 at a variety of wavelengths results in P n → Zr d charge transfer. This excitation appears to accelerate catalysis by promoting substrate insertion at the Zr–P bond based on experimental observations.

Introduction Wider utilization of hydrophosphination has been stymied by limits in substrate scope, reaction times, and broader synthetic viability, despite significant advances from global researchers in the last five years.1 Unlike comparatively popular hydroamination,2-8 hydrophosphination is not as well developed. Like hydroamination, many reported systems rely on precious metals, but earthabundant metals for catalytic hydrophosphination have emerged in recent years.9-21 Catalytic hydrophosphination is still thwarted by substrate underrepresentation for both the unsaturated component and phosphine. The limited commercial availability of phosphines compared to amines is doubtlessly a contributing factor. Regardless, the long reaction times at elevated temperatures for many systems and limited selectivity have hindered the progress of metal-catalyzed hydrophosphination.1, 22-23 Indeed, milder conditions tend to promote greater selectivity. Photolysis represents a selective energetic input that may accelerate reactions without loss in selectivity. In a previous report, [κ5-N,N,N,N,C(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH2]Zr (1) demonstrated good conversions at ambient temperature for alkene hydrophosphination with primary phosphines, but the main limitation was relatively lengthy (ca. 12 h) reaction times.24 Nevertheless, the conversions, substrate scope, and mild conditions have been among the best reported for this transformation. For example, 89% conversion (72% isolated) of styrene to PhCH2CH2PHPh occurred after 12 h at ambient temperature with catalytic 1, which was the most active at the time of publication.24 More recent examples reach conversions greater than 90% for styrene but require either elevated temperatures12, 18, 25-31 or sec-

ondary phosphines,12, 15-16, 29-32 and only a handful of these examples will proceed in under four hours.25, 31-32 Compound 1 is one of the (still) uncommon primary phosphine hydrophosphination catalysts.16, 25-28, 31, 33-38 Leaders in this field, including Carpentier, Sarazin, and Trifonov, have pushed the performance of hydrophosphination catalysts, and the most active systems for styrene hydrophosphination to date reach quantitative conversions of styrene after 3 h at 70 °C for PhPH225 and 30 min at 60 °C for Ph2PH26, 39 using calcium- and ytterbium-based catalysts. These reports represent substantial improvement for a benchmark substrate with respect to reaction time and conversion. While metal-catalyzed hydrophosphination is gaining ground in activity, simple substrates like unactivated alkenes, remain a challenge.1 These substrate limitations hamper broader use in synthesis. Mechanistic aspects of metal-catalyzed hydrophosphination have been explored in depth for several systems,22 and these studies, despite being informative, have not yet revealed how to circumvent some of the critical challenges of the reaction. Oddly, though, reports of metal-catalyzed hydrophosphination that require light are quite rare.40-42 For our recent work with iron, photolysis is critical for catalyst activation rather than turnover (i.e., photoactivation vs. photocatalysis).40, 42 Thus, our report on the double hydrophosphination of alkynes with 1 is exceptional in that the light is necessary for turnover (eq. 1).41

ACS Paragon Plus Environment

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Si

Me3Si Me3Si

Zr N

N N

N 1

R

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R' + 2 PhPH2

LED

PhHP

R

PHPh

R'

(1)

Little is known about the impact of light on this system, but the observation that catalytic hydrophosphination in eq. 1 does not proceed in the dark is consistent with photocatalysis rather than photoactivation.41 That observation led to the hypothesis that irradiation may be a general route to accelerated hydrophosphination catalyzed by 1, and we set out to test that hypothesis and better understand the effects of irradiation on this system. Photocatalysis with 1 appears to be general and fast with TDDFT and spectroscopic data indicating an LMCT excitation responsible for catalysis. Understanding of the role of light with 1 is itself of interest, but such understanding may offer general strategies for further development.

Results and discussion

Figure 1: UV-vis absorbance of 2 in different solvents

Photocatalytic hydrophosphination Reaction of styrene with two equivalents of PhPH2 and 5 mol % of 1 at ambient temperature under irradiation with a commercially available, visible light, 9-Watt LED lamp gave nearly quantitative conversion to PhCH2CH2PHPh in two hours (Table 1, entry c). Reactions run without direct irradiation reach 18% and 89% conversion after two and twelve hours, respectively, consistent with our original report (Table 1, entries a and b).24 Reactions run in the dark give only 4% conversion (Table 1, entry f). Colorless 1 would appear to have no significant activity upon visible light irradiation, but the key zirconium phosphido intermediate, (N3N)ZrPHPh (2), is yellow.43 The UVvis absorbance spectrum of 2 in hexanes revealed absorption bands at 364.5 nm, 290.5 nm, and a high-energy band at approximately 210 nm (Figure 1). While no absorption bands appeared completely in the visible, it was the trailing end of the 364.5 nm absorption feature that would be excited upon visible light irradiation and appeared to be responsible for the enhanced hydrophosphination reactivity in our previous report (Table 1).41

Because the absorption of interest was centered in the ultraviolet region (Figure 1), efforts turned to a more efficient delivery of photons with wavelengths close to 365 nm. Commercially available BL-type fluorescent “blacklights” emit over a fairly narrow range centered around 370 nm (see Supporting Information for details). Irradiation of a 1:2 styrene:PhPH2 mixture with 5 mol % of 1 using a compact fluorescent blacklight resulted in quantitative consumption of styrene in 90 minutes at ambient temperature (Table 1, entry d). Furthermore, irradiation of the same reaction using a 253.7 nm photoreactor decreased the time to reach quantitative conversion to 30 minutes (Table 1, entry e). Table 1: Catalytic hydrophosphination of styrene with 1 to form PhCH2CH2PHPh. PHPh

5 mol % 1 2 PhPH2 +

C6D6, light ambient temperature

Entry

Light source

Time

% Conversion

a

ambient light

12 h

89

b

ambient light

2h

18

c

LED

2h

97

d

blacklight

90 min

100

e

253.7 nma

30 min

100

f

dark

2h

4

Conditions: 20 equiv. of styrene, 40 equiv. of PhPH2, 1 equiv. of 1 in ca 0.5 mL benzene-d6, ambient temperature. Percent conversion was measured by integration of the 1H NMR spectra. aPhotoreactor operates at 35 °C. Reactions run in the absence of 1 show only trace amounts of product under otherwise identical conditions for any light source. Type of glass (quartz vs. borosilicate) gave no detectable change in conversions.


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ACS Catalysis

A variety of styrene derivatives were screened for catalytic hydrophosphination with 1 under ultraviolet irradiation from the 253.7 nm photoreactor and blacklight (Table 2). Both turnover number and turnover frequency for all reactions were substantially higher under irradiation than prior reports, and these reactions maintained the high selectivity for secondary phosphine product formation when using two equivalents of PhPH2 as seen previously.24, 44-45 Both catalytic hydrophosphination under blacklight (TON = 20 and TOF = 13.3 h-1) and 253.7 nm (TON = 20 and TOF = 40 h-1) lamps substantially outperform thermal hydrophosphination of styrene with 1 (TON = 18 and TOF = 1.5 h-1). The greater reactivity at 253.7 nm may be a result of relative light intensity rather than the specific wavelength (vide infra). The enhanced reactivity of photoactivated 1 is a step function compared to reported hydrophosphination catalysts. To date, reported catalysts for styrene derivatives only reach conversions greater than 90% under forcing conditions, and only two reports using PhPH2 had a reaction time on the scale of minutes for reactions run at elevated temperatures.26, 39 Table 2: Catalytic hydrophosphination of styrene derivatives, Michael acceptors, diene substrates, and norbornene with 1 under blacklight.

Entry a b

Product

Conv. (%) by light sourcea 100% (30 min, 253.7 nm) 100% (90 min, blacklight) 100% (30 min, 253.7 nm)

100% (30 min, 253.7 nm) 100% (90 min, blacklight)

d


f

100% (30 min, 253.7 nm) 98% (90 min, blacklight) 100% (30 min, 253.7 nm)

g

h


j k

Beyond styrene derivatives, there are a set of common substrates in hydrophosphination catalysis. The most prominent are Michael acceptors because they are poised for nucleophilic attack from a late-metal phosphido intermediate,22, 33, 46-51 although outer-sphere Michael additions have been proposed.52 As anticipated, these substrates showed increased reactivity under ultraviolet irradiation as well (Table 2, entries k and l). Unactivated substrates remain a significant challenge in hydrophosphination catalysis, and only a few catalysts have realized conversions with these.22, 41 The enhanced reactivity under photolysis for typical substrates (e.g., styrene derivatives) invited a deeper exploration of unactivated alkenes, and the reactivity of 1 under photolysis vastly exceeds prior art (Table 3). To give this claim context, consider the performance of 1 in the hydrophosphination of 1-hexene. Under thermal conditions, 47% conversion was measured after four days with heating at 60 °C.24 Under photolysis at 253.7 nm at ambient temperature, the consumption approaches double that previously reported in a quarter of the reaction time. The reactivity of unactivated alkenes under photolysis with 1 (Table 3) now rivals the reactivity of styrene derivatives under thermal conditions with 1. Table 3: Catalytic hydrophosphination of unactivated alkenes with 1 under blacklight.

Entry

Product

100% (60 min, 253.7 nm) 100% (120 min, blacklight) 97% (3.5 h, 253.7 nm) 96% (120 min, blacklight) 100% (120 min, 253.7 nm) 85% (120 min, blacklight)

Conv. (%) sourcea

by

a

81% (253.7 nm) 59% (blacklight)

b

78% (253.7 nm) 50% (blacklight)

c

73% (253.7 nm) 60% (blacklight)

d

73% (253.7 nm) 45% (blacklight)

100% (90 min, blacklight) 100% (120 min, 253.7 nm) 84% (90 min, blacklight)

i

Conditions: 20 equiv. of substrate, 40 equiv. of PhPH2, 1 equiv. of 1 in ca 0.5 mL benzene-d6 at ambient temperature. Percent conversion was measured by integration of the 1H NMR spectra. aThe 253.7 nm photoreactor operates at 35 °C.

100% (90 min, blacklight)

c

e


l

e f

light

56% (253.7 nm) 63% (blacklight) 81% (253.7 nm) 61% (blacklight)

Conditions: 20 equiv. of unsaturated substrate, 40 equiv. of PhPH2, 1 equiv. of 1 in ca 0.5 mL benzene-d6 at ambient temperature. Percent conversion was measured by integration of


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the 1H NMR spectra. aThe 253.7 nm photoreactor operates at 35 °C.

Catalytic hydrophosphination of unactivated alkenes at ambient temperature with 1 under photolysis does require longer reaction times than those for styrene substrates and provides the secondary phosphine products in less impressive yields (Table 3). Despite the modest conversions, it is worth underscoring that the only other hydrophosphination catalysts that report attempts of hydrophosphination with 1-hexene form products in trace amounts with either primary or secondary phosphines, despite more forcing conditions.12, 52-53 This represents substantial progress over previous reports of thermal hydrophosphination with respect to reaction time, temperature, selectivity, and substrate scope.54 Furthermore, catalytic hydrophosphination with 1 of unactivated alkenes and primary phosphines exclusively selects for the secondary phosphine product. Primary phosphines are underreported in catalytic hydrophosphination, despite a resurgence in interest over the last several years.24, 27, 41, 44-45, 55-56 Expansion of catalytic hydrophosphination to include primary phosphines beyond PhPH2 would therefore be of value. Photocatalytic hydrophosphination undertaken with CyPH2 (Cy = cyclohexyl) and MesPH2 (Mes = 2,4,6-trimethylphenyl) using 1 gave high conversions (Table 4). Table 4: Catalytic hydrophosphination of unactivated alkenes with 1 under blacklight.

Product

Conv. (%)a by light source

Entry

R’PH2

a

CyPH2

98% (5 h, 253.7 nm) 90% (24 h, blacklight)

b

CyPH2

98% (24 h, 253.7 nm) 68% (24 h, blacklight)

c

CyPH2

95% (24 h, 253.7 nm) 53% (24 h, blacklight)

d

MesPH2

95% (24 h, 253.7 nm) 83% (24 h, blacklight)

Conditions: 20 equiv. of unsaturated substrate, 40 equiv. of R’PH2, 1 equiv. of 1 in ca 0.5 mL benzene-d6 at ambient temperature. Percent conversion was measured by integration of the 1H NMR spectra. aThe 253.7 nm photoreactor operates at 35 °C.

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In dehydrocoupling catalysis with 1, steric factors have limited substrate scope, and CyPH2 was the largest primary phosphine for which productive chemistry was observed.24, 43 In prior efforts at catalysis, MesPH2 and 1 were entirely unreactive. Hydrophosphination reactivity with CyPH2 is substantially improved under irradiation. For example, hydrophosphination of 2,3-dimethyl-1,3butadiene reaches nearly quantitative conversion after 24 hours at ambient temperature (Table 4, entry c), whereas the previous report required three days at 60 °C to achieve 85%.24 Despite the added bulk over PhPH2, catalytic hydrophosphination with MesPH2 provides outstanding yields of the product phosphine under irradiation. Control reactions of either CyPH2 or MesPH2 with alkene in the presence of 1 at ambient temperature under ambient light fail to provide detectable levels of products, which is consistent with prior work and strongly supports that light is necessary for enhanced catalytic activity. Greater reactivity in catalytic hydrophosphination allows for the possibility of greater synthetic utility. Heterocycles are an important target, and catalytic hydrophosphination of alkenyl and alkynyl phosphines has been reported to make secondary or tertiary rings.35-36, 57-58 Given the enhanced reactivity of 1 under irradiation with unactivated alkenes, it was suspected that a direct preparation of phosphacycles could be undertaken despite the fact that sequential diene hydrophosphination to make cyclic phosphines is unknown. Treatment of 1,4-pentadiene with one equivalent of PhPH2 and 5 mol % of 1 under irradiation from a 253.7 nm mercury arc lamp provided primarily phosphorinane 4 with some alkenyl phosphine 3 (Scheme 1). Secondary phosphine 3 arises from a single intermolecular hydrophosphination of 1,4-pentadiene, which was noted during observation of the catalytic reaction by 31P NMR spectroscopy. This product undergoes an intramolecular hydrophosphination to provide the cyclic phosphine 4 exclusively as the six-membered ring. In previous reports, intramolecular hydrophosphination of 3 gave exclusively the phospholane product(s).35-36, 57-58 Selective formation of a six- over a five-membered ring is the result of 1,2-insertion versus 2,1-insertion during catalysis with 1. However, it is known that derivatives of 3 can spontaneously ring-close upon irradiation.34, 59-60 To test if this were a non-catalytic pathway compound 3 was synthesized independently. Treatment of 3 with 5 mol % of 1 under irradiation provided high conversions (85%) to 4 at ambient temperature (Scheme 1). Control reactions of 3 under irradiation in the absence of 1 also provide 4 in up to 35% conversion. Despite some competitive ring closure under photolysis, the overall reaction from 1,4-pentadiene fails without 1. Moreover, control reactions with 1 in the dark provides scarce (~4%) product formation, illustrating the importance of light for this system. Of minor note, this photocatalytic reaction is more efficient than prior reports of intramolecular hydrophosphination of 3,35-36, 57-58 but the difference in product selectivity make such a claim modest at best. More important, this tandem inter/intramolecular hydrophosphination with 1 is the first example of hydrophosphination in the direct synthesis of heterocycles from


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ACS Catalysis

commercial starting materials and represents new synthetic value for this kind of catalysis. Ph P + PhPH2

5 mol % 1 C6D6, 22 h ambient temperature 253.7 nm irradiation

PHPh 3

PHPh 4 61%

3 15%

5 mol % 1 C6D6, 90 min ambient temperature 253.7 nm irradiation

Ph P

4 85%

Scheme 1: Catalytic hydrophosphination of 1,4pentadiene to produce alkenyl phosphine 3 and phosphorinane 4

Role of irradiation It was hypothesized that enhanced catalytic hydrophosphination with 1 arose from the photochemistry of phosphido compound 2 via an excited state accessed by irradiation at 364.5 nm. Indeed, photoluminescence of 2 was first observed under blacklight irradiation of NMR-scale reactions. Photoluminescence lifetime experiments of 2 showed fluoresence decay that was fit to a triexponential function producing lifetimes of 1.30 ns (47% relative amplitude), 6.31 ns (24% relative amplitude) and 37.7 ns (29% relative amplitude), indicating a singlet excited state may be involved in the photochemical reaction (see Supporting Information for details). A singlet excited state in photoluminescence experiments is consistent with previous studies on catalytic hydrophosphination with 1 in the presence of radical traps.41 Initial transient absorption (TA) spectroscopy experiments confirm the fluoresence lifetimes and provide additional insight into the excited state dynamics of 2. Picosecond TA taken over a 5.5-ns window revealed lifetimes of 24.6 ps and 1.2 ns along with a longer-lived lifetime that could not be resolved within the experimental window, which likely represents the 6.31 ns lifetime seen in time-resolved photoluminescence measurements. An additional feature was observed in nanosecond TA with a 4.1 μs lifetime that likely corresponds to a triplet state accessed by 2. These initial data demonstrate the complex photophysics of 2 and reaction mixtures containing 2 that merit future attention. We hypothesize that light irradiation by either blacklight or the 253.7 nm mercury arc lamp promotes excitation into high-lying excited states followed by relaxation to the lowest lying excited state (364.5 nm) following Kasha’s rule.61 It is through this low-lying excited state that catalysis is proposed to occur. Exploration of the electronic structure was undertaken through time-dependent density functional theory (TDDFT) modeling (see Supporting Information for details). Calculation of the absorption energies and intensities with the BLYP functional followed by convolution with Gaussians with 2250 cm-1 fwhm reveal a low-energy band composed of one electronic transition and a high-energy band composed of many electronic transitions. Calculations with four different functionals were made, which gave consistent results. BLYP is discussed here for simplicity, but complete results are presented in

the Supporting Information. The low- and high-energy bands are consistent with the experimentally observed bands at 27,350 cm-1 (364.5 nm) and 34,450 cm-1 (290.5 nm) in hexanes (Figure 2, top). Additionally, the BLYP functional predicts the high energy band being approximately five times more intense than the low energy band, consistent with experiment. Accurate calculation of the relative intensities signifies the TDDFT-computed absorbance spectrum is derived from the correct orbitals. Inspection of the TDDFT-computed transition responsible for the low-energy band reveals that a one-electron excitation from a phosphorus-based orbital to a zirconium-based orbital is the primary contributor (Figure 2, bottom). The donor orbital was found to be primarily composed of the P 3p orbital, while the acceptor orbitals was found to be primarily composed of the Zr 4d orbital based upon the Löwdin reduced orbital population analysis.62 Based upon these data, the experimental band at 27,350 cm-1 is assigned to a one-electron excitation from P n → to Zr d.

Figure 2: TDDFT-predicted absorbance spectrum at the BLYP/def2-TZVP level of theory in the gas phase for 2 (red spectrum, top) and experimental absorbance spectrum for 2 in hexanes (blue spectrum, top). Visual representation of donor and acceptor orbitals involved in the one-electron excitation that gives rise to the low energy band (bottom).


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Efforts to fully optimize the excited state geometry failed due to state crossings along the excited state potential energy surface. To circumvent this issue, we varied the Zr–P bond in 0.1 Å increments and calculated the ground and excited state energies along the Zr–P stretching coordinate. The Zr–P bond length in BLYP/def2-TZVP optimized structure of 2 is 2.745 Å (Figure 3A), consistent with the crystallographically determined value of 2.726 Å. Shortening or lengthening of the Zr–P bond increases the ground state energy, as expected for a fully optimized ground state structure. The P n → Zr d excited state lies 22,200 cm-1 above the ground state minimum (Zr–P = 2.745 Å) and decreases along the Zr–P stretching coordinate to 19,100 cm-1 above the ground state minimum at a Zr–P bond length of 3.445 Å (Figure 3B). The BLYP/def2-TZVP computed Stokes shift is 7,500 cm-1, which agrees well with the experimentally determined Stokes shift of 7,300 cm-1 in hexanes (see Supporting Information for details). This close agreement between experiment and theory strongly suggests the Zr–P bond of 2 lengthens 0.7 Å to 3.4 Å in the excited state prior to fluorescent emission. It seems likely that the significant lengthening of the Zr–P bond in the excited state gives rise to the enhanced photocatalytic activity of 2.

Figure 3: BLYP/def2-TZVP predicted ground state energies (red circles) and BLYP/def2-TZVP TDDFT-predicted excited state energies (blue diamonds) as a function of Zr–P bond length. The ground state structure has a Zr–P bond length of 2.745 Å while the excited state reaches a minimum at a Zr–P bond length of 3.445 Å. Inset: Expanded view of the P n Zr d excited state energy as a function of Zr–P bond length.

The results of computational analysis indicate how excitation that results in n→d charge transfer impacts the reactivity of this system. First, the loss of electron density from phosphorus would diminish possible nucleophilic behavior of the phosphide and thereby discourage nucleophilic attack on the substrate, if it were occurring. Second, the weakened Zr–P bond would favor substrate insertion. Insertion chemistry is consistent with what is known about early-metal hydrophosphination catalysts22 and previous studies on catalytic hydrophosphination with 1.63 Whether it is reduced nucleophilicity at phosphorus or a weakened

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Zr–P bond, greater reactivity with substrates otherwise inert to hydrophosphination catalysis is observed. To test for insertion-based reactivity, a Hammett analysis was conducted on competition experiments between styrene substrates during photocatalysis (see Supporting Information for details). Results of this analysis confirm empirical observations that electron-rich styrene substrates react faster, consistent with an insertion-based mechanism. It is important to note that in the first report of thermal hydrophosphination using 1 and primary phosphines, a Hammett analysis revealed the reverse trend, namely that electron-deficient substrates were preferred, which implies some degree of nucleophilic attack from the phosphide of 2.24, 64 That dichotomy may indicate different mechanisms are accessible during hydrophosphination without direct irradiation. Further study of this step has been thwarted by the apparent turnover-limiting dependence on photoexcitation of 1. Nevertheless, these observations not only speak to the exceptional behavior of 1, but also suggest that photoexcitation may faciliate insertion broadly and could avail substantial increases in the activity of other known catalysts. Photocatalytic hydrophosphination with 1 appeared to depend on wavelength, but the critical feature is intensity. It was suspected that higher conversions from photolysis under irradiation at 253.7 nm were the result of increased intensity from the higher operating power rather than the specific wavelength, regardless of the difference in extinction coefficients between the affected absorption bands. As expected, filtering the light intensity from this lamp resulted in a decrease in the catalytic turnover. This relationship explains the original observation and avails a route for improvement. Visible light irradiation from a 9Watt bulb provides enhancement because the 364.5 nm feature extends into the visible region, but the 13-Watt blacklight provides better conversions because it is closer to λmax for the absorption band and of a higher wattage (i.e., intensity). The operating temperature of the 253.7 nm photoreactor is slightly higher than ambient temperature due to the heat given off by the mercury arc lamps. However, catalytic hydrophosphination run at elevated temperatures in an oil bath under irradiation from ultraviolet light showed a diminished catalytic turnover.65 This observation appeared related to the distance of the sample from the light source. Because light intensity is inversely proportional to distance squared, the distance of the reaction to the comercially available blacklight bulb was probed. A series of parallel reactions, run at variable distances from the surface of the blacklight, gave product conversion proportional to distance. The reaction with greatest conversion, as measured at a 20 minute reaction time by 31P and 1H NMR spectroscopy, was closest to the lamp itself. The commerically available blacklight shows no emission at energies greater than 360 nm, which is nearly optimal for the 364.5 nm absorption band but the photon output (i.e., intensity) is limited. As a final test for increased activity by increased light intensity, a 9-W UV-A ultraviolet lamp with spectral energy distribution centered


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ACS Catalysis

at 360 nm was acquired and tested. Reaction of styrene with PhPH2 and 5 mol % of 1 at ambient temperature66 under irradiation from this lamp provided complete product formation in 20 minutes (eq. 2) and as little as 12 minutes for more concentrated reaction mixtures. This observation demonstrates that careful selection of wavelength and control of light intensity can lead to optimal catalysis.

(2) Conclusions In summary, irradiation during catalytic hydrophosphination greatly increases the activity of 1 for a range of alkenes as well as availing reactivity with sterically encumbered primary phosphines. Of particular note, 1 exhibits substantially greater activity with unactivated alkenes, substrates that have been largely absent from hydrophosphination.1 This enhancement avails new reactivity, namely the tandem inter/intramolecular hydrophosphination of 1,4-pentadiene to give the heterocyclic phosphorinane product. Spectroscopic and computational efforts indicate an n→d charge transfer event that results in weakening of the Zr–P bond and favoring insertion as evidenced by a Hammett analysis. The observation that photoexcitation of a ubiquitous metal-phosphido feature (i.e., LMCT) suggests that this behavior may be general. Naturally, such a broad statement depends on the photophysics of individual catalysts and chemical outcomes that such excitation would enable. Regardless, it is an enticing possibility that the reactivity of some if not many pre-existing catalysts could be enhanced by photolysis. It is a strategy that we will continue to explore.

Experimental considerations General methods All air-sensitive manipulations were performed under a positive pressure of nitrogen using standard Schlenk line or in a M. Braun glove box. Dry, oxygen-free solvents were employed throughout. Benzene-d6 was purchased then degassed and dried over NaK alloy and distilled under reduced pressure. PhPH2 was purchased from Strem Chemicals and used without further purification. Compound 1 was prepared according to the literature procedure.43 All other chemicals were obtained from commercial suppliers and dried by conventional means. NMR spectra were recorded with a Bruker AXR 500 MHz spectrometer in benzene-d6 and are reported with reference to residual solvent signals (C6D6, δ 7.16 and 128.0) and to an external 85% H3PO4 (δ 0.0) standard for 31P NMR spectra. ESI-mass spectra were collected on an Applied Biosystems 4000QTrap Pro. IR data were collected on a Bruker Alpha FTIR spectrometer in hexanes. Absorption spectra were recorded with a QuantaMaster 4 fluorescence

spectrophotometer (PTI, Edison, NJ, USA) as hexanes solutions. Compound 2 was excited at either 310 nm or 360 nm and excitation and emission slits were both set to 1 nm. Time resolved photoluminescence was collected on a Horiba Fluorolog TCSPC instrument using a 287 nm DeltaDiode for excitation. Picosecond and nanosecond transient absorption (TA) measurements were performed using a Coherent Libra amplifier and TOPAS-C optical parametric amplifier as the excitation source and an Ultrafastsystems Helios/EOS system for detection as described previously.67 A 1 kHz pulse train of 150-fs excitation pulses at 290 nm were generated by an optical parametric amplifier (TOPAS-C, Light Conversion) pumped by 96% of a Coherent Libra Ti:Sapphire system (~1.0 W at 800 nm). The resulting pump beam was attenuated to 0.5 mW for TA experiments. Samples were prepared to have ~0.3 AU at 290 nm in toluene in a 2-mm cuvette.

General procedure for hydrophosphination reactions A borosilicate NMR tube was charged with 0.1 mmol primary phosphine and 0.05 mmol alkene or diene in the presence of 5 mol % of 1 in benzene-d6 solvent. The solutions were reacted at ambient temperature for noted time period under irradiation. The consumption of substrate to product was monitored by 1H NMR spectroscopy. Conversions were determined by substrate consumption through integration of 1H NMR spectra. Reactions run in brand-new NMR tubes showed identical conversions as those in reused, washed NMR tubes. Synthesis of 3 A scintillation vial containing 391.1 mg (3.55 mmol) of PhPH2 in ca 8 mL of diethyl ether at –30 °C was dropwise given 2.20 mL of a 1.6 M butyllithium solution in hexanes (3.52 mmol). The contents stirred at ambient temperature for 15 minutes. The contents were cooled to –30 °C and given 0.45 mL (3.80 mmol) of 5-bromo-pent-1-ene. The contents were stirred for 1 hour at ambient temperature. Volatiles were removed under reduced pressure. The crude reaction mixture was dissolved in hexanes, filtered, and concentrated. Distillation (70–75 °C) under reduced pressure gave 3 as a colorless oil (481 mg, 2.58 mmol, 73%). Spectra match those previously reported.34 Computational methods All electronic structure calculations were performed in the ORCA 4.0 software package on the 380 node IBM Bluemoon cluster at the Vermont Advanced Computing Core.68 A structural model of 2 was prepared from the crystal structure of 269 and optimized using either the generalized gradient approximation (GGA) density functionals PBE70 or BLYP71-72, or the hybrid density functionals PBE070, 73 or B3LYP71-72, 74 with the def2-TZVP75 basis set which uses an extended core potential for Zr and tight SCF convergence criteria. The RIJCOSX76 method was used to speed up the calculation of the Coulomb and exchange terms for both of the hybrid functionals. The orca_plot utility program was used to generate molecular orbitals (MOs) and these were plotted in Visual Molecular Dynamics (VMD)77 with isodensity values of ±0.09 au. Time-dependent DFT (TDDFT) calculations were performed to assess the MOs involved in the transition at 27,350 cm-1. TDDFT was used to calculate the first 20 elec-


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tronic excited states within an expansion space of 100 vectors for each gas-phase optimized structure using its corresponding density functional. The ORCA_mapspc utility program was employed to generate TDDFT-predicted Abs spectra by convoluting Gaussian-shaped bands with full width at half-maximum (fwhm) values of 2250 cm-1. Seventeen structural models were generated for 2 with zirconium-phosphorus bond lengths varied between 2.345 Å and 3.945 Å in 0.1 Å increments to investigate the influence of the Zr–P bond length on the ground and excited state energies. Each structural model was partially optimized using the BLYP functional and def2-TZVP basis set while constraining the Zr–P bond length. The ground state energy was taken from the total SCF energy after partial optimization. TDDFT was performed as described above for each partially optimized structure, and the excited state energy was calculated as the sum of the ground state energy and transition energy for the P n → to Zr d transition. The Stokes shift was calculated as the difference between the P n → to Zr d transition energy of the fully optimized structure and the P n → to Zr d transition energy of the model that gives the minimum excited state energy as compared to the ground state minimum.

ASSOCIATED CONTENT Experimental procedures and characterization data. This material is available free of charge via the Internet at http:// pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest

ACKNOWLEDGMENT This work was supported by the US National Science Foundation (CHE-1565658 to R.W., CHE-1428633 to E.R.Y., and DMR1506248 to M.D.L.). The authors would like to thank Prof. Dr. Madalina Furis for assistance measuring the spectral energy distribution of the blacklight lamp and Biswash Thakuri for assistance with spectroscopic measurements.

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Light-Driven, Zirconium-Catalyzed Hydrophosphination with Primary

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