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Synthetic and Spectroscopic Investigations Enabled by Modular Synthesis of Molecular Phosphaalkyne Precursors Wesley J. Transue,y Junyu Yang,y Matthew Nava,y Ivan V. Sergeyev,z Timothy J. Barnum,y Michael C. McCarthy,{ and Christopher C. Cummins∗,y yDepartment of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA zBruker BioSpin Corporation, Billerica, MA 01821, USA {Harvard{Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA Received October 27, 2018; E-mail: [email protected] PPh3 Cl Abstract: A series of dibenzo-7-phosphanorbornadiene com- P 2 Ph3P=CHR R P 20–80 °C RC P pounds, Ph3PC(R)PA (1-R; A = C14H10, anthracene; R = Me, – [Ph3PCH2R]Cl – PPh , A Et, iPr, sBu), are reported to be capable of thermal fragmen- 3 tation to generate alkyl-substituted phosphaalkynes (RC≡P) ClPA 1-R concomitant with triphenylphosphine and anthracene. Facile preparation of these molecular precursors proceeds by treatment Scheme 1. Synthesis and fragmentation of alkyl-substituted phos- phaalkyne molecular precursors, Ph3P=C(R)PA (1-R; R = H, Me, of ClPA with the appropriate ylide Ph P=CHR (2 equiv). For i s 3 Et, Pr, Bu; A = C14H10). methyl, ethyl, and isopropyl substituents, the phosphaalkyne conversions are measured to be 56{73% in solution by quan- 31 titative P NMR spectroscopy. In the case of compound 1- H, 1-R compounds have been found to spontaneously release Me, the kinetic profile of its spontaneous unimolecular frag- the RC≡P payload into solution at or below room temper- mentation is investigated by an Eyring analysis. The resulting ature. This series of phosphaalkyne precursors has allowed 1-phosphapropyne is directly detected by solution NMR spec- us to pursue further exploratory reactivity and spectroscopic troscopy and gas phase rotational microwave spectroscopy. The studies of RC≡P compounds, underscoring the value of the latter technique allows for the first time measurement of the molecular precursor approach to accessing the chemistry of phosphorus-31 nuclear spin-rotation coupling tensor. The nu- reactive intermediates. clear spin-rotation coupling provides a link between rotational and NMR spectroscopies, and is contextualized in relation to the chemical shift anisotropy. Results and Discussion Substituent Investigations Interested in expanding the scope of 1-R compounds be- Introduction yond the phosphaethyne precursor, we set out to synthesize analogs with alkyl substituents. We envisioned access to Phosphaalkynes are a well known class of low-coordinate λ3- these RC≡P precursors through treatment of ClPA with phosphines, especially as building blocks in organophospho- an appropriate Wittig reagent (Scheme 1), the same syn- rus chemistry. 1 They have been extensively used in the syn- thetic route as devised for 1-H. 25 Accordingly, treatment thesis of both coordination complexes and phosphines, and of a thawing THF solution of ClPA with a thawing, or- 2{4 26 have a rich and useful cycloaddition chemistry. The earli- ange THF solution of Ph3P=CHMe (2 equiv) gave rapid est reports of their synthesis relied on aggressive experimen- bleaching and formation of ethyltriphenylphosphonium chlo- tal conditions involving electric discharge 5 or high (900 ◦C) ride as a colorless precipitate. Analysis of the supernatant by temperatures, 6 but milder preparations have since been 31Pf1Hg NMR spectroscopy revealed a pair of doublet res- 7{13 14{17 developed and have spurred numerous synthetic onances with chemical shifts of δ 197 (Pbridge) and 28 ppm 6,18{24 2 and spectroscopic studies. In pursuit of new and (Pylide) with a scalar coupling of JPP = 173.0 Hz, con- mild methods for generation of low-coordinate phosphorus sistent with successful formation of 1-Me. Repetition in species, we have recently reported the synthesis and thermal THF-d8 and rapid acquisition of a series of one- and two- ◦ behavior of Ph3PC(H)PA (1-H; A = C14H10 or anthracene), dimensional NMR spectra at 0 C allowed this new species a compound demonstrated to fragment into phosphaethyne to be confidently assigned as 1-Me. (HC≡P), triphenylphosphine, and anthracene upon mild The low temperature used in these NMR studies was nec- heating (80 ◦C). 25 The gradual release of HCP at elevated essary due to the instability of the precursor at ambient temperature from this precursor enabled its 1,3-dipolar cy- temperatures, spontaneously fragmenting to form 1-phos- cloaddition with azide, unavailable through more traditional phapropyne. The dramatic decrease of precursor stability low-temperature manipulation techniques due to competi- upon alkyl substitution had not been anticipated, but it par- tive polymerization. alleled the behavior of R2NPA compounds, which fragment From our further studies on phosphaalkyne generation more easily with increasingly large alkyl groups. 30 The frag- with this platform, we now report the synthesis and ther- mentation of 1-R at 25 ◦C was also remarkable as it allowed mal behavior of a series of alkyl-substituted Ph3PC(R)PA direct observation of the resultant MeC≡P by NMR spec- (1-R; R = Me, Et, iPr, sBu) compounds. In contrast to 1- troscopy (31Pf1Hg δ {64 ppm), 12,28 a consequence of the 1 31 Table 1. Phosphaalkyne P NMR chemical shifts and per- 38 cent conversion from 1-R precursors in THF solution after ometry. Such a shift could occur through a closed-shell 2 h at 22 ◦C.a intermediate or though an open-shell intermediate akin to [1,3]-sigmatropic alkyl shifts. 39 Subsequent rate-limiting R δ / ppm Conversionb [2+1]-cheletropic cycloelimination releases an unusual ylide- H 12,25 {32.0 27 |c substituted phosphinidene intermediate from the anthracene Me {63.5 12 56% platform. Rapid fragmentation of this ylide provides triph- Et {64.5 12 73% enylphosphine and 1-phosphapropyne. iPr {65.7 28 58% Alternative fragmentation sequences include initial tri- 40 sBu {61.2 13% phenylphosphine release or a single-step coarctate mech- tBu |d |d anism. We are now in a position to provide experimental support for initial anthracene release to generate a phos- a After 2 h at 22 ◦C, complete consumption of all 1-R com- pounds was observed by NMR spectroscopy. b Quantified by phinidene. Heating 1-H in the presence of excess 1,3-cyclo- ◦ inverse-gated decoupled 31Pf1Hg NMR spectroscopy after 2 h hexadiene (CHD) to 80 C for 3 h resulted in the detection of ◦ at 22 C by integration against PPh3 with a recycle delay of a new species identified as 2 by a series of NMR experiments at least 8T using Cr(acac) as a relaxation reagent. 29 c No 1 3 (eq 1, SI S.1.5). appreciable fragmention at 22 ◦C over 2 h. d Not observed. § PPh3 PPh CHD 80 °C, 5 h 3 H increased solution stability of this phosphaalkyne in rela- 1-H P (1) – tion to HC≡P. Although anthracene and triphenylphosphine A H P were released quantitatively, the conversion to 1-phospha- propyne was found to be 56% after allowing development 2 of the solution for 2 h at 25 ◦C by quantitative integration Product 2 suggested direct capture of the intermediate phos- against PPh3 using Cr(acac)3 as a paramagnetic relaxation phinidene to produce a new 7-phosphanorbornene frame- reagent. 29 The fate of the rest of the phosphorus was not work. Initial PPh3 release would be expected to re- obvious, and has not been tracked down. tain the dibenzo-7-phosphanorbornadiene unit to produce Successful phosphaalkyne generation was not limited to a norbornen-7-yl substituted RPA compound. Single-step MeC≡P from 1-Me. Further investigations into alkyl sub- coarctate fragmentation would similarly not produce 2, in- stituents have also revealed 1-Et and 1-iPr and 1-sBu to stead proceeding directly to HC≡P from 1-H without an fragment over 2 h at 22 ◦C to provide solutions of 1-phos- intermediate for interception by 1,3-cyclohexadiene. pha-1-butyne (EtC≡P) and 3-methyl-1-phospha-1-butyne Experimental investigations of the kinetic profile of 1- (iPrC≡P) and 3-methyl-1-phospha-1-pentyne (sBuC≡P), Me decay were consistent with the mechanism proposed in respectively (Table 1). The percent conversion rose upon Figure 1. Unimolecular decomposition of 1-Me was ob- switching from a methyl to an ethyl substituent; how- served in THF solution, and proceeded with a half life of ◦ ever, further increases in steric bulk were deleterious to 12.5(6) min at 25 C. An Eyring analysis (Figure S.21) re- z phosphaalkyne production. Attempts to form tBuC≡P vealed kinetic parameters for this fragmentation of ∆H = z through 1-tBu failed as ClPA was not found to react with 23.0(9) kcal/mol and ∆S = 4.6(3.1) cal/mol·K. This bar- t Ph3P=C(H) Bu under our conditions, likely a consequence rier to fragmentation was nicely in agreement with the of the steric demands of a tert-butyl substituent. DFT predictions, and smaller than that previously de- termined for 1-H (∆Hz = 25.1(8) kcal/mol and ∆Sz = 25,41 Mechanistic Insight {3.6(2.2) cal/mol·K), as expected from the observed de- crease in stability of 1-Me. Density functional theory (DFT) calculations using ORCA 4.0 31,32 with the M06-2X functional, 33 Def2-TZVP ba- Bonding Analysis sis set, 34,35 and RIJCOSX appromation 36,37 uncovered an identical mechanism for phosphaalkyne release along the The diminished barrier to fragmentation observed for 1-Me closed-shell singlet potential energy surface (PES) to that in comparison to 1-H likely results from an increased π- reported for 1-H (Figure 1). 25 This mechanism involves donicity of the ylide carbon lone pair due to the α-alkyl an initial [1,3]-sigmatropic shift of the phosphorus bridge group, mirroring the behavior of dialkylamide-substituted 30 from a norbornadiene to an intermediate norcaradiene ge- R2NPA compounds.

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