Phospholones from Diacetylenic Ketones: Synthesis, Properties, and

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Phospholones from diacetylenic ketones – synthesis, properties and reactivity Martin Obermeier, and Anna I Arkhypchuk J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00076 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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The Journal of Organic Chemistry

Phospholones from diacetylenic ketones – synthesis, properties and reactivity Martin Obermeier, Anna I. Arkhypchuk* Uppsala University, Department of Chemistry - Ångström Laboratory, Box 523, 75120 Uppsala, Sweden [email protected] Table of Contents/Abstract Graphic

Mes*

H P +

O OEt OEt

P

O R

1 step

R (EtO)2(O)P

R

Mes* R P H O

37% - 96%

Abstract: The reactivity of phosphanyl phosphonates towards diacetylenic ketones was studied. Reactions resulted in the selective formation of phospholones via phosphaalkene intermediates. Phospholones were obtained in yields of 37–96% depending on the substituent on the acetylenic unit. Reduction of the phenyl substituted phospholone resulted in the formation of a persubstituted phosphole bearing a hydroxyl group in position 3 in 64% yield, and its oxidation lead to oxaphospholone in 77% yield. Both of these modifications led to substantial changes in the optoelectronic properties of the compounds and bathochromic shifts of the longest wavelength absorption maximum. Introduction Five-membered heterocycles containing double bonds in conjugation with exocyclic carbonyl groups, such as 3-pyrrolone, 3(2H)-furanone, and thiophen-3(2H)-one (Figure 1), are important

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intermediates in the synthesis of biologically active compounds that show high antitumor and antiviral activities, as well as anti-inflammatory properties.1-6 These heterocyclic subunits can be found in biologically active products such as vermelhotin, jatrophone, and geiparvarin and have recently drawn a lot of attention.5, 7-8 The phosphorus analog of these heterocycles, 1,2-dihydro3H-phosphol-3-one, has made only sporadic appearances in the literature,9-10 and has been studied in much less detail due to its complicated synthesis. On the other hand, another 5-membered phosphorus heterocycle family, the 1H-phospholes, are well known and are believed to be highly promising for applications including optoelectronic materials,11-12 organic light-emitting diodes and organic field-effect transistors, 13-15 and organic solar cells.16-18 Interest in the 1H-phospholes is enhanced by the possibility of fine-tuning their optical properties by modifying the tetrahedral phosphorus center. The geometry of this phosphorus atom does not allow conjugation of the phosphorus lone pair with the butadiene unit, with the low aromaticity of the phospholes being attributable to hyperconjugation of the exocyclic σ*-orbital with the butadiene fragment.19 However, this opens up the possibility to change the properties of these compounds by altering the phosphorus substitution pattern, for example by metal coordination, oxidation, or formation of phosphonium salts (quaternisation of the P-center).20-23 Effects on the π-system caused by changes at the P-center are often referred to as “doping effects”.21,

24

Such modifications are easy to

perform, and can be carried out as the final step of a synthesis, allowing the preparation of large numbers of compounds while avoiding tedious and time-consuming synthetic pathways. S

O

N

thiophen-3(2H)-one

P

O

O

3(2H)-pyrrolone O

3H-phosphol-3-one

O

3(2H)-furanone

P 1H-phosphole


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Figure 1. Five membered heterocycles containing a double bond in conjugation with an exocyclic carbonyl group. Several synthetic approaches to phospholes are available,11, 25 with the Fagan-Nugent method as well as its titanium version being widely used.26, 27 Both of these strategies allow a wide variety of substituents in the 2 and 5 positions of the final phosphole, but prevent the introduction of new substituents at the 3 and 4 positions. This is because these positions are occupied by saturated alkane bridges, which are required to bring the starting acetylenes into close proximity with each other to increase the yield and selectivity of the formation of the zirconocycle or titanocycle that is later transformed into the phosphole.28-29 When more complicated phosphole substitution patterns are desired, more complex synthetic procedures need to be applied. Recently, we have reported a method for the preparation of persubstituted 1,2-oxa-phospholes and ethylene-bridged bisphospholes starting from readily available diynones and tungsten-coordinated phosphanyl phosphonates.30-31 This approach allowed the preparation of the target compounds in one step and in moderate to high yields. The final compounds showed interesting optoelectrochemical properties, and can be used as electrochromic switches.32 Unfortunately, all attempts to remove the tungsten protecting groups from the P-centers to allow the use of doping strategies on these complexes were unsuccessful, outlining the necessity of developing tungstenfree strategies. In this paper, we report our studies on reactions between diacetylenic ketones bearing aromatic substituents on the acetylene termini with metal-free, unprotected phosphanyl phosphonates, which were recently synthesized in our laboratory.33 In contrast to the previously reported tungsten-protected system, the reaction of the metal-free phosphanyl phosphonate 1 with diacetylenic ketones results in the formation of phospholones featuring an exocyclic C=C double



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bond as well as a carbonyl functionality. In addition, a detailed study of the structure-properties relationships of the compounds obtained by oxidation and reduction of the phospholones is presented. Results and Discussion Phosphanyl phosphonate 1 was prepared according to the literature procedure, then treated with 1 eq. of n-BuLi. Deprotonation took place immediately, with

31P

NMR showing that the initial

doublets at -89 and 35 ppm (1JPP = 222 Hz) completely disappeared, being replaced with a new set at -120 and 71 ppm (1JPP = 615 Hz). Addition of ketone 2a to this reaction mixture resulted in an instant color change from bright lemon yellow to purple-red, and 31P NMR showed the formation of a broad multiplet at 250 ppm and a doublet at 27 ppm (JPP = 35 Hz), with the first of these being characteristic of the formation of a P=C double bond. Aqueous work up and purification resulted in the unexpectedly clean formation of a bright orange species with 31P NMR shifts of 11.1 and 17.2 ppm (both singlets). Analysis of 1H, 13C, and 31P NMR spectra, as well as the use of various 2D NMR techniques, suggested the formation of the phospholone heterocycle 3a. The presence of a doublet at 8.11ppm (3JHP = 18 Hz) in the 1H NMR spectrum points towards the selective formation of the cis isomer. Restricted rotation around the P–C bond of the bulky Mes* (2,4,6-tritert-butyl phenyl) group results in the broadening of the proton signals from the Mes* at room temperature, and their complete disappearance at elevated temperatures (ca 40°C). On the other hand, lowering the temperature to -50 °C allowed two sets of signals to be resolved for the aromatic protons and the tBu groups in the ortho-positions of the Mes* group. Compound 3a was formed with a yield of 96% and showed remarkable stability towards oxygen.


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Mes*

H P

Mes* R P

O

O OEt 1 OEt P

BuLi

+ R

R

2a-d

S

R=

2,3a

2,3b

2,3c

R

H

THF, -45C, 30 min

(EtO)2(O)P

S S 2,3d

O 3a-d

3a: 96% 3b: 89% 3c: 30% 3d: 37%

Scheme 1. Formation of phospholones 3a-d from phosphanyl phosphonate 1 and diacetylenic ketones 2a-d. Inspired by the selective formation of 3a, we treated phosphanyl phosphonate 1 with a series of ketones (2b-d). Clean formation of phospholones 3b-d was observed in all cases; however, the stability of the final compounds showed a dependence on the nature of the R group of the ketone (Scheme 1). Ketones bearing all-carbon aromatic rings (2a and 2b) resulted in phospholones 3a,b with high yields (96% and 89%, respectively), while ketones with heterocyclic substituents on the acetylene termini (2c,d) gave much lower isolated yields of phospholes 3c,d (30% and 37%, respectively). This was primarily due to the substantial amount of decomposition that occurred during chromatographic purification. Although all of the phospholones were initially formed as cis-isomers, in the cases of compounds 3c and d partial isomerization to the E-isomers was observed during column chromatography, as well as when solutions were exposed to light. A proposed mechanism for the formation of compounds 3a-d is depicted in Scheme 2.



Mes* 1-Li R

P

O OEt + OEt P

Mes* [2+2]

O

O OEt P P OEt

R

Mes* R P

Mes* R P

(EtO)2(O)P

O C2

Mes* R HO P OH

Mes*

R

(EtO)2(O)P

P(O)(OEt)2 P Mes*

R 4

R (EtO)2(O)P

H 3

P

O

O P OEt R OEt B2 Mes*

Mes* R P O

H 2O

R

O C1

Mes*

O

O P OEt R OEt B1

A

R

P

R

R

(EtO)2(O)P

R

Mes*

O

R

2

R

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P

D

Mes*

O P OEt OEt

PH

E

O

O P OEt OEt

R O P OEt R OEt B 3

R

R

O

R

OH

R

(EtO)2(O)P

Scheme 2. Proposed reaction mechanism for reaction of 1 with diacetylenic ketones. In the first step of the reaction, compound 1 is deprotonated by base (n-BuLi). The size of the one bond coupling constant (1JPP = 615 Hz) suggests the presence of both a P=P double bond and a negative charge on the oxygen of the phosphonate group. 1-Li undergoes [2+2] cycloaddition with a triple bond of the ketone, resulting in the formation of intermediate A. Ring opening of A gives intermediate B, which possesses several resonance forms (B1, B2, and B3) in solution. 31P NMR of the crude reaction mixture of 1-Li with 2a confirms formation of B and suggests B1 as the main component, since the observed resonance at 250 ppm (broad) is typical for a P=C double bond and the resonance at 27 ppm (doublet with JPP = 35 Hz) is in a position that indicates a phosphonate. The broadening of the signals in the 31P NMR spectrum may be explained by the presence of small quantities of B2 and B3, which are in rapid equilibria with B1. The reaction mixture containing B is stable under inert conditions for long periods, with no decomposition or final product 3 being observed even after several days. This suggests that the formation of the final phospholone 3


P

R

Mes* R P H O

3

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requires aqueous work up; it should be noted that using the tungsten-protected analogue of 1 resulted in the immediate formation of intermediate C and its subsequent dimerization to provide the final bisphosphole (in the present case compound 4 would correspond to this reaction outcome, Scheme 2).30-31 During the aqueous work up, B1 is protonated (intermediate D) and undergoes keto-enole tautomerization to form the secondary phosphine E. Intramolecular addition of the P-H over the triple bond results in the formation of the final product (Scheme 2). In fact, similar addition reactions – hydrophosphinations – have previously been observed by other groups and resulted in the predominant formation of the Z isomer,34 similar to our case.

A series of quenching

experiments with TIPSCl, TESCl, and TMSCl were performed to confirm the structure of B1 (Scheme 3). Mes*

P

Mes*

O

Ph O P OEt B1 OEt

TMSCl

Ph

P

OTMS 3a

Ph O P OEt OEt 5a

Ph

Scheme 3. Formation of 3a upon aqueous work up. The first two compounds showed no reactivity with B1 even under extended reaction times. Addition of TMSCl resulted in a sharpening of all peaks in the

31P

NMR spectrum and the

formation of two sets of signals that can be assigned to the trans-phosphaalkene (doublets at δ = 261 and 24 ppm, 3JPP = 34 Hz) and cis-phosphaalkene (doublets at δ = 252 and 15 ppm, 3JPP = 39 Hz), respectively (Scheme 3). No products that could be assigned to the quenching of intermediates B2 or B3 were found. The ratio of the phosphaalkene isomers in 5a appeared to be dependent on light exposure and should not be taken into account when assigning the structure of B1. All attempts to isolate 5a were unsuccessful as the TMS group appeared to be very labile under both



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acidic and basic conditions. As such, compound 5a underwent spontaneous deprotection to either re-form B1 or its protonated analogue (intermediate D), followed by cyclisation to form compound 3a (Scheme 3). Mes* Ph P H

Ph (EtO)2(O)P

O H 2O 2* H 2N

NH2

*Mes O P Ph (EtO)2(O)P

O 3a

Ph H

O 6a 77%

*Mes NaBH4

Ph (EtO)2(O)P

O H 2O 2* H 2N

O NH2

or O2/silica

OH

Ph (EtO)2(O)P

Mes* Ph P OH

7a 64%

O

Ph

(EtO)2(O)P

O 9a 24%

NaBH4 or BH3 Mes* Ph P

P

Ph

O Ph

P

Mes* Ph

(EtO)2(O)P O 8a 30%

Scheme 4. Exploring the reactivity of 3a in oxidation and reduction reactions. Compounds 3a-d are interesting systems in that their photophysical properties can be influenced by two different approaches. They contain a trivalent phosphorus atom, the post-synthetic modification of which is a well-known method of influencing the properties of phosphorus compounds.12-14,

20-21

In addition, 3a-d also contain ketone functionalities conjugated with

exocyclic double bonds, which on similar systems gave access to phospholes via conjugate reduction. Bearing both these ideas in mind, the reactivity of heterocycle 3a towards oxidants as well as reductants was tested (Scheme 4). Compound 3a appeared to be non-reactive towards mild oxidants like tBuOOH and Me3N→O, whereas the application of a 30% aqueous solution of H2O2 resulted in the full decomposition of 3a. Refluxing 3a in DCM in the presence of 5 eq. of finely ground H2O2-urea complex for 3 days resulted in the clean oxidation of 3a at the phosphorus center, and compound 6a was isolated in a 77% yield after chromatography. Long reaction times


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were a result of the low solubility of the oxidant in DCM, as well as the hindered access to the P center due to the bulk of the Mes* and the substituents on the 2 and 5 positions of the heterocycle. More rapid oxidation of 3a was achieved using mCPBA (30 min at room temperature); however, in this case product 6a could not be separated from the mCBA formed during the reaction. Oxidation of 3a can be conveniently followed by 31P NMR, where the disappearance of the signals of the starting material occurs in parallel with the growth of new doublets at +32.7 and 8.8 ppm (3JPP = 52 Hz). Significant changes also occur in the 1H NMR spectrum upon oxidation to 6a, with the spectrum of the product being more complicated than that of 3a. The introduction of another substituent to the already crowded phosphorus center results in the termination of rotation of the Mes* group around the P-C bond. As such, separate signals are observed for the tBu groups in the ortho-positions (singlets at 1.25 and 1.22 ppm) as well as for the protons in the meta-positions of Mes* (doublets of doublets at 7.32 and 6.78 ppm with coupling constants to phosphorus being 4JHP = 4 Hz and to each other 4JHH = 2 Hz). Reduction of 3a can be achieved within several hours by treatment with NaBH4, and results in the formation of phosphole 7a, which can be isolated in a 64% yield after chromatography on Al2O3 (Scheme 4). The 31P NMR spectrum of 7a features a signal with a characteristic phosphole shift at 2.1 ppm (3JPP = 14 Hz), as well as a signal for the phosphonate group at 18.4 ppm. Reduction of the exocyclic double bond is also confirmed by changes in the 1H NMR spectrum, where the doublet of vinyl protons at 8.11 ppm are no longer observed while a new doublet at 3.93 ppm (3JHP = 14 Hz) corresponding to the two benzylic protons can be found. The signal of the hydroxyl proton is observed at 9.86 ppm as a relatively sharp singlet; the shape of this signal as well as its unusual downfield position can be explained by the presence of hydrogen bonding to the oxygen of the phosphonate group. Formation of 7a is also accompanied by release of the steric strain



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around the P-center, with the Mes* substituent now being able to freely rotate around the P-C bond. This is shown by only one signal being observed for the two meta-protons (doublet at 7.26 ppm, 4JHP = 3Hz) as well as one signal for both ortho-tBu groups (singlet at 1.27 ppm) in the 1H NMR. Compound 7a was notably oxygen sensitive and oxidized rapidly on silica gel. The compound isolated after oxidation has two doublets in its 31P NMR spectrum, at 50.6 (heterocyclic phosphorus, 3JPP = 59 Hz) and 9.4 ppm (phosphonate group), and appeared to be not the expected phosphole oxide but the rearranged derivative 8a. This features a Mes* group that cannot freely rotate around its C-P bond due to a clash with the benzylic substituent in position 5 of the heterocycle (see Scheme 5). Careful analysis of the 1H NMR spectrum of compound 8a allows assignment of the relative positions of all of the groups, indicating that it is the cis-isomer; i.e., the Mes* group and benzylic group are on the same side of the heterocycle plane. Approach from the hindered side

H

Mes* H Ph

P

O

O

Mes* Ph

H

(EtO)2(O)P

Ph

(EtO)2(O)P

Mes* P

O

Ph Ph

Ph (EtO)2(O)P

H

O

H

O

Mes* P O

Ph

Ph

H

(EtO)2(O)P

H

(EtO)2(O)P OH

O H Ph

O 9a

OH

Ph

P

(EtO)2(O)P

O 8a

Mes* P O Ph F

Mes* P O Ph

O G

Approach from the less crouded side

Scheme 5. Mechanism of stereoselective formation of 8a and 9a. Formation of the more sterically strained isomer can be explained by the slightly acidic conditions under which it formed. The first step of the transformation of 7a to 8a includes the oxidation of the P center. The resulting phosphole oxide features a very sterically crowded P center that likely


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results in significant distortions to the diene moiety, making orbital overlap less efficient. As such, the entire system is forced to undergo keto-enol tautomerisation, which establishes a fully conjugated ketone-ethene unit. The rate-determining step for acid-promoted keto-enol tautomerisation is protonation of the sp2-hybridised carbon, which requires the proton source to come relatively close to the sp2 center. This means that steric interactions start to play a critical role, with the bulky Mes* group hindering the approach of the proton source from one side. As a result, only the product resulting from protonation opposite the Mes* is observed (Scheme 5). Intrigued by the chemistry of these heterocycles, we decided to reduce 6a that was obtained by oxidation of 3a. Treatment of 6a with 10 eq. of NaBH4 in THF gave a single product, which had a slightly shifted 31P NMR spectrum compared to 8a, with doublets at 42.9 and 9.3 ppm (3JPP = 63 Hz). Symmetric signals corresponding to the Mes* group are found in the 1H NMR spectrum of 9a, suggesting assignment of trans-substitution to the heterocycle; i.e., the Mes* and benzylic groups are on opposite sides of the heterocycle plane. Such a reaction outcome can be explained by taking into account that keto-enol tautomerisation of the enol, formed after the 1,4-conjugated reduction of 6a by NaBH4, takes place under very basic conditions. In such cases, deprotonation of the enol is rapid and the rate-determining step is the re-hybridization of the flat sp2 carbon to an sp3 carbon with tetrahedral geometry, which is then followed by rapid protonation. Pyramidalization of carbon 5 of the heterocycle leads to a change in the relative positions of the substituents. Two possible intermediates can be suggested: structure G (Scheme 5), where the electron lone pair is trans to the Mes* group; and structure F (Scheme 5), where the Mes* and electron pair orbital are cis. The first intermediate (G) would suffer from steric hindrance, as in order to achieve pyramidal geometry the benzyl substituent would need to move closer to the Mes*. On the other hand, F would experience no significant steric hindrance. This means that



formation of G would be much slower than that of F, if it occurs at all. Intermediate F leads to the formation of the observed product 9a after protonation. Conjugation of the newly formed ketone to an endocyclic double bond likely deactivates it towards further reduction under the present conditions. Compounds 8a and 9a both suffer from substantial acid sensitivity, decomposing upon prolonged exposure to silica gel as well as in solutions containing traces of acid (e.g., CDCl3 solutions). However, they are stable in the solid state or in acid free solutions (e.g., CD2Cl2). UV/Vis spectroscopic and electrochemical characterization: The electronic absorption spectra of selected compounds are presented in Table 1. Phospholone 3a is a bright orange-red compound with an absorption maximum of 448 nm. The λmax of 3a is bathochromically shifted by more than 100 nm compared to dibenzylideneacetone, likely due to contributions from the trivalent phosphorus center and the Mes* group.35 Changing the terminal aromatic group of the starting ketones from phenyl to the larger naphthalene or the heterocyclic thiophene and thieno[3,2-b]thiophene groups leads to bathochromic shifts in λmax to 460 (3b), 480 (3c), and 510 nm (3d), respectively. This observation suggests that the lowest energy absorptions can be attributed not only to the heterocyclic core but are extended over the entire system, with contributions from the aromatic substituents. Oxidation of phospholone 3a to 6a leads to a dramatic change in the UV/Vis spectra, with a hypsochromic shift from 445 nm (3a) to 332 nm (6a) being observed. This change is likely caused by blocking the interaction between the phosphorus lone pair and the π-system, since the λmax of 6a is close to that of dibenzylidenacetone.35 Phosphole 7a also undergoes a hypsochromic shift compared with 3a, with a λmax of 372 nm. This is because of the decrease in the length of the π-system, although the lone pair of the phosphorus is still involved. Blocking this integration leads to the rearrangement and


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formation of the final compound, 8a, which has a relatively small and isolated π-system as well as no interaction with the phosphorus lone pair. Table 1. Optical and electrochemical properties of compounds 3a-d and 6a-9a, and their 31P NMR chemical shifts. Chemical Chemical Reduction shift, shift, potentialb, heterocyclic phosphate eV P c, ppm P c, ppm -1.73 -17.3 11.2

3J PP

Compound

λmax nm

λon nm

Oxidation potentialb, eV

3a

448

560

0.88i, 1.15i

6a

332

415

-

-1.63

32.7

8.8

52

7a

372

440

0.55i

-1.58

2.1

18.3

14

8a

314

365

-

-1.61

50.6d

9.4 d

59

9a

313

360

-

-1.52i

42.9 d

9.3 d

63

3b

460

560

0.90i

-1.71

-14.5

10.5

0

3c

480

590

n.a.

n.a.

-16.1

11.3

0

3d

510

605

n.a.

n.a.

-15.9

11.3

0

a,

a,

coupling constant c, Hz 0

λon - onset of absorption, [a] – Measured in dry CH2Cl2. [b] – Measured using 1 mM solutions of the analyte in CH2Cl2 (0.1 M NBu4PF6), glassy C electrode, υ = 100 mVs-1. All potentials are given versus Fc+/0. Reported value corresponds to E1/2 = (Epa + Epc)/2 if nothing else is stated. [i] – peak is irreversible. [n.a.] – not possible to measure CV of the compound under conditions stated. [c] – Spectra measured in CDCl3 using 400 MHz NMR magnet. [d] – Spectra measured in CD2Cl2 due to sensitivity of the compound to acid traces. The electrochemical behavior of the compounds was studied by cyclic voltammetry, and the results of the study are summarized in Table 1. Compounds that feature trivalent phosphorus in the heterocycle undergo irreversible oxidations at potentials of 0.88 and 1.15 eV for 3a, and 0.55 eV



for 6a. The oxidation potential of 7a is similar to the that of the other phospholes reported in the literature.12 Compounds containing conjugated ketone functionalities feature reversible reductions at ca. -1.71 eV for compounds with trivalent phosphorus atoms (3a and 3b) and at slightly lower potentials (ca. -1.61 eV for 6a and 8a, ca. -1.52 eV for 9a) for compounds containing pentavalent heteroatoms. Conclusion We have shown that the treatment of bisacetylenic ketones 2a-d with lithiated phosphanyl phosphonate 1-Li results in the selective formation of phosholones 3a-d after aqueous work up. In cases where the starting ketones bear aromatic terminal substituents such as phenyl or naphthalene groups, phospholones can be obtained with very high yields (96 and 89% for 3a and 3b, respectively). In cases where ketones are substituted with heteroaromatic groups, the yields of the phospholones are significantly lower: 30% (3c) and 37% (3d). Formation of phospholones proceeds in two steps, during which the phosphaalkene intermediate is formed first. Aqueous work up results in the quenching of the phosphaalkene intermediate and the formation of the secondary phosphine after rearrangement, which then undergoes addition over the triple bond to form the heterocycle featuring the cis-exocyclic double bond selectively. The chemical behavior of compound 3a was studied in reactions with oxidants and reducing agents. Addition of mCPBA or H2O2-urea to 3a resulted in oxidation of the phosphorus atom and a change in the electronic absorption properties of the compound, with λmax shifting from 445 nm (3a) to 340 nm (6a). Reduction of 3a resulted in the formation of the phosphole in 64% yield. Phosphole 7a appeared to be unstable and prone to oxidation at the phosphorus center. Formed in this way, heterocycle 8a was proven to be not the phosphole oxide but its rearranged isomer, where the benzyl substituent in position 5 of the heterocycle and the Mes* group occupy positions cis to each other. On the


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other hand, reduction of 7a in the presence of excess NaBH4 results in the conjugated reduction of the exocyclic double bond and the formation of 9a, which is a diastereoisomer to 8a where the benzyl group and the Mes* occupy cis positions relative to each other. Selective formation of isomers 8a and 9a can be explained by the different conditions under which keto-enol tautomerization takes place. Overall we have developed an easy one-step synthesis of the phospholones and studied possibilities of their post-synthetic modification, opening the door to investigations on their biological activity and the possibilities of future applications in molecular electronics. Experimental Section Material and Methods. All reactions were carried out under argon atmospheres. Glassware was thoroughly flame-dried, and ketones were dried prior to use. THF and Et2O were freshly distilled over Na/benzophenone under nitrogen. DCM was freshly distilled from CaH2. NMR spectra were recorded on a JEOL (400YH magnet) Resonance 400 MHz spectrometer. Chemical shifts (δ) are reported in ppm and coupling constants (J) in Hz. 1H NMR shifts are referenced to the residual protic solvent signal and 31P NMR spectra externally to 85% H3PO4(aq). Ketones 2a36-37, 2b38 and 2c39 as well as phosphanyl phosphonate 133 were prepared according by literature procedures. 1,5-bis(thieno[3,2-b]thiophen-2-yl)penta-1,4-diyn-3-one, 2d: Ketone 2d was prepared in two steps starting from 2-(2,2-dibromovinyl)thieno[3,2-b]thiophene, which was prepared according to a literature procedure.40 This was first converted to the alcohol-containing analogue of 2d, which was then oxidized to yield the final product. Both steps are described below. 2-(2,2-Dibromovinyl)thieno[3,2-b]thiophene (0.74 g, 2.3 mmol, 1 eq) was dissolved in 100 ml of diethyl ether and cooled to -78 °C. n-BuLi (1.9 ml, 4.8 mmol, 2.1 eq, 2.5 M solution in hexanes) was then added dropwise and the reaction mixture was stirred for 2 h, allowed slowly to warm to



r.t. and stirred for an additional 30 min. After this, the reaction mixture was cooled back to -78 °C and 70 µl (67 mg, 1.15 mmol, 0.5 eq) of methyl formate (anhydrous) was added in one portion. The reaction mixture was allowed to warm to r.t. and stirred for 2 h, after which it was quenched by addition of brine and extracted with diethyl ether (3×75 ml). The combined organic phases were dried over MgSO4. Solvent was removed and the resulting residue was subjected to column chromatography on silica gel (dry loading) using pentane:diethyl ether (1:1 mixture) as eluent. Rf = 0.77. Yield: 220 mg, 26%, brown powder. 1H NMR (400 MHz, CDCl3): δ = 7.50 (d, J = 5 Hz, 2H), 7.45 (s, 2H), 7.21 (dd, J = 5 Hz, J = 1 Hz, 2H), 5.65 (bs, 1H), 2.50 (bs, 1H) ppm. 13C {1H} NMR (101 MHz, CDCl3): δ = 140.5, 138.3, 129.8, 125.4, 123.0, 119.4, 90.1, 79.0, 53.5 ppm. HRMS: calc. for C17H8OS4Ag, [M+Ag]+ 464.8495, found 464.8496. 1,5-bis(thieno[3,2-b]thiophen-2-yl)penta-1,4-diyn-3-ol (220 mg, 0.62 mmol, 1 eq) was dissolved in DCM (50 ml) and BaMnO4 (280 mg, 1.1 mmol, 1.8 eq, 80% technical grade) was added in one portion. The reaction mixture was stirred for 15 h at r.t. and then filtered through a silica plug using DCM as eluent. After solvent removal, the residue was subjected to column chromatography on silica gel using DCM as eluent. Rf (DCM) = 0.86. This compound has low solubility in most solvents and cannot be completely purified from the starting alcohol (see above for characterization of the alcohol). Yield: 190 mg, 86%, bright yellow powder. 1H NMR (400 MHz, CDCl3): δ =7.77 (s, 2H), 7.66 (d, J = 5 Hz, 2H), 7.28 (dd, J = 5 Hz, J = 0.5 Hz, 2H). 13C {1H} NMR (101 MHz, CDCl3): δ = 159.2, 144.1, 139.0, 132.7, 130.0, 120.5, 119.7, 119.5, 95.0, 87.1 ppm. HRMS: calc. for C17H7OS4, [M+H]+ 354.9374, found 354.9375. General procedure for preparation of phospholones 3a-d: A solution of the starting phosphanyl phosphonate 1 (1 eq) in 10 ml THF is cooled down to ca 40°C, and a solution of n-BuLi (1.05 eq, 1.6 M or 2.5 M in hexane) is added drop wise. After this,


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the reaction mixture is taken out of the cooling bath and allowed to warm to r.t. The solution gradually turns bright yellow. After ca 30 min, the reaction is cooled to ca 0 °C and the ketone (1 eq pre-dissolved in 2–5 ml THF) is added. The reaction is allowed to warm to r.t, and stirred for an additional 1 h. During this time, the solution turns dark red-purple. After this, brine (ca 25 ml) is added and the reaction mixture is extracted with diethyl ether (3×25 ml), and the combined organic phases are washed with brine and dried over MgSO4. After evaporation of the solvent, the crude material is obtained as red-orange dark solid. Column chromatography on silica gel (exact conditions are given for each compound) gives pure products. Diethyl(Z)-(5-benzylidene-4-oxo-2-phenyl-1-(2,4,6-tri-tert-butylphenyl)-4,5-dihydro-1Hphosphol-3-yl)phosphonate, 3a: For this reaction, 1 (0.755 g, 1.8 mmol, 1 eq), n-BuLi (0.75 ml, 1.05 eq, 2.5 M solution in hexane), and ketone 2a (0.42 g, 1 eq, 1.8 mmol) were used. The pure compound was usually obtained after the reaction without purification by chromatography (see the NMR spectra in the ESI). When additional purification was needed, chromatography on silica gel using a gradient from pure DCM to 2% MeOH in DCM (Rf (DCM) = 0.05, Rf (2%MeOH) = 0.9) was applied. Yield: 96%, 1.118 g, bright orange solid. 31P {1H} NMR (162 MHz, CDCl3): δ = 11.1 (s), -17.2 (s) ppm. 1H NMR (400 MHz, CDCl3): δ = 8.11 (d, 3JHP = 18 Hz, 1H, CH=), 7.41 – 7.36 (m, 2H, Mes*), 7.23 – 7.16 (m, 5H, Ph), 7.11 – 7.03 (m, 5H, Ph), 4.10 – 3.87 (m, 4H, OCH2), 1.30 (bs, 18H, o-tBu), 1.25 (s, 9H, p-tBu), 1.12 – 1.07 (m, 6H, OCH2CH3) ppm. 13C {1H} NMR (101 MHz, CDCl3): δ = 190.3 (dd, J = 14, 9 Hz), 185.3 (dd, J = 35, 6 Hz), 152.9 (d, J = 3 Hz), 144.8 (d, J = 14 Hz), 136.8 (d, J = 6 Hz), 136.7 (d, J = 12 Hz), 135.0 (dd, J = 21, 12 Hz), 134.4 (dd, J = 2, 1 Hz), 131.4 (dd, J = 188, 15 Hz)131.3 (s), 131.3 (s), 130.1 (d, J = 1 Hz), 128.9 (d, J = 2 Hz), 128.3 (s), 127.9 – 127.8 (m), 127.5 (s), 127.5 (s), 121.6 (d, J = 54 Hz), 121.6 (d, J = 49 Hz), 62.9 – 61.3 (m), 40.3 – 39.7 (m), 35.0 (d, J = 1 Hz), 33.9 (s), 31.2 (s) ppm. FT-IR (neat): 2961, 2905,



2866, 1660, 1588, 1567, 1478, 1444, 1392, 1361, 1259, 1243, 1179 cm-1. HRMS: calc. for C39H50O4P2H, [M+H]+ 645.3257, found 645.3272. Diethyl-(2-(naphthalen-1-yl)-5-(naphthalen-1-ylmethylene)-4-oxo-1-(2,4,6-tri-tertbutylphenyl)-4,5-dihydro-1H-phosphol-3-yl)phosphonate, 3b: For this reaction, 1 (25 mg, 0.06 mmol, 1 eq), n-BuLi (40 µl, 1.05 eq, 1.6 M solution in hexane), and ketone 2b (20 mg, 1 eq, 0.06 mmol) were used. The pure compound was usually obtained after the reaction without purification by chromatography (see the NMR spectra in the ESI). When additional purification was needed, chromatography on silica gel using a gradient from pure DCM to 1% MeOH in DCM (Rf (DCM) = 0.1, Rf (1%MeOH) = 0.9) was applied. Yield: 89%, 40 mg, red solid. 31P {1H} NMR (162 MHz, CDCl3): δ =10.5 (s), -14.5 (s) ppm. 1H NMR (400 MHz, CDCl3): δ = 9.09 (d, J = 17 Hz, 1H), 8.29 (d, J = 8 Hz, 1H), 7.98 (d, J = 8 Hz, 1H), 7.78 (d, J = 8 Hz, 2H), 7.70 (d, J = 8 Hz, 2H), 7.56-7.60 (m, 2H), 7.52 – 7.45 (m, 3H), 7.20 (d, J = 6 Hz, 1H), 7.19 – 7.11 (m, 2H), 7.05 (dd, J = 8 Hz, J = 8 Hz, 1H), 6.72 (d, J = 7 Hz, 1H), 3.97 – 3.88 (m, 1H), 3.87 – 3.76 (m, 2H), 3.65 – 3.53 (m, 1H), 1.29 (s, 9H), 1.12 (s, 9H), 1.06 (s, 9H), 0.92 (t, J = 7 Hz, 3H), 0.83 (t, J = 7 Hz, 3H) ppm. 13C {1H} NMR (101 MHz, CDCl3): δ = 191.3 (dd, J = 14, 6 Hz), 183.3 (dd, J = 37, 4 Hz), 161.73 (d, J = 34 Hz), 157.5 (d, J = 10 Hz), 152.2 (d, J = 3 Hz), 141.7 (d, J = 14 Hz), 136.6 (dd, J = 20, 12 Hz), 136.4 (d, J = 13 Hz), 134.6 (dd, J = 11, 5 Hz), 134.5 (d, J = 13 Hz), 133. 5 (s), 133.0 (d, J = 1 Hz), 132.3 (d, J = 1 Hz), 131.0 (s), 131.0 (s), 129.8 (dd, J = 210, 11 Hz), 128.8 (s), 128.2 (d, J = 1 Hz), 128 (s), 127.0 (s), 126.5 (s), 126.3 (s), 126.0 (s), 125.3 (s), 124.9 (s), 124.4 (s), 123.8 (s), 123.5 (s), 123.4 (s), 122.7 (dd, J = 52, 2 Hz), 62.0 (d, J = 24 Hz), 62.0 (d, J = 24 Hz), 40.1 (s), 39.0 (d, J = 8 Hz), 34.7 (d, J = 1 Hz), 33.8 (d, J = 4 Hz), 33.7 (d, J = 7 Hz), 31.1 (s), 16.1 (d, J = 7 Hz), 16.0 (d, J = 7 Hz) ppm. HRMS: calc. for C47H54O4P2H, [M+H]+ 745.3570, found 745.3573.


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Diethyl-(4-oxo-2-(thiophen-2-yl)-5-(thiophen-2-ylmethylene)-1-(2,4,6-tri-tert-butylphenyl)4,5-dihydro-1H-phosphol-3-yl)phosphonate, 3c: For this reaction, 1 (50 mg, 0.12 mmol, 1 eq), n-BuLi (51 µl, 1.05 eq, 2.5 M solution in hexane) and ketone 2c (29 mg, 1 eq, 0.12 mmol) were used. Chromatography on silica gel using a gradient from 25% diethyl ether in pentane to 50% diethyl ether in pentane (Rf (50% Et2O in pentane) = 0.4) gave the final product. Compound was partially light sensitive and prone to isomerization around the C=C double bond. Yield: 30%, 23 mg, deep red solid. 31P {1H} NMR (162 MHz, CDCl3): δ = 11.2 (s), -16.0 (s) ppm. 1H NMR (400 MHz, CDCl3): δ = 8.13 (d, J = 18 Hz, 1H), 7.51 (bs, 2H), 7.37 (d, J = 5 Hz, 1H), 7.33 (d, J = 4 Hz, 1H), 7.27 – 7.24 (m, 1H), 7.00 (dd, J = 5 Hz, J = 4 Hz, 1H), 6.82 – 6.77 (m, 2H), 4.18 – 4.00 (m, 4H), 1.33 (bs, 18H), 1.31 (s, 9H), 1.22 – 1.17 (m, 6H) ppm.13C {1H} NMR (101 MHz, CDCl3): δ = 188.9 (dd, J = 13, 12 Hz), 177.6 (dd, J = 32, 3 Hz), 153.8 (d, J = 3 Hz), 153.4 (d, J = 3 Hz), 139.5 (dd, J = 2, 1 Hz), 138.9 (d, J = 9 Hz), 136.4 (dd, J = 13, 7 Hz), 136.2 (d, J = 21 Hz), 135.2 (d, J = 152 Hz), 133.6 (d, J = 2 Hz), 132.8 (dd, J = 18, 12 Hz), 132.3 (d, J =1 Hz), 130.44 (dd, J = 4, 1 Hz), 129.4 (dd, J = 186, 18 Hz), 129.1 (d, J = 2 Hz), 128.2 (d, J = 1 Hz), 127.1 (d, J = 1 Hz), 127.00 (d, J = 1 Hz), 120.0 (dd, J = 50, 2 Hz), 62.51 – 62.26 (m), 40.42 – 40.17 (m), 35.1 (d, J = 1 Hz), 34.1 (s), 34.0 (s), 31.3 (s), 16.4 (s), 16.3 (s). HRMS: calc. for C35H46O4P2S2H, [M+H]+ 657.2386, found 657.2387. Diethyl-(4-oxo-2-(thieno[3,2-b]thiophen-2-yl)-5-(thieno[3,2-b]thiophen-2-ylmethylene)-1(2,4,6-tri-tert-butylphenyl)-4,5-dihydro-1H-phosphol-3-yl)phosphonate, 3d: For this reaction, 1 (25 mg, 0.06 mmol, 1 eq), n-BuLi (40 µl, 1.05 eq, 1.6 M solution in hexane), and ketone 2d (21 mg, 1 eq, 0.06 mmol) were used. Chromatography on silica gel using a gradient from pure DCM to 5% MeOH in DCM (Rf (DCM) = 0, Rf (5%MeOH) = 1) gave the final product. Compound was partially light sensitive and prone to isomerization around the C=C double bond. Yield: 37%, 17



mg, deep red-brown solid. 31P {1H} NMR (162 MHz, CDCl3): δ = 11.3 (s), -15.8 (s) ppm. 1H NMR (400 MHz, CDCl3): δ = 8.14 (d, J = 18 Hz, 1H), 7.58 (bs, 2H), 7.52 (d, J = 1 Hz, 1H), 7.41 (d, J = 5 Hz, 1H), 7.36 (d, J = 5 Hz, 1H), 7.15 (d, J = 5 Hz, 1H), 7.04 (d, J = 5 Hz, 1H), 6.96 (s, 1H), 4.23 – 3.98 (m, 4H), 1.38 (s, 18H), 1.32 (s, 9H), 1.20 – 1.13 (m, 6H) ppm. 13C {1H} NMR (101 MHz, CDCl3): δ = 188.7 (dd, J = 14, 10 Hz), 176.8 (dd, J = 18, 15 Hz), 154.1 (d, J = 4 Hz), 153.7 (d, J = 4 Hz), 144.9 (s), 142.3 (s), 141.6 (s), 141.4 (d, J = 8 Hz), 140.0 (s), 139.1 (s), 138.0 (dd, J = 13, 6 Hz), 137.0 (d, J = 23 Hz), 136.3 (d, J = 16 Hz), 132.9 (dd, J = 17, 12 Hz), 130.5 (s), 129.5 (dd, J = 188, 18 Hz), 129.1 (s), 129.0 (s), 125.6 (s), 122.7 (s), 120.6 (d, J = 52 Hz), 119.4 (d, J = 22 Hz), 119.2 (s), 62.81 – 62.31 (m), 40. 5 (s), 40.4 (s), 39.6 (d, J = 4 Hz), 34.2 (s), 34.2 (s), 31.3 (s), 16.3 (d, J = 7 Hz) ppm. HRMS: calc. for C39H46O4P2S4Na, [M+Na]+ 791.1646, found 791.1646. Reactivity of 3a towards oxidants and reductants. Diethyl-(5-benzylidene-1-oxido-4-oxo-2-phenyl-1-(2,4,6-tri-tert-butylphenyl)-4,5dihydrophosphol-3-yl)phosphonate, 6a: To a solution of 2a (22 mg, 0.034 mmol, 1 eq) in 10 ml of DCM was added a finely ground complex of hydrogen peroxide with urea (16 mg, 5 eq). The suspension was refluxed for until full conversion was achieved according to 31P NMR (~3 days). After cooling to r.t., the reaction mixture was washed with water and brine, and then dried over MgSO4. Evaporation of the solvent gave the final product. Yield: 77%, 17 mg, pale yellow solid. Alternatively, compound 6a can be prepared by oxidation of 3a with 1 eq of m-CPBA in DCM at room temperature. Full conversion is achieved after 30 min with clean formation of 6a. Unfortunately, 6a runs together with byproduct – m-CBA under all chromatography conditions we tried, and so cannot be obtained in pure form by this protocol. 31P {1H} NMR (162 MHz, CDCl3): δ = 32.7 (d, 3JPP = 52 Hz), 8.8 (d, 3JPP = 52 Hz) ppm. 1H NMR (400 MHz, CDCl3): δ = 7.95 (d, J


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= 33 Hz, 1H), 7.53 – 7.48 (m, 2H), 7.44 – 7.35 (m, 5H), 7.32 (dd, J = 4 Hz, J = 2 Hz, 1H), 7.22 (t, J = 7 Hz, 1H), 7.05 (d, J = 8 Hz, 1H), 7.03 (d, J = 8 Hz, 1H), 6.78 (dd, J = 5 Hz, J = 2 Hz, 1H), 4.22 – 4.09 (m, 1H), 4.06 – 3.91 (m, 2H), 3.88 – 3.76 (m, 1H), 1.25 (s, 9H), 1.22 (s, 9H), 1.17 (t, J = 7 Hz, 3H), 1.04 (t, J = 7 Hz, 3H), 0.99 (s, 9H) ppm. 13C {1H} NMR (101 MHz, CDCl3): δ = 187.6 (dd, J = 40, 11 Hz), 170.8 (d, J = 64 Hz), 165.7 (d, J = 7 Hz), 158.7 (d, J = 10 Hz), 153.1 (d, J = 4 Hz), 151.1 (d, J = 4 Hz), 140.4 (dd, J = 180, 5 Hz), 134.3 (dd, J = 6, 6 Hz), 133.2 (dd, J = 4, 1 Hz), 131.5 (s), 131.4 (s), 130.4 (dd, J = 112, 7 Hz), 128. 6 (d, J = 2 Hz), 128.5 (s), 127.7 (s), 127.7 (s), 125.1 (d, J = 13 Hz), 124.5 (d, J = 14 Hz), 121.3 (dd, J = 106, 3 Hz), 62.9 (d, J = 6 Hz), 62.2 (d, J = 6 Hz), 40.5 (d, J = 3 Hz), 40.1 (d, J = 4 Hz), 34.2 (s), 34.0 (s), 33.0 (s), 30.7 (s), 16.3 (d, J = 7 Hz), 16.0 (d, J = 7 Hz) ppm. FT-IR (neat): 2964, 2907, 2869, 1686, 1592, 1573, 1477, 1441, 1429, 1393, 1361, 1258, 1237, 1211, 1179 cm-1. HRMS: calc. for C39H50O5P2Na, [M+Na]+ 683.3031, found 683.3059. Diethyl(5-benzyl-4-hydroxy-2-phenyl-1-(2,4,6-tri-tert-butylphenyl)-1H-phosphol-3yl)phosphonate, 7a: To a solution of 3a (28 mg, 0.043 mmol) in 10 ml of THF at r.t. was added 10 eq of NaBH4 (16 mg, 0.43 mmol). Full conversion according to 31P NMR was achieved after 2.5 h. The reaction mixture was quenched by direct application to a silica gel column. Chromatography using 50% diethyl ether in pentane gives the final product (Rf (50% Et2O in pentane) = 0.6, Rf (DCM) = 0.8). Compound was oxygen sensitive in solution. Yield: 64%, 18 mg, pale yellow solid. Alternatively, compound 7a can be prepared by treatment of 3a (1 eq, 200 mg, 0.31 mmol) dissolved in dry THF (10 mL) with BH3·THF (5 eq, 1.55 mmol, 1 M solution in THF). Reaction mixture was stirred for 2 days at room temperature under argon. After complete conversion, the solvent was removed in vacuo and the residue was quenched with degassed NH4Cl, washed with degassed water, and extracted with DCM under inert conditions. Chromatography on



silica gel (50% diethyl ether in pentane e) gives the final product. Yield: 46%, 92 mg, pale yellow solid. 31P {1H} NMR (162 MHz, CDCl3): δ = 18.4 (d, 3JPP = 14 Hz), 2.1 (d, 3JPP = 14 Hz) ppm. 1H NMR (400 MHz, CDCl3): δ = 9.86 (s, 1H), 7.26 (d, J = 3 Hz, 2H), 7.21 – 7.16 (m, 4H), 7.14 – 7.11 (m, 1H), 7.11 – 7.07 (m, 2H), 7.05 – 7.00 (m, 3H), 4.08 – 3.83 (m, 4H), 3.93 (d, J = 14 Hz, 2H), 1.27 (s, 18H), 1.20 (s, 9H), 1.09 (t, J = 8 Hz, 6H) ppm. 13C {1H} NMR (101 MHz, CDCl3): δ = 159.6 (d, J = 12 Hz), 158.7 (dd, J = 7, 4 Hz), 154.0 (dd, J = 31, 16 Hz), 152.9 (d, J = 3 Hz), 149.9 (s), 141.5 (s), 135.4 (dd, J = 10, 5 Hz), 129.1 (d, J = 11 Hz), 128.7 (s), 128.0 (s), 127.0 (s), 126.9 (s), 125.5 (s), 122.5 (d, J = 10 Hz), 112.3 (dd, J = 13, 13 Hz), 62.2 (d, J = 5 Hz), 38.8 (d, J = 4 Hz), 34.9 (s), 33.2 (d, J = 3 Hz), 31.7 (d, J = 14 Hz), 31.0 (s), 15.9 (d, J = 7 Hz) ppm. FT-IR (neat): 3370 (very broad band), 2962, 2906, 2871, 1588, 1567, 1493, 1478, 1454, 1444, 1391, 1361, 1260, 1237, 1211, 1190 cm-1. HRMS: calc. for C39H52O4P2H, [M+H]+ 647.3414, found 647.3417. Diethyl-(5-benzyl-1-oxido-4-oxo-2-phenyl-1-(2,4,6-tri-tert-butylphenyl)-4,5dihydrophosphol-3-yl)phosphonate, 8a: To a solution of 6a (23 mg, 0.036 mmol) in THF (ca 10 ml) was added a finely ground complex of hydrogen peroxide with urea (33 mg, 0.36 mmol, 10 eq). The suspension was stirred at r.t. until full conversion was achieved according to 31P NMR (~2 h). After cooling to r.t., the reaction mixture was diluted with DCM, washed with water and brine, and then dried over MgSO4. Evaporation of the solvent gave the crude product. The final product was obtained by chromatography on aluminum oxide using a gradient from pentane:DCM (1:1) to pure DCM (Rf (DCM ) = 0.85). Compound 8a is moderately acid sensitive and undergoes fast decomposition when applied to silica gel or treated with CDCl3. Yield: 30%, 7 mg, pale yellow solid. 31P {1H} NMR (162 MHz, CDCl3): δ = 51.6 (d, 3JPP = 60 Hz), 9.5 (d, 3JPP = 60 Hz) ppm. 31P NMR (162 MHz, DCM-d2): δ = 50.6 (d, 3JPP = 59 Hz), 9.4 (d, 3JPP = 59 Hz) ppm. 1H NMR (400


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MHz, DCM-d2): δ = 7.49 (dd, J = 5 Hz, J = 2 Hz, 1H), 7.45 (dd, J = 8 Hz, J = 1 Hz, 2H), 7.41 (d, J = 8 Hz, 2H), 7.35 – 7.28 (m, 2H), 7.28 – 7.19 (m, 2H), 7.09 (d, J = 8 Hz, 1H), 7.07 (d, J = 8 Hz, 1H), 6.82 (dd, J = 5 Hz, J = 2 Hz, 1H), 4.25 – 4.13 (m, 1H), 4.13 – 4.00 (m, 1H), 3.90 – 3.72 (m, 2H), 3.68 (dd, J = 6 Hz, J = 2 Hz, 1H), 3.63 – 3.54 (m, 1H), 3.42 (ddd, J = 15 Hz, J = 6 Hz, J = 6 Hz, 1H), 1.58 (bs, 9H), 1.17 (bs, 9H), 1.14 (dt, J = 7 Hz, J = 1 Hz, 3H), 1.01 (bs, 9H), 0.87 (td, J = 7 Hz, J = 1 Hz, 3H) ppm. 13C {1H} NMR (101 MHz, DCM-d2): δ = 199.2 (dd, J = 24, 10 Hz), 170.6 (dd, J = 71, 2 Hz), 158.1 (d, J = 8 Hz), 154.0 (d, J = 4 Hz), 141.4 (d, J = 8 Hz), 134.3 (dd, J = 178, 6 Hz), 131.6 (d, J = 6 Hz), 131.0 (dd, J = 6, 1 Hz), 130.9 (s), 129.6 (s), 128.6 (s), 128.6 (s), 128.3 (s), 127.4 (s), 126.5 (s), 125.5 (d, J = 12 Hz), 122.8 (d, J = 13 Hz), 119.4 (dd, J = 97, 3 Hz), 63.6 (d, J = 6 Hz), 62.5 (d, J = 7 Hz), 55.4 (dd, J = 60, 5 Hz), 40.8 (d, J = 3 Hz), 39.9 (d, J = 3 Hz), 39.2 (d, J = 3 Hz), 34.5 (d, J = 1 Hz), 33.1 (s), 33.0 (d, J = 3 Hz), 30.7 (s), 16.0 (d, J = 6 Hz), 15.5 (d, J = 7 Hz) ppm. FT-IR (neat): 2962, 2927, 2909, 2872, 2854, 1668, 1648, 1596, 1576, 1540, 1469, 1444, 1412, 1397, 1364, 1258 cm-1. HRMS: calc. for C39H52O5P2Na, [M+Na]+ 685.3188, found 685.3183. Diethyl-(5-benzyl-1-oxido-4-oxo-2-phenyl-1-(2,4,6-tri-tert-butylphenyl)-4,5dihydrophosphol-3-yl)phosphonate, 9a: To a solution of 5a (200 mg, 0.3 mmol) in THF (10 ml) was added excess NaBH4 (ca 10 eq, 115 mg) in one portion, and the reaction mixture was stirred at r.t. until full conversion was determined by 31P NMR (~2 h). The reaction mixture was diluted with diethyl ether and washed with saturated NH4Cl solution, then extracted with diethyl ether (3×25 ml), washed with brine, and dried over MgSO4. The final product was obtained by chromatography on aluminum oxide with DCM (Rf (DCM) = 0.7). Compound 9a is moderately acid sensitive and undergoes fast decomposition when applied to silica gel or treated with CDCl3. Alternatively, crude 6a obtained via mCPBA protocol can be used for this reaction. No difference



in the yield or complications during the purification were observed. Yield: 24%, 47 mg, white solid. 31P {1H} NMR (162 MHz, DCM-d2): δ = 42.9 (d, 3JPP = 63 Hz), 9.3 (d, 3JPP = 63 Hz) ppm. 1H

NMR (400 MHz, DCM-d2): δ = 7.51 – 7.48 (m, 2H), 7.30 – 7.20 (m, 6H), 7.14 – 7.08 (m, 4H),

4.20 - 4.09 (m, 1H), 4.08 – 3.95 (m, 1H), 3.89 – 3.71 (m, 2H), 3.57 (ddd, J = 15 Hz, J = 11 Hz, J = 4 Hz, 1H), 3.42 (dd, J = 9 Hz, J = 4 Hz, 1H), 2.99 (ddd, J = 15 Hz, J = 9 Hz, J = 9 Hz, 1H), 1.28 (s, 18H), 1.16 (t, J = 6 Hz, 3H), 1.15 (s, 9H), 0.89 (t, J = 7 Hz, 3H) ppm. 13C {1H} NMR (101 MHz, DCM-d2): δ 198.4 (dd, J = 22, 10 Hz), 171.1 (dd, J = 68, 3 Hz), 161.1 (s), 153.3 (d, J = 4 Hz), 138.9 (d, J = 9 Hz), 137.0 (dd, J = 177, 8 Hz), 132.1 (dd, J = 6, 6 Hz), 130.8 (dd, J = 5, 1 Hz), 130.7 (d, J = 1 Hz), 129.6 (s), 128.3 (s), 127.5 (s), 126.6 (s), 123.9 (d, J = 13 Hz), 121.7 (dd, J = 101, 3 Hz), 63.5 (d, J = 6 Hz), 62.5 (d, J = 7 Hz), 50.1 (dd, J = 65, 5 Hz), 39.9 (d, J = 3 Hz), 35.0 (d, J = 4 Hz), 34.4 (d, J = 1 Hz), 33.0 (s), 30.7 (s), 16.1 (d, J = 7 Hz), 15.6 (d, J = 7 Hz) ppm. FTIR (neat): 2961, 2924, 2870, 2857, 1678, 1648, 1594, 1540, 1466, 1442, 1431, 1393, 1362, 1259, 1233 cm-1. HRMS: calc. for C39H53O5P2, [M+H]+ 663.3363, found 663.3373. Acknowledgments The authors would like to thank Prof. Dr. Sascha Ott for valuable discussions and support during this project. Dr. Joshua Green is acknowledged for reading the manuscript and polishing the language. Supporting Information containing NMR spectra of all of the compounds, figures of UV-Vis spectra and CV are available free of charge via the Internet at http://pubs.acs.org. References. 1.

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Phospholones from Diacetylenic Ketones: Synthesis, Properties, and

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