Rate constants and Arrhenius parameters for H-atom abstraction from Bu₃SnH by the 2,2-dimethylvinyl radical in PhMe. Kinetic evidence for an entirely free radical mechanism for the O-directed hydrostannation of alkynols with stannanes and Et₃B/O₂

Abstract

Using the 2,2-dimethylvinyl radical 6 as a horological calibrant for the α-cyclopropyl-β-tributylstannylvinyl radicals 2a and 13 in PhMe, the k values and Arrhenius parameters for their cyclopropane ring-openings have been estimated by competition kinetics over a 293–353 K temperature range. The high log A values (14.95 and 14.55) for these reactions only satisfactorily align with a unimolecular, β-scissive, E_H1 radical ring-opening being rate-determining, and the radicals 3a (R = Bu) and 14 undergoing H-atom abstraction from the stannane to give 4a and 15. The log A data for these two reactions only endorse a totally free radical mechanism for the O-directed free radical hydrostannation of dialkyl acetylenes with stannanes and Et₃B/O₂. An estimated k_{H-atom abstraction Bu3SnH PhMe 293 K} of 1.96 × 10⁸ mol⁻¹ s⁻¹ is proposed for 6 in PhMe, along with an estimated k_{H-atom abstraction Ph3SnH PhMe 293 K} of 1.36 × 10⁹ mol⁻¹ s⁻¹.

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Introduction

Quantifying the rate constants and Arrhenius parameters for solution-phase free radical reactions of established synthetic worth is often a highly rewarding endeavour, since such information can frequently guide the design of efficient new synthetic pathways based upon those processes,¹ while also providing important new mechanistic insights² into the detailed inner workings of such reactions.

In that very connection, we recently had cause to kinetically re-investigate the mechanism³ of the O-directed free radical hydrostannation reaction of dialkyl acetylenes⁴ using “radical clock” competition methods,⁵ due to a recent series of papers⁶ having postulated that O₂-generated stannylvinyl cations are key synthetic intermediates in these reactions; these forming from stannylvinyl radical precursors by single electron transfer (SET) to O₂, and subsequently undergoing facile ionic reduction by the stannane, to provide the allylically-oxygenated trisubstituted (Z)-vinylstannane products alongside regenerated O₂ (see section 1.6 of the ESI‡ for more detail).

It was felt that if the O-directed free radical hydrostannation of alkynols 1 and 12 (Schemes 1 and 2) could be studied with Bu₃SnH and cat. Et₃B/O₂ in PhMe, over a fairly wide temperature range, the product allenyltin : vinyltin ratios might yield rate constants and log A values for the ensuing cyclopropane ring-openings. The magnitude of that log A data might then give important clues as to the molecularity of the rate-determining step of these ring-openings, and reveal whether the mechanistic pathway to 4a and 15 was unimolecular, and exclusively free radical in its nature,³ or whether it proceeded via a putative α-cyclopropyl β-stannylvinyl cation and a cationic reduction, as would be advocated by the proponents⁶ of the stannylvinyl cation theory.

Scheme 1 Use of the radical 6 as a calibrating free radical “clock” for α-cyclopropyl-β-stannylvinyl radical probe 2a (R = Bu).

Scheme 2 Synthesis of probe 12 and its k_ring-opening values.

A key assumption in doing such work would be that the intermediary stannylvinyl radicals^4d2a (Scheme 1) and 13 (Scheme 2) would be calibratable with the k_{H-atom abstraction} value for a typical vinyl radical such as 6 from Bu₃SnH in pentane and PhMe.

Although rate constants have long been known for the abstraction of a H-atom from Bu₃SnH by several vinylic radicals,^7,8 only one set of Arrhenius parameters has so far emerged from such studies.⁷ That work is due to Ingold et al.⁷ who measured the rate at which the 2,2-dimethylvinyl radical (6) abstracted a H-atom from Bu₃SnH in pentane; a solvent rarely used in free radical chemistry.

Importantly, Ingold's study⁷ yielded a k_{H-atom abstraction 298 K} value of 2.96 × 10⁸ mol⁻¹ s⁻¹, an E_a of 1.624 ± 0.407 kcal mol⁻¹, and a log A of 9.67 ± 0.33 (A = 4.67 × 10⁹ mol⁻¹ s⁻¹) for this process⁷ (see Scheme 1 and the Ingold ESI‡⁷). Ingold generated his 2,2-dimethylvinyl radical 6 by laser flash photolysis (LFP) of 3-methyl-but-2-enoyl peroxide (8) at 308 nm;⁷ a process now widely accepted^9–12 to produce the highly reactive 6 alongside the much more delocalised and less reactive 3,3-dimethylacryloyloxy radical (9). Both radicals are thought to emerge from a concerted two-bond homolytic cleavage reaction occurring within the photoexcited S₁ form of peroxide 8 ([(Me)₂C=C(H)–C(O)O]₂), on a reaction timescale of 0.4 ps, given recent LFP and CIDNP-NMR studies of related acyl peroxides.^9–12

Most importantly, Ingold's k_{H-atom abstraction Bu3SnH 298 K} value⁷ for 6 aligned very well with Branchi, Galli and Gentili's⁸ independent k determination of 3.7 × 10⁸ mol⁻¹ s⁻¹ for the encounter of a fluorenyl vinyl radical with Bu₃SnH at 298 K in MeCN : MeOH (40 : 60 v/v); the latter radical itself having been generated from a vinylic bromide precursor by LFP. This means that Ingold's log A, E_a and k_{H-atom abstraction Bu3SnH} data⁷ for 6 can be relied upon for k calculations and radical probe calibrations (accepting a 25% level of error in the E_a).

Given the dependability of Ingold's Arrhenius parameters for the 2,2-dimethylvinyl radical (6) in pentane,⁷ we set about using these to horologically calibrate the two stannylvinyl radical reporter probes 2a ^3b,c (R = Bu) and 13 as free radical “clocks”⁵ in PhMe, for a series of competition experiments aimed at establishing the relative rates of the two competing reactions shown in Schemes 1 and 2. Namely: (a) the E_H1 stannylvinyl radical-induced cyclopropane ring-opening of radicals 2a and 13 and (b) the S_H2 H-atom abstraction event involving Bu₃SnH and radicals 2a and 13, to give the vinyltins 5a and 16.

While conceptually analogous to the novel k determinations of Baines,¹³ Newcomb¹⁴ and Crich¹⁵ using other free radical “clocks”,⁵ the two reporter probes, (Z)-2a (R = Bu) (Scheme 1) and 13 (see Scheme 2) are themselves unique and conceptually new, having been purposely designed to allow an estimate of the k values for an event that has hitherto resisted k quantification by other means, namely, the radical ring-opening of α-cyclopropyl-β-tributylstannylvinyl radicals.

Results and discussion

Our precise experimental method is detailed here. It used the 2,2-dimethylvinyl radical (6) as a horological calibrant for the α-cyclopropyl-β-tri-n-butylstannylvinyl radical (Z)-2a (R = Bu) in pentane, with 2a ^3b,c itself being generated by an O-directed free radical hydrostannation^4,5,16–19 of the alkynol 1 ^3b,c with Bu₃SnH/cat. Et₃B^3b,4b,6 over a temperature range of 20–30 °C. Accordingly, at 298 K (25 °C), the radical 2a (R = Bu) was assigned Ingold's k_{H-atom abstraction} value for the reaction of 6 with Bu₃SnH in pentane,⁷ which is 2.96 × 10⁸ mol⁻¹ s⁻¹. From Ingold's log A of 9.67 and his E_a of +1.624 kcal mol⁻¹ (6794.816 J mol⁻¹) for 6,⁷ the corresponding Bu₃SnH k_{H-atom abstraction} values were calculated for 6/2a in pentane at 293 K and 303 K. These calculated values were then used alongside Ingold's experimentally-determined k_{H-atom abstraction} value at 298 K, to allow a reasonably accurate experimental quantification of the k_ring-opening values (Scheme 1) for the α-cyclopropyl-β-tri-n-butylstannylvinyl radical 2a (R = Bu) in pentane at 293, 298 and 303 K using Baines’ proven method for α-cyclopropylvinyl radicals.¹³ The Baines formula of eqn (1) equates the ratio of the vinyltin : allenyltin products in such radical “clock” experiments⁵ to the ratio of the k values for H-atom abstraction and cyclopropane ring-opening:

Of course, the latter expression rearranges to that in eqn (2):

Following collation of the three experimentally-derived values (Scheme 1 entries 1–3) for the log k_{ring-opening2ain pentane}v 1/T in the form of an Arrhenius plot (see ESI‡), it was possible to deduce a log A of 13.274 (frequency factor A = 1.88 × 10¹³ s⁻¹) for the ring-opening of 2a (R = Bu) in pentane, and a mean E_a of +7.18 kcal mol⁻¹. The high magnitude of the log A for this ring-opening of 2a (R = Bu) unambiguously confirmed that it was a unimolecular E_H1 free radical ring cleavage process that was leading to the radical 3a (R = Bu), which then H-atom abstracted from the Bu₃SnH. Such a log A most definitely did not align with a stannylvinyl cation E1-ring-opening/reduction mechanism having led to 4a,⁶ nor a bimolecular S_N2 stannylvinyl cation reduction, as would be invoked by advocates of the stannylvinyl cation mechanistic theory⁶ (see ESI‡).

Significantly, however, our experimentally-derived k_ring-opening value of 9.47 × 10⁷ s⁻¹ for 2a (R = Bu) in pentane at 298 K, and its accompanying log A of 13.274, did align very satisfactorily with Newcomb's k_ring-opening value^14a of 1.0 × 10⁸ s⁻¹ for the cyclopropylcarbinyl radical in THF at 298 K, and the log A of 13.15 that these workers reported for this process, which lends considerable confidence to the entirely free radical mechanistic proposal that is being advanced here (see Scheme 1).

By comparing the experimentally-derived vinyltin : allenyltin ratios 5a : 4a (R = Bu) for the hydrostannation of 1 in pentane at 273, 298 and 303 K with the corresponding data gathered in PhMe, we were able to show that the rate of H-atom abstraction from Bu₃SnH by the stannylvinyl radical 2a (R = Bu)/6 is approximately 1.47 times slower in PhMe than it is in pentane, which confirmed a noticeable solvent effect. Moreover, when the experimentally-determined rate constants obtained for 2a (R = Bu)/6 in PhMe were collated in the form of an Arrhenius plot (see ESI‡), this led to an E_a of +1.599 kcal mol⁻¹ (i.e. 1.6 kcal mol⁻¹) or 6693.84 J mol⁻¹ being determined for the H-atom abstraction event involving 6/2a and Bu₃SnH in PhMe. The resulting log A of 9.4826 (A = 3.04 × 10⁹ mol⁻¹ s⁻¹) also allowed a ΔS^‡_{298 K} of −17.148 e.u. or −71.75 J K⁻¹ mol⁻¹ to be deduced, which showed that the rate-determining step for this H-atom transfer was bimolecular and S_H2.

From the experimentally-derived log A (9.4826 i.e. 9.48) and E_a (6693.84 J mol⁻¹) data gathered on 2a (R = Bu) in PhMe, the theoretical k_{H-atom abstraction} values could now be calculated for the reaction of the 2,2-dimethylvinyl radical 6/2a with Bu₃SnH in PhMe at the higher temperatures of 313, 333 and 353 K (see Scheme 1). The availability of this log A and these k_{H-atom abstraction} values now allowed a complete experimental determination of the k_ring-opening values for the α-cyclopropyl-β-tri-n-butylstannylvinyl radical 2a (R = Bu) in PhMe over the temperature range 20–80 °C (293–353 K) at 0.2 M Bu₃SnH concentration, and this k data is tabulated in Scheme 1.

An Arrhenius plot of the experimentally-derived log k_ring-opening data for 2a (R = Bu) in PhMe vs. 1/T gave a straight line output (see Fig. 1 and ESI‡) from which a log A of 14.951 (A = 8.93 × 10¹⁴ s⁻¹) and an E_a of +9.47 kcal mol⁻¹ (i.e. 9.5 kcal mol⁻¹) could both be deduced for the ring-opening of 2a over the 293–353 K temperature range studied. The high mean log A for this cyclopropane ring-opening, and its substantially sized positive entropy of activation at 333 K (ΔS^‡_{333 K} = +32.09 J K⁻¹ mol⁻¹ or +7.67 e.u.) both immediately ruled out a stannylvinyl cation E1-ring-opening/reduction or a bimolecular ionic reduction mechanism⁶as having led to4a (see section 1.6 of the ESI‡ for an in depth discussion of these two invalid ionic mechanisms). Observations that were further supported by our previous unsuccessful cation-trappings with H₂O in 4 : 1 THF : H₂O.^3b,c

Fig. 1 Arrhenius plot of log k_ring-opening of 2avs. 1/T from the reaction of 1 with Bu₃SnH/cat. Et₃B/O₂ in PhMe over 293–353 K.

Instead, our newly derived kinetic parameters only satisfactorily aligned with an entirely homolytic, unimolecular, E_H1 fissive mechanism operating in the rate-determining step (Scheme 1), in which a very loose activated complex of the radical 2 was singularly transforming into the radical 3avia a product-like transition state in which cyclopropane bond-cleavage was already very advanced. The resulting stannylhomoallenyl radical 3a then H-atom abstracted from the Bu₃SnH to ultimately yield 4a.

Critically, the log A for this ring-opening of the α-cyclopropyl-β-stannylvinyl radical 2a (R = Bu) in PhMe aligned very well with the typical log A values (13.29–16.11) recorded by Frey²⁰ for the unimolecular gas phase pyrolytic C–C bond homolyses of various cyclopropanes, which are always associated with large positive ΔS^‡ values, due to the increased bond-loosening and much greater mobility that is experienced by such activated cyclopropane rings as they fissively transit into their initial biradical products.

We next elected to synthesize the sterically less encumbered chiral cyclopropylpropargylic alcohol 12 by the route shown in Scheme 2. This featured a catalytic Carreira alkynylation²¹ as a key step. The alkynol 12 was then subjected to an O-directed hydrostannation^4,6,16–19 with Bu₃SnH/cat. Et₃B in PhMe, to generate 13, which now permitted an estimate of the k_ring-opening for its cyclopropane ring over a range of temperatures (Scheme 2).

Once again, it was assumed that the k_{H-atom abstraction} values for 13 would very closely mirror those for 2a/6. If one is prepared to accept this key kinetic assumption, with the usual experimental caveats of course, then an Arrhenius plot of the resulting log k_ring-opening data v 1/T (see Fig. 2) reveals a log A of 14.549 (A = 3.54 × 10¹⁴ s⁻¹), a ΔS^‡_{333 K} of +24.39 J K⁻¹ mol⁻¹ (+5.83 e.u.), and an E_a of +9.92 kcal mol⁻¹ (i.e. +9.9 kcal mol⁻¹).

Fig. 2 Arrhenius plot of log k_ring-opening of 13vs. 1/T from the reaction of 12 with Bu₃SnH/cat. Et₃B/O₂ in PhMe over 313–353 K.

Critically, the above log A and ΔS^‡_{333 K} data definitively ruled out a stannylvinyl cation reduction mechanism⁶ as having afforded 15 (see section 2.2 of the ESI‡ for a more detailed and in depth discussion of this point).

Significant also was the fact that our experimentally derived E_a of +9.9 kcal mol⁻¹ was close in magnitude to the E_a of +10.7 kcal mol⁻¹ calculated by Guo et al.²² for the closely related unimolecular radical-induced ring-opening²² of radical 17 (Scheme 3).

Scheme 3 Guo's calculations for the ring-opening of radical 17.²²

While it is tempting to try to estimate the k values for the reaction of the β-triphenylstannylvinyl radical 2b (R = Ph) (Scheme 1) with Ph₃SnH at different temperatures, by assuming that the k_ring-opening values for 2a and 2b would be identical, current EPR evidence suggests that β-triphenylstannylvinyl radicals are much more highly stabilised^3dand potentially far less reactive than their β-trialkylstannylvinyl radical counterparts, which are generally unobservable by low temperature EPR spectroscopy.²³

This is not the case with β-triphenylstannylvinyl radicals^3d generated by the O-directed alkyne hydrostannation with Ph₃SnH/cat. Et₃B/O₂.⁴ A process that has now allowed many such radicals to be routinely observed by EPR spectroscopy at low temperatures in PhMe and THF,^3d due to the much greater lifetimes of β-triphenylstannylvinyl radicals in solution, even in the presence of excess Ph₃SnH.

Possibly this enhanced longevity and much greater stability of β-triphenylstannylvinyl radicals is due to increased negative hyperconjugation (SOMO → σ*_C–Sn) in such radicals (due to the electron-withdrawing Ph groups present on the Sn), as well as the reduced positive σ_C–Sn → SOMO hyperconjugation they experience.^17e,f,g

Now even though it is not possible to reliably use the k_ring-opening values for 2a to directly calibrate 2b, a simple relative comparison of the 4b : 5b product ratio of entry 10 in Scheme 1 with the ratio of 4a : 5a obtained in entry 4, does suggest that the 2,2-dimethylvinyl radical 6 will likely react with Ph₃SnH at a rate which is at least 6.95 times faster than the corresponding reaction with Bu₃SnH in PhMe at 20 °C. This, in turn, points to a k_{H-atom abstraction} value of no less than 1.36 × 10⁹ mol⁻¹ s⁻¹ for 6 and 2b from Ph₃SnH in PhMe (Scheme 1). While this relative k_{H-atom abstraction} value for Ph₃SnH can only ever be considered tentative, and a conservative minimal estimate at best, it does nevertheless confirm that such H-atom transfers do proceed at a very fast rate that is approximately an order of magnitude less than a diffusion-controlled reaction in PhMe (k_{diffusion PhMe 293 K} = 1.101 × 10¹⁰ mol⁻¹ s⁻¹). The availability of this k_{H-atom abstraction Ph3SnH} value for 2b/6 has allowed a tentative estimate of the k_ring-opening value for 2b, which has clearly confirmed that the radical 2b has a lower level of reactivity with respect to its unimolecular ring-opening than 2a.

Our collective findings to date do very strongly suggest that it is the fast rate of formation and trapping of β-triphenylstannylvinyl radicals, and their much lower tendency to β-scissively revert back into the starting propargyloxy O-coordinated tin radical, that is responsible for Ph₃SnH generally outperforming Bu₃SnH^6,18 as a hydrostannylating reagent with most propargylically-oxygenated dialkylacetylene substrates under the rt Et₃B-initiated reaction conditions.

It is also pertinent to point out that just because β-tributylstannylvinyl radicals are far less stable and more reactive than their β-triphenylstannylvinyl radical counterparts, this does not necessarily impose on them the requirement to preferentially engage in a fast bimolecular H-atom abstraction event with Bu₃SnH. Such enhanced reactivity for β-tributylstannylvinyl radicals could manifest itself in other ways, such as through increased unimolecular β-scissive dissociation back into the starting alkyne in the form of its O-complexed Bu₃Sn radical. This, in turn, might explain the generally lower levels of conversion^4a,b,24 that one typically sees with Bu₃SnH/cat. Et₃B in most O-directed⁴ and non-directed²⁴ alkyne free radical hydrostannations.

Although the latter may be synthetically detrimental to a significant number of intended applications,⁴ equally well, the enhanced reactivity of many β-tributylstannylvinyl radicals might sometimes be of direct benefit to certain tandem radical cyclisation processes.¹⁷ One case in point is Alabugin's brilliant O-directed hydrostannylative route to benzofluorenes from oligoalkynes,^17a where Bu₃SnH/AIBN was found to vastly outperform Ph₃SnH/AIBN in PhMe in the tandem stannylvinyl radical cyclisation process conducted on a diyne model test substrate (86% yield vs. 40% yield). However, for most rt O-directed⁴ and non-directed²⁴ dialkylacetylene hydrostannations with Et₃B initiation, it is Ph₃SnH^4a,b that usually outperforms Bu₃SnH, and this enhanced performance is almost certainly attributable to the higher stability of most β-triphenylstannylvinyl radical intermediates, which allows for their much more effective bimolecular trapping by the Ph₃SnH at the fast, near diffusion-controlled, rates that we are seeing here.

Of further note in our current studies is the significant 5-fold rate acceleration seen for the ring-opening of 2a at 80 °C (k_ring-opening = 1.29 × 10⁹ mol⁻¹ s⁻¹) relative to 13 (k_ring-opening = 2.66 × 10⁸ mol⁻¹ s⁻¹). Such a marked increase in the rate of ring-opening of 2a possibly points to the potential constant recurrence of temporary transient internal MeO–C=O :  → Sn electron-donating events helping to accelerate the E_H1 cyclopropane ring-opening event, by strongly reinforcing the σ_C–Sn → SOMO positive hyperconjugative interaction.¹⁷ Such Thorpe–Ingold-induced internal coordination in 2a might also be impeding the aforementioned reverse unimolecular β-scissive (R)₃Sn˙ elimination back into the starting alkyne O-coordinated tin radical. Also, the much lower tendency of the stannylvinyl radical 2b (R = Ph) to engage in E_H1 elimination to give the ring-cleaved 3b might simply be a reflection of the much higher stability of 2b, reduced conformational mobility induced by the Ph₃Sn group, and the superior H-donor power of Ph₃SnH. While our k_ring-opening and k_{H-atom abstraction} data for 2a and 13 in PhMe are all based on Ingold's k and log A data for 6 in pentane,⁷ clearly, our values will potentially be modifiable in the future, should improved k calibration data appear.

Conclusions

We expect that our new k_{H-atom abstraction} data for the reaction of the 2,2-dimethylvinyl radical (6) with Bu₃SnH and Ph₃SnH in PhMe will aid much future synthetic planning with vinyl radicals in the commonly used solvent PhMe.

Significantly, our new kinetic and log A work on the cyclopropane ring-openings of the β-stannylvinyl radicals derived from the probes 1 and 12 have further ruled out the hypothesised intermediacy of stannylvinyl cations⁶ in these Et₃B/O₂ radical-initiated alkyne hydrostannation reactions and, as such, the present work has confirmed an entirely free radical mechanism³ for the O-directed free radical hydrostannation of propargylically-oxygenated dialkylacetylenes (see sections 1.5 and 1.6 of the ESI‡ for more detailed discussion).⁴

In the paper that accompanies this,²⁵ other probe trapping studies will be described in THF : H₂O that further invalidate the stannylvinyl cationic mechanistic theory⁶ of alkyne hydrostannation under the Et₃B/O₂-initiated reaction conditions. This work and the EPR studies that accompany it²⁵ provide further new insights into the complex mechanistic events that proceed alongside these highly stereoselective, entirely free radical, O-directed hydrostannation reactions.^3a,26

Experimental

General information

Unless stated otherwise, all reactions were run in dry solvents under an N₂ atmosphere. Dry pentane was freshly distilled from CaH₂ under an N₂ atmosphere and dry PhMe was used as supplied by Sigma-Aldrich. Both anhydrous solvents were taken out by dry syringe under an N₂ atmosphere. Ph₃SnH was purchased from Sigma-Aldrich and used as supplied; it was always handled in a glove-bag under N₂. Bu₃SnH was purchased from Alfa and was used as supplied. It was also periodically tested on a known thiocarbonyl imidazolide substrate that typically deoxygenates in >95% yield; if a yield of this magnitude was obtained, then the Bu₃SnH was used for the experiments reported. SiO₂ flash chromatography was carried out using Fluorochem silica gel 60 Å, and petrol refers to the 40–60 °C b.p. fraction; it was distilled prior to use for chromatography. HPLC grade EtOAc was used for all chromatographic purifications. TLC analysis and preparative TLC were performed on Merck glass-backed TLC plates coated with silica gel 60 F₂₅₄. NMR analyses were carried out using the QUB School of Chemistry Bruker Avance III HD Ascend 600 instrument operating at a frequency of 600.1337 MHz. Although the 600.13 MHz ¹H spectra of 4a and 5a in CDCl₃ (referenced upon tetramethylsilane (TMS) at δ 0.00 ppm, residual CHCl₃ at δ 7.23 ppm) were previously published in ref. 3c (see: H. A. Watson, S. Manaviazar, H. G. Steeds and K. J. Hale, Tetrahedron, 2020, 76, 131061), we have included these spectra here in considerably abridged form, along with some of the previous spectra of 4b and 5b, in order to allow the readers of the present paper to conveniently gauge the new kinetic ratio determinations that we are presenting here for the very first time. Clearly, there are minor changes in the chemical shifts observed, in the new spectra, as one would expect.

Experimental procedures for generating α-stannylvinyl radical 2a and stannyl homoallenyl radical 3aen route to 4a and 5a

General procedure for the O-directed hydrostannation of 1 with Bu₃SnH in pentane at various temperatures to obtain the 4a : 5a ratio.
To a round-bottomed flask containing a well-stirred solution of the cyclopropylacetylenic alcohol 1 (196.2 mg, 1 mmol) in dry pentane (10 mL) under N₂ was added Bu₃SnH (0.54 mL, 2 mmol) dropwise via syringe over 1 min. To this stirred mixture at the desired temperature (20, 25 and 30 °C) was successively added Et₃B (0.1 mL, 1 M in hex, 0.1 mmol, 0.1 equiv.) dropwise via syringe followed by air (5 mL) from a syringe 5 min later. The reactants were stirred at the requisite temperature for 24 h, after which, the reaction flask was transferred to a rotary evaporator and the solvent removed in vacuo. A ¹H NMR spectrum was recorded of a portion of the crude reaction mixture in CDCl₃ to ascertain the crude ratio of products. Each reaction temperature was examined a minimum of three times and the average product ratio of 4a : 5a was taken to determine of the rate constant k_ring-opening for the (Z)-2a→3a (R = Bu) conversion at the designated temperature.

General procedure for the O-directed hydrostannation of 1 with Bu₃SnH in PhMe at various temperatures to obtain the 4a : 5a ratio.
To a small round-bottomed flask containing a well-stirred solution of the cyclopropylacetylenic alcohol 1 (196.2 mg, 1 mmol) in dry PhMe (10 mL) under N₂ was added Bu₃SnH (0.54 mL, 2 mmol) dropwise via syringe over 1 min. To this stirred mixture at the desired temperature (20, 25, 30, 40, 60 and 80 °C) was successively added Et₃B (0.1 mL, 1 M in hex, 0.1 mmol) (0.1 equiv.) dropwise via syringe followed by air (5 mL, from a syringe) 5 min later. The reactants were then maintained at the desired temperature with stirring for 24 h, after which, the reaction flask was transferred to a rotary evaporator and solvent removed in vacuo. A ¹H NMR spectrum was recorded of a portion of the crude reaction mixture in CDCl₃ to ascertain the crude ratio of products. Each reaction temperature was examined a minimum of 2–4 times and the average product ratio of 4a : 5a (R = Bu) was taken to determine of k_ring-opening for the (Z)-2a→3a conversion in PhMe at the designated temperature.

Synthetic route to the (R)-1-(tert-butyldiphenylsilyloxy)-4-cyclopropylbut-3-yn-2-ol (12)

Synthesis of aldehyde 11.
To a round-bottomed flask containing ethylene glycol (20 mL, 357.7 mmol, 7 equiv.) in dry CH₂Cl₂ (200 mL) under N₂ was added imidazole (6.69 g, 102.2 mmol, 2 equiv.) in one portion with vigorous stirring. THF (40 mL) was then added via syringe, and the reaction mixture was cooled to 0 °C using an ice bath. t-Butyldiphenylsilyl chloride (13.3 mL, 51.146 mmol, 1 equiv.) was then added dropwise over 30 min via syringe. When the addition was complete, the ice bath was removed and the reactants were allowed to stir at rt for 18 h before the reaction was diluted with CH₂Cl₂ (200 mL) and quenched with saturated aq. NaHCO₃ solution (100 mL) and H₂O (200 mL). The aqueous layer was washed with CH₂Cl₂ (50 mL × 3) and the combined organic layers were dried with MgSO₄, filtered and concentrated in vacuo. The oily residue was purified by gradient elution SiO₂ flash chromatography with petrol–EtOAc (50 : 1 → 25 : 1 → 20 : 1 → 10 : 1 → 5 : 1) to give the O–silyl ether 17 (10.67 g, 69%) as a slightly impure oil. This technical grade alcohol 17 was then used directly for the oxidation step.

To a stirred −78 °C solution of (COCl)₂ (2.83 mL, 33.05 mmol, 1 equiv.) in dry CH₂Cl₂ (187 mL) under N₂ was added DMSO (4.7 mL, 66.1 mmol, 2 equiv.) dropwise via syringe over 3 min. Stirring was continued at −78 °C for a further 30 min before a solution of the aforementioned alcohol 17 (9.93 g, 33.05 mmol, 1 equiv.) in dry CH₂Cl₂ (20 mL) was added dropwise via syringe over 15 min. After a further 7 min of stirring at −78 °C, Et₃N (20.7 mL, 148.717 mmol, 4.5 equiv.) was added dropwise over 3 min and the reaction mixture then allowed to warm from −78 °C to rt, whereupon it was stirred for 2 h. The solvents were then removed in vacuo on the rotary evaporator. The crude residue of the aldehyde 11 was then suspended in petrol–EtOAc (4 : 1, 500 mL), and the solid Et₃NHCl filtered off under vacuum. The filtrate was concentrated in vacuo and the syrupy residue was purified by gradient elution SiO₂ flash chromatography with petrol–EtOAc (20 : 1 → 10 : 1) to give the aldehyde 11 (6.73 g, 68%) as an oil.

(R)-1-(tert-Butyldiphenylsilyloxy)-4-cyclopropylbut-3-yn-2-ol (12).
To solid Zn(OTf)₂ (2.05 g, 5.649 mmol, 0.3 equiv.) and (−)-N-methylephedrine (1.35 g, 7.532 mmol, 0.4 equiv.) in a small pear-shaped flask under N₂ was successively added PhMe (20 mL) and Et₃N (3.94 mL, 28.246 mmol, 1.5 equiv.) by syringe. Cyclopropylacetylene (5.62 mL, 66.284 mmol, 3.52 equiv.) was then added by syringe maintaining the N₂ atmosphere throughout. The reactants were stirred vigorously at rt for 2 h whereafter a solution of aldehyde 11 (5.62 g, 18.831 mmol) (which had been pre-dried by coevaporation from PhMe × 2) in PhMe (7.1 mL) was added via syringe, along with a 1 mL rinse of the flask with more dry PhMe. The flask containing the reactants was next transferred to an oil bath and vigorously stirred at 40 °C for 22 h. The reaction mixture was then quenched by the addition of saturated aq. NH₄Cl solution (50 mL) and diluted with EtOAc (50 mL). The organic extract was separated, and the aqueous layer was further extracted with more EtOAc (2 × 50 mL). The combined organic extracts were washed with H₂O (50 mL), dried over MgSO₄, filtered and concentrated in vacuo. The crude residue was purified by SiO₂ flash chromatography with petrol–EtOAc (25 : 1) to give the alkynol 12 (4.33 g, 63%) as a thick oil. ¹H NMR of 12 (600.13 MHz, CDCl₃) δ: 7.73–7.64 (m, 4H, –Ph), 7.46–7.34 (m, 6H, Ph), 4.43 (m, 1H, H2), 3.765–3.75 (dd, J = 10.2, 3.6 Hz, 1H, H1a), 3.67 (dd, J = 10.2, 6.6 Hz, 1H, H1b), 2.58 (d, J = 5.4 Hz, 1H, –OH), 1.23 (m, 1H, H5), 1.07 (s, 9H, t-Bu), 0.74 (m, 1H, H6a), 0.66 (m, 1H, H6b). ¹³C NMR of 12 (150.9 MHz, CDCl₃) δ: 135.6 (m-CH of Ph), 135.5 (m-CH of Ph), 133.0 (q of Ph), 132.9 (q of Ph), 129.9 (p-CH of Ph), 129.8 (p-CH of Ph), 127.8 (o-CH of Ph), 127.77 (o-CH of Ph), 89.4 (C3), 73.2 (C4), 67.9 (C2), 63.2 (C1), 26.8 (Me groups of t-Bu), 26.6 (C5), 19.2 (q carbon of t-Bu), 8.11 (C6a), 8.10 (C6b) ppm.

When run under identical conditions on 1 g (3.351 mmol) scale, with respect to aldehyde 11, the yield of 12 (0.87 g) was found to improve to 71%, possibly due to improved stirring.

General procedure for the O-directed hydrostannation of alkynol 12 with Bu₃SnH in PhMe at various temperatures to obtain the 15 : 16 ratio

For each of these kinetic runs, a 1 M solution of Et₃B in PhMe was freshly prepared by addition of Et₃B (0.2 mL, 1 M solution in hexanes) to dry PhMe (2 mL) under N₂; an aliquot of that solution was then taken and used as the reaction initiator, adhering to the general procedure set out below.

A small pear-shaped flask was charged with the alkynol 12 (100.0 mg, 0.275 mmol) and the contents of this flask were co-evaporated twice from dry PhMe (5 mL). After the second evaporation had taken place, a N₂ atmosphere was introduced into the flask, whilst it was attached to the rotary evaporator. Whilst maintaining the counter-flow of N₂ from the N₂-filled balloon connected to the rotary evaporator, an open 3-way tap, fitted with an N₂-filled balloon emitting N₂, was used to cap the reaction flask that was being removed, to preserve the N₂ atmosphere inside the flask. That flask was then placed under high vacuum for 30 min, whereafter a N₂ atmosphere was re-introduced by means of the 3-way tap (which now had a rubber septum fitted to its vertical gas inlet). To that dried residue of the 12 was added dry PhMe (2.64 mL) via syringe, followed by Bu₃SnH (0.15 mL, 0.55 mmol), and the reactants were stirred at rt to ensure proper mixing. The flask containing 12, Bu₃SnH and PhMe was then placed in an oil bath at the requisite temperature between 40 and 80 °C, and a small aliquot of Et₃B (0.1 mL, 1 M solution in hex, ca. 0.1 equiv.) was added dropwise over several seconds. Air (5 mL) from a syringe was then introduced into the reaction vessel, whilst the N₂ atmosphere was maintained. The reactants were then stirred at the requisite temperature for reaction times that varied between 19–21 hours, before they were concentrated in vacuo. In all cases, TLC analysis indicated that the reactions did not progress much further after 1.5–2 h, and starting alkynol 12 always remained at reaction end, but the prolonged heating did help to decompose the tin and borane by-products, to make the crude NMR analysis easier. The allenyltin and vinyltin products 15 and 16 were much faster-moving than the starting alkynol 12, and the allenyltin diastereomers 15 were themselves slightly faster-moving than the vinyltin product 16 on TLC. The ratio of 15 : 16 in the crude concentrated reaction mixture was then determined by high field NMR spectroscopy in CDCl₃ and this ratio was subsequently used alongside the theoretical or experimentally determined k_{H-atom abstraction} values in Scheme 2, to determine the k_ring-opening values for the conversion of 13 into 14. Each reaction temperature was examined a minimum of 2–4 times and the average product ratio of 15 : 16 was taken to determine of k_ring-opening for the 13 into 14 conversion in PhMe at the designated temperature.

In an attempt to obtain analytically pure samples of the two products 15 and 16, several of the aforementioned crude reaction mixtures were combined and partially purified by gradient-elution SiO₂ flash chromatography using petrol–EtOAc (80 : 1 → 40 : 1 → 20 : 1 → 10 : 1) as the eluent. A second flash chromatographic purification of this partially purified mixture (enriched in the stannylallene 15) was then performed with petrol–Et₂O (150 : 1 → 100 : 1) as the eluent, to isolate 15 in reasonably pure condition. A third analytical column with neat CH₂Cl₂ was then performed to allow isolation of the allene 15 as a 1 : 1 diastereomeric mixture in near pure condition. The spectral data for this mixture of the two diastereosiomers of 15 is reported now in full: ¹H NMR of 15 (600.13 MHz, CDCl₃) δ: 7.71–7.63 (m, 4H, Ph), 7.46–7.34 (m, 6H, Ph), 4.77 (td, J = 6.6 and 3.0 Hz, 1H, H5 geometric isomer 1), 4.715 (td, J = 6.6 and 3.0 Hz, 1H, H5 geometric isomer 2), 4.383 (complex m, 1H, H2 both diastereomers), 3.68 (dd, J = 10.2, 3.0 Hz, 1H, H1a diastereomer 1) partially superimposed upon 3.66 (dd, J = 10.2, 3.6 Hz, 1H, H1a diastereomer 2), 3.49 (dd, J = 8.4, 4.8 Hz, 1H, H1b, diastereomer 1) partially superimposed upon 3.48 (dd, J = 8.4, 4.8 Hz, 1H, H1b diastereomer 2), 2.71 (d, J = 3.0 Hz, 1H, OH, diastereomer 1) superimposed upon 2.706 (d, J = 3.0 Hz, 1H, OH, diastereomer 2), 1.64 and 1.45 (m, 2H, H6a, H6b both diastereomers), 1.40–1.23 (complex m, 18 H, –CH₂– regions of Bu₃Sn, both diastereomers), 1.066 and 1.064 (2 × s, 9H, t-Bu, TBDPS, both diastereomers), 0.92 (t, J = 7.8 Hz, 9H, Me of Bu₃Sn, superimposed upon m, 3H, H7, diastereomer 1), 0.86 (t, J = 7.2 Hz, 9H, Me of Bu₃Sn, superimposed upon m, 3H, H7, diastereomer 2) ppm. ¹³C NMR of 15 (150.9 MHz, CDCl₃) δ: 200.77 and 200.64 (1 × C4, both diastereomers), 135.56 and 135.54 (2 × m-CH of Ph, both diastereomers), 133.3 (1 × quaternary C of Ph, both diastereomers), 129.74 and 129.72 (1 × p-CH of Ph, both diastereomers), 127.71 (2 × o-CH of Ph carbons of both diastereomers) 96.28 and 96.22 (1 × C5, both diastereomers), 86.17 and 86.07 (1 × C3, both diastereomers), 72.94 and 72.73 (C2, both diastereomers), 68.74 and 68.43 (1 × C1, both diastereomers), 29.0, 27.84 and 27.28 (–CH₂– groups of Bu₃Sn, both diastereomers), 26.86 and 26.83 (t-Bu, both diastereomers), 21.65 and 21.60 (C6, of both diastereomers), 19.2 (quaternary C, t-Bu), 17.51 (–CH₂– groups of Bu₃Sn, ¹J¹¹⁹Sn¹³C = 336.5 Hz, ¹J¹¹⁷Sn¹³C = 321.4 Hz, –SnC̲H₂– of Bu₃Sn, both diastereomers), 14.0 and 13.9 (C7–Me of both diastereomers) 13.69, 13.65 and 13.59 (Me groups of Bu₃Sn groups, both diastereomers), 10.88 and 10.83 (CH₂– of Bu₃Sn, both diastereomers) ppm.

Unfortunately, we were never able to obtain a satisfactory ¹H NMR spectrum of the pure vinyltin product 16 of the hydrostannation of 12. Nonetheless, this did not prove especially problematical for the kinetic task at hand, since it was possible to readily determine the crude ratios of 15 : 16 from the ¹H NMR spectra run of the crude reaction mixtures. In this regard, the olefinic H4 peak of the vinyltin 16 clearly stood out, it resonating as a dd (J = 10.2 and 1.2 Hz) at δ 5.55 ppm in CDCl₃. Its identity was readily confirmed by the small allylic coupling between H4 and H2 (⁴J = 1.2 H), and the large J coupling (³J = 10.2 Hz) with the cyclopropane CH (H5). The vinyltin geometry could be readily assigned from the large ^119/117Sn–¹H J couplings (¹¹⁹Sn–¹H = 131.4 and 111.6 Hz) that accompanied this resonance.

General procedure for the O-directed hydrostannation of 1 with Ph₃SnH in PhMe at various temperatures to obtain the 4b : 5b ratio

A 1 M solution of Ph₃SnH in PhMe was prepared by accurately weighing out Ph₃SnH, into an open-necked round-bottomed flask containing a magnetic stirring bar, inside a glove bag filled with dry N₂. The reaction vessel was then capped with a closed 3-way tap possessing a Quickfit male joint, while still inside the glove bag. The sealed flask was then removed from the glove bag and connected to a vacuum line via a 3-way tap, which was also fitted with an N₂-filled balloon. The reaction flask was then sequentially evacuated and purged with N₂ from the balloon before it was clamped over a magnetic stirrer. Dry PhMe was then added to give a 1 M solution. An aliquot of that freshly prepared solution of Ph₃SnH (2 mL, 1 M in PhMe, 2 mmol) was then added to the flask containing the acetylene 1 (196.2 mg, 1 mmol) and a magnetic stirring bar under N₂. To this stirred mixture of the Ph₃SnH and 1 at the desired temperature (20, 40, 60 and 80 °C) was then added Et₃B (0.1 mL, 1 M in hex, 0.1 mmol) (0.1 equiv.) dropwise via syringe, followed by air (5 mL, from a syringe) 5 min later. The reactants were then stirred at the designated temperature for 24 h, after which, the reaction flask was transferred to a rotary evaporator and the solvent removed in vacuo. A ¹H NMR spectrum was recorded of a tiny portion of the crude reaction mixture in CDCl₃ to ascertain the crude ratio of products. The remaining crude concentrated residue was then purified by gradient-elution SiO₂ flash chromatography using initially 3 : 1 → 2 : 1 → 1 : 1 petrol : CH₂Cl₂ to remove excess tin hydride, and then 30 : 1 petrol : EtOAc to yield the allenylstannane product 4b as a clear oil. Finally the eluent was changed to 25 : 1 petrol : EtOAc to obtain the essentially pure vinylstannane product 5b as a white amorphous solid. Each reaction temperature was examined a minimum of two/four times and the average product ratio of 4b : 5b (R = Ph) was taken. This protocol allowed estimation of the k_{H-atom abstraction Ph3SnH} (i.e.(Z)-2b→5b [R = Ph]) at 293 K (20 °C).

Data availability

The experimental data supporting this article can be found in the Experimental section of this paper and in the ESI.‡ The ESI‡ provides NMR spectra and product ratio determinations for 4a : 5a, 4b : 5b and 15 : 16. The ESI‡ also contains the theoretical rate constant calculations that were performed, and our experimental rate constant determinations, and the Arrhenius Plots that were associated with these studies in Excel format. The ESI‡ also provides details of how the log A, E_a, ΔS^‡ and ΔG^‡ data were calculated from the experimentally-derived data gathered in these plots. Finally, the ESI‡ contains a detailed mechanistic interpretation of the new kinetic data gathered. Citations to references 26–43 can be found in the ESI.‡

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the Leverhulme Trust (RPG-2015-438), QUB, LJMU, and the NI DfE for financial support (Ph.D Studentship to HAW), and Dr Joseph S. Vyle of QUB for generous gifts of chemicals.

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Footnotes

† Dedicated to the memory of Dr Clive W. Bird FRSC, former Reader in Chemistry at King's College London (University of London); a truly outstanding organic chemist of extraordinary chemical insight and teaching ability. Clive was a genuinely good human being who helped all around him. He was inventor of the now famous “Bird Aromaticity Index”.

‡ Electronic supplementary information (ESI) available: Full experimental details, calculations and NMR data supporting the work. See DOI: https://doi.org/10.1039/d4ob01846j

§ Current address: Halazar Pharma Ltd, Edgware, Middlesex, HA8 7RB, UK. E-mail: E-mail: k.hale120@btinternet.com

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