The formation of the seven-membered rings is relatively sluggish and an excess of the alkyne was often used to drive the reacton to completion in a reasonable timeframe (four equivalents of alkyne required to form 7 in two days). Although the alkyne is relatively volatile and can be recycled for future experiments, we sought to improve the improve the efficiency of the reation. The rate-determining step for the formation of the 7-membered ring from the fivemembered precursor is the the attack of the electrophilic alkyne by the nitrogen centre (TS3 in Scheme 5). We therefore rationalised that a Lewis acidic borane could bind to the alkyne and activate it (thereby lowering the energy of TS3), with the caveat that the borane would have to be sterically bulky enough to preclude binding to the nucleophilic nitrogen centre on the five-membered ring and quenching its reactivity. We opted to use tris(pentafluorophenyl)borane, B(C6F5)3, which is highly Lewis acidic and relatively bulky, and is widely employed as a main-group Lewis acid catalyst and as a component of frustrated Lewis pair catalysts. Five-membered heterocycle 4 (R = i Pr) was thus combined with one equivalent of (C(CO2Et))2 and one equivalent of B(C6F5)3 in toluene (Scheme 7). The reaction resulted in the rapid formation (<1 hour) of 7•BCF, in which B(C6F5)3 is bound to an ester group in the corresponding seven-membered ring 7. This adduct was characterised by SXRD (Figure ) and multinuclear NMR spectroscopy, with 7•BCF having a subtly different 31 P{ 1 H} NMR chemical shift to 7 (60.0 ppm (7•BCF), 60.4 ppm ( )). The fact that the borane is bound to this particular ester group out of the four available is consistent with the electron-rich nature of this oxygen centre due to the aforementioned resonance structure β (Scheme 3B). The bound borane increases the contribution from resonance β relative to the free 7; this can be seen in the longer P1-C13 bond distance in 7•BCF (1.7791(15) Å vs. 1.7465( ) Å in 7). The 7•BCF could be readily converted to spectroscopically pure 7 by addition of the nucleophilic amine 4-dimethylaminopyridine (DMAP). The same reaction was then monitored with a substoichiometric amount of B(C6F5)3 to assess whether the borance could be used catalytically; the rate of reaction of 4 with one equivalent of (C(CO2Et))2 with 20 mol% of B(C6F5)3 was compared to the same reaction without B(C6F5)3 (Figure ). B(C6F5)3 was indeed observed to have a catalytic effect, with the reaction containing 20 mol% B(C6F5)3 reaching 82% conversion in under two days, and full conversion within five days; whereas the reaction without B(C6F5)3 had reached only 21% and 43% conversion at these same timepoints, respectively. ). Similarly to 4, the inclusion of B(C6F5)3 presumably lowers the enery barrier to the first step by activating the alkyne to attack from 6, allowing 11 to form. However, the binding of the borane would also raise the energy of the second step (attack of the nucleophilic carbon to the phosphorus centre) as the carbon is less nucleophilic. In this case with R = t Bu substituents, the energy barrier for the subsequent steps is presumably raised to such an extent that this now become rate-limiting and inaccessible, preventing formation of the analogous sevenmembered ring, and stopping the reaction at 11. ), C26-C27-C28: 131.9(5). 11 crystallised with two product molecules and one hexane molecule in the asymmetric unit; for clarity, only one product molecule is shown here, and the hexane molecule has been omitted. Thermal ellipsoids were drawn at the 50% probability level. In good agreement with the structure of I2 (and I1) predicted by DFT, 11 features a partially allenic, partially bent structure, although the bound borane increases the contribution from the allenic structure. The N2-C26 [1.440( ) Å] and C26-C27 [1.302(6) Å] bond lengths are consistent with an N-C single bond and C=C double bond, while the C27-C28 [1.384(6) Å], being somewhere single and double bond character. The C26-C27-C28 bond angle is 131.9(5)°, consistent with a geometry somewhere between that of an allene and alkene. In the solution phase, C27 also possesses a very downfield shift (205.9 ppm) in the 13 C{ 1 H} NMR spectrum of 11, in line with other reported allenes. DFT and NBO analyses of the structure of 11 (see SI for details) is in agreement with the observed crystallographic data. The N2-C26 and C26-C27 bonds possess WBI values of 0.96 and 1.82 respectively, while the WBI for C27-C28 (1.36) is indicative of its partial single-and double-bond character. The HOMO of 11 is located on the lone pair of C27, which resides in an sp 3 orbital. 11 can also be viewed as an example of frustrated Lewis pair (FLP) capture of diethyl acetylenedicarboxylate, in which 6 is acting as the Lewis base and B(C6F5)3 acting as the Lewis acid. While capture of alkynes with FLPs is common in the literature, including in FLP-catalysed hydrogenations, non-terminal alkynes predominantly undergo 1,2addition with FLPs to form an alkene-linked zwitterion. The preference here for 1,4-addition is presumably due to the sterically bulky alkyne, and bulky nitrogen centre on 6 disfavouring 1,2-addition, as well as the ability to form a strong boron-oxygen bond via 1,4-addition. The structurally similar alkyne dimethyl acetylenedicarboxylate has indeed been shown to undergo unusual reactivity with FLPs, including 1,4-addition.