the nickel - catalyzed 
 reductive coupling of aldehydes with alkynes 
 has been widely studied in both intra- and intermolecular forms across 
 a broad range of substrates , reducing agents , and ligands . numerous reports employing organozinc , organoborane , and organosilane reducing agents with phosphine and n - heterocyclic carbene ( nhc ) ligands have appeared . in the first report 
 of nickel - catalyzed aldehyde alkyne reductive couplings , the 
 reductive cyclization of an ynal was demonstrated employing phosphine 
 ligands and diethylzinc as the reducing agent . in that report , the generation of a metallacyclic intermediate 
 derived from oxidative cyclization of ni(0 ) with the ynal substrate , 
 followed by conversion of the metallacycle to product by a transmetalation/-hydride 
 elimination / reductive elimination sequence , was proposed . many subsequent 
 developments in nickel - catalyzed reductive couplings of aldehydes 
 and alkynes have suggested related mechanistic pathways involving 
 metallacyclic intermediates , followed by a late - stage involvement 
 of the reducing agent . more recent developments described procedures 
 of broader scope involving different reducing agent and ligand combinations , 
 such as the use of et3b - mediated couplings with phosphine 
 ligands from jamison and the use of silanes 
 with n - heterocyclic carbene ligands from our laboratories . these improved methods allow for challenging intermolecular couplings and macrocyclizations , whereas the prior advances were largely limited to five- and six - membered 
 ring cyclizations . the pathway for the generally proposed mechanism 
 involving silane reducing agents , which is the focus of this study , 
 is depicted in scheme 1 . key pieces of data have largely supported the metallacycle 
 mechanism 
 in reactions of this type . stoichiometric generation of the metallacycle 
 derived from ni(cod)2 , pcy3 , benzaldehyde , and 
 butyne was described by ogoshi ( scheme 2 ) . in this case , a dimeric form of the metallacycle 
 was characterized by crystallographic analysis , although no implications 
 of metallacycle aggregation have been suggested as a significant mechanistic 
 feature . computational studies have provided considerable insight 
 into processes of this type , including studies of intermolecular couplings 
 employing phosphine ligands and organoborane reducing agents as well 
 as the use of nhc ligands and organosilane reducing agents . in both cases , the formation of a metallacycle , 
 followed by subsequent reduction , was found to be the operative pathway 
 among several proposed reactive pathways . recent experimental work 
 included an initial rates analysis of intramolecular ynal reductive 
 couplings employing pcy3 as the ligand with triethylsilane 
 as the reducing agent . in that study , 
 the reaction was found to be first order in both ynal and nickel and 
 zeroth order in silane . additionally , the report included an in situ 
 ir study , which showed that direct addition of the silane to the nickel 
 phosphine catalyst does not occur on the time scale of catalytic reactions 
 unless both the aldehyde and alkyne are present . all of the above studies , across a broad range of ligands 
 and reducing agents , are unified in the conclusion that aldehyde alkyne 
 reductive couplings proceed through a metallacycle intermediate before 
 reduction to the corresponding silyl - protected allylic alcohols . prior to much of the above mechanistic insight being obtained , montgomery reported in 2004 that the silane - mediated reductive couplings 
 of aldehydes and alkynes using nhc ligands showed a substantial increase 
 in scope in comparison to the earlier reported silane - mediated phosphine - promoted 
 variant . in that study , a series of crossover 
 experiments were described , employing a mixture of et3sid 
 and pr3sih . studies of this type are exceptionally useful 
 in determining the molecularity of silane addition reactions . observation of only the noncrossover products 
 ( those incorporating et3si with d or incorporating pr3si with h ) illustrates that the mechanism involves addition 
 of the r3si unit and the h / d unit from a single molecule 
 of silane into a single molecule of product . alternatively , observation 
 of the above two compounds in combination with crossover products 
 ( those incorporating et3si with h or incorporating pr3si with d ) illustrates that the r3si unit and h / d 
 unit must come from two different molecules of silane . in interpreting 
 studies of this type , first , it must be demonstrated that the rates of addition of et3sid and pr3sih are comparable . this can be accomplished 
 by evaluating early time points or by judging the ratio of et3si- and pr3si - containing products when an excess 
 of both silanes is employed . additionally , in the event of crossover 
 products being observed , it must be confirmed that neither the starting 
 silanes nor the silyl - containing products scramble under the reaction 
 conditions . in all crossover experiments described in this study , 
 when a mixture of et3sid and pr3sih is employed , 
 the distribution of the four products is provided as relative percentages , 
 and total crossover is provided as the sum of percentages of the two 
 crossover products . therefore , reactions that proceed with complete 
 crossover are described here as having 50% total crossover . as the 
 ms analysis of these experiments can be somewhat cumbersome using 
 low - resolution ms methods , a detailed experimental description along 
 with an excel spreadsheet for simple calculation of crossover percentages 
 is provided as supporting information to 
 enable facile evaluation of crossover by standard gc / ms instrumentation . to summarize the key features of the crossover experiments from 
 the 2004 study , an ynal reductive cyclization proceeded without significant 
 formation of crossover products ( generating only products with r = 
 et , x = d and r = pr , x = h ) using imes as the ligand , whereas the 
 corresponding reaction with pbu3 proceeded with substantial 
 crossover product formation ( generating all four products with 36% 
 total crossover ) ( table 1 ) . the control experiments 
 described above illustrated that the crossover observed with pbu3 as the ligand was derived from kinetic selectivities during 
 coupling , and not through a scrambling pathway of the starting silanes 
 or final products . the corresponding intermolecular addition of benzaldehyde 
 and phenylpropyne proceeded without crossover utilizing imes as the 
 ligand , but the pbu3-promoted intermolecular process was 
 not efficient , thus precluding its analysis in that study . to explain the crossover observed in the intramolecular couplings 
 employing pbu3 and organosilanes , it was originally proposed 
 that phosphines and nhc ligands likely proceed via different mechanisms . the absence of crossover in nhc - ligated systems 
 was consistent with the metallacycle mechanism ( scheme 1 ) , whereas with phosphines , an alternate mechanism might involve 
 generation of a nickel hydride or nickel silyl species , related to 
 the types of species well established in work from martin , as the active catalyst . addition of a nickel 
 hydride or silyl species to the ynal , followed by a -bond metathesis 
 reaction with the silane , would allow crossover products to be observed . 
 however , the more recent theoretical studies , kinetic analyses , and in situ ir studies cast doubt on these possibilities . while these 
 more recent mechanistic analyses all point to a metallacycle mechanism 
 of the type depicted above , the 2004 crossover study was seemingly 
 inconsistent with the involvement of the metallacycle mechanism ( scheme 1 ) for the phosphine - promoted pathway . in this 
 study , we describe an exhaustive evaluation of crossover 
 experiments with a phosphine ligand ( pcy3 ) that exhibits 
 crossover product formation in some cases while maintaining a broad 
 substrate scope . new insights are provided into the characteristics 
 of ligands , substrates , and reaction conditions that promote crossover 
 in silane - mediated reductive couplings of aldehydes and alkynes . on 
 the basis of these data , a theoretical study was undertaken which 
 provides a modified mechanistic proposal that explains the crossover 
 effects and is consistent with recently described mechanistic analyses . in the initial 
 evaluation of silane 
 crossover experiments , significant crossover was observed in experiments 
 involving five - membered ring cyclizations of ynal 1a using 
 a catalyst generated from ni(cod)2 ( 10 mol % ) and pbu3 ( 20 mol % ) . due to the limited scope of pbu3-promoted 
 couplings , extensive variation of the ynal substrate however , more recent mechanistic studies illustrated that pcy3 performs as a much more versatile ligand in aldehyde alkyne 
 reductive couplings with silane reducing agents . on the basis of this 
 finding , an extensive study of crossover effects was undertaken to 
 evaluate the impacts of phosphine sterics , ring size formed , molecularity 
 ( inter vs intra ) , alkyne sterics , temperature , silane concentration , 
 and catalyst loading . in the cases where crossover was observed , recovered 
 silane was analyzed to confirm that scrambling of the silanes had 
 not occurred . in all cases procedures with 
 isolated yields , following the crossover general procedure , are provided 
 for the preparation of all authentic standards in the supporting information . the role of phosphine 
 structure was only studied to a limited extent since most classes 
 of phosphines are inefficient across a broad range of couplings . however , 
 as a benchmark to compare previously reported data using pbu3 with new data using the more versatile pcy3 ligand , five - membered 
 cyclizations of two representative ynals were examined ( table 2 ) . with both substrates 1a and 1b , more extensive crossover was observed when pbu3 was employed , while the formation of crossover products with both 
 ligands was minimally impacted as the steric influence of the alkyne 
 substituent was increased . with most of the reaction variables studied 
 below , pbu3-promoted couplings were inefficient , so data 
 will only be presented with pcy3 in the evaluation of variations 
 of substrates and reaction conditions . as previously reported , all 
 intermolecular and intramolecular couplings examined with the n - heterocyclic carbene ligand imes proceeded without significant 
 formation of the crossover products , as the examples depicted below 
 illustrate . variations in the size of the formed ring and intermolecular 
 couplings 
 were next examined using ni(cod)2 ( 10 mol % ) and pcy3 ( 20 mol % ) at rt ( table 3 ) . while 
 the extent of crossover was relatively small in five - membered ring 
 cyclizations , there was a subtle but consistent trend toward diminishing 
 crossover as ring size increases . in the formation of a seven - membered 
 ring , essentially no crossover was observed . in an intermolecular 
 coupling of benzaldehyde and phenylpropyne , the steric features surrounding the 
 alkyne were next considered 
 ( table 4 ) . in intramolecular couplings , there 
 was a clear trend from terminal alkynes to highly hindered internal 
 alkynes showing that increased sterics diminishes crossover . a comparison 
 of a simple phenyl substituent on the alkyne terminus compared with 
 a 3,5-di - tert - butylphenyl substituent illustrated 
 that the trend is maintained even with little variation in electronic 
 bias . the influence of the alkyne steric bulk was also seen in intermolecular 
 reductive couplings , wherein increasing sterics of the distal substituent 
 ( r = me , et , or ph ) leads to diminished formation of crossover 
 products . variations in temperature led to 
 substantial changes in crossover , 
 with the formation of crossover products increasing as the temperature 
 was lowered ( table 5 ) . with substrates 1a and 1c that lead to five- or six - membered 
 ring closure , the small extent of crossover seen at rt was substantially 
 increased as the temperature was lowered . substrates that show no 
 significant crossover at rt , such as intermolecular combinations ( 2a with 3a ) and seven - membered ring closures 
 ( 1d ) , display significant levels of crossover at 25 
 c . the observed trends in the formation of crossover show a 
 direct relationship with the temperature , supporting an equilibration 
 between competing pathways . an increase in crossover was observed as the silane concentration 
 increased ( table 6 ) . finally , variation in 
 catalyst loading introduced a minimal impact on crossover in experiments 
 ranging from 5 to 20 mol % nickel , in both intra- and intermolecular 
 couplings ( table 7 ) . in competition experiments employing mixtures of et3sid and pr3sih the 
 changes that lead to the most significant increase in crossover products 
 include a decrease in ligand size , a decrease in alkyne substituent 
 size , the formation of entropically favored ring sizes , and lower 
 reaction temperature . a smaller contribution from silane concentration 
 also plays a minor role , whereas the concentration of catalyst displays 
 essentially no effect . on the basis of these outcomes , a computational 
 evaluation of possible mechanisms was undertaken to further investigate 
 the origin of these effects . as part of 
 a recent study from 
 houk and montgomery on the control of regiochemistry in silane - mediated 
 aldehyde  alkyne reductive couplings , a complete energy profile of a model reaction employing an n - heterocyclic carbene ligand with acetaldehyde and 2-butyne 
 as substrates was described . in this sequence , the lowest energy pathway 
 was found to involve oxidative cyclization of ni(0 ) -complex 4 to ni(ii ) metallacycle 5 as the rate- and regioselectivity - determining 
 step of the process ( scheme 3 , monomer pathway ) . -bond metathesis with silane affords nickel hydride intermediate 6 , which undergoes reductive elimination to afford product . 
 in a mechanism of this type , no crossover is expected , and this is 
 consistent with experimental observations since nhc ligands do not 
 provide crossover products with any substrates examined . to evaluate the origin 
 of silane crossover in phosphine - promoted 
 ynal cyclization pathways , we have now computationally evaluated the 
 cyclization of ynal 1 g to product 10 ( scheme 4 ) . the geometries 
 were optimized with b3lyp and a mixed 
 basis set of lanl2dz for nickel and 6 - 31g(d ) for other atoms . single - point 
 energies were determined with m06 and 
 a basis set of sdd for nickel and 6 - 311+g(d , p ) for other atoms . these 
 are the same levels of methods as those we used in the recent computational 
 study of regioselectivity of the intermolecular reductive coupling 
 reaction . the pme3 ligand 
 and me3sih were employed in the calculations ( see later 
 for computations on the effects of the ligand ) . the potential energy 
 profiles of the oxidative addition and the -bond metathesis 
 steps involving the monomeric and dimeric metallacycles are shown 
 in figure 1 . potential energy surfaces of the monomeric 
 and dimeric pathways 
 ( shown in purple and green , respectively ) calculated with m06/sdd-6 - 311+g(d , p)//b3lyp / lanl2dz-6 - 31g(d ) . oxidative cyclization of the alkyne 
 and aldehyde gives a five - membered 
 metallacycle , 12 . this step ( ts1 ) requires 26.4 kcal / mol 
 in terms of gibbs free energy and is the rate - determining step in 
 the catalytic cycle . upon coordination with a trimethylsilane molecule , 
 complex the -bond metathesis transition state 
 ts2 involves a four - coordinated square planar ni . the h and si atoms 
 in the silane are both in the same plane as the metallacycle and the 
 phosphine . a nickel(ii ) hydride intermediate , 14 , is 
 formed after the -bond metathesis . complex 14 undergoes 
 facile reductive elimination ( ts3 ) to form product complex 15 , which then undergoes ligand exchange to regenerate reactant complex 11 . optimized geometries 
 of intermediates and transition states in 
 the ( a ) monomeric and ( b ) dimeric -bond metathesis pathways . upon consideration of the dimeric 
 form of the proposed metallacycle , 
 which had been isolated and characterized by ogoshi ( scheme 2 ) , a new possible pathway 
 emerged as a possible candidate that might be consistent with crossover 
 results , theoretical evaluations , kinetic studies , and in situ ir 
 analyses ( scheme 3 ) . rather than direct participation 
 of a monomeric metallacycle species , 5 , in the silane - mediated 
 -bond metathesis reaction to afford products without crossover , 
 dimerization of 5 to structure 8 a likely sequence for the dimer pathway 
 involves ligand exchange in 8 , followed by two sequential 
 -bond metathesis reactions to provide intermediate 6 , which would be formed with crossover of the silane mixture employed 
 via the involvement of 9 following the first addition 
 of silane . the conversion of structure 9 to noncrossover 
 products is possible through pathways involving silyl migration . the 
 crossover analysis can not distinguish between noncrossover products 
 derived from the monomer pathway vs those derived from the dimer pathway 
 accompanied by internal silyl migration . under some sets of conditions 
 ( i.e. , table 5 , entry 1 ) , crossover pathways 
 dominate , suggesting that the manifold leading to crossover can proceed 
 without internal silyl migration in some instances . an evaluation 
 of alternative pathways for the -bond metathesis step in the 
 dimer - initiated sequence is provided in the supporting 
 information . the initial -bond metathesis step 
 in the dimeric pathway 
 for the 1 g to 10 conversion was evaluated 
 computationally ( shown in green in figure 1 ) . optimized geometries of selected structures in the dimeric pathway 
 are shown in figure 2b . the formation of metallacycle 
 dimer 17 from the monomer 12 is highly exergonic 
 by 20.2 kcal / mol . the dimerization occurs via a 
 van der waals complex , 16 , which is 4.4 kcal / mol more 
 stable than the monomer metallacycle 12 . the dimerization 
 transition state ts4 requires a low barrier with respect to the van 
 der waals complex 16 . the intermolecular ni o 
 distances decrease from 4.15  in the van der waals complex 16 and 3.62  in ts4 to only 1.96  in the metallacycle 
 dimer 17 , indicative of strong ni  o interactions . 
 in comparison , the intramolecular ni  o bonds 
 in 17 are only slightly shorter ( 1.91  ) . in the 
 dimer complex 17 , although the 16 e ni prefers square planar geometry , the pme3 ligands are 
 bent out of the plane to avoid steric repulsions . this presumably 
 promotes the dissociative ligand exchange which replaces a pme3 ligand with me3sih to form complex 18 . due to similar steric constraints , the si in the -bond metathesis 
 transition state ts5 is not coplanar with the metallacycle . the energy 
 required for ligand exchange ( 17  18 ) and the more crowded steric environment in ts5 than in ts2 lead 
 to a higher barrier for -bond metathesis in the dimeric pathway 
 than that in the monomeric pathway . the computed potential energy surfaces indicated that , after the 
 formation of metallacycle 12 , the formation of dimeric 
 intermediate 17 and the monomeric -bond metathesis 
 via ts2 both require very low barriers and are highly exothermic . thus , the exact ratio between these pathways is expected to be affected 
 by nonstatistical dynamic effects . once 
 the dimer 17 is formed , the complex may undergo dimeric 
 -bond metathesis via ts5 to the crossover products or dissociate 
 to 12 and then undergo monomeric -bond metathesis 
 via ts2 . calculations indicated the dimeric process ( 17  ts5 ) requires a 5.0 kcal / mol higher free energy barrier 
 than the monomeric process ( 17  12  ts2 ) . considering the error range of dft calculations on 
 these challenging species , these results suggest the dimeric and monomeric 
 pathways are competing at room temperature . to investigate the 
 effects of the ligand and substituents on crossover , 
 we calculated the dissociation energy of the dimeric metallacyclic 
 intermediate 17 ( g1712 ) and the barrier of -bond 
 metathesis from the dimeric intermediate ( g17ts5 ) for the 
 reactions of ynal 1 g with different phosphine and nhc 
 ligands ( table 8) . the metallacyclic dimer 
 with the bulkier p(i - pr)3 ligand ( entry 
 2 ) is much easier to dissociate to the monomeric metallacycle 12 than the pme3-ligated complex , while the size 
 of the phosphine ligand has small effects on the barrier for dimeric 
 -bond metathesis . thus , bulkier phosphine ligands are expected 
 to promote the monomeric pathway , leading to lower crossover ratios 
 ( see table 2 for the experimental observation 
 of the effects of the ligand size ) . calculations with a small model 
 nhc ligand ( iph ) indicated a very high barrier for dimeric -bond 
 metathesis ( entry 3 ) . the strong coordination of nhc to the metal 
 makes the ligand exchange step ( 17  18 ) highly unfavorable compared to that with phosphine ligands . in 
 addition , the dimer dissociation is much more facile with bulkier 
 nhcs , e.g. , imes ( entry 4 ) . in work from ogoshi , structural evaluation 
 of nickel  nhc metallacycles derived from cyclopropyl ring - opening 
 provide a clear experimental precedent that the monomer / dimer equilibrium 
 is highly sensitive to the steric environment of the substrate . in summary , strong coordination to the metal 
 and the steric bulk of nhc ligands substantially raise the barrier 
 of the dimeric pathway and promote dissociation to the monomeric metallacycle . this is consistent with the absence of crossover products observed 
 in the nhc - ligated systems ( see table 1 ) . we also investigated the reaction of ynal 1a with 
 a terminal phenyl group ( entry 5 ) . increasing the size of the r group from me to ph has small effects on the stability of 
 the dimer complex , while it noticeably increases the barrier for dimeric 
 -bond metathesis . this is due to steric repulsions between 
 r and sime3 groups in ts5 ( see the supporting information for the optimized geometry 
 of the transition state ) . this is again consistent with the experimentally 
 observed alkyne steric effects ( see table 4 ) . on the basis of the above computational 
 evaluation , we envisioned that the previously reported dimeric metallacycle 
 reported from ogoshi ( scheme 2 ) would be interesting to examine in reactions with silanes , 
 especially those with a ligand  substrate structure combination 
 relevant to crossover studies . it should 
 be noted that reductive coupling reactions that proceed with extensive 
 crossover typically are less efficient than protocols that proceed 
 without crossover . we interpret this observation as arising from the 
 thermodynamic stability of dimeric metallacycles , leading to slower 
 catalytic turnovers . this analysis is consistent with the observation 
 that the most effective catalyst systems , such as those involving 
 bulky nhc ligands , proceed without observed crossover . therefore , 
 a metallacycle presynthesized as the dimer would likely show reduced 
 reactivity in comparison to the most effective catalytic protocols . to explore the stoichiometric behavior of the ogoshi metallacycle 20 , this purple crystalline material was used to prepare a 
 0.01 m solution in thf , and crossover experiments were conducted with 
 10 equiv each of et3sid and pr3sih ( scheme 5a ) . while conversions were low , the expected product 21 was obtained with 44% total crossover , which exceeds that 
 seen in a typical catalytic experiment . repeating the experiment in 
 the presence of benzaldehyde and 1,5-cyclooctadiene improved the reaction 
 efficiency , and the total crossover decreased to 19% , which is very 
 close to that observed in a standard catalytic crossover experiment 
 ( entries 5 and 6 , table 6 ) . these results are consistent with crossover resulting from 
 the dimer , whereas benzaldehyde may stabilize coordinatively unsaturated 
 intermediates that allow the dimer to more efficiently access the 
 monomeric metallacycle . to gauge whether benzaldehyde 
 promotes metallacycle cleavage back 
 to aldehyde and alkyne , an experiment was conducted with the addition 
 of p - tolualdehyde and triethylsilane to a thf solution 
 of 20 ( scheme 5b ) . this experiment 
 afforded only benzaldehyde - derived product 21a , illustrating 
 that the dimer 20 is chemically competent and that metallacycle 
 cleavage to free aldehyde and alkyne does not occur under catalytically 
 relevant conditions . this is consistent 
 with the computed energy profile shown in figure 1 , which indicated the monomeric and dimeric -bond metathesis 
 pathways both require a lower barrier than the reverse reaction to 
 cleave the metallacycle via ts1 . finally , to probe the kinetic competence 
 of dimer 20 , the reductive coupling of p - tolualdehyde , 4-octyne , and triethylsilane was conducted using 20 as the catalyst in 5 mol % concentration ( scheme 5c ) . while the reaction proceeded efficiently , the 
 earliest time points displayed an initial burst of product 21a , followed by the appearance of 22 . this experiment 
 illustrates that the dimer conversion to product is faster than the 
 rate of the complete catalytic cycle starting from aldehyde and alkyne . considering 
 the proposed 
 dimer pathway within the context of the theoretical analysis and studies 
 of the independently prepared dimer , it is instructive to consider 
 the observed crossover data ( tables 27 ) compared with the proposed mechanistic pathway 
 ( scheme 3 and figure 1 ) . increasing either the ligand size ( table 2 ) or alkyne sterics ( table 4 ) results in a 
 substantially decreased level of crossover . these changes have a major 
 influence on the mechanistic path followed , and the influence of sterics 
 on the monomer / dimer equilibrium and on the activation barrier for 
 dimer consumption has been elucidated through a computational evaluation 
 ( table 8) . the temperature effects ( table 5 ) are also substantial , and the effect originates 
 from influencing the keq for the monomer / dimer 
 equilibrium . the dimeric pathway becomes more favorable at low temperature 
 since the dimer is enthalpically favored but entropically disfavored . the ring size and molecularity ( table 3 ) undoubtedly 
 influence the rate of formation of the initially produced monomeric 
 metallacycle . less favorable ring sizes that diminish the rate of 
 metallacycle formation should disfavor the involvement of a pathway 
 that involves metallacycle dimerization . additionally , smaller ring 
 sizes will lead to less steric repulsion with the ligand during dimer 
 assembly . the extent of crossover increases as the silane concentration 
 increases ( table 6 ) . consumption of both the 
 monomer and dimer requires silane addition ; however , the barrier for 
 silane addition to the monomer ( ts2 , figure 1 ) is extremely small , whereas the barrier for silane addition to 
 the dimer ( ts5 , figure 1 ) is substantial . therefore , 
 the increase in silane concentration would be expected to more dramatically 
 increase the rate of dimer conversion to product compared with the 
 minimal impact of the silane concentration on conversion of monomer 
 to product , consistent with the observations . the very small influence 
 of the catalyst concentration ( table 7 ) is 
 perhaps surprising given that the monomer however , 
 all crossover experiments were performed with a constant 1:2 metal : ligand 
 ratio at constant volume . therefore , the concentration of free cod 
 and phosphine in solution increases as the active catalyst concentration 
 increases , and these simultaneous changes may affect both the monomer dimer 
 equilibrium ( 12 to 17 , figure 1 ) and the ligand dissociation required for the dimer conversion 
 to product ( 17 to 18 , figure 1 ) . the experimental parameters that most impact the extent 
 of crossover , namely , ligand and substrate sterics , ring size , temperature , 
 and silane concentration , are all readily explained by the proposed 
 dual involvement of monomer and dimer pathways . in summary , the involvement of a novel dimeric pathway has been 
 evaluated in nickel - catalyzed silane - mediated reductive couplings 
 of ynals . the analysis of silane crossover ( using et3sid 
 and pr3sih ) was extensively studied across a range of ligand 
 and substrate variations , including various ring sizes and intermolecular 
 versions . the effects of temperature variation and the concentration 
 of silane and catalyst were also examined . a computational study comparing 
 monomeric and dimeric pathways , both involving initial formation of 
 a metallacycle from oxidative cyclization of a nickel(0 ) aldehyde  alkyne 
 -complex , provides evidence for the competitive nature of the 
 two pathways . identification of the key steps that determine the predominance 
 of the monomeric or dimeric pathways is supported by the silane crossover 
 experiments . the independent synthesis of a dimeric metallacycle confirmed 
 that its conversion to product upon treatment with silane proceeds 
 predominantly through a crossover pathway . a general and useful 
 trend that can be seen from these studies 
 is that the most efficient protocols , proceeding in high yields with 
 the broadest range of substrates , correlate to the reaction conditions 
 that minimize crossover . we attribute this effect to the considerable 
 stability of the metallacycle dimers of structure type 8 ( scheme 3 ) . protocols that proceed predominantly 
 via the monomer pathway , such as silane - based procedures employing 
 bulky nhc ligands or borane - based procedures , which would likely impede metallacycle dimerization 
 due to coordination of the borane to the metallacycle oxygen , are 
 the most robust methods across a range of substrate combinations . the analysis reported herein of the dimeric pathways and their influence 
 on catalytic cycle efficiency can thus serve as a guide to predicting 
 optimum catalyst the silane 
 crossover technique provides a simple and rapid analysis method for 
 evaluating the complex mechanistic trends highlighted in this study .
the 
 mechanism of nickel(0)-catalyzed reductive coupling of aldehydes 
 and alkynes has been studied . 
 extensive double - labeling crossover 
 studies have been conducted . while previous studies illustrated that 
 phosphine- and n - heterocyclic carbene - derived catalysts 
 exhibited differing behavior , 
 the origin of these effects has now 
 been evaluated in detail . 
 many variables , including ligand class , 
 sterics of the ligand and alkyne , temperature , and ring size being 
 formed in intramolecular versions , all influence the extent of crossover 
 observed . 
 a computational evaluation of these effects suggests that 
 dimerization of a key metallacyclic intermediate provides the origin 
 of crossover . 
 protocols that proceed with crossover are typically 
 less efficient than those without crossover given the thermodynamic 
 stability and low reactivity of the dimeric metallacycles involved 
 in crossover pathways .