allylsilanes are versatile 
 nucleophiles in organic synthesis that 
 possess numerous favorable properties , including low toxicity and 
 high functional group compatibility . we 
 recently described a novel synthesis of allyl- ( and vinyl- ) silanes 
 by palladium - catalyzed silylation of terminal alkenes using me3sii . this silyl - heck reaction provides an expedient entry into these 
 important nucleophiles using widely available alkene starting materials . although our initial conditions provided allylsilanes 
 with good to excellent selectivities for the trans isomers and were 
 functional - group tolerant , the yields of these reactions were moderate 
 ( typically 60% ) due to competitive isomerization of the alkene 
 starting material . guided by rational ligand design , we have sought improved 
 catalysts 
 that minimize competitive alkene isomerization and increase the yield 
 of the allylsilane . we now report a second - generation ligand for the 
 palladium - catalyzed silyl - heck reaction that converts a broad range 
 of terminal alkenes into allylsilanes with dramatically improved yields . in addition , these studies have deepened our understanding of the 
 product - determining step that leads to allylsilane formation . quantitative 
 studies of the steric and electronic properties of the various ligands 
 investigated , as well as stoichiometric experiments , provide insight 
 into the palladium species involved in the oxidative addition of me3si  i . these studies culminated in isolation and characterization 
 of the first monomeric palladium complex from the oxidative addition 
 of a silicon , these studies have not 
 only greatly improved the synthetic utility of the silyl - heck reaction 
 in the preparation of allylsilanes , but also significantly increased 
 our understanding of the process . in our 
 previous study , we identified bupph2 as an effective ligand for silyl - heck reactions . this ligand 
 was vastly superior to both all - aryl phosphines , such as ph3p , as well as all - alkyl phosphines , such cy3p and bu3p . we rationalized that the effectiveness of bupph2 was due to it being sufficiently electron - rich 
 to favor oxidative addition of the me3si  i bond 
 ( in contrast to ph3p ) , while being able to accommodate 
 the large silyl group in the pd  sime3 intermediates 
 involved in the reaction ( by rotation of the flat phenyl groups , in 
 contrast to cy3p and bu3p ) . we recognized , however , that while the inclusion 
 of phenyl groups might be necessary for steric reasons , the inductive 
 electron - withdrawing nature of these aromatic substituents dramatically 
 lowers the electron - density of the ligand compared to all - alkyl congeners 
 ( e.g. , cy3p and bu3p ) . large , highly 
 electron - rich ligands have been shown to promote 
 oxidative addition . as it was likely that 
 competition between oxidative addition to the si i bond ( leading 
 to productive silyl - heck products ) and background isomerization of 
 the alkene starting material ( likely by trace metal hydride species ) 
 was responsible for the limited yields of allylsilanes in our original 
 report , we hypothesized that catalysts employing larger and more electron - rich 
 phosphine ligands would further improve the reaction . specifically , 
 we hoped that by increasing the electron - donating character of the 
 arene groups on the ligand backbone , a more efficient catalyst might 
 arise . at the same time , additional substituents on the aromatic groups 
 would increase the size of the ligand , potentially leading to further 
 improvements in the reaction . to investigate the 
 possibility that larger , more electron - rich ligands would lead to 
 more efficient allylsilane synthesis , we prepared a series of diaryl - tert - butyl phosphine ligands with varied electronic and 
 steric properties of the aryl groups . these ligands were prepared 
 by addition of the aromatic organolithium reagent to dichloro - tert - butylphosphine ( eq 1 ) . further details regarding ligand synthesis are provided in the supporting information ( si).1 to assess 
 the effect of ligand design on the silyl - heck reaction , we selected 
 the coupling of 1-decene and me3sii as our model system 
 ( table 1 ) . our initial investigation compared 
 the use of bupph2 ( 3 ) and bup(4-c6h4ome)2 ( 6 ) as ligands under palladium - catalyzed 
 conditions . consistent with our hypothesis , the reaction involving 
 the more electron - rich ligand 6 provided markedly higher 
 yield of desired product 2 compared to that using bupph2 ( entries 1 vs 4 ) . further , 
 when ligands bearing meta electron - withdrawing substituents were employed 
 ( 4 and 5 , entries 2 and 3 ) , lower yields 
 were observed . to further increase the electron - donor ability 
 of the ligand , as well as its size , para - anisole 
 derivatives bearing meta alkyl groups were prepared ( ligands 7 and 8 , entries 5 and 6 ) . the yield of desired 
 product 2 improved with both methyl and tert - butyl substituents , and increased with the size of the alkyl group . 
 with 3,5-di - tert - butyl-4-methoxyphenyl - derived ligand 8 , the product , 
 we also prepared the ligand series bearing para - dimethylamino 
 substitution ( ligands 911 ) . in this 
 series , the reaction involving ligand 9 , which lacked meta substitution , 
 provided similar yields of 2 compared to bupph2 ( entry 7 ) . in contrast , the ligand 
 bearing meta - methyl groups ( 10 ) provided 
 increased yields ( entry 8) , similar to dimethylanisole ligand 7 . surprisingly , however , the 3,5-di - iso - propyl-4-dimethylaminophenyl 
 derived 11 provided vastly superior results , giving the 
 desired product in nearly quantitative yield ( entry 9 ) . while 
 highly encouraging , this latter result was also perplexing . examination of molecular models revealed that the flanking isopropyl 
 groups force the dimethylamino group into a conformation where the 
 nonbonding electron pair of the nitrogen is perpendicular to the -system 
 of the aromatic ring ( figure 2 ) . in such a 
 conformation , little electronic benefit from the dimethyl amino group 
 is expected . yield 
 reflects total silylated products ( allylsilane + trace vinyl silane ) ; 
 in all cases , allylsilane / vinylsilane  93:7 , e - allyl / z - allyl  85:15 . accordingly , we prepared one final 
 series of ligands bearing only meta - alkyl substituents 
 on the aromatic rings ( ligands 1214 ) . pleasantly , and surprisingly , 
 this series of ligands was also highly effective in the model transformation . ligands bearing either methyl or isopropyl groups showed significant 
 improvement over bupph2 ( entries 
 10 and 11 ) . most significantly , however , the ligand bearing meta , meta - di - tert - butylphenyl 
 groups ( 14 ) provided product 1 in 99% assay 
 yield ( entry 12 ) . as ligand 14 can be easily prepared 
 on multigram scale in only a single step from commercially available 
 materials and is air stable ( no detectable oxidation was observed 
 by p nmr after one year of storage on the benchtop under 
 air ) , we selected this ligand to explore as a second generation ligand 
 for the preparation of allylsilanes . vastly 
 improved yields of allylsilanes are obtained with a broad range of 
 -olefin substrates ( figure 3 ) . in general , 
 high yields , good e / z selectivities , and outstanding 
 allyl / vinyl selectivities were observed . for example , the model substrate gave rise to allylsilane 2 in 98% isolated yield as a 84:16 mixture of e / z isomers , with only trace amounts ( 7% ) of the vinylsilane 
 isomer . similar results were obtained for other n - alkyl -olefins ( e.g. , 15 and 16 ) . the allyl vs vinyl and e - allyl vs z - allyl selectivity are very similar to the first - generation catalyst 
 system . importantly , however , and in contrast 
 to the earlier system using bupph2 , almost no alkene isomerization of the starting material 
 was observed . this not only significantly improves the yield of the 
 allylsilane , but also greatly simplifies purification of the product . the silyl - heck reaction also demonstrates a broad range of functional 
 group compatibility , although in some cases slightly greater catalyst 
 loadings where required to achieve high yields ( see figure 3 ) . protected alcohols , including pivaloyl esters 
 and silyl ethers alkyl bromides can also be used , but higher catalyst loading and 
 a modified metal to ligand ratio ( 1:1 ) are required to obtain desired 
 product 21 in modest yield . alkyl boronic esters participate 
 smoothly in this reaction giving a potentially versatile dimetalloid 
 ( 22 ) . scope of silyl - heck reaction to form allylsilanes with 
 second - generation 
 ligand . malonates are tolerated in the 
 reaction , as shown by 23 . in this particular case , a 
 small amount of alkene isomerization 
 ( 11% ) was observed , wherein the alkene moved into conjugation with 
 the diester group , giving the homoallylsilane . a similar result was 
 obtained with the use of homoallylbenzene . in this case , product 24 was contaminated with approximately 8% of the e - homoallylsilane , wherein the alkene moved into conjugation with the 
 phenyl group . we believe that the fact that such a low amount of alkene 
 isomers are observed , even in the these highly thermodynamically favorable 
 cases , suggests that the observed allylsilanes are the kinetic products 
 of the silyl - heck reaction and are not the result of postreaction 
 alkene isomerization . complex , natural product derived substrates 
 can also be silylated 
 using this protocol . for example , silylation of an allylated cholesterol 
 derivative gave rise to allylsilane 25 in excellent yield . 
 importantly , this example also demonstrates that terminal alkenes 
 are silylated selectively over internal alkenes , and that stereogenic 
 centers at the homoallylic position of the starting material can be 
 maintained with full stereochemical fidelity during the reaction . the effect of steric bulk proximal to the site of -hydride - elimination 
 was studied by the preparation of allylsilanes 2628 . in particular , we desired to understand the effects of 
 steric crowding on allyl- vs vinylsilane selectivity . with modest 
 levels of steric encumbrance ( tertiary carbon center ) at either the 
 - or -position relative to the terminal alkene starting 
 material , no erosion of selectivity for allylsilanes in fact , even branching at 
 the -position resulted in good selectivity for allylsilanes 
 as long as there was a hydrogen atom available for -hydride - elimination 
 ( 28 ) . notably , however , in these more sterically encumbered 
 cases , a slight increase in catalyst loading was required to maintain 
 high yield ( see figure 3 ) . we attribute this 
 requirement to increased difficulty in the migratory insertion step . in contrast for example , the presence of a fully substituted 
 carbon at the -position of the starting material noticeably 
 decreased selectivity , leading to a 72:28 mixture of allyl and vinyl 
 isomers ( 29 , see discussion below ) . interestingly , however , when trimethylallylsilane was used , the product 
 symmetry allows for the production of unsaturated disilane 30 as a single isomer . with our previous catalyst system , we 
 observed poor allyl vs vinyl 
 selectivity in the formation of silyl ether 31 . as we 
 believed this to be an informative reaction in understanding the factors 
 controlling allyl vs vinyl selectivity , we reinvestigated this reaction 
 using the second - generation catalyst ( figure 3 ) . while the yield of 31 is greatly improved using the 
 new catalyst , silyl - heck 
 reactions with aryl - substituted allyl substrates . finally , in our previous study , aryl - substituted allyl compounds 
 were a particularly challenging class of substrates due to the propensity 
 of the alkene of the starting material to migrate into conjugation 
 with the aromatic group . second - generation ligand 14 provides 
 vastly improved silyl - heck reactions across a range of such substrates 
 ( 3237 , figure 4 ) . with electron - neutral and heteroaromatic substrates , very little 
 alkene isomerization was observed ( 33 and 37 ) . with highly electron - rich allyl aromatic substrates , alkene isomerization 
 could not be fully suppressed , resulting in slightly diminished yields 
 ( 32 , 3436 ) . however , 
 in all cases , reactions involving ligand 14 were dramatically 
 improved over those using bupph2 . in these cases , only the e - allylsilanes are observed . 
 in no case , were z - allyl- or vinylsilane was detected 
 ( nmr and gc / ms ) . as mentioned above , the limited 
 amount of alkene isomerization observed in the formation of compounds 23 and 24 suggests that the observed allyl / vinyl 
 ratio is likely kinetic in nature . to further determine if the allylsilane 
 products might arise from isomerization of vinylsilanes under the 
 reaction conditions , we independently prepared vinylsilane 38 and added it to the silyl - heck reaction of 1-hexene and me3sii ( scheme 1 ) . after the reaction , allylsilane 16 ( arising from 1-hexene ) was observed in quantitative yield 
 ( > 95% , gc / ms ) . in addition , vinylsilane 38 remained 
 unchanged , 
 being returned from the reaction in > 95% yield ( gc / ms ) . importantly , 
 allylsilane 15 , which is the product of alkene isomerization 
 of 38 , was not detected . these results support the proposal 
 that the observed allyl / vinyl ratios from the silyl - heck reaction 
 are kinetic in nature , and implicates -hydride elimination 
 from the alkyl - palladium intermediate as the product - determining step . allylsilanes are typically 
 the vastly major product in this class 
 of silyl - heck reactions . these products result from -hydride 
 elimination away from the silicon center , and in principle either 
 steric or electronic factors could contribute to the observed selectivity . the observed ratio of allyl- and vinylsilanes in the formation of tert - butyl substituted product 29 ( figure 3 ) sheds considerable light on the nature of this 
 selectivity . considering allyl - palladium intermediate 38 that 
 forms 
 en route to 29 , -hydride elimination can occur 
 either toward the tert - butyl group or the trimethylsilyl 
 group in an otherwise symmetrical molecule ( figure 5 ) . as the tert - butyl group ( a - value = 4.9 kcal / mol ) is larger than the trimethylsilyl group ( a - value = 2.5 kcal / mol ) , strictly steric selectivity would 
 favor formation of vinylsilane product vinyl-29 . in contrast , due to the  silicon effect 
 and the fact that the hydrogen atoms  to the silicon center 
 have increased acidity and decreased hydricity , electronic control would favor -elimination toward 
 the tert - butyl group . the fact that allylsilane allyl-29 is the dominant product suggests a significant electronic 
 contribution in product determination in the silyl - heck reaction . the importance of electronic factors in -hydride elimination 
 have been documented in other palladium - catalyzed heck - type reactions . importantly , in substrates lacking such bulk substituents , 
 such 
 as those leading to allylsilanes 2 and 1528 , the large trimethylsilyl group is the most 
 sterically demanding in the proximity of the palladium center in the 
 metal alkyl intermediate . in such cases , steric and electronic factors 
 reinforce each other , helping to explain the high allylsilane selectivity 
 observed . finally , we believe electronic factors also explain 
 the low allyl / vinyl 
 ratio observed in the formation of product 31 ( figure 3 ) . however , unlike 31 , the formation of compound 27 vastly favors formation 
 of the allyl isomer , ruling out steric factors as an explanation for 
 the observed selectivity in 31 . instead , we believe that 
 the poor selectivity observed in the formation of 31 is 
 primarily electronic in nature and is due to the lowered hydricity 
 of the hydrogen atoms  to the electron - withdrawing ether and 
 aromatic group . this deeper understanding of the importance 
 of electronic , as well 
 as steric , factors in product selection will undoubtedly be important 
 in planning synthetic applications of the silyl - heck reaction in complex 
 allylsilane synthesis . having demonstrated 
 that ligand 14 is vastly superior to the earlier system 
 in the formation of allylsilanes , we wanted to better understand how 
 the electronic and steric properties of the various ligands contributed 
 to their performance in the silyl - heck reaction . p 
 bond = 2.28  . to gauge 
 the steric properties of the ligands , we obtained x - ray 
 crystal structures of the ligands that could be crystallized ( in all , 
 8 of the 12 ligands studied ) . using these structures , we computed 
 the equivalent cone angle ( eca ) , employing the method of guzei and 
 assuming a metal - phosphine bond distance of 2.28  ( table 2 ) . from these data , and in 
 comparison to the data presented in table 1 further , 
 the best preforming ligands ( 8 , 11 , and 14 ) have the largest computed cone angles . steric data , however , do not provide a complete explanation 
 for 
 the variance in ligand performance . in particular , these data do not 
 explain the superiority of ligands 11 and 14 ( eca 149 for both ) over ligand 8 ( eca 153 ) . 
 in particular , ligands 8 and 14 differ only 
 by the presence of a para - methoxy group on the aromatic 
 rings . the difference in the ecas appears to be too small ( and is 
 in opposite trend ) to explain the difference in reactivity . to gauge 
 the -donor ability of the ligands , each was converted to 
 the phosphine selenide by treatment with elemental ( black ) selenium 
 in cdcl3 . previous studies have shown that the -donor 
 ability of phosphine ligands is directly proportional to the inverse 
 of the p  se coupling constant in 
 the nmr spectra ( jp  se ) . measurement of this coupling constant for 
 each of the in situ generated phosphine selenides first , 
 as expected , the substituted ligands bearing electron - donor groups 
 are in fact more electron - rich than the parent bupph2 . second , while electron - rich ligands give 
 generally improved catalytic performance , only those ligands that 
 are also sterically bulky provide highly improved results compared 
 to bupph2 . for example , ligand 9 bearing the n , n - dimethylaniline group is measured to be the most electron - rich , 
 but its small cone angle and exposed basic amine make it only negligibly 
 better than bupph2 . third , 11 and 14 are two of the most electron - rich ligands 
 and appear to have greater -donor ability than ligand 8 . in the context of the silyl - heck reaction , the ligand data 
 outlined 
 above correlate very strongly with the observed trends in yield . the 
 two most effective ligands ( 11 and 14 ) are 
 the two with the best combination of electron - richness and large steric 
 size . these factors are expected to favor oxidative addition , and 
 are consistent with our hypothesis that led to the rational design 
 of these second - generation ligands . a more subtle result that 
 emerges from the electronic analysis 
 of the ligands is the comparative lack of electron - richness of ligand 8 , particularly compared to ligand 14 , which 
 lacks the presumably electron - donating para - methoxy 
 group . one possible explanation for this surprising result is that 
 the large flanking alkyl groups of ligand 8 restrict 
 the conformation of the methoxy group to be out of plane with the 
 aromatic ring , so that the beneficial overlap between the nonbonding 
 electrons on the oxygen and the -system of aromatic ring is 
 poor and offset by the inductive electron - withdrawing effect of the 
 oxygen atom . we believe that this is the first time that the 
 electronic properties 
 of these large phosphine substituents have been quantified in a systematic 
 way . beyond the scope of the silyl - heck reaction , these data might 
 inform on ligand design in other classes of novel sterically demanding , 
 electron - rich phosphine ligands . the silyl - heck reactions with this 
 second - generation ligand ( as well 
 as with the other high - yielding ligands presented herein ) are particularly 
 sensitive to the ligand / metal ( l / m ) ratio employed . empirically ( as 
 reflected in figure 2 ) , l / m ratios between 
 1:1 and 1.5:1 provide the highest yields of allylsilanes . with l / m 
 ratios at or above 2:1 , a pronounced drop in reactivity is observed . these results are suggestive of the importance of low - valent metal 
 centers in the catalytic reaction . to further probe the relevance of low - valent palladium complexes 
 in silyl - heck reactions using our second - generation ligands , stoichiometric 
 experiments were undertaken . for example , combining ligand 7 with precatalyst 1 and excess me3sii in 
 a solvent mixture of toluene , benzene , and pentane at 35 c 
 results in the formation of light green crystals , which were determined 
 to be oxidative addition complex 39 using x - ray crystallography 
 ( figure 6 ) . this thermally and air - sensitive 
 t - shaped monophosphine complex has an open coordination site opposite 
 the silicon ligand ( no intramolecular interactions , including agostic 
 interactions with the tert - butyl group , nor intermolecular 
 interactions are observed in this open coordination site ) . this compound 
 is the first monomeric palladium complex resulting from oxidative 
 addition of a silicon halide bond to be characterized , and 
 is also the first such complex to be supported by a phosphine ancillary 
 ligand . this complex was fully characterized 
 in solution ( h , c and p nmr 
 in cd2cl2 at 0 c ) , and the data are consistent 
 with the structure shown in figure 6 . the use of 2 mol % of this complex with an additional 1 
 mol % of 7 as catalyst ( mimicking the optimized metal / ligand 
 ratio ) provided allylsilane 2 in 72% yield , which is 
 comparable to the in situ generated catalyst ( table 1 ) . further , complex 39 is the major product observed 
 ( p nmr ) in stoichiometric experiments performed in dce 
 ( 1 equiv each of 1 and 7 , 15 equiv each 
 of me3sii and et3n ) , which is the solvent of 
 choice for the catalytic reaction . combined , these experiments establish 
 the relevance of complex 39 under catalytically relevant 
 conditions . finally and significantly , a highly similar signal is 
 observed by p nmr when ligand 14 is used 
 in the stoichiometric reaction ( in dce , as above ) , suggesting that 
 an analogous complex is formed with the optimal ligand . to date , however , we have been unable to crystallize 
 the complex using the more soluble ligand 14 . these results , along with the steric and electronic data 
 outlined 
 above and the optimized reaction conditions , are highly suggestive 
 that the active catalyst is a palladium  monophosphine ( l1 m ) complex . such a catalytic intermediate parallels those 
 involved in oxidative additions of carbon  halogen bonds using 
 large , electron - rich phosphines . moreover , 
 the structure of 39 , particularly the disposition of 
 the aromatic groups on the phosphine , clearly illustrates how this 
 class of diaryl - tert - butylphosphine ligands accommodates 
 the steric bulk of the large me3si group . the rotation 
 of these groups away from the silicon center is consistent with our 
 previous hypothesis for why these ligands are superior in the silyl - heck 
 reaction . using rational ligand design , we have developed an easily prepared , 
 bench - stable second - generation ligand for the preparation of allylsilanes 
 using the silyl - heck reaction . unlike the previously reported system , 
 the new ligand provides extremely high yields of allylsilane products 
 using simple terminal -olefin substrates , with good e / z selectivity , very little formation of vinylsilanes , 
 and only trace isomerization of the alkene starting materials . quantitative 
 studies demonstrate that this new ligand is much larger and more electron - rich 
 than the first - generation ligand , and the isolation of an oxidative 
 addition complex of me3sii sheds significant light onto 
 the effectiveness of this ligand class . overall , we believe that this 
 new ligand provides a simple , direct , and highly useful means to access 
 high - value allylsilanes from readily available starting materials 
 using the silyl - heck reaction .
using 
 rational ligand design , we have developed of a second - generation 
 ligand , bis(3,5-di - tert - butylphenyl)(tert - butyl)phosphine , for the preparation of allylsilanes using the palladium - catalyzed 
 silyl - heck reaction . 
 this new ligand provides nearly complete suppression 
 of starting material alkene isomerization that was observed with our 
 first - generation catalyst , providing vastly improved yields of allylsilanes 
 from simple alkene starting materials . 
 the studies quantifying the 
 electronic and steric properties of the new ligand are described . 
 finally , we report an x - ray crystal structure of a palladium complex 
 resulting from the oxidative addition of me3sii using an 
 analogous ligand that provides significant insight into the nature 
 of the catalytic system .