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Class Note for CHEM 6311 with Professor Albright at UH


Class Note for CHEM 6311 with Professor Albright at UH

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Date Created: 02/06/15
XI Cycloaddition reactions AThese are reactions of two acyclic molecules to form a cyclic molecule lThere are a very large number of these reactions in the organic world 2They are extremely useful in a synthetic sense since two CC bonds are formed 3We will not have time to give a comprehensive treat ment a Chapters l0 and l l in Lowry amp Richardson do this bYou do not have to read all of this material BWe will concentrate on only one reaction the dimerization of ethylene to cyclobutane l Energy considerations aThe two CC 039 bonds formed are stronger than the two CC 7 bonds broken i This is the result despite the ring strain ii Using Benson s tables H H H H H H I I a Hmquot H H H H H H 111 AH 19 kcalmol bAS however is negative i Two molecules become one ii Using Benson s tableszA5298 44 cal mol O K cThus AG298 59 kcalmol i The reaction is still exothermic and should be ob servable however ii Although AH l9 kcalmol AH3l3 for this reaction as shown is incredibly high about 43 kcalmol a There is no way that it will be observed experimen tally under normal conditions b One must use very forcing conditions high tem peratures high pressure 2 Mechanistic questions aThe activation barrier is a function of the reaction mechanism 257 b For the thermally driven reaction considered above i One clue about the reaction mechanism is that under some conditions polymers are formed ie CH2CH2CH2CH2n ii Another clue as we shall see later is that the reac tion is completely or almost completely stereo nonselective cThe photochemical reaction is intriguingly different i It is quite general olefins bearing substituents of almost any kind will still produce a cyclobutane ii There is virtually no activation energy associated with the reaction if one disregards the amount of energy required to excite the olefin dimer to the excited state iii Furthermore the photochemical reaction not only is stereoselective but it is also stereospeci c C Before we look at the mechanism for the thermal and photochemical dimerization reaction of olefins it is useful to first consider another much more simple reaction H2 D2 gt 2HD lThis could be considered to be a special sort of cy cloaddition reaction H H H H T H H l l gt I DD l5 396 D D 112 2 Many experiments have been carried out on this reac tion a Part of the reason for this is that although experimentaly Ea 4 kcalmol an essentially exact MO calculation has given Ea l52 kcalmol bYet for the dissociation reaction HH gt 2H0 AG only l09 kcalmol 3 One possibility that people considered was that this might be a radical chain mechanism ie HH gt H H initiation H D2 gt HD D chain propagation D H2 gt HD H 258 H0 H0 gt H2 H D gt HD chain termination D D gt D2 a Recall that the activation energy comes from mea suring the rates of reaction i If there was a constant relativer large concentration of H radicals around or more probably if the num ber of chain propagation steps was very large com pared to the number of chain termination events then the overall rate would be given by the chain propaga tion reaction ii We saw earlier in the semester that Ea l l3 kcal mol for H H2 gt H2 H iii Alternatively if the H concentration is small or the chain propagation sequence relative to chain termina tion is small then Ea will approach the bond dissocation energy of H2 I04 kcalmol bThus limits for the activation energy expected for a free radical mechanism would be I l3 lt Ea lt l04 kcalmol a rather large range cActually since the AG and AG for each step of this reaction is known with some precision one can calcu late the concentration of and that must be present to give the observed reaction rate at a par ticular temperature i This concentration turns out to be pretty large ii Researchers have looked long and hard to find any evidence for H and D but none has been found iii Thus the concentration of free radicals recall this is in the gas phase is too low to give the experimentally observed rate 4 Returning to the theoretical calculation why is there a gigantic activation energy associated with this reaction a Let us construct aWalsh diagram an orbital correla tion diagram for this reaction i There are two mirror planes of symmetry which are always present in the reaction 259 113 W you 352 SS 32 ii There are several noteworthy points about this diagram a The orbitals for square H2D2 are exactly like the p MOs of square cyclobutadiene and for the same reason that cyclobutadiene distorts to a rectanglethe square H2D2 geometry is a high point on the surface b The point of intersection of the AS and SA orbitals is exactly l2 of the distance energetically between AS and SA at the ground state geometry i However this point of degeneracy is above the RE of H H ii The energy of stabilization of SA and SS is less than the energy by which AS and AA is destabilized at the ground state c The HOMO AS crosses the LUMO SA i This istherefore a symmetry forbidden reaction ii The two electrons in AS somehow need to jump into the SA orbital iii Therefore the amount of energy that 260 two electrons in AS need to go up is greater than the energy need to break the HH bond iethe energy difference between AS at the ground state and the RE of HThusthe potential energy surface for the H2 D2 reaction is very replusive Shown below are two views of the H2 H2 reaction Each sheet is worth 005 atomic units 30 kcalmolThese are all potential energies so a surface with the most negative value is the most stable one When rl r2 that plane defines all possible rectangular shapes The line given by rl r2 R defines all possible squares andfinally the geometries for R 0 are those for all linear ar rangements of the H2 molecules 205 95 quot3990 200 421 These calculations are taken from jAm Chem Soc 98 6427 I976 261 1 225 220 2J5 L90 225 RI 25 E ZB 20 85 an Il4b As can be seen there are two tubes of low energy which correspond to the two ways H2 units can be combined in this coordinate system Each tube is well insulatedthe potential steeply rises beyond the tube bAn alternative bimolecular reaction path the only other one forms a tetrahedral molecule at the TS D D H Eh c gt I vFH gt H 39 r H H D I DD Ilz 115 D i By applying group theory one can easily get the MOS for the H4 tetrahedron ii Again this is a symmetry forbidden reaction with an extremely large activation barrier 262 cAn alternative to the dimerization reaction is given 139 by H H H H H H H E H gt H H gtH H Bl DD I D D quot397 iiThis pathway involves no HOMOLUMO crossing symmetry allowed iii There are however two problems with this approach a This is a termolecular collision i No known reactions proceed via a real threebody collision ii Furthermorethe ASZli must be extremely negative and since AGi AH3l3 TASZli AH3l3 would have to be extremely small b Calculations have given an Ea AHZli for this reac tion to be 67 kcalmol While this is certainly smaller than l5l kcalmol it is nonetheless is somewhat too large compared to an experimental barrier of AG1 43 kcalmol iiiThe working assumptions by the experimental physical chemists suggesting this in I973 were 263 a Disregard the theoretical estimate for the three body collision b Get around entropy effects by postulating the existance of a dimer a van derWaals dimer formed in a preequilibrium step t HH H H I HquotH 4 H H 4gtH H H H 9 DD D D 113 H H H H 5The very large difference between theory and experi ment has been explained only after much effort I986 aAll of the reactions were carried out in glass pyrex tubes b It was found that if one carefully treats the inner glass surface then there is no formation of HD from H2 D2 i This is done by passing CH33SiCl through it ii The silylilating reagent reacts with OH groups etc cTherefore the glass surface itself is catal ing the reaction i It causes HH and DD bond breakage to form H D radicals close to the surface ii These then immediately react with H2D2 by the surface to produce HD d So there is m disagreement between theory and experiment eWhat is ironic is that organic chemists who do vapor phase pyrrolysis reactions pyrex tubes have known for over 20 years before this time that preparation of the glass surface is necessary or else one will get all kinds of extraneous side reactions catalysized by the surface D Olefin dimerization the thermal reaction I Let us first consider what will happen in a concerted reaction path 264 H H H quotcH Pkg CAHH I Ll gt I L 119 r H HI H HI lI 39 H aThe two CC 039 bonds are formed at the same rate iea leastmotion path bAgain two mirror planes of symmetry are conserved i In the reactant we are breaking the two bonds so we need to make the two 7 and two 75 orbitals symmetry correct with respect to m m m2 ii Likewise in the product we have formed two CC bondsthus the two CC 039 and 5 orbitals must be symmetry correct with respect to m and m2 iii Therefore linear combinations of the T and 75 orbitals are taken m1m2 g 115 1112 5 5 The splitting energy difference between these com binations is small since the distance between the two olefins is initially long iv Similarly the CC 039 and 5 orbitals in the product are symmetry correct with respect to m2 M not ml so linear combinations are again taken of them 265 cAn orbital correlation diagram for this concerted process then would be note 039 lower than TE 5 higher than TE m1m2 rquot1m2 W 1114 55 266 i The HOMO and LU MO cross look carefully at the orbital shapes ii Just as with H2 D2 this is symmetry forbidden iii There must be a lower activation energy process 2The nonconcerted thermal reaction pathway aThe most simple would be to allow one CC bond to form at a faster rate than the other I H H 39C H IH H l c H 9 l CH 939 39 39C H H I H H H AH Iy mz y mz 1115 b For the same reason as the H2 D2 reaction the crossing point for the concerted reaction occurs at a higher energy than the energy of a p orbital therefore H should lie at a much lower energy than H H H C the TSfor a concerted rxn 1116 H H H H Futhermorethe mirror plane of symmetry ml is now not present and so the reaction becomes symmetry allowed Work this out for yourself cAn idealized potential energy surface for the reac tion looks like this the energy units are arbitary H H H H H H H V Fl 7 13921 c c c c Is I r1 u H H H H I noquot Hh H H l 5 r2 I 1117 H H H H H 1 r l H quot H l H f f CC c c r m r h H H H H 1 H H H 267 i Although formation of the diradical will certainly be a very endothermic reaction once it is formed it can wander around ii Large regions of space spanning 9 92 93 are extremely flatThis is called a twixtxl surface H 2 H H H 1113 1 10 6 0 g Q H H 4H iii It seems reasonable that the diradical would be formed via H bccltH H I H H7c c H 1115 H H c HI c H Hcc H H H39 H H iv Thereforethere are three mechanistic possibilities for this reaction Potential Enegry Potential Enegry Potential Enegry T gt I concerted synchronous Reaction Path T 1120 li l concerted asynchronous Reaction Path nonconcerted Reaction Path 268 As pointed out earlier there is good theoretical evi dence that a concerted synchronous where both CC 6 bonds are formed at the same rate is not possible But this does not exclude a concerted asynchronous path where the bonds form at different rates from one where there is a discrete tetramethylene intermediate dThe experimental evidence for this route comes from direct observation of what is most liker the diradical intermediate by femtosecond spectroscopy l l l l l londetector Time of flight T Signal Molecular spnuer Femtosecond pulses O II2I See Science 266 I359 I994The experimental setup is given by two lasers one beam is used to irradiate the molecular beam of cyclopentanone This under goes a very wellknown decarbonylation process called the Norrishtype occleavageto produce CO cyclobutane and ethyleneThe other laser ionizes the species produced which then are massselected using a mass spectrometer What comes out from this experi ment is then a series of mass spectra of the species separated by femtoseconds For this case of cyclopentanone ones sees 269 Diradical Parent k I l22 r I f I f I j I 39 I 39l 1 2 3 z 4 5 6 7 5 E SE 5 Time of flight us r I k I 28 41 55 56 84 Mass amu Notice that the peak associated with the cyclopentanone 84 amu dies out quickly with the production of a new peak at 56 amu which is the molecular weight corresponding to tetramethylene The kinetics associated with its production and decay is shown in part a of the figure below A B Parent quot Parent Diradical U Diradical I 7 p 39 12 70640 is 2 120220 is I O O o 1 l l v I 39 I f 39 l J 1 J l i I l i g l f l 0 1000 2000 O 1000 2000 Time delay ts C hik u l x hbquot Diradical r 221400t200ls 270 Time delay ts The buildup time is c ISO i 30 fs and the decay time is 3952 700 i 40 fs for this compound This clearly is an intermediate and not a transition state species The precursor cyclopentanone decays with a T l20 i 20 fsWhen cyclobutanone is used then the interme diate speciestrimethylene has a much shorter lifetime I 3952 l20 i 20 fs see part bwhereas stabilizing the teramethylene diradical by putting methyl substitu ents which can undergo hyperconjugation greatly increases the lifetime 3952 I400 i 200 fs see part c Furthermore2 I90 fs for l3 cyclopentadiyl why This is all consistent with the formation of diradical intermediates of varying lifetimes The situation for tetramethylene is shown below The barrier height for the diradical is estimated to be 4 kcalmol Diradical Intermediate arbOn W L39 g 55 5 5355 3335323 332 39 390 5 quotIII s II39 Os Aquot39IIII O09 I II39 o 3 z fi lllllzz I 7III II 33 i quot3950 I II 39397IIlIIl lt9gt i I I 0 O 39 O O O Q s s 0 A 9 I 23a e Other evidence in favor of the diradical path i Substituents that stabilize radicals accelerate the reaction a Thus for X OR halogen or alkyl which stabilize radicals dimerizations age faster H 1124 H X b In fact for F2CCF2 the radical is so stable that polymerization is a major side reaction ii The regioselectivity is consistent with radicals 271 CI CI bl 3 4 lt39 bl N CI III The reaction IS stereononseIectIve Cl Cl H CI Cl CH2 1125 gtlt Cl A Cl H Cl 1125 a The diradical has a long enough lifetime to rotate freely 17m has been estimated to be 200 fs Chem Phys Letts 3 03 249 I999 b The products are formed in H ratio from either set of reactants f Sometimes a zwitterionic intermediate is formed FE j 6 D 39AD D D WD 0R NR2 SR etc A 1127 A CN C0R C02R N02 etc g Evidence for a zwitterionic pathway comes from a variety of sources i Hammett type substituent effects A 272 CM CM NC Ar Ar NC CN 1123 CN CN p 71 The very negative value of p is consistent with a zwitterion being formed so that electron density is lost from the arene and builds up on one of the tetracyanoethylene carbon atoms ii Solvent effects in the above reactionthe rate in CH3CN 63 x l04 times greater than the rate in cyclohexane iii The formation of intensely colored solutions charge transfer complexes We hhhhhhhhh f 7f low wavelength absorptions iv Secondary D isotope effects 1129 H2 Hz 5 Ncozme EtD e HZ ammlquotle 302Me EtDquot N ll slow f t as H H1 H1 N Hr N no Me 2 H1 cone CDZMe H1 1130 vi The interception of intermediates H oat Ncozme Eto e H NC ZM G 4 I 1 slow H C02M9 H H quotCOZMe 1131 H20 0HZ H0 H Em HNcone Em H eNcone H N H H cone H H COZMe 273 Potential Energy kcalmol vii A nice computational example is provided by the reaction of ketene CH2CO with imine CH2NH In the gas phase this is a single step reaction where the CN bond is almost fully formed and the CC bond is quite long If the calculations are done using a dielec tric continuum model representing aqueous solvation a very different picture emerges The two paths are shown below where the numbers in parenthesis for the structures comes from the gasphase mCNCC 393 317 p 703 D 971 D 174 n 721 D 1063 D I l4 gt 1132 Reaction Path 274 H In solution a very stabile zwiterionic intermediate is formed which then must undergo rotation around the NC bond and CC ring closure to the product in the ratedetermining step In the gas phase this intermedi ate is not formed Notice that aqueous sovation appreciably stabilizes the transition state for ring closure Geometrically there is not much difference between the two transition states E Olefin dimerization the photochemical reaction I Clues which suggest that a totally different mechanism must occur aA remarkable range of substituents can be used eg l LE ch CH3 ch CH3 hv I I ma CH3 CH3 ch bThe reaction is stereospecificwhich strongly sug gests a concerted mechanism CH3 hv CH3 Hac hv J J 2 Let us go back to the original correlation diagram that we developed and consider what would happen in the first excited state 1134 275 AS z SA SS 33 M 3 33 a Excitation of an electron from the SA orbita whic is strongly antibonding between the ethylenes to the AS orbital which is strongly bonding between the ethylenes consequently will allow the olefins to ap proach each other This is called the exciplexThere is a neat experimental example of this phenomena on the next page which is derived from a series of very fanciful molecules called pagodanes see Pure and Applied Chem 67 673 I995 Oxidation of the bis olefin causes the CC bonds to stretch by approxi mately 0 l 0A and a collapse of the nonbonded CC distances between the two olefins by 055A Notice that oxidation of the totally saturated pagodane by two electrons also leads to the same intermediate Here the metrical changes are opposite in sign but nearly the same In magnitude 276 a2765 a157 b133 b155A bThe reaction between the rst excited state of the olefin dimer and the rst excited state of the cyclobutane is symmetry allowed cThere is one problem however with this scheme i Notice that the one electron in SA must rise up very high in energy to the 5 eveThis should energetically be very unfavorable ii Another way to put this is that the lSt excited state of an olefin lies in the UV range gt I65 nm absorp tion the lSt excited state of an alkane 5 gt 5 l35nm is much higher iii There then should be no driving force for this reaction since the exciplex can return back by fores cence to ground state olefin dimer dThere is a way around this but we have to look at the properties of the electronic state in a molecule 1 gt WIW2 a product function of orbitals i There is a symmetry associated with each statethis can be determined by multiplying each orbital function times another according to the following rules SXSS SXAA AXAS 277 ii Thus the first few excited states of the olefin dimer and cyclobutane are the following a For the olefin dimer m1m2 m1m2 AA l AA AS 39l39 39l39 39H39 AS 1136 SA SA SS 1 Hl39 39Hl I i H SS this IS the SS SS2 SS SS1 ss1 SS2 ground state of SA2 SA1 SA1 SA2 SA2 As2 cyclobutane As1 AA1 AS1 AA1 lll Ill I ll Ill Ill 55 AA As As AA 55 states 9 II 112 W P W5 W6 b for cyclobutane m1m2 m1m2 AA l AA SA 39l39 39l39 39H39 SA 1137 AS H l H H AS SS H H H l H SS SS SS2 SS ss 2 3 552 this is the ground AS AA1 SAlz state of the olefin Ill I III III Ill 1 dimer SS AA SA SA AA SS states gt 111 112 113 II 1 5 396 iii One can then energetically order the relative energies for the ground and excited electronic states of the olefin dimer and cyclobutane It is now im perative to correlate states of the same symmetry The hypothetical scheme then is 278 5 SS2SA2 AA 9 SA SA AA SS2SA1AS1 1138 SS2SA2 ss 55 SS2AS2 a The ground state of reactant should correlate directly dashed line to the doubly excited state of product and viceversa b However these two states have the same total symmetry so they avoid each other solid line c In the thermal concerted addition this then is the source of the gigantic barrier one goes from P to U directly via the high energyT d In the photochemical process one has photochemi cal excitation from P to Q i Q must then rise in energy and at sometime point R cross states from AA to SS ii This then immediately decays from R to S and S to T to U without activation the state symmetry always remains SS e But how can we test this mechanism i So far no one has been able to do this 279 ii The problem is that no activation energies can be measured a We would expect that the crossing from Q to R and perhaps from S to T would have a large negative ASzlz from related state crossings in thermal reactions b The diagram is simplified in that we have not shown vibrational levels excitation to Q does mt produce Q in the first vibrational level c If you look closely at the relative phases of the MO s Q should have a CC equilibrium distance far shorter than point P The equilibrium distance for the doubly excited state should be even less d The only thing that holds the SS ground state together is being caught in the solvent cage Since there are no strong interactions between two ethyl enes or solventethylene dimer point P should be in a very shallow well e An idealized onedimensional P E surface for the concerted reaction redrawn according to the consid erations above is given below 55 5515A2 SS2AS2 55 AA 5515A1AS1 5515A1A51 AA R S conical intersectiqn39 5515A2 55 E 55 ss AS2 280 i Note vertical excitation from P to Q produces a vibrationally excited state Q which can decay via R in any number of places without any AHt contribution ii The equilibrium distance for the doubly excited SS state is actually point S halfway between reactants and productsThis decays toT from the conical inter section of the two surfaces f In general the potential energy surface of a photo chemical process can be represented by This is a representation of two competing thermal and photochemical processes In the photochemical world life is much more difficult due to surface crossings internal conversion conical intersections and the like i One might think that the photochemistry of H2 D2 should follow the ole n dimerization since there is a relationship between the two thermal processes Unfortunately this is not the case ii The potential energy surface for the lowest doubly excited state of the H2 H2 reaction is shown on the next page It is strongly repulsive in all directions In fact the H2 distances are quite long compared to the ground state see previous PE surfaces The surface for the lowest singly excited state is shown by two views on the next page 281 L7OL75 485 3393 H i 39 4 R k H This is the lowest douny excited state I39ere are two views of ie owest singly excited ate I I43 H 282 iii Notice that there is a low energy cell formed This is the exciplexThe exciplex is a square of I SA dimensions This can be compared to H2 in the ground state where the HH distance is 074A FThe DielsAlder reaction This reaction dates back to O Diels and KAlderj Liebig sAnn Chem460 98 I928The parent reaction is Q gt E 1144 This was reported by LJoshel and LW ButzjAm Chem Soc 63 3350 I94 This has been a reaction which has been around for a long time It is very useful because it is a very stereospecific reaction which tolerates many functional groups and so it is extensively used to build larger cyclic molecules from to acyclic pieces lThere are again three basic paths that one can consider for the reaction T i r T I J gt I l concerted Kx 39 synchronous T l r 39 I gt I I concerted K I asynchronous x 1145 x r nonconcerted 2 Since the reaction is stereospecificthe nonconcerted path seems rather unlikely However an argument devel oped for the two possible variants of the concerted reaction between two physical organic chemists MJS 283 Dewar and Ken Houk 3The first proposal of a reaction mechanism was given by Wasserman in I935 He envisioned a concerted reaction with CC bond distances of 20A in the transition state A early theoretical treatment of this reaction was given by Evans and Warhurst in I938 who also predicted a con certed process with activation barrier of 36 kcalmol Experimentally the reaction proceeds with a barrier of 275 kcalmol and it is exothermic AH by 384 kcalmol 4 In more modern times R BWoodward and TJ KatZ proposed that the reaction was asynchronous concerted and Dewar at the time I959 argued strongly that it was a concerted synchronous path Woodward and R Hoffmann proposed a set of orbital symmetry rules in I965 and on this basis a concerted synchronous path for the DielsAlder reaction was preferred On the basis of MO calculations and some philosophical discourse Dewar in I974 reversed himself and argued very strongly for the asynchronous path In fact in I984 he proclaimed in the title of a JACS article Multibond Reactions Cannot Normally Be Synchronous In I986 it became recognized that there were in fact two transition states for this reaction that could be IocatedThis was work done in Ken Houk s group who was a former student of R B Woodward A very interesting humorous account of the history of this as well as related mechanistic arguments can be found in Acct Chem 288I I995 Please get a copy and read it 5At the best level of theory the concerted synchronous transition state has an activation barrier of 29 kcalmol and is 5 7 kcalmol lower in energy than the asychronous oneThe structures are shown below 1397 1362 II46 Concerted TS of the DialsAlder Reaction Stepwise TS of the DialsAlder Reaction CASSCF321 G CASSCF631G CASSCF321 G CASSCF631G 284 6 Now that estimate of the energy differences has stood the test of several more advanced computational tech niques however that is not to say that the situation could not change in the future An independent measure is derived from the computed and experiment values of secondary Disotope effects 102 101 106107 rquot 3 1021 00 1 031 06 H O96O94 093096 O98098 quot p 1 05 097 O93O96 097095 39 111110 cg 099 097 092096 110110 I I47a lt 02 Dz Calculated KIE at CASSCF631G HF631G39 02 CN lt r i 02 Exptl 091 098 089 via concerted TS 088 098 087 via stepwise TS 093 113 106 W W Dz 02 Exptl 079 098 078 vua concerted TS 084 097 081 via stepwise TS 090 118 107 The clear pattern is that the synchronous path is closer to experiment CN f Cir I I47b 7The femtosecond spectroscopy of Zewail was used to study the retro DielsAlder reaction See jAm Chem Soc I I8 8755 I996The reactions studied were the decom position of norbornene and norboradiene g y eW 285 8The results are shown below Let us only consider the results for norbornene those for norbornadiene are very similar A species with the molecular weight of the precursor 94 amu rises and decays with a time constant of I60 i 20 fs There is another species at 66 amu which rises with a time constant of 30 i 5 fs and decays at 220 i 20 fsThis is a real intermediate since its intensity drops off with time and it has the same molecular weight as cyclopentadiene A careful analysis was made of the decay of the 94 amu peak and the rise of the 66 amu peak They do not match and in fact differ by a factor of 5 Thereforethe 94 and 66 amu peaks must represent separate trajectories reaction paths 10 o s o s m lauumu t Intensity s in 7 on s o is m Mouhmu Intensity 66 amu 400 200 0 200 400 600 800 1000 1200 Time Delay fs I I48 9The interpretation of these results was initially thought to be that there were two reaction paths a concerted path leads to the species decaying with the 220 fs rate since the time constant for stretching a CC bond is 40 fs andthereforethis is exactly in the range of the 286 30 fs buildup So this is then a concerted asynchronous route The species with the I60 fs lifetime is a diradical intermediate This is certainly a novel mecha nism Later MO calculations were reported by Houk etal PureApp Chem 70 I947 I998 which offered an alternative explanation The initial photon pulse produces a highly vibrationally excited state where one electron has been promoted from the T to the n orbital This then decays into a conical intersection Either one bond breaks to produce a diradical inter mediate which then undergoes closure to form a variety of products Alternatively two bonds are broken to form excited state cyclopentadiene which then decays The full scheme is shown below zb 1rn 184 kcalmol conical intersection biradical l l collapsed products 1 220 fs quot2 l0 The moral of this story at this point in time is then that the femtosecond spectroscopy study of this reaction does not correlate with what occurs in the ground state thermal reaction For most DielsAlder reactions the synchronous concerted path is the most reasonable 30 f5 1149 287 GThis is only a very small part of the story about cycloaddi tion reactions l Basically one must always deal with a spectrum of mechanistic behvior r1 A B A B C D gt C D A B A B r2 1150 r1 r2 2What makes cycloaddition reactions so useful and a study of their mechanisms so important is that the stere ochemistry including regiochemistry see below can be predicted and used to form much larger organic molecules r1 A quot D A c A c B C B D ltBDgt 1151 r2 In other words cycloadditions form an important tool to synthesize natural products etc But you will have to take a couple more organic classes physical organic and synthesis to learn about this 288


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