σ 1 * - σ 2 * σ 1 * + σ 2 * σ 1 - σ 2 σ 1 + σ 2

Transcription

1 EMITY 0- EXM I NWE I.. a. rbitrarily shown is the endo mode of the approach in parallel planes with p-orbitals at the termini pointing toward one another. The reacting orbitals of the reactants will be ψ through ψ of,-butadiene and χ through χ of the allyl system. The reacting orbitals of the product are σ, σ *, σ, σ * of the newly formed single bonds, π, π* of the double bond, and p for the p-orbital on the or sp -hybridized positively charged carbon. These orbitals are shown below. Because the σ orbitals are neither symmetric nor anti-symmetric, sums and differences of them are formed. The symmetry plane is perpendicular to the page; it bisects - of the diene and of allyl. ETNT POUT σ * - σ * σ * σ * ψ ψ χ π* ψ ψ χ χ π p σ - σ σ σ b. There are four symmetric and three anti-symmetric orbitals for the reactants and also for the product (see above). Using an assumption made in lecture (which is not rigorous but gets to the correct correlations), the orbitals correlate in order of their stability. ψ χ ψ σ * - σ * σ * σ * π* χ p ψ χ ψ π σ - σ σ σ c. For the reactants in the ground-state, there are four π-electrons in,-butadiene, two in ψ and two in ψ ; there are also two π-electrons in the allyl cation, both in χ. For the product, there are two electrons, each, in the combination σ orbitals and two more in the π orbital. ccording to the "advice" on the exam, the reactant first excited state is the one in which one electron from butadiene's ψ is promoted to the antibonding ψ ; for the product, one electron has been excited from π to π*.

2 (ψ ) (ψ ) (χ ) (χ ) (σ σ ) (σ - σ )(π) (p) first excited state? (ψ ) (ψ ) (χ ) (ψ ) (σ σ ) (σ - σ ) (π) (π*) first excited state? ground state (ψ ) (ψ ) (χ ) (σ σ ) (σ - σ ) (π) ground state d. Note the smooth correlation of the reactant and product ground-states (above). On the other hand, the first excited states correlate, but only after passing over a symmetry-imposed barrier. (The intended correlation of both of the first excited states with a high excited state of the partner does not occur because of the noncrossing rule.) The clusion: the thermal reaction is symmetry-allowed; a certed photochemical reaction is symmetry-forbidden.. In the endo mode, we examine possible interactions between the OMO of one component with the LUMO of the other. s shown to the right, the OMO of butadiene with the LUMO of allyl gives no sedary orbital interaction (neither favorable nor unfavorable) but the other interaction (OMO of allyl with LUMO of butadiene) shows an anti-bonding sedary orbital interaction that will destabilize the endo mode. In this reaction, exo is favored. χ ψ χ ψ. One way to do this be to look at the reaction of a cyclic diene with a cyclic allylic cation, as shown to the right. With such a pair of reactants, exo and endo approaches lead to different stereoisomers. nother way to do this is with acyclic reactants that have suitable stereochemical features at the termini of both reactants. uch a combination would be the two E,E reactants shown here. endo E,E exo E,E endo exo. The stepwise process here generates an allylic carbocation intermediate whose stability is about the make - make - same as that of the reactants; furthermore, a new single bond has been made. In trast, stepwise processes for a diene plus a dienophile necessarily produce an intermediate that is either a diradical or a dipolar ion (zwitterion) B. The k= and k- processes look reasonable, as each produces and unstrained product. In fact, the k= process, in the right-to-left direction, is the very fast closure of the EZZE triene that uisgen found to occur at -0 ; and the k= closure was reported by him to occur rapidly at 0. In trast to these, the potential k=, which orbital symmetry predicts will occur, would produce a very strained trans-fused bicyclic product.

3 or () k= Z E k= or ZZZZ () Z E k= ().. This is a k= opening of a cyclohexadiene to a hexatriene. Orbital symmetry predicts rotation (which is what is shown on the exam).. This is a k= opening of a cyclobutene to a butadiene. Orbital symmetry predicts rotation which would lead to a strained ZZZE tetraene, not what is what is shown on the exam.. Two important reasons (there may be others) are: (a) a planar eight-membered ring would have internal bond angles of ; this is so much larger than the ideal sp angle of 0 as to cause significant angle strain; (b) a planar structure would have eight π-electrons, a non-ückel number; it would be anti-aromatic if it stayed as a regular octagon with equal bond lengths (as in benzene). uch a species is predicted to be a triplet diradical.. It would seem reasonable that bond-switch occurs by first flattening the ring (as shown below) followed by a reorganization of the eight π-electrons; the transition state for the latter is, in fact, the anti-aromatic delocalized species described in Part. The high activation energy is a result of angle strain and antiaromaticity. flatten bond shift. The, ubstituted compound provides a means of tinguishing between processes (a) and (b). The symmetry-allowed k= gives a cyclooctatraene with the alkyls at the ends of a = bond; the symmetryforbidden k= gives an isomer (a bondshift isomer, in fact) in which the alkyls straddle a - single bond. k= k=. s established by epuy and ristol and as developed by chleyer and chöllkopf, the preferred rotatory opening of a cyclopropyl halide (or related compound) occurs with the groups trans to the leaving group l rotating outward. In the bicyclo [..0] Br Br and [..0] cases cussed in class, it -gl O is the s at the bridgehead that Br O outward. Thus, for isomer to lose chloride but not bromide, it must have the structure shown here; similarly Br isomer B is its epimer. B l -gbr l O O l

4 II.. fter hydrogenation over Lindlar catalyst, the resulting EZZE tetraene undergoes k= rotatory closure. Two formations are available for the cyclized product - each is set up for a k= rotatory closure to a bicyclo[..0] system (as also analyzed in Problem IB). One of these bicyclic structures is the ester of Endiandric cid F and the other G (the correct names, instead of Ponndorf). Note that Endiandric cid F s ester, after Ph Z Z E E k= OOMe ' = ring flip ' ' k= k= F G ' ' rotation of the transoid diene to a cisoid formation, is set up for intramolecular iels-lder reaction to the diene of the six-membered ring (as shown below); this produces the ester of Endiandric cid B. imilarly, the ester of Endiandric cid G can undergo intramolecular iels-lder reaction (as shown on the next page) and produce. Ph F ' Ph ' intramolecular iels-lder Ph B ' MeOO G MeOO intramolecular iels-lder MeOO B.. Yes, a photochemically-allowed process is [ σ s π s ] which will result in the retention of figuration found in the two stereoisomers of -butene.. No, the photochemically-allowed electrocyclic opening for k= involves rotation, but both cis- and trans- open by clean rotation.. I suggest that you read the article for details, but what the authors suggest is this: there is photochemical excitation to a ydberg state (some higher value of principal quantum number n) followed by "internal version to the ground state with close to unit efficiency." It is this "hot ground-state" that is responsible for the observed rotation. OK? OK... Isomers a and b ought to be formed in equal amounts, and each should give two iels-lder adducts, also in equal amounts, as shown here. Thus, barring some bizarre sedary isotope effect, one expects % each of adducts, B,, and. a b a MeOO MeOO MeOO b MeOO MeOO MeOO B OOMe OO. From square planar c, one would again expect % each of the four cycloadducts.. o, the experiment did produce all four adducts in equal yield, thus standing in the way of a tinction

5 between rectangular and square planar. Thus, a route that will produce only one of the rectangular structures (if the rectangular hypothesis is correct) is needed. ecomposition of azo compound by path (a) would be a symmetry-forbidden [ π s π s ] process; in trast, path (b) is a six-electron [ π s σ s σ s ] reaction (a reverse iels-lder reaction) that we ll talk more about in andout VI. Thus path (b) leading to b seems preferable.. ooray for us! We analyzed the situation correctly. The adducts from b (i.e., and ) were produced. Not only does this rule out square planar c as the structure but it says that the barrier separating a and b is substantial - it is large enough to hinder such equilibrium if b is trapped by a reactive partner.. With a less reactive dienophile, all four isomers of adduct were formed in equal yield. If the experiment described in Parts and had not been done, this experiment would not have allowed a tinction between rectangular and square planar. But in light of the fact that a barrier has been established between a and b, using a less reactive trapping agent allows the initially formed b to equilibrate fully with a before undergoing cycloaddition with methyl acrylate. III... Two features that destabilize relative to are the strained cyclopropane rings and the loss of aromaticity.. enumbering the carbons of as shown here points the way to a different retro iels-lder reaction. Nevertheless, the product is identical to on the exam. same as. One can establish forward iels-lder to followed by the reverse by using a substituted reactant. The authors of this work chose to break the symmetry using methyl (as shown on the next page) or deuterium. bridgehead substituent winds up on a terminal carbon of the diene unit. (There are also other positions to w h i c h t h e s y mmetry-breaking substituent could be attached.) Note that this problem is very similar to Exercise VIII on the propellane rearrangement (andout V, p. ). B.. a. [] []

6 b. c. One route to tricyanobenzenes and 9 is through iels-lder reaction of,-dicyanocyclobutadiene with cyanoacetylene (two ways) followed by symmetry-forbidden opening of the resulting ewar benzenes. iels lder 9 iels lder nother route would be via iels-lder reactions of,-dicyanocyclobutadiene with cyanoactylene. iels- lder iels- lder iels- lder iels- lder.. When isobenzofuran undergoes iels-lder reaction (via its furan moiety) with a dienophile a=b, it generates an aromatic system; this is not the case with furan itself. O a b O a b. This is a case of kinetic vs. thermodynamic trol. The first product shown (endo) is formed faster than the other and predominates at short reaction times; this is expected because the endo transition state is stabilized by sedary orbital overlap (or other factors). The sed product (exo) must be the more stable of the two, as it predominates at long reaction times. The faster-formed kinetic product reverts to the addends which then combine to give the more stable product; over time, it accumulates such that it is the nearly exclusive material in the mixture after three months.. This experiment introduces us to Frank-Gerritt Klärner (Universität Essen) who, in addition to John Baldwin and a few others, qualifies as one of oering's "intrepid purists" for his reliance on deuterium (a minimally perturbing substituent) to study reaction stereochemistry. We'll encounter much of his work later in the semester, for it was he and Baldwin who performed the experiments that produced tradictory results, as cussed in general terms in the lecture of February. [To found the story even further, I can add that Klärner was a co-author with oering on the paper from which I quoted the passage about "intrepid purists" and "soldiering pragmatists": J. m. hem. oc. 00,,.] nd now on to the answers.. This stereochemical outcome is precisely what would be expected from a [] iels-lder reaction that is suprafacial on both components. [Both dienes are shown, below, in cisoid formations.] endo 0a exo 0b. s shown above, 0a is the endo product and 0b the exo in this reaction. uriously, this case "violates" the expectation that endo will usually occur faster than exo; and it is interesting that this,-butadiene dimerization is the very reaction that Woodward and offmann used in developing their ideas on sedary orbital interactions. Go figure!. The :: ratio of ring stereoisomers () is sistent with a stepwise [] cycloaddition featuring a

7 diradical (or similar intermediate) that is long-lived enough to allow complete randomization of stereochemistry. In the drawing below, I've oriented the partners as was done in andout V for the [] reaction of acrylonitrile. There are no mechanistic inferences, yet. Orient dienes so as to produce trans deuteriums at the new ' bond Orient dienes so as to produce cis deuteriums at the new ' bond ' no rotation about a not formed ' no rotation about not formed b about both and ' c about both and ' not formed If we then allow bonding to produce a diradical, the outcome is shown in the figure on the next page. Unlike the oering case cussed in class, in which there was not complete rotation about the or bond, the results of this experiment can be best explained by assuming complete equilibration of the initially formed bis-allylic radicals. For some reason, most likely steric,* there is no closure to a cis-,- divinylcyclopropane. On the other hand, all closures to trans-,-divinylcyclopropane occur equally fast, leading to a :: ratio of isomers a, b, and c. Note, also, that the formation of a and c, the former with no rotation and the latter with rotation about both bonds is insistent with expectations for a certed [ π s π a ] reaction. [Product b could have been formed by this certed route, but there's no way that one could rationalize a certed process for one product and a stepwise process for the other two in such a minimally perturbed system. Note, finally, the complete retention of stereochemistry in the vinyl groups of all three products; this suggests that rotation about the former = to equilibrate the diradicals is much faster than rotation about the partial π-bond in the two allylic radicals. *I don t really believe this. fter all, the oering work on acrylonitrile dimerization, cussed in class, showed roughly 0/0 of the cis and trans cycloadducts. The vinyl groups, here, can t provide steric problems greater than cyano. Perhaps, instead, cis-,-divinylcyclobutanes are formed and undergo a ope rearrangement to (Z,Z)-,-cyclooctadiene. Klärner did report minor amounts of this by-product. ' ' make ' bond about a ' b about ' about ' ' make ' bond ' about c about ' b