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A Fischer carbene complex typically has a low-oxidation-state, relatively electron-rich middle/late transition metal with strong pi-acceptor ligands such as CO. The carbene is treated as a neutral two-electron donor; metal-to-carbene back-donation contributes to its bond, but its carbon remains electrophilic. An OR or NR${}_2$ substituent stabilizes that carbon by heteroatom donation. Here the metal is Cr(0), $d^6$, in an 18-electron pentacarbonyl complex.

A Schrock carbene complex usually contains an early metal in a high oxidation state, often $d^0$, with electron-donating ligands rather than a shell of pi-accepting carbonyls. Its alkylidene has substantial dianionic character in ionic counting, and its carbon is correspondingly nucleophilic. Alkyl substituents are common; heteroatom stabilization of an electrophilic carbon is not the defining feature. These are bonding/electronic limiting descriptions rather than a rule based only on the drawn metal-carbon double bond.

Coordinate one diphenylacetylene and insert it into Rh-Ar. Syn migration puts the new Rh-C and Ar-C bonds on the same face of the former alkyne. Repeat alkyne coordination/insertion into the resulting Rh-vinyl bond. The chain after two insertions isNumbering runs from the metal toward the initiating aryl group. Each insertion is syn: Rh and the continuing chain lie on the same side of the newly made double bond, giving the displayed cis relationship of the two Ph substituents at each inserted unit. The second alkyne inserts at the Rh end; it is not appended by coupling two free vinyl radicals.

To obtain F, promote PPh${}_3$ dissociation, allowing G's dienyl group to fold and form the new C${}_1$-C${}_4$ bond. A four-pi-electron electrocyclic ring closure gives a cyclobutenyl ligand, stabilized by eta-three coordination of its allylic C${}_1$-C${}_2$-C${}_3$ fragment to Rh. The substituted C${}_4$ is the saturated ring carbon bearing Ar and Ph.

The thermal four-pi closure is conrotatory. Ring closure plus eta-three binding restores a 16-electron Rh(I) centre after phosphine loss; no net oxidation-state change is needed. F is an eta-three cyclobutenyl complex, not an eta-four cyclobutadiene with five substituents squeezed onto four unsaturated carbons.

The two substituents are $-\mathrm{C}{\mathrm{H}}_2-\mathrm{O}-\mathrm{C}(\mathrm{O})-\mathrm{CH}=\mathrm{CH}-\mathrm{Ph}$, with the alkene E, and $-\mathrm{C}(\mathrm{OH})(\mathrm{C}{\mathrm{H}}_3{)}_2$. The free bond at the left of each drawing attaches to the cyclohexene.

The A-D COSY correlation and $16.2\mathrm{Hz}$ coupling identify the trans cinnamate double bond. Five aromatic protons and four phenyl-carbon environments identify its monosubstituted phenyl ring. F is a two-proton OCH${}_2$ signal; its HMBC correlation to carbonyl 1 joins that methylene through oxygen to the ester. The two three-proton singlets N/O, their correlations to oxygenated quaternary carbon 10, and the missing OH resonance give the tertiary-alcohol substituent.

On the ester-containing side chain, read outward from the cyclohexene:A is the downfield beta-vinylic proton and D the alpha-vinylic proton next to the ester.

The phenyl ortho pair is B on carbon 7; the meta pair is C on carbon 6; its para proton is the remaining C on carbon 5. Thus the C envelope integrates to three protons, while B integrates to two. Carbon 3 is the unprotonated ipso site.

In the other substituent, 10 is the quaternary C-OH carbon. The methyl assignments are N on 13 and O on 16, reading the very closely spaced HSQC cross peaks together with the positive methyl responses in DEPT-135. Their mutual long-range correlations and correlations to 10 support the gem-dimethyl group. The two methyl carbons are distinguishable in this chiral molecular environment; their exact stereotopic names are not determined here.

Number ring positions $r_1$ through $r_6$ around the ring, with $r_1=r_2$ the double bond. These auxiliary ring-position labels are distinct from the printed carbon-signal numbers.

The HSQC pairs J and M with 17 and places L on 12. It also places K and one unresolved member of G/H/I on one of the closely spaced allylic carbons, with the other two G/H/I protons on the other. COSY connects the vinylic E to the adjacent allylic part and connects the methine L to its neighboring methylenes; HMBC links F to the substituted alkene end and the gem-dimethyl group to the methine end.

The printed overlap does not justify a unique assignment of every G/H/I letter or a forced 14-versus-15 assignment to those two ring positions. The drawing retains those alternatives explicitly. In the half-chair, L is pseudoaxial; the resolved large/small couplings are consistent with pseudoaxial/pseudoequatorial methylene partners, but exact axial letter labels within the overlapped group remain uncertain.

The constitution is [4-(2-hydroxypropan-2-yl)cyclohex-1-en-1-yl]methyl (E)-3-phenylprop-2-enoate, with absolute configuration at the saturated substituted ring carbon unspecified.

The OCH${}_2$ side chain is attached to the unprotonated alkene carbon 4: F correlates through the ring junction to the alkene carbons and an allylic methylene, while its correlation to 1 fixes the ester linkage. The $\mathrm{C}(\mathrm{OH})(\mathrm{C}{\mathrm{H}}_3{)}_2$ side chain is attached to methine carbon 12, as shown by the methyl-to-12 HMBC correlations and the contiguous ring COSY network.

The formula check gives 19 carbons and 24 hydrogens including OH. Its degree of unsaturation is eight: four from phenyl, one ester carbonyl, one cinnamate C=C, one cyclohexene C=C and one ring. No additional double bond or hidden second carbonyl is needed.

At night, nitrate radical addition and ozone attack are relevant oxidations of isoprene; direct daytime photochemistry is absent, though small OH sources can still exist. The nitrate-containing products identify the NO${}_3$-addition route. Add NO${}_3$ to the terminal carbon next to the methyl-substituted alkene carbon, producing a nitrooxy substituent and an allylic radical. Resonance moves the radical to the far terminal carbon while shifting the double bond. Oxygen trapping gives a peroxy radical with the carbon skeletonA nitrate-forming termination branch, for example the organic-nitrate branch of reaction with NO, replaces the terminal peroxyl group by a nitrate ester, giving ${\mathrm{O}}_2\mathrm{NOC}{\mathrm{H}}_2-\mathrm{C}(\mathrm{C}{\mathrm{H}}_3)=\mathrm{CH}-\mathrm{C}{\mathrm{H}}_2\mathrm{ON}{\mathrm{O}}_2$. An alternative branch forms an alkoxy radical, such as peroxy-radical reaction with NO${}_3$ producing RO${}^{\bullet }$, NO${}_2$ and O${}_2$. Reaction of the terminal CH${}_2$O${}^{\bullet }$ group with O${}_2$ then gives an aldehyde plus HO${}_2$, producing ${\mathrm{O}}_2\mathrm{NOC}{\mathrm{H}}_2-\mathrm{C}(\mathrm{C}{\mathrm{H}}_3)=\mathrm{CH}-\mathrm{CHO}$. These are covalent O–NO${}_2$ ester groups, not C–NO${}_2$ nitro groups. The delocalized radical can be trapped after conformational change, so this mechanism does not require one universal alkene geometry for both product channels. Product structures alone do not fix a unique branching ratio.

Amide base removes a proton alpha to the nitrile, giving a resonance-stabilized carbon nucleophile. Removal of the available aryl proton ortho to chlorine, together with chloride elimination, generates a benzyne between the chlorine-bearing carbon and its unsubstituted ortho neighbour. The side-chain nitrile carbanion attacks the former chlorine-bearing aryne carbon intramolecularly; one aryne pi bond shifts to the neighbouring aryl carbon. Protonation of that aryl anion by ammonia restores the aromatic ring. The newly formed C–C bond closes the four-membered ring of the cyanobenzocyclobutane product. Two deprotonation processes are required to generate both the nitrile nucleophile and the aryne. The nitrile group is retained, and the product is not formed by simple replacement of aryl chlorine with an external amide nucleophile.

Base first gives phenoxide. Use its para-carbanion/quinoid resonance contributor: the para carbon bearing the tether attacks the terminal chloromethylene carbon in an intramolecular substitution, ejecting chloride. This forms a five-membered ring at that carbon and produces a para-spirocyclohexadienone, with the phenoxide oxygen becoming a carbonyl oxygen. The first stage is a dearomatizing spirocyclization of a phenoxide.

On acidification, carbonyl protonation makes the dienone electrophilic. A tether C–C bond attached to the spiro centre migrates to the adjacent dienone carbon, with the accompanying pi-bond shifts; loss of a proton restores aromaticity. This dienone-phenol rearrangement converts the spiro five-membered ring into the fused six-membered ring while regenerating the phenolic OH. The apparent bond formation meta to OH is thus explained by para spirocyclization followed by skeletal migration, rather than requiring the final fused connectivity to form directly in the base step.

Activate the oxime hydroxyl oxygen with thionyl chloride to give a good N–O leaving group. In the Beckmann rearrangement branch, the C–C bond anti to that leaving group migrates onto nitrogen as the N–O bond breaks. Here the migrating carbon is the spiro centre. Inserting N between that centre and the former oxime carbon expands the five-membered oxime ring to a six-membered nitrilium ion ring. Water attacks the electrophilic nitrilium carbon, followed by proton transfers and tautomerization to the observed spiro lactam. The lactam carbonyl oxygen is introduced in the aqueous work-up.

The competing Beckmann fragmentation cleaves the same anti C–C bond instead of completing migration onto N. Formation of the C≡N bond gives an open-chain nitrile and a tertiary carbocation at the former spiro centre. Loss of a proton from the adjacent chain carbon then gives the exocyclic alkene attached to the retained cyclopentane. The key distinction is N insertion and ring expansion for the lactam, versus C–C cleavage followed by elimination for the unsaturated nitrile. Stabilization of the resulting tertiary carbocation makes fragmentation plausible.

Copper-assisted loss of nitrogen from the diazo group forms a metal carbenoid at the carbon bearing the ester substituent. That tethered electrophilic carbon adds across the nearby exocyclic alkene in an intramolecular cyclopropanation. Two C–C bonds form: one to the alkene ring carbon and one to its terminal methylene. The existing tether thereby also becomes the five-membered ketone ring in the product; the methyl ester stays attached to the former diazo carbon.

Cyclopropanation is suprafacial on the reacting alkene face. A folded tether geometry that permits the short intramolecular closure selects the approach face and accounts for the shown relationship of the angular methyl group and the new cyclopropane methylene bridge. This is a stereoselective conformational ring-closure model, not a freely rotating intermediate carbocation: the two new bonds are formed in the same cyclopropanation event. A detailed quantitative preference between folded conformers would require an energetic model beyond the structure alone.

A to B is a photochemical six-electron electrocyclic ring opening; B to C is a thermal six-electron electrocyclic ring closure. The broken or formed sigma bond is between the two methyl-bearing saturated carbons of the cyclohexadiene. Both transformations conserve the carbon skeleton. The light-induced and thermal processes differ in their orbital occupation and therefore in their stereochemical selection rule.

Addition consumes the carbon–carbon double bond of vinyl chloride and retains chlorine as a side substituent. The interior repeat of A is thereforenamely polyvinyl chloride. Benzoyl peroxide supplies initiating free radicals; head-to-tail addition gives the usual backbone connectivity. End groups and tacticity are not uniquely specified by this repeat formula.

B contains both $-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}(\mathrm{Cl})-$ and $-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}(\mathrm{OCOC}{\mathrm{H}}_3)-$ monomeric units distributed along the same carbon backbone: a vinyl chloride-vinyl acetate copolymer. One possible local segment isThe ester is attached through oxygen to the backbone carbon. A random copolymer should not be drawn as two long homopolymer blocks. The stated $75:25$ monomer feed is not enough to prove an incorporated $75:25$ unit ratio: monomer reactivity ratios and conversion affect composition and sequence statistics. Under an explicitly ideal equal-incorporation assumption that ratio could be used as a schematic average, not a periodic repeat.

Saponification cleaves the acetate ester side groups while the intended carbon backbone remains intact:The ideal target C is therefore a vinyl chloride-vinyl alcohol copolymer, containing $-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}(\mathrm{Cl})-$ and $-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}(\mathrm{OH})-$ units in the original sequence positions. The carbon–oxygen bond to the backbone remains; hydrolysis removes the acetyl group from oxygen. Incomplete hydrolysis would leave a three-component population of chloride, acetate and alcohol units. New hydroxyl groups can strengthen adhesion and coating interactions through hydrogen bonds.

Only one bromine is replaced by hydrogen. Writing the unchanged protected chain as $R=\mathrm{THPOC}{\mathrm{H}}_2\mathrm{CH}(\mathrm{Me})$, the product is $R\mathrm{CH}=\mathrm{C}(\mathrm{Me})\mathrm{CH}=\mathrm{CHBr}$, retaining the original internal alkene geometry and forming predominantly the $Z$ terminal bromoalkene: bromine and the carbon chain lie on the same side of that new stereodefined double bond. The formula is consequently ${\mathrm{C}}_{13}{\mathrm{H}}_{21}\mathrm{Br}{\mathrm{O}}_2$.

Phosphine dissociation exposes Pd(0), which undergoes preferential oxidative addition into the less hindered C–Br bond, the bromine trans to the carbon chain in the gem-dibromoalkene. hydride transfer from tributyltin hydride replaces the palladium bromide ligand, producing tributyltin bromide and an alkenyl–Pd–H intermediate. reductive elimination joins the alkenyl carbon and hydride, regenerating Pd(0). Retention of the alkenyl geometry through this sequence puts the surviving bromine cis to the chain. The conjugated carbon skeleton and protecting group remain intact; this is not addition of tin hydride across either alkene. The stereoselective monohydrogenolysis is established for this reagent combination.^[ref]

The first equivalent of titanocene methylidene methylenates the ester: the carbonyl oxygen and titanium form an oxatitanacyclobutane, whose fragmentation gives $\mathrm{PhC}(\mathrm{OMe})=\mathrm{C}{\mathrm{H}}_2$ and a titanium–oxygen product. The second equivalent adds across this electron-rich double bond. The initial adduct is a titanacyclobutane with cyclic connectivity $\mathrm{C}{\mathrm{p}}_2\mathrm{Ti}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}(\mathrm{Ph})(\mathrm{OMe})-\mathrm{C}{\mathrm{H}}_2-$, closing from the last methylene to titanium. The ester's phenyl-substituted carbon lies opposite Ti; the two new methylenes originate from the two methylidene equivalents.

Beta-methoxy elimination opens that metallacycle to B, $\mathrm{C}{\mathrm{p}}_2\mathrm{Ti}(\mathrm{OMe})[\mathrm{C}{\mathrm{H}}_2\mathrm{C}(\mathrm{Ph})=\mathrm{C}{\mathrm{H}}_2]$, the allyltitanium reagent. The C–OMe bond breaks, OMe transfers to Ti, one Ti–C bond becomes the new C=C bond, and the other Ti–C bond remains as the allylic nucleophile. The cyclic adduct is therefore a precursor to the opened reagent B; treating B as merely the one-equivalent enol ether loses the second reagent carbon.

The aldehyde oxygen coordinates to titanium. Allylic carbon transfer to its carbonyl carbon, accompanied by movement of the C=O pi electrons to oxygen, gives the titanium alkoxide $\mathrm{PhC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{CH}(\mathrm{O}-\mathrm{Ti})\mathrm{C}{\mathrm{H}}_2\mathrm{C}(\mathrm{Ph})=\mathrm{C}{\mathrm{H}}_2$. Acidic aqueous workup cleaves O–Ti and protonates oxygen, giving $\mathrm{PhC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{CH}(\mathrm{OH})\mathrm{C}{\mathrm{H}}_2\mathrm{C}(\mathrm{Ph})=\mathrm{C}{\mathrm{H}}_2$. A six-membered allyl-transfer arrangement is a convenient mechanistic model. No enantioselectivity is imposed by these achiral reagents.

Schwartz reagent adds syn across the terminal alkene, placing zirconium at its terminal carbon and hydrogen at the substituted carbon. The organozirconium intermediate is $\mathrm{PhCH}(\mathrm{OMe})\mathrm{CH}(\mathrm{Me})\mathrm{C}{\mathrm{H}}_2\mathrm{ZrC}{\mathrm{p}}_2\mathrm{Cl}$. The two faces of the achiral starting alkene give enantiomeric organometallic intermediates.

Boron trifluoride coordinates the ether oxygen, making the benzylic C–OMe bond susceptible to cleavage. The C–Zr bond then transfers its carbon terminus intramolecularly to the benzylic carbon as the activated methoxy group departs, closing a cyclopropane. This can be represented as a concerted, strongly assisted displacement rather than requiring a freely equilibrating long-lived benzylic carbocation. hydrozirconation fixes the relative disposition of the new C–H and C–Zr bonds, and the preferred closure conformation places the bulky phenyl and methyl groups on opposite faces. The result is racemic trans-1-methyl-2-phenylcyclopropane, with much less steric crowding than the cis alternative. The ring contains the original two alkene carbons and the original benzylic carbon; no carbene carbon is added.

Let $\mathrm{Ar}=2,6$-dimethylphenyl and $\mathrm{A}{\mathrm{r}}^{\prime }=2$-methoxyphenyl. After conversion of the Pd(OAc)${}_2$ precatalyst into an active phosphine–Pd(0) species, oxidative addition of $\mathrm{A}{\mathrm{r}}^{\prime }\mathrm{Cl}$ gives ${\mathrm{L}}_{\mathrm{n}}\mathrm{P}{\mathrm{d}}^{\mathrm{II}}(\mathrm{A}{\mathrm{r}}^{\prime })\mathrm{Cl}$. Carbonate-assisted replacement of chloride by the alcohol's alkoxide produces ${\mathrm{L}}_{\mathrm{n}}\mathrm{P}{\mathrm{d}}^{\mathrm{II}}(\mathrm{A}{\mathrm{r}}^{\prime })[\mathrm{OC}(\mathrm{Me}{)}_2\mathrm{Ar}]$.

In the key beta-carbon elimination, the alkoxide oxygen forms a C=O bond while the bond between its carbinol carbon and $\mathrm{Ar}$ cleaves, transferring $\mathrm{Ar}$ to palladium. This gives acetone and a diarylpalladium(II) complex, ${\mathrm{L}}_{\mathrm{n}}\mathrm{P}{\mathrm{d}}^{\mathrm{II}}(\mathrm{A}{\mathrm{r}}^{\prime })(\mathrm{Ar})$. reductive elimination creates the new biaryl bond and regenerates Pd(0). Thus the tertiary alcohol functions as an aryl donor by carbon–carbon bond cleavage, rather than being incorporated as a carbinol substituent. The overall organic products are the cross-coupled biaryl and acetone. The carbinol carbon has no hydrogen, so ordinary beta-hydride elimination from this alkoxide cannot explain the observed aryl transfer. The bulky electron-rich phosphine helps access the aryl-chloride oxidative addition and the sterically congested biaryl-forming elimination.

acetone first forms a proline enamine by reversible iminium formation and alpha-deprotonation. The enamine attacks the aromatic aldehyde in a hydrogen-bonded arrangement involving the catalyst's carboxylic acid. hydrolysis of the resulting iminium releases the beta-hydroxyketone and regenerates proline. The usual L-proline arrangement gives the $R$ secondary alcohol.

The sterically crowded aryl–amide axis is selected at the same bond-forming step: rapid rotation in the aldehyde permits the catalyst to react preferentially with the conformer that puts the smaller carbonyl-oxygen side of the amide towards the approaching aldol transition structure and the bulky diisopropylamino side away. A chair-like enamine–aldehyde model minimizes the interaction of the amide with the catalyst-side axial hydrogen; reversing the amide orientation creates the less favourable diastereomeric transition state. Replacing planar CHO by tetrahedral CH(OH)CH${}_2$COMe then raises the rotation barrier and traps the axis. This is a dynamic kinetic resolution, not resolution of two configurationally stable starting atropisomers.

To specify the predicted major axis without relying on a flat bond-line formula, look from the aromatic ipso carbon towards the amide carbonyl carbon. Put the higher-priority aryl path, the trimethylsilyl-bearing ortho side, at nine o'clock. The preferred model puts the carbonyl oxygen at approximately six o'clock and the diisopropylamino group at twelve o'clock. This is $S_a$ using the near-to-far high-priority convention; oxygen outranks nitrogen at the far end. Coupled with the alcohol assignment, the model predicts $(R,S_a)$. The drawing defines the configuration even if an alternative viewing direction is used. The dynamic-resolution mechanism is established by the primary aldol study; the product-axis assignment here is a stereochemical-model prediction for the printed substrate.^[ref]