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Chapter 2
The Synthesis of Novel
Dihydronaphthalenes and Benzofluorenes
Thischapterisconcernedwiththesynthesisandapplicationsofdihydronaphthalenes
andbenzofluorenes.Thischapterdescribesliteraturemethodsforthepreparationof
dihydronaphthalenesandbenzofluorenes.Effortstodiscovernewsyntheticmethods
forthesynthesisofdihydronaphthalenesandbenzofluorenesarethendescribed.
2.1 Introduction to Dihydronaphthalenes
The 1,2-dihydronaphthalene ring system is present in various natural products of
therapeutic importance including: cannabisins 48, isolated from the fruits of
Cannabissativa[1].6,7-dehydrosempervirol49[2],isolatedfromtherootsofSalvia
apianaandnegundinB50[3],isolatedfromtherootsofVitexnegundo.Nafoxidene
51 is a class of biologically active dihydronaphthalene and its analogues can be
prepared from 1-(4-benzyloxyphenyl)-6-methoxy-2-phenyl-3,4-dihydronaphtha-
lene[4–6].Dihydronaphthalenederivativesare used asfluorescent ligands for the
estrogenreceptor[7]andexhibitactivityasHepatitisCNS5Bpolymeraseinhibitors
[8].Recently,dihydronaphthaleneswerefoundtobepotentandselectiveinhibitors
ofaldosteronesynthase(CYP11B2)forthetreatmentofcongestiveheartfailureand
myocardialfibrosisFig. 2.1[9].
Asdihydronaphthalenederivativesareusefulstartingmaterialsforthesynthesis
of biologically active cyclic molecules [10], numerous traditional synthetic
approachestothesecompoundshavebeen reported[11–13,45].Amongthem the
dearomatisation ofnaphthalene derivativesbythenucleophilicadditionofcertain
organometallicreagentsisoneofthemostusefulandconvenientmethods[14–18].A
drawbackofthenucleophilicadditionmethodisthedifficultyinapplicationtoawide
range of substrates. Dihydronaphthalenes are known as useful building blocks in
organicsynthesis[7,19–27].Theycanundergobromination[28],cyclopropanation
[29, 30], dipolar cycloaddition [31, 32] and epoxidation [33] reactions to afford
S.A.Shahzad,NovelSelenium-MediatedRearrangementsandCyclisations, 13
SpringerTheses,DOI:10.1007/978-3-642-33173-2_2,
(cid:2)Springer-VerlagBerlinHeidelberg2013
14 2 TheSynthesisofNovelDihydronaphthalenesandBenzofluorenes
R2O X OH H3CO
H3CO
OH
R1O X H
HO OH
48 49 50 51
R3O H
OR1 NegundinB OCH3 N O
X=-CONH(CH2)2-C6H4OH(4-) OH Nafoxidene
R1,R2,R3=H,Me,Ac
Fig.2.1 Dihydronaphthalenenaturalproducts
O K(2.4eq) O Li(2.4eq) O OH
or
FeCl3 K(2.4eq)
75%
75%
55 52 53 54
O O O
Li(2eq),liquid Limetal(2eq),
NH3excess,MeI liquidNH3excess
75% 75%
58 56 57
Scheme2.1 TheBirchreduction
useful products. Rhodium- and palladium-catalysed asymmetric ring-opening
reactions ofoxabenzonorbornadienes byvarious alcohol,amine andalkyl nucleo-
philesafforddihydronaphthalenederivativesingoodyields[34,35].Theconversion
of a- and b-tetralone into dihydronapthalenes was accomplished by palladium-
catalysedcouplingofGrignardreagentswithinsitu-generatedenolphosphates[36].
The gold(I)-catalysed intramolecular rearrangement of vinylidenecyclopropanes
alsogivesdihydronaphthalenes[37].Themetal-ammoniareductionofnaphthalene
anditsderivativeshasalsobeenextensivelyinvestigated[38,39].Additionalmetal-
catalystshavealsobeenusedintheformationofdihydronaphthalenes,suchasthose
usedintheBirchreductionandthereductivemethylationof1-and2-acetylnaph-
thalenesintodihydronaphthalenesasdescribedbyRaoandSundar.Ithasbeenfound
that 1-acetylnaphthalene is reduced to the 3,4-dihydronaphthalene 57 whilst the
2-acetylnaphthalene gives the corresponding 1,2,3,4-tetrahydronaphthalene 53
(Scheme 2.1)[40].However,anhydrousferricchloridehasbeenfoundtolimitthe
reductiontothedihydro-stagewith2-acetylnaphthalene.
1,2-Dihydronaphthalene derivatives have been synthesised by the thermal
cyclisations of alkenyliodonium tetrafluoroborates. The required alkenyliodonium
salts60possessinganaromaticgroupwerepreparedfromalkenylsilanes59using
iodosylbenzene inthepresence ofboron trifluoride-diethyletherat0 (cid:3)C followed
byquenchingwithaqueoussodiumtetrafluoroborate,affordinga77 %yieldofthe
vinyliodonium tetrafluoroborate 60. Intramolecular aromatic vinylation of the
iodonium salts 60 was found to occur smoothly on gentle heating at 40 (cid:3)C for
2.1 IntroductiontoDihydronaphthalenes 15
ICoHdo2Csyl2lb,eBnFz3e•neOE(3t2e(q3),eq) CHC3l03,m4i0noC
R1 59 SiMe3R2 R1=H0,CoCl,,B7rh;R2=H,MeR1 60 I+PhRB2F4¯ 65% R1 61 R2
Scheme2.2 Hypervalentiodine-mediatedsynthesisofdihydronaphthalene61
0.5 h in a sealed tube and provided dihydronaphthalene 61 in 65 % yield
(Scheme 2.2) [41].
1,2-Dihydronaphthalenes can also be prepared in moderate yields from
allenylsilanes and benzylic cations by a one-step intermolecular cyclisation. For
example, treatment of alcohol 62 with an excess of allenylsilane 63 and 2.0
equivalents of tin(IV) chloride in dichloromethane at 0 (cid:3)C afforded dihydro-
naphthalene64in65 %yield.Thereactioncanalsobecarriedoutusinganexcess
ofbenzylicalcohol,forexample,treating2.0equivalentsofalcohol62withallene
63 afforded dihydronaphthalene 64 in 79 % yield. The use of an excess of one of
thereactantsisadrawback.Anotherdrawbackisthatsubstitutiononthearomatic
ring controls whether dihydronaphthalene or spirodecatrienone products are
formed (Scheme 2.3) [42].
The transition metal-promoted insertion of carbonyl compounds to carbon–
carbontriplebondscanprovideusefulroutetodihydronaphthalenes.Oxidationof
diethyl a-benzylmalonate 65 by manganese(III)acetate in acetic acid at 70 (cid:3)C in
the presence of alkynes 66 leads to dihydronaphthalene derivatives 67 in low to
good yields (Scheme 2.4) [43].
In2002,Harrowven[44]reportedtheconversionof4-arylalk-1-en-1-ylmethyl
ethers68todihydronaphthalenesundercatalyticreactioncondition.Cyclisationis
accomplishedbywarmingatoluenesolutionofthesubstrate68with1,2-ethanediol
andcatalyticpara-toluenesulfonicacidat80 (cid:3)Cwhichproceedsviainsituformation
ofa1,3-dioxolane.Reactionsgenerallygivegoodyields(66–91 %)andhavebeen
successful with electron rich, unsubstituted and halogenated arenes, the latter
requiringextendedreactiontimes(Scheme 2.5).
Suzuki and co-workers [45] described the thermal ring expansion of various
alkenylbenzocyclobutenolderivativesintodihydronaphthalenes.Thereactionswere
carriedoutat110 (cid:3)Cintoluene.Thermolysisof70showedareactivitydependence
ontheRgroup(Scheme 2.6).Therelativepropensitytoformeitherdihydronaph-
thalene71ornaphthalenederivative72isdependentuponthenatureofthesubstrate.
Substrateswithsilylormethylethergroupsunderwentsmoothrearrangementtogive
thedihydronaphthalene71.WhenRisanacetyl,thenaphthalene72bwasobtainedas
themainproduct.Thestereochemistryofthethermalconversionofalkenylbenzo-
cyclobutenol into either cis-dihydronaphthalene or trans-dihydronaphthalene was
alsostudied.
Yamamoto[46]describedthepreparationof1,2-dihydronaphthalene75bythe
reactionofo-(alkynyl)benzaldehydesoro-(alkynyl)phenylketones73witholefins
74 using 10 mol % Cu(OTf) at 80 (cid:3)C in THF. This Cu(OTf) -catalysed cyclo-
2 2
addition reaction affords dihydronaphthalene derivatives 75 bearing a ketone
16 2 TheSynthesisofNovelDihydronaphthalenesandBenzofluorenes
OH
OMe Si(t-Bu)Me2
MeO OMe Si(t-Bu)Me
2 SnCl (2equiv) HO
4
•
62
63 MeO
Et OH Et
OMe OMe
HO Si(t-Bu)Me HO Si(t-Bu)Me2
2
MeO
MeO
Et 64 Et
Scheme2.3 Synthesisofdihydronaphthalene64frombenzylicalcohol62
COOEt
COOEt Mn(OAc)3 COOEt
R R1
COOEt 13-92% R1
65 66 67
R=alkyl,aryl,Me Si;R1=H,alkyl,Ph,COOEt R
3
Scheme2.4 Manganesemediatedsynthesisofdihydronaphthalene67
R 1equiv.(CH OH) ,10mol%p-TsOH
2 2 R
68 OMe toluene,80oC,4 24h,66 91%yield 69
Scheme2.5 Acid-catalysedcyclisationof68todihydronaphthalene69
OR″ R2 Toluene,110oC OR″ R1 OR″ R1
R′ R1 R′ R′
OR R=TBDMS,R1=R2=H,71a=95% R2 R2
R=Me,R1=R2=H,71b=68%,72a=27% OR
70 R=Ac,R1=R2=H,72b=85% 71 72
R=TBDMS,R1=R2=Me,71c=94%,72c=5%
Scheme2.6 Thermalringexpansionof70intodihydronaphthalenes
function at the 1-position in 26–90 % yields. The process is reasonably general
with regard to the types of substituents on the olefin that can be employed and
alkyne can also be used with a range of substitution patterns (Scheme 2.7).
The formation of dihydronaphthalene derivatives from x-arylalkyne 76 can be
catalysed by the use of 0.1 mol % Hg(OTf) -(TMU) (TMU = tetramethylurea)
2 3
complex in acetonitrile at room temperature. Under these conditions, various
dihydronaphthalene derivatives are formed in good yields along with smaller
amounts of by-products. However, the choice of substitution pattern on the
substrate is crucial for the success of this process (Scheme 2.8) [47].
2.1 IntroductiontoDihydronaphthalenes 17
O R3 O R
R1 10mol%Cu(OTf)2,THF R2R4
R2 R4
80oC,10h,26-90%yield
73 R 74 R3
75 R1
R=H,Bu,Ph;R1=H,Me,Ph R2=R4=H,alkyl,aryl;R3=H,Me
Scheme2.7 Cu(OTf) -catalysed[4?2]cycloadditionofo-alkynylbenzeneswithalkenes
2
OMe OMe
Hg(OTf) —(TMU) (0.1mol%)
2 3
OMe CH CN,r.t,3.3h, 95% OMe
3
76 77
Scheme2.8 Mercurictriflate-(TMU) -catalysedcyclisationofx-arylalkyne
3
R2 R2
CO2Et Mn(OAc)3,AcOH,80oC,10h
CO2Et
78 R1 CO2Et 60—70% R1 CO2Et
R1=H,Me,OMe,Cl,Br,NO2 79
EtO2C CO2Et R2=H,Me,Ph
R2 R2
CO2Et
78a RE1tO2C CO2Et 78b R1 CO2Et
Scheme2.9 Manganeseacetatemediatedfree-radicalcyclisationreactionofalkylenecyclopropanes
Chenandco-workers[48]reportedaconvenientsynthesisof3,4-dihydronaphth-
alen-2-yl-malonicesters79inmoderateyieldbythereactionofarylidenecyclopro-
panes78withdiethylmalonateinthepresenceofMn(OAc) .Thereactionisproposed
3
to proceed by the b-scission of the C–C bond in the cyclopropane ring in 78a to
generate78b.Subsequentintramolecularradicalcyclisationof78b producescyclic
product79withthelossofaprotonandoxidationinthepresenceofanothermolecule
ofMn(OAc) (Scheme 2.9).
3
Theregio-andstereoselectivering-openingadditionofalkyl-orallylzirconium
reagentsto7-oxabenzonorbornadienes80asdescribedinScheme 2.10iscatalysed
by10 mol %NiBr (dppe)and20 mol %ZnpowderindryTHFat50 (cid:3)C[49].Under
2
theseconditions,awiderangeofcis-2-alkyl-orallyl-1,2-dihydronaphthalenes82are
formed in good yields (49–82 %). The nickel-catalysed transmetalation of alkyl-
zirconiumreagentstoformnickel(II)alkylintermediate81cispostulatedtoproceed
throughtheformationofapalkenenickelcomplex81b.Thecatalyticcycleinvolves
18 2 TheSynthesisofNovelDihydronaphthalenesandBenzofluorenes
O R OH
[NiBr2(dppe)](10mol%),Zn(20mol%) R
Cl(Cp)2Zr
THF,50oC,12h,49—82%
80 81 R=alkyl,benzyl,SiMe3,cyclohexyl,cyclopentyl 82
Scheme2.10 Synthesis of dihydronaphthalene 82 by nickel-catalysed addition of alkyl
zirconiumreagents81tooxabenzonorbornadienes
initial coordination of 7-oxabenzonorbornadiene (via the exo face of the carbon–
carbon double bond) to the Ni center in 81b followed by the addition of the or-
ganonickelspeciesintothedoublebondresultingintheformationofintermediate
81c.Subsequentb-oxyeliminationleadstointermediate81d,andtransmetalation
withCp ZrClBrgivesthenickel(II)catalystandzirconiumalkoxide82a.Thelatter
2
is converted to the final desired alkyl product 82 by protonation after workup
(Scheme 2.11).
Ichikawaandco-workers[50]haveshownthat2,2-difluorovinylketonesbearing
anarylgroupcanbecyclisedto4-fluorinated3-acyl-1,2-dihydronaphthalenesusing
1equivalenttrimethylsilylatingagent[Me SiOTforMe SiB(OTf) ].Theresulting
3 3 4
dihydronaphthalene84issubjectedtoasubstitution-cyclodehydrationprocessora
Nazarov-type cyclisation to construct fused polycyclic systems. The process is
believedtoproceedthroughthegenerationofthea-fluorocarbocation83afollowed
by Friedel–Crafts cyclisation. For example, 4-fluorinated 3-acyl-1,2-dihydro-
naphthalene 84 was formed in 84 % yield via a Friedel–Crafts-type alkylation
accompanied by the loss of a fluoride ion. 4,5-Dihydrobenzo[g]indazoles 85 and
5,6-dihydrobenzo[h]quinazolines 86 have been obtained in good yields by the
reaction of 84 with both hydrazines and amidines as bifunctional nucleophiles in
benzeneat reflux, respectively (Scheme 2.12).
Alternatively,1,2-disubstituted-3,4-dihydronaphthalenes89areformedin36–90 %
yieldsbythecycloadditionreactionofvinylarenes87withelectron-deficientalkynes88
suchasdiethylacetylenedicarboxylateandmethylphenylpropiolate[51].Thesereac-
tionwereconductedat110 (cid:3)CinthepresenceofDMF-DMA(N,N-dimethylformamide
dimethylacetal)asanorganocatalyst(Scheme 2.13).Thisorganocatalysedmethodol-
ogyexhibitstheadvantagesofsubstrateversatilityandmildreactionconditions.
Anewsynthesisofdihydronaphthalenefroma-tetralonewasdisclosedbySingh
and co-workers [52]. When ketoxime 91 was treated with (stoichiometric) triphen-
ylphosphineandaceticanhydrideintolueneatreflux,completeconversiontoproduct
92wasobserved.Thismethodologyinvolvesaphosphine-mediatedreductiveacyla-
tionofoximesandtheresultingdihydronaphthalene92bearingenamideisisolatedin
goodyields(upto89 %)withexcellentpurity(Scheme 2.14)[52].
Reactions of arylsubstituted propargylic alcohols catalysed by a simple Lewis
or Brønsted acid have been developed for the selective synthesis of di- and
tetrahydronaphthalene systems [53]. Treatment of a variety of aryl substituted
propargylic alcohols 93 with toluenesulfonic acid in nitromethane at 80 (cid:3)C
afforded the corresponding 1,2-dihydronaphthalenes 95 formed through the
2.1 IntroductiontoDihydronaphthalenes 19
R
Cl(Cp) Zr
2
P Br
Ni Cp ZrClBr
OZr(Cp)2Cl P Br 2
R
82a P=PPh2
R
P
Ni
Br 81a
P
Cp ZrClBr
2
ZnBr
2
R
P ZnBr3
Ni
81d P
P 80
O Ni
P Br
R P
Ni O
P
R
81b
O
81c
Scheme2.11 Proposedcatalyticcycleforthepreparationof82
O OLA
O Ph
H3C Ph LA H3C Ph H3C Ph H3C
CF2 0oC CF2 F RNHNH2 NN
R
85
83 83a 84
NH CRNH2
Ph
H3C N
N R
86
Scheme2.12 Synthesisofdihydronaphthaleneanditsapplicationtofusedheterocycles
R2 COOR3
DMF-DMA(20mol%) COOR3
R1 R3OOC COOR3 R1
110oC,5h,36-90%
87 88 89 R2
R1=Me,Cl,Br;R2=H,Me;R3=Me,Et
Scheme2.13 Synthesisofdihydronaphthalenebycycloadditionreactionof87with88
20 2 TheSynthesisofNovelDihydronaphthalenesandBenzofluorenes
O NOH NHAc
NH2OH•HCl R3P,Ac2O
NaOAc,MeOH toluene,reflux
89%yieldfromketone
90 91 92
R=Ph,Et,n-Bu,n-Oct,cyclohexyl
Scheme2.14 Synthesisofdihydronaphthalene92froma-tetralone90
E E E
E E E E
5mol%TsOH•H2O E 5FemCol6l%•6H2O isomerisation
Ph CH3NO2,80oC Ph • 80oC Ph
63-93%yield 0→5oC,
Ph PhOH 37—95% Ph Ph Ph
95 93 94 95
Scheme2.15 Synthesisof95fromaryl-substitutedpropargylicalcohols93
intramolecular Friedel–Crafts reaction followed by successive isomerisation in
moderateto excellent yields, dependingon the nucleophilicityof the aryl nucleus
involved and the nature of substituents at the propargylic position. Selective
preparationof95couldbeachievedbyusingFeCl •6H Oat0–5 (cid:3)C.Itwasalso
3 2
feasible to isolate 94 in good yields using TsOH as a catalyst if the reaction was
carried out at room temperature. Remarkably, both 93 and 95 were converted to
spiro-skeletons, when using FeCl • 6 H O at 80 (cid:3)C (Scheme 2.15).
3 2
A range of dihydronaphthalenes was accomplished by the palladium-catalysed
de-aromatisation reaction of naphthalene derivatives with allyltributylstannane
(Scheme 2.16) [54]. The allylative de-aromatisation reactions of naphthalene
derivatives 96 with allyltributylstannane have been performed in the presence of
[Pd (dba) ](5 mol %)andPPh (20 mol %).Thesimplesubstrates96underwent
2 3 3
thede-aromatisationreactionsmoothlytoafford97inhighyields(74–87 %)Neither
theelectron-donatinggroupnortheelectron-withdrawinggrouponthearomaticring
exertedastronginfluenceonthereaction(exceptintermsofthereactiontimes).The
proposed mechanism involves the formation of g3-allylpalladium chloride inter-
mediate96bbyoxidativeadditionof96atoaPd(0)species,followedbyreaction
withallyltributylstannanetogenerateabis(g3-allyl)palladiumintermediate96cupon
ligandexchange.Isomerisationof96cwouldoccurtogiveabis(g3-allyl)palladium
intermediate96d.Theresultingallyl-Pdcomplexundergoesreductiveeliminationto
formthedearomatisedproduct97andregeneratethePd(0)catalyst(Scheme 2.17).
Nickelcanefficientlycatalysethecyclisationofalkenes,andalkynestoafforda
seriesofsubstituteddihydronaphthalenesthatcannotbepreparedfromthereadily
availablestartingmaterials[55].In2009,XieandQiudescribedthepreparationof
wide range of dihydronaphthalenes in good yields from nickel-catalysed three-
component [2 ? 2 ? 2] carboannulation reaction of arynes, activated alkenes,
and alkynes [56]. This work offers an exceptionally efficient route to 1,2-
dihydronaphthlenesfromreadilyavailablestartingmaterials.Variousalkyneswere
2.1 IntroductiontoDihydronaphthalenes 21
Cl [Pd2(dba)3](5mol%)
PPh (20mol%),CH Cl
3 2 2
rt,2 24h
SnBu
3
74—89%yields 97
R 96 R
Scheme2.16 Synthesis of 97 by palladium-catalysed de-aromatisation of naphthalene deriva-
tiveswithallyltributylstannane
Cl
L Pd
Cl Pd Cl L
L
SnBu3
96 96a 96b
Pd(0)
Pd
Pd
BuSnCl
3
97
96d 96c
Scheme2.17 Mechanismforthede-aromatisationof96(allchargesareomitted)
TMS [Ni(cod)2](5mol%),CsF(3equiv.) R2
R1 R2 R3 R4 R1
OTf CH3CN,r.t,upto78%yield R4
98 99 100 R2=CO2Me,CO2n-Bu,CN R3 101
R3=alkyl,allyl,methoxy;R4=aryl,alkyl,CO2Me
Scheme2.18 Nickel-catalysedsynthesisof101viaamulti-componentreaction
compatiblewiththisnickel-catalysedcarboannulationreactionandgavethedesired
dihydronaphthalenes101inverygoodyields.Thebestresultswereobtainedusing
5 mol % [Ni(cod) ] and 3 equivalents cesium fluoride in acetonitrile at room
2
temperature.Ifunactivatedalkeneswereused,noneofthedesiredproducts101were
detected.Functionalisedaryneprecursorswithelectron-donatinggroupswereless
effective, producing dihydronaphthalene derivatives 101 in moderate yields
(Scheme 2.18).
In 2009, the rhodium-catalysed reaction of 1,6-enynes 103 with 2-
bromophenylboronicacids102hasbeenutilizedbyTongetal.[57].Toconstructa
multi-substituted dihydronaphthalene scaffold. A screen of reaction conditions
revealedthat5 mol %Rh(CO) (acac),triphenylphosphine,potassiumcarbonatein
2
a dioxane and water mixture at 100 (cid:3)C for 3–5 h afforded dihydronaphthalene
22 2 TheSynthesisofNovelDihydronaphthalenesandBenzofluorenes
R
Br Rh(CO) (acac),PPh ,K CO
2 3 2 3
X
B(OH)2 Y X dioxane:H2O(20:1)
32-75%yield R Y
X=O,N;Y=O,CH
2 104
102 103
Scheme2.19 Rhodium-catalysedsynthesisofdihydronaphthalenescaffold104
scaffold104ingoodyieldsformostsubstrates.This[2 ? 2 ? 2]cycloadditionof
1,6-enynes with 2-bromophenylboronic acids involves the Rh-catalysed regiose-
lective insertion of an alkyne into an arylrhodium(I) species and the oxidative
additionofC–Brbondsintheadjacentphenylringtotheresultingvinylrhodium(I)
species as key steps (Scheme 2.19).
In 2009, Yao and co-workers [58] exploited scope of isochromenylium tetra-
fluoroborates as precursors of various dihydronaphthalenes. Direct metal-free
treatmentofisochromenyliumtetrafluoroborate106withalkenesinacetonitrileat
either 25 or 60 (cid:3)C afforded a diverse range of dihydronaphthalenes 107 via mild
cascade reactions. Reaction of 106 with the monosubstituted, disubstituted, and
trisubstitutedolefinsaswellaswithcyclicalkenesdelivereddesiredproducts107
successfully in 48–79 % yields (Scheme 2.20).
Inan attempt toinducechirality on dihydronaphthaleneringsystems, Cho and
co-workers used chiral (S,S0)-(R,R0)-C -ferriphos 108 as ligand and [Rh(cod)Cl]
2 2
as a catalyst in tetrahydropyrane at 80 (cid:3)C. In the presence of a rhodium catalyst
generated in situ from [Rh(cod)Cl] and (S,S0)-(R,R0)-C2-ferriphos 108, the
2
asymmetric ring-opening reaction of azabenzonorbornadienes 109 with various
aliphaticandaromaticaminesproceededwithhighenantioselectivity(upto99 %
ee) to give the corresponding 1,2-diamine substituted dihydronaphthalene deriv-
atives110inhighyields.Experimentsrevealedthatthenatureofthechiralligand
hasthesignificantimpactonthereactivityofthecatalystandtheuseofexcess(2.2
equiv. to Rh) of the chiral ligand plays an important role to increase the enanti-
oselectivity in the ring-opening reactions of azabenzonorbornadienes with amine
nucleophiles (Scheme 2.21) [59].
In 2010, Ohwada and co-workers [60] disclosed the acid-catalysed cyclisation
of arylacetoacetates to afford 3,4-dihydronapththalene derivatives in Brønsted
superacids. For example, methyl 2-aceto-4-phenylbutyrate 111 underwent the
cyclisationinthepresenceof10equivalentstrifluoromethylsulfonicacid(TFSA)
to afford 1-methyl-2-carbomethoxy-3,4-dihydronaphthalene 112 and 1-methyl-
3,4-dihydronaphthalene-2-carboxylic acid 113 in 87 % combined yield. In the
same communication, they also reported thermochemical data on the acid-
catalysed cyclisation of arylacetoacetates. Thermochemical data shows that
activation of arylacetoacetates toward cyclisation by a strong acid, and the
electron-withdrawing nature of the O-protonated ester functionality significantly
increases the electrophilicity of the ketone moiety (Scheme 2.22) [60].