handout complete

Aromatic and Heterocyclic ChemistryLectures 1-6 2nd Year Michaelmas 2009

Dr Jonathan Burton

CRL Office 6, 1st floor

E-mail: [email protected]

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Synopsis

Lectures 1-6: J. W. Burton

1. The Origin of Aromaticity and General Characteristics of Aromatic Compoundsa. kinetic and thermodynamic stability of benzeneb. substitution not additionc. resonance energy by hydrogenationd. Hückel rule of linear and cyclic polyenes – 1,3,5 hexatriene vs benzenee. cyclobutadiene and antiaromaticity

2. Examples of Aromaticitya. Frost circleb. examples of 3,4,5,6,7,8-membered aromatic compounds including cations and

anions e.g. tropylium cation, acidity of cyclopentadiene, 18-annulene.

3. Electrophilic Aromatic Substitutiona. Revision of 1st yearb. Hammond’s postulate to explain EASc. mechanistic evidence for EAS

i. isolation of arenium ionii. Kinetic Isotope Effect (briefly)

4. Electrophilic Aromatic Substitution cont.a. Substituent effects including halogensb. Effects of two substituentsc. Ispo attack and reversible reactions

5. Nucleophilic Aromatic Substitutiona. Problems with SN1 and SN2 reaction on aromaticsb. Diazonium salts and SN1 reactions – radical mechanismsc. SNAr by addition/elimination mechanismd. Meisenheimer intermediatese. Substituent effectsf. Vicarious Nucleophilic Substitution (briefly)g. SRN1 – mechanism and examples (briefly)

6. Arynesa. Evidence for benzyne – Roberts experiment, trapping experiments, etc.b. Structure of (ortho)-benzynec. Generation of benzyne – various methodsd. Substituent effectse. meta- and para-benzynes (briefly)

Lectures 7-12: D. M. Hodgson

7. Reactions with metals: Ortho Lithiation, Palladium Catalysed Couplings, ChromiumComplexes and Birch Reductions.

8. Introduction of Functional Groups

9. Pyridine: Synthesis and Reactions

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10. Pyrrole, Thiophene and Furan: Synthesis and Reactions.

11. Indole: Synthesis and Reactions.

Suggested Reading

Advanced Organic Chemistry: Reactions Mechanisms, and Structures; J. March.

Advanced Organic Chemistry Parts A and B; F. Carey and R. Sundberg.

Aromatic Substitution Reactions; L. Stock

Frontier Orbitals and Organic Chemical Reactions; I. Fleming.

Aromatic Chemistry; M. Sainsbury.

Reactive Intermediates; C. Moody and G. Whitham.

Heterocyclic Chemistry; J. Joule, K. Mills and G. Smith.

Aromatic Heterocyclic Chemistry; D. Davies.

N.B. Other general organic texts also cover the basic chemistry of organic compounds.

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Origin of Aromaticity

Typical reactions of alkenes

Typical reactions of benzene

■ benzene is kinetically stable

■ retains aromatic sextet of electrons in substitution reactions

■ does not behave like a “normal” polyene or alkene

■ benzene is both kinetically and thermodynamically very stable

■ how do we quantify this stability?

Quantification of stability – heats of hydrogenation

hydrogH = -120 kJmol-1

hydrogH =3 x -120 = -360 kJmol-1

(hypothetical, 1,3,5-cyclohexatriene)

hydrogH = -210 kJmol-1

■ benzene ≈150 kJmol-1 more stable than expected – (represents stability over hypothetical 1,3,5-cyclohextriene) – termed the empirical resonance energy (values vary enormously)

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Energy

■ empirical resonance energy is not the true resonance energy as this would be the differencebetween benzene and a symmetrical non-delocalised cyclohexatriene unit which does not exist

■ we know that delocalisation is stabilising, but how much more stabilising is the delocalisation inbenzene – should compare benzene with a real molecule – we will use 1,3,5-hexatriene

■ require a theory which explains the stability of benzene

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Understanding Aromaticity - the Molecular Orbital Approach

Hückel Theory

■ Hückel’s Rule: planar, monocyclic, completely conjugated hydrocarbons will be aromatic whenthe ring contains (4n +2) π-electrons (n = 0, 1, 2….positive integers)

Corollary

■ planar, monocyclic, completely conjugated hydrocarbons will be anti-aromatic when the ringcontains (4n) π-electrons (n = 0, 1, 2….positive integers)

Hückel Molecular Orbital Theory (HMOT)

■ applicable to conjugated planar cyclic and acyclic systems

■ only the π-system is included; the σ-framework is ignored (in reality σ-framework affects π-system)

■ used to calculate the wave functions (ψi) and hence relative energies by the LCAO method

i.e. ψi = c1φ1 + c2φ2 + c3φ3 + c4φ4 + c5φ5 …..

■ HMOT solves energy (Ei) and coefficients ci

■ there are now many more sophisticated methods for calculating the stabilisation energy inconjugated systems; however, HMOT is adequate for our purposes.

HMOT in Action

■ For cyclic and acyclic systems:

Molecular Orbital Energies = Ei = α + mjβ

■ α = coulomb integral - energy associated with electron in an isolated 2p orbital (albeit in the molecular environment) – α is negative (stabilising) and is the same for any p-orbital in π-system

■ β = resonance integral – energy associated with having electrons shared by atoms in the form of a covalent bond – β is negative (stabilising) and is set to zero for non-adjacent atoms.

■ (all overlap integrals S assumed to be zero, electron correlation ignored)

■ linear polyenes mj = 2cos[jπ/(n+1)] j = 1, 2……n (n = number of carbon atoms in conjugated system)

■ cyclic polyenes mj = 2cos(2jπ/n) j = 0, ±1, ±2……±[(n-1)/2] for odd n,±n/2 for even n

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Examples

■ ethene

■ two 2p atomic orbitals give 2π molecular orbitals; n = 2, j = 1 or 2

■ mj = 2cos(π/3) = 1 2cos(2π/3) = -1

■ energy E = α ± β

■ stabilisation energy Estab = 2(α + β) Note: the energy of two isolated p orbitals is 2α – we have gained 2β of stabilisation

■ allyl System

■ three 2p atomic orbitals give 3π molecular orbitals; n = 3, j = 1, 2 or 3

■ mj = 2cos(π/4) = √2 2cos(π/2) = 0 2cos(3 π/4) = -√2

■ energy E = α + √2β α α - √2β

E = +

E = -

E = +

E = -

■ allyl cation stabilisation energy Estab =2α + 2.8β ■ allyl radical would have stabilisation energyi.e. more stable than ethene (Estab ethene =2α + 2β) Estab = 3α + 2.8β

take home message – conjugation is stabilising

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■ 1,3,5-Hexatriene vs Benzene

■ six 2p atomic orbitals give 6π molecular orbitals; n = 6, j = 1, 2, 3, 4, 5, 6

■ mj = 2cos(π/7) = 1.80 2cos(2π/7) = 1.25 2cos(3π/7) = 0.45...... and the corresponding negative values

■ energy E = α + 1.80β α + 1.25β α + 0.45β α – 0.45β…..etc

MO no.nodes

energy HMOT – MO (calculated)

ψ6 5 α - 1.80β

ψ5 4 α - 1.25β

ψ4 3 α - 0.45β

ψ3 2 α + 0.45β

ψ2 1 α + 1.25β

ψ1 0 α + 1.80β

■ stabilisation energy Estab= 2(3α + 3.5β) = 6α + 7β

■ stabilisation with respect to 3 x ethene = (6α + 7β) – 3(2α + 2β) = β

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Benzene

■ Cyclic polyenes mj = 2cos(2jπ/n) j = 0 ±1, ±±[(n-1)/2] for odd n; ±n/2 for even n

■ six 2p atomic orbitals give 6π molecular orbitals; n = 6, j = 0 ±1, ±2, ±3

■ mj = 2cos(0) = 2, 2cos(±2π/6) = 1, 2cos(±4π/6) = -1, 2cos(±6π/6)= -2

■ energy E = α + 2β, α + β, α - β, α – 2β

MOno.

nodesenergy HMOT – MO (calculated)

ψ6 6 α - 2β

ψ4,5 4 α - β

ψ2,3 2 α + β

ψ1 0 α + 2β

■ stabilisation energy Estab= 2(α + 2β) + 4(α + β) = 6α + 8β

■ stabilisation energy with respect to 1,3,5-hexatriene = (6α + 8β) – (6α + 7β) = β

■ stabilisation energy with respect to 3 x ethene = (6α + 8β) – 3(2α + 2β) = 2β

■ HMOT predicts benzene is more stable than hexatriene and than 3 x ethene

■ aromatic compounds are those with a π-system lower in energy than that of acycliccounterpart

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■ Cyclobutadiene vs 1,3-butadiene

■ four 2p atomic orbitals give 4π molecular orbitals; n = 4, j = 0 ±1, ±2

■ mj = 2cos(0) = 2 2cos(±2π/4) = 0 2cos(±4π/4) = -2

■ energy E = α + 2β α α – 2β

cyclobutadiene 1,3-butadiene

MOno.

nodesenergy energy

ψ4 4 α - 2β α – 1.62β

ψ2,3 2 α

α – 0.62β

α + 0.62β

ψ1 0 α + 2β α + 1.62β

Stabilisation energy Estab = 4α + 4β i.e. same as 2 x etheneStabilisation energy

Estab = 4α + 4.4β

■ cyclobutadiene is less stable than 1,3-butadiene

■ anti-aromatic compounds are those with a π-system higher in energy than that of acycliccounterpart

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Frost-Musulin Diagram – Frost Circle

■ simple method to find the energies of the molecular orbitals for an aromatic compound

■ inscribe the regular polygon, with one vertex pointing down, inside a circle of radius 2β, centred

at energy α

■ each intersection of the polygon with the circumference of the circle corresponds to the energy

of a molecular orbital.

General Characteristics of Aromatic Compoundsi) planar fully conjugates cyclic polyenes

ii) more stable than acyclic analogues

iii) bonds of nearly equal length i.e. not alternating single and double bonds

iv) undergo substitution reactions (rather than addition reactions)

v) support a diamagnetic ring current

> >

> >

magnetic field induces electrons in π-system to circulate setting

up a secondary magnetic field opposed to the first magnetic

field

aromatic protons of C6H6 see applied field plus induced field

and hence require higher frequency to bring them into

resonance and they appear at higher chemical shift (δ)

Good test for aromatic character of a compound

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Examples of Aromatic and Anti-Aromatic Compounds■ Hückel’s rule [(4n +2) π-electrons for aromatic compounds; 4n π-electrons for anti-aromatic

compounds] holds for anions, cations and neutrals

Cyclopropenium cation

■ (4n +2), n = 0, 2π electrons

■ insoluble in non-polar solvents; 1 signal in 1H NMR H = 11.1 ppm - aromatic and a cation

■ compare with cyclopropyl cation which is subject to rearrangement to the allyl cation

Synthesis

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Cyclopropenones

Cyclobutene dications


δH = 3.68 ppm allMe groupsequivalent. Megroups α to 2 positive charges

δH = 2.1 ppm

Benzene


δH = 7.26 ppm, planar molecule;

bond length = 1.39 Å

C-C sp3-sp3 = 1.54 ÅC-C sp3-sp2 = 1.50 ÅC-C sp3-sp = 1.47 ÅC-C sp2-sp2 = 1.46 ÅC=C sp2-sp2 = 1.34 Å

isoelectronic with pyridine:

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Cyclopentadienyl Anion


■ quick reminder: pKa is a measure of the position of equilibrium between an acid and its

conjugate base; pKa = -log10Ka.

■ most important factor in acid strength is the stability of the conjugate base A-

■ for a strong acid the conjugate base A- is stable the equilibrium lies to the RHS and the pKa is low

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Furan, pyrrole and thiophene


■ Cyclopentadienide anion is isoelectronic with furan pyrrole and thiophene

■ in each case the (one of the) lone pair(s) is parallel to the p-orbitals and part of the π-system

■ all 3 are aromatic, show ring currents and undergo electrophilic aromatic substitution

Tropylium Cation


■ ionic compound

■ mp 205 °C insoluble in nonpolar solvents

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Tropone


O O

HBr

OH

Br

pKa = -0.6

OH

pKa = -6.2

■ some aromatic character

■ more basic than acetone

Tropolones


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Cyclobutadiene

■(4n), n = 1, 4π electrons – anti-aromatic

■ (HMOT predicts triplet ground state for cyclobutadiene – cyclobutadiene is actually a singletground state and is rectangular not square)

■ only possible to isolate at very low temperatures in an argon matrix

Synthesis

■ Cyclobutadiene can be stabilised with very bulky substituents

H

■ stable up to 150 °C but very reactive towards O2

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Cyclopropenium anion

■ (4n), n = 1, 4π electrons – anti-aromatic

■ stabilisation energy Estab= 4α +2β

■ stabilisation energy of allyl anionEstab= 4α +2√2β

■ rate of proton exchangecyclopropane/cyclopropene = 10000

Cyclopentadienyl cation


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Cyclopentadienone

Cycloheptatrienyl anion and Cyclooctatetraene


probably not planar

similar pKa to normal 1,4-diene

■ non-aromatic, alternating bond lengths, distorts to avoid planar anti-aromatic conformation,

■normal reactivity of polyene

Other Examples

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[18]-Annulene

■ (4n+2), n = 4, 18π electrons –aromatic

Cu(OAc)2pyridine, 85 °C

Glaser coupling

hexadiyne

tBuOK

base catalysedisomerisation

(allene intermediates)

H = 9.3

H = -3.0

18 electronsbright red

HH

H2/Pd

■ ring large enough to be planar without steric congestion of inner protons

■ 2 signals in 1H NMR consistent with ring current

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Benzene■ model for arene reactions – aromaticity makes benzene a thermodynamic sink

■ Some isomers of C6H6

Numbering and Nomenclature

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Reactions

Electrophilic Aromatic Substitution (revision)

■ what does transition state look like for attack of E+ on aromatic ring?

■ rates of chemical reactions are controlled by free energy of transition state therefore

information about transition state structure is crucial

■ transition states have transitory existence cannot make experimental measurements on them

Hammond’s Postulate

“If two states, as, for example, a transition sateand an unstable intermediate, occurconsecutively during a reaction process andhave nearly the same energy content, theirinterconversion will involve only a smallreorganisation of molecular structure.”

■ The transition state resembles the structure(intermediate or substrate or product) to whichit is closest in energy

(i.e. transition state resembles intermediate

arenium ion, therefore what stabilises the

arenium ion stabilises the transition state.)

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Mechanistic Evidence

Isolation of intermediates

Isotope Effects

■ effect on rate of reaction by substitution of an element (most commonly H) for a heavier isotope

(D or T)

■ very useful mechanistic probe

Example

■ transfer of H-atom 9 times faster than D

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■ examine potential energy curves for C-H(D) bond – these are the same for C-H and C-D bonds

(Morse potentials)

■ due to lower zero point energy for vibration of C-D bond with respect to C-H bond, C-D bond is

stronger than C-H bond

■ stretching frequency of bond

k

21

21

mm

mm

hE

2

10

■ µC-H = 0.92 (ca. 1) µC-D = 1.72 (ca. 2)

■ Eo CH > Eo CD hence C-D bond stronger than C-H bond

■ Crude analysis (which assumes bond 100% broken at transition state) gives kH/kD ≈ 7 (normally

2-7) – primary kinetic isotope effect – (more later)

dissociation

potentialenergy

bond distance

C-H

C-D

■ take home message: expect kinetic isotope effect if break C-H bond in rate determining step

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Nitration of Benzene

■ kH/kD = 1

■ hence C-H bond not involved in r.d.s. - consistent with step 1 being rate determining

■ no kinetic isotope for nitration of benzene excludes the following mechanisms

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■ if there is a kinetic isotope effect there is some reversibility in step 1

■ normally 1st step is rate determining step – lose aromaticity

■ 2nd step is fast – regain aromaticity

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Substituent Effects

■ substituent Y affects both the rate and regiochemistry of the reaction

■ electron donating groups activate the aromatic ring (i.e. substrate reacts faster than benzene)

and are ortho and para directing

■ electron withdrawing groups activate the aromatic ring (i.e. substrate reacts slowed than

benzene) and are meta directing

■ halogens are mildly deactivating and direct o/p

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■ Y = electron donating group, ortho and para attack favoured i.e. the intermediate (and hence

the transition state leading to the intermediate) is stabilised by electron donation from Y (not

possible for meta attack)

■ Y = Electron withdrawing group –o and p attack disfavoured i.e. the intermediate (and hence the

transition state leading to the intermediate) is destabilised by electron withdrawing group Y (not

possible for meta attack), and hence get meta attack by default

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■ curly arrows (in a molecular orbital context) illustrate the electron distribution in the frontier

orbital

■ we are interested in the stability of the competing intermediates, and, although curly arrows

give us the correct answer we really should compare the total energy of the π-system – we can do

this using Hückel theory

■ Y = EDG i.e. a group carrying a lone pair or a C-H or C-C bond – we will model these as the benzyl

anion and will compare their energies – look at three filled MOs, ψ1, ψ2 and ψ3

MO ortho Eψortho meta Eψmeta para Eψpara

ψ3E

0.45β

E

0β

E

0.52β

ψ3

E

1.25β E

1.18β

E

1.0β

ψ1

E1.80β

E 1.90β E 1.93β

Estab =(6α) + 7β Estab =(6α) + 6.16β Estab =(6α) + 6.9β

■ The intermediates from ortho and para attack are more stable than those from meta attack

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■ Y = EWG - we will model these groups as the benzyl cation and will compare the energies of the

sigma complexes - look at two filled MOs, ψ1 and ψ2

MO ortho Eψortho meta Eψmeta para Eψpara

ψ3

E

1.25β E

1.18β

E

1.0β

ψ1

E1.80β

E 1.90β E 1.93β

Estab =(4α) + 6.1β Estab =(4α) + 6.16β Estab =(4α) + 5.86β

■ intermediate from meta attack is more stable than those from ortho and para attack – we will

continue to use the curly arrow method.

Halogens

■ mildly deactivating as they are electronegative and withdraw electron density from the ring

through the σ-framework (falls off with distances)

■ halogens direct ortho and para as they have lone pairs in high energy orbitals which stabilise the

intermediates for ortho/para attack

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■ fluorobenzene has similar reactivity to benzene – although it is the most electronegative of the

halogens if has a 2p lone pair which overlaps efficiently with the benzene ring

Examples

Two Substituents

■ reinforcing

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■ opposing - if similar reactivity will get mixtures of compounds; however, a strong activator

beats a weak activator.

■ activating ability – rough order

■ all other things being equal a 3rd group is least likely to enter between two groups meta to one

another

Ipso attack

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Reversible reactions

■ electrophilic aromatic substitution is generally an irreversible process but there are some

exceptions

SO3H

H / H2O + SO3

SH

OOHO

Me

tBuCl / AlCl3

Me

tBu tBu

Me

tButBuCl / AlCl3heat

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Nucleophilic Aromatic Substitution

SN2: aliphatic vs aromatic

Aliphatic

■ nucleophile attacks C-L σ* resulting in inversion of configuration

Aromatic

■ no possibility of nucleophile attacking backside of C-L σ* (transition geometry impossible)

■ LUMO is π* not σ*

■ attacking electron rich arene with electron rich nucleophile

SN1: aliphatic vs aromatic

Aliphatic

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Aromatic

Examples

36

SNAr – Addition – Elimination Mechanism

■ nucleophile attacks LUMO, electron withdrawing groups lower energy of LUMO and stabilise the

negative charge in the intermediate

■ best to have electron withdrawing group(s), ortho and/or para to the leaving group

Evidence

Isolation of intermediates

■ for halogens as leaving groups, rate of reaction follows kF > kCl > kBr (c.f. rate of SN2 reacitons kI >

kBr > kCl > kF)

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■ r.d.s. is attack of nucleophile on aromatic ring therefore bond strength to leaving group is not so

important in influencing the rate

■ fluorine is the most electronegative element, polarises sigma bond, lowers LUMO, therefore

fastest rate

Example

NO2

X

O2N

HN

+

NO2

N

O2N

F Cl Br I

3300 4.3 4.3 1relative rate

X =

NO2

X

O2N

Ph Me

HN +

NO2

N

O2N

Ph Me

F Cl Br

1 15 46relative rate

X =

Mechanism

2

d[P][I]

d[t]k 1 2 -1

d[I][A][B] - [I] - [I]

dtk k k = 0 (steady state approximation)

1

-1 2

[A][B][I]

k

k k

1 2

1 2

d[P] [A][B]

d[t]

k k

k k

38

■ if k2 >> k-1 step 1 is the rate determining step,

loss of X- is faster than loss of R2NH

1 2

2

d[P] [A][B]

d[t]

k k

k

do not break C-X bond in r.d.s

■ if k-1 >> k2 i.e. the second step is rate

determining – intermediate reverts to starting

material faster than progressing to product

1 2

1

d[P] [A][B]

d[t]

k k

k

in r.d.s break C-X bond and hence fastest with

weaker bond

A + B

I

P

k1

k2

A + B

I

P

k2k1

k-1

■ piperidine is a worse leaving group then

methyl aniline ■ step 2 is r.d.s for methyl aniline

■ step 1 is r.d.s for piperidine

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Substitutent Effects

■ electron withdrawing groups ortho/para to leaving group enhance rate

■ electron donating groups ortho/para to leaving group retard rate

■ substituents meta to the leaving group have less influence

Other Examples

Sanger’s Reagent – N-terminus labelling of peptides

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Vicarious Nucleophilic Substitution - (nucleophilic substitution of hydrogen)

mechanism

LG = Cl, Br, PhO, PhS etc; EWG = SO2Ph, SO2NR2, CN, CO2Et

Examples

SRN1 – nucleophilic substitution by unimolecular radical mechanism

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Mechanism – radical chain process

Nucleophile can be:

O

, enolates, ,

O

■ electron donating and electron withdrawing groups are tolerated

■ 1 electron reducible groups (e.g. NO2) are not tolerated

■ hν accelerates the initiation

■ single electron transfer to aromatics facilitates other reactions

■ equivalent of SN2 at a tertiary centre

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Arynes■ removal of two hydrogen atoms from benzene leaving two electrons to be distributed between

two orbitals gives rise to the various arynes

•

•

ortho-benzyne

(benzyne)

relative energy = 0 kJmol-1

•

•

bestrepresented

as

meta-benzyne

relative energy

ca. 64 kJmol-1

•

•

para-benzyne

relative energy

ca. 130 kJmol-1

ortho-Benzyne

Synthesis

Evidence

■ IR spectrum of benzyne in an argon matrix shows C-C bond is 0.05 Å shorter than in benzene

■ Roberts isotope labelling experiment – disproves direct substitution or SNAr addition/elimination

mechanism

43

■ trapping experiments

■ isolation of complexes

Stucture of ortho-benzyne

■ best represented as an alkyne with a very strained triple bond

■ benzyne has a very small HOMO-LUMO gap (the “triple” bond is very weak)

■ the LUMO energy is very low and arynes are very electrophilic; they are uncharged their

reactions tend to be dominated by orbital control (i.e. they are very “soft”)

■ carbene like – two electrons in two orbitals

Generation

■ relative reactivity for formation of benzyne with alkyl lithiums is F > Cl > Br > I; actual rate of

generation of benzyne depends on solvent, base and leaving group

44

■ lithium halogen exchange is very fast for Br and I and loss of X- is r.d.s. hence rate is Br>Cl>F

■ mildest method of benzyne generation involves hypervalent iodine intermediates

■ generation from zwitterions

NN

O

O

NH2

O

OH

explosive

heat or h + CO2 + N2

45

■ thermal and photochemical fragmentations

■ oxidative fragmentation

46

Orientation of attack on arynes from ortho-disubstituted substrates

Note: EWG and EDG in the above examples refer to inductive effects

■ aryne orbitals are orthogonal to the aromatic π-system hence substituents exert influence

inductively through σ-framework

Orientation of attack on arynes from meta-disubstituted substrates


47

Orientation of attack on arynes from para-disubstituted substrates


Examples

48

Further examples

49

O

OEt

OSiMe3

OTf

+

CsF, MeCN80 °C

O

EtO2CF-

O

OEt

O

+

O

EtO2C

O

O OEt

O

OEtO

Application to the total synthesis of (-)-quinocarcin an antitumour antibiotic

50

Cycloadditions

■ benzynes readily undergo Diels-Alder reactions

51

meta-Benzyne

para-Benzyne

■ the Bergman cyclisation

52

Mechanism of action of calicheamicin γ1I

■ highly potent antitumour agent

■ enediyne constrained in 10-membered ring

■ ends of enediyne brought in close proximity by conformational change on conjugate addition of

thiolate

■ Bergman cyclisation occurs to give a diradical which abstracts hydrogen atoms from DNA

causing DNA damage

handout complete

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