Fluorinated dibenzofuran and dibenzothiophene derivatives

1. A compound of the formula I

in which

W denotes O or S,

Y denotes F, Cl, CF 3 , OCF 3 or OCF 2 H,

with the proviso that, if W denotes O, Y cannot be F,

X 1 , X 2 denote H or F,

with the proviso that at least one radical X 1 or X 2 denotes F,

R denotes an alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, one or more CH 2 groups in these radicals may each be replaced, independently of one another, by —C≡C—, —CF 2 O—, —OCF 2 —, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by halogen,

A on each occurrence, identically or differently, denotes a radical selected from the following groups:

a) 1,4-phenylene, in which, in addition, one or two CH groups may be replaced by N and in which, in addition, one or more H atoms may be replaced by a group L,

b) the group consisting of trans-1,4-cyclohexylene and 1,4-cyclohexenylene, in which, in addition, one or more non-adjacent CH 2 groups may be replaced by —O— and/or —S— and in which, in addition, one or more H atoms may be replaced by F or Cl, and

c) the group consisting of tetrahydropyran-2,5-diyl, 1,3-dioxane-2,5-diyl, tetrahydrofuran-2,5-diyl, cyclobutane-1,3-diyl, piperidine-1,4-diyl, thiophene-2,5-diyl and selenophene-2,5-diyl, each of which may also be mono- or polysubstituted by a group L,

L on each occurrence, identically or differently, denotes F, Cl, CN, SCN, SF 5 or straight-chain or branched, and in each case optionally fluorinated alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy having 1 to 12 C atoms, and

Z on each occurrence, identically or differently, denotes a single bond, —CF 2 O—, —OCF 2 —, —CH 2 O—, —OCH 2 —, —(CO)O—, —O(CO)—, —(CH 2 ) 4 —, —CH 2 CH 2 —, —CF 2 —CF 2 —, —CF 2 —CH 2 —, —CH 2 —CF 2 —, —CH═CH—, —CF═CF—, —CF═CH—, —CH═CF—, —(CH 2 ) 3 O—, —O(CH 2 ) 3 —, —C≡C—, —O—, —CH 2 —, —(CH 2 ) 3 — or —CF 2 —,

m denotes 0, 1 or 2.

2. A compound according to claim 1 , wherein X 1 and X 2 both denote F.

3. A compound according to claim 1 , wherein m denotes 0 or 1.

4. A compound according to claim 1 , wherein Y in formula I denotes CF 3 or OCF 3 .

5. A compound according to claim 4 , wherein Y denotes OCF 3 .

6. A compound according to claim 1 , wherein the compound of the formula I is selected from the compounds of sub-formulae Ia to Id,

in which R, A, Z and Y have the meanings indicated for formula I in claim 1 .

7. A compound according to claim 1 , wherein W denotes O.

8. A compound according to claim 1 , wherein the compound is selected from the compounds of the following formulae:

9. Liquid-crystalline medium comprising at least two compounds, which comprises at least one compound according to claim 1 .

10. Electro-optical display element containing a liquid-crystalline medium according to claim 9 .

11. Process for the preparation of a compound of the formula I according to claim 1 , comprising converting a compound of the formula II into a compound of the formula I

where R, A, Z, X 1 , X 2 , W, Y and m have the meaning indicated under claim 1 and G denotes —OH or —SG′, where G′ denotes H or a protecting group.