Process for the preparation of 2-halogenopyridinealdehydes and novel 2-halogenopyridinealdehydes

A process for the preparation of a compound of the formula ##STR51## in which X denotes chlorine or bromine,

• Q denotes hydrogen, halogen, C.sub.1 -C.sub.8 -alkyl, cyano-C.sub.1 -C.sub.8 -alkyl, C.sub.1 -C.sub.4 -alkoxy-C.sub.1 -C.sub.8 -alkyl, C.sub.3 -C.sub.10 -cycloalkyl, C.sub.6 -C.sub.12 -aralkyl, C.sub.6 -C.sub.12 -aryl, hetaryl having 5 to 7 ring members and 1 to 3 hetero atoms selected from the group consisting of N, O, or S,
• R.sup.1, R.sup.2 denote formyl, cyano, hydroxyl, halogen, C.sub.1 -C.sub.8 -alkyl, halogeno-C.sub.1 -C.sub.8 -alkyl, cyano-C.sub.1 -C.sub.8 -alkyl, C.sub.1 -C.sub.4 -alkoxy-C.sub.1 -C.sub.8 alkyl, C.sub.3 -C.sub.10 -cycloalkyl, ##STR52## C.sub.6 -C.sub.12 aralkyl, C.sub.6 -C.sub.12 -aryl, C.sub.6 -C.sub.12 -aryloxy, hetaryl having 5 to 7 ring members and 1 to 3 hetero atoms selected from the group consisting of N, O, or S, hetaryloxy having 5 to 7 ring members and 1 to 3 hetero atoms selected from the group consisting of N, O, or S in the hetaryl moiety,
• with the restriction that at least one of the two substituents R.sup.1 and R.sup.2 represents formyl,
• R denotes halogen, C.sub.1 -C.sub.4 -alkoxy, phenoxy, --NR'R" and
• R', R" denote, independently of cacti other, C.sub.1 -C.sub.4 -alkyl or both substituents together denote C.sub.4 -C.sub.6 -alkylene which can be interrupted by an oxygen atom, which comprises reacting a compound of the formula ##STR53## in which the dashed line indicates the two possible positions of a C.dbd.C bond,
• Q' can assume the meanings defined for Q or denotes hydroxyl or C.sub.1 -C.sub.4 -alkoxy,
• P denotes --CN, --CONH.sub.2 or --CH.dbd.NOH and
• R.sup.1', R.sup.2' denote, independently of each other, hydrogen, cyano, halogen, --CONH.sub.2, C.sub.1 -C.sub.8 -alkyl, halogeno-C.sub.1 -C.sub.8 -alkyl, cyano-C.sub.1 -C.sub.8 -alkyl, C.sub.1 -C.sub.4 -alkoxy-C.sub.1 -C.sub.8 -alkyl, hydroxy-C.sub.1 -C.sub.8 -alkyl, C.sub.1 -C.sub.8 -alkoxy, C.sub.3 -C.sub.10 -cycloakyl, ##STR54## carboxyl, C.sub.1 -C.sub.8 -acyloxy, C.sub.6 -C.sub.12 -aralkyl, C.sub.6 -C.sub.12 -aryl, C.sub.6 -C.sub.12 -aryloxy, hetaryl having 5 to 7 ring members and 1 to 3 hetero atoms selected from the group consisting of N, O, or S, hetaryloxy having 5 to 7 ring members and 1 to 3 hetero atoms selected from the group consisting of N, O, or S in the hetaryl moiety with the restriction that at least one of the two substituents R.sup.1' and R.sup.2' represents hydrogen and
• R has the meaning specified in formula (I)
• with a Vilsmeier reagent wherein when R.sup.1' or R.sup.2' is hydrogen and P is CN at least 2 mols of the Vilsmeier reagent are used per mol of compound (II) and when R.sup.1' or R.sup.2' are hydrogen and P is CN, and at least 3 mols of Vilsmeier reagent per mol of compound (II) are employed.