Herbicidally active pyridyl-/pyrimidyl-pyrazine derivatives

A method, comprising:

• applying as an hericide a compound of formula (I)
• or a salt thereof, wherein:
• X1 is N or CR1;
• R1 is selected from the group consisting of hydrogen, halogen, cyano, C1-C6alkyl, C3-C6cycloalkyl, C2-C6alkenyl, C2-C6alkynyl, C1-C6alkoxy, —C(O)OC1-C6alkyl, —S(O)pC1-C6alkyl, NR6R7, C1-C6haloalkoxy and C1-C6haloalkyl;
• R2 is selected from the group consisting of halogen, cyano, nitro, C1-C6alkyl, C1-C6haloalkyl, C2-C6alkenyl, C2-C6alkynyl, trimethylsilylC2-C6alkynyl-, C3-C6cycloalkyl, C5-C6cycloalkenyl, —C(O)OC1-C6alkyl, —S(O)p(C1-C6alkyl), C1-C6alkoxy, C1-C6haloalkoxy, —O—(CRaRb)qR9, or phenyl;
• R3 is selected from the group consisting of hydrogen, C1-C6alkyl, C2-C6alkenyl, C2-C6alkynyl, C2-C6haloalkenyl, C1-C6alkoxyC1-C3alkyl-, C1-C6haloalkyl- and —(CRaRb)qR5;
• Ra is hydrogen or C1-C2 alkyl;
• Rb is hydrogen or C1-C2 alkyl;
• R4 is hydrogen, C1-C6alkyl, C2-C6alkenyl, C2-C6alkynyl, C2-C6haloalkenyl, C1-C6alkoxyC1-C3alkyl-, C1-C6haloalkyl- and —(CRaRb)qR5;
• R5 is hydroxy, —C(O)OC1-C6alkyl, —C3-C10cycloalkyl, -aryl, or -heteroaryl wherein said aryl and heteroaryl are optionally substituted by 1 to 3 independent R8;
• or R3 and R4 together with the nitrogen to which they are attached, form a saturated or partially unsaturated 4-6 membered ring system optionally containing 1 or 2 further heteroatoms independently selected from S, O and N, wherein said ring is optionally substituted by 1 to 3 R8;
• R6 and R7 are independently selected from the group consisting of hydrogen, C1-C6alkyl and —C(O)OC1-C6alkyl;
• each R8 is independently selected from the group consisting of halogen, C1-C6 alkyl and C1-C6alkoxy-, C1-C6 haloalkyl, C1-C6 haloalkoxy-, cyano and S(O)p(C1-C6alkyl);
• R9 is —C(O)ORc, —OC(O)Rc, —C3-C6cycloalkyl, or an -aryl, -aryloxy, -heteroaryl, -heteroaryloxy or -heterocyclyl ring, wherein said ring is optionally substituted by 1 to 3 independent R8;
• Rc is hydrogen or C1-C4alkyl;
• n is 0 or 1;
• p is 0, 1, or 2;
• and
• q is 0, 1, or 2.