Abstract
2-Phosphanyl-substituted imidazolium salts 2-PR2(4,5-Cl-Im)[OTf] (9a,b[OTf]) (4,5-Cl-Im = 4,5-dichloro-1,3-bis(2,6-di-isopropylphenyl)-imidazolium) (a: R = Cy, b: R = Ph) are prepared from the reaction of R2PCl (R = Cy, Ph) with NHC 8 (4,5-dichloro-1,3-bis(2,6-di-isopropylphenyl)-imidazolin-2-ylidene) in the presence of Me3SiOTf. 5-Phospanyl-substituted imidazolium salts 5-PR2(2,4-Cl-Im)[OTf] (10a,b[OTf]) are obtained in quantitative yield when a slight excess of the NHC 8 is used. 5-Phosphonio-substituted imidazolium salts 5-PR2Me(2,4-Cl-Im)[OTf]2 (14a,b[OTf]2) and 5-PR2F(2,4-Cl-Im)[OTf]2 (16a,b[OTf]2) result from methylation reaction or oxidation of 10a,b[OTf] with XeF2 and subsequent fluoride abstraction. According to our quantum chemical studies the Cl1 atom at the 2-position at the imidazolium ring of dication 14b(2+) carries a slightly positive charge and is therefore accessible for nucleophilic attack. Accordingly, the reaction of 14a,b[OTf]2 and 16a,b[OTf]2 with R3P (R = Cy, Ph) affords cationic 5-phosphonio-substituted NHCs 5-PR2Me(4-Cl-NHC)[OTf] (17a,b[OTf]) and 5-PR2F(4-Cl-NHC)[OTf] (18a,b[OTf]) via a SN2(Cl)-type reaction. A series of transition metal complexes such as [AuCl(5-PPh2Me(4-Cl-NHC))][OTf] (19[OTf]), [CuBr(5-PPh2Me(4-Cl-NHC))][OTf] (20[OTf]), [AuCl(5-PPh2F(4-Cl-NHC))[OTf] (21[OTf]) and [RhCl(cod)(5-PPh2Me(4-Cl-NHC))][OTf] (23[OTf]) are prepared to prove the coordination abilities of carbenes 17b[OTf] and 18b[OTf]. The isolation of a rare example of a tricationic bis-carbene silver complex [Ag(5-PPh2Me(4-Cl-NHC))2][OTf]3 (22[OTf]3) is achieved by reacting 14b[OTf] with Cy3P in the presence of AgOTf. NHC 17b[OTf] represents a very effective dehydrocoupling reagent for secondary (R2PH, R = Ph, Cy, (i)Bu) and primary (RPH2, R = Ph, Cy) phosphanes to give diphosphanes of type R4P2 (R = Ph, Cy, (i)Bu) and oligophosphanes R4P4, R5P5 (R = Ph, Cy), respectively. Methylation of 17b(+) and subsequent deprotonation reaction with LDA affords the cationic NHO (N-heterocyclic olefin) 35(+) of which the gold complex 36(+) is readily accessible via the reaction with AuCl(tht).
Highlights
The application of N-heterocyclic carbenes (NHCs) in phosphorus chemistry has led to some remarkable discoveries in recent years and is a growing field with considerable impact
Very few examples of 4, 4,5-bisphosphoryl-9 and 4,5bis-phosphanyl-10 substituted imidazolium salts and 4-phosphanido-substituted NHC11 are reported in the literature so far
Increasing the amount of NHC 8 to 1.2 equivalents in the reaction leads to the quantitative formation of 5-phosphanylsubstituted imidazolium salts 10a,b[OTf ] (10a[OTf ]: 92%; 10b [OTf ]: 97%)
Summary
Scheme 2 Preparation of 9a,b[OTf ], (i): C6H5F, r.t., 6 h, 9a[OTf ] 91%; 9b [OTf ] 99%; preparation of 10a,b[OTf ]; (ii): o-C6H4F2, r.t. 14 h, 10a[OTf ] 92%; 10b[OTf ] 97%; conversion of 9a,b[OTf ] to 10a,b[OTf ], (iii) 0.3 eq 8, o-C6H4F2, r.t. 30 h, quantitative. Scheme 1 Preparation of 2, 4, and 7[OTf ] (Mes = 1,3,5-trimethylphenyl, Dipp = 2,6-diisopropylphenyl, Ph = phenyl, HMDS = bis(trimethylsilyl) amide); for all examples, only one representative Lewis structure is depicted. Very few examples of 4-, 4,5-bisphosphoryl-9 and 4,5bis-phosphanyl-10 substituted imidazolium salts and 4-phosphanido-substituted NHC11 are reported in the literature so far. The aforementioned compounds exhibit either one or two Lewis basic functionalities. There is no example of a cationic NHC featuring a phosphonium functionality at the 4/5-position, which should have a significant influence on the reactivity of the carbene
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