Synthesis and characterization of platinum(II)-bis(boryl) catalyst precursors for diboration of alkynes and diynes: Molecular structures of cis-[(PPh(3))(2)Pt(B-4-Bu(t)cat)(2)], cis-[(PPh(3))(2)Pt(Bcat)(2)], cis-[(dppe)Pt(Bcat)(2)], cis-[(dppb)Pt(Bcat)(2)], (E)-(4-MeOC(6)H(4))C(Bcat)=CH(Bcat), (Z)-(C6H5)C(Bcat)=C(C6H5)(Bcat), and (Z,Z)-(4-MeOC(6)H(4))C(Bcat)=C(Bcat)C(Bcat)=C(4-MeOC(6)H(4))(Bcat) (cat equals 1,2-O2C6H4; dppe equals Ph(2)PCH(2)CH(2)PPh(2); dppb equals Ph(2)P(CH2)(4)PPh(2))

The reactions of the B-B-bonded compounds B-2(cat)(2) (cat = 1,2-O2C6H4) (1a), B-2(4-Bu(t)-cat)(2) (1b), and B-2(OCMe(2)CMe(2)O)(2) (1c) with the Pt(O)-bis(phosphine) complex [(PPh(3))(2)Pt(eta-C2H4)] (4) via oxidative addition of the B-B bond yield cis-bis(boryl) Pt(II) complexes. The molecular structures of cis-[(PPh(3))(2)Pt(Bcat)(2)]. C6D6 (3a) and cis-[(PPh(3))(2)Pt(B-4-Bu(t)cat)(2)] (3b) were determined by single-crystal X-ray diffraction. Reaction of 3a with 1 equiv of the bidentate phosphine dppe (Ph(2)PCH(2)CH(2)PPh(2)) or dppb (Ph(2)P(CH2)(4)PPh(2)) proceeds smoothly in toluene to give cis-[(dppe)Pt(Bcat)(2)] (5a) and cis-[(dppb)Pt(Bcat)(2)] (5b), respectively, which have also been characterized by X-ray diffraction. Compounds 3a,b and 4 are highly active catalyst precursors for the diboration of alkynes and 1,3-diynes. X-ray crystal structures of (E)-(4-MeOC(6)H(4))C(Bcat)=CH(Bcat) (10c), (Z)-(C6H5)C(Bcat)=C(C6H5)(Bcat) (10e), and (Z,Z)-(4-MeOC(6)H(4))C(Bcat)=C(Bcat)C(Bcat)=C(4-MeOC(6)H(4))(Bcat) (14a) confirm the cis stereochemistry of the baron substituents in three representative cases, namely, products of the catalyzed diboration of the terminal alkyne 4-MeOC(6)H(4)C=CH, the internal alkyne PhC=CPh, and the tetraboration of the diyne 4,4'-MeOC(6)H(4)C=CC=CC(6)H(4)OMe. The presence of the C=CSiMe(3) moiety in catalytic reactions gives rise to additional products other than those derived from the diboration of the alkyne group. Metathetical reactions involving the diboron reagent and products derived from C-Si bond cleavage give rise to novel tris(boronate esters) as a result of the subsequent diboration of the C=CB(OR(2)) moieties formed by this competing process. The absence of catalytic activity using compound 5a, the extremely law activity of 5b, and the strong decrease in activity of 3b in the presence of added PPh(3) suggests that phosphine dissociation is a critical step in the catalytic pathway.