Influence of hydrogen bonding in competition with lattice interactions on carbonyl coordination at phosphorus. Implications for phosphoryl transfer activated states.

A series of phosphorus compounds containing carboxyl groups that serve as mimics for amino acid residues was synthesized. The series was composed of the phosphonium salts 1A, 1B, and 2, the anionic phosphines 3A and 3B, and the anionic phosphine oxide 4. X-ray structural analysis revealed that P-O coordination occurred in the presence of extensive hydrogen bonding and led to pseudo or regular trigonal bipyramidal geometries. (31)P chemical shifts indicated retention of the basic coordination geometries in solution. The two forms observed for 1 and 3 revealed the influence of hydrogen bonding on the P-O donor interactions while 2 and 4 showed the influence of molecular packing effects in competition with hydrogen bonding interactions. The results suggest that phosphoryl transfer enzyme mechanisms should benefit by taking into account P-O donor interactions by residues at active sites that can be manipulated by hydrogen bonding and molecular packing effects in enhancing nucleophilic attack at phosphorus centers.

Pseudoheptacoordination and pseudohexacoordination in tris(2-N,N-dimethylbenzylamino)phosphane.

The phosphane (C(6)H(4)-2-CH(2)NMe(2))(3)P (1) upon recrystallization from various solvents yielded the structurally different forms 1A, 1C, 1B(1), and 1B(2). Phosphane oxide (C(6)H(4)-2-CH(2)NOMe(2))(3)PO (2) was obtained from 1 by oxidation with hydrogen peroxide. X-ray analysis provided molecular structures for 1A, 1B(1), 1B(2), and 2. Phosphanes 1A and 1B(1) have pseudohexacoordinate frameworks as a result of the formation of two P-N donor interactions, 1B(2) has a pseudoheptacoordinate geometry due to the presence of three P-N interactions, and 2 resides in a tetrahedral geometry. The presence of the flexible dimethylaminobenzyl group in 1A, 1C, 1B(1), and 1B(2) is reasoned to be responsible for this variation in coordination geometry. Phosphane oxide 2 has very strong donor oxygen atoms from N-oxide groups but they are involved in competition with the presence of hydrogen bonding, which results in the lack of donor coordination. High-resolution (1)H, (13)C, and (31)P NMR measurements are also reported. The results provide evidence for the low-energy threshold required to allow hypercoordinated phosphorus to alter coordination geometry.

Conversion of tricoordinate to hexacoordinate phosphorus. Formation of a phosphorane-phosphatrane system.

Reaction of RPCl(2) with tris(2-hydroxy-3-tert-butyl-5-methylbenzyl)amine (4) led to the formation of a tricoordinated phosphonite (1) when R = Ph and to a hexacoordinated phosphorane-phosphatrane (2) when R = Et. The X-ray structures showed that the unreacted hydroxyl group in 1 oxidatively added to phosphorus in 2 leading to the formation of three additional bonds, a P[bond]O, a P[bond]H, and a P[bond]N linkage. In solution, (31)P measurements assisted by solid-state (31)P measurements revealed that each of the compounds existed in both structural forms. VT (31)P established equilibria where the solid-state structures predominated in each case. This is the first example of a conversion of three-coordinate to six-coordinate phosphorus on going from the solid to the solution state and the existence of these two disparate geometries in equilibrium with one another in solution. In the absence of steric protection with the use of an analogous amine (5) without tert-butyl groups, a hydrolysis reaction occurred with PhPCl(2). X-ray analysis revealed an anionic phenylphosphinate structure (3) hydrogen bonded in a cage-like arrangement with the protonated amine. Similar hydrolysis reactions take place with 1 and 2 but much more slowly.

P-O donor action from carboxylate anions with phosphorus in the presence of hydrogen bonding. A model for phosphoryl-transfer enzymes.

A series of phosphorus compounds (1-3) containing anionic carboxylate groups were synthesized by treatment of the respective neutral precursor acid forms B-D with amines, which also served to introduce hydrogen-bonding interactions. The compounds, subjected to X-ray structure analysis, resulted in hexacoordinated anionic phosphoranates 1A and 1B, a pseudo-trigonal-bipyramidal anionic phosphine (2), and a trigonal-bipyramidal anionic phosphine oxide (3). The structures revealed that P-O donor coordination was present in all members of the anionic series 1-3 and resulted in stronger interactions than existed in the precursor neutral acid forms B-D as measured by the presence of shorter P-O distances. Evaluation of the energies of the donor interactions relative to the energies of the hydrogen bonds that were present showed that the donor energies now exceeded the hydrogen bond strengths. (31)P chemical shifts indicated that the basic coordination geometries were retained in solution. Both 1A and 1B are chiral and exist as racemates. The results suggest that mechanisms of phosphoryl-transfer enzymes should benefit by taking into account donor interactions at phosphorus by residues at active sites in addition to the inclusion of hydrogen bonding. Reference is made to specific phosphoryl-transfer enzymes.

Chloro- and Fluoro-Substituted Phosphites, Phosphates, and Phosphoranes Exhibiting Sulfur and Oxygen Coordination(1).

New cyclic phosphoranes, O(2)S[Me(t-Bu)C(6)H(2)O](2)PCl(3) (1), O(2)S[Me(t-Bu)C(6)H(2)O](2)P(OC(6)H(4)-m-CF(3))(3) (2), and O(2)S[(t-Bu)(2)C(6)H(2)O](2)PCl(3) (3), containing sulfone donor groups and halogen substituents were synthesized by oxidative addition reactions of a diol with a tricoordinated phosphorus precursor. Cyclic phosphates, O(2)S[(t-Bu)(2)C(6)H(2)O](2)P(O)Cl (4) and O(2)S[Me(t-Bu)C(6)H(2)O](2)P(O)(OC(6)H(4)-m-CF(3)) (5), resulted from hydrolysis reactions of 3 and 2, respectively. Phosphate O(2)S[Me(t-Bu)C(6)H(2)O](2)P(O)(OC(6)F(5)) (6) was prepared from a known phosphorane precursor and independently from the reaction of an N-oxide molecule with a parent phosphite, O(2)S[Me(t-Bu)C(6)H(2)O](2)P(OC(6)F(5)). Two additional compounds containing a sulfur atom in place of the sulfone group, a cyclic phosphite, S[Me(2)C(6)H(2)O](2)P(OC(6)F(5)) (7), and a cyclic phosphate, S[Me(2)C(6)H(2)O](2)P(O)Cl (8), were synthesized. X-ray analysis and NMR data were obtained on all of these compounds to establish the role of electronegative ligands in promoting donor interaction. Compounds 1-3 were hexacoordinate as a consequence of oxygen atom coordination while 7 and 8 were trigonal bipyramidal due to sulfur atom coordination. The chlorine atom and the pentafluorophenoxy group promote similar degrees of coordination in these compounds. The results support the conclusion that the sulfur atom and oxygen atom of the sulfone group show similar coordination abilities in donor action with phosphoranes containing strongly electronegative ligands while in the presence of less electronegative ligands sulfur is the stronger coordinating agent. With phosphates and phosphites the sulfur atom exerts the stronger donor action independent of the electronegativity of the attached ligands.

Conformational Preference and Donor Atom Interaction Leading to Hexacoordination vs Pentacoordination in Bicyclic Tetraoxyphosphoranes(1).

New bicyclic tetraoxyphosphoranes all containing a six-membered oxaphosphorinane ring, C(6)H(8)(CH(2)O)(2)P(OC(12)H(8))(OXyl) (1), (C(6)H(4)O)(2)P(OC(12)H(8))(OXyl) (2), CH(2)[(t-Bu)(2)C(6)H(2)O](2)P(OC(12)H(8))(OXyl) (3), O(2)S[(t-Bu)MeC(6)H(2)O](2)P(OC(12)H(8))(OXyl) (4), and S[(t-Bu)MeC(6)H(2)O](2)P(OC(12)H(8))(OXyl) (5), were synthesized by the oxidative addition reaction of the cyclic phosphine P(OC(12)H(8))(OXyl) (6) with an appropriate diol in the presence of N-chlorodiisopropylamine. X-ray analysis revealed trigonal bipyramidal (TBP) geometries for 1-4 where the dioxa ring varied in size from six- to eight-membered. With a sulfur donor atom as part of an eight-membered ring in place of a potential oxygen donor atom of a sulfone group as in 4, the X-ray study of 5 showed the formation of a hexacoordinated structure via a P-S interaction. Ring constraints are evaluated to give an order of conformational flexibility associated with the (TBP) tetraoxyphosphoranes 4 > 3 approximately 1 > 2 which parallels the degree of shielding from (31)P NMR chemical shifts: 4 > 3 > 1 > 2. The six- and seven-membered dioxa rings in 1 and 2, respectively, are positioned at axial-equatorial sites, whereas the eight-membered dioxa ring in 3 and 4 occupies diequatorial sites of a TBP. V-T (1)H NMR data give barriers to xylyl group rotation about the C-OXyl bond. The geometry of 5 is located along a coordinate from square pyramidal toward octahedral to the extent of 60.7%. Achieving hexacoordination in bicyclic tetraoxyphosphoranes of reduced electrophilicity relative to bicyclic pentaoxyphosphoranes appears to be dependent on the presence of a sufficiently strong donor atom.

Increased Coordination via Sulfur Donor Action in Cyclic Pentaoxyphosphoranes and the Parent Cyclic Phosphite. Influence of Pentafluorophenoxy Ligands(1).

The pentafluorophenoxy ligand was introduced into the new cyclic pentaoxyphosphoranes S[(t-Bu)MeC(6)H(2)O](2)P(OC(6)F(5))(O(2)C(6)Cl(4)) (1), S[(t-Bu)MeC(6)H(2)O](2)P(OC(6)F(5))(O(2)C(14)H(8)) (2), and S[(t-Bu)MeC(6)H(2)O](2)P(OC(6)F(5))(3) (3). X-ray analysis revealed hexacoordinate structures formed by sulfur donor action present as a bridging atom in flexible eight-membered rings for 1-3. X-ray analysis showed that sulfur coordination also occurred with the same type of ring system as part of the phosphite S[(t-Bu)MeC(6)H(2)O](2)P(OC(6)F(5)) (4) to give a pseudo-trigonal-bipyramidal geometry. The pentafluorophenoxy ligand present in the oxyphosphoranes 1-3 as well as in the phosphite 4 acts comparably to a chlorine atom in its ability to enhance phosphorus electrophilicity as measured by the degree of P-S coordination and geometrical displacement toward a more highly coordinated state. The phosphite 4 has a P-S donor distance of 2.876(2) Å, considerably longer than the range of P-S distances from 2.366(3) to 2.495(2) Å obtained for the pentaoxyphosphoranes 1-3. These data express quantitatively the relative electrophilicity of phosphorus as a function of coordination number and substituent composition.

Molecular Structures of Three-, Four-, and Five-Coordinate Phosphorus Compounds Containing Salicylate Ligands(1).

The new cyclic compound 2,2'-sulfurylbis(4-methyl-6-tert-butylphenyl) methyl 2-benzoate phosphite, O(2)S[(t-Bu)MeC(6)H(4)O](2)(OC(6)H(4)CO(2)Me)P (3), containing a salicylate ligand was synthesized from 2,2'-sulfurylbis(4-methyl-6-tert-butylphenyl) chlorophosphite and methyl salicylate in the presence of triethylamine in ether solution. X-ray analyses of bis(methyl salicylate-O)phenylphosphine, (OC(6)H(4)CO(2)Me)(2)PPh (1), and bis(methylsalicylato-O)phenyl(tetrachlorophenylene-1,2-dioxy)phosphorane, (O(2)C(6)Cl(4))(OC(6)H(4)CO(2)Me)(2)PPh (2), as well as that for 3 were obtained. The phosphane 1 has a pseudo trigonal bipyramidal (TBP) structure due to coordination of a carbonyl oxygen atom at an axial site. The cyclic phosphorane 2 and the phosphite 3 lack any coordination from salicylate ligands. This results in a TBP geometry and a pyramidal geometry respectively for 2 and 3. Comparisons with X-ray structures for carboxylate-containing phosphorus compounds exhibiting oxygen coordination show the formation of four- and five-membered cyclic systems. Thus, 1 appears to be the first example of formation of a six-membered ring via carbonyl oxygen coordination. Reference is made to the tyrosyl-tRNA synthetase system where it is proposed that carbonyl oxygen atom coordination is a likely occurrence in the transition state on the basis of the analysis presented in this work.

Cyclic Three-, Four-, Five-, and Six-Coordinate Nitrogen-Containing Phosphorus Compounds Varying in Ring Size from Five- to Ten-Membered. P-N Donor Action(1).

New nitrogen-containing phosphorus compounds 1 and 3-5 were prepared by the reaction of a nitrogen-containing phenol with PhPCl(2). Hydrolysis of 1 gave an acyclic anionic phosphinate hydrogen bonded to an ammonium component (2). Use of a nitrogen-containing diol with P(OPh)(3) resulted in oxidative addition to give hexacoordinate pentaoxyphosphorus compound 6 exhibiting P-N donor action. X-ray analyses performed on all six phosphorus compounds revealed a variety of geometries extending from three- to six-coordinate with ring sizes varying from five- to ten-membered. The structure of 3 is displaced toward a trigonal bipyramid (TBP) as a result of weak P-N donor action. As a consequence of N-C bond cleavage, 1 forms as a bicyclic phosphorane with the nitrogen atom located at an equatorial site of a TBP. In the formation of the tetracoordinate cyclic phosphinate 5, a P-C bond is formed at the expense of O-C bond cleavage of the reactant diol. (1)H and (31)P NMR spectra indicated that the basic coordination structures were retained in solution. It is concluded that the more elusive donor action found for nitrogen relative to sulfur and oxygen is a consequence of bond cleavage reactions. However, with sufficient phosphorus electrophilicity in higher valent states, P-N donor action is achievable as found in the pentaoxyphosphorane (6) in this study while more modest donor action takes place in the lower coordinate state present in 3.

Hexacoordination via Sulfur Donor Action in Bicyclic Pentaoxyphosphoranes(1).

New bicyclic oxyphosphoranes, S[(t-Bu)MeC(6)H(2)O](2)P(OC(6)H(5))(O(2)C(6)H(3)F) (1) and S[(t-Bu)MeC(6)H(2)O](2)P(OC(6)H(5))(O(2)C(6)H(4)) (3), were synthesized by displacement reactions of a monocyclic pentaoxyphosphorane by a diol, and S[(t-Bu)(2)C(6)H(2)O](2)P(OCH(2)CF(3))(O(2)C(6)Cl(4)) (2) and S[(t-Bu)(2)C(6)H(2)O](2)P(C(6)H(5))(O(2)C(6)Cl(4)) (4), by oxidative addition reactions of a phosphite or phosphine with tetrachlorobenzoquinone. X-ray studies revealed hexacoordinated structures formed by the presence of a sulfur donor atom incorporated in a flexible eight-membered ring. The structures were displaced along a coordinate from a square pyramid toward an octahedron. (31)P and (1)H NMR data are also reported. Comparisons are made between bicyclic tetraoxyphosphoranes and monocyclic and bicyclic pentaoxyphosphoranes which show the importance of ligand electronegativity in increasing the degree of hexacoordination. Of the various series now studied, the extent of sulfur donor atom coordination increases in the following order: phosphates < phosphites < oxyphosphoranes. It is concluded that, in general, sulfur donor atom coordination will take place with phosphorus in any of the common coordination geometries in the presence of sufficiently electronegative ligands.

Axial Site Occupancy by the Least Electronegative Ligands in Trigonal Bipyramidal Tetraoxyphosphoranes(1).

Analogous to the formation of CH(2)[(t-Bu)(2)C(6)H(2)O](2)P(Ph)(O(2)C(6)Cl(4)) (1), the new bicyclic tetraoxyphosphoranes CH(2)[(t-Bu)(2)C(6)H(2)O](2)P(Et)(O(2)C(6)Cl(4)) (3) and CH(2)[ClC(6)H(3)O](2)P(Ph)(O(2)C(6)Cl(4)) (4) were synthesized by the oxidative addition of the appropriate cyclic phosphines with o-tetrachlorobenzoquinone. For the formation of CH(2)[(t-Bu)(2)C(6)H(2)O](2)P(Ph)(O(2)C(2)Ph(2)) (2), a similar reaction was followed with the use of benzil (PhCOCOPh) in place of o-tetrachlorobenzoquinone. X-ray analysis of 1-3 revealed trigonal bipyramidal geometries and provided evidence for the first series of complexes in the absence of ring strain in which the least electronegative group, ethyl or phenyl, is located in an axial position, in violation of the electronegativity rule. Thus, the two oxygen-containing ring systems occupied two different sets of positions in the trigonal bipyramid (TBP) with the eight-membered rings at diequatorial sites. X-ray analysis of 4 revealed a trigonal bipyramidal geometry with electron-withdrawing chlorine substituents on each ring assumed the more conventional geometry with the rings occupying axial-equatorial positions and the phenyl group located in the remaining equatorial site. The fact that molecular mechanics calculations favorably reproduced the observed geometries suggests that a steric contribution associated with the ring tert-butyl groups for 1-3 is partly responsible in favoring diequatorial ring occupancy for the eight-membered ring. NMR data supported rigid pentacoordinated structures in solution at 23 degrees C. Phosphorane 1 crystallizes in the orthorhombic space group Fdd2 with a = 44.787(5) Å, b = 34.648(8) Å, c = 10.3709(9) Å, and Z = 16. Phosphorane 2 crystallizes in the orthorhombic space group Pna2(1) with a = 20.658(8) Å, b = 10.342(2) Å, c = 19.879(6) Å, and Z = 4. Phosphorane 3 crystallizes in the orthorhombic space group Pcmn with a = 9.807(2) Å, b = 16.632(4) Å, c = 23.355(3) Å, and Z = 4. Phosphorane 4 crystallizes in the monoclinic space group C2/c with a = 35.699(5) Å, b = 12.187(2) Å, c = 14.284(3) Å, beta = 107.08(1) degrees, and Z = 8. The final conventional unweighted residuals are 0.0395 (1), 0.0518 (2), 0.0540 (3), and 0.0868 (4).

Synthesis and Molecular Structures of Pyridine-Containing Large-Membered Cyclic Bis(alkoxy)silanes(1).

The new cyclic silanes [(C(5)H(3)N)(CH(2)O)(2)SiMe(2)](2) (1) and (C(5)H(3)N)(CH(2)CPh(2)O)(2)SiMe(2) (2) containing 16-membered and 10-membered rings, respectively, were prepared by the condensation reaction of Me(2)SiCl(2) with an appropriate pyridine diol in the presence of Et(3)N. X-ray studies show that the dimeric formulation for 1 represents a tetracoordinate cyclic silane, whereas 2 has a geometry halfway from a tetrahedron toward a trigonal bipyramid (TBP) as a result of Si-N(ax) donor action. (29)Si and (1)H NMR indicate retention of the coordination geometry for 2 in solution that undergoes rapid Si-N cleavage and ring rearrangement. In comparison with other silanes containing five- and six-membered rings that exhibit nitrogen or oxygen coordination, the presence of larger rings, as in 2 and related silanes having sulfur coordination, indicates that retention of donor action persists, thus largely ruling out ring size as a dominant factor controlling the possibility of donor action at silicon. The dimeric silane 1 crystallizes in the triclinic space group P&onemacr; with a = 6.347(3) Å, b = 12.455(4) Å, c = 14.289(5) Å, alpha = 101.63(3) degrees, beta = 102.99(3) degrees, gamma = 104.71(3) degrees, and Z = 2. The cyclic silane 2 crystallizes in the triclinic space group P&onemacr; with a = 9.733(4) Å, b = 10.938(2) Å, c = 14.312(3) Å, alpha = 89.03(2) degrees, beta = 74.59(3) degrees, gamma = 79.24(3) degrees, and Z = 2. The final conventional unweighted residuals are 0.040 (1) and 0.039 (2).