Synthesis and properties of SMAPs 1-phospha-4-silabicyclo[2.2.2

Synthesis and properties of a new class of trialkylphosphine ligands SMAPs (1-phospha-4-silabicyclo[2.2.2]octane derivatives, named after s ilicon-constrained m onodentate a lkyl p hosphine) with Me 3 P-like steric demand around the phosphorus center are described. A new feature of this class of ligands is the presence of a site for functionalization at the backside of the P -lone pair, which is not the case for Me 3 P. The SMAP ligands contain phosphorus and silicon atoms at each bridgehead of the bicyclo[2.2.2]octane framework. The molecular constraint of the bicyclic framework makes the steric demand around the phosphorus center as small as that of Me 3 P and projects the P -lone pair and the Si -substituent (R) in diametrically opposite directions on the straight line defined by the two bridgehead atoms. SMAP derivatives bearing Si substituents with varied electronic natures are obtainable by transforming the parent compound 4-phenyl-phospha-4-silabicyclo

1. Introduction 2. Synthesis of Ph-SMAP 3. X-ray crystal structure analysis of Ph-SMAP derivatives 4. Electronic properties of Ph-SMAP (DFT calculations) 5. Structure modification of SMAP at the bridgehead silicon

Introduction
Trialkylphosphines have found wide application in coordination chemistry and organometallic chemistry as metal-coordinating ligands with strong σ-donating ability.One ligand with an extremely low steric demand is trimethylphosphine (Me 3 P).We have designed a new class of trialkylphosphine ligands 1 (SMAP, named after silicon-constrained monodentate alkylphosphine) with Me 3 P-like steric demand around the phosphorus center (Figure 1).A new feature of this class of ligands is the presence of a site for functionalization at the backside of the P-lone pair, which is not the case for Me 3 P.The SMAP ligands 1 contain phosphorus and silicon atoms at each bridgehead of the bicyclo[2.2.2]octane framework.The molecular constraint of the bicyclic framework makes the steric demand around the phosphorus center as small as that of Me 3 P and projects the P-lone pair and the Si-substituent (R) in diametrically opposite directions on the straight line defined by the two bridgehead atoms (Figure 1).P-donor ligands that can be functionalized with such a directional constraint are rarely found and are limited to phosphaalkynes (2), 2 phosphabenzenes (3), 3 bicyclic phosphites (4) 4 and phosphatriptycenes (5). 5 To the best of our knowledge, no analogous trialkylphosphine ligand exists. 6MAP derivatives (1a-1g, Figure 1) bearing Si substituents with varied electronic natures are obtainable by transforming the parent compound Ph-SMAP (1a) through Si-Ph bond cleavage, and they constitute a class of electronically tunable trialkylphosphines.7 DFT calculations indicated that the parent ligand Ph-SMAP is similar to Me 3 P in donor power and that the tunable range of the donor power overlaps those of (t-Bu) 3 P and RAr 2 P (R: alkyl, Ar: aryl).This account describes the synthesis and properties of the SMAP derivatives.

Synthesis of Ph-SMAP
The synthesis of Ph-SMAP (1a) is illustrated in Scheme 1. Phenyltrivinylsilane (6) was converted into triol 7 through three-fold hydroboration with diisopinocampheylborane followed by H 2 O 2 /NaOH oxidation. 8The hydroboration proceeded with complete regioselectivity to give the primary alcohol against the electronic requirement of the silicon atom to induce the opposite selectivity.Mesylation of 7 and subsequent substitution with iodide anion afforded tris(2iodoethyl)phenylsilane (8).Then [5+1] annulation between the triiodide (8) and dilithium salt 9 of PhPH 2 -BH 3 produced the borane complex of monocyclic tertiary phosphine (9) as a mixture of cis-and trans-isomers (cis:trans = ca.60:40).Upon heating with excess 1-octene in refluxing DME, the isomeric mixture of 9 was transformed into phosphonium salt 10. 10,11 Subsequent reductive cleavage of the P-Ph bond of 10 with lithium naphthalenide followed by reaction with sulfur afforded phosphine sulfide 11.Alternatively, addition of BH Solid Ph-SMAP is highly air-stable, with no detectable oxidation observed after exposure to air for several days.Moreover, being almost odorless, Ph-SMAP does not produce the noxious phosphine odor characteristic of volatile phosphines.

X-ray crystal structure analysis of Ph-SMAP derivatives
Single crystal X-ray diffraction analysis revealed a rod-like shape of Ph-SMAP (1a) and Ph-SMAP-BH 3 (12) (Figure 2). 14,15Analyses also showed that the bicyclic cage possesses some flexibility and twists toward chiral C 3 -symmetric conformations.In free phosphine 1a, the values for the average C-P-C and P-C-C-Si dihedral angles, and the P-Si distance are 100.9°,15.5° and 3.105 Å, respectively.In BH 3 complex 12, the P atom bonds to the B atom with a distance of 1.922(2) Å. 16 The average C-P-C angle is enlarged to 104.2°.Such a slight enlargement of the angles around the P atom is typical for the metal coordination of a P-donor ligand.The BH 3 coordination also causes shrinkage of the cage as indicated by enlargement of the P-C-C-Si dihedral angles (22.3°, averaged) and shortening of the P-Si distance (3.031 Å).Although the cage possesses some flexibility for twisting and stretching, almost no bending of the longest molecular axes was observed for both 1a and 12.

Electronic properties of Ph-SMAP (DFT calculations)
DFT calculations [B3LYP/6-31G(d,p)] indicated that Ph-SMAP possesses an electron-donating ability as strong as that of Me 3 P, and replacement of the Si atom of Ph-SMAP with a carbon atom drastically decreases the donor power.We optimized the geometry of Ph-SMAP (Figure 4a) and evaluated donor ability by the value of the molecular electrostatic potential minimum V min (kcal•mol -1 ) according to Koga's method. 18A larger negative V min value corresponds to stronger electron-donating ability of a phosphine.For comparison, we also performed calculations for 4-phenyl-1-phosphabicyclo[2.2.2]octane (13, Figure 4b), an analog of Ph-SMAP that has a bridgehead carbon atom instead of the Si atom. 19As shown in Table 1, the V min (-43.14kcal•mol -1 ) of Ph-SMAP is much more negative than the value of monoaryldialkylphosphine PhMe 2 P and is in the range for trialkylphosphines, being between the values of Me 3 P and Et 3 P.However, the V min of 13 is less negative than that of PhMe 2 P. The drastic decrease in donor ability upon placement of a carbon atom at the bridgehead may be due to the increase in s-character of the P-lone pair caused by the strain in the 1phosphabicyclo[2.2.2]octane cage.The strain is evident from the comparison of the C-P-C angles of the optimized structures; the avarage angle of 13 (96.2°) is much smaller than that of Ph-SMAP (99.7°), and the latter is almost the same as the values of Me 3 P (99.4°) and Et 3 P (99.5°).

Structure modification of SMAP at the bridgehead silicon
SMAP derivatives with a substituted phenyl group on the bridgehead silicon were synthesized through a hydrosilane-type compound H-SMAP sulfide ( 14), a pivotal compound of diverse reactivity.The hydrosilane 3 was obtained as an air-stable crystalline material from Ph-SMAP sulfide (11a) through protodesilation with TfOH followed by reduction with LiAlH 4 (Scheme 2).Note that the Si-Ph bond in Ph-SMAP sulfide was stable against protodesilation when compared to that in acyclic phenylsilane Ph-SiBu 3 .While the latter was totally cleaved on treatment with 1.1 eq of TfOH in CH 2 Cl 2 at 0°C for 3 h, the former needed 8 eq of TfOH and a reaction time of 15 h at 25°C for 100% cleavage.The low reactivity of Ph-SMAP sulfide likely is due to instability of the leaving non-planar bridgehead silyl cation.The palladium-catalyzed hydrosilane-iodoarene coupling developed by Masuda was applied to H-SMAP sulfide (14) to afford a series of SMAP sulfides (11b-g). 20The yield of the cross-coupling product depended on the iodoarenes and the reaction conditions.Generally the yield was improved (20%~63%) by slow addition of a solution of hydrosilane 3 into a solution of the catalyst and iodoarene.Reduction of the phosphine sulfides with Si 2 Cl 6 afforded the corresponding phosphines (Ar-SMAPs, 1b-g).These phosphines are air-stable, crystalline, colorless solids that do not produce a noxious phosphine odor.

Electronic substituent effect at the Si-Ph group on P-donor power
DFT calculations [B3LYP/6-31G(d,p)] indicated that the electronic character of the P-lone pair of the SMAP derivatives is strongly influenced by the distal Si substituents. 21We again used the molecular electrostatic potential minimum V min (kcal/mol) value associated with the phosphine lone pair region as a quantitative measure of donor power (Table 2). 17Calculations indicated that the donor powers of 4-MeO-Ph-SMAP (1c) and 4-Me-Ph-SMAP (1d) apparently were stronger than those of Me 3 P and Ph-SMAP (1a), and comparable with those of Bu 3 P and (i-Pr) 3 P (entries 3-6).The para-Me 2 N group exerts a much larger effect, increasing the donor ability of the aniline-type derivative 4-Me 2 N-Ph-SMAP (1b) even more than (t-Bu) 3 P, which is one of the σdonor ligands possessing the strongest donor power (entries 1, 2).In contrast, substitution at the para-position with an electron-withdrawing group decreased donor power; the donating power of 4-Cl-Ph-SMAP (1e) and 4-CF 3 -Ph-SMAP (1f) were comparable with monoaryldialkylphosphine Me 2 PhP (entries 9-11).Donor power of 3,5-(CF 3 ) 2 -Ph-SMAP (1g) is as weak as that of MePh 2 P (entries 12, 13).Figure 5 shows that the substitution effect at the para-position of Ph-SMAP on V min is proportional to Hammett's substituent constant σ.The 1 J( 31 P, 77 Se) measurements for phosphine selenides provided experimental evidence for the substituent effect at the bridgehead silicon atom on phosphine donor power.The selenides were prepared by heating a mixture of the phosphine and selenium in C 6 D 6 in an NMR tube, and the 1 J( 31 P, 77 Se) values were measured in situ.It is commonly accepted that the stronger the donor power is, the smaller the 1 J( 31 P, 77 Se) value. 22Values for SMAP selenides (15a-g) and Me 3 P=Se and Bu 3 P=Se are listed in Table 1.The 1 J( 31 P, 77 Se) values for the SMAP selenides increase by 12.9 Hz upon examination of the selenide (15b) possessing the strongest donating aryl-SMAP (1b) to that (15g) of the weakest donating one (1g).Figure 3 demonstrates that 1 J( 31 P, 77 Se) values for SMAP selenides 15a-g show good linear correlation with the theoretical measures (V min 's).Figure 3

Conclusions
We presented a new class of trialkylphosphines SMAPs with steric and electronic features that guarantee its wide application as a robust ligand for transition metal coordination.SMAP derivatives with various Si-substituents were synthesized by transforming Ph-SMAP through Si-Ph bond cleavage.The donor properties of the P-lone pair varied over a wide range depending upon the electronic nature of the Si substituent, with retention of the steric demand around the phosphorus center.These characteristics should allow SMAPs to find diverse utility in mechanistic studies of organometallic reactions and in the development of efficient catalytic reactions.In addition, the molecular rigidity and flexibility of functionalization allows the preparation of a sophisticated series of SMAP ligands providing useful components for supramolecular architectures based on coordination chemistry.

Figure 1 .
Figure 1.Structures of SMAP and related P-donor ligands.

Figure 3 Figure 3 .
Figure 3 represents densely packed crystal structures of 1a and 12.The former consists of the two enantiomeric molecules (P2 1 /n), while only a single enantiomer is involved in the latter (chiral space group: P2 1 ) 17 (This is a case of chiral crystallization of an achiral molecule with

a Data taken from ref 18 Figure 5 .
Figure 5.A plot of Vmin vs Hammett's substituent constants σ's for SMAP derivatives.
also shows that the values for Me 3 P=Se and Bu 3 P=Se are far below the correlation line for 15a-g.This drastic deviation is attributable to the change in steric environment around the P center.

Figure 6 .
Figure 6.A plot of 1 J( 31 P, 77 Se) vs V min of the phosphine derivatives.The correlation line is associated with 1a-g/15a-g.

Table 1 .
Results of DFT calculations for various tertiary phosphines a Values of optimized structures.b Data are taken from ref 18.